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Vol 11: The Classics - Part 2
I then put into the hive, instead of a thick, rectangular
piece of wax, a thin and narrow, knife-edged ridge, coloured
with vermilion. The bees instantly began on both sides to
excavate little basins near to each other, in the same way as
before; but the ridge of wax was so thin, that the bottoms
of the basins, if they had been excavated to the same depth
as in the former experiment, would have broken into each
other from the opposite sides. The bees, however, did not
suffer this to happen, and they stopped their excavations in
due time; so that the basins, as soon as thej^ had been a little
deepened, came to have flat bases ; and these flat bases,
formed by thin little plates of the vermilion wax left un-
gnawed, were situated, as far as the eye could judge, exactly
along the planes of imaginary intersection between the
basins on the opposite sides of the ridge of wax. In some
parts, only small portions, in other parts, large portions of a
rhombic plate were thus left between the opposed basins,
but the work, from the unnatural state of things, had not
been neatly performed. The bees must have worked at very
nearly the same rate in circularly gnawing away and deep-
ening the basins on both sides of the ridge of vermilion wax,
in order to have thus succeeded in leaving flat plates between
the basins, by stopping work at the planes of intersection.
Considering how flexible thin wax is, I do not see that
there is any difficulty in the bees, whilst at work on the two
sides of a strip of wax, perceiving when they have gnawed
the wax away to the proper thinness, and then stopping their
work. In ordinary combs it has appeared to me that the
bees do not always succeed in working at exactly the same
rate from the opposite sides- for I have noticed half-com-
pleted rhombs at the base of a just commenced cell, which
were slightly concave on one side, where I suppose that the
284 ORIGIN OF SPECIES
bees had excavated too quickl}', and convex on the opposed
side where the bees had worked less quickly. In one well
marked instance, I put the comb back into the hive, and
allowed the bees to go on working for a short time, and
again examined the cell, and I found that the rhombic plate
had been completed, and had become perfectly flat: it w-as
absolutely impossible, from the extreme thinness of the little
plate, that they could have effected this by gnawing away
the convex side; and I suspect that the bees in such cases
stand on opposite sides and push and bend the ductile and
warm wax (which as I have tried is easily done) into its
proper intermediate plane, and thus flatten it.
From the experiment of the ridge of vermilion wax we
can see that, if the bees were to build for themselves a thin
wall of wax, they could make their cells of the proper shape,
by standing at the proper distance from each other, by exca-
vating at the same rate, and by endeavouring to make equal
spherical hollows, but never allowing the sp-heres to break
into each other. Now bees, as may be clearly seen by exam-
ining the edge of a growing comb, do make a rough, circum-
ferential wall or rim all round the comb; and they gnaw this
away from the opposite sides, always working circularly as
they deepen each cell. They do not make the whole three-
sided pyramidal base of any one cell at the same time, but
only that one rhombic plate which stands on the extreme
growing margin, or the two plates, as the case may be ; and
they never complete the upper edges of the rhombic plates,
until the hexagonal walls are commenced. Some of these
statements differ from those made by the justly celebrated
elder Huber, but I am convinced of their accuracy; and if
I had space, I could show that they are conformable with
my theory.
Ruber's statement, that the very first cell is excavated out
of a little parallel-sided wall of wax, is not, as far as I have
seen, strictly correct; the first commencement having always
been a little hood of wax; but I will not here enter on de-
tails. We see how important a part excavation plays in the
construction of the cells; but it would be a great error to
suppose that the bees cannot build up a rough wall of wax in
the proper position — that is, along the plane of intersection
CELD-MAKING INSTINCT 285
between two adjoining spheres. I have several specimens
showing clearly that they can do this. Even in the rude
circumferential rim or wall of wax round a growing comb,
flexures may sometimes be observed, corresponding in posi-
tion to the planes of the rhombic basal plates of future cells.
But the rough wall of wax has in every case to be finished
off, by being largely gnawed away on both sides. The
manner in which the bees build is curious; they always make
the first rough wall from ten to twenty times thicker than
the excessively thin finished wall of the cell, which will
ultimately be left. We shall understand how they work, by
supposing masons first to pile up a broad ridge of cement,
and then to begin cutting it away equally on both sides near
the ground, till a smooth, very thin wall is left in the middle ;
the masons always piling up the cut-away cement, and
adding fresh cement on the summit of the ridge. We shall
thus have a thin wall steadily growing upward but always
crowned bv a gigantic coping. From all the cells, both those
just commenced and those completed, being thus crowned by
a strong coping of wax, the bees can cluster and crawl over
the com.b without injuring the delicate hexagonal walls.
These walls, as Professor Miller has kindly ascertained for
me, vary greatly in thickness; being, on an average of
twelve measurements made near the border of the comb,
■j^^ of an inch in thickness ; whereas the basal rhomboidal
plates are thicker, nearly in the proportion of three to two,
having a mean thickness, from twenty-one measurements,
of -j^-g of an inch. By the above singular manner of build-
ing, strength is continually given to the comb, with the ut-
most ultimate economy of wax.
It seems at first to add to the difficulty of understanding
how the cells are made, that a multitude of bees all work
together ; one bee after working a short time at one cell
going to another, so that, as Huber has stated, a score of in-
dividuals work even at the commencement of the first cell.
I was able practically to show this fact, by covering the
edges of the hexagonal walls of a single cell, or the extreme
margin of the circumferential rim of a growing comb, with
an extremely thin layer of melted vermilion wax ; and I in-
variably found that the Colour was most delicately diffused
286 ORIGIN OF SPECIES
by the bees — as delicately as a painter could have done it
with his brush — by atoms of the coloured wax having been
taken from the spot on which it had been placed, and worked
into the growing edges of the cells all round. The work
of construction seems to be a sort of balance struck between
many bees, all instinctively standing at the same relative
distance from each other, all trying to sweep equal spheres,
and then building up, or leaving ungnawed, the planes of
intersection between these spheres. It was really curious to
note in cases of difficulty, as when two pieces of comb met
at an angle, how often the bees would pull down and rebuild
in different ways the same cell, sometimes recurring to a
shape which they had at first rejected.
When bees have a place on which they can stand in their
proper positions for working, — for instance, on a slip of
wood, placed directly under the middle of a comb growing
downwards, so that the comb has to be built over one face
of the slip — in this case the bees can lay the foundations
of one wall of a new hexagon, in its strictly proper place,
projecting beyond the other completed cells. It suffices that
the bees should be enabled to stand at their proper relative
distances from each other and from the walls of the last
completed cells, and then, by striking imaginary spheres,
they can build up a wall intermediate between two adjoin-
ing spheres; but, as far as 1 have seen, they never gnaw
away and finish off the angles of a cell till a large part both
of that cell and of the adjoining cells, has been built. This
capacity in bees of laying down under certain circumstances
a rough wall in its proper place between two just-commenced
cells, is important, as it bears on a fact, which seems at
first subversive of the foregoing theory; namely, that the
cells on the extreme margin of wasp-combs are sometimes
strictly hexagonal; but I have not space here to enter on
this subject. Noi does there seem to me any great difficulty
in a single insect (as in the case of a queen-wasp) making
hexagonal cells, if she were to work alternately on the in-
side and outside of two or three cells commenced at the
same time, always standing at the proper relative distance
from the parts of the cells just begun, sweeping spheres or
cylinders, and building up intermediate planes.
CELL-MAKING INSTINCT 287
As natural selection acts only by the accumulation of
slight modifications of structure or instinct, each profitable
to the individual under its conditions of life, it may reason-
ably be asked, how a long and graduated succession of modi-
fied architectural instincts, all tending towards the present
perfect plan of construction, could have profited the progeni-
tors of the hive-bee? I think the answer is not difficult:
cells constructed like those of the bee or the wasp gain in
strength, and save much in labour and space, and in the ma-
terials of which they are constructed. With respect to the
formation of wax, it is known that bees are often hard
pressed to get sufficient nectar, and I am informed by Mr,
Tegetmeier that it has been experimentally proved that from
twelve to fifteen pounds of dry sugar are consumed by a
hive of bees for the secretion of a pound of wax; so that
a prodigious quantity of fluid nectar must be collected and
consumed by the bees in a hive for the secretion of the wax
necessary for the construction of their combs. Moreover,
many bees have to remain idle for many days during the
process of secretion. A large store of honey is indispensable
to support a large stock of bees during the winter; and the
security of the hive is known mainly to depend on a large
number of bees being supported. Hence the saving of wax
by largely saving honey and the time consumed in collect-
ing the honey must be an important element of success to
any family of bees. Of course the success of the species
may be dependent on the number of its enemies, or parasites,
or on quite distinct causes, and so be altogether independent
of the quantity of honey which the bees can collect. But
let us suppose that this latter circumstance determined, as
it probably often has determined, whether a bee allied to
our humble-bees could exist in large numbers in any coun-
try; and let us further suppose that the community lived
through the winter, and consequently required a store of
honey: there can in this case be no doubt that it would be
an advantage to our imaginary humble-bee, if a slight modi-
fication in her instincts led her to make her waxen cells
near together, so as to intersect a little; for a wall in com-
mon even to two adjoining cells would save some little labour
and wax. Hence it would continually be more and more
288 ORIGIN OF SPECIES
advantageous to our humble-bees, if they were to make their
cells more and more regular, nearer together, and aggre-
gated into a mass, like the cells of the Melipona; for in this
case a large part of the bounding surface of each cell would
serve to bound the adjoining cells, and much labour and wax
would be saved. Again, from the same cause, it would be
advantageous to the Melipona, if she were to make her cells
closer together, and more regular in every way than at pres-
ent; for then, as we have seen, the spherical surfaces would
wholly disappear and be replaced by plane surfaces; and
the Melipona would make a comb as perfect as that of the
hive-bee. Beyond this stage of perfection in architecture,
natural selection could not lead; for the comb of the hive-
bee, as far as we can see, is absolutely perfect in economis-
ing labour and wax.
Thus, as I believe, the most wonderful of all known in-
stincts, that of the hive-bee, can be explained by natural
selection having taken advantage of numerous, successive,
slight modifications of simpler instincts ; natural selection
having, by slow degrees, more and more perfectly led the
bees to sweep equal spheres at a given distance from each
other in a double layer, and to build up and excavate the wax
along the planes of intersection ; the bees, of course, no more
knowing that they swept their spheres at one particular dis-
tance from each other, than they know what are the several
angles of the hexagonal prisms and of the basal rhombic
plates ; the motive power of the process of natural selection
having been the construction of cells of due strength and of
the proper size and shape for the larvae, this being effected
with the greatest possible economy of labour and wax; that
individual swarm which thus made the best cells with least
labour, and least waste of honey in the secretion of wax,
having succeeded best, and having transmitted their newly
acquired economical instincts to new swarms, which in their
turn will have had the best chance of succeeding in the
struggle for existence.
OBJECTIONS TO THE THEORY 289
OBJECTIONS TO THE THEORY OF NATURAL SELECTION AS APPLIED
TO INSTINCTS: NEUTER AND STERILE INSECTS
It has been objected to the foregoing view of the origin of
instincts that "the variations of structure and of instinct must
have been simultaneous and accurately adjusted to each other
as a modification in the one without an immediate correspond-
ing change in the other would have been fatal." The force
of this objection rests entirely on the assumption that the
changes in the instincts and structure are abrupt. To take
as an illustration the case of the larger titmouse (Parus
major) alluded to in a previous chapter; this bird often holds
the seeds of the yew between its feet on a branch, and ham-
mers with its beak till it gets at the kernel. Now what spe-
cial difficulty would there be in natural selection preserving
all the slight individual variations in the shape of the beak,
which were better and better adapted to break open the seeds,
until a beak was formed, as well constructed for this purpose
as that of the nuthatch, at the same time that habit, or com-
pulsion, or spontaneous variations of taste, led the bird to
become more and more of a seed-eater? In this case the beak
is supposed to be slowly modified by natural selection, subse-
quently to, but in accordance with, slowly changing habits
or taste ; but let the feet of the titmouse vary and grow larger
from correlation with the beak, or from any other unknown
cause, and it is not improbable that such larger feet would
lead the bird to climb more and more until it acquired the
remarkable climbing instinct and power of the nuthatch. In
this case a gradual change of structure is supposed to lead to
changed instinctive habits. To take one more case : few
instincts are more remarkable than that which leads the swift
of the Eastern Islands to make its nest wholly of inspissated
saliva. Some birds build their nests of mud, believed to be
moistened with saliva; and one of the swifts of North
America makes its nest (as I have seen) of sticks aggluti-
nated with saliva, and even with flakes of this substance. Is
it then very improbable that the natural selection of individual
swifts, which secreted more and more saliva, should at last
produce a species with instincts leading it to neglect other
materials, and to make its nest exclusively of inspissated
J — lie XI
290 ORIGIN OF SPECIES
saliva ? And so in other cases. It must, however, be admitted
that in many instances we cannot conjecture whether it was
instinct or structure which first varied.
No doubt many instincts of very difficult explanation could
be opposed to the theory of natural selection — cases, in which
we cannot see how an instinct could have originated; cases,
in which no intermediate gradations are known to exist ;
cases of instincts of such tritiing importance, that they could
hardly liave been acted on by natural selection; cases of in-
stincts almost identically the same in animals so remote in
the scale of nature, that we cannot account for their simi-
larity by inheritance from a common progenitor, and conse-
quently must believe that they were independently acquired
through natural selection. I will not here enter on these
several cases, but will confine myself to one special difficulty,
which at first appeared to me insuperable, and actually fatal
to the whole theory. I allude to the neuters or sterile females
in insect-communities ; for these neuters often differ widely
in instinct and in structure from both the males and fertile
females, and yet, from being sterile, they cannot propagate
their kind.
The subject well deserves to be discussed at great length,
but I will here take only a single case, that of working or
sterile ants. How the workers have been rendered sterile
is a difficulty; but not much greater than that of any other
striking modification of structure ; for it can be shown that
some insects and other articulate animals in a state of nature
occasionally become sterile ; and if such insects had been
social, and it had been profitable to the community that a
number should have been annually born capable of work, but
incapable of procreation, I can see no especial difficulty in
this having been effected through natural selection. But I
must pass over this preliminary difficulty. The great difficulty
lies in the working ants differing widely from both the males
and the fertile females in structure, as in the shape of the
thorax, and in being destitute of wings and sometimes of
eyes, and in instinct. As far as instinct alone is concerned,
the wonderful difference in this respect between the workers
and the perfect females, would have been better exemplified
by the hive-bee. If a working ant or other neuter insect had
OBJECTIONS TO THE THEORY 291
been an ordinary animal, I should have unhesitatingly as-
sumed that all its characters had been slowly acquired through
natural selection; namely, by individuals having been born
with slight profitable modifications, which were inherited by
the offspring; and that these again varied and again were
selected, and so onwards. But with the working ant we have
an insect dift'ering greatly from its parents, yet absolutely
sterile, so that it could never have transmitted successively
acquired modifications of structure or instinct to its progeny.
It may well be asked how is it possible to reconcile this case
with the theory of natural selection?
First, let it be remembered that we have innumerable in-
stances, both in our domestic productions and in those in a
state of nature, of all sorts of differences of inherited struc-
ture which are correlated with certain ages, and with either
sex. We have dift'erences correlated not only with one sex,
but with that short period when the reproductive system is
active, as in the nuptial plumage of many birds, and in the
hooked jaws of the male salmon. We have even slight dif-
ferences in the horns of different breeds of cattle in relation
to an artificially imperfect state of the male sex; for oxen
of certain breeds have longer horns than the oxen of other
breeds, relatively to the length of the horns in both the bulls
and cows of these same breeds. Hence I can see no great
difficulty in any character becoming correlated with the sterile
condition of certain members of insect-communities : the dif-
ficulty lies in understanding how such correlated modifications
of structure could have been slowly accumulated by natural
selection.
This difficulty, though appearing insuperable, is lessened,
or, as I believe, disappears, when it is remembered that selec-
tion may be applied to the family, as well as to the individual,
and may thus gain the desired end. Breeders of cattle wish
the flesh and fat to be well marbled together: an animal thus
characterised has been slaughtered, but the breeder has gone
with confidence to the same stock and has succeeded. Such
faith may be placed in the power of selection, that a breed
of cattle, always yielding oxen with extraordinarily long
horns, could, it is probable, be formed by carefully watching
Which individual bulls and cows, when matched, produce oxen
292 ORIGIN OF SPECIES
with the longest horns ; and yet no one ox would ever have
propagated its kind. Here is a better and real illustration:
according to M. Verlot, some varieties of the double annual
Stock from having been long and carefully selected to the
right degree, always produce a large proportion of seedlings
bearing double and quite sterile flowers ; but they likewise
yield some single and fertile plants. These latter, by which
alone the variety can be propagated, may be compared with
the fertile male and female ants, and the double sterile plants
with the neuters of the same community. As with the varie-
ties of the stock, so with social insects, selection has been
applied to the family, and not to the individual, for the sake
of gaining a serviceable end. Hence we may conclude that
slight modifications of structure or of instinct, correlated
with the sterile condition of certain members of the com-
munity, have proved advantageous : consequently the fertile
males and females have flourished, and transmitted to their
fertile offspring a tendency to produce sterile members with
the same modifications. This process must have been re-
peated many times, until that prodigious amount of difference
between the fertile and sterile females of the same species
has been produced, which we see in many social insects.
But we have not as yet touched on the acme of the diffi-
culty; namely, the fact that the neuters of several ants differ,
not only from the fertile females and males, but from each
other, sometimes to an almost incredible degree, and are thus
divided into two or even three castes. The castes, moreover,
do not commonly graduate into each other, but are perfectly
well defined ; being as distinct from each other as are any two
species of the same genus, or rather as any two genera of the
same family. Thus in Eciton, there are working and soldier
neuters, with jaws and instincts extraordinarily different: in
Cryptocerus, the workers of one caste alone carry a wonder-
ful sort of shield on their heads, the use of which is quite
unknown: m the Mexican Myrmecocystus, the workers of
one caste never leave the nest ; they are fed by the workers
of another caste, and they have an enormously developed ab-
domen which secretes a sort of honey, supplying the place of
that excreted by the aphides, or the domestic cattle as they
may be called, which our European ants guard and imprison.
OBJECTIONS TO THE THEORY 293
It will indeed be thought that I have an overweening con-
fidence in the principle of natural selection, when I do not
admit that such wonderful and well-established facts at once
annihilate the theory. In the simpler case of neuter insects
all of one caste, which, as I believe, have been rendered dif-
ferent from the fertile males and females through natural
selection, we may conclude from the analogy of ordinary
variations, that the successive, slight, profitable modifications
did not first arise in all the neuters in the same nest, but in
some few alone ; and that by the survival of the communities
with females which produced most neuters having the ad-
vantageous modification, all the neuters ultimately came to be
thus characterized. According to this view we ought occa-
sionally to find in the same nest neuter insects, presenting
gradations of structure ; and this we do find, even not rarely
considering how few neuter insects out of Europe have been
carefully examined. Mr. F. Smith has shown that the neuters
of several British ants differ surprisingly from each other in
size and sometimes in colour; and that the extreme forms can
be linked together by individuals taken out of the same nest:
I have myself compared perfect gradations of this kind. It
sometimes happens that the larger or the smaller sized
workers are the most numerous ; or that both large and small
are numerous, whilst those of an intermediate size are scanty
in numbers. Formica flava has larger and smaller workers,
with some few of intermediate size ; and, in this species, as
Mr. F. Smith has observed, the larger workers have simple
eyes (ocelli), which though small can be plainly distinguished,
whereas the smaller workers have their ocelli rudimentary.
Having carefully dissected several specimens of these
workers, I can affirm that the eyes are far more rudi-
mentary in the smaller workers than can be accounted
for merely by their proportionally lesser size; and I fully
believe, though I dare not assert so positively, that the workers
of intermediate size have their ocelli in an exactly inter-
mediate condition. So that here we have two bodies of sterile
workers in the same nest, differing not only in size, but in
their organs of vision, yet connected by some few members
in an intermediate conditio h. I may digress by adding, that
if the smaller workers had been the most useful to the com-
294 ORIGIN OF SPECIES
munity, and those males and females had been continually
selected, which produced more and more of the smaller
workers, until all the workers were in this condition; we
should then have had a species of ant with neuters in nearly
the same condition as those of Myrmica. For the workers of
Myrmica have not even rudiments of ocelli, though the male
and female ants of this genus have well-developed ocelli.
I may give one other case : so confidently did I expect
occasionally to find gradations of important structures be-
tween the different castes of neuters in the same species, that
I gladly availed myself of Mr. F. Smith's offer of numerous
specimens from the same nest of the driver ant (Anomma)
of West Africa. The reader will perhaps best appreciate the
amount of difference in these workers, by my giving not the
actual measurements, but a strictly accurate illustration: the
difference was the same as if we were to see a set of work-
men building a house, of whom many were five feet four
inches high, and many sixteen feet high; but we must in
addition suppose that the larger workmen had heads four
instead of three times as big as those of the smaller men,
and jaws nearly five times as big. The jaws, moreover, of
the working ants of the several sizes differed wonderfully in
shape, and in the form and number of the teeth. But the
important fact for us is, that, though the workers can be
grouped into castes of different sizes, yet they graduate in-
sensibly into each other, as does the widely-different struc-
ture of their jaws. I speak confidently on this latter point,
as Sir J. Lubbock made drawings for me, with the camera
lucida, of the jaws which I dissected from the workers of
the several sizes. Mr. Bates, in his interesting 'Jvlaturalist on
the Amazons,' has described analogous cases.
With these facts before me, I believe that natural selec-
tion, by acting on the fertile ants or parents, could form a
species which should regularly produce neuters, all of large
size with one form of jaw, or all of small size with widely
different jaws; or lastly, and this is the greatest difficulty,
one set of workers of one size and structure, and simultane-
ously another set of workers of a different size and struc-
ture; — a graduated series having first been formed, as in the
case of the driver ant, and then the extreme forms having
OBJECTIONS TO THE THEORY 295
been produced in greater and greater numbers, through the
survival of the parents which generated them, until none
with an intermediate structure were produced.
An analogous explanation has been given by Mr. Wallace,
of the equally complex case, of certain Malayan Butterflies
regularly appearing under two or even three distinct female
forms ; and by Fritz Miiller, of certain Brazilian cri:staceans
likewise appearing under two widely distinct male forms.
But this subject need not here be discussed.
I have now explained how, as I believe, the wonderful fact
of two distinctly defined castes of sterile workers existing in
the same nest, both widely different from each other and from
their parents, has originated. We can see how useful their
production may have been to a social community of ants, on
the same principle that the division of labour is useful to
civilised man. Ants, however, work by inherited instincts
and by inherited organs or tools, whilst man works by
acquired knowledge and manufactured instruments. But I
must confess, that, with all my faith in natural selection, I
should never have anticipated that this principle could
have been efficient in so high a degree, had not the case of
these neuter insects led me to this conclusion. I have, there-
fore, discussed this case, at some little but wholly insufficient
length, in order to show the power of natural selection, and
likewise because this is by far the most serious special dif-
ficulty which my theory has encountered. The case, also, is
very interesting, as it proves that with animals, as with
plants, any amount of modification may be effected by the
accumulation of numerous, slight, spontaneous variations,
which are in any way profitable, without exercise or habit
having been brought into play. For peculiar habits confined
to the workers or sterile females, however long they might
be followed, could not possibly affect the males and fertile
females, which alone leave descendants. I am surprised that
no one has hitherto advanced this demonstrative case of
neuter insects, against the well-known doctrine of inherited
habit, as advanced by Lamarck.
296 ORIGIN OF SPECIES
SUMMARY
I have endeavored in this chapter briefly to shov^r that the
mental qualities of our domestic animals vary, and that the
variations are inherited. Still more briefly I have attempted
to show^ that instincts vary slightly in a state of nature. No
one vi'ill dispute that instincts are of the highest importance
to each animal. Therefore there is no real difficulty, under
changing conditions of life, in natural selection accumulating
to any extent slight modifications of instinct which are in
any way useful. In many cases habit or use and disuse have
probably come into play. I do not pretend that the facts
given in this chapter strengthen in any great degree my
theory; but none of the cases of difficulty, to the best of my
judgment, annihilate it. On the other hand, the fact that
instincts are not always absolutely perfect and are liable to
mistakes : — that no instinct can be shown to have been pro-
duced for the good of other animals, though animals take
advantage of the instincts of others ; — that the canon in
natural history, of "Natura non facit saltum," is applicable
to instincts as well as to corporeal structure, and is plainly
explicable on the foregoing views, but is otherwise inexplic-
able, — all tend to corroborate the theory of natural selection.
This theory is also strengthened by some few other facts in
regard to instincts ; as by that common case of closely allied,
but distinct, species, when inhabiting distant parts of the
world and living under considerably different conditions of
life, yet often retaining nearly the same instincts. For in-
stance, we can understand, on the principle of inheritance,
how it is that the thrush of tropical South America lines its
nest with mud, in the same peculiar manner as does our
British thrush ; how it is that the Hornbills of Africa and
India have the same extraordinary instinct of plastering up
and imprisoning the females in a hole in a tree, with only a
small hole left in the plaster through which the males feed
them and their young when hatched ; how it is that the male
wrens (Troglodytes) of North America build "cock-nests,"
to roost in, like the males of our Kitty-wrens, — a habit wholly
unlike that of any other known bird. Finally, it may not be
a logical deduction, but to my imagination it is far more satis-
SUMMARY 297
factory to look at such instincts as the young cuckoo ejecting
its foster-brothers, — ants making slaves, — the larvae of ichneu-
monidse feeding v^^ithin the live bodies of caterpillars, — not
as specially endowed or created instincts, but as small conse-
quences of one general law leading to the advancement of all
organic beings, — namely, multiply, vary, let the strongest live
and the weakest die.
CHAPTER IX
Hybridism
Distinction between the sterility of first crosses and of hybrids —
Sterility various in degree, not universal, affected by close inter-
breeding, removed by domestication — Laws governing the ster-
ility of hybrids — Sterility not a special endowment, but incidental
on other differences, not accumulated by natural selection —
Causes of the sterility of first crosses and of hybrids — Parallel-
ism between the effects of changed conditions of life and of
crossing — Dimorphism and Trimorphism — Fertility of varieties
when crossed, and of their mongrel offspring not universal —
Hybrids and mongrels compared independently of their fertility
— Summary.
THE view commonly entertained by naturalists is that
species, when intercrossed, have been specially en-
dowed with sterility, in order to prevent their con-
fusion. This view certainly seems at first highly probable,
for species living together could hardly have been kept dis-
tinct had they been capable of freely crossing. The subject
is in many ways important for us, more especially as the
sterility of species when first crossed, and that of their hybrid
offspring, cannot have been acquired, as I shall show, by the
preservation of successive profitable degrees of sterility. It
is an incidental result of differences in the reproductive sys-
tems of the parent-species.
In treating this subject, two classes of facts, to a large
extent fundamentally different, have generally been con-
founded; namely, the sterility of species when first crossed,
and the sterility of the hybrids produced from them.
Pure species have of course their organs of reproduction
in a perfect condition, yet when intercrossed they produce
either few or no offspring. Hybrids, on the other hand, have
their reproductive organs functionally impotent, as may be
clearly seen in the state of the male element in both plants
and animals; though the formative organs themselves are
298
DEGREES OF STERILITY 299
perfect in structure, as far as the microscope reveals. In the
first case the two sexual elements which go to form the
embryo are perfect; in the second case they are either not at
all developed, or are imperfectly developed. This distinction
is important, when the cause of the sterility, which is common
to the two cases, has to be considered. The distinction prob-
ably has been slurred over, owing to the sterility in both cases
being looked on as a special endowment, beyond the province
of our reasoning powers.
The fertility of varieties, that is of the forms known or
believed to be descended from common parents, when crossed,
and likewise the fertility of their mongrel offspring, is, with
reference to my theory, of equal importance with the sterility
of species ; for it seems to make a broad and clear distinction
between varieties and species.
Degrees of Sterility. — First, for the sterility of species
when crossed and of their hybrid offspring. It is impossible
to study the several memoirs and works of those two con-
scientious and admirable observers, Kolreuter and Gartner,
who almost devoted their lives to this subject, without being
deeply impressed with the high generality of some degree of
sterility. Kolreuter makes the rule universal ; but then he
cuts the knot, for in ten cases in which he found two forms,
considered by most authors as distinct species, quite fertile
together, he unhesitatingly ranks them as varieties. Gartner,
also, makes the rule equally universal ; and he disputes the
entire fertility of Kolreuter's ten cases. But in these and in
many other cases, Gartner is obliged carefully to count the
seeds, in order to show that there is any degree of sterility. He
always compares the maximum number of seeds produced by
two species when first crossed, and the maximum produced
by their hybrid offspring, with the average number produced
by their pure parent-species in a state of nature. But causes
of serious error here intervene : a plant, to be hybridised,
must be castrated, and, what is often more important, must
be secluded in order to prevent pollen being brought to it
by insects from other plants. Nearly all the plants experi-
mented on by Gartner were potted, and were kept in a
chamber in his house. That these processes are often in-
jurious to the fertility^ of a plant cannot be doubted; for
300 ORIGIN OF SPECIES
Gartner gives in his table about a score of cases of plants
which he castrated, and artificially fertilised with their own
pollen, and (excluding all cases such as the Leguminosae, in
which there is an acknowledged difficulty in the manipula-
tion) half of these twenty plants had their fertility in some
degree impaired. Moreover, as Gartner repeatedly crossed
some forms, such as the common red and blue pimpernels
(Anagallis arvensis and coeulea), which the best botanists
rank as varieties, and found them absolutely sterile, we may
doubt whether many species are really so sterile, when inter-
crossed, as he believed.
It is certain, on the one hand, that the sterility of various
species when crossed is so different in degree and graduates
away so insensibly, and, on the other hand, that the fertility
of pure species is so easily affected by various circumstances,
that for all practical purposes it is most difficult to say where
perfect fertility ends and sterility begins. I think no better
evidence of this can be required than that the two most ex-
perienced observers who have ever lived, namely Kolreuter
and Gartner, arrived at diametrically opposite conclusions in
regard to some of the very same forms. It is also most in-
structive to compare — but I have not space here to enter into
details — the evidence advanced by our best botanists on the
question whether certain doubtful forms should be ranked as
species or varieties, with the evidence from fertility adduced
by different hybridisers, or by the same observer from ex-
periments made during different years. It can thus be shown
that neither sterility nor fertility affords any certain distinc-
tion between species and varieties. The evidence from this
source graduates away, and is doubtful in the same degree as
is the evidence derived from other constitutional and struc-
tural differences.
In regard to the sterility of hybrids in successive genera-
tions ; though Gartner was enabled to rear some hybrids, care-
fully guarding them from a cross with either pure parent, for
six or seven, and in one case for ten generations, yet he
asserts positively that their fertility never increases, but gen-
erally decreases greatly and suddenly. With respect to this
decrease, it may first be noticed that when any deviation in
structure or constitution is common to both parents, this is
DEGREES OF STERILITY 301
often transmitted in an augmented degree to the offspring;
and both sexual elements in hybrid plants are already affected
in some degree. But I believe that their fertility has been
diminished in nearly all these cases by an independent cause,
namely, by too close interbreeding. I have made so many
experiments and collected so many facts, showing on the one
hand that an occasional cross with a distinct individual or
variety increases the vigour and fertility of the offspring, and
on the other hand that very close interbreeding lessens their
vigour and fertility, that I cannot doubt the correctness of
this conclusion. Hybrids are seldom raised by experimental-
ists in great numbers ; and as the parent-species, or other
allied hybrids, generally grow in the same garden, the visits
of insects must be carefully prevented during the flowering
season ; hence hybrids, if left to themselves, will generally be
fertilised during each generation by pollen from the same
flower; and this would probably be injurious to their fertility,
already lessened by their hybrid origin. I am strengthened
in this conviction by a remarkable statement repeatedly made
by Gartner, namely, that if even the less fertile hybrids be
artificially fertilised with hybrid pollen of the same kind, their
fertility, nothwithstanding the frequent ill effects from manip-
ulation, sometimes decidedly increases, and goes on increas-
ing. Now, in the process of artificial fertilisation, pollen Is
as often taken by chance (as I know from my own experi-
ence) from the anthers of another flower, as from the anthers
of the flower itself which is to be fertilised; so that a cross
between two flowers, though probably often on the same
plant, would be thus effected. Moreover, whenever compli-
cated experiments are in progress, so careful an observer as
Gartner would have castrated his hybrids, and this would
have ensured in each generation a cross with pollen from
a distinct flower, either from the same plant or from another
plant of the same hybrid nature. And thus, the strange fact
of an increase of fertility in the successive generations of
artificially fertilised hybrids, in contrast with those spon-
taneously self-fertilised, may, as I believe, be accounted for
by too close interbreeding having been avoided.
Now let us turn to the results arrived at by a third most
experienced hybridiser, hamely, the Hon. and Rev. W. Her-
302 ORIGIN OF SPECIES
bert. He is as emphatic in his conclusion that some hybrids
are perfectly fertile — as fertile as the pure parent-species —
as are Kolreuter and Gartner that some degree of sterility
between distinct species is a universal law of nature. He
experimented on some of the very same species as did
Gartner. The difference in their results may, I think, be in
part accounted for by Herbert's great horticultural skill, and
by his having hot-houses at his command. Of his many im-
portant statements I will here give only a single one as an
example, namely, that "every ovule in a pod of Crinum
capense fertilised by C. revolutum produced a plant, which
I never saw to occur in a case of its natural fecundation."
So that here we have perfect or even more than com-
monly perfect fertility, in a first cross between two distinct
species.
This case of the Crinum leads me to refer to a singular
fact, namely, that individual plants of certain species of
Lobelia, Verbascum and Passiflora, can easily be fertilised by
pollen from a distinct species, but not by pollen from the
same plant, though this pollen can be proved to be perfectly
sound by fertilising other plants or species. In the genus
Hippeastrum, in Corydalis as shown by Professor Hilde-
brand, in various orchids as shown by Mr. Scott and Fritz
Miiller, all the individuals are in this peculiar condition. So
that with some species, certain abnormal individuals, and in
other species all the individuals, can actually be hybridised
much more readily than they can be fertilised by pollen from
the same individual plant ! To give one instance, a bulb of
Hippeastrum aulicum produced four flowers; three were fer-
tilised by Herbert with their own pollen, and the fourth was
subsequently fertilised by the pollen of a compound hybrid
descended from three distinct species: the result was that
"the ovaries of the three first flowers soon ceased to grow,
and after a few days perished entirely, whereas the pod im-
pregnated by the pollen of the hybrid made vigorous growth
and rapid progress to maturity, and bore good seed, which
vegetated freely." Mr. Herbert tried similar experiments
during many years, and always with the same result. These
cases serve to show on what slight and mysterious causes the
lesser or greater fertility of a species sometimes depends.
DEGREES OF STERILITY 303
The practical experiments of horticulturists, though not
made with scientific precision, deserve some notice. It is
notorious in how complicated a manner the species of Pelar-
gonium, Fuchsia, Calceolaria, Petunia, Rhododendron, &c.,
have been crossed, yet many of these hybrids seed freely.
For instance, Herbert asserts that a hybrid from Calceolaria
integrifolia and plantaginea, species most widely dissimilar
in general habit, "reproduces itself as perfectly as if it had
been a natural species from the mountains of Chili." I have
taken some pains to ascertain the degree of fertility of som£
of the complex crosses of Rhododendrons, and I am assured
that many of them are perfectly fertile. Mr. C. Noble, for
instance, informs me that he raises stocks for grafting from
a hybrid between Rhod. ponticum and catawbiense, and that
this hybrid "seeds as freely as it is possible to imagine." Had
hybrids, when fairly treated, always gone on decreasing in
fertility in each successive generation, as Gartner believed
to be the case, the fact would have been notorious to nursery-
men. Horticulturists raise large beds of the same hybrid, and
such alone are fairly treated, for by insect-agency the several
individuals are allowed to cross freely with each other, and
the injurious influence of close interbreeding is thus pre-
vented. Any one may readily convince himself of the effici-
ency of insect-agency by examining the flowers of the more
sterile kinds of hybrid Rhododendrons, which produce no
pollen, for he will find on their stigmas plenty of pollen
brought from other flowers.
In regard to animals, much fewer experiments have been
carefully tried than with plants. If our systematic arrange-
ments can be trusted, that is, if the genera of animals are as
distinct from each other as are the genera of plants, then
we may infer that animals more widely distinct in the scale
of nature can be crossed more easily than in the case of
plants; but the hybrids themselves are, I think, more sterile.
It should, however, be borne in mind that, owing to few
animals breeding freely under confinement, few experiments
have been fairly tried : for instance, the canary-bird has been
crossed with nine distinct species of finches, but, as not one
of these breeds freely in confinement, we have no right to
expect that the first ci'osses between them and the canary.
304 ORIGIN OF SPECIES
or that their hybrids, should be perfectly fertile. Again, with
respect to the fertility in successive generations of the more
fertile hybrid animals, I hardly know of an instance in which
two families of the same hybrid have been raised at the same
time from different parents, so as to avoid the ill effects of
close interbreeding. On the contrary, brothers and sisters
have usually been crossed in each successive generation, in
opposition to the constantly repeated admonition of every
breeder. And in this case, it is not at all surprising that the
inherent sterility in the hybrids should have gone on in-
creasing.
Although I know of hardly any thoroughly well-authen-
ticated cases of perfectly fertile hybrid animals, I have reason
to believe that the hybrids from Cervulus vaginalis and Ree-
vesii, and from Phasianus colchicus with P. torquatus, are
perfectly fertile. M. Ouatrefages states that the hybrids from
two moths (Bombyx cynthia and arrindia) were proved in
Paris to be fertile inter se for eight generations. It has lately
been asserted that two such distinct species as the hare and
rabbit, when they can be got to breed together, produce off-
spring, which are highly fertile when crossed with one of
the parent-species. The hybrids from the common and Chi-
nese geese (A. cygnoides), species which are so different that
they are generally ranked in distinct genera, have often bred
in this country with either pure parent, and in one single in-
stance they have bred inter se. This was effected by Mr.
Eyton, who raised two hybrids from the same parents, but
from different hatches; and from these two birds he raised
no less than eight hybrids (grandchildren of the pure geess)
from one nest. In India, however, these cross-bred geese
must be far more fertile ; for I am assured by two eminently
capable judges, namely Mr. Blyth and Capt. Hutton, that
whole flocks of these crossed geese are kept in various parts
of the country ; and as they are kept for profit, where neither
pure parent-species exists, they must certainly be highly or
perfectly fertile.
With our domesticated animals, the various races when
crossed together are quite fertile ; yet in many cases they are
descended from two or more wild species. From this fact we
must conclude either that the aboriginal parent-species at
LAWS GOVERNING THE STERILITY 305
first produced perfectly fertile hybrids, or that the hybrids
subsequently reared under domestication became quite fertile.
This latter alternative, which was first propounded by Pallas,
seems by far the most probable, and can, indeed, hardly be
doubted. It is, for instance, almost certain that our dogs are
descended from several wild stocks; yet, with perhaps the
exception of certain indigenous domestic dogs of South
America, all are quite fertile together; but analogy makes
me greatly doubt, whether the several aboriginal species would
at first have freely bred together and have produced quite
fertile hybrids. So again I have lately acquired decisive evi- \
dence that the crossed offspring from the Indian humped and
common cattle are inter se perfectly fertile; and from the
observations by Riitimeyer on their important osteological
differences, as well as from those by Mr. Blyth on their dif-
ferences in habits, voice, constitution, &c., these two forms
must be regarded as good and distinct species. The same re-
marks may be extended to the two chief races of the pig.
We must, therefore, either give up the belief of the universal
sterility of species when crossed; or we must look at this
sterility in animals, not as an indelible characteristic, but as
one capable of being removed by domestication.
Finally, considering all the ascertained facts on the inter-
crossing of plants and animals, it may be concluded that some
degree of sterility, both in first crosses and in hybrids, is an
extremely general result ; but that it cannot, under our present
state of knowledge, be considered as absolutely universal.
LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF
HYBRIDS.
We will now consider a little more in detail the laws gov-
erning the sterility of first crosses and of hybrids. Our chief
object will be to see whether or not these laws indicate that
species have been specially endowed with this quality, in order
to prevent their crossing and blending together in utter con-
fusion. The following conclusions are drawn up chiefly from
Gartner's admirable work' on the hybridisation of plants. I
have taken much pains to ascertain how far they apply to
animals, and, considering how scanty our knowledge is in re-
306 ORIGIN OF SPECIES
gard to hybrid animals, I have been surprised to find how
generally the same rules apply to both kingdoms.
It has been already remarked, that the degree of fertility,
both of first crosses and of hybrids, graduates from zero to
perfect fertility. It is surprising in how many curious ways
this gradation can be shown; but only the barest outline of
the facts can here be given. When pollen from a plant of
one family is placed on the stigma of a plant of a distinct
family, it exerts no more influence than so much inorganic
dust. From this absolute zero of fertility, the pollen of dif-
ferent species applied to the stigma of some one species of
the same genus, yields a perfect gradation in the number of
seeds produced, up to nearly complete or even quite complete
fertility; and, as we have seen, in certain abnormal cases,
even to an excess of fertility, beyond that which the plant's
own pollen produces. So in hybrids themselves, there are
some which never have produced, and probably never would
produce, even with the pollen of the pure parents, a single
fertile seed: but in some of these cases a first trace of fer-
tility may be detected, by the pollen of one of the pure parent-
species causing the flower of the hybrid to wither earlier
than it otherwise would have done ; and the early withering
of the flower is well known to be a sigTi of incipient fertilisa-
tion. From this extreme degree of sterility we have self-
fertilised hybrids producing a greater and greater number of
seeds up to perfect fertility.
The hybrids raised from two species which are very diffi-
cult to cross, and which rarely produce any offspring, are
generally very sterile ; but the parallelism between the diffi-
culty of making a first cross, and the sterility of the hybrids
thus produced — two classes of facts which are generally con-
founded together — is by no means strict. There are many
cases, in which two pure species, as in the genus Verbascum,
can be united with unusual facility, and produce numerous
hybrid-offspring, yet these hybrids are remarkably sterile.
On the other hand, there are species which can be crossed
very rarely, or with extreme difficulty, but the hybrids, when
at last produced, are very fertile. Even within the limits of
the same genus, for instance in Dianthus, these two opposite
cases occur.
LAWS GOVERNING THE STERILITY 307
The fertility, both of first crosses and of hybrids, is more
easily affected by unfavorable conditions, than is that of
pure species. But the fertility of first crosses is likewise in-
nately variable ; for it is not always the same in degree when
the same two species are crossed under the same circum-
stances; it depends in part upon the constitution of the in-
dividuals which happen to have been chosen for the experi-
ment. So it is with hybrids, for their degree of fertility is
often found to differ greatly in the several individuals raised
from seed out of the same capsule and exposed to the same
conditions.
By the term systematic affinity is meant, the general re-
semblance between species in structure and constitution. Now
the fertility of first crosses, and of the hybrids produced from
them, is largely governed by their systematic affinity. This
is clearly shown by hybrids never having been raised between
species ranked by systematists in distinct families ; and on
the other hand, by very closely allied species generally uniting
with facility. But the correspondence between systematic
affinity and the facility of crossing is by no means strict. A
multitude of cases could be given of very closely allied species
which will not unite, or only with extreme difficulty; and on
the other hand of very distinct species which unite with the
utmost facility. In the same family there may be a genus,
as Dianthus, in which very many species can' most readily be
crossed ; and another genus, as Silene, in which the most per-
severing efforts have failed tO' produce between extremely
close species a single hybrid. Even within the limits of the
same genus, we meet with this same difference ; for instance,
the many species of Nicotiana have been' more largely crossed
than the species of almost any other genus ; but Gartner
found that N. acuminata, which is not a particularly distinct
species, obstinately failed to fertilise, or to be fertilised by no
less than eight other species of Nicotiana. Many analogous
facts could be given.
No one has been able to point out what kind or what
amount of difference, in any recognisable character, is suf-
ficient to prevent two species crossing. It can be shown that
plants most widely different in habit and general appearance,
and having strongly ntarked differences in every part of the
308 ORIGIN OF SPECIES
flower, even in the pollen, in the fruit, and in the cotyledons,
can be crossed. Annual and perennial plants, deciduous and
evergreen trees, plants inhabiting different stations and fitted
for extremely different cHmates, can often be crossed with
ease.
By a reciprocal cross between two species, I mean the case,
for instance, of a female-ass being first crossed by a stallion,
and then a mare by a male-ass; these two species may then
be said to have been reciprocally crossed. There is often the
widest possible difference in the facility of making reciprocal
crosses. Such cases are highly important, for they prove
that the capacity in any two species to cross is often com-
pletely independent of their systematic affinity, that is of any
difference in their structure or constitution, excepting in
their reproductive systems. The diversity of the result in
reciprocal crosses between the same two species was long
ago observed by Kolreuter. To give an instance: Mirabilis
jalapa can easily be fertilised by the pollen of M. longiflora,
and the hybrids thus produced are sufficiently fertile; but
Kolreuter tried more than two hundred times, during eight
following years, to fertilise reciprocally M. longiflora with
the pollen of M. jalapa, and utterly failed. Several other
equally striking cases could be given. Thuret has observed
the same fact with certain sea-weeds or Fuci. Gartner,
moreover, found that this difference of facility in making
reciprocal crosses is extremely common in a lesser degree.
He has observed it even between closely related forms (as
Matthiola annua and glabra) which many botanists rank only
as varieties. It is also a remarkable fact, that hybrids raised
from reciprocal crosses, though of course compounded of the
very same two species, the one species having first been used
as the father and then as the mother, though they rarely
differ in external characters, yet generally differ in fertility
in a small, and occasionally in a high degree.
Several other singular rules could be given from Gartner:
for instance, some species have a remarkable power of cross-
ing with other species ; other species of the same genus have
a remarkable power of impressing their likeness on their
hybrid offspring; but these two powers do not at all neces-
sarily go together. There are certain hybrids which, instead
LAWS GOVERNING THE STERILITY 309
of having, as is usual, an intermediate character between their
two parents, always closely resemble one of them ; and such
hybrids, though externally so like one of their pure parent-
species, are with rare exceptions extremely sterile. So again
amongst hybrids which are usually intermediate in" structure
between their parents, exceptional and abnormal individuals
sometimes are born, which closely resemble one of their pure
parents ; and these hybrids are almost always utterly sterile,
even when the other hybrids raised from seed from the same
capsule have a considerable degree of fertility. These facts
show how completely the fertility of a hybrid may be inde-
pendent of its external resemblance to either pure parent.
Considering the several rules now given, which govern the
fertility of first crosses and of hybrids, we see that when
forms, which must be considered as good and distinct species,
are united, their fertility graduates from zero to perfect fer-
tility, or even to fertility under certain conditions in excess;
that their fertility, besides being eminently susceptible to
favourable and unfavourable conditions, is innately variable;
that it is by no means always the same in degree in the first
cross and in the hybrids produced from this cross ; that the
fertility of hybrids is not related to the degree in which they
resemble in external appearance either parent; and lastly,
that the facility of making a first cross between any two
species is not always governed by their systematic affinity or
degree of resemblance to each other. This latter statement
is clearly proved by the difference in the result of reciprocal
crosses between the same two species, for, according as the
one species or the other is used as the father or the mother,
there is generally some difference, and occasionally the widest
possible difference, in the facility of effecting an union. The
hybrids, moreover, produced from reciprocal crosses often
differ in fertility.
Now do these complex and singular rules indicate that
species have been endowed with sterility simply to prevent
their becoming confounded in nature? I think not. For
why should the sterility be so extremely different in degree,
when various species are crossed, all of which we must sup-
pose it would be equally important to keep from blending to-
gether? Why should the degree of sterility be innately vari-
310 ORIGIN OF SPECIES
able in the individuals of the same species? Why should
some species cross with facility, and yet produce very sterile
hybrids; and other species cross with extreme difficulty, and
yet produce fairly fertile hybrids? Why should there often
be so great a difference in the result of a reciprocal cross
between the same two species ? Why, it may even be asked,
has the production of hybrids been permitted? To grant to
species the special power of producing hybrids, and then to
stop their further propagation by different degrees of sterility,
not strictly related to the facility of the first union between
their parents, seems a strange arrangement.
The foregoing rules and facts, on the other hand, appear to
me clearly to indicate that the sterility both of first crosses
and of hybrids is simply incidental or dependent on unknown
dififerences in their reproductive systems ; the differences be-
ing of so peculiar and limited a nature, that, in reciprocal
crosses between the same two species, the male sexual ele-
ment of the one will often freely act on the female sexual
element of the other, but not in a reversed direction. It will
be advisable to explain a little more fully by an example what
I mean by sterility being incidental on other differences, and
not a specially endowed quality. As the capacity of one
plant to be grafted or budded on another is unimportant for
their welfare in a state of nature, I presume that no one will
suppose that this capacity is a specially endowed quality, but
will admit that it is incidental on differences in the laws of
growth of the two plants. We can sometimes see the reason
why one tree will not take on another, from differences in
their rate of growth, in the hardness of their wood, in the
period of the flow or nature of their sap, &c. ; but in a multi-
tude of cases we can assign no reason whatever. Great di-
versity in the size of two plants, one being woody and the other
herbaceous, one being evergreen and the other decidu-
ous, an adaptation to widely different climates, do not
always prevent the two grafting together. As in hybridisa-
tion, so with grafting, the capacity is limited by systematic
affinity, for no one has been able to graft together trees be-
longing to quite distinct families; and, on the other hand,
closely allied species, and varieties of the same species, can
usually, but not invariably, be grafted with ease. But this
LAWS GOVERNING THE STERILITY 311
capacity, as in hybridisation, is by no means absolutely gov-
erned by systematic affinity. Although many distinct genera
within the same family have been grafted together, in other
cases species of the same genus will not take on each other.
The pear can be grafted far more readily on the quince,
which is ranked as a distinct genus, than on the apple, which
is a member of the same genus. Even different varieties of
the pear take with different degrees of facility on the quince ;
so do difi'erent varieties of the apricot and peach on certain
varieties of the plum.
As Gartner found that there was sometimes an innate dif
ference in different individuals of the same two species in
crossing; so Sageret believes this to be the case with different
individuals of the same two species in being grafted together.
As in reciprocal crosses, the facility of effecting an union is
often very far from equal, so it sometimes is in grafting; the
common gooseberry, for instance, cannot be grafted on the
currant, whereas the currant will take, though with difficulty,
on the gooseberry.
We have seen that the sterility of hybrids, which have
their reproductive organs in an imperfect condition, is a dif-
ferent case from the difficulty of uniting two pure species,
which have their reproductive organs perfect; yet these two
distinct classes of cases run to a large extent parallel. Some-
thing analogous occurs in grafting; for Thouin found that
three species of Robinia, which seeded freely on their own
roots, and which could be grafted with no great difficulty on
a fourth species, when thus grafted were rendered barren.
On the other hand, certain species of Sorbus, when grafted
on other species yielded twice as much fruit as when on their
own roots. We are reminded by this latter fact of the extra-
ordinary cases of Hippeastrum, Passiflora, &c., which seed
much more freely when fertilised with the pollen of a dis-
tinct species, than when fertilised with pollen from the same
plant.
We thus see, that, although there is a clear and great dif-
ference between the mere adhesion of grafted stocks, and the
union of the male and female elements in the act of repro-
duction, yet that there is & rude degree of parallelism in the
results of grafting and of crossing distinct species. And as
312 ORIGIN OF SPECIES
we must look at the curious and complex laws governing the
facility with which trees can be grafted on each other as in-
cidental on unknown differences in their vegetative systems,
so I believe that the still more complex laws governing the
facility of first crosses are incidental on unknown differences
in their reproductive systems. These differences in both
cases, follow to a certain extent, as might have been expected,
systematic affinity, by which term every kind of resemblance
and dissimilarity between organic beings is attempted to be
expressed. The facts by no means seem to indicate that the
greater or lesser difficulty of either grafting or crossing vari-
ous species has been a special endowment ; although in the
case of crossing, the difficulty is as important for the endur-
ance and stability of specific forms, as in the case of graft-
ing it is unimportant for their welfare.
ORIGIN AND CAUSES OF THE STERILITY OF FIRST CROSSES
AND OF HYBRIDS
At one time it appeared to me probable, as it has to others,
that the sterility of first crosses and of hybrids might have
been slowly acquired through the natural selection of slightly
lessened degrees of fertility, which, like any other variation,
spontaneously appeared in certain individuals of one variety
when crossed with those of another variety. For it would
clearly be advantageous to two varieties or incipient species,
if they could be kept from blending, on the same principle
that, when man is selecting at the same time two varieties,
it is necessary that he should keep them separate. In the
first place, it may be remarked that species inhabiting dis-
tinct regions are often sterile when crossed ; now it could
clearly have been of no advantage to such separated species
to have been rendered mutually sterile, and consequently this
could not have been effected through natural selection ; but
it may perhaps be argued, that, if a species was rendered
sterile with some one compatriot, sterility with other species
would follow as a necessary contingency. In the second
place, it is almost as much opposed to the theory of natural
selection as to that of special creation, that in reciprocal
crosses the male element of one form should have been ren-
CAUSES OF THE STERILITY 313
dered utterly impotent on a second form, whilst at the same
time the male element of this second form is enabled freely
to fertilise the first form ; for this peculiar state of the repro-
ductive system could hardly have been advantageous to either
species.
In considering the probability of natural selection having
come into action, in rendering species mutually sterile, the
greatest difficulty M^ill be found to lie in the existence of many
graduated steps from slightly lessened fertility to absolute
sterility. It may be admitted that it would profit an incipient
species, if it were rendered in some slight degree sterile when
crossed with its parent form or with some other variety; for
thus fewer bastardised and deteriorated offspring would be
produced to commingle their blood with the new species in
process of formation. But he who will take the trouble to
reflect on the steps by which this first degree of sterility
could be increased through natural selection to that high de-
gree which is common with so many species, and which is
universal with species which have been differentiated to a
generic or family rank, will find the subject extraordinarily
complex. After mature reflection it seems to me that this
could not have been effected through natural selection. Take
the case of any two species which, when crossed, produced
few and sterile offspring; now, what is there which could
favour the survival of those individuals which happened to
be endowed in a slightly higher degree with mutual infer-
tility, and which thus approached by one small step towards
absolute sterility? Yet an advance of this kind, if the theory
of natural selection be brought to bear, must have incessantly
occurred with many species, for a multitude are mutually
quite barren. With sterile neuter insects we have reason to
believe that modifications in their structure and fertility
have been slowly accumulated by natural selection, frgm an
advantage having been thus indirectly given to the com-
munity to which they belonged over other communities of the
same species; but an individual animal not belonging to a
social community, if rendered slightly sterile when crossed
with some other variety, would not thus itself gain any ad-
vantage or indirectly give any advantage to the other individ-
uals of the same variety, thus leading to their preservation.
314 ORIGIN OF SPECIES
But it would be superfluous to discuss this question in de-
tail ; for with plants we have conclusive evidence that the
sterility of crossed species must be due to some principle,
quite independent of natural selection. Both Gartner and
Kolreuter have proved that in genera including numerous
species, a series can be formed from species which when
crossed yield fewer and fewer seeds, to species which never
produce a single seed, but yet are affected by the pollen of
certain other species, for the germen swells. It is here mani-
festly impossible to select the more sterile individuals, which
have already ceased to yield seeds ; so that this acme of ster-
ility, when the germen alone is affected, cannot have been
gained through selection ; and from the laws governing the
various grades of sterility being so uniform throughout the
animal and vegetable kingdoms, we may infer that the cause,
whatever it may be, is the same or nearly the same in all
cases.
We will now look a little closer at the probable nature of
the differences between species which induce sterility in first
crosses and in hybrids. In the case of first crosses, the
greater or less difficulty in effecting an union and in obtain-
ing offspring apparently depends on several distinct causes.
There must sometimes be a physical impossibility in the male
element reaching the ovule, as would be the case with a plant
having a pistil too long for the pollen-tubes to reach the
ovarium. It has also been observed that when the pollen of
one species is placed on the stigma of a distantly allied spe-
cies, though the pollen-tubes protrude, they do not penetrate
the stigmatic surface. Again, the male element may reach the
female element but be incapable of causing an embryo to be
developed, as seems to have been the case with some of Thu-
ret's experiments on Fuci. No explanation can be given of
these facts, any more than why certain trees cannot be grafted
on others. Lastly an embryo may be developed, and then perish
at an early period. This latter alternative has npt been suf-
ficiently attended to; but I believe, from observations com-
municated to me by Mr. Hewitt, who has had great experi-
ence in hybridising pheasants and fowls, that the early death
of the embryo is a very frequent cause of sterility in first
CAUSES OF THE STERILITY 315
crosses. Mr. Salter has recently given the results of an ex-
amination of about 500 eggs produced from various crosses
between three species of Gallus and their hybrids ; the ma-
jority of these eggs had been fertilised; and in the majority
of the fertilised eggs, the embryos had either been partially
developed and had then perished, or had become nearly ma-
ture, but the young chickens had been unable to break through
the shells. Of the chickens which were born, more than four-
fifths died within the first few days, or at latest weeks, "with-
out any obvious cause, apparently from mere inability to
live ;" so that from the 500 eggs only twelve chickens were
reared. With plants, hybridised embryos probably often
perish in a like manner ; at least it is known that hybrids
raised from very distinct species are sometimes weak and
dwarfed, and perish at an early age ; of which fact Max
Wichura has recently given some striking cases with hybrid
willows. It may be here worth noticing that in some cases of
parthenogenesis, the embryos within the eggs of silk moths
which had not been fertilised, pass through their early stages
of development and then perish like the embryos produced by
a cross between distinct species. Until becoming acquainted
with these facts, I was unwilling to believe in the frequent
early death of hybrid embryos ; for hybrids, when once born,
are generally healthy and long-lived, as we see in the case
of the common mule. Hybrids, however, are differently cir-
cumstanced before and after birth; when born and living in
a country where their two parents live, they are generally
placed under suitable conditions of life. But a hybrid par-
takes of only half of the nature and constitution of its
mother; it may therefore before birth, as long as it is nour-
ished within its mother's womb, or within the egg or seed
produced by the mother, be exposed to conditions in some de-
gree unsuitable, and consequently be liable to perish at an early
period ; more especially as all very young beings are eminently
sensitive to injurious or unnatural conditions of life. But af-
ter all, the cause more probably lies in some imperfection
in the original act of impregnation, causing the embryo to be
imperfectly developed, rather than in the conditions to which
it is subsequently exposed.
In regard to the sterility of hybrids, in which the sexual
316 ORIGIN OF SPECIES
elements are imperfectly developed, the case is somewhat dif-
ferent. I have more than once alluded to a large body of
facts showing that, when animals and plants are removed
from their natural conditions, they are extremely liable to
have their reproductive systems seriously affected. This, in
fact, is the great bar to the domestication of animals. Be-
tween the sterility thus superinduced and that of hybrids,
there are many points of similarity. In both cases the steril-
ity is independent of general health, and is often accompanied
by excess of size or great luxuriance. In both cases the
sterility occurs in various degrees; in both, the male element
is the most liable to be affected; but sometimes the female
more than the male. In both, the tendency goes to a certain
extent with systematic afifinity, for whole groups of animals
and plants are rendered impotent by the same unnatural con-
ditions ; and whole groups of species tend to produce sterile
hybrids. On the other hand, one species in a group will some-
times resist great changes of conditions with unimpaired
f ertiHty ; and certain species in a group will produce unusually
fertile hybrids. No one can tell, till he tries, whether any
particular animal will breed under confinement, or any exotic
plant seed freely under culture ; nor can he tell till he tries,
whether any two species of a genus will produce more or
less sterile hybrids. Lastly, when organic beings are placed
during several generations under conditions not natural to
them, they are extremely liable to vary, which seems to be
partly due to their reproductive systems having been specially
affected, though in a lesser degree than when sterility ensues.
So it is with hybrids, for their offspring in successive genera-
tions are eminently liable to vary, as every experimentalist
has observed.
Thus we see that when organic beings are placed under new
and unnatural conditions, and when hybrids are produced
by the unnatural crossing of two species, the reproductive
system, independently of the general state of health, is af-
fected in a very similar manner. In the one case, the condi-
tions of life have been disturbed, though often in so slight
a degree as to be inappreciable by us ; in the other case, or
that of hybrids, the external conditions have remained the
same, but the organisation has been disturbed by two dis-
CAUSES OF THE STERILITY 317
tinct structures and constitutions, including of course the
reproductive systems, having been blended into one. Foi
it is scarcely possible that tw^o organisations should be
compounded into one, without some disturbance occur-
ring in the development, or periodical action, or mutual
relations of the different parts and organs one to another or
to the conditions of life. When hybrids are able to breed
inter se, they transmit to their offspring from generation to
generation the same compounded organisation, and hence we
need not be surprised that their sterility, though in some
degree variable, does not diminish ; it is even apt to increase,
this being generally the result, as before explained, of too
close interbreeding. The above view of the sterility of hy-
brids being caused by two constitutions being compounded
into one has been strongly maintained by Max Wichura.
It must, however, be owned that we cannot understand, on
the above or any other view, several facts with respect to the
sterility of hybrids ; for instance, the unequal fertility of hy-
brids produced from reciprocal crosses ; or the increased ster-
ility in those hybrids which occasionally and exceptionally
resemble closely either pure parent. Nor do I pretend that
the foregoing remarks go to the root of the matter; no ex-
planation is offered why an organism, when placed under nat-
ural conditions, is rendered sterile. All that I have attempted
to show is, that in two cases, in some respects allied, sterility
is the common result, — in the one case from the conditions
of life having been disturbed, in the other case from the
organisation having been disturbed by two organisations
being compounded into one.
A similar parallelism holds good with an allied yet very dif-
ferent class of facts. It is an old and almost universal be-
lief founded on a considerable body of evidence, which I have
elsewhere given, that slight changes in the conditions of life
are beneficial to all living things. We see this acted on by
farmers and gardeners in their frequent exchanges of seed,
tubers, &c., from one soil or climate to another, and back
again. During the convalescence of animals, great benefit
is derived from almost aiiy change in their habits of life.
Again, both with plants and animals, there is the clearest
evidence that a cross between individuals of the same spe-
318 ORIGIN OF SPECIES
cies, which differ to a certain extent, gives vigour and fer-
tility to the offspring; and that close interbreeding continued
during several generations between the nearest relations, if
these be kept under the same conditions of life, almost always
leads to decreased size, weakness, or sterility.
Hence it seems that, on the one hand, slight changes in the
conditions of life benefit all organic beings, and on the other
hand, that slight crosses, that is crosses between the males
and females of the same species, which have been subjected
to slightly different conditions, or which have slightly varied,
give vigour and fertility to the offspring. But, as we have
seen, organic beings long habituated to certain uniform condi-
tions under a state of nature, when subjected, as under con-
finement, to a considerable change in their conditions, very
frequently are rendered more or less sterile ; and we know
that a cross between two forms, that have become widely or
specifically different, produce hybrids which are almost al-
ways in some degree sterile. I am fully persuaded that this
double parallelism is by no means an accident or an illusion.
He who is able to explain why the elephant and a multitude
of other animals are incapable of breeding when kept under
only partial confinement in their native country, will be
able to explain the primary cause of hybrids being so gener-
ally sterile. He will at the same time be able to explain
how it is that the races of some of our domesticated animals,
which have often been subjected to new and not uniform con-
ditions, are quite fertile together, although they are descended
from distinct species, which would probably have been sterile
if aboriginally crossed. The above two parallel series of
facts seem to be connected together by some common but
unknown bond, which is essentially related to the principle of
life ; this principle, according to Mr. Herbert Spencer, being
that life depends on, or consists in, the incessant action and
reaction of various forces, which, as throughout nature, are
always tending towards an equilibrium; and when this ten-
dency is slightly disturbed by any change, the vital forces
gain in power.
DIMORPHISM AND TRIMORPHISM 319
RECIPROCAL DIMORPHISM AND TRIMORPHISM
This subject may be here briefly discussed, and will be
found to throw some light on hybridism. Several plants be-
longing to distinct orders present two forms, which exist
in about equal numbers and which differ in no respect ex-
cept in their reproductive organs ; one form having a long
pistil with short stamens, the other a short pistil with long
stamens; the two having differently sized pollen-grains.
With trimorphic plants there are three forms likewise differ-
ing in the lengths of their pistils and stamens, in the size
and colour of the pollen-grains, and in some other respects;
and as in each of the three forms there are two sets of sta-
mens, the three forms possess altogether six sets of stamens
and three kinds of pistils. These organs are so proportioned
in length to each other, that half the stamens in two of the
forms stand on a level with the stigma of the third form.
Now I have shown, and the result has been confirmed by
other observers, that, in order to obtain full fertility with
these plants, it is necessary that the stigma of the one form
should be fertilised by pollen taken from the stamens of cor-
responding height in another form. So that with dimorphic
species two unions, which may be called legitimate, are
fully fertile; and two, which may be called illegitimate,
are more or less infertile. With trimorphic species six
unions are legitimate, or fully fertile, — and twelve are ille-
gitimate, or more or less infertile.
The infertility which may be observed in various dimorphic
and trimorphic plants, when they are illegitimately fertilised,
that is by pollen taken from stamens not corresponding in
height with the pistil, differs much in degree, up to absolute
and utter sterility; just in the same manner as occurs in
crossing distinct species. As the degree of sterility in the
latter case depends in an eminent degree on the conditions
of life being more or less favourable, so I have found it
with illegitimate unions. It is well known that if pollen of a
distinct species be placed on the stigma of a flower, and its
own pollen be afterwards, even after a considerable interval
of time, placed on the same stigma, its action is so strongly
prepotent that it generally annihilates the effect of the foreiga
320 ORIGIN OF SPECIES
pollen; so it is with the pollen of the several forms of the
same species, for legitimate pollen is strongly prepotent over
illegitimate pollen, when both are placed on the same stigma.
I ascertained this by fertilising several flowers, first ille-
gitimately, and twenty-four hours afterwards legitimately,
with pollen taken from a peculiarly coloured variety, and
all the seedlings were similarly coloured ; this shows that
the legitimate pollen, though applied twenty-four hours sub-
sequently, had wholly destroyed or prevented the action of
the previously applied illegitimate pollen. Again, as in
making reciprocal crosses between the same two species,
there is occasionally a great difference in the result, so the
same thing occurs with trimorphic plants; for instance, the
mid-styled form of Lythrum salicaria was illegitimately fer-
tilised with the greatest ease by pollen from the longer sta-
mens of the short-styled form, and yielded many seeds ; but
the latter form did not yield a single seed when fertilised by
the longer stamens of the mid-styled form.
In all these respects, and in others which might be added,
the forms of the same undoubted species when illegitimately
united behave in exactly the same manner as do two distinct
species when crossed. This led me carefully to observe
during four years many seedlings, raised from several illegiti-
mate unions. The chief result is that these illegitimate plants,
as they may be called, are not fully fertile. It is possible to
raise from dimorphic species, both long-styled and short-
styled illegitimate plants, and from trimorphic plants all three
illegitimate forms. These can then be properly united in a
legitimate manner. When this is done, there is no apparent
reason why they should not yield as many seeds as did their
parents when legitimately fertilised. But such is not the
case. They are all infertile, in various degrees; some being
so utterly and incurably sterile that they did not yield dur-
ing four seasons a single seed or even seed-capsule. The
sterility of these illegitimate plants, when united with each
other in a legitimate manner, may be strictly compared with
that of hybrids when crossed inter se. If, on the other hand,
a hybrid is crossed with either pure parent-species, the steril-
ity is usually much lessened; and so it is when an illegitimate
plant is fertilised by a legitimate plant. In the same man-
DIMORPHISM AND TRIMORPHISM 321
ner as the sterility of hybrids does not always run parallel
with the difficulty of making the first cross between the two
parent-species, so the sterility of certain illegitimate plants
was unusually great, whilst the sterility of the union from
which they were derived was by no means great. With hy-
brids raised from the same seed-capsule the degree of ster-
ility is innately variable, so it is in a marked manner with
illegitimate plants. Lastly, many hybrids are profuse and
persistent flowerers, whilst other and more sterile hybrids
produce few flowers, and are weak, miserable dwarfs;
exactly similar cases occur with the illegitimate offspring of
various dimorphic and trimorphic plants.
Altogether there is the closest identity in character and
behaviour between illegitimate plants and hybrids. It is
hardly an exaggeration to maintain that illegitimate plants are
hybrids, produced within the limits of the same species by
the improper union of certain forms, whilst ordinary hybrids
are produced from an improper union between so-called dis-
tinct species. We have also already seen that there is the
closest similarity in all respects between first illegitimate
unions and first crosses between distinct species. This will
perhaps be made more fully apparent by an illustration ; we
may suppose that a botanist found two well-marked varieties
(and such occur) of the long-styled form of the trimorphic
Lythrum salicaria, and that he determined to try by cross-
ing whether they were specifically distinct. He would find
that they yielded only about one-fifth of the proper number of
seed, and that they behaved in all the other above specified
respects as if they had been two distinct species. But to make
the case sure, he would raise plants from his supposed hy-
bridized seed, and he wpuld find that the seedlings were mis-
erably dwarfed and utterly sterile, and that they behaved in
all other respects like ordinary hybrids. He might then main-
tain that he had actually proved, in accordance with the
common view, that his two varieties were as good and as
distinct species as any in the world; but he would be com-
pletely mistaken.
The facts now given oh dimorphic and trimorphic plants
are important, because they show us, first, that the physio-
logical test of lessened fertility, both in first crosses and in
K — HC XI
322 ORIGIN OF SPECIES
hybrids, is no safe criterion of specific distinction; secondly,
because we may conclude that there is some unknown bond
which connects the infertility of illegitimate unions with
that of their illegitimate offspring, and we are led to extend
the same view to first crosses and hybrids; thirdly, because
we find, and this seems to me of especial importance, that
two or three forms of the same species may exist and may
differ in no respect whatever, either in structure or in con-
stitution, relatively to external conditions, and yet be sterile
when united in certain ways. For we must remember that
it is the union of the sexual elements of individuals of the
same form, for instance, of two long-styled forms, which
results in sterility; whilst it is the union of the sexual
elements proper to two distinct forms which is fertile. Hence
the case appears at first sight exactly the reverse of what
occurs, in the ordinary unions of the individuals of the same •
species and with crosses between distinct species. It is,
however, doubtful whether this is really so ; but I will not
enlarge on this obscure subject.
We may, however, infer as probable from the consideration
of dimorphic and trimorphic plants, that the sterility of dis-
tinct species when crossed and of their hybrid progeny, de-
pends exclusively on the nature of their sexual elements, and
not on any difference in their structure or general constitu-
tion. We are also led to this same conclusion by considering
reciprocal crosses, in which the male of one species cannot
be united, or can be unitetl with great difficulty, with the
female of a second species, whilst the converse cross can be
effected with perfect facility. That excellent observer, Gart-
ner, likewise concluded that species when crossed are sterile
owing to differences confined to their reproductive systems.
FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR
MONGREL OFFSPRING, NOT UNIVERSAL
It may be urged, as an overwhelming argument, that there
must be some essential distinction between species and vari-
eties, inasmuch as the latter, however much they may differ
from each other in external appearance, cross with perfect
facility, and yield perfectly fertile offspring. With some
FERTILITY OF VARIETIES 323
exceptions, presently to be given, I fully admit that this is
the rule. But the subject is surrounded by difficulties, for,
looking to varieties produced under nature, if two forms
hitherto reputed to be varieties be found in any degree sterile
together, they are at once ranked by most naturalists as
species. For instance, the blue and red pimpernel, which
are considered by most botanists as varieties, are said by
Gartner to be quite sterile when crossed, and he conse-
quently ranks them as undoubted species. If we thus argue
in a .circle, the fertility of all varieties produced under
nature will assuredly have to be granted.
If we turn to varieties, produced, or supposed to have been
produced, under domestication, we are still involved in some
doubt. For when it is stated, for instance, that certain South
American indigenous domestic dogs do not readily unite with
European dogs, the explanation which will occur to every
one, and probably the true one, is that they are descended
from aboriginally distinct species. Nevertheless the perfect
fertility of so many domestic races, differing widely from
each other in appearance, for instance those of the pigeon,
or of the cabbage, is a remarkable fact ; more especially when
we reflect how many species there are, which, though re-
sembling each other most closely, are utterly sterile when
intercrossed. Several considerations, however, render the
fertility of domestic varieties less remarkable. In the first
place, it may be observed that the amount of external differ-
ence between two species is no sure guide to their degree of
mutual sterility, so that similar differences in the case of
varieties would be no sure guide. It is certain that with
species the cause lies exclusively in differences in their sex-
ual constitution. Now the varying conditions to which do-
mesticated animals and cultivated plants have been subjected,
have had so little tendency towards modifying the repro-
ductive system in a manner leading to mutual sterility, that
we have good grounds for admitting the directly opposite
doctrine of Pallas, namely, that such conditions generally
eliminate this tendency ; so that the domesticated descendants
of species, which in their natural state probably would have
been in some degree sterile when crossed, become perfectly
fertile together. With plants, so far is cultivation from giving
334 ORIGIN OF SPECIES
a tendency towards sterility between distinct species, that in
several well-authenticated cases already alluded to, certain
plants have been affected in an opposite manner, for they have
become self-impotent whilst still retaining the capacity of
fertilising, and being fertilised by, other species. If the
Pallasian doctrine of the elimination of sterility through
long-continued domestication be admitted, and it can hardly
be rejected, it becomes in the highest degree improbable that
similar conditions, long-continued should likewise induce this
tendency; though in certain cases, with species having a
peculiar constitution, sterility might occasionally be thus
caused. Thus, as I believe, we can understand why with
domesticated animals varieties have not been produced which
are mutually sterile ; and why with plants only a few such
cases, immediately to be given, have been observed.
The real difficulty in our present subject is not, as it ap-
pears to me, why domestic varieties have not become mutually
infertile when crossed, but why this has so generally occurred
with natural varieties, as soon as they have been permanently
modified in a sufficient degree to take rank as species. We
are far from precisely knowing the cause; nor is this sur-
prising, seeing how profoundly ignorant we are in regard
to the normal and abnormal action of the reproductive sys-
tem. But we can see that species, owing to their struggle
for existence with numerous competitors, will have been
exposed during long periods of time to more uniform condi-
tions, than have domestic varieties ; and this may well make
a wide difference in the result. For we know how com-
monly wild animals and plants, when taken from their natural
conditions and subjected to captivity, are rendered sterile;
and the reproductive functions of organic beings which have
always lived under natural conditions would probably in like
manner be eminently sensitive to the influence of an un-
natural cross. Domesticated productions, on the other hand,
which, as shown by the mere fact of their domestication, were
not originally highly sensitive to changes in their conditions
of life, and which can now generally resist with undiminished
fertility repeated changes of conditions, might be expected
to produce varieties, which would be little liable to have
their reproductive powers injuriously affected by the act
FERTILITY OP VARIETIES 325
of crossing with other varieties which had originated in a
like manner.
I have as yet spoken as if the varieties of the same species
were invariably fertile when intercrossed. But it is im-
possible to resist the evidence of the existence of a certain
amount of sterility in the few following cases, which I will
briefly abstract. The evidence is at least as good as that
from which we believe in the sterility of a multitude of spe-
cies. The evidence is, also, derived from hostile witnesses,
who in all other cases consider fertility and sterility as safe
criterions of specific distinction. Gartner kept during sev-
eral years a dwarf kind of maize with yellow seeds, 3rd a
tall variety with red seeds growing near each other in his
garden ; and although these plants have separated sexes, they
never naturally crossed. He then fertilised thirteen flowers
of the one kind with pollen of the other; but only a single
head produced any seed, and this one head produced only
five grains. Manipulation in this case could not have been
injurious, as the plants have separated sexes. No one, I
believe, has suspected that these varieties of maize are dis-
tinct species ; and it is important to notice that the hybrid
plants thus raised were themselves perfectly fertile ; so that
even Gartner did not venture to consider the two varieties
as specifically different.
Girou de Buzareingues crossed three varieties of gourd,
which like the maize has separated sexes, and he asserts
that their mutual fertilisation is by so much the less easy as
their dififerences are greater. How far these experiments
may be trusted, I know not; but the forms experimented
on are ranked by Sageret, who mainly founds his classifica-
tion by the test of infertility, as varieties, and Naudin has
come to the same conclusion.
The following case is far more remarkable, and seems
at first incredible ; but it is the result of an astonishing num-
ber of experiments made during many years on nine species
of Verbascum, by so good an observer and so hostile a wit-
ness as Gartner : namely that the yellow and white varieties
when crossed .produce less 5eed than the similarly coloured
varieties of the same species. Moreover, he asserts that
when yellow and white ^varieties of one species are crossed
826 ORIGIN OF SPECIES
with yellow and white varieties of a distinct species, more
seed is produced by the crosses between the similarly coloured
flowers, than between those which are differently coloured.
Mr. Scott also has experimented on the species and varieties
of Verbascum ; and although unable to confirm Gartner's
results on the crossing of the distinct species, he finds that
the dissimilarly coloured varieties of the same species yield
fewer seeds, in the proportion of 86 to loo, than the similarly
coloured varieties. Yet these varieties differ in no respect
except in the colour of their flowers; and one variety can
sometimes be raised from the seed of another.
Kolreuter, whose accuracy has been confirmed by every
subsequent observer, has proved the remarkable fact, that
one particular variety of the common tobacco was more
fertile than the other varieties, when crossed with a widely
distinct species. He experimented on five forms which are
commonly reputed to be varieties, and which he tested by
the severest trial, namely, by reciprocal crosses, and he found
their mongrel offspring perfectly fertile. But one of these
five varieties, when used either as the father or mother, and
crossed with the Nicotiana glutinosa, always yielded hybrids
not so sterile as those which were produced from the four
other varieties when crossed with N, glutinosa. Hence the
reproductive system of this one variety must have been
in some manner and in some degree modified.
From these facts it can no longer be maintained that var-
ieties when crossed are invariably quite fertile. From the
great difficulty of ascertaining the infertility of varieties in
a state of nature, for a supposed variety, if proved to be in-
fertile in any degree, would almost universally be ranked as
a species ; — from man attending only to external characters
in his domestic varieties, and from such varieties not hav-
ing been exposed for very long periods to uniform conditions
of life ; — from these several considerations we may conclude
that fertility does not constitute a fundamental distinction
between varieties and species when crossed. The general
sterility of crossed species may safely be looked at, not as a
special acquirement or endowment, but as incidental on
changes of an unknown nature in their sexual elements.
HYBRIDS AND MONGRELS COMPARED 327
HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF
THEIR FERTILITY
Independently of the question of fertility, the offspring
of species and of varieties when crossed may be compared
in several other respects. Gartner, whose strong wish it was
to draw a distinct line between species and varieties, could
find very few, and, as it seems to me, quite unimportant dif-
ferences between the so-called hybrid offspring of species,
and the so-called mongrel oft'spring of varieties. And, on the
other hand, they agree most closely in many important re-
spects.
I shall here discuss this subject with extreme brevity. The
most important distinction is, that in the first generation
mongrels are more variable than hybrids ; but Gartner admits
that hybrids from species which have long been cultivated are
often variable in the first generation ; and I have myself seen
striking instances of this fact. Gartner further admits that
hybrids between very closely allied species are more variable
than those from very distinct species ; and this shows that
the difference in the degree of variability graduates away.
When mongrels and the more fertile hybrids are propagated
for several generations, an extreme amount of variability in
the offspring in both cases is notorious; but some few in-
stances of both hybrids and mongrels long retaining a uniform
character could be given. The variability, however, in the
successive generations of mongrels is, perhaps, greater than
in hybrids.
This greater variability in mongrels than in hybrids does
not seem at all surprising. For the parents of mongrels
are varieties, and mostly domestic varieties (very few ex-
periments having been tried on natural varieties,) and this
implies that there has been recent variability, which would
often continue and would augment that arising from the act
of crossing. The slight variability of hybrids in the first
generation, in contrast with that in the succeeding genera-
tions, is a curious fact and deserves attention. For it bears
on the view which I have taken of one of the causes of
ordinary variability; namely, that the reproductive system
from being eminently sensitive to changed conditions of life.
328 ORIGIN OF SPECIES
fails under these circumstances to perform its proper func-
tion of producing offspring closely similar in all respects
to the parent-form. Now hybrids in the first generation are
descended from species (excluding those long-cultivated)
which have not had their reproductive systems in any way
affected, and they are not variable; but hybrids themselves
have their reproductive systems seriously affected, and their
descendants are highly variable.
But to return to our comparison of mongrels and hybrids:
Gartner states that mongrels are more liable than hybrids
to revert to either parent-form; but this, if it be true, is cer-
tainly only a difference in degree. Moreover, Gartner ex-
pressly states that hybrids from long cultivated plants are
more subject to reversion than hybrids from species in their
natural state; and this probably explains the singular differ-
ence in the results arrived at by different observers: thus
Max Wichura doubts whether hybrids ever revert to their
parent-forms, and he experimented on uncultivated species
of willows; whilst Naudin, on the other hand, insists in the
strongest terms on the almost universal tendency to reversion
in hybrids, and he experimented chiefly on cultivated plants.
Gartner further states that when any two species, although
most closely allied to each other, are crossed with a third
species, the hybrids are widely different from each other;
whereas if two very distinct varieties of one species are
crossed with another species, the hybrids do not differ much.
But this conclusion, as far as I can make out, is founded
on a single experiment; and seems directly opposed to the
results of several experiments made by Kolreuter.
Such alone are the unimportant differences which Gartner
is able to point out between hybrid and mongrel plants. On
the other hand, the degrees and kinds of resemblance in
mongrels and in hybrids to their respective parents, more
especially in hybrids produced from nearly related species,
follow according to Gartner the same laws. When two
species are crossed, one has sometimes a prepotent power
of impressing its likeness on the hybrid. So I believe it to
be with varieties of plants ; and with animals one variety cer-
tainly often has this prepotent power over another variety.
Hybrid plants produced from a reciprocal cross, generally
HYBRIDS AND MONGRELS COMPARED 329
resemble each other closely; and so it is with mongrel plants
from a reciprocal cross. Both hybrids and mongrels can be
reduced to either pure parent-form, by repeated crosses in
successive generations with either parent.
These several remarks are apparently applicable to ani-
mals; but the subject is here much complicated, partly owing
to the existence of secondary sexual characters; but more
especially owing to prepotency in transmitting likeness run-
ning more strongly in one sex than in the other, both when
one species is crossed with another, and when one variety is
crossed with another variety. For instance, I think those
authors are right who maintain that the ass has a prepo-
tent power over the horse, so that both the mule and the
hinny resemble more closely the ass than the horse;
but that the prepotency runs more strongly in the male
than in the female ass, so that the mule, which is the
offspring of the male ass and mare, is more like an ass,
than is the hinny, which is the offspring of the female
ass and stallion.
Much stress has been laid by some authors on the sup-
posed fact, that it is only with mongrels that the offspring
are not intermediate in character, but closely resemble one
of their parents; but this does sometimes occur with hy-
brids, yet I grant much less frequently than with mongrels.
Looking to the cases which I have collected of cross-bred
animals closely resembling one parent, the resemblances
seem chiefly confined to characters almost monstrous in their
nature, and which have suddenly appeared — such as albinism,
melanism, deficiency of tail or horns, or additional fingers
and toes; and do not relate to characters which have been
slowly acquired through selection. A tendency to sudden
reversions to the perfect character of either parent would,
also, be much more likely to occur with mongrels, which are
descended from varieties often suddenly produced and serai-
monstrous in character, than with hybrids, which are de-
scended from species slowly and naturally produced. On the
whole, I entirely agree with Dr. Prosper Lucas, who, after
arranging an enormous body of facts with respect to animals,
comes to the conclusion that the laws of resemblance of the
child to its parents are the same, whether the two parents
330 ORIGIN OF SPECIES
differ little or much from each other, namely, in the union
of individuals of the same variety, or of different varieties,
or of distinct species.
Independently of the question of fertility and sterility, in
all other respects there seems to be a general and close simi-
larity in the offspring of crossed species, and of crossed vari-
eties. If we look at species as having been specially created,
and at varieties as having been produced by secondary laws,
this similarity would be an astonishing fact. But it har-
monises perfectly with the view that there is no essential
distinction between species and varieties.
SUMMARY OF CHAPTER.
First crosses between forms, sufficiently distinct to be
ranked as species, and their hybrids, are very generally,
but not universally, sterile. The sterility is of all degrees,
and is often so slight that the most careful experimentalists
have arrived at diametrically opposite conclusions in ranking
forms by this test. The sterility is innately variable in indi-
viduals of the same species, and is eminently susceptible to
the action of favourable and unfavourable conditions. The
degree of sterility does not strictly follow systematic affinity,
but is governed by several curious and complex laws. It is
generally different, and S£)metimes widely different in
reciprocal crosses between the same two species. It is not
always equal in degree in a first cross and in the hybrids
produced from this cross.
In the same manner as in grafting trees, the capacity in
one species or variety to take on another, is incidental on
differences, generally of an unknown nature, in their vege-
tative systems, so in crossing, the greater or less facility of
one species to unite with another is incidental on unknown
differences in their reproductive systems. There is no more
reason to think that species have been specially endowed
with various degrees of sterility to prevent their crossing
and blending in nature, than to think that trees have been
specially endowed with various and somewhat analogous
degrees of difficulty in being grafted together in order to pre-
vent their inarching in our forests.
SUMMARY 331
The sterility of first crosses and of their hybrid progeny
has not been acquired through natural selection. In the
case of first crosses it seems to depend on several circum-
stances ; in some instances in chief part on the early death
of the embryo. In the case of hybrids, it apparently depends
on their whole organisation having been disturbed by being
compounded from two distinct forms; the sterility being
closely allied to that which so frequently affects pure species,
when exposed to new and unnatural conditions of life. He
who will explain these latter cases will be able to explain
the sterility of hybrids. This view is strongly supported by
a parallelism of another kind: namely, that, firstly, slight
changes in the conditions of life add to the vigour and fertil-
ity of all organic beings; and secondly, that the crossing of
forms, which have been exposed to slightly different condi-
tions of life or which have varied, favours the size, vigour,
and fertility of their offspring. The facts given on the
sterility of the illegitimate unions of dimorphic and trimor-
phic plants and of their illegitimate progeny, perhaps ren-
der it probable that some unknown bond in all cases connects
the degree of fertility of first unions with that of their
offspring. The consideration of these facts on dimorphism,
as well as of the results of reciprocal crosses, clearly leads
to the conclusion that the primary cause of the sterility
of crossed species is confined to differences in their sexual
elements. But why, in the case of distinct species, the sexual
elements should so generally have become more or less modi-
fied, leading to their mutual infertility, we do not know;
but it seems to stand in some close relation to species hav-
ing been exposed for long periods of time to nearly uniform
conditions of life.
It is not surprising that the difficulty in crossing any two
species, and the sterility of their hybrid offspring, should
in most cases correspond, even if due to distinct causes : for
both depend on the amount of difference between the species
which are crossed. Nor is it surprising that the facility of
effecting a first cross, and the fertility of the hybrids thus
produced, and the capacity of being grafted together — though
this latter capacity evidently depends on widely different cir-
cumstances — should all run, to a certain extent, parallel with
332 ORIGIN OF SPECIES
the systematic affinity of the forms subjected to experiment;
for systematic affinity includes resemblances of all kinds.
First crosses between forms known to be varieties, or suffi-
ciently alike to be considered as varieties, and their mon-
grel offspring, are very generally, but not, as is so often
stated, invariably fertile. Nor is this almost universal and
perfect fertility surprising, when it is remembered how
liable we are to argue in a circle with respect to varieties
in a state of nature ; and when we remember that the greater
number of varieties have been produced under domestication
by the selection of mere external differences, and that they
have not been long exposed to uniform conditions of life. It
should also be especially kept in mind, that long-continued
domestication tends to eliminate sterility, and is therefore
little likely to induce this same quality. Independently of the
question of fertility, in all other respects there is the closest
general resemblance between hybrids and mongrels, — in their
variability, in their power of absorbing each other by re-
peated crosses, and in their inheritance of characters from
both parent-forms. Finally, then, although we are as ig-
norant of the precise cause of the sterility of first crosses
and of hybrids as we are why animals and plants removed
from their natural conditions become sterile, yet the facts
given in this chapter do not seem to me opposed to the belief
that species aboriginally existed as varieties.
CHAPTER X
On the Imperfection of the Geological Record
On the absence of intermediate varieties at the present day — On the
nature of extinct intermediate varieties ; on their number — On
the lapse of time, as inferred from the rate of denudation and
of deposition — On the lapse of time as estimated by years —
On the poorness of our palaeontological collections — On the in-
termittence of geological formations — On the denudation of
granitic areas — On the absence of intermediate varieties in any
one formation — On the sudden appearance of groups of species
— On their sudden appearance in the lowest known fossiliferous
strata — Antiquity of the habitable earth.
IN the sixth chapter I enumerated the chief objections
which might be justly urged against the views main-
tained in this volume. Most of them have now been dis-
cussed. One, namely the distinctness of specific forms, and
their not being blended together by innumerable transitional
links, is a very obvious difficulty. I assigned reasons why
such links do not commonly occur at the present day under
the circumstances apparently most favourable for their pres-
ence, namely on an extensive and continuous area with grad-
uated physical conditions. I endeavoured to show, that the
life of each species depends in a more important manner on
the presence of other already defined organic forms, than on
climate, and, therefore, that the really governing conditions
of life do not graduate away quite insensibly like heat or
moisture. I endeavoured, also, to show that intermediate va-
rieties, from existing in lesser numbers than the forms which
they connect, will generally be beaten out and exterminated
during the course of further modification and improvement.
The main cause, however, of innumerable intermediate links
not now occurring everywhere throughout nature, depends on
the very process of natural selection, through which new va-
rieties continually take the places of and supplant their
parent-forms. But just in proportion as this process of ex-
333
334 ORIGIN OF SPECIES
termination has acted on an enormous scale, so must the
number of intermediate varieties, which have formerly ex-
isted, be truly enormous. Why then is not every geological
formation and every stratum full of such intermediate links?
Geology assuredly does not reveal any such finely-graduated
organic chain; and this, perhaps, is the most obvious and
serious objection which can be urged against the theory. The
explanation lies, as I believe, in the extreme imperfection of
the geological record.
In the first place, it should always be borne in mind what
sort of intermediate forms must, on the theory, have formerly
existed. I have found it difficult, when looking at any two
species, to avoid picturing to myself forms directly intermedi-
ate between them. But this is a wholly false view; we should
always look for forms intermediate between each species and
a common but unknown progenitor ; and the progenitor will
generally have differed in some respects from all its modified
descendants. To give a simple illustration: the fantail and
pouter pigeons are both descended from the rock-pigeon; if
we possessed all the intermediate varieties which have ever
existed, we should have an extremely close series between
both and the rock-pigeon ; but we should have no varieties
directly intermediate between the fantail and pouter ; none,
for instance, combining a tail somewhat expanded with a crop
somewhat enlarged, the characteristic features of these two
breeds. These two breeds, moreover, have become so much
modified, that, if we had no historical or indirect evidence
regarding their origin, it would not have been possible to
have determined, from a mere comparison of their structure
with that of the rock-pigeon, C. livia, whether they had de-
scended from this species or from some other allied form,
such as C. oenas.
So, with natural species, if we look to forms very distinct,
for instance to the horse and tapir, we have no reason to
suppose that links directly intermediate between them ever
existed, but between each and an unknown common parent.
The common parent will have had in its whole organisation
much general resemblance to the tapir and to the horse ; but
in some points of structure may have differed considerably
from both, even perhaps more than they differ from each
THE LAPSE OF TIME 335
other. Hence, in all such cases, we should be unable to rec-
ognise the parent-form of any two or more species, even if
we closely compared the structure of the parent with that of
its modified descendants, unless at the same time we had a
nearly perfect chain of the intermediate links.
It is just possible by the theory, that one of two living
forms might have descended from the other; for instance, a
horse from a tapir ; and in this case direct intermediate links
will have existed between them. But such a case would im-
ply that one form had remained for a very long period unal-
tered, whilst its descendants had undergone a vast amount
of change ; and the principle of competition between organism
and organism, between child and parent, will render this a
very rare event ; for in all cases the new and improved forms
of life tend to supplant the old and unimproved forms.
By the theory of natural selection all living species have
been connected with the parent-species of each genus, by dif-
ferences not greater than we see between the natural and
domestic varieties of the same species at the present day ; and
these parent-species, now generally extinct, have in their
turn been similarly connected with more ancient forms; and
so on backwards, always converging to the common ancestor
of each great class. So that the number of intermediate and
transitional links, between all living and extinct species, must
have been inconceivably great. But assuredly, if this theory
be true, such have lived upon the earth.
ON THE LAPSE OF TIME^ AS INFERRED FROM THE RATE OF
DEPOSITION AND EXTENT OF DENUDATION
Independently of our not finding fossil remains of such in-
finitely numerous connecting links, it may be objected that
time cannot have sufficed for so great an amount of organic
change, all changes having been effected slowly. It is hardly
possible for me to recall to the reader who is not a practical
geologist, the facts leading the mind feebly to comprehend the
lapse of time. He who can read Sir Charles Lyell's grand
work on the Principles of Geology, which the future historian
will recognise as having produced a revolution in natural
science, and yet does n<3t admit how vast have been the past
336 ORIGIN OF SPECIES
periods of time, may at once close this volume Not that it
suffices to study the Principles of Geology, or to read special
treatises by different observers on separate formations, and
to mark how each author attempts to give an inadequate idea
of the duration of each formation, or even of each stratum.
We can best gain some idea of past time by knowing the
agencies at work, and learning how deeply the surface of the
land has been denuded, and how much sediment has been de-
posited. As Lyell has well remarked, the extent and thick-
ness of our sedimentary formations are the result and the
measure of the denudation which the earth's crust has else-
where undergone. Therefore a man should examine for
himself the great piles of superimposed strata, and watch the
rivulets bringing down mud^ and the waves wearing away the
sea-cliffs, in order to comprehend something about the dura-
tion of past time, the monuments of which we see all
around us.
It is good to wander along the coast, when formed of mod-
erately hard rocks, and mark the process of degradation. The
tides in most cases reach the cliffs only for a short time twice
a day, and the waves eat into them only when they are
charged with sand or pebbles ; for there is good evidence that
pure water effects nothing in wearing away rock. At last
the base of the cliff is undermined, huge fragments fall down,
and these, remaining fixed, have to be worn away atom by
atom, until after being reduced in size they can be rolled
about by the waves, and then they are more quickly ground
into pebbles, sand, or mud. But how often do we see along
the bases of retreating cliffs rounded boulders, all thickly
clothed by marine productions, showing how little they are
abraded and how seldom they are rolled about ! Moreover,
if we follow for a few miles any line of rocky cliff, which is
undergoing degradation, we find that it is only here and there,
along a short length or round a promontory, that the cliffs
are at the present time suffering. The 'appearance of the sur-
face and the vegetation show that elsewhere years have
elapsed since the waters washed their base.
We have, however, recently learnt from the observations
of Ramsay, in the van of many excellent observers — of Jukes,
Geikie, Croll, and others, that subaerial degradation is a
THE LAPSE OF TIME 337
much more important agency than coast-action, or the power
of the waves. The whole surface of the land is exposed to
the chemical action of the air and of the rain-water with its
dissolved carbonic acid, and in colder countries to frost; the
disintegrated matter is carried down even gentle slopes dur-
ing heavy rain, and to a greater extent than might be sup-
posed, especially in arid districts, by the wind; it is then
transported by the streams and rivers, which when rapid
deepen their channels, and triturate the fragments. On a
rainy day, even in a gently undulating country, we see the
effects of subaerial degradation in the muddy rills which flow
down every slope. Messrs. Ramsay and Whitaker have
shown, and the observation is a most striking one, that the
great lines of escarpment in the Wealden district and those
ranging across England, which formerly were looked at as
ancient sea-coasts, cannot have been thus formed, for each
line is composed of one and the same formation, whilst our
sea-cliffs are everywhere formed by the intersection of vari-
ous formations. This being the case, we are compelled to
admit that the escarpments owe their origin in chief part to
the rocks of which they are composed having resisted subae-
rial denudation better than the surrounding surface ; this sur-
face consequently has been gradually lowered, with the lines
of harder rock left projecting. Nothing impresses the mind
with the vast duration of time, according to our ideas of time,
more forcibly than the conviction thus gained that subaerial
agencies which apparently have so little power, and which
seem to work so slowly, have produced great results.
When thus impressed with the slow rate at which the land
is worn away through subaerial and littoral action, it is good,
in order to appreciate the past duration of time, to consider
on the one hand, the masses of rock which have been re-
moved over many extensive areas, and on the other hand the
thickness of our sedimentary formations. I remember hav-
ing been much struck when viewing volcanic islands, which
have been worn by the waves and pared all round into per-
pendicular cliffs of one or two thousand feet in height; for
the gentle slope of the lava-streams, due to their formerly
liquid state, showed at a glance how far the hard, rocky beds
had once extended into -the open ocean. The same story is
338 ORIGIN OF SPECIES
told still more plainly by faults, — those great cracks along
which the strata have been upheaved on one side, or thrown
down on the other, to the height or depth of thousands of
feet; for since the crust cracked, and it makes no great dif-
ference whether the upheaval was sudden, or, as most geolo-
gists now believe, was slow and effected by many starts, the
surface of the land has been so completely planed down that
no trace of these vast dislocations is externally visible. The
Craven fault, for instance, extends for upwards of 30 miles,
and along this line the vertical displacement of the strata
varies from 600 to 3000 feet. Professor Ramsay has pub-
lished an account of a downthrow in Anglesea of 2300 feet ;
and he informs me that he fully believes' that there is one in
Merionethshire of 12,000 feet; yet in these cases there is
nothing on the surface of the land to show such prodigious
movements ; the pile of rocks on either side of the crack
having been smoothly swept away.
On the other hand, in all parts of the world the piles of
sedimentary strata are of wonderful thickness. In the Cor-
dillera I estimated one mass of conglomerate at ten thou-
sand feet ; and although conglomerates have probably been
accumulated at a quicker rate than finer sediments, yet from
being formed of worn and rounded pebbles, each of which
bears the stamp of time, they are good to show how slowly
the mass must have been heaped together. Professor Ramsay
has given me the maximum thickness, from actual measure-
ment in most cases, of the successive formations in different
parts of Great Britain ; and this is the result : —
Feet
Palaeozoic strata (not inclviding igneous beds) 57, 154
Secondary strata 13,190
Tertiary strata 2,240
— making altogether 72,584 feet ; that is, very nearly thirteen
and three-quarters British miles. Some of the formations,
which are represented in England by thin beds, are thousands
of feet in thickness on the Continent. Moreover, between
each successive formation, we have, in the opinion of most
geologists, blank periods of enormous length. So that the
lofty pile of sedimentary roclvs in Britain gives but an inade-
quate idea of the time which has elapsed during their accu-
THE LAPSE OF TIME 339
niulation. The consideration of these various facts impresses
the mind almost in the same manner as does the vain en-
deavour to grapple with the idea of eternity.
Nevertheless this impression is partly false. Mr. Croll, in
an interesting paper, remarks that we do not err "in forming
too great a conception of the length of geological periods,"
but in estimating them by years. When geologists look at
large and complicated phenomena, and then at the figures rep-
resenting several million years, the two produce a totally
different effect on the mind, and the figures are at once pro-
nounced too small. In regard to subaerial denudation, Mr.
Croll shows, by calculating the known amount of sediment
annually brought down by certain rivers, relatively to their
areas of drainage, that looo feet of solid rock, as it became
gradually disintegrated, would thus be removed from the
mean level of the whole area in the course of six million
years.
This seems an astonishing result, and some considera-
tions lead to the suspicion that it may be too large, but even
if halved or quartered it is still very surprising. Few of us,
however, know what a million really means : Mr. Croll gives
the following illustration : take a narrow strip of paper, 83
feet 4 inches in length, and stretch it along the wall of a large
hall ; then mark off at one end the tenth of an inch. This
tenth of an inch will represent one hundred years, and the
entire strip a million years. But let it be borne in mind, in
relation to the subject of this work, what a hundred years
implies, represented as it is by a measure utterly insignificant
in a hall of the above dimensions. Several eminent breeders,
during a single lifetime, have so largely modified some of the
higher animals, which propagate their kind much more slowly
than most of the lower animals, that they have formed what
well deserves to be called a new sub-breed. Few men have
attended with due care to any one strain for more than half '
a century, so that a hundred years represents the work of two
breeders in succession. It is not to be supposed that species
in a state of nature ever change so quickly as domestic ani-
mals under the guidance of methodical selection. The com-
parison would be in every way fairer with the effects which
follow from unconscious selection, that is the preservation of
340 ORIGIN OF SPECIES
the most useful or beautiful animals, with no intention of
modifying the breed; but by this process of unconscious
selection, various breeds have been sensibly changed in the
course of two or three centuries.
Species, however, probably change much more slowly, and
within the same country only a few change at the same time.
This slowness follows from all the inhabitants of the same
country being already so well adapted to each other, that new
places in the polity of nature do not occur until after long
intervals, due to the occurrence of physical changes of some
kind, or through the immigration of new forms. Moreover
variations or individual differences of the right nature, by
which some of the inhabitants might be better fitted to their
new places under the altered circumstances, would not always
occur at once. Unfortunately we have no means of deter-
mining, according to the standard of years, how long a
period it takes to modify a species; but to the subject of time
we must return.
ON THE POORNESS OF PAL.50NT0L0GICAL COLLECTIONS
Now let us turn to our richest geological museums, and
what a paltry display we behold ! That our collections are
imperfect is admitted by every one. The remark of that ad-
mirable palaeontologist, Edward Forbes, should never be for-
gotten, namely, that very many fossil species are known and
named from single and often broken specimens, or from a
few specimens collected on some one spot. Only a small por-
tion of the surface of the earth has been geologically ex-
plored, and no part with sufificient care, as the important
discoveries made every year in Europe prove. No organism
wholly soft can be preserved. Shells and bones decay and
disappear when left on the bottom of the sea, where sediment
is not accumulating. We probably take a quite erroneous
view, when we assume that sediment is being deposited over
nearly the whole bed of the sea, at a rate sufficiently quick
to embed and preserve fossil remains. Throughout an enor-
mously large proportion of the ocean, the bright blue tint of
the water bespeaks its purity. The many cases on record of
a formartion conformably covered, after an immense interval
PALyEONTOLOGICAL COLLECTIONS 341
of time^ oy another and later formation, without the under-
lying bed having suffered in the interval any wear and tear,
seem explicable only on the view of the bottom of the sea not
rarely lying for ages in an unaltered condition. The remains
which do become embedded, if in sand or gravel, will, when
the beds are upraised, generally be dissolved by the percola-
tion of rain-water charged with carbonic acid. Some of the
many kinds of animals which live on the beach between high
and low water mark seem to be rarely preserved. For in-
stance, the several species of the Chthamalinse (a sub-family
of sessile cirripedes) coat the rocks all over the world in
infinite numbers ; they are all strictly littoral, with the excep-
tion of a single Mediterranean species, which inhabits deep
water, and this has been found fossil in Sicily, whereas not
one other species has hitherto been found in any tertiary
formation ; yet it is known that the genus Chthamalus ex-
isted during the Chalk period. Lastly, many great deposits
requiring a vast length of time for their accumulation, are
entirely destitute of organic remains, without our being able
to assign any reason: one of the most striking instances is
that of the Flysch formation, which consists of shale and
sandstone, several thousand, occasionally even six thousand
feet in thickness, and extending for at least 300 miles from
Vienna to Switzerland; and although this great mass has
been most carefully searched, no fossils, except a few vege-
table remains, have been found.
With respect to the terrestrial productions which lived
during the Secondary and Palaeozoic periods, it is superfluous
to state that our evidence is fragmentary in an extreme de-
gree. For instance, until recently not a land-shell was known
belonging to either of these vast periods, with the exception
of one species discovered by Sir C. Lyell and Dr. Dawson in
the carboniferous strata of North America; but now land-
shells hav^ been found in the lias. In regard to mammifer-
ous remains, a glance at the historical table published in
Lyell's Manual will bring home the truth, how accidental and
rare is their preservation, far better than pages of detail.
Nor is their rarity surprising, when we remember how large
a proportion of the bones of tertiary mammals have been
discovered either in caves or in lacustrine deposits ; and that
342 ORIGIN OF SPECIES
not a cave or true lacustrine bed is known belonging to the
age of our secondary or palaeozoic formations.
But the imperfection in the geological record largely re-
sults from another and more important cause than any of the
foregoing; namely, from the several formations being sep-
arated from each other by wide intervals of time. This doc-
trine has been emphatically admitted by many geologists and
palaeontologists, who, like E. Forbes, entirely disbelieve in
the change of species. When we see the formations tabulated
in written works, or when we follow them in nature, it is
difficult to avoid believing that they are closely consecutive.
But we know, for instance, from Sir R. Murchison's great
work on Russia, what wide gaps there are in that country
between the superimposed formations ; so it is in North
America, and in many other parts of the world. The most
skilful geologist, if his attention had been confined exclusively
to these large territories, would never have suspected that,
during the periods which were blank and barren in his own
country, great piles of sediment, charged with new and pe-
culiar forms of life, had elsewhere been accumulated. And
if, in each separate territory, hardly any idea can be formed
of the length of time which has elapsed between the consecu-
tive formations, we may infer that this could nowhere be
ascertained. The frequent and great changes in the mineral-
ogical composition of consecutive formations, generally im-
plying great changes in the geography of the surrounding
lands, whence the sediment was derived, accord with the
belief of vast intervals of time having elapsed between each
formation.
We can, I think, see why the geological formations of each
region are almost invariably intermittent; that is, have not
followed each other in close sequence. Scarcely any fact
struck me more when examining many hundred miles of the
South American coasts, which have been upraised several
hundred feet within the recent period, than the absence of
any recent deposits sufficiently extensive to last for even a
short geological period. Along the whole west coast, which
is inhabited by a peculiar marine fauna, tertiary beds are so
poorly developed, that no record of several successive and
peculiar marine faunas will probably be preserved to a distant
PALiEONTOLOGICAL COLLECTIONS 343
age. A little reflection will explain why, along the rising
coast of the western side of South America, no extensive
formations with recent or tertiary remains can anywhere be
found, though the supply of sediment must for ages have
been great, from the enormous degradation of the coast-rocks
and from muddy streams entering the sea. The explanation,
no doubt, is, that the littoral and sub-littoral deposits are
continually worn away, as soon as they are brought up by
the slow and gradual rising of the land within the grinding
action of the coast-waves.
We may, I think, conclude that sediment must be accumu-
lated in extremely thick, solid, or extensive masses, in order
to withstand the incessant action of the waves, when first
upraised and during successive oscillations of level, as well as
the subsequent subaerial degradation. Such thick and ex-
tensive accumulations of sediment may be formed in two
ways ; either in profound depths of the sea, in which case
the bottom will not be inhabited by so many and such varied
forms of life, as the more shallow seas ; and the mass when
upraised will give an imperfect record of the organisms
which existed in the neighbourhood during the period of its
accumulation. Or, sediment may be deposited to any thick-
ness and extent over a shallow bottom, if it continue slowly
to subside. In this latter case, as long as the rate of subsi-
dence and the supply of sediment nearly balance each other,
the sea will remain shallow and favourable for many and
varied forms, and thus a rich fossiliferous formation, thick
enough, when upraised, to resist a large amount of denuda-
tion, may be formed.
I am convinced that nearly all our ancient formations,
which are throughout the greater part of their thickness rich
in fossils, have thus been formed during subsidence. Since
publishing my views on this subject in 1845, I have watched
the progress of Geology, and have been surprised to note how
author after author, in treating of this or that great forma-
tion, has come to the conclusion that it was accumulated
during subsidence. I may add, that the only ancient tertiary
formation on the west coast of South America, which has
been bulky enough to resist such degradation as it has as yet
suffered, but which will hardly last to a distant geological
344 ORIGIN OF SPECIES
age, was deposited during a downward oscillation of level,
and thus gained considerable thickness.
All geological facts tell us plainly that each area has under-
gone numerous slow oscillations of level, and apparently these
oscillations have affected wide spaces. Consequently, forma-
tions rich in fossils and sufficiently thick and extensive to
resist subsequent degradation, will have been formed over
wide spaces during periods of subsidence, but only where the
supply of sediment was sufficient to keep the sea shallow and
to embed and preserve the remains before they had time to
decay. On the other hand, as long as the bed of the sea
remains stationary, thick deposits cannot have been accumu-
lated in the shallow parts, which are the most favourable to
life. Still less can this have happened during the alternate
periods of elevation ; or, to speak more accurately, the beds
which were then accumulated will generally have been de-
stroyed by being upraised and brought within the limits of
the coast-action.
These remarks apply chiefly to littoral and sub-littoral de-
posits. In the case of an extensive and shallow sea, such as
that within a large part of the Malay Archipelago, where the
depth varies from 30 or 40 to 60 fathoms, a widely extended
formation might be formed during a period of elevation, and
yet not suffer excessively from denudation during its slow
upheaval; but the thickness of the formation could not be
great, for owing to the elevatory movement it would be less
than the depth in which it was formed ; nor would the deposit
be much consolidated, nor be capped by overlying formations,
so that it would run a good chance of being worn away by
atmospheric degradation and by the action of the sea during
subsequent oscillations of level. It has, however, been sug-
gested by Mr. Hopkins, that if one part of the area, after
rising and before being denuded, subsided, the deposit formed
during the rising movement, though not thick, might after-
wards become protected by fresh accumulations, and thus be
preserved for a long period.
Mr. Hopkins also expresses his belief that sedimentary beds
of considerable horizontal extent have rarely been completely
destroyed. But all geologists, excepting the few who believe
that our present metamorphic schists and plutonic rocks once
PAL/EONTOLOGICAL COLLECTIONS 345
formed the primordial nucleus of the globe, will admit that
these latter rocks have been stript of their covering to an
enormous extent. For it is scarcely possible that such rocks
could have been solidified and crystallized whilst uncovered;
but if the metamorphic action occurred at profound depths of
the ocean, the former protecting mantle of rock may not have
been very thick. Admitting then that gneiss, mica-schist,
granite, diorite, &c., were once necessarily covered up, how
can we account for the naked and extensive areas of such
rocks in many parts of the world, except on the belief that
they have subsequently been completely denuded of all over-
lying strata? That such extensive areas do exist cannot be
doubted ; the granitic region of Parime is described by Hum-
boldt as being at least nineteen times as large as Switzerland.
South of the Amazon, Boue colours an area composed of
rocks of this nature as equal to that of Spain, France, Italy,
part of Germany, and the British Islands, all conjoined. This
region has not been carefully explored, but from the concur-
rent testimony of travellers, the granitic area is very large;
thus. Von Eschwege gives a detailed section of these rocks,
stretching from Rio de Janeiro for 260 geographical miles
inland in a straight line; and I travelled for 150 miles in
another direction, and saw nothing but granitic rocks. Nu-
merous specimens, collected along the whole coast from near
Rio Janeiro to the mouth of the Plata, a distance of iioo geo-
graphical miles, were examined by me, and they all belonged
to this class. Inland, along the whole northern bank of the
Plata I saw, besides modern tertiary beds, only one small
patch of slightly metamorphosed rock, which alone could
have formed a part of the original capping of the granitic
series. Turning to a well-known region, namely, to the
United States and Canada, as shown in Professor H. D.
Rogers's beautiful map, I have estimated the areas by cutting
out and weighing the paper, and T find that the metamorphic
(excluding "the semi-metamorphic") and granitic rocks ex-
ceed, in the proportion of 19 to 12-5. the whole of the newer
Palaeozoic formations. In many regions the metamorphic and
granitic rocks would be found much more widely extended
than they appear to be, if all the sedimentary beds were re-
moved which rest unconformably on them, and which could
346 ORIGIN OF SPECIES
not have formed part of the original mantle under which they
were crystallized. Hence it is probable that in some parts
of the world whole formations have been completely de-
nuded, with not a wreck left behind.
One remark is here worth a passing notice. During periods
of elevation the area of the land and of the adjoining shoal
parts of the sea will be increased, and new stations will often
be formed : — all circumstances favourable, as previously ex-
plained, for the formation of new varieties and species ; but
during such periods there will generally be a blank in the
geological record. On the other hand, during subsidence, the
inhabited area and number of inhabitants will decrease (ex-
cepting on the shores of a continent when first broken up into
an archipelago), and consequently during subsidence, though
there will be much extinction, few new varieties or species
will be formed; and it is during these very periods of subsi-
dence, that the deposits which are richest in fossils have been
accumulated.
ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIETIES
IN ANY SINGLE FORMATION
From these several considerations, it cannot be doubted
that the geological record, viewed as a whole, is extremely
imperfect ; but if we confine our attention to any one forma-
tion, it becomes much more difficult to understand why we do
not therein find closely graduated varieties between the allied
species which lived at its commencement and at its close.
Several cases are on record of the same species presenting
varieties in the upper and lower parts of the same formation ;
thus, Trautschold gives a number of instances with Ammo-
nites ; and Hilgendorf has described a most curious case of
ten graduated forms of Planorbis multiformis in the succes-
sive beds of a fresh-water formation in Switzerland. Although
each formation has indisputably required a vast number of
years for its deposition, several reasons can be given why
each should not commonly include a graduated series of links
between the species which lived at its commencement and
close; but I cannot assign due proportional weight to the
following considerations.
ABSENCE OF INTERMEDIATE VARIETIES 347
Although each formation may mark a very long lapse of
years, each probably is short compared with the period requi-
site to change one species into another. I am aware that two
palaeontologists, whose opinions are worthy of much defer-
ence, namely Bronn and Woodward, have concluded that the
average duration of each formation is twice or thrice as long
as the average duration of specific forms. But insuperable
difficulties, as it seems to me, prevent us from coming to any
just conclusion on this head. When we see a species first
appearing in the middle of any formation, it would be rash
in the extreme to infer that it had not elsewhere previously
existed. So again when we find a species disappearing before
the last layers have been deposited, it would be equally rash
to suppose that it then became extinct. We forget how small
the area of Europe is compared with the rest of the world;
nor have the several stages of the same formation throughout
Europe been correlated with perfect accuracy.
We may safely infer that with marine animals of all kinds
there has been a large amount of migration due to climatal
and other changes; and when we see a species first appearing
in any formation, the probability is that it only then first im-
migrated into that area. It is well known, for instance, that
several species appear somewhat earlier in the palaeozoic beds
of North America than in those of Europe; time having ap-
parently been required for their migration from the American
to the European seas. In examining the latest deposits in
various quarters of the world, it has everywhere been noted,
that some few still existing species are common in the de-
posit, but have become extinct in the immediately surround-
ing sea; or, conversely, that some are now abundant in the
neighbouring sea, but are rare or absent in this particular
deposit. It is an excellent lesson to reflect on the ascer-
tained amount of migration of the inhabitants of Europe dur-
ing the glacial epoch, which forms only a part of one whole
geological period; and likewise to reflect on the changes of
level, on the extreme change of climate, and on the great
lapse of time, all included within the same glacial period.
Yet it may be doubted whether, in any quarter of the world,
sedimentary deposits, including fossil remains, have gone on
accumulating within the same area during the whole of this
348 ORIGIN OF SPECIES
period. It is not, for instance, probable that sediment was
deposited during the whole of the glacial period near the
mouth of the Mississippi, within that limit of depth at which
marine animals can best flourish : for we know that great
geographical changes occurred in other parts of America dur-
ing this space of time. When such beds as were deposited in
shallow water near the mouth of the Mississippi during some
part of the glacial period shall have been upraised, organic
remains will probably first appear and disappear at different
levels, owing to the migrations of species and to geographical
changes. And in the distant future, a geologist, examining
those beds, would be tempted to conclude that the average
duration of life of the embedded fossils had been less than
that of the glacial period, instead of having been really far
greater, that is, extending from before the glacial epoch to
the present day.
In order to get a perfect gradation between two forms in
the upper and lower parts of the same formation, the deposit
must have gone on continuously accumulating during a long
period, sufficient for the slow process of modification ; hence
the deposit must be a very thick one; and the species under-
going change must have lived in the same district throughout
the whole time. But we have seen that a thick formation,
fossiliferous throughout its entire thickness, can accumulate
only during a period of subsidence ; and to keep the depth ap-
proximately the same, which is necessary that the same
marine species may live on the same space, the supply of
sediment must nearly counterbalance the amount of subsi-
dence. But this same movement of subsidence will tend to
submerge the area whence the sediment is derived, and thus
diminish the supply, whilst the downward movement con-
tinues. In fact, this nearly exact balancing between the
supply of sediment and the amount of subsidence is probably
a rare contingency; for it has been observed by more than
one palaeontologist, that very thick deposits are usually
barren of organic remains, except near their upper or lower
limits.
It would seem that each separate formation, like the whole
pile of formations in any country, has generally been inter-
mittent in its accumulation. When we see, as is so often the
ABSENCE OF INTERMEDIATE VARIETIES 349
case, a formation composed of beds of widely different min-
eralogical composition, we may reasonably suspect that the
process of deposition has been more or less interrupted. Nor
will the closest inspection of a formation give us any idea of
the length of time which its deposition may have consumed.
Many instances could be given of beds only a few feet
in thickness, representing formations, which are elsewhere
thousands of feet in thickness, and which must have required
an enormous period for their accumulation ; yet no one igno-
rant of this fact would have even suspected the vast lapse of
time represented by the thinner formation. Many cases could
be given of the lower beds of a formation having been up-
raised, denuded, submerged, and then re-covered by the upper
beds of the same formation, — facts, showing what wide, yet
easily overlooked, intervals have occurred in its accumula-
tion. In other cases we have the plainest evidence in great
fossilised trees, still standing upright as they grew, of many
long intervals of time and changes of level during the process
of deposition, which would not have been suspected, had not
the trees been preserved : thus Sir C. Lyell and Dr. Dawson
found carboniferous beds 1400 feet thick in Nova Scotia, with
ancient root-bearing strata, one above the other at no less
than sixty-eight different levels. Hence, when the same
species occurs at the bottom, middle, and top of a formation,
the probability is that it has not lived on the same spot during
the whole period of deposition, but has disappeared and reap-
peared, perhaps many times, during the same geological
period. Consequently if it were to undergo a considerable
amount of modification during the deposition of any one geo-
logical formation, a section would not include all the fine
intermediate gradations which must on our theory have ex-
isted, but abrupt, though perhaps slight, changes of form.
It is all-important to remember that naturalists have no
golden rule by which to distinguish species and varieties ;
they grant some little variability to each species, but when
they meet with a somewhat greater amount of difference be-
tween any two forms, they rank both as species, unless they
are enabled to connect them together by the closest inter-
mediate gradations; and this, from the reasons just assigned,
we can seldom hope to effect in any one geological section.
350 ORIGIN OP SPECIES
Supposing B and C to be two species, and a third, A, to be
found in an older and underlying bed; even if A were strictly-
intermediate between B and C, it would simply be ranked as a
third and distinct species, unless at the same time it could be
closely connected by intermediate varieties with either one or
both forms. Nor should it be forgotten, as before explained,
that A might be the actual progenitor of B and C, and yet
would not necessarily be strictly intermediate between them
in all respects. So that we might obtain the parent-species
and its several modified descendants from the lower and
upper beds of the same formation, and unless we obtained
numerous transitional gradations, we should not recognise
their blood-relationship, and should consequently rank them
as distinct species.
It is notorious on what excessively slight differences many
palaeontologists have founded their species ; and they do this
the more readily if the specimens come from different sub-
stages of the same formation. Some experienced concholo-
gists are now sinking many of the very fine species of
D'Orbigny and others into the rank of varieties ; and on this
view we do find the kind of evidence of change which on the
theory we ought to find. Look again at the later tertiary de-
posits, which include many shells believed by the majority of
naturalists to be identical with existing species ; but some ex-
cell«'.nt naturalists, as Agassiz and Pictet, maintain that all
these tertiary species are specifically distinct, though the dis-
tinction is admitted to be very slight ; so that here, unless we
believe that these eminent naturalists have been misled by
their imaginations, and that these late tertiary species really
present no difference whatever from their living representa-
tives, or unless we admit, in opposition to the judgment of
most naturalists, that these tertiary species are all truly dis-
tinct from the recent, we have evidence of the frequent oc-
currence of slight modifications of the kind required. If we
look to rather wider intervals of time, namely, to distinct but
consecutive stages of the same great formation, we find that
the embedded fossils, though universally ranked as specific-
ally different, yet are far more closely related to each other
than are the species found in more widely separated forma-
tions ; so that here again we have undoubted evidence of
ABSENCE OF INTERMEDIATE VARIETIES 351
change in the direction required by the theory ; but to this
latter subject I shall return in the following chapter.
With animals and plants that propagate rapidly and do not
wander much, there is reason to suspect, as we have formerly
seen, that their varieties are generally at first local ; and that
such local varieties do not spread widely and supplant
their parent-forms until they have been modified and per-
fected in some considerable degree. According to this view,
the chance of discovering in a formation in any one country
all the early stages of transition between any two forms, is
small, for the successive changes are supposed to have been
local or confined to some one spot. Most marine animals
have a wide range ; and we have seen that with plants it is
those which have the widest range, that oftenest present va-
rieties ; so that, with shells and other marine animals, it is
probable that those which had the widest range, far exceed-
ing the limits of the known geological formations in Europe,
have oftenest given rise, first to local varieties and ultimately
to new species ; and this again would greatly lessen the
chance of our being able to trace the stages of transition in
any one geological formation.
It is a more important consideration, leading to the same
result, as lately insisted on by Dr. Falconer, namely, that the
period during which each species underwent modification,
though long as measured by years, was probably short in
comparison with that during which it remained without un-
dergoing any change.
It should not be forgotten, that at the present day, with
perfect specimens for examination, two forms can seldom be
connected by intermediate varieties, and thus proved to be
the same species, until many specimens are collected from
many places; and with fossil species this can rarely be done.
We shall, perhaps, best perceive the improbability of our
being enabled to connect species by numerous, fine, inter-
mediate, fossil links, by asking ourselves whether, for in-
stance, geologists at some future period will be able to prove
that our different breeds of cattle, sheep, horses, and dogs are
descended from a single stock or from several aboriginal
stocks ; or, again, whether certain sea-shells inhabiting the
shores of North America, which are ranked by some con-
352 ORIGIN OF SPECIES
chologists as distinct species from their European representa-
tives, and by other conchologists as only varieties, are really
varieties, or are, as it is called, specifically distinct. This
could be effected by the future geologist only by his discov-
ering in a fossil state numerous intermediate gradations; and
such success is improbable in the highest degree.
It has been asserted over and over again, by writers who
believe in the immutability of species, that geology yields
no linking forms. This assertion, as we shall see in the next
chapter, is certainly erroneous. As Sir J. Lubbock has re-
marked, "Every species is a link between other allied forms."
If we take a genus having a score of species, recent and ex-
tinct, and destroy four-fifths of them, no one doubts that the
remainder will stand much more distinct from each other.
If the extreme forms in the genus happen to have been thus
destroyed, the genus itself will stand more distinct from
other allied genera. What geological research has not re-
vealed, is the former existence of infinitely numerous grada-
tions, as fine as existing varieties, connecting together nearly
all existing and extinct species. But this ought not to be ex-
pected; yet this has been repeatedly advanced as a most
serious objection against my views.
It may be worth while to sum up the foregoing remarks on
the causes of the imperfection of the geological record under
an imaginary illustration. The Malay Archipelago is about
the size of Europe from the North Cape to the Mediter-
ranean, and from Britain to Russia; and therefore equals all
the geological formations which have been examined with any
accuracy, excepting those of the United States of America.
I fully agree with Mr. Godwin-Austen, that the present con-
dition of the Malay Archipelago, with its numerous large
islands separated by wide and shallow seas, probably repre-
sents the former state of Europe, whilst most of our forma-
tions were accumulating. The Malay Archipelago is one of
the richest regions in organic beings ; yet if all the species
were to be collected which have ever lived there, how im-
perfectly would they represent the natural history of the
world !
But we have every reason to believe that the terrestrial
productions of the archipelago would be preserved in an ex-
ABSENCE OF INTERMEDIATE VARIETIES 553
tremely imperfect manner in the formations which we sup-
pose to be there accumulating. Not many of the strictly
littoral animals, or of those which lived on naked submarine
rocks, would be embedded ; and those embedded in gravel or
sand would not endure to a distant epoch. Wherever sedi-
ment did not accumulate on the bed of the sea, or where it
did not accumulate at a sufficient rate to protect organic
bodies from decay, no remains could be preserved.
Formations rich in fossils of many kinds, and of thickness
sufficient to last to an age as distant in futurity as the sec-
ondary formations lie in the past, would generally be formed
in the archipelago only during periods of subsidence. These
periods of subsidence would be separated from each other
by immense intervals of time, during which the area would
be either stationary or rising; whilst rising, the fossiliferous
formations on the steeper shores would be destroyed, almost
as soon as accumulated, by the incessant coast-action, as we
now see on the shores of South America. Even throughout
the extensive and shallow seas within the archipelago, sedi-
mentary beds could hardly be accumulated of great thickness
during the periods of elevation, or become capped and pro-
tected by subsequent deposits, so as to have a good chance of
enduring to a very distant future. During the periods
of subsidence, there would probably be much extinction
of life ; during the periods of elevation, there would be much
variation, but the geological record would then be less
perfect.
It may be doubted whether the duration of any one great
period of subsidence over the whole or part of the archipel-
ago, together with a contemporaneous accumulation of sedi-
ment, would exceed the average duration of the same specific
forms; and these contingencies are indispensable for the pres-
ervation of all the transitional gradations between any two
or more species. If such gradations were not all fully pre-
served, transitional varieties would merely appear as so many
new, though closely allied species. It is also probable that
each great period of subsidence would be interrupted by os-
cillations of level, and that slight climatal changes would
intervene during such lengthy periods ; and in these cases the
inhabitants of the archipelago would migrate, and no closely
L — HC XI
354 ORIGIN OF SPECIES
consecutive record of their modifications could be preserved
in any one formation.
Very many of the marine inhabitants of the archipelago
now range thousands of miles beyond its confines ; and anal-
ogy plainly leads to the belief that it would be chiefly these
far-ranging species, though only some of them, which would
oftenest produce new varieties ; and the varieties would at
first be local or confined to one place, but if possessed of any
decided advantage, or when further modified and improved,
they would slowly spread and supplant their parent-forms.
When such varieties returned to their ancient homes, as they
would differ from their former state in a nearly uniform,
though perhaps extremely slight degree, and as they would
be found embedded in slightly different sub-stages of the
same formation, they would, according to the principles fol-
lowed by many palaeontologists, be Kanked as new and distinct
species.
If then there be some degree of truth in these remarks, we
have no right to expect to find, in our geological formations,
an infinite number of those fine transitional forms which, on
our theory, have connected all the past and present species
of the same group into one long and branching chain of life.
We ought only to look for a few links, and such assuredly
we do find — some more distantly, some more closely, related
to each other; and these links, let them be ever so close, if
found in different stages of the same formation, would, by
many palaeontologists, be ranked as distinct species. But I
do not pretend that I should ever have suspected how poor
was the record in the best preserved geological sections, had
not the absence of innumerable transitional links between the
species which lived at the commencement and close of each
formation, pressed so hardly on my theory.
ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF
ALLIED SPECIES
The abrupt manner in which whole groups of species sud-
denly appear in certain formations, has been urged by several
palaeontologists — for instance, by Agassiz, Pictet, and Sedg-
wick — as a fatal objection to the belief in the transmutation
APPEARANCE OF WHOLE GROUPS 355
of species. If numerous species, belonging to the same gen-
era or families, have really started into life at once, the fact
would be fatal to the theory of evolution through natural
selection. For the development by this means of a group of
forms, all of which are descended from some one progenitor,
must have been an extremely slow process ; and the progeni-
tors must have lived long before their modified descendants.
But we continually overrate the perfection of the geological
record, and falsely infer, because certain genera or families
have not been found beneath a certain stage, that they did
not exist before that stage. In all cases positive palseonto-
logical evidence may be implicitly trusted ; negative evidence
is worthless, as experience has so often shown. We contin-
ually forget how large the world is, com.pared with the area
over which our geological formations have been carefully ex-
amined ; we forget that groups of species may elsewhere have
long existed, and have slowly multiplied, before they invaded
the ancient archipelagoes of Europe and the United States.
We do not make due allowance for the intervals of time
which have elapsed between our consecutive formations, —
longer perhaps in many cases than the time required for the
accumulation of each formation. These intervals will have
given time for the multiplication of species from some one
parent- form: and in the succeeding formation, such groups
or species will appear as if suddenly created.
I may here recall a remark formerly made, namely, that it
might require a long succession of ages to adapt an organism
to some new and peculiar line of life, for instance, to fly
through the air ; and consequently that the transitional forms
would often long remain confined to some one region; but
that, when this adaptation had once been effected, and a few
species had thus acquired a great advantage over other or-
ganisms, a comparatively short time would be necessary to
produce many divergent forms, which would spread rapidly
and widely, throughout the world. Professor Pictet, in his
excellent Review of this work, in commenting on early
transitional forms, and taking birds as an illustration, cannot
see how the successive modifications of the anterior limbs of
a supposed prototype could possibly have been of any advan-
tage. But look at the penguins of the Southern Ocean; have
356 ORIGIN OF SPECIES
not these birds their front limbs in this precise intermediate
state of "neither true arms nor true wings"? Yet these birds
hold their place victoriously in the battle for life ; for they
exist in infinite numbers and of many kinds. I do" not sup-
pose that we here see the real transitional grades through
which the wings of birds have passed ; but what special diffi-
culty is there in believing that it might profit the modified
descendants of the penguin, first to become enabled to flap
along the surface of the sea like the logger-headed duck, and
ultimately to rise from its surface and glide through the air?
I will now give a few examples to illustrate the foregoing
remarks, and to show how liable we are to error in supposing
that whole groups of species have suddenly been produced.
Even in so short an interval as that between the first and
second editions of Pictet's great work on Palaeontology, pub-
lished in 1844-46 and 1853-57, the conclusions on the first ap-
pearance and disappearance of several groups of animals
have been considerably modified ; and a third edition would
require still further changes. I may recall the well-known
fact that in geological treatises, published not many years
ago, mammals were always spoken of as having abruptly
come in at the commencement of the tertiary series. And
now one of the richest known accumulations of fossil mam-
mals belongs to the middle of the secondary series; and true
mammals have been discovered in the new red sandstone at
nearly the commencement of this great series. Cuvier used
to urge that no monkey occurred in any tertiary stratum ; but
now extinct species have been discovered in India, South
America, and in Europe, as far back as the miocene stage.
Had it not been for the rare accident of the preservation of
footsteps in the new red sandstone of the United States, who
would have ventured to suppose that no less than at least
thirty different bird-like animals, some of gigantic size, existed
during that period? Not a fragment of bone has been dis-
covered in these beds. Not long ago, paljeontologists main-
tained that the whole class of birds came suddenly into ex-
istence during the eocene period; but now we know, on the
authority of Professor Owen, that a bird certainly lived dur-
ing the deposition of the upper greensand ; and still more re-
cently, that strange bird, the Archeopteryx, with a long
APPEARANCE OF WHOLE GROUPS 357
lizard-like tail, bearing a pair of feathers on each joint, and
with its wings furnished with two free claws, has been dis-
covered in the oolitic slates of Solenhofen. Hardly any recent
discovery shows more forcibly than this, how little we as yet
know of the former inhabitants of the world.
I may give another instance, which, from having passed
under my own eyes, has much struck me. In a memoir on
Fossil Sessile Cirripedes, I stated that, from the large number
of existing and extinct tertiary species ; from the extraordi-
nary abundance of the individuals of many species all over
the world, from the Arctic regions to the equator, inhabiting
various zones of depths from the upper tidal limits to 50
fathoms ; from the perfect manner in which specimens are
preserved in the oldest tertiary beds; from the ease with
which even a fragment of a valve can be recognized ; from
all these circumstances, I inferred that, had sessile cirripedes
existed during the secondary periods, they would certainly
have been preserved and discovered ; and as not one species
had then been discovered in beds of this age, I concluded that
this great group had been suddenly developed at the com-
mencement of the tertiary series. This was a sore trouble
to me, adding as I then thought one more instance of the
abrupt appearance of a great group of species. But my work
had hardly been published, when a skilful palaeontologist, M.
Bosquet, sent me a drawing of a perfect specimen of an un-
mistakeable sessile cirripede, which he had himself extracted
from the chalk of Belgium. And, as if to make the case as
striking as possible, this cirripede was a Chthamalus, a very
common, large, and ubiquitous genus, of which not one
species has as yet been found even in any tertiary stratum.
Still more recently, a Pyrgoma, a member of a distinct sub-
family of sessile cirripedes, has been discovered by Mr,
Woodward in the upper chalk ; so that we now have abundant
evidence of the existence of this group of animals during the
secondary period.
The case most frequently insisted on by palaeontologists of
the apparently sudden appearance of a whole group of species,
is that of the teleostean fishes, low down, according to Agas-
siz, in the Chalk period. This group includes the large ma-
jority of existing species.- But certain Jurassic and Triassic
358 ORIGIN OF SPECIES
forms are now commonly admitted to be teleostean ; and even
some palaeozoic forms have thus been classed by one high
authority. If the teleosteans had really appeared suddenly in
the northern hemisphere at the commencement of the chalk
formation, the fact would have been highly remarkable ; but
it would not have formed an insuperable difficulty, unless it
could likewise have been shown that at the same period the
species were suddenly and simultaneously developed in other
quarters of the world. It is almost superfluous to remark
that hardly any fossil-fish are known from south of the
equator; and by running through Pictet's Palaeontology it will
be seen that very few species are known from several forma-
tions in Europe. Some few families of fish now have a con-
fined range ; the teleostean fishes might formerly have had a
similarly confined range, and after having been largely de-
veloped in some one sea, have spread widely. Nor have we
any right to suppose that the seas of the world have always
been so freely open from south to north as they are at pres-
ent. Even at this day, if the Malay Archipelago were con-
verted into land, the tropical parts of the Indian Ocean would
form a large and perfectly enclosed basin, in which any great
group of marine animals might be multiplied; and here they
would remain confined, until some of the species became
adapted to a cooler climate, and were enabled to double the
Southern capes of Africa or Australia, and thus reach other
and distant seas.
From these considerations, from our ignorance of the geol-
ogy of other countries beyond the confines of Europe and the
United States, and from the revolution in our palaeontological
knowledge effected by the discoveries of the last dozen years,
it seems to me to be about as rash to dogmatize on the suc-
cession of organic forms throughout the world, as it would
be for a naturalist to land for five minutes on a barren point
in Australia, and then to discuss the number and range of its
productions.
SUDDEN APPEARANCE OF GROUPS 359
ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES
IN THE LOWEST KNOWN FOSSILIFEROUS STRATA
There is another and allied difficulty, which is much more
serious. I allude to the manner in which species belonging
to several of the main divisions of the animal kingdom sud-
denly appear in the lowest known fossiliferous rocks. Most
of the arguments which have convinced me that all the ex-
isting species of the same group are descended from a single
progenitor, apply with equal force to the earliest known
species. For instance, it cannot be doubted that all the
Cambrian and Silurian trilobites are descended from some
one crustacean, which must have lived long before the Cam-
brian age, and which probably differed greatly from any
known animal. Some of the most ancient animals, as the
Nautilus, Lingula, &c., do not differ much from living species ;
and it cannot on our theory be supposed, that these old spe-
cies were the progenitors of all the species belonging to the
same groups which have subsequently appeared, for they are
not in any degree intermediate in character.
Consequently, if the theory be true, it is indisputable that
before the lowest Cambrian stratum was deposited long peri-
ods elapsed, as long as, or probably far longer than, the whole
interval from the Cambrian age to the present day ; and that
during these vast periods the world swarmed with living
creatures. Here we encounter a formidable objection; for it
seems doubtful whether the earth, in a fit state for the habi-
tation of living creatures, has lasted long enough. Sir W.
Thompson concludes that the consolidation of the crust can
hardly have occurred less than 20 or more than 400 million
years ago, but probably not less than 98 or more than 200
million years. These very wide limits show how doubtful
the data are ; and other elements may have hereafter to be
introduced into the problem. Mr. Croll estimates that about
60 million years have elapsed since the Cambrian period, but
this, judging from the small amount of organic change since
the commencement of the Glacial epoch, appears a very short
time for the many and great mutations of life, which have
certainly occurred since the Cambrian formation ; and the
previous 140 million years. can hardly be considered as suffi-
360 ORIGIN OF SPECIES
cient for the development of the varied forms of life which
already existed during the Cambrian period. It is, however,
probable, as Sir William Thompson insists, that the world at
a very early period was subjected to more rapid and violent
changes in its physical conditions than those now occurring;
and such changes would have tended to induce changes at a
corresponding rate in the organisms which then existed.
To the question why we do not find rich fossiliferous de-
posits belonging to these assumed earliest periods prior to the
Cambrian system, I can give no satisfactory answer. Sev-
eral eminent geologists, with Sir R. Murchison at their head,
were until recently convinced that we beheld in the organic
remains of the lowest Silurian stratum the first dawn of life.
Other highly competent judges, as Lyell and E. Forbes, have
disputed this conclusion. We should not forget that only a
small portion of the world is known with accuracy. Not very
long ago M. Barrande added another and lower stage,
abounding with new and peculiar species, beneath the then
known Silurian system; and now, still lower down in the
Lower Cambrian formation, Mr. Hicks has found in South
Wales beds rich in' trilobites, and containing various molluscs
and annelids. The presence of phosphatic nodules and bitu-
minous matter, even in some of the lowest azoic rocks, prob-
ably indicates life at these periods ; and the existence of the
Eozoon in the Laurentian formation of Canada is generally
admitted. There are three great series of strata beneath the
Silurian system in Canada, in the lowest of which the Eozoon
is found. Sir W. Logan states that their "united thickness
'may possibly far surpass that of all the succeeding rocks,
'from the base of the palaeozoic series to the present time. We
'are thus carried back to a period so remote that the appear-
'ance of the so-called Primordial fauna (of Barrande) may
'by some be considered as a comparatively modern event."
The Eozoon belongs to the most lowly organised of all
classes of animals, but is highly organised for its class; it
existed in countless numbers, and, as Dr. Dawson has re-
marked, certainly preyed on other minute organic beings,
which must have lived in great numbers. Thus the words,
which I wrote in 1859, about the existence of living beings
long before the Cambrian period, and which are almost the
SUDDEN APPEARANCE OF GROUPS 361
same with those since used by Sir W. Logan, have proved
true. Nevertheless, the difficulty of assigning any good
reason for the absence of vast piles of strata rich in fossils
beneath the Cambrian system is very great. It does not seem
probable that the most ancient beds have been quite w^orn
away by denudation, or that their fossils have been wholly
obliterated by metamorphic action, for if this had been the
case we should have found only small remnants of the forma-
tions next succeeding them in age, and these would always
have existed in a partially metamorphosed condition. But
the descriptions which we possess of the Silurian deposits
over immense territories in Russia and in North America, do
not support the view, that the older a formation is, the more
invariably it has suffered extreme denudation and meta-
morphism.
The case at present must remain inexplicable ; and may
be truly urged as a valid argument against the views here
entertained. To show that it may hereafter receive some
explanation, I will give the following hypothesis. From the
nature of the organic remains which do not appear to have
inhabited profound depths, in the several formations of
Europe and of the United States; and from the amount of
sediment, miles in thickness, of which the formations are
composed, we may infer that from first to last large islands
or tracts of land, whence the sediment was derived, occurred
in the neighbourhood of the now existing continents of
Europe and North America. The same view has since been
maintained by Agassiz and others. But we do not know
what was the state of things in the intervals between the
several successive formations ; whether Europe and the
United States during these intervals existed as dry land, or
as a submarine surface near land, on which sediment was
not deposited, or as the bed on an open and unfathomable sea.
Looking to the existing oceans, which are thrice as exten-
sive as the land, we see them studded with many islands;
but hardly one truly oceanic island (with the exception of
New Zealand, if this can be called a truly oceanic island) is
as yet known to afford even' a remnant of any palaeozoic and
secondary formation. Hence vre may perhaps infer that
during the palaeozoic anci secondary periods, neither conti-
362 ORIGIN OF SPECIES
nents nor continental islands existed where our oceans now
extend; for had they existed, palaeozoic and secondary forma-
tions would in all probability have been accumulated from
sediment derived from their wear and tear ; and these would
have been at least partially upheaved by the oscillations of
level, which must have intervened during these enormously
long periods. If then we may infer anything from these
facts, we may infer that, where our oceans now extend,
oceans have extended from the remotest period of which we
have any record; and on the other hand, that where conti-
nents now exist, large tracts of land have existed, subjected
no doubt to great oscillations of level, since the Cambrian
period. The colored map appended to my volume on Coral
Reefs, led me to conclude that the great oceans are still
mainly areas of subsidence, the great archipelagoes still areas
of oscillations of level, and the continents areas of elevation.
But we have no reason to assume that things have thus
remained from the beginning of the world. Our continents
seem to have been formed by a preponderance, during many
oscillations of level, of the force of elevation ; but may not
the areas of preponderant movement have changed in the
lapse of ages ? At a period long antecedent to the Cambrian
epoch, continents may have existed where oceans are now
spread out; and clear and open oceans may have existed
where our continents now stand. Nor should we be justified
in assuming that if, for instance, the bed of the Pacific Ocean
were now converted into a continent we should there find
sedimentary formations in a recognisable condition older
than the Cambrian strata, supposing such to have been for-
merly deposited ; for it might well happen that strata which
had subsided some miles nearer to the centre of the earth,
and which had been pressed on by an enormous weight of
super-incumbent water, might have undergone far more
metamorphic action than strata which have always remained
nearer to the surface. The immense areas in some parts of the
world, for instance in South America, of naked metamorphic
rocks, which must have been heated under great pressure,
have always seemed to me to require some special explana-
tion ; and we may perhaps believe that we see in these large
areas, the many formations long anterior to the Cambrian
SUDDEN APPEARANCE OF GROUPS 363
epoch in a completely metamorphosed and denuded con-
dition.
The several difficulties here discussed, namely — that,
though we find in our geological formations many links be-
tween the species which now exist and which formerly
existed, we do not find infinitely numerous fine transitional
forms closely joining them all together ; — the sudden man-
ner in which several groups of species first appear in our
European formations; — the almost entire absence, as at
present known, of formations rich in fossils beneath the
Cambrian strata, — are all undoubtedly of the most serious
nature. We see this in the fact that the most eminent
palaeontologists, namely, Cuvier, Agassiz, Barrande, Pictet,
Falconer, E. Forbes, &:c., and all our greatest geologists, as
Lyell, Murchison, Sedgwick, &c., have unanimously, often
vehemently, maintained the immutability of species. But
Sir Charles Lyell now gives the support of his high author-
ity to the other side ; and most geologists and palaeontologists
are much shaken in their former belief. Those who believe
that the geological record is in any degree perfect, will un-
doubtedly at once reject the theory. For my part, follow-
ing out Lyell's metaphor, I look at the geological record
as a history of the world imperfectly kept, and written in
a changing dialect; of this history we possess the last vol-
ume alone, relating only to two or three countries. Of this
volume, only here and there a short chapter has been pre-
served; and of each page, only here and there a few lines.
Each word of the slowly-changing language, more pr less
different in the successive chapters, may represent the forms
of life, which are entombed in our consecutive formations,
and which falsely appear to have been abruptly introduced.
On this view, the difficulties above discussed are greatly
diminished, or even disappear.
CHAPTER XI
On the Geological Succession of Organic Beings
On the slow and successive appearance of new species — On their
different rates of change — Species once lost do not reappear —
Groups of species follow the same general rules in their appear-
ance and disappearance as do single species — On extinction —
On simultaneous changes in the forms of life throughout the
world— On the affinities of extinct species to each other and to
living species — On the state of development of ancient forms —
On the succession of the same types within the same areas —
Summary of preceding and present chapter.
IET US now see whether the several facts and laws relat-
. ing to the geological succession of organic beings
^ accord best with the common view of the immutability
of species, or with that of their slow and gradual modifica-
tion, through variation and natural selection.
New species have appeared very slowly, one after another,
both on the land and in the waters. Lyell has shown that
it is hardly possible to resist the evidence on this head in the
case of the several tertiary stages ; and every year tends
to fill up the blanks between the stages, and to make the pro-
portion between the lost and existing forms more gradual.
In some of the most recent beds, though undoubtedly of high
antiquity if measured by years, only one or two species are
extinct, and only one or two are new, having appeared there
for the first time, either locally, or, as far as we know, on
the face of the earth. The secondary formations are more
broken; but, as Bronn has remarked, neither the appear-
ance nor disappearance of the many species embedded in
each formation has been simultaneous.
Species belonging to different genera and classes have not
changed at the same rate, or in the same degree. In the
older tertiary beds a few living shells may still be found in
the midst of a multitude of extinct forms. Falconer has
364
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 365
given a striking instance of a similar fact, for an existing
crocodile is associated with many lost mammals and reptiles
in the sub-Himalayan deposits. The Silurian Lingula differs
but little from the living species of this genus ; whereas most
of the other Silurian Molluscs and all the Crustaceans have
changed greatly. The productions of the land seem to have
changed at a quicker rate than those of the sea, of which
a striking instance has been observed in Switzerland. There
is some reason to believe that organisms high in the scale,
change more quickly than those that are low: though there
are exceptions to this rule. The amount of organic change,
as Pictet has remarked, is not the same in each successive
so-called formation. Yet if we compare any but the most
closely related formations, all the species will be found to
have undergone some change. When a species has once dis-
appeared from the face of the earth, we have no reason to
believe that the same identical form ever reappears. The
strongest apparent exception to this latter rule is that of
the so-called "colonies" of M. Barrande, which intrude for a
period in the midst of an older formation, and then allow
the pre-existing fauna to reappear ; but Lyell's explanation,
namely, that it is a case of temporary migration from a
distinct geographical province, seems satisfactory.
These several facts accord well with our theory, which
includes no fixed law of development, causing all the in-
habitants of an area to change abruptly, or simultaneously,
or to an equal degree. The process of modification must be
slow, and will generally affect only a few species at the
same time ; for the variability of each species is independent
of that of all others. Whether such variations or individual
differences as may arise will be accumulated through natural
selection in a greater or less degree, thus causing a greater
or less amount of permanent modification, will depend on
many complex contingencies — on the variations being of a
beneficial nature, on the freedom of intercrossing, on the
slowly changing physical conditions of the country, on the
immigration of new colonists, and on the nature of the other
inhabitants with which the varying species come into com-
petition. Hence it is by no means surprising that one species
should retain the same identical form much longer than
366 ORIGIN OF SPECIES
others; or, if changing, should change in a less degree. We
find similar relations between the existing inhabitants of dis-
tinct countries; for instance, the land-shells and coleopterous
insects of Madeira have come to differ considerably from
their nearest allies on the continent of Europe, whereas the
marine shells and birds have remained unaltered. We can
perhaps understand the apparently quicker rate of change in
terrestrial and in more highly organised productions com-
pared with marine and lower productions, by the more com-
plex relations of the higher beings to their organic and in-
organic conditions of life, as explained in a former chapter.
When many of the inhabitants of any area have become
modified and improved, we can understand, on the principle
of competition, and from the all-important relations of or-
ganism to organism in the struggle for life, that any form
which did not become in some degree modified and improved,
would be liable to extermination. Hence we see why all
the species in the same region do at last, if we look to long
enough intervals of time, become modified, for otherwise
they would become extinct.
In members of the same class the average amount of
change during long and equal periods of time, may, perhaps,
be nearly the same; but as the accumulation of enduring
formation, rich in fossils, depends on great masses of sedi-
ment being deposited on subsiding areas, our formations have
been almost necessarily accumulated at wide and irregularly
intermittent intervals of time ; consequently the amount of
organic change exhibited by the fossils embedded in consecu-
tive formations is not equal. Each formation, on this view,
does not mark a new and complete act of creation, but only
an occasional scene, taken almost at hazard in an ever
slowly changing drama.
We can clearly understand why a species when once lost
should never reappear, even if the very same conditions of
life, organic and inorganic, should recur. For though the
offspring of one species might be adapted (and no doubt
this has occurred in innumerable instances) to fill the place
of another species in the economy of nature, and thus sup-
plant it ; yet the two forms — the old and the new — would
not be identically the same; for both would almost certainly.
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 367
inherit different characters from their distinct progenitors ;
and organisms already differing would vary in a different
manner. For instance, it is possible, if all our fantail
pigeons were destroyed, that fanciers might make a new
breed hardly distinguishable from the present breed ; but
if the parent rock-pigeon were likewise destroyed, and under
nature we have every reason to believe that parent-forms
are generally supplanted and exterminated by their improved
offspring, it is incredible that a fantail, identical with the
existing breed, could be raised from any other species of
pigeon, or even from any other well-established race of the
domestic pigeon, for the successive variations would almost
certainly be in some degree different, and the newly-formed
variety would probably inherit from its progenitor some char-
acteristic differences.
Groups of species, that is, genera and families, follow the
same general rules in their appearance and disappearance as
do single species, changing more or less quickly, and in a
greater or lesser degree. A group, when it has once dis-
appeared, never reappears ; that is, its existence, as long as it
lasts, is continuous. I am aware that there are some ap-
parent exceptions to this rule, but the exceptions are surpris-
ingly few, so few that E. Forbes, Pictet, and Woodward
(though all strongly opposed to such views as I maintain)
admit its truth ; and the rule strictly accords with the theory.
For all the species of the same group, however long it may
have lasted, are the modified descendants one from the other,
and all from a common progenitor. In the genus Lingula,
for instance, the species which have successively appeared at all
ages must have been connected by an unbroken series of gen-
erations, from the lowest Silurian stratum to the present day.
We have seen in the last chapter that whole groups of
species sometimes falsely appear to have been abruptly devel-
oped; and I have attempted to give an explanation of this
fact, which if true would be fatal to my views. But such
cases are certainly exceptional ; the general rule being a
gradual increase in number, until the group reaches its maxi-
mum, and then, sooner or later, a gradual decrease. If the
number of the species included within a genus, or the number
of the genera within a family, be represented by a vertical
368 ORIGIN OF SPECIES
line of varying thickness, ascending through the successive
geological formations, in which the species are found, the
line will sometimes falsely appear to begin at its lower end,
not in a sharp point, but abruptly ; it then gradually thickens
upwards, often keeping of equal thickness for a space, and
ultimately thins out in the upper beds, marking the decrease
and final extinction of the species. This gradual increase in
number of the species of a group is strictly conformable
with the theory, for the species of the same genus,
and the genera of the same family, can increase only slowly
and progressively; the process of modification and the pro-
duction of a number of allied forms necessarily being a slow
and gradual process, — one species first giving rise to two
or three varieties, these being slowly converted into species,
which in their turn produce by equally slow steps other
varieties and species, and so on, like the branching of a great
tree from a single stem, till the group becomes large.
ON EXTINCTION
We have as yet only spoken incidentally of the disappear-
ance of species and of groups of species. On the theory of
natural selection, the extinction of old forms and the pro-
duction of new and improved forms are intimately con-
nected together. The old notion of all the inhabitants of the
earth having been swept away by catastrophes at successive
periods is very generally given up, even by those geologists,
as Elie de Beaumont, Murchison, Barrande, &c., whose gen-
eral views would naturally lead them to this conclusion.
On the contrary, we have every reason to believe, from the
study of the tertiary formations, that species and groups of
species gradually disappear, one after another, first from one
spot, then from another, and finally from the world. In
some few cases, however, as by the breaking of an isthmus
and the consequent irruption of a multitude of new inhabi-
tants into an adjoining sea, or by the final subsidence of an
island, the process of extinction may have been rapid. Both
single species and whole groups of species last for very un-
equal periods ; some groups, as we have seen, have endured
from the earliest known dawn of life to the present day;
EXTINCTION 369
some have disappeared before the close of the palaeozoic
period. No fixed law seems to determine the length of time
during which any single species or any single genus en-
dures. There is reason to believe that the extinction of a
whole group of species is generally a slower process than
their production : if their appearance and disappearance be
represented, as before, by a vertical line of varying thickness
the line is found to taper more gradually at its upper end,
which marks the progress of extermination, than at its
lower end, which marks the first appearance and the early
increase in number of the species. In some cases, however,
the extermination of whole groups, as of ammonites, towards
the close of the secondary period, has been wonderfully
sudden.
The extinction of species has been involved in the most
gratuitous mystery. Some authors have even supposed that,
as the individual has a definite length of life, so have species
a definite duration. No one can have marvelled more than I
have done at the extinction of species. When I found in La
Plata the tooth of a horse embedded with the remains of
Mastodon, Megatherium, Toxodon, and other extinct mon-
sters, which all co-existed with still living shells at a very
late geological period, I was filled with astonishment; for,
seeing that the horse, since its introduction by the Span-
iards into South America, has run wild over the whole coun-
try and has increased in numbers at an unparalleled rate, I
asked myself what could so recently have exterminated the
former horse under conditions of life apparently so favour-
able. But my astonishment was groundless. Professor
Owen soon perceived that the tooth, though so like that of
the existing horse, belonged to an extinct species. Had this
horse been still living, but in some degree rare, no naturalist
would have felt the least surprise at its rarity; for rarity
is the attribute of a vast number of species of all classes, in
all countries. If we ask ourselves why this or that species
is rare, we answer that something is unfavourable in its
conditions of life ; but what that something is we can hardly
ever tell. On the supposition of the fossil horse still existing
as a rare species, we might have felt certain, from the
analogy of all other mammals, even of the slow-breeding
370 ORIGIN OF SPECIES
elephant, and from the history of the naturalisation of
the domestic horse in South America, that under more
favourable conditions it would in a very few years have
stocked the whole continent. But we could not have told
what the unfavourable conditions were which checked its in-
crease, whether some one or several contingencies, and at
what period of the horse's life, and in what degree they
severally acted. If the conditions had gone on, however
slowly, becoming less and less favourable, we assuredly
should not have perceived the fact, yet the fossil horse would
certainly have become rarer and rarer, and finally extinct;
— its place being seized on by some more successful com-
petitor.
It is most difficult always to remember that the increase
of every creature is constantly being checked by unperceived
hostile agencies; and that these same unperceived agencies
are amply sufficient to cause rarity, and finally extinction.
So little is this subject understood, that I have heard sur-
prise repeatedly expressed at such great monsters as the
Mastodon and the more ancient Dinosaurians having be-
come extinct; as if mere bodily strength gave victory in the
battle of life. Mere size, on the contrary, would in some
cases determine, as has been remarked by Owen, quicker
extermination from the greater amount of requisite food.
Before man inhabited India or Africa, some cause must
have checked the continued increase of the existing ele-
phant. A highly capable judge. Dr. Falconer, believes that
it is chiefly insects which, from incessantly harassing and
weakening the elephant in India, check its increase ; and
this was Bruce's conclusion with respect to the African ele-
phant in Abyssinia. It is certain that insects and blood-
sucking bats determine the existence of the larger natural-
ized quadrupeds in several parts of S. America.
We see in many cases in the more recent tertiary forma-
tions, that rarity precedes extinction; and we know that this
has been the progress of events with those animals which
have been exterminated, either locally or wholly, through
man's agency. I may repeat what I published in 1845,
namely, that to admit that species generally become rare
before they become extinct — to feel no surprise at the rarity
EXTINCTJON 371
of a species, and yet to marvel greatly when the species
ceases to exist, is much the same as to admit that sickness
in the individual is the forerunner of death — to feel no sur-
prise at sickness, but, when the sick man dies, to wonder and
to suspect that he died by some deed of violence.
The theory of natural selection is grounded on the belief
that each new variety and ultimately each new species, is
produced and maintained by having some advantage over
those with which it comes into competition; and the conse-
quent extinction of the less favoured forms almost inevitably
follows. It is the same with our domestic productions; when
a new and slightly improved variety has been raised, it at
first supplants the less improved varieties in the same neigh-
bourhood ; when much improved it is transported far and
near, like our short-horn cattle, and takes the place of other
breeds in other countries. Thus the appearance of new
forms and the disappearance of old forms, both those natu-
rally and those artificially produced, are bound together. In
flourishing groups, the number of new specific forms which
have been produced within a given time has at some periods
probably been greater than the number of the old specific
forms which have been exterminated ; but we know that spe-
cies have not gone on indefinitely increasing, at least during
the later geological epochs, so that, looking to later times,
we may believe that the production of new^ forms has caused
the extinction of about the same number of old forms.
The competition will generally be most severe, as formerly
explained and illustrated by examples, between the forms
which are most like each other in all respects. Hence the
improved and modified descendants of a species will gener-
ally cause the extermination of the parent species; and if
many new forms have been developed from any one species,
the nearest allies of that species, i.e., the species of the same
genus, will be the most liable to extermination. Thus, as I
believe, a number of new species descended from one species,
that is a new genus, comes to supplant an old genus, belong-
ing to the same family. But it must often have happened
that a new species belonging to some one group has seized
on the place occupied by a species belonging to a distinct
group, and thus have caused its extermination. If many
372 ORIGIN OF SPECIES
allied forms be developed from the successful intruder, many
will have to yield their places; and it will generally be the
allied forms, which will suffer from some inherited inferior-
ity in common. But whether it be species belonging to the
same or to a distinct class, which have yielded their places
to other modified and improved species, a few of the sufferers
may often be preserved for a long time, from being fitted to
some pecwliar line of life, or from inhabiting some distant
and isolated station, where they will have escaped severe
competition. For instance, some species of Trigonia, a great
genus of shells in the secondary formations, survive in the
Australian seas; and a few members of the great and almost
extinct group of Ganoid fishes still inhabit our fresh waters.
Therefore the utter extinction of a group is generally, as
we have seen, a slower process than its production.
With respect to the apparently sudden extermination of
whole families or orders, as of Trilobites at the close of the
palaeozoic period and of Ammonites at the close of the sec-
ondary period, we must remember what has been already
said on the probable wide intervals of time between our con-
secutive formations; and in these intervals there may have
been much slow extermination. Moreover, when, by sud-
den immigration or by unusually rapid development, many
species of a new group have taken possession of an area,
many of the older species will have been exterminated in a
correspondingly rapid manner; and the forms which thus
yield their places will commonly be allied, for they will par-
take of the same inferiority in common.
Thus, as it seems to me, the manner in which single species
and whole groups of species become extinct accord well with
the theory of natural selection. We need not marvel at ex-
tinction ; if we must marvel, let it be at our own presumption
in imagining for a moment that we understand the many
complex contingencies on which the existence of each spe-
cies depends. If we forget for an instant that each species
tends to increase inordinately, and that some check is always
in action, yet seldom perceived by us, the whole economy of
nature will be utterly obscured. Whenever we can precisely
say why this species is more abundant in individuals than
that; why this species and not another can be naturalised in
FORMS OF LIFE CHANGING 373
a given country; then, and not until then, we may justly feel
surprise why we cannot account for the extinction of any
particular species or group of species.
ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANE-
OUSLY THROUGHOUT THE WORLD
Scarcely any palaeontological discovery is more striking
than the fact that the forms of life change almost simulta-
neously throughout the world. Thus our European Chalk
formation can be recognised in many distinct regions, under
the most different climates, where not a fragment of the
mineral chalk itself can be found; namely in North America,
in equatorial South America, in Tierra del Fuego, at the
Cape of Good Hope, and in the peninsula of India. For at
these distant points, the organic remains in certain beds pre-
sent an unmistakeable resemblance to those of the Chalk. It
is not that the same species are met with; for in some cases
not one species is identically the same, but they belong to the
same families, genera, and sections of genera, and sometimes
are similarly characterised in such trifling points as mere
superficial sculpture. Moreover, other forms, which are not
found in the Chalk of Europe, but which occur in the forma-
tions either above or below, occur in the same order at these
distant points of the world. In the several successive palaeo-
zoic formations of Russia, Western Europe, and North
America, a similar parallelism in the forms of life has been
observed by several authors; so it is, according to Lyell, with
the European and North American tertiary deposits. Even
if the few fossil species which are common to the Old and
New Worlds were kept wholly out of view, the general par-
allelism in the successive forms of life, in the palaeozoic and
tertiary stages, would still be manifest, and the several for-
mations could be easily correlated.
These observations, however, relate to the marine inhabi-
tants of the world : we have not sufficient data to judge
whether the productions of the land and of fresh water at
distant points change in the same parallel manner. We may
doubt whether they have thus changed: if the Megatherium,
Mylodon, Macrauchenia, and Toxodon had been brought to
374 ORIGIN OF SPECIES
Europe from I.a Plata, without any information in regard
to their geological position, no one would have suspected
that they had co-existed with sea-shells all still living; but
as these anomalous monsters co-existed with the Mastodon
and Horse, it might at least have been inferred that they
had lived during one of the later tertiary stages.
When the marine forms of life are spoken of as having
changed simultaneously throughout the world, it must not be
supposed that this expression relates to the same year, or to
the same country, or even that it has a very strict geological
sense; for if all the marine animals now living in Europe,
and all those that lived in Europe during the pleistocene
period (a very remote period as measured by years, includ-
ing the whole glacial epoch) were compared with those now
existing in South America or in Australia, the most skilful
naturalist would hardly be able to say whether the present
or the pleistocene inhabitants of Europe resembled most
closely those of the southern hemisphere. So, again, several
highly competent observers maintain that the existing pro-
ductions of the United States are more closely related to
those which lived in Europe during certain late tertiary
stages, than to the present inhabitants of Europe ; and if this
be so, it is evident that fossiliferous beds now deposited on
the shores of North America would hereafter be liable to be
classed with somewhat older European beds. Nevertheless,
looking to a remotely future epoch, there can be little doubt
that all the more modern marine formations, namely, the
upper pliocene, the pleistocene and strictly modern beds of
Europe, North and South America, and Australia, from con-
taining fossil remains in some degree allied, and from not
including those forms which are found only in the older
underlying deposits, would be correctly ranked as simulta-
neous in a geological sense.
The fact of the forms of life changing simultaneously, in
the above large sense, at distant parts of the world, has
greatly struck those admirable observers, MM. de Verneuil
and d'Archiac. After referring to the parallelism of the
palaeozoic forms of life in various parts of Europe, they add,
"If, struck by this strange sequence, we turn our attention
to North America, and there discover a series of analogous
FORMS OF LIFE CHANGING 375
phenomena, it will appear certain that all these modifications
of species, their extinction, and the introduction of new ones,
cannot be owing to mere changes in marine currents or other
causes more or less local and temporary, but depend on gen-
eral laws which govern the whole animal kingdom." M.
Barrande has made forcible remarks to precisely the same
effect. It is, indeed, quite futile to look to changes of cur-
rents, climate, or other physical conditions, as the cause of
these great mutations in the forms of life throughout the
world, under the most different climates. We must, as Bar-
rande has remarked, look to some special law. We shall see
this more clearly when we treat of the present distribution
of organic beings, and find how slight is the relation between
the physical conditions of various countries and the nature
of their inhabitants.
This great fact of the parallel succession of the forms of
life throughout the world, is explicable on the theory of
natural selection. New species are formed by having some
advantage over older forms ; and the forms, which are al-
ready dominant, or have some advantage over the other
forms in their own country, give birth to the greatest num-
ber of new varieties or incipient species. We have distinct
evidence on this head, in the plants which are dominant, that
is, which are commonest and most widely diffused, producing
the greatest number of new varieties. It is also natural that
the dominant, varying, and far-spreading species, which have
already invaded to a certain extent the territories of other
species, should be those which would have the best chance
of spreading still further, and of giving rise in new countries
to other new varieties and species. The process of diffusion
would often be very slow, depending on climatal and geo-
graphical changes, on strange accidents, and on the gradual
acclimatisation of new species to the various climates
through which they might have to pass, but in the course
of time the dominant forms would generally succeed in
.spreading and would ultimately prevail. The diffusion
would, it is probable, be slower with the terrestrial inhabi-
tants of the distinct continents than with the marine inhabi-
tants of the continuous sea. We might therefore expect to
find, as we do find, a less strict degree of parallelism in the
376 ORIGIN OF SPECIES
succession of the productions of the land than with those of
the sea.
Thus, as it seems to me, the parallel, and, taken in a large
sense, simultaneous, succession of the same forms of life
throughout the world, accords well with the principle of new
species having been formed by dominant species spreading
widely and varying; the new species thus produced being
themselves dominant, owing to their having had some ad-
vantage over their already dominant parents, as well as over
other species, and again spreading, varying, and producing
new forms. The old forms which are beaten and which
yield their places to the new and victorious forms, will gen-
erally be allied in groups, from inheriting some inferiority
in common ; and therefore, as new and improved groups
spread throughout the world, old groups disappear from the
world ; and the succession of forms everywhere tends to
correspond both in their first appearance and final disappear-
ance.
There is one other remark connected with this subject
worth making. I have given my reasons for believing that
most of our great formations, rich in fossils, were deposited
during periods of subsidence; and that blank intervals of
vast duration, as far as fossils are concerned, occurred dur-
ing the periods when the bed of the sea was either stationary
or rising, and likewise when sediment was not thrown down
quickly enough to embed and preserve organic remains.
During these long and blank intervals I suppose that the in-
habitants of each region underwent a considerable amount
of modification and extinction, and that there was much
migration from other parts of the world. As we have rea-
son to believe that large areas are affected by the same move-
ment, it is probable that strictly contemporaneous formations
have often been accumulated over very wide spaces in the
same quarter of the world ; but we are very far from having
any right to conclude that this has invariably been the case,
and that large areas have invariably been affected by the
same movements. When two formations have been deposited
in two regions during nearly, but not exactly, the same
period, we should find in both, from the causes explained in
the foregoing paragraphs, the same general succession in
AFFINITIES OF EXTINCT SPECIES 377
the forms of life; but the species would not exactly corre-
spond; for there will have been a little more time in the one
region than in the other for modification, extinction, and
immigration.
I suspect that cases of this nature occur in Europe. Mr.
Prestwich, in his admirable Memoirs on the eocene deposits
of England and France, is able to draw a close general par-
allelism between the successive stages in the two countries ;
but when he compares certain stages in England with those
in France, although he finds in both a curious accordance in
the numbers of the species belonging to the same genera, yet
the species themselves differ in a manner very difficult to
account for considering the proximity of the two areas,—.
unless, indeed, it be assumed that an istihmus separated two
seas inhabited by distinct, but contemporaneous, faunas.
Lyell has made similar observations on some of the later ter-
tiary formations. Barrande, also, shows Jiat there is a strik-
ing general parallelism in the successive Silurian deposits of
Bohemia and Scandinavia ; nevertheless he finds a surprising
amount of difference in the species. If the several forma-
tions in these regions have not been deposited during the
same exact periods, — a formation in one region often cor-
responding with a blank interval in the other, — and if in
both regions the species have gone on slowly changing dur-
ing the accumulation of the several formations and during
the long intervals of time between them; in this case the sev-
eral formations in the two regions could be arranged in the
same order, in accordance with the general succession of the
forms of life, and the order would falsely appear to be
strictly parallel ; nevertheless the species would not be all
the same in the apparently corresponding stages in the two
regions.
ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER
AND TO LIVING FORMS
Let us now look to the mutual affinities of extinct and
living species. All fall intp a few grand classes; and this
fact is at once explained on the principle of descent. The
more ancient any form is, the more, as a general rule, it dif-
378 ORIGIN OF SPECIES
fers from living forms. But, as Buckland long ago re-
marked, extinct species can all be classed either in still ex-
isting groups, or between them. That the extinct forms of
life help to fill \ip the intervals between existing genera,
families, and orders, is certainly true; but as this statement
has often been ignored or even denied, it may be well to
make some remarks on this subject, andf to give some in-
stances. If we confine our attention either to the living or
to the extinct species of the same class, the series is far less
perfect that if we combine both into one general system. In
the writings of Professor Owen we continually meet with
the expression of generalised forms, as applied to extinct
animals ; and in the writings of Agassiz, of prophetic or syn-
thetic types; and these terms imply that such forms are in
fact intermediate or connecting links. Another distinguished
palaeontologist, M. Gaudry, has shown in the most striking
manner that many of the fossil mammals discovered by him
in Attica serve to break down the intervals between existing
genera. Cuvier ranked the Ruminants and Pachyderms, as
two of the most distinct orders of mammals: but so many
fossil links have been disentombed that Owen has had to
alter the whole classification, and has placed certain pachy-
derms in the same sub-order with ruminants ; for example, he
dissolves by gradations the apparently wide interval between
the pig and the camel. The Ungulata or hoofed quadrupeds
are now divided into the even-toed or odd-toed divisions;
but the Macrauchenia of S. America connects to a certain
extent these two grand divisions. No one will deny that
the Hipparion is intermediate between the existing horse
and certain older ungulate forms. What a wonderful con-
necting link in the chain of mammals is the Typotherium
from S. America, as the name given to it by Professor Ger-
vais expresses, and which cannot be placed in any existing
order. The Sirenia form a very distinct group of mammals,
and one of the most remarkable peculiarities in the existing
dugong and lamentin is the entire absence of hind limbs
without even a rudiment being left ; but the extinct Hali-
therium had, according to Professor Flower, an ossified
thigh-bone "articulated to a well-defined acetabulum in the
pelvis," and it thus makes some approach to ordinary hoofed
AFFINITIES OF EXTINCT SPECIES 379
quadrupeds, to which the Sirenia are in other respects allied.
3 he cetaceans or whales are widely different from all other
mammals, but the tertiary Zeuglodon and Squalodon, which
have been placed by some naturalists in an order by them-
selves, are considered by Professor Huxley to be undoubt-
edly cetaceans, "and to constitute connecting links with the
aquatic carnivora."
Even the wide intetval between birds and reptiles has been
shown by the naturalist just quoted to be partially bridged
over in the most unexpected manner, on the one hand, by the
ostrich and extinct Archeopteryx, and on the other hand, by
the Compsognathus, one of the Dinosaurians — that group
v/hich includes the most gigantic of all terrestrial reptiles.
Turning to the Invertebrata, Barrande asserts, a higher au-
thority could not be named, that he is every day taught that,
although palaeozoic animals can certainly be classed under
existing groups, yet that at this ancient period the groups
were not so distinctly separated from each other as they
now are.
Some writers have objected to any extinct species, or
group of species, being considered as intermediate between
any two living species, or groups of species. If by this term
it is meant that an extinct form is directly intermediate in
all its characters between two living forms or groups, the
objection is probably valid. But in a natural classification
many fossil species certainly stand between living species,
and some extinct genera between living genera, even be-
tween genera belonging to distinct families. The most com-
mon case, especially with respect to very distinct groups,
such as fish and reptiles, seems to be, that, supposing them
to be distinguished at the present day by a score of char-
acters, the ancient members are separated by a somewhat
lesser number of characters; so that the two groups formerly
made a somewhat nearer approach to each other than they
now do.
It is a common belief that the more ancient a form is, by
so much the more it tends to connect by some of its char-
acters groups now widely separated from each other. This
remark no doubt must be restricted to those groups which
have undergone much change in the course of geological
380 ORIGIN OF SPECIES
ages ; and it would be difficult to prove the truth of the propo-
sition, for every now and then a living animal, as the Lepi-
dosiren, is discovered having affinities directed towards very
distinct groups. Yet if we compare the older Reptiles and
Batrachians, the older Fish, the older Cephalopods, and the
eocene Mammals, with the more recent members of the same
classes, we must admit that there is truth in the remark.
Let us see how far these several facts and inferences ac-
cord with the theory of descent with modification. As the
subject is somewhat complex, I must request the reader to
turn to the diagram in the fourth chapter. We may suppose
that the numbered letters in italics represent genera, and the
dotted lines diverging from them the species in each genus.
The diagram is much too simple, too few genera and too few
species being given, but this is unimportant for us. The
horizontal lines may represent successive geological forma-
tions, and all the forms beneath the uppermost line may be
considered as extinct. The three existing genera a", g", p^\
will form a small family ; &" and f* a closely allied family
or sub-family; and o", e", w", a third family. These three
families, together with the many extinct genera on the sev-
eral lines of descent diverging from the parent-form (A)
will form an order, for all will have inherited something in
common from their ancient progenitor. On the principle of
the continued tendency to divergence of character, which
was formerly illustrated by this diagram, the more recent
any form is, the more it will generally differ from its ancient
progenitor. Hence we can understand the rule that the most
ancient fossils differ most from existing forms. We must
not, however, assume that divergence of character is a neces-
sary contingency; it depends solely on the descendants from
a species being thus enabled to seize on many and different
places in the economy of nature. Therefore it is quite pos-
sible, as we have seen in the case of some Silurian forms,
that a species might go on being slightly modified in relation
to its slightly altered conditions of life, and yet retain
throughout a vast period the same general characteristics.
This is represented in the diagram by the letter f".
All the many forms, extinct and recent, descended from
(A), make, as before remarked, one order; and this order,
AFFINITIES OF EXTINCT SPECIES 381
from the continued effects of extinction and divergence of
character, has become divided into several sub-famiHes and
famiHes, some of which are supposed to have perished at
different periods, and some to have endured to the present
day.
By looking at the diagram we can see that if many of the
extinct forms supposed to be imbedded in the successive
formations, were discovered at several points low down in
the series, the three existing families on the uppermost line
would be rendered less distinct from each other. If, for in-
stance, the genera a^ a', a^", f, m^, nf, m", were disinterred,
these three families would be so closely linked together that
they probably would have to be united into one great fam-
ily, in nearly the same manner as has occurred with rumi-
nants and certain pachyderms. Yet he who objected to con-
sider as intermediate the extinct genera, which thus link
together the living genera of three families, would be partly
justified, for they are intermediate, not directly, but only by
a long and circuitous course through many widely different
forms. If many extinct forms were to be discovered above
one of the horizontal lines or geological formations — for in-
stance, above No. VI. — but none from beneath this line, then
only two of the families (those on the left hand, a", &c., and
&," &c.) would have to be united into one; and there would
remain two families, which would be less distinct from each
other than they were before the discovery of the fossils.
So again if the three families formed of eight genera (a" to
m"), on the uppermost line, be supposed to differ from each
other by half-a-dozen important characters, then the fami-
lies which existed at the period marked VI. would certainly
have differed from each other by a less number of char-
acters; for they would at this early stage of descent have
diverged in a less degree from their common progenitor.
Thus it comes that ancient and extinct genera are often in a
greater or less degree intermediate in character between
their modified descendants, or between their collateral
relations.
Under nature the process will be far more complicated
than is represented in the diagram ; for the groups will have
been more numerous; they will have endured for extremely
382 ORIGIN OF SPECIES
unequal lengths of time, and will have been modified in vari-
ous degrees. As we possess only the last volume of the geo-
logical record, and that in a very broken condition, we have
no right to expect, except in rare cases, to fill up the wide
intervals in the natural system, and thus to unite distinct
families or orders. All that we have a right to expect is,
that those groups which have, within known geological peri-
ods, undergone much modification, si'iould in the older for-
mations make some slight approach to each other; so that
the older members should differ less from each other in some
of their characters than do the existing members of the
same groups; and this by the concurrent evidence of our best
palaeontologists is frequently the case.
Thus, on the theory of descent with modification, the main
facts with respect to the mutual affinities of the extinct
forms of life to each other and to living forms, are explained
in a satisfactory manner. And they are wholly inexplicable
on any other view.
On this same theory, it is evident that the fauna during
any one great period in the earth's history will be inter-
mediate in general character between that which preceded
and that which succeeded it. Thus the species which lived
at the sixth great stage of descent in the diagram are the
modified offspring of those which lived at the fifth stage,
and are the parents of those which became still more modi-
fied at the seventh stage; hence they could hardly fail to be
nearly intermediate in character between the forms of life
above and below. We must, however, allow for the entire
extinction of some preceding forms, and in any one region
for the immigration of new forms from other regions, and
for a large amount of modification during the long and blank
interval between the successive formations. Subject to these
allowances, the fauna of each geological period undoubtedly
is intermediate in character, between the preceding and suc-
ceeding faunas. I need give only one instance, namely, the
manner in which the fossils of the Devonian system, when
this system was first discovered, were at once recognised by
palaeontologists as intermediate in character between those
of the overlying carboniferous, and underlying Silurian sys-
tems. But each fauna is not necessarily exactly intermediate,
AFFINITIES OF EXTINCT SPECIES 383
as unequal intervals of time have elapsed between consecu-
tive formations.
It is no real objection to the truth of the statement that
the fauna of each period as a whole is nearly intermediate
in character between the preceding and succeeding faunas,
that certain genera offer exceptions to the rule. For in-
stance, the species of mastodons and elephants, when ar-
ranged by Dr. Falconer in two series, — in the first place
according to their mutual affinities, and in the second place
according to their periods of existence, — do not accord in
arrangement. The species extreme in character are not the
oldest or the most recent; nor are those which are interme-
diate in character, intermediate in age. But supposing for
an instant, in this and other such cases, that the record of
the first appearance and disappearance of the species was
complete, which is far from the case, we have no reason to
believe that forms successively produced necessarily endure
for corresponding lengths of time. A very ancient form
may occasionally have lasted much longer than a form else-
where subsequently produced, especially in the case of terres-
trial productions inhabiting separated districts. To compare
small things with great; if the principal living and extinct
races of the domestic pigeon were arranged in serial affinity,
this arrangement would not closely accord with the order in
time of their production, and even less with the order of
their disappearance; for the parent rock-pigeon still lives;
and many varieties between the rock-pigeon and the carrier
have become extinct ; and carriers which are extreme in the
important character of length of back originated earlier than
short-beaked tumblers, which are at the opposite end of the
series in this respect.
Closely connected with the statement, that the organic re-
mains from an intermediate formation are in some degree
intermediate in character, is the fact, insisted on by all
palaeontologists, that fossils from two consecutive formations
are far more closely related to each other, than are the fos-
sils from two remote formations. Pictet gives as a well-
known instance, the general, resemblance of the organic re-
mains from the several stages of the Chalk formation,
though the species are distinct in each stage. This fact
384 ORIGIN OF SPECIES
alone, from its generality, seems to have shaken Professor
Pictet in his belief in the immutability of species. He who
is acquainted with the distribution of existing species over
the globe, will not attempt to account for the close resem-
blance of distinct species in closely consecutive formations,
by the physical conditions of the ancient areas having re-
mained nearly the same. Let it be remembered that the
forms of life, at least those inhabiting the sea, have changed
almost simultaneously throughout the world, and therefore
under the most different climates and conditions. Consider
the prodigious vicissitudes of climate during the pleistocene
period, which includes the whole glacial epoch, and note how
little the specific forms of the inhabitants of the sea have
been affected.
On the theory of descent, the full meaning of the fossil
remains from closely consecutive formations being closely
related, though ranked as distinct species, is obvious. As
the accumulation of each formation has often been inter-
rupted, and as long blank intervals have intervened between
successive formations, we ought not to expect to find, as I
attempted to show in the last chapter, in any one or in any
two formations, all the intermediate varieties between the
species which appeared at the commencement and close of
these periods : but we ought to find after intervals, very long
as measured by years, but only moderately long as measured
geologically, closely allied forms, or, as they have been called
by some authors, representative species ; and these assuredly
we do find. We find, in short, such evidence of the slow
and scarcely sensible mutations of specific forms, as we have
the right to expect.
ON THE STATE OF DEVELOPMENT OF ANCIENT COMPARED
WITH LIVING FORMS
We have seen in the fourth chapter that the degree of
differentiation and specialisation of the parts in organic
beings, when arrived at maturity, is the best standard, as yet
suggested, of their degree of perfection or highness. We
have also seen that, as the specialisation of parts is an ad-
vantage to each being, so natural selection will tend to render
STATE OF DEVELOPMENT COMPARED 385
the organisation of each being more specialised and perfect,
and in this sense higher; not but that it may leave many
creatures with simple and unimproved structures fitted for
simple conditions of life, and in some cases will even de-
grade or simplify the organisation, yet leaving such degraded
beings better fitted for their new walks of life. In another
and more general manner, new species become superior to
their predecessors; for they have to beat in the struggle for
life all the older forms, with which they come into close
competition. We may therefore conclude that if under a
nearly similar climate the eocene inhabitants of the world
could be put into competition with the existing inhabitants,
the former would be beaten and exterminated by the latter,
as would the secondary by the eocene, and the palaeozoic by
the secondary forms. So that by this fundamental test of
victory in the battle for life, as well as by the standard of
the specialisation of organs, modern forms ought, on the
theory of natural selection, to stand higher than ancient
forms. Is this the case? A large majority of palaeon-
tologists would answer in the affirmative ; and it seems
that this answer must be admitted as true, though difficult
of proof.
It is no valid objection to this conclusion, that certain
Brachiopods have been but slightly modified from an ex-
tremely remote geological epoch; and that certain land and
fresh-water shells have remained nearly the same, from the
time when, as far as is known, they first appeared. It is not
an insuperable difficulty that Foraminifera have not, as in-
sisted on by Dr. Carpenter, progressed in organisation since
even the Laurentian epoch; for some organisms would have
to remain fitted for simple conditions of life, and what could
be better fitted for this end than these lowly organised Pro-
tozoa? Such objections as the above would be fatal to my
view, if it included advance in organisation as a necessary
contingent. They would likewise be fatal, if the above Fora-
minifera, for instance, could be proved to have first come
into existence during the Laurentian epoch, or the above
Brachiopods during the Caipbrian formation; for in this
case, there would not have been time sufficient for the de-
velopment of these organisms up to the standard which they
M — HC XI
386 ORIGIN OF SPECIES
had then reached. When advanced up to any given point,
there is no necessity, on the theory of natural selection, for
their further continued progress; though they vi^ill, during
each successive age, have to be slightly modified, so as to
hold their places in relation to slight changes in their condi-
tions. The foregoing objections hinge on the question
whether we really know how old the world is, and at what
period the various forms of life first appeared; and this may
well be disputed.
The problem whether organisation on the whole has ad-
vanced is in many ways excessively intricate. The geological
record, at all times imperfect, does not extend far enough
back, to show with unmistakeable clearness that within the
known history of the world organisation has largely ad-
vanced. Even at the present day, looking to members of the
same class, naturalists are not unanimous which forms ought
to be ranked as highest: thus, some look at the selaceans or
sharks, from their approach in some important points of
structure to reptiles, as the highest fish ; others look at the
teleosteans as the highest. The ganoids stand intermediate
between the selaceans and teleosteans; the latter at the
present day are largely preponderant in number ; but for-
merly selaceans and ganoids alone existed ; and in this case,
according to the standard of highness chosen, so will it be
said that fishes have advanced or retrograded in organisa-
tion. To attempt to compare members of distinct types in
the scale of highness seems hopeless; who will decide whether
a cuttle-fish be higher than a bee — that insect which the
great V^on Baer believed to be "in fact more highly organised
than a fish, although upon another type" ? In the complex
Struggle for life it is quite credible that crustaceans, not very
high in their own class, might beat cephalopods, the highest
molluscs ; and such crustaceans, though not highly developed,
would stand very high in the scale of invertebrate animals, if
judged by the most decisive of all trials — the law of battle.
Beside these inherent difficulties in deciding which forms
are the most advanced in organisation, we ought not solely
to compare the highest members of a class at any two
periods — though undoubtedly this is one and perhaps the
most important element in striking a balance — but we ought
STATE OF DEVELOPMENT COMPARED 387
to compare all the members, high and low, at the two periods.
At an ancient epoch the highest and lowest molluscoidal ani-
mals, namely, cephalopods and brachiopods, swarmed in
numbers ; at the present time both groups are greatly re-
duced, whilst others, intermediate in organisation, have
largely increased; consequently some naturalists maintain
that molluscs were formerly more highly developed than at
present ; but a stronger case can be made out on the oppo-
site side, by considering the vast reduction of the brachio-
pods, and the fact that our existing cephalopods, though few
in number, are more highly organised than their ancient rep-
resentatives. We ought also to compare the relative propor-
tional numbers at any two periods of the high and low classes
throughout the world: if, for instance, at the present day
fifty thousand kinds of vertebrate animals exist, and if we
knew that at some former period only ten thousand kinds
existed, we ought to look at this increase in number in the
highest class, which implies a great displacement of lowei
forms, as a decided advance in the organisation of the world.
We thus see how hopelessly difficult it is to compare with
perfect fairness under such extremely complex relations, the
standard of organisation of the imperfectly-known faunas
of successive periods.
We shall appreciate this difficulty more clearly, by looking
to certain existing faunas and floras. From the extraordi-
nary manner in which European productions have recently
spread over New Zealand, and have seized on places which
must have been previously occupied by the indigenes, we
must believe, that if all the animals and plants of Great
Britain were set free in New Zealand, a multitude of British
forms would in the course of time become thoroughly nat-
uralised there, and would exterminate many of the natives.
On the other hand, from the fact that hardly a single inhabi-
tant of the southern hemisphere has become wild in any part
of Europe, we may well doubt whether, if all the productions
of New Zealand were set free in Great Britain, any consid-
erable number would be enabled to seize on places now occu-
pied by our native plants and animals. Under this point of
view, the productions of Great Britain stand much higher in
the scale than those of N^w Zealand. Yet the most skilful
388 ORIGIN OF SPECIES
naturalist, from an examination of the species of the two
countries, could not have foreseen this result.
Agassiz and several other highly competent judges insist
that ancient animals resemble to a certain extent the em-
bryos of recent animals belonging to the same classes; and
that the geological succession of extinct forms is nearly par-
allel with the embryological development of existing forms.
This view accords admirably well with our theory. In a
future chapter I shall attempt to show that the adult differs
from its embryo, owing to variations having supervened at a
not early age, and having been inherited at a corresponding
age. This process, whilst it leaves the embryo almost unal-
tered, continually adds, in the course of successive genera-
tions, more and more difference to the adult. Thus the
embryo comes to be left as a sort of picture, preserved by
nature, of the former and less modified condition of the
species. This view may be true, and yet may never be
capable of proof. Seeing, for instance, that the oldest known
mammals, reptiles, and fishes strictly belong to their proper
classes, though some of these old forms are in a slight de-
gree less distinct from each other than are the typical mem-
bers of the same groups at the present day, it would be vain
to look for animals having the common embryological char-
acter of the Vertebrata, until beds rich in fossils are discov-
ered far beneath the lowest Cambrian strata — a discovery of
which the chance is small.
ON THE SUCCESSION OF THE SAME TYPES WITHIN THE
SAME AREAS^ DURING THE LATER TERTIARY PERIODS
Mr. Clift many years ago showed that the fossil mammals
from the Australian caves were closely allied to the living
marsupials of that continent. In South America, a similar
relationship is manifest, even to an uneducated eye, in the
gigantic pieces of armour, like those of the armadillo, found
in several parts of La Plata ; and Professor Owen has shown
in the most striking manner that most of the fossil mammals,
buried there in such numbers, are related to South American
types. This relationship is even more clearly seen in the
wonderful collection of fossil bones made by MM. Lund and
SUCCESSION OF SAME TYPES 389
Clausen in the caves of Brazil. I was so much impressed
with these facts that I strongly insisted, in 1839 and 1845,
on this "law of the succession of types,"' — on "this won-
derful relationship in the same continent between the dead
and the living." Professor Owen has subsequently extended
the same generalisation to the mammals of the Old World.
We see the same law in this author's restorations of the
extinct and gigantic birds of New Zealand. We see it also
in the birds of the caves of Brazil. Mr. Woodward has
shown that the same law holds good with sea-shells, but,
from the wide distribution of most molluscs, it is not well
displayed by them. Other cases could be added, as the rela-
tion between the extinct and living land-shells of Madeira;
and between the extinct and living brackish water-shells of
the Aralo-Caspian Sea.
Now what does this remarkable law of the succession of
the same types within the same areas mean ? He would be
a bold man who, after comparing the present climate of Aus-
tralia and of parts of South America, under the same lati-
tude, would attempt to account, on the one hand through
dissimilar physical conditions, for the dissimilarity of the
inhabitants of these two continents; and, on the other hand
through similarity of conditions, for the uniformity of the
same types in each continent during the later tertiary periods.
Nor can it be pretended that it is an immutable law that
marsupials should have been chiefly or solely produced in
Australia; or that Edentata and other American types should
have been solely produced in South America. For we know
that Europe in ancient times was peopled by numerous mar-
supials ; and I have shown in the publications above alluded
to, that in America the law of distribution of terrestrial
mammals was formerly different from what it now is. North
America formerly partook strongly of the present character
of the southern half of the continent; and the southern half
was formerly more closely allied, than it is at present, to the
northern half. In a similar manner we know, from Falconer
and Cautley's discoveries, that Northern India was formerly
more closely related in its mammals to Africa than it is at
the present time. Analogous facts could be given in rela-
tion to the distribution of marine animals.
390 ORIGIN OF SPECIES
On the theory of descent with modification, the great law
of the long enduring, but not immutable, succession of the
same types within the same areas, is at once explained ; for
the inhabitants of each quarter of the world will obviously
tend to leave in that quarter, during the next succeeding
period of time, closely allied though in some degree modified
descendants. If the inhabitants of one continent formerly
differed greatly from those of another continent, so will
their modified descendants still differ in nearly the same
manner and degree. But after very long intervals of time,
and after great geographical changes, permitting much inter-
migration, the feebler will yield to the more dominant forms,
and there will be nothing immutable in the distribution of
organic beings.
It may be asked in ridicule, whether I suppose that the
megatherium and other allied huge monsters, which formerly
lived in South America, have left behind them the sloth,
armadillo, and anteater, as their degenerate descendants.
This cannot for an instant be admitted. These huge animals
have become wholly extinct, and have left no progeny. But
in the caves of Brazil, there are many extinct species which
are closely allied in size and in all other characters to the
species still living in South America; and some of these
fossils may have been the actual progenitors of the living
species.
It must not be forgotten that, on our theory, all the
species of the same genus are the descendants of some one
species; so that, if six genera, each having eight species, be
found in one geological formation, and in a succeeding
formation there be six other allied or representative genera
each with the same number of species, then we may con-
clude that generally only one species of each of the older
genera has left modified descendants, which constitute the /
new genera containing the several species; the other seven
species of each old genus having died out and left no progeny.
Or, and this will be a far commoner case, two or three spe-
cies in two or three alone of the six older genera will be
the parents of the new genera: the other species and the other
old genera having become utterly extinct. In failing orders,
with the genera and species decreasing in numbers as is the
SUMMARY 391
case with the Edentata of South America, still fewer genera
and species will leave modified blood-descendants.
SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS
I have attempted to show that the geological record is ex-
tremely imperfect; that only a small portion of the globe has
been geologically explored with care; that only certain
classes of organic beings have been largely preserved in a
fossil stale ; that the number both of specimens and of spe-
cies, preserved in our museums, is absolutely as nothing com-
pared with the number of generations which must have
passed away even during a single formation ; that, owing to
subsidence being almost necessary for the accumulation of
deposits rich in fossil species of inany kinds, and thick enough
to outlast future degradation, great intervals of time must
have elapsed between most of our successive formations;
that there has probably been more extinction during the
periods of subsidence, and more variation during the periods
of elevation, and during the latter the record will have been
least perfectly kept ; that each single formation has not been
continuously deposited; that the duration of each formation
is probably short compared with the average duration of
specific forms: that migration has played an important part
in the first appearance of new forms in any one area and
formation ; that widely ranging species are those which have
varied most frequently, and have oftenest given rise to new
species ; that varieties have at first been local ; and lastly,
although each species must have passed through numerous
transitional stages, it is probable that the periods, during
which each underwent modification, though many and long
as measured by years, have been short in comparison witli
the periods during which each remained in an unchanged
condition. These causes, taken conjointly, will to a large ex-
tent explain why — though we do find many links — we do not
find interminable varieties, connecting together all extinct
and existing forms by the finest graduated steps. It should
also be constantly borne in mind that any linking variety
between two forms, which might be found, would be ranked,
unless the whole chain could be perfectly restored, as a new
392 ORIGIN OF SPECIES
and distinct species ; for it is not pretended that we have
any sure criterion by which species and varieties can be
discriminated.
He who rejects this view of the imperfection of the geo-
logical record, will rightly reject the whole theory. For he
may ask in vain where are the numberless transitional links
which must formerly have connected the closely allied or
representative species, found in the successive stages of the
same great formation? He may disbelieve in the immense
intervals of time which must have elapsed between our con-
secutive formations; he may overlook how important a part
migration has played, when the formations of any one great
region, as those of Europe, are considered; he may urge the
apparent, but often falsely apparent, sudden coming in of
whole groups of species. He may ask where are the remains
of those infinitely numerous organisms which must have ex-
isted long before the Cambrian system was deposited? We
now know that at least one animal did then exist ; but I can
answer this last question only by supposing that where our
oceans now extend they have extended for an enormous
period, and where our oscillating continents now stand they
have stood since the commencement of the Cambrian system;
but that, long before that epoch, the world presented a widely
different aspect; and that the older continents, formed of
formations older than any known to us, exist now only as
remnants in a metamorphosed condition, or lie still buried
under the ocean.
Passing from these difficulties, the other great leading
facts in palaeontology agree admirably with the theory of
descent with modification through variation and natural
selection. We can thus understand how it is that new spe-
cies come in slowly and successively; how species of dif-
ferent classes do not necessarily change together, or at the
same rate, or in the same degree ; yet in the long run that all
undergo modification to some extent. The extinction of old
forms is the almost inevitable consequence of the production
of new forms. We can understand why, when a species has
once disappeared, it never reappears. Groups of species in^
crease in numbers slowly, and endure for unequal periods
of time; for the process of modification is necessarily slow,
SUMMARY 393
and depends on many complex contingencies. The dominant
species belonging to large and dominant groups tend to leave
many modified descendants, which form new sub-groups and
groups. As these are formed, the species of the less vig-
orous groups, from their inferiority inherited from a com-
mon progenitor, tend to become extinct together, and to leave
no modified offspring on the face of the earth. But the utter
extinction of a whole group of species has sometimes been a
slow process, from the survival of a few descendants, lin-
gering in protected and isolated situations. When a group
has once wholly disappeared, it does not reappear; for the
link of generation has been broken.
We can understand how it is that dominant forms which
spread widely and yield the greatest number of varieties tend
to people the world with allied, but modified, descendants;
and these will generally succeed in displacing the groups
which are their inferiors in the struggle for existence.
Hence, after long intervals of time, the productions of the
world appear to have changed simultaneously.
We can understand how it is that all the forms of life,
ancient and recent, make together a few grand classes. We
can understand, from the continued tendency to divergence
of character, why the more ancient a form is, the more it
generally differs from those now living; why ancient and
extinct forms often tend to fill up gaps between existing
forms, sometimes blending two groups, previously classed
as distinct, into one; but more commonly bringing them only
a little closer together. The more ancient a form is, the
more often it stands in some degree intermediate between
groups now distinct; for the more ancient a form is, the
more nearly it will be related to, and consequently resemble,
the common progenitor of groups, since become widely
divergent. Extinct forms are seldom directly intermediate
between existing forms ; but are intermediate only by a long
and circuitous course through other extinct and different
forms. We can clearly see why the organic remains of
closely consecutive formations are closely allied; for they
are closely linked together by' generation. We can clearly
see why the remains of an intermediate formation are inter-
mediate in chr.racter.
394 ORIGIN OF SPECIES
The inhabitants of the world at each successive period in
its history have beaten their predecessors in the race for
life, and are, in so far, higher in the scale, and their struc-
ture has generally become more specialised; and this may
account for the common belief held by so many palaeontolo-
gists, that organisation on the whole has progressed. Extinct
and ancient animals resemble to a certain extent the embryos
of the more recent animals belonging to the same classes,
and this wonderful fact receives a simple explanation accord-
ing to our views. The succession of the same types of
structure within the same areas during the later geological
periods ceases to be mysterious, and is intelligible on the
principle of inheritance.
If then the geological record be as imperfect as many be-
lieve, and it may at least be asserted that the record cannot
be proved to be much more perfect, the main objections to
the theory of natural selection are greatly diminished or dis-
appear. On the other hand, all the chief laws of palaeontology
plainly proclaim, as it seems to me, that species have been
produced by ordinary generation : old forms having been sup-
planted by new and improved forms of life, the products of
Variation and the Survival of the Fittest.
CHAPTER XII
Geographical Distribution
Present distribution cannot be accounted for by differences in physi-
cal conditions — Importance of barriers — Affinity of the produc-
tions of the same continent — Centres of creation — Means of
dispersal, by changes of climate and of the level of the land, and
by occasional means — Dispersal during the Glacial period —
Alternate Glacial periods in the North and South.
IN considering the distribution of organic beings over the
face of the globe, the first great fact which strikes us is,
that neither the similarity nor the dissimilarity of the
inhabitants of various regions can be wholly accounted for by
climatal and other physical conditions. Of late, almost every
author who has studied the subject has come to this conclu-
sion. The case of America alone would almost suffice to
prove its truth ; for if we exclude the arctic and northern
temperate parts, all authors agree that one of the most fun-
damental divisions in geographical distribution is that .be-
tween the New and Old Worlds; yet if we travel over the
vast American continent, from the central parts of the
United States to its extreme southern point, we meet with
the most diversified conditions ; humid districts, arid deserts,
lofty mountains, grassy plains, forests, marshes, lakes, and
great rivers, under almost every temperature. There is
hardly a climate or condition in the Old World which can-
not be paralleled in the New — at least as closely as the same
species generally require. No doubt small areas can be
pointed out in the Old World hotter than any in the New
World ; but these are not inhabited by a fauna different from
that of the surrounding districts ; for it is rare to find a group
of organisms confined to a small area, of which the con-
ditions are peculiar in only a slight degree. Notwithstand-
ing this general parallelism in the conditions of the Old and
New Worlds, how widely different are their living pro-
ductions !
395
396 ORIGIN OF SPECIES
In the southern hemisphere, if we compare large tracts of
land in Australia, South Africa, and western South America,
between latitudes 25° and 35°, we shall find parts extremely
similar in all their conditions, yet it would not be possible to
point out three faunas and floras more utterly dissimilar.
Or, again, we may compare the productions of South Amer-
ica south of lat, 35° with those north of 25°, which conse-
quently are separated by a space of ten degrees of latitude,
and are exposed to considerably different conditions ; yet they
are incomparably more closely related to each other than
they are to the productions of Australia or Africa under
nearly the same climate. Analogous facts could be given
with respect to the inhabitants of the sea.
A second great fact which strikes us in our general review
is, that barriers of any kind, or obstacles to free migration,
are related in a close and important manner to the differ-
ences between the productions of various regions. We see
this in the great differences in nearly all the terrestrial pro-
ductions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins, and where, under
a slightly different climate, there might have been free mi-
gration for the northern temperate forms, as there now is
for the strictly arctic productions. We see the same fact in
the great difference between the inhabitants of Australia,
Africa, and South America under the same latitude; for
these countries are almost as much isolated from each other
as is possible. On each continent, also, we see the same
fact ; for on the opposite sides of lofty and continuous moun-
tain-ranges, of great deserts and even of large rivers, we
find different productions ; though as mountain-chains, des-
erts, &c., are not as impassable, or likely to have endured so
long, as the oceans separating continents, the differences are
very inferior in degree to those characteristic of distinct
continents.
Turning to the sea, we find the same law. The marine
inhabitants of the eastern and western shores of South
America are very distinct, with extremely few shells, Crus-
tacea, or echinodermata in common ; but Dr. Giinther has
recently shown that about thirty per cent, of the fishes are
the same on the opposite sides of the isthmus of Panama;
GEOGRAPHICAL DISTRIBUTION 397
and this fact has led naturalists to believe that the isthmus
was formerly open. Westward of the shores of America, a
wide space of open ocean extends, with not an island as a
halting-place for emigrants ; here we have a barrier of an-
other kind, and as soon as this is passed we meet in the east-
ern islands of the Pacific with another and totally distinct
fauna. So that three marine faunas range far northward
and southward in parallel lines not far from each other,
under corresponding climates; but from being separated from
each other by impassable barriers, either of land or open sea,
they are almost wholly distinct. On the other hand, proceed-
ing still farther westward from the eastern islands of the
tropical parts of the Pacific, we encounter no impassable
barriers, and we have innumerable islands as halting-places,
or continuous coasts, until, after travelling over a hemisphere,
we come to the shores of Africa ; and over this vast space
we meet with no well-defined and distinct marine faunas.
Although so few marine animals are common to the above-
named three approximate faunas of Eastern and Western
America and the eastern Pacific islands, yet manj' fishes
range from the Pacific into the Indian Ocean, and many
shells are common to the eastern islands of the Pacific and
the eastern shores of Africa on almost exactly opposite
meridians of longitude.
A third great fact, partly included in the foregoing state-
ment, is the affinity of the productions of the same continent
or of the same sea, though the species themselves are dis-
tinct at different points and stations. It is a law of the
widest generality, and every continent offers innumerable
instances. Nevertheless, the naturalist, in travelling, for
instance, from north to south, never fails to be struck by
the manner in which successive groups of beings, specifically
distinct, though nearly related, replace each other. He hears
from closely allied, yet distinct kinds of birds, notes nearly
similar, and sees their nests similarly constructed, but not
quite alike, with eggs coloured in nearly the same manner.
The plains near the Straits of Magellan are inhabited by
one species of Rhea (American ostrich), and northward the
plains of La Plata by another species of the same genus ; and
not by a true ostrich or- emu, like those inhabiting Africa
398 ORIGIN OF SPECIES
and Australia under the same latitude On these same plains
of La Plata we see the agouti and bizcacha, animals having
nearly the same habits as our hares and rabbits, and belong-
ing to the same order of Rodents, but they plainly display
an American type of structure. We ascend the lofty peaks
of the Cordillera, and we find an alpine species of bizcacha;
we look to the waters, and we do not find the beaver or
musk-rat, but the coypu and capybara, rodents of the S.
American type. Innumerable other instances could be given.
If we look to the islands off the American shore, however
much they may differ in geological structure, the inhabitants
are essentially American, though they may be all peculiar
species. We may look back to past ages, as shown in the
last chapter, and we find American types then prevailing on
the American continent and in the American seas. We see
in these facts some deep organic bond, throughout space and
time, over the same areas of land and water, independently
of physical conditions. The naturalist must be dull who is
not led to inquire what this bond is.
The bond is simply inheritance, that cause which alone,
as far as we positively know, produces organisms quite like
each other, or, as we see in the case of varieties, nearly
alike. The dissimilarity of the inhabitants of different re-
gions may be attributed to modification through variation
and natural selection, and probably in a subordinate degree
to the definite influence of different physical conditions. The
degrees of dissimilarity will depend on the migration of the
more dominant forms of life from one region into another
having been more or less effectually prevented, at periods
more or less remote ; — on the nature and number of the for-
mer immigrants ; — and on the action of the inhabitants on
each other in leading to the preservation of different modifi-
cations ; the relation of organism to organism in the struggle
for life being, as I have already often remarked, the most
important of all relations. Thus the high importance of
barriers comes into play by checking migration ; as does time
for the slow process of modification through natural selec-
tion. Widely-ranging species, abounding in individuals,
which have already triumphed over many competitors in
their own widely-extended homes, will have the best chance
GEOGRAPHICAL DISTRIBUTION 399
of seizing on new places, when they spread into new coun-
tries. In their new homes they will be exposed to new con-
ditions, and will frequently undergo further modification and
improvement; and thus they will become still further vic-
torious, and will produce groups of modified descendants.
On this principle of inheritance with modification we can
understand how it is that sections of genera, whole genera,
and even families, are confined to the same areas, as is so
commonly and notoriously the case.
There is no evidence, as was remarked in the last chapter,
of the existence of any law of necessary development. As
the variability of each species is an independent property,
and will be taken advantage of by natural selection, only so
far as it profits each individual in its complex struggle for
life, so the amount of modification in different species will
be no uniform quantity. If a number of species, after hav-
ing long competed with each other in their old home, were
to migrate in a body into a new and afterwards isolated
country, they would be little liable to modification ; for
neither migration nor isolation in themselves effect anything.
These principles come into play only by bringing organisms
into new relations with each other and in a lesser degree
with the surrounding physical conditions. As we have seen
in the last chapter that some forms have retained nearly the
same character from an enormously remote geological period,
so certain species have migrated over vast spaces, and have
not become greatly or at all modified.
According to these views, it is obvious that the several
species of the same genus, though inhabiting the most dis-
tant quarters of the world, must originally have proceeded
from the same source, as they are descended from the same
progenitor. In the case of those species which have under-
gone during whole geological periods little modification,
there is not much difficulty in believing that they have mi-
grated from the same region ; for during the vast geographi-
cal and climatal changes which have supervened since ancient
times, almost any amount of migration is possible. But in
many other cases, in which we have reason to believe that
the species of a genus have been produced within compara-
tively recent times, there Ls great difficulty on this head. It
400 ORIGIN OF SPECIES
is also obvious that the individuals of the same species,
though now inhabiting distant and isolated regions, must have
proceeded from one spot, where their parents were first pro-
duced: for, as has been explained, it is incredible that indi-
viduals identically the same shoufd have been produced from
parents specifically distinct.
Single Centres of supposed Creation. — We are thus
brought to the question which has been largely discussed by
naturalists, namely, whether species have been created at
one or more points of the earth's surface. Undoubtedly
there are many cases of extreme difficulty in understanding
how the same species could possibly have migrated from
some one point to the several distant and isolated points,
where now found. Nevertheless the simplicity of the view
that each species was first produced within a single region
captivates the mind. He who rejects it, rejects the vera
causa of ordinary generation with subsequent migration, and
calls in the agency of a miracle. It is universally admitted,
that in most cases the area inhabited by a species is con-
tinuous ; and that when a plant or animal inhabits two points
so distant from each other, or with an interval of such a
nature, that the space could not have been easily passed over
by migration, the fact is given as something remarkable and
exceptional. The incapacity of migrating across a wide sea
is more clear in the case of terrestrial mammals than perhaps
with any other organic beings ; and, accordingly, we find no
inexplicable instances of the same mammals inhabiting dis-
tant points of the world. No geologist feels any difficulty in
Great Britain possessing the same quadrupeds with the rest
of Europe, for they were no doubt once united. But if the
same species can be produced at two separate points, why do
we not find a single mammal common to Europe and Aus-
tralia or South America? The conditions of life are nearly
the same, so that a multitude of European animals and plants
have become naturalised in America and Australia ; and
some of the aboriginal plants" are identically the same at
these distant points of the northern and southern hemi-
spheres. The answer, as I believe, is, that mammals have
not been able to migrate, whereas some plants, from their
varied means of dispersal, have migrated across the wide and
CENTRES OF SUPPOSED CREATION 401
broken interspaces. The great and striking influence of bar-
riers of all kinds, is intelligible only on the view that the
great majority of species have been produced on one side,
and have not been able to migrate to the opposite side.
Some few families, many sub-families, very many genera,
and a still greater number of sections of genera, are con-
fined to a single region ; and it has been observed by several
naturalists that the most natural genera, or those genera in
which the species are most closely related to each other, are
generally confined to the same country, or if they have a
wide range that their range is continuous. What a strange
anomaly it would be, if a directly opposite rule were to pre-
vail, when we go down one step lower in the series, namely,
to the individuals of the same species, and these had not
been, at least at first, confined to some one region !
Hence it seems to me, as it has to many other naturalists,
that the view of each species having been produced in one
area alone, and having subsequently migrated from that area
as far as its powers of migration and subsistence under past
and present conditions permitted, is the most probable. Un-
doubtedly many cases occur, in which we cannot explain how
the same species could have passed from one point to the
other. But the geographical and climatal changes which
have certainly occurred within recent geological times, must
have rendered discontinuous the formerly continuous range
of many species. So that we are reduced to consider whether
the exceptions to continuity of range are so numerous and
of so grave a nature, that we ought to give up the belief,
rendered probable by general considerations, that each species
has been produced within one area, and has migrated thence
as far as it could. It would be hopelessly tedious to discuss
all the exceptional cases of the same species, now living at
distant and separated points, nor do I for a moment pretend
that any explanation could be offered of many instances.
But, after some preliminary remarks, I will discuss a few of
the most striking classes of facts ; namely, the existence of
the same species on the summits of distant mountain ranges,
and at distant points in the arctic and antarctic regions; and
secondly (in the following chapter), the wide distribution of
fresh-water productions ; .and thirdly, the occurrence of the
402 ORIGIN OF SPECIES
same terrestrial species on islands and on the nearest main-
land, though separated by hundreds of miles of open sea.
If the existence of the same species at distant and isolated
points of the earth's surface, can in many instances be ex-
plained on the view of each species having migrated from a
single birthplace ; then, considering our ignorance with re-
spect to former climatal and geographical changes and to
the various occasional means of transport, the belief that a
single birthplace is the law, seems to me incomparably the
safest.
In discussing this subject, we shall be enabled at the same
time to consider a point equally important for us, namely,
whether the several species of a genus which must on our
theory all be descended from a common progenitor, can have
migrated, undergoing modification during their migration,
from some one area. If, when most of the species inhabiting
one region are different from those of another region, thoiigh
closely allied to them, it can be shown that migration from
the one region to the other has probably occurred at some
former period, our general view will be much strengthened ;
for the explanation is obvious on the principle of descent
with modification. A volcanic island, for instance, upheaved
and formed at th^ distance of a few hundreds of miles from
a continent, would probably receive from it in the course of
time a few colonists, and their descendants, though modified,
would still be related by inheritance to the inhabitants of
that continent. Cases of this nature are common, and are,
as we shall hereafter see, inexplicable on the theory of inde-
pendent creation. This view of the relation of the species
of one region to those of another, does not differ much from
that advanced by Mr. Wallace, who concludes that "every
species has come into existence coincident both in space and
time with a pre-existing closely allied species." And it is
now well known that he attributes this coincidence to descent
v/ith modification.
The question of single or multiple centres of creation dif-
fers from another though allied question, — namely, whether
all the individuals of the same species are descended from a
single pair, or single hermaphrodite, or whether, as some
authors suppose, from many individuals simultaneously ere-
MEANS OF DISPERSAL 403
ated. With organic beinp:s which never intercross, if such
exist, each species must be descended from a succession of
modified varieties, that have supplanted each other, but have
never blended with other individuals or varieties of the same
species ; so that, at each successive stage of modification, all
the individuals of the same form will be descended from a
single parent. But in the great majority of cases, namely,
with all organisms which habitually unite for each birth, or
which occasionally intercross, the individuals of the same
species inhabiting the same area will be kept nearly uniform
by intercrossing; so that many individuals will go on simul-
taneously changing, and the whole amount of modification at
each stage will not be due to descent from a single parent.
To illustrate what I mean: our English race-horses differ
from the horses of every other breed ; but they do not owe
their difference and superiority to descent from any single
pair, but to continued care in the selecting and training of
many individuals during each generation.
Before discussing the three classes of facts, which I have
selected as presenting the greatest amount of difficulty on the
theory of "single centres of creation," I must say a few
words on the means of dispersal.
MEANS OF DISPERSAL
Sir C. Lyell and other authors have ably treated this sub-
ject. I can give here only the briefest abstract of the more
important facts. Change of climate must have had a power-
ful influence on migration. A region now impassable to cer-
tain organisms from the nature of its climate, might have
been a high road for migration, when the climate was dif-
ferent. I shall, however, presently have to discuss this
branch of the subject in some detail. Changes of level in
the land must also have been highly influential: a narrow
isthmus now separates two marine faunas; submerge it, or
let it formerly have been submerged, and the two faunas
will now blend together, or may formerly have blended
Where the sea now extends^ land may at a former period
have connected islands or possibly even continents together,
and thus have allowed torestrial productions to pass from
404 ORIGIN OF SPECIES
one to the other. No geologist disputes that great muta-
tions of level have occurred within the period of existing
organisms. Edward Forbes insisted that all the islands in
the Atlantic must have been recently connected with Europe
or Africa, and Europe likewise with America. Other authors
have thus hypothetically bridged over every ocean, and
united almost every island with some mainland. If indeed
the arguments used by Forbes are to be trusted, it must be
admitted that scarcely a single island exists which has not
recently been united to some continent. This view cuts the
Gordian knot of the dispersal of the same species to the most
distant points, and removes many a difficulty; but to the best
of my judgment we are not authorised in admitting such
enormous geographical changes within the period of existing
species. It seems to me that we have abundant evidence of
great oscillations in the level of the land or sea ; but not of
such vast changes in the position and extension of our con-
tinents, as to have united them within the recent period to
each other and to the several intervening oceanic islands.
I freely admit the former existence of many islands, now
buried beneath the sea, which may have served as halting-
places for plants and for many animals during their migra-
tion. In the coral-producing oceans such sunken islands are
now marked by rings of coral or atolls standing over them.
"Whenever it is fully admitted, as it will some day be, that
each species has proceeded from a single birthplace, and
when in the course of time we know something definite about
the means of distribution, we shall be enabled to speculate
with security on the former extension of the land. But I do
not believe that it will ever be proved that within the recent
period most of our continents which now stand quite sep-
arate, have been continuously, or almost continuously united
with each other, and with the many existing oceanic islands.
Several facts in distribution — such as the great difference in
the marine faunas on the opposite sides of almost every con-
tinent, — the close relation of the tertiary inhabitants of sev-
eral lands and even seas to their present inhabitants, — the
degree of affinity between the mammals inhabiting islands
with those of the nearest continent, being in part determined
(as we shall hereafter see) by the depth of the intervening
MEANS OF DISPERSAL 405
ocean. — these and other such facts are opposed to the admis-
sion of such prodigious geographical revolutions within the
recent period, as are necessary on the view advanced by
Forbes and admitted by his followers. The nature and rela-
tive proportions of the inhabitants of oceanic islands are
likewise opposed to the belief of their former continuity with
continents. Nor does the almost universally volcanic com-
position of such islands favour the admission that they are
the wrecks of sunken continents; — if they had originally
existed as continental mountain ranges, some at least of the
islands would have been formed, like other mountain sum-
mits, of granite, metamorphic schists, old fossiliferous and
other rocks, instead of consisting of mere piles of volcanic
matter.
I must now say a few words on what are called accidental
means, but which more properly should be called occasional
means of distribution. I shall here confine myself to plants.
In botanical works, this or that plant is often stated to be ill
adapted for wide dissemination ; but the greater or less facili-
ties for transport across the sea may be said to be almost
wholly unknown. Until I tried, with Mr. Berkeley's aid, a
few experiments, it was not even known how far seeds
could resist the injurious action of sea-water. To my sur-
prise I found that out of 87 kinds, 64 germinated after an
immersion of 28 days, and a few survived an immersion of
137 days. It deserves notice that certain orders were far
more injured than others: nine Leguminosae were tried, and,
with one exception, they resisted the salt-water badly ; seven
species of the allied orders, Hydrophyllaceae and Polemo-
niaceae, were all killed by a month's immersion. For con-
venience' sake I chiefly tried small seeds without the cap-
sule or fruit ; and as all of these sank in a few days they
could not have been floated across wide spaces of the sea,
whether or not they were injured by the salt-water. After-
wards I tried some larger fruits, capsules, &c., and some of
these floated for a long time. It is well known what a dif-
ference there is in the buoyancy of green and seasoned tim-
ber; and it occurred to me that floods would often wash into
the sea dried plants or branches with seed-capsules or fruit
attached to them. Hence I was led to drv the stems and
406 ORIGIN OF SPECIES
branches of 94 plants with ripe fruit, and to place them on
sea-water. The majority sank quickly, but some which,
whilst green, floated for a very short time, when dried floated
much longer; for instance, ripe hazel-nuts sank immediately,
but when dried they floated for 90 days, and afterwards when
planted germinated; an asparagus-plant with ripe berries
floated for 23 days, when dried it floated for 85 days, and
the seeds afterwards germinated ; the ripe seeds of Helosci-
adium sank in two days, when dried they floated for above
90 days, and afterwards germinated. Altogether, out of the
94 dried plants, 18 floated for above 28 days ; and some of
the 18 floated for a very much longer period. So that as -1^
kinds of seeds germinated after an immersion of 28 days ;
and as ^f distinct species with ripe fruit (but not all the same
species as in the foregoing experiment) floated, after being
dried, for above 28 days, we may conclude, as far as anything
can be inferred from these scanty facts, that the seeds of y^^*^
kinds of plants of any country might be floated by sea-cur-
rents during 28 days, and would retain their power of ger-
mination. In Johnston's Physical Atlas, the average rate of
the several Atlantic currents is 33 miles per diem (some cur-
rents running at the rate of 60 miles per diem) ; on this
average, the seeds of ■^^ plants belonging to one country
might be floated across 924 miles of sea to another country,
and when stranded, if blown by an inland gale to a favour-
able spot, would germinate.
Subsequently to my experiments^ M. Martens tried similar
ones, but in a much better manner, for he placed the seeds
in a box in the actual sea, so that they were alternately wet
and exposed to the air like really floating plants. He tried
98 seeds, mostly different from mine ; but he chose many
large fruits and likewise seeds from plants which live near
the sea ; and this would have favoured both the average
length of their flotation and their resistance to the injurious
action of the salt-water. On the other hand, he did not pre-
viously dry the plants or branches with the fruit; and this,
as we have seen, would have caused some of them to 'i:ave
floated much longer. The result was that -J-l of his seeds of
different kinds floated for 42 days, and were then capable of
germination. But I do not doubt that plants exposed to the
MEANS OF DISPERSAL 407
waves v.'ould float for a less time than those protected from
violent movement as in our experiments. Therefore it would
perhaps be safer to assume that the seeds of about ^\ plants
of a flora, after having been dried, could be floated across a
space of sea 900 miles in width, and would then germinate.
The fact of the larger fruits often floating longer than the
small, is interesting ; as plants with large seeds or fruit which,
as Alph. de Candolle has shown, generally have restricted
ranges, could hardly be transported by any other means.
Seeds may be occasionally transported in another manner.
Drift timber is thrown up on most islands, even on those in
the midst of the widest oceans ; and the natives of the coral-
islands in the Pacific procure stones for their tools, solely
from the roots of drifted trees, these stones being a valuable
royal tax. I find that when irregularly shaped stones are
embedded in the roots of trees, small parcels of earth are fre-
quently enclosed in their interstices and behind them, — so
perfectly that not a particle could be washed away during the
longest transport: out of one small portion of earth thus
completely enclosed by the roots of an oak about 50 years
old, three dicotyledonous plants germinated; I am certain of
the accuracy of this observation. Again, I can show that
the carcases of birds, when floating on the sea, sometimes
escape being immediately devoured : and many kinds of seeds
in the crops of floating birds long retain their vitality: peas
and vetches, for mstance, are killed by even a few days' im-
mersion in sea-water ; but some taken out of the crop of a
pigeon, which had floated on artificial sea-water for 30 days,
to my surprise nearly all germinated.
Living birds can hardly fail to be highly effective agents
in the transportation of seeds. I could give many facts
showing how frequently birds of many kinds are blown by
gales to vast distances across the ocean. We may safely
assume that under such circumstances their rate of flight
would often be 25 miles an hour ; and some authors have
given a far higher estimate. I have never seen an instance
of nutritious seeds passing through the intestines of a bird ;
but hard .seeds of fruit pass uninjured through even the di-
gestive organs of a turkey. In the course of two months, I
picked tip in my garden J2 kinds of seeds, out of the excre-
408 ORIGIN OF SPECIES
meat of small birds, and these seemed perfect, and some of
them, which were tried, germinated. But the following fact
is more important: the crops of birds do not secrete gastric
juice, and do not, as I know by trial, injure in the least the
germination of seeds; now, after a bird has found and de-
voured a large supply of food, it is positively asserted that
all the grains do not pass into the gizzard for twelve or even
eighteen hours. A bird in this interval might easily be
blown to the distance of 500 miles, and hawks are known to
look out for tired birds, and the contents of their torn crops
might thus readily get scattered. Some hawks and owls
bolt their prey whole, and, after an interval of from twelve
to twenty hours, disgorge pellets, which, as I know from
experiments made in the Zoological Gardens, include seeds
capable of germination. Some seeds of the oat, wheat, mil-
let, canary, hemp, clover, and beet germinated after having
been from twelve to twenty-one hours in the stomachs of
different birds of prey ; and two seeds of beet grew after hav-
ing been thus retained for two days and fourteen hours.
Fresh-water fish, I find, eat seeds of many land and water
plants ; fish are frequently devoured by birds, and thus the
seeds might be transported from place to place. I forced
many kinds of seeds into the stomachs of dead fish, and then
gave their bodies to fishing-eagles, storks, and pelicans ;
these birds, after an interval of many hours, either rejected
the seeds in pellets or passed them in their excrement; and
several of these seeds retained the power of germination.
Certain seeds, however, were always killed by this process.
Locusts are sometimes blown to great distances from the
land ; I myself caught one 370 miles from the coast of Africa,
and have heard of others caught at greater distances. The
Rev. R. T. Lowe informed Sir C. Lyell that in November
1844 swarms of locusts visted the island of Madeira. They
were in countless numbers, as thick as the flakes of snow in
the heaviest snowstorm, and extended upwards as far as
could be seen with a telescope. During two or three days
they slowly careered round and round in an immense ellipse,
at least five or six miles in diameter, and at night alighted
on the taller trees, which were completely coated with them.
They then disappeared over the sea, as suddenly as they had
MEANS OF DISPERSAL 409
appeared, and have not since visited the island. Now, in
parts of Natal it is beheved by some farmers, though on in-
sufficient evidence, that injurious seeds are introduced into
their grassland in the dung left by the great flights of locusts .
which often visit that country. In consequence of this be-
lief Mr. Weale sent me in a letter a small packet of the dried
pellets, out of which I extracted under the microscope several
seeds, and raised from them seven grass plants, belonging to
two species, of two genera. Hence a swarm of locusts, such
as that which visited Madeira, might readily be the means of
introducing several kinds of plants into an island lying far
from the mainland.
Although the beaks and feet of birds are generally clean,
earth sometimes adheres to them : in one case I removed
sixty-one grains, and in another case twenty-two grains of
dry argillaceous earth from the foot of a partridge, and in
the earth there was a pebble as large as the seed of a vetch.
Here is a better case : the leg of a woodcock was sent to me
by a friend, with a little cake of dry earth attached to the
shank, weighing only nine grains ; and this contained a seed
of the toad-rush (Juncus bufonius) which germinated and
flowered. Mr. Swaysland, of Brighton, who during the last
forty years has paid close attention to our migratory birds,
informs me that he has often shot wagtails (Motacillae),
wheatears, and whincats (Saxicolse), on their first arrival
on our shores, before they had alighted; and he has several
times noticed little cakes of earth attached to their feet.
Many facts could be given showing how generally soil is
charged with seeds. For instance, Prof. Newton sent me
the leg of a red-legged partridge (Caccabis rufa) which had
been wounded and could not fly, with a ball of hard earth
adhering to it, and weighing six and a half ounces. The
earth had been kept for three years, but when broken,
watered and placed under a bell-glass, no less than 82 plants
sprung from it: these consisted of 12 monocotyledons, includ-
ing the common oat, and at least one kind of grass, and of 70
dicotyledons, which consisted, judging from the young leaves,
of at least three distinct species. With such facts before us,
can we doubt that the many birds which are annually blown
by gales across great spaces of ocean, and which annually
410 ORIGIN OF SPECIES
migrate — for instance, the millions of quails across the Medi-
terranean — must occasionally transport a few seeds embedded
in dirt adhering to their feet or beaks? But I shall have to
recur to this subject.
As icebergs are known to be sometimes loaded with earth
and stones, and have even carried brushwood, bones, and the
nest of a land-bird, it can hardly be doubted that they must
occasionally, as suggested by Lyell, have transported seeds
form one part to another of the arctic and antarctic regions ;
and during the Glacial period from one part of the now tem-
perate regions to another. In the Azores, from the large
number of plants common to Europe, in comparison with the
species on the other islands of the Atlantic, which stand
nearer to the mainland, and (as remarked by Mr. H. C.
Watson) from their somewhat northern character in com-
parison with the latitude, I suspected that these islands had
been partly stocked by ice-borne seeds, during the Glacial
epoch. At my request Sir C, Lyell wrote to M. Hartung
to inquire whether he had observed erratic boulders on these
islands, and he answered that he had found large fragments
of granite and other rocks, which do not occur in the archi-
pelago. Hence we may safely infer that icebergs formerly
landed their rocky burthens on the shores of these mid-ocean
islands, and it is at least possible that they may have brought
thither some few seeds of northern plants.
Considering that these several means of transport, and that
other means, which without doubt remain to be discovered,
have been in action year after year for tens of thousands of
years, it would, I think, be a marvellous fact if many plants
had not thus become widely transported. These means of
transport are sometimes called accidental, but this is not
strictly correct : the currents of the sea are not accidental,
nor is the direction of prevalent gales of wind. It should be
observed that scarcely any means of transport would carry
seeds for very great distances : for seeds do not retain their
vitality when exposed for a great length of time to the action
of sea-water; nor could they be long carried in the crops or
intestines of birds. These means, however, would suffice for
occasional transport across tracts of sea some hundred miles
in breadth, or from island to island, or from a continent to a
DISPERSAL DURING GLACIAL PERIOD 411
neighbouring island, but not from one distant continent to
another The floras of distant continents would not by such
means become mingled ; but would remain as distinct as they
now are. The currents, from their course, would never
bring seeds from North America to Britain, though they
might and do bring seeds from the West Indies to our west-
ern shores, where, if not killed by their long immersion in
salt-water, they could not endure our climate. Almost every
year, one or two land-birds are blown across the whole At-
lantic Ocean, from North America to the western shores of
Ireland and England; but seeds could be transported by these
rare wanderers only by one means, namely, by dirt adhering
to their feet or beaks, which is in itself a rare accident.
Even in this case, how small would be the chance of a seed
falling on favourable soil, and coming to maturity ! But it
would be a great error to argue that because a well-stocked
island, like Great Britain, has not, as far as is known (and
it would be very difficult to prove this), received within the
last few centuries, through occasional means of transport,
immigrants from Europe or any other continent, that a
poorly-stocked island, though standing more remote from the
mainland, would not receive colonists by similar means. Out
of a hundred kinds of seeds or animals transported to an
island, even if far less well-stocked than Britain, perhaps
not more than one would be so well fitted to its new home,
as to become naturalised. But this is no valid argument
against what would be effected by occasional means of trans-
port, during the long lapse of geological time, whilst the
island was being upheaved, and before it had become fully
stocked with inhabitants. On almost bare land, with few or
no destructive insects or birds living there, nearly every seed
which chanced to arrive, if fitted for the climate, would ger-
minate and survive.
DISPERSAL DURING THE GLACIAL PERIOD
The identity of many plants and animals, on mountain-
summits, separated from each other by hundreds of miles of
lowlands, where Alpine species could not possibly exist, is
one of the most striking cases known of the same species
412 ORIGIN OF SPECIES
living at distant points, without the apparent possibility of
their having migrated from one point to the other. It is in-
deed a remarkable fact to see so many plants of the same
species living on the snowy regions of the Alps or Pyrenees,
and in the extreme northern parts of Europe ; but it is far
more remarkable, that the plants on the White Mountains,
in the United States of America, are all the same with those
of Labrador, and nearly all the same, as we hear from Asa
Gray, with those on the loftiest mountains of Europe. Even
as long ago as 1747, such facts led Gmelin to conclude that
the same species must have been independently created at
many distinct points; and we might have remained in this
same belief, had not Agassiz and others called vivid atten-
tion to the Glacial period, which, as we shall immediately
see, affords a simple explanation of these facts. We have
evidence of almost every conceivable kind, organic and in-
organic, that, within a very recent geological period, central
Europe and North America suffered under an arctic climate.
The ruins of a house burnt by fire do not tell their tale more
plainly than do the mountains of Scotland and Wales, with
their scored flanks, polished surfaces, and perched boulders,
of the icy streams with which their valleys were lately filled.
So greatly has the climate of Europe changed, that in North-
ern Italy, gigantic moraines, left by old glaciers, are now
clothed by the vine and maize. Throughout a large part of
the United States, erratic boulders and scored rocks plainly
reveal a former cold period.
The former influence of the glacial climate on the distribu-
tion of the inhabitants of Europe, as explained by Edward
Forbes, is substantially as follows. But we shall follow the
changes more readily, by supposing a new glacial period
slowly to come on, and then pass away, as formerly occurred.
As the cold came on, and as each more southern zone be-
came fitted for the inhabitants of the north, these would take
the places of the former inhabitants of the temperate regions.
The latter, at the same time, would travel further and fur-
ther southward, unless they were stopped by barriers, in
which case they would perish. The mountains would become
covered with snow and ice, and their former Alpine inhabit-
ants would descend to the plains. By the time that the cold
DISPERSAL DURING GLACIAL PERIOD 413
had reached its maximum, we should have an arctic fauna
and flora, covering the central parts of Europe, as far south
as the Alps and Pyrenees, and even stretching into Spain.
The now temperate regions of the United States would like-
wise be covered by arctic plants and animals and these would
be nearly the same with those of Europe; for the present
circumpolar inhabitants, which we suppose to have every-
where travelled southward, are remarkably uniform round
the world.
As the warmth returned, the arctic forms would retreat
northward, closely followed up in their retreat by the produc-
tions of the more temperate regions. And as the snow
melted from the bases of the mountains, the arctic forms
would seize on the cleared and thawed ground, always as-
cending, as the warmth increased and the snow still further
disappeared, higher and higher, whilst their brethren were
pursuing their northern journey. Hence, when the warmth
had fully returned, the same species, which had lately lived
together on the European and North American lowlands,
would again be found in the arctic regions of the Old and
New Worlds, and on many isolated mountain-summits far
distant from each other.
Thus we can understand the identity of many plants at
points so immensely remote as the mountains of the United
States and those of Europe. We can thus also understand
the fact that the Alpine plants of each mountain-range are
more especially related to the arctic forms living due north
or nearly due north of them : for the first migration when
the cold came on, and the re-migration on the returning
warmth, would generally have been due south and north.
The Alpine plants, for example, of Scotland, as remarked
by Mr. H. C. Watson, and those of the Pyrenees, as re-
marked by Ramond, are more especially allied to the plants
of northern Scandinavia ; those of the United States to Lab-
rador ; those of the mountains of Siberia to the arctic regions
of that country. These views, grounded as they are on the
perfectly well-ascertained occurrence of a former Glacial
period, seem to me to explain in so satisfactory a manner
the present distribution of the Alpine and Arctic productions
of Europe and America, that when in other regions we find
414 ORIGIN OF SPECIES
the same species on distant mountain-summits, we may al-
most conclude, without other evidence, that a colder climate
formerly permitted their migration across the intervening
lowlands, now become too warm for their existence.
As the arctic forms moved first southward and afterwards
backwards to the north, in unison with the changing climate,
they will not have been exposed during their long migrations
to any great diversity of temperature; and as they all mi-
grated in a body together, their mutual relations will not
have been much disturbed. Hence, in accordance with the
principles inculcated in this volume, these forms will not have
been liable to much modification. But with the Alpine pro-
ductions, left isolated from the moment of the returning
warmth, first at the bases and ultimately on the summits of
the mountains, the case vv^ill have been somewhat different;
for it is not likely that all the same arctic species will have
been left on mountain-ranges far distant from each other,
and have survived there ever since ; they will also in all prob-
ability, have become mingled with ancient Alpine species,
which must have existed on the mountains before the com-
mencement of the Glacial epoch, and which during the cold-
est period will have been temporarily driven down to the
plains ; they will, also, have been subsequently exposed to
somewhat different climatal influences. Their mutual rela-
tions will thus have been in some degree disturbed; conse-
quently they will have been liable to modification ; and they
have been modified; for if we compare the present Alpine
plants and animals of the several great European mountain-
ranges one with another, though many of the species remain
identically the same, some exist as varieties, some as doubt-
ful forms or sub-species, and some as distinct yet closely
allied species representing each other on the several ranges.
In the foregoing illustration I have assumed that at the
commencement of our imaginary Glacial period, the arctic
productions were as uniform round the polar regions as they
are at the present day. Eut it is also necessary to assume
that many sub-arctic and some few temperate forms were
the same round the world, for some of the species which
now exist on the lower mountain-slopes and on the plains of
North America and Europe are the same; and it may be
DISPERSAL DURING GLACIAL PERIOD 415
asked how I account for this degree of uniformity in the
sub-arctic and temperate forms round the world, at the com-
mencement of the real Glacial period. At the present day,
the sub-arctic and northern temperate productions of the Old
and New Worlds are separated from each other by the whole
Atlantic Ocean and by the northern part of the Pacific.
During the Glacial period, when the inhabitants of the Old
and New Worlds lived farther southwards than they do at
present, they must have been still more completely separated
from each other by wider spaces of ocean ; so that it may
well be asked how the same species could then or previously
have entered the two continents. The explanation, I believe,
lies in the nature of the climate before the commencement of
the Glacial period. At this, the newer Pliocene period, the
majority of the inhabitants of the world were specifically the
same as now, and we have good reason to believe that the
climate was warmer than at the present day. Hence we
may suppose that the organisms which now live under lati-
tude 60°, lived during the Pliocene period farther north
under the Polar Circle, in latitude 66°-67° ; and that the
present arctic productions then lived on the broken land still
nearer to the pole. Now, if we look at a terrestrial globe,
we see under the Polar Circle that there is almost continuous
land from western Europe, through Siberia, to eastern Amer-
ica. And this continuity of the circumpolar land, with the
consequent freedom under a more favourable climate for
intermigration, will account for the supposed uniformity of
the sub-arctic and temperate productions of the Old and New
Worlds, at a period anterior to the Glacial epoch.
Believing, from reasons before alluded to, that our conti-
nents have long remained in nearly the same relative posi-
tion, though subjected to great oscillations of level, I am
strongly inclined to extend the above view, and to infer that
during some still earlier and still warmer period, such as the
older Pliocene period, a large number of the same plants and
animals inhabited the almost continuous circumpolar land ;
and that these plants and animals, both in the Old and New
Worlds, began slowly to migrate southwards as the climate
became less warm, long before the commencement of the
Glacial period. We now see, as I believe, their descendants.
416 ORIGIN OF SPECIES
mostly in a modified condition, in the central parts of Europe
and the United States. On this view we can understand the
relationship with very little identity, between the productions
of North America and Europe, — a relationship which is
highly remarkable, considering the distance of the two areas,
and their separation by the whole Atlantic Ocean. We can
further understand the singular fact remarked on by several
observers that the productions of Europe and America dur-
ing the later tertiary stages were more closely related to
each other than they are at the present time; for during
these warmer periods the northern parts of the Old and New
Worlds will have been almost continuously united by land,
serving as a bridge, since rendered impassable by cold, for
the intermigration of their inhabitants.
During the slowly decreasing warmth of the Pliocene
period, as soon as the species in common, which inhabited
the New and Old Worlds, migrated south of the Polar
Circle, they will have been completely cut off from each
other. This separation, as far as the more temperate produc-
tions are concerned, must have taken place long ages ago.
As the plants and animals migrated southward, they will
have become mingled in the one great region with the native
American productions, and would have had to compete with
them; and in the other great region, with those of the Old
World. Consequently we have here everything favourable
for much modification, — for far more modification than with
the Alpine productions, left isolated, within a much more
recent period, on the several mountain-ranges and on the
arctic lands of Europe and N. America. Hence it has come,
that when we compare the now living productions of the tem-
perate regions of the New and Old Worlds, we find very few
identical species (though Asa Gray has lately shown that
more plants are identical than was formerly supposed), but
we find in every great class many forms, which some nat-
uralists rank as geographical races, and others as distinct
species; and a host of closely allied or representative forms
which are ranked by all naturalists as specifically distinct.
As on the land, so in the waters of the sea, a slow south-
ern migration of a marine fauna, which, during the Pliocene
or even a somewhat earlier period, was nearly uniform along
ALTERNATE GLACIAL PERIODS 417
the continuous shores of the Polar Circle, will account, on
the theory of modification, for many closely allied forms now
living in marine areas completely sundered. Thus, I think,
we can understand the presence of some closely allied, still
existing and extinct tertiary forms, on the eastern and west-
ern shores of temperate North America ; and the still more
striking fact of many closely allied crustaceans (as described
in Dana's admirable work), some fish and other marine ani-
mals, inhabiting the Mediterranean and the seas of Japan, —
these two areas being now completely separated by the
breadth of a whole continent and by wide spaces of ocean.
These cases of close relationship in species either now or
formerly inhabiting the seas on the eastern and western
shores of North America, the Mediterranean and Japan, and
the temperate lands of North America and Europe, are inex-
plicable on the theory of creation. We cannot maintain that
such species have been created alike, in correspondence with
the nearly similar physical conditions of the areas; for if we
compare, for instance, certain parts of South America with
parts of South Africa or Australia, we see countries closely
similar in all their physical conditions, with their inhabitants
utterly dissimilar.
ALTERNATE GLACIAL PERIODS IN THE NORTH AND SOUTH
But we must return to our more immediate subject. I am
convinced that Forbes' view may be largely extended. In
Europe we meet with the plainest evidence of the Glacial
period, from the western shores of Britain to the Oural range,
and southward to the Pyrenees. We may infer from the
frozen mammals and nature of the mountain vegetation, that
Siberia was similarly affected. In the Lebanon, according
to Dr. Hooker, perpetual snow formerly covered the central
axis, and fed glaciers which rolled 4000 feet down the val-
leys. The same observer has recently found great moraines
at a low level on the Atlas range in N. Africa. Along the
Himalaya, at points 900 miles apart, glaciers have left the
marks of their former low descent; and in Sikkim, Dr.
Hooker saw maize growing on ancient and gigantic moraines.
Southward of the Asiatie continent, on the opposite side of
X — HC xt
41S ORIGIN OF SPECIES
the equator, we know, fiom the excellent researches of Dr.
J. Haast and Dr. Hector, that in New Zealand immense
glaciers formerly descended to a low level ; and the same
plants found by Dr. Hooker on widely separated mountains'
in this island tell the same story of a former cold period.
From facts communicated to me by the Rev. W. B. Clarke,
it appears also that there are traces of former glacial action
on the mountains of the south-eastern corner of Australia.
Looking to America ; in the northern half, ice-borne frag-
ments of rock have been observed on the eastern side of the
continent, as far south as lat. 36°-37°, and on the shores of
the Pacific, where the climate is now so dilYerent, as far
south as lat. 46°. Erratic boulders have, also, been noticed
on the Rocky Mountains. In the Cordillera of South Amer-
ica, nearly under the equator, glaciers once extended far be-
low their present level. In Central Chile I examined a vast
mound of detritus with great boulders, crossing the Portillo
valley, which there can hardly be a doubt once formed a huge
moraine ; and Mr. D. Forbes informs me that he found in
various parts of the Cordillera, from lat. 13° to 30° S., at
about the height of 12,000 feet, deeply-furrowed rocks, re-
sembling those with which he was familiar in Norway, and
likewise great masses of detritus, including grooved pebbles.
Along this whole space of the Cordillera true glaciers do not
now exist even at much more considerable heights. Farther
south on both sides of the continent, from lat. 41° to the
southernmost extremity, we have the clearest evidence of
former glacial action, in numerous immense boulders trans-
ported far from their parent source.
From these several facts, namely from the glacial action
having extended all round the northern and southern hemi-
spheres — from the period having been in a geological sense
recent in both hemispheres — from its having lasted in both
during a great length of time, as may be inferred from the
amount of work effected — and lastly from glaciers having
recently descended to a low level along the whole line of the
Cordillera, it at one time appeared to me that we could not
avoid the conclusion that the temperature of the whole world
had been simultaneously lowered during the Glacial period.
But now Mr. Croll, in a series of admirable memoirs, has
ALTERNATE GLACIAL PERIODS 419
attempted to show that a glacial condition of climate is the
result of various physical causes, brought into operation by
an increase in the eccentricity of the earth's orbit. All these
causes tend towards the same end ; but the most powerful
appears to be the indirect influence of the eccentricity of the
orbit upon oceanic currents. According to Mr. CroU, cold
periods regularly recur every ten or fifteen thousand years;
and these at long intervals are extremely severe, owing to
certain contingencies, of which the most important, as Sir C.
Lyell has shown, is the relative position of the land and
water. Mr. Croll believes that the last great Glacial period
occurred about 240,000 years ago, and endured with slight
alterations of climate for about 160,000 years. With respect
to more ancient Glacial periods, several geologists are con-
vinced from direct evidence that such occurred during the
Miocene and Eocene formations, not to mention still more
ancient formations. But the most important result for us,
arrived at by Mr. Croll, is that whenever the northern hemi-
sphere passes through a cold period the temperature
of the southern hemisphere is actually raised, with the win-
ters rendered much milder, chiefly through changes in the
direction of the ocean-currents. So conversely it will be
with the northern hemisphere, whilst the southern passes
through a Glacial period. This conclusion throws so much
light on geographical distribution that I am strongly inclined
to trust in it; but I will first give the facts, which demand
an explanation.
In South America, Dr. Hooker has shown that besides
many closely allied species, between forty and fifty of the
flowering plants of Tierra del Fuego, forming no inconsider-
able part of its scanty flora, are common to North America
and Europe, enormously remote as these areas in opposite
hemispheres are from each other. On the lofty mountains
of equatorial America a host of peculiar species belonging
to European genera occur. On the Organ mountains of
Brazil, some few temperate European, some Antarctic, and
some Andean genera were found by Gardner, which do not
exist in the low intervening hot countries. On the Silla of
Caraccas, the illustrious Humboldt long ago found species
belonging to genera characteristic of the Cordillera.
420 ORIGIN OF SPECIES
In Africa, several forms characteristic of Europe and some
few representatives of the flora of the Cape of Good Hope
occur on the mountains of Abyssinia. At the Cape of Good
Hope a very few European species, believed not to have been
introduced by man, and on the mountains several representa-
tive European forms are found, which have not been dis-
covered in the intertropical parts of Africa. Dr. Hooker
has also lately shown that several of the plants living on the
upper parts of the lofty island of Fernando Po and on the
neighbouring Cameroon mountains, in the Gulf of Guinea,
are closely related to those on the mountains of Abyssinia,
and likewise to those of temperate Europe. It now also
appears, as I hear from Dr. Hooker, that some of these same
temperate plants have been discovered by the Rev. R. T.
Lowe on the mountains of the Cape Verde islands. This
extension of the same temperate forms, almost under the
equator, across the whole continent of Africa and to the
mountains of the Cape Verde archipelago, is one of the most
astonishing facts ever recorded in the distribution of plants.
On the Himalaya, and on the isolated mountain-ranges of
the peninsula of India, on the heights of Ceylon, and on the
volcanic cones of Java, many plants occur, either identically
the same or representing each other, and at the same time
representing plants of Europe, not found in the intervening
hot lowlands. A list of the genera of plants collected on
the loftier peaks of Java, raises a picture of a collection made
on a hillock in Europe ! Still more striking is the fact that
peculiar Australian forms are represented by certain plants
growing on the summits of the mountains of Borneo. Some
of these Australian forms, as I hear from Dr. Hooker, ex-
tend along the heights of the peninsula of Malacca, and are
thinly scattered on the one hand over India, and on the other
hand as far north as Japan.
On the southern mountains of Australia, Dr. F. Miiller has
discovered several European species; other species, not in-
troduced by man, occur on the lowlands ; and a long list can
be given, as I am informed by Dr. Hooker, of European
genera, found in Australia, but not in the intermediate torrid
regions. In the admirable 'Introduction to the Flora of New
Zealand,' by Dr. Hooker, analogous and striking facts are
ALTERNATE GLACIAL PERIODS 42^
given in regard to the plants of that large island. Hence we
see that certain plants growing on the more lofty mountains
of the tropics in all parts of the world, and on the temperate
plains of the north and south, are either the same species or
varieties of the same species. It should, however, be ob-
served that these plants are not strictly arctic forms; for, as
Mr. H. C. Watson has remarked, "in receding from polar
towards equatorial latitudes, the Alpine or mountain floras
really become less and less Arctic." Besides these identical
and closely allied forms, many species inhabiting the same
widely sundered areas, belong to genera not now found in
the intermediate tropical lowlands.
These brief remarks apply to plants alone ; but some few
analogous facts could be given in regard to terrestrial ani-
mals. In marine productions, similar cases likewise occur;
as an example, I may quote a statement by the highest
authority. Prof. Dana, that "it is certainly a wonderful fact
that New Zealand should have a closer resemblance in its
Crustacea to Great Britain, its antipode, than to any other
part of the world." Sir J. Richardson, also, speaks of the re-
appearance on the shores of New Zealand, Tasmania, &c., of
northern forms of fish. Dr. Hooker informs me that
twenty-five species of Algae are common to New Zealand
and to Europe, but have not been found in the intermediate
tropical seas.
From the foregoing facts, namely, the presence of tem-
perate forms on the highlands across the whole of equatorial
Africa, and along the Peninsula of India, to Ceylon and the
Malay Archipelago, and in a less well-marked manner across
the wide expanse of tropical South America, it appears
almost certain that at some former period, no doubt during
the most severe part of a Glacial period, the lowlands of
these great continents were everywhere tenanted under the
equator by a considerable number of temperate forms. At
this period the equatorial climate at the level of the sea was
probably about the same with that now experienced at the
height of from five to six thousand feet under the same lati-
tude, or perhaps even rather cooler. During this, the coldest
period, the lowlands under the equator must have been
clothed with a mingled tropical and temperate vegetation.
422 ORIGIN OF SPECIES
like that described by Hooker as growing luxuriantly at the
height of from four to five thousand feet on the lower slopes
of the Himalayas, but with perhaps a still greater prepon-
derance of temperate forms. So again in the mountainous
islands of Fernando Po, in the Gulf of Guinea, Mr. Mann
found temperate European forms beginning to appear at the
height of about five thousand feet. On the mountains of
Panama, at the height of only two thousand feet, Dr. See-
mann found the vegetation like that of Mexico, "with forms
of the torrid zone harmoniously blended with those of the
temperate."
Now let us see whether Mr. Croll's conclusion that when
the northern hemisphere suffered from the extreme cold of
the great Glacial period, the southern hemisphere was actu-
ally warmer, throws any clear light on the present apparently
inexplicable distribution of various organisms in the tem-
perate parts of both hemispheres, and on the mountains of
the tropics. The Glacial period, as measured by years, must
have been very long; and when we remember over what vast
spaces some naturalised plants and animals have spread
within a few centuries, this period will have been ample for
any amount of migration. As the cold became more and
more intense, we know that Arctic forms invaded the tem-
perate regions; and, from the facts just given, there can
hardly be a doubt that some of the more vigorous, dominant
and widest-spreading temperate forms invaded the equa-
torial lowlands. The inhabitants of these hot lowlands would
at the same time have migrated to the tropical and sub-
tropical regions of the south, for the southern hemisphere
was at this period warmer. On the decline of the Glacial
period, as both hemispheres gradually recovered their former
temperatures, the northern temperate forms living on the
lowlands under the equator, would have been driven to their
former homes or have been destroyed, being replaced by the
equatorial forms returning from the south. Some, however,
of the northern temperate forms would almost certainly have
ascended any adjoining high land, where, if sufficiently lofty,
they would have long survived like the Arctic forms on the
mountains of Europe. They might have survived, even if
the climate was not perfectly fitted for them, for the change
ALTERNATE GLACIAL PERIODS 423
of temperature must have been very slow, and plants un-
doubtedly possess a certain capacity for acclimatisation, as
shown by their transmitting to their offspring different con-
stitutional powers of resisting heat and cold.
In the regular course of events the southern hemisphere
would in its turn be subjected to a severe Glacial period, with
the northern hemisphere rendered warmer; and then the
southern temperate forms would invade the equatorial low-
lands. The northern forms which had before been left on
the mountains would now descend and mingle with the south-
ern forms. These latter, when the warmth returned, would
return to their former homes, leaving some few species on
the mountains, and carrying southward with them some of
the northern temperate forms which had descended from
their mountain fastnesses. Thus, we should have some few
species identically the same in the northern and southern
temperate zones and on the mountains of the intermediate
tropical regions. But the species left during a long time on)
these mountains, or in opposite hemispheres, would have to
compete with many new forms and would be exposed to
somewhat different physical conditions ; hence they would
be eminently liable to modification, and would generally now
exist as varieties or as representative species; and this is the
case. We must, also, bear in mind the occurrence in both
hemispheres of former Glacial periods; for these will ac-
count, in accordance with the same principles, for the many
quite distinct species inhabiting the same widely separated
areas, and belonging to genera not now found in the inter-
mediate torrid zones.
It is a remarkable fact strongly insisted on by Hooker in
regard to America, and by Alph. de Candolle in regard to
Australia, that many more identical or slightly modified spe-
cies have migrated from the north to the south, than in a
reversed direction. We see, however, a few southern forms
on the mountains of Borneo and Abyssinia. I suspect that
this preponderant migration from the north to the south is
due to the greater extent of land in the north, and to the
northern forms having existed in their own homes in greater
numbers, and having consequently been advanced through
natural selection and competition to a higher stage of per-
424 ORIGIN OF SPECIES
fection, or dominating power, than the southern forms. And
thus, when the two sets became commingled in the equatorial
regions, during the alternations of the Glacial periods, the
northern forms were the more powerful and were able to
hold their places on the mountains, and afterwards to mi-
grate southward with the southern forms; but not so the
southern in regard to the northern forms. In the same
manner at the present day, we see that very many European
productions cover the ground in La Plata, New Zealand, and
to a lesser degree in Australia, and have beaten the natives;
whereas extremely few southern forms have become natu-
ralised in any part of the northern hemisphere, though hides,
wool, and other objects likely to carry seeds have been
largely imported into Europe during the last two or three
centuries from La Plata and during the last forty or fifty
years from Australia. The Neilgherrie mountains in India,
however, offer a partial exception; for here, as I hear from
Dr. Hooker, Australian forms are rapidly sowing themselves
and becoming naturalised. Before the last great Glacial
period, no doubt the intertropical mountains were stocked
with endemic Alpine forms; but these have almost every-
where yielded to the more dominant forms generated in the
larger areas and more efficient workshops of the north. In
many islands the native productions are nearly equalled, or
even outnumbered, by those which have become i.aturalised;
and this is the first stage towards their extinction. Moun-
tains are islands on the land, and their inhabitants have
yielded to those produced within the larger areas of the
north, just in the same way as the inhabitants of real islands
have everywhere yielded and are still yielding to continental
forms naturalised through man's agency.
The same principles apply to the distribution of terrestrial
animals and of marine productions, in the northern and
southern temperate zones, and on the intertropical mountains.
When, during the height of the Glacial period, the ocean-
currents were widely different to what they now are, some
of the inhabitants of the temperate seas might have reached
the equator ; of these a few would perhaps at once be able to
migrate southward, by keeping to the cooler currents, whilst
others might remain and survive in the colder depths until
ALTERNATE GLACIAL PERIODS 425
the southern hemisphere was in its turn subjected to a glacial
climate and permitted their further progress; in nearly the
same manner as, according to Forbes, isolated spaces inhab-
ited by Arctic productions exist to the present day in the
deeper parts of the northern temperate seas.
I am far from supposing that all the difficulties in regard
to the distribution and affinities of the identical and allied
species, which now live so widely separated in the north and
south, and sometimes on the intermediate mountain-ranges,
are removed on the views above given. The exact lines of
migration cannot be indicated. We cannot say why certain
species and not others have migrated; why certain species
have been modified and have given rise to new forms, whilst
others have remained unaltered. We cannot hope to explain
such facts, until we can say why one species and not another
becomes naturalised by man's agency in a foreign land; why
one species ranges twice or thrice as far, and is twice or
thrice as common, as another species within their own homes.
Various special difficulties also remain to be solved; for
instance, the occurrence, as shown by Dr. Hooker, of the
same plants at points so enormously remote as Kerguelen
Land, New Zealand, and Fuegia; but icebergs, as suggested
by Lyell, may have been concerned in their dispersal. The
existence at these and other distant points of the southern
hemisphere, of species, which, though distinct, belong to
genera exclusively confined to the south, is a more remark-
able case. Some of these species are so distinct, that we
cannot suppose that there has been time since the commence-
ment of the last Glacial period for their migration and sub-
sequent modification to the necessary degree. The facts
seem to indicate that distinct species belonging to the same
genera have migrated in radiating lines from a common
genera; and I am inclined to look in the southern, as in the
northern hemisphere, to a former and warmer period, before
the commencement of the last Glacial period, when the Ant-
arctic lands, now covered with ice, supported a highly
peculiar and isolated flora. It may be suspected that before
this flora was exterminated during the last Glacial epoch, a
few forms had been already widely dispersed to various
points of the southern hemisphere by occasional means of
426 ORIGIN OF SPECIES
transport, and by the aid as halting-places, of now sunken
islands. Thus the southern shores of America, Australia,
and New Zealand may have become slightly tinted by the
same peculiar forms of life.
Sir C. Lyell in a striking passage has speculated, in lan-
guage almost identical with mine, on the effects of great
alterations of climate throughout the world on geographical
distribution. And we have now seen that Mr. Croll's conclu-
sion that successive Glacial periods in the one hemisphere
coincide with warmer periods in the opposite hemisphere,
together with the admission of the slow modification of spe-
cies, explains a multitude of facts in the distribution of the
same and of the allied forms of life in all parts of the globe.
The living waters have flowed during one period from the
north and during another from the south, and in both cases
have reached the equator; but the stream of life has flowed
with greater force from the north than in the opposite direc-
tion, and has consequently more freely inundated the south.
As the tide leaves its drift in horizontal lines, rising higher
on the shores where the tide rises highest, so have the living
waters left their living drift on our mountain summits, in a
line gently rising from the Arctic lowlands to great altitude
under the equator. The various beings thus left stranded
may be compared with savage races of man, driven up and
surviving in the mountain fastnesses of almost every land,
which serves as a record, full of interest to us, of the former
inhabitants of the surrounding lowlands.
CHAPTER XIII
Geographical Distribution — continued
Distribution of fresh-water productions — On the inhabitants of
oceanic islands — Absence of Batrachians and of terrestrial Mam-
mals—On the relation of the inhabitants of islands to those of
the nearest mainland— On colonisation from the nearest source
with subsequent modification — Summary of the last and present
chapter.
FRESH-WATER PRODUCTIONS
AS lakes and river-systems are separated from each
l\ other by barriers of land, it might have been thought
-^-M^ that fresh-water productions would not have ranged
widely within the same country, and as the sea is apparently
a still more formidable barrier, that they would never have
extended to distant countries. But the case is exactly the re-
verse. Not only have many fresh-water species, belonging
to different classes, an enormous range, but allied species
prevail in a remarkable manner throughout the world. When
first collecting in the fresh waters of Brazil, I well remember
feeling much surprise at the similarity of the fresh-water
insects, shells, &c., and at the dissimilarity of the surround-
ing terrestrial beings, compared with those of Britain.
But the wide ranging power of fresh-water productions
can, I think, in most cases be explained by their having be-
come fitted, in a manner highly useful to them, for short
and frequent migrations from pond to pond, or from stream
to stream, within their own countries; and liability to wide
dispersal would follow from this capacity as an almost neces-
sary consequence. We can here consider only a few cases ;
of these, some of the most difficult to explain are presented
by fish. It was formerly believed that the same fresh-water
species never existed on two continents distant from each
other. But Dr. Giinther has lately shown that the Galaxias
427
428 ORIGIN OF SPECIES
attenuatus inhabits Tasmania, New Zealand, the Falkland
Islands, and the mainland of South America. This is a won-
derful case, and probably indicates dispersal from an Ant-
arctic centre during a former warm period. This case, how-
ever, is rendered in some degree less surprising by the spe-
cies of this genus having the power of crossing by some
unknown means considerable spaces of open ocean : thus
there is one species common to New Zealand and to the
Auckland Islands, though separated by a distance of about
230 miles. On the same continent fresh-water fish often
range widely, and as if capriciously; for in two adjoining
river-systems some of the species may be the same, and some
wholly different.
It is probable that they are occasionally transported by
what may be called accidental means. Thus fishes still alive
are not very rarely dropped at distant points by whirlwinds ;
and it is known that the ova retain their vitality for a con-
siderable time after removal from the water. Their dispersal
may, however, be mainly attributed to changes in the level
of the land within the recent period, causing rivers to flow
into each other. Instances, also, could be given of this
having occurred during floods, without any change of level.
The wide difl'erence of the fish on the opposite sides of most
mountain-ranges, which are continuous, and which conse-
quently must from an early period have completely prevented
the inosculation of the river-system on the two sides, leads to
the same conclusion. Some fresh-water fish belong to very
ancient forms, and in such cases there will have been ample
time for great geographical changes, and consequently time
and means for much migration. Moreover Dr. Giinther has
recently been led by several considerations to infer that with
fishes the same forms have a long endurance. Salt-water
fish can with care be slowly accustomed to live in fresh
water ; and, according to Valenciennes, there is hardly a
single group of which all the members are confined to fresh
water, so that a marine species belonging to a fresh-water
group might travel far along the shores of the sea, and
could, it is probable, become adapted without much difficulty
to the fresh waters of a distant land.
Some species of fresh-water shells have very wide ranges.
FRESH-WATER PRODUCTIONS 429
and allied species which, on our theory, are descended from
a common parent, and must have proceeded from a single
source, prevail throughout the world. Their distribution at
first perplexed me much, as their ova are not likely to be
transported by birds; and the ova, as well as the adults, are
immediately killed by sea-water. I could not even under-
stand how some naturalised species have spread rapidly
throughout the same country. But two facts, which I have
observed — and many others no doubt will be discovered —
throw some light on this subject. When ducks suddenly
emerge from a pond covered with duck-weed, I have twice
seen these little plants adhering to their backs ; and it has
happened to me, in removing a little duck-weed from one
aquarium to another, that I have unintentionally stocked the
one with fresh-water shells from the other. But another
agency is perhaps more effectual : I suspended the feet of a
duck in an aquarium, where many ova of fresh-water shells
were hatching; and I found that numbers of the extremely
minute and just-hatched shells crawled on the feet, and clung
to them so firmly that when taken out of the water they
could not be jarred off, though at a somewhat more advanced
age they would voluntarily drop off. These just-hatched
molluscs, though aquatic in their nature, survived on the
duck's feet, in damp air, from twelve to twenty hours; and
in this length of time a duck or heron might Hy at least six
or seven hundred miles, and if blown across the sea to an
oceanic island, or to any other distant point, would be sure
to alight on a pool or rivulet. Sir Charles Lyell informs me
that a Dytiscus has been caught with an Ancylus (a fresh-
water shell like a limpet) firmly adhering to it; and a water-
beetle of the same family, a Colymbetes, once flew on board
the 'Beagle,' when forty-five miles distant from the nearest
land : how much farther it might have been blown by a
favouring gale no one can tell.
With respect to plants, it has long been known what enor-
mous ranges many fresh-water, and even marsh species,
have, both over continents and to the most remote oceanic
islands. This is strikingly illustrated, according to Alph. de
Candolle, in those large groups of terrestrial plants, which
have very few aquatic .members ; for the latter seem immedi-
430 ORIGIN OF SPECIES
ately to acquire, as if in consequence, a wide range. I think
favourable means of dispersal explain this fact. I have be-
fore mentioned that earth occasionally adheres in some quan-
tity to the feet and beaks of birds. Wading birds, which
frequent the muddy edges of ponds, if suddenly flushed,
would be the most likely to have muddy feet. Birds of this
order wander more than those of any other; and they are
occasionally found on the most remote and barren islands
of the open ocean; they would not be likely to alight on the
surface of the sea, so that any dirt on their feet would not be
washed off; and when gaining the land, they would be sure to
fly to their natural fresh-water haunts. I do not believe thac
botanists are aware how charged the mud of ponds is with
seeds; I have tried several little experiments, but will here
give only the most striking case : I took in February three
table-spoonfuls of mud from three different points, beneath
water, on the edge of a little pond : this mud when dried
weighed only 6}i ounces; I kept it covered up in my study
for six months, pulling up and counting each plant as it
grew; the plants were of many kinds, and were altogether
537 in number ; and yet the viscid mud was all contained in
a breakfast cup ! Considering these facts, I think it would
be an inexplicable circumstance if water-birds did not trans-
port the seeds of fresh-water plants to unstocked ponds and
streams, situated at very distant points. The same agency
may have come into play with the eggs of some of the
smaller fresh-water animals.
Other and unknown agencies probably have also played a
part. I have stated that fresh-water fish eat some kinds of
seeds, though they reject many other kinds after having
swallowed them; even small fish swallow seeds of moderate
size, as of the yellow water-lily and Potamogeton. Herons
and other birds, century after century, have gone on daily
devouring fish ; they then take flight and go to other waters,
or are blown across the sea ; and we have seen that seeds
retain their power of germination, when rejected many hours
afterwards in pellets or in the excrement. When I saw the
great size of the seeds of that fine water-lily, the Nelumbium,
and remembered Alph. de Candolle's remarks on the distribu-
tion of this plant, I though that the means of its dispersal
INHABITANTS OF OCEANIC ISLANDS 431
must remain inexplicable ; but Audubon states that he found
the seeds of the great southern water-lily (probably, accord-
ing to Dr. Hooker, the Nelumbium luteum ) in a heron's
stomach. Now this bird must often have flown with its
stomach thus well stocked to distant ponds, and then getting
a hearty meal of fish, analogy makes me believe that it
would have rejected the seeds in a pellet in a fit state for
germination.
In considering these several means of distribution, it should
be remembered that when a pond or stream is first formed,
for instance, on a rising islet, it will be unoccupied ; and a
single seed or egg will have a good chance of succeeding.
Although there will always be a struggle for life between
the inhabitants of the same pond, however few in kind, yet
as the number even in a well-stocked pond is small in com-
parison with the number of species inhabiting an equal area
of land, the competition between them will probably be less
severe than between terrestrial species ; consequently an in-
truder from the waters of a foreign country would have a
better chance of seizing on a new place, than in the case
of terrestrial colonists. We should also remember that many
fresh-water productions are low in the scale of nature, and
we have reason to believe that such beings become modified
more slowly than the high ; and this will give time for the
migration of aquatic species. We should not forget the
probability of many fresh-water forms having formerly
ranged continuously over immense areas, and then having
become extinct at intermediate points. But the wide distri-
bution of fresh-water plants and of the lower animals,
whether retaining the same identical form or in some degree
modified, apparently depends in main part on the wide dis-
persal of their seeds and eggs by animals, more especially by
fresh-water birds, which have great powers of flight, and
naturally travel from one piece of water to another.
ON THE INHABITANTS OF OCEANIC ISLANDS
We now come to the last of the three classes of facts,
which I have selected as presenting the greatest amount of
difficulty with respect to distribution, on the view that not
432 ORIGIN OF SPECIES
only all the individuals of the same species have migrated
from some one area, but that allied species, although now
inhabiting the most distant points, have proceeded from a
single area, — the birthplace of their early progenitors. I
have already given my reason for disbelieving in continental
extensions within the period of existing species, on so enor-
mous a scale that all the many islands of the several oceans
were thus stocked with their present terrestrial inhabitants.
This view removes many difficulties, but it does not accord
with all the facts in regard to the productions of islands. In
the following remarks I shall not confine myself to the mere
question of dispersal, but shall consider some other cases
bearing on the truth of the two theories of independent crea-
tion and of descent with modification.
The species of all kinds which inhabit oceanic islands are
few in number compared with those on equal continental
areas : Alph. de Candolle admits this for plants, and Wollas-
ton for insects. New Zealand, for instance, with its lofty
mountains and diversified stations, extending over 780 miles
of latitude, together with the outlying islands of Auckland,
Campbell and Chatham, contain altogether only 960 kinds of
flowering plants ; if we compare this moderate number with
the species which swarm over equal areas in South-Western
Australia or at the Cape of Good Hope, we must admit that
some cause, independently of different physical conditions,
has given rise to so great a difference in number. Even the
uniform county of Cambridge has 847 plants, and the little
island of Anglesea 764, but a few ferns and a few intro-
duced plants are included in these numbers, and the compari-
son in some other respects is not quite fair. We have
evidence that the barren island of Ascension aboriginally
possessed less than half-a-dozen flowering plants; yet many
species have now become naturalised on it, as they have in
New Zealand and on every other oceanic island which can
be named.
In St. Helena there is reason to believe that the natu-
ralised plants and animals have nearly or quite extermi-
nated many native productions. He who admits the doctrine
of the creation of each separate species, will have to admit
that a sufficient number of the best adapted plants and ani-
INHABITANTS OF OCEANIC ISLANDS 433
mals were not created for oceanic islands; for man has unin-
tentionally stocked them far more fully and perfectly than
did nature.
Although in oceanic islands the species are few in number,
the proportion of endemic kinds (i.e. those found nowhere
else in the world) is often extremely large. If we compare,
for instance, the number of endemic land-shells in Madeira,
or of endemic birds in the Galapagos Archipelago, with the
number found on any continent, and then compare the area
of the island with that of the continent, we shall see that this
is true. This fact might have been theoretically expected,
for, as already explained, species occasionally arriving after
long intervals of time in the new and isolated district, and
having to compete with new associates, would be eminently
liable to modification, and would often produce groups of
modified descendants. But it by no means follows that, be-
cause in an island nearly all the species of one class are
peculiar, those of another class, or of another section of the
same class, are peculiar; and this difiference seems to depend
partly on the species which are not modified having immi-
grated in a body, so that their mutual relations have not
been much disturbed ; and partly on the frequent arrival of
unmodified immigrants from the mother-country, with which
the insular forms have intercrossed. It should be borne in
mind that the offspring of such crosses would certainly gain
in vigour ; so that even an occasional cross would produce
more effect than might have been anticipated. I will give a
few illustrations of the foregoing remarks : in the Galapagos
Islands there are 26 land-birds; of these 21 (or perhaps 23)
are peculiar, whereas of the 11 marine birds only 2 are
peculiar ; and it is obvious that marine birds could arrive at
these islands much more easily and frequently than land-
birds. Bermuda, on the other hand, which lies at about the
same distance from North America as the Galapagos Islands
do from South America, and which has a very peculiar soil,
does not possess a single endemic land-bird ; and we know
from Mr. J. M. Jones' admirable account of Bermuda, that
very many North American birds occasionally or even fre-
quently visit this island. Almost every year, as I am in-
formed by Mr. E. V. Harcourt, many European and African
434 ORIGIN OF SPECIES
birds are blown to Madeira; this island is inhabited by 99
kinds, of which one alone is peculiar, though very closely
related to a European form ; and three or four other species
are confined to this island and to the Canaries. So that the
Islands of Bermuda and Madeira have been stocked from
the neighbouring continents with birds, which for long ages
have there struggled together, and have become mutually
co-adapted. Hence when settled in their new homes, each
kind will have been kept by the others to its proper place
and habits, and will consequently have been but little liable
to modification. Any tendency to modification will also have
been checked by intercrossing with the unmodified immi-
grants, often arriving from the mother-country. Madeira
again is inhabited by a wonderful number of peculiar land-
shells, whereas not one species of sea-shell is peculiar to its
shores ; now, though we do not know how sea-shells are dis-
persed, yet we can see that their eggs or larvae, perhaps at-
tached to seaweed or floating timber, or to the feet of wading-
birds, might be transported across three or four hundred
miles of open sea far more easily than land-shells. The dif-
ferent orders of insects inhabiting Madeira present nearly
parallel cases.
Oceanic islands are sometimes deficient in animals of cer-
tain whole classes, and their places are occupied by other
classes ; thus in the Galapagos Islands reptiles, and in New
Zealand gigantic wingless birds, take, or recently took, the
place of mammals. Although New Zealand is here spoken
of as an oceanic island, it is in some degree doubtful whether
it should be so ranked ; it is of large size, and is not sep-
arated from Australia by a profoundly deep sea ; from its
geological character and the direction of its mountain-ranges,
the Rev. W. B. Clarke has lately maintained that this island,
as well as New Caledonia, should be considered as appur-
tenances of Australia. Turning to plants. Dr. Hooker has
shown that in the Galapagos Islands the proportional num-
bers of the different orders are very different from what they
are elsewhere. All such differences in number, and the ab-
sence of certain whole groups of animals and plants, are gen-
erally accounted for by supposed differences in the physical
conditions of the islands; but this explanation is not a little
ABSENCE OF BATRACHIANS 435
doubtful. Facility of immigration seems to have been fully
as important as the nature of the conditions.
Many remarkable little facts could be given M^ith respect
to the inhabitants of oceanic islands. For instance, in cer-
tain islands not tenanted by a single mammal, some of the
endemic plants have beautifully hooked seeds; yet few rela-
tions are more manifest than that hooks serve for the trans-
portal of seeds in the wool or fur of quadrupeds. But a
hooked seed might be carried to an island by other means;
and the plant then becoming modified would form an endemic
species, still retaining its hooks, which would form a useless
appendage like the shrivelled wings under the soldered wing-
covers of many insular beetles. Again, islands often possess
trees or bushes belonging to orders which elsewhere include
only herbaceous species ; now trees, as Alph. de Candolle
has shown, generally have, whatever the cause may be, con-
fined ranges. Hence trees would be little likely to reach dis-
tant oceanic islands ; and an herbaceous plant, which had no
chance of successfully competing with the many fully devel-
oped trees growing on a continent, might, when established
on an island, gain an advantage over other herbaceous plants
by growing taller and taller and overtopping them. In this
case, natural selection would tend to add to the stature of the
plant, to whatever order it belonged, and thus first convert
it into a bush and then into a tree.
ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS ON
OCEANIC ISLANDS.
With respect to the absence of whole orders of animals on
oceanic islands, Bory St. Vincent long ago remarked that
Batrachians (frogs, toads, newts) are never found on any of
the many islands with which the great oceans are studded.
I have taken pains to verify this assertion, and have found
it true, with the exception of New Zealand, New Caledonia,
the Andaman Islands, and perhaps the Salomon Islands and
the Seychelles. But I have already remarked that it is
doubtful whether New Zealand and New Caledonia ought to
be classed as oceanic islands; and this is still more doubtful
with respect to the Andaman and Salomon groups and the
436 .ORIGIN OF SPECIES
Seychelles. This general absence of frogs, toads, and newts
on so many true oceanic islands cannot be accounted for by
their physical conditions : indeed it seems that islands are
peculiarly fitted for these animals; for frogs have been intro-
duced into Madeira, the Azores, and Mauritius, and have
multiplied so as to become a nuisance. But as these animals
and their spawn are immediately killed (with the exception,
as far as known, of one Indian species) by sea-water, there
would be great difficulty in their transportal across the sea,
and therefore we can see why they do not exist on strictly
oceanic islands. But why, on the theory of creation, they
should not have been created there, it would be very difficult
to explain.
Mammals offer another and similar case. I have carefully
searched the oldest voyages, and have not found a single
instance, free from doubt, of a terrestrial mammal (excluding
domesticated animals kept by the natives) inhabiting an island
situated above 300 miles from a continent or great continental
island; and many islands situated at a much less distance are
equally barren. The Falkland Islands, which are inhabited
by a wolf-like fox, come nearest to an exception; but this
group cannot be considered as oceanic, as it lies on a bank
in connection with the mainland at the distance of about 280
miles ; moreover, icebergs formerly brought boulders to its
western shores, and they may have formerly transported
foxes, as now frequently happens in the arctic regions. Yet
it cannot be said that small islands will not support at least
small mammals, for they occur in many parts of the world
on very small islands, when lying close to a continent ; and
hardly an island can be named on which our smaller quadru-
peds have not become naturalised and greatly multiplied. It
cannot be said, on the ordinary view of creation, that there
has not been time for the creation of mammals ; many vol-
canic islands are sufficiently ancient, as shown by the stu-
pendous degradation which they have suffered, and by their
tertiary strata : there has also been time for the production
of endemic species belonging to other classes ; and on conti-
nents it is known that new species of mammals appear and
disappear at a quicker rate than other and lower animals.
Although terrestrial mammals do not occur on oceanic
ABShisCii OF BA'iRACHIANS 437
islands, aerial mammals do occur on almost every island.
New Zealand possesses two bats found nowhere else in the
world: Norfolk Island, the Viti Archipelago, the Bonin
Islands, the Caroline and Marianne Archipelagoes, and Mau-
ritius, all possess their peculiar bats. Why, it may be asked,
has the supposed creative force produced bats and no other
mammals on remote islands ? On my view this question can
easily be answered ; for no terrestrial mammal can be trans-
ported across a wide space of sea, but bats can fly across.
Bats have been seen wandering by day far over the Atlantic
Ocean ; and two North American species either regularly or
occasionally visit Bermuda, at the distance of 600 miles from
the mainland. I hear from Mr. Tomes, who has specially
studied this family, that many species have enormous ranges,
and are found on continents and on far distant islands.
Hence we have only to suppose that such wandering species
have been modified in their new homes in relation to their
new position, and we can understand the presence of endemic
bats on oceanic islands, with the absence of all other terres-
trial mammals.
Another interesting relation exists, namely between the
depth of the sea separating islands from each other or from
the nearest continent, and the degree of affinity of their mam-
malian inhabitants. Mr. Windsor Earl has made some strik-
ing observations on this head, since greatly extended by Mr.
Wallace's admirable researches, in regard to the great Malay
Archipelago, which is traversed near Celebes by a space of
deep ocean, and this separates two widely distinct mam-
malian faunas. On either side the islands stand on a mod-
erately shallow submarine bank, and these islands are inhab-
ited by the same or by closely allied quadrupeds. I have not
as yet had time to follow up this subject in all quarters of
the world ; but as far as I have gone, the relation holds good.
For instance, Britain is separated by a shallow channel from
Europe, and the mammals are the same on both sides ; and so
it is with all the islands near the shores of Australia. The
West Indian Islands, on the other hand, stand on a deeply
submerged bank, nearly 1000 fathoms in depth, and here we
find American forms, but the species and even the genera are
quite distinct. As the an:;punt of modification which animals
438 ORIGIN OF SPECIES
of all kinds undergo partly depends on the lapse of time, and
as the islands which are separated from each other or from
the mainland by shallow channels, are more likely to have
been continuously united within a recent period than the
islands separated by deeper channels, we can understand how
it is that a relation exists between the depth of the sea sep-
arating two mammalijin faunas, and the degree of their
affinity, — a relation which is quite inexplicable on the theory
of independent acts of creation.
The foregoing statements in regard to the inhabitants of
oceanic islands, — namely, the fewness of the species, with a
large proportion consisting of endemic forms — the members
oi certain groups, but not those of other groups in the same
Class, having been modified — the absence of certain whole
orders, as of batrachians and of terrestrial mammals, not-
withstanding the presence of aerial bats, — the singular pro-
portions of certain orders of plants, — herbaceous forms
having been devel ped mto trees, &c., — seem to me to accord
better with the benef in the efficiency of occasional means of
transport, carried on during a long course of time, than with
the belief in th former -onnection of all oceanic islands with
the nearest continent; for on this latter view it is probable
that the various classes would have immigrated more uni-
formly, and from the species having entered in a body their
mutual relations would not have been much disturbed, and
consequently they would either have not been modified, or all
the species in a more equable manner.
I do not deny that there are many and serious difficulties
in understanding how many of the inhabitants of the more
remote islands, whether .^till retaining the same specific form
or subsequently modified, have reached their present homes.
But the probability of other islands having once existed as
halting-places, of which not a wreck now remains, must not
be overlooked. I will specify one difficult case. Almost all
oceanic islands, even the most isolated and smallest, are in-
habited by land-shells, generally by endemic species, but
sometimes by species found elsewhere, — striking instances of
which have been given by Dr. A. A. Gould in relation to the
Pacific. Now it is notorious that land-shells are easily killed
by sea-water ; their eggs, at least such as I have tried, sim: in
INHABITANTS OF ISLANDS 439
it and are killed. Yet there must be some unknown, but
occasionally efficient means for their transportal. Would the
just-hatched young sometimes adhere to the feet of birds
roosting on the ground, and thus get transported? It oc-
curred to me that land-shells, when hybernating and having a
membranous diaphragm over the mouth of the shell, might
be floated in chinks of drifted timber across moderately wide
arms of the sea. And I find that several species in this state
withstand uninjured an immersion in sea- water during seven
days : one shell, the Helix pomatia, after having been thus
treated and again hybernating was put into sea-water for
twenty days, and perfectly recovered. During this length of
time the shell might have been carried by a marine current
of average swiftness, to a distance of 660 geographical miles.
As this Helix has a thick calcareous operculum, I removed
it, and when it had formed a new membranous one, I again
immersed it for fourteen days in sea-water, and again it
recovered and crawled away. Baron Aucapitaine has since
tried similar experiments; he placed 100 land-shells, belong-
ing to ten species, in a box pierced with holes, and immersed
it for a fortnight in the sea. Out of the hundred shells,
twenty-seven recovered. The presence of an operculum
seems to have been of importance, as out of twelve specimens
of Cyclostoma elegans, which is thus furnished, eleven re-
vived. It is remarkable, seeing how well the Helix pomatia
resisted with me the salt-water, that not one of fifty-four
specimens belonging to four other species of Helix tried by
Aucapitaine, recovered. It is, however, not at all probable
that land-shells have often been thus transported ; the feet
of birds offer a more probable method.
ON THE RELATIONS OF THE INHABITANTS OF ISLANDS TO
THOSE OF THE NEAREST MAINLAND
The most striking and important fact for us is the affinity
of the species which inhabit islands to those of the nearest
mainland, without being actually the same. Numerous in-
stances could be given. The Galapagos Archipelago, situ-
ated under the equator, lies at the distance of between 500
and 600 miles from the shores of South America. Here
440 ORIGIN OF SPECIES
almost every product of the land and of the water bears the
unmistakeable stamp of the American continent. There are
twenty-six land-birds; of these, twenty-one, or perhaps
twenty-three, are ranked as distinct species, and would com-
monly be assumed to have been here created: yet the close
affinity of most of these birds to American species is mani-
fest in every character, in their habits, gestures, and tones
of voice. So it is with the other animals, and with a large
proportion of the plants, as shown by Dr. Hooker in his
admirable Flora of this archipelago. The naturalist, looking
at the inhabitants of these volcanic islands in the Pacific,
distant several hundred miles from the continent, feels that
he is standing on American land. Why should this be so?
why should the species which are supposed to have been
created in the Galapagos Archipelago, and nowhere else,
bear so plainly the stamp of affinity to those created in
America? There is nothing in the conditions of life, in the
geological nature of the islands, in their height or climate,
or in the proportions in which the several classes are asso-
ciated together, which closely resembles the conditions of
the South American coast: in fact, there is a considerable
dissimilarity in all these respects. On the other hand, there
is a considerable degree of resemblance in the volcanic na-
ture of the soil, in the climate, height and size of the islands,
between the Galapagos and Cape Verde Archipelagoes : but
what an entire and absolute difference in their inhabitants!
The inhabitants of the Cape Verde Islands are related to
those of Africa, like those of the Galapagos to America.
Facts such as these, admit of no sort of explanation on the
ordinary view of independent creation : whereas on the view
here maintained, it is obvious that the Galapagos Islands
would be likely to receive colonists from America, whether
by occasional means of transport or (though I do not believe
in this doctrine) by formerly continuous land, and the Cape
Verde Islands from Africa ; such colonists would be liable to
modification, — the principle of inheritance still betraying
their original birthplace.
Many analogous facts could be given : indeed it is an al-
most universal rule that the endemic productions of islands
are related to those of the nearest continent, or of the near-
INHABITANTS OF ISLANDS 441
est large island. The exceptions are few, and most of them
can be explained. Thus although Kerguelen Land stands
nearer to Africa than to America, the plants are related, and
that very closely, as we know from Dr. Hooker's account,
to those of America : but on the view that this island has
been mainly stocked by seeds brought with earth and stones
on icebergs, drifted by the prevailing currents, this anomaly
disappears. New Zealand in its endemic planes is much
more closely related to Australia, the nearest mainland, than
to any other region : and this is what might have been ex-
pected ; but it is also plainly related to South America, which,
although the next nearest continent, is so enormously remote,
that the fact becomes an anomaly. But this difficulty par-
tially disappears on the view that New Zealand, South
America, and the other southern lands have been stocked in
part from a nearly intermediate though distant point, namely
from the antarctic islands, when they were clothed with vege-
tation, during a warmer tertiary period, before the com-
mencement of the last Glacial period. The affinity, which
though feeble, I am assured by Dr. Hooker is real, between
the flora of the south-western corner of Australia and of the
Cape of Good Hope, is a far more remarkable case : but this
affinity is confined to the plants, and will, no doubt, some day
be explained.
The same law which has determined the relationship be-
tween the inhabitants of islands and the nearest mainland, is
sometimes displayed on a small scale, but in a most interest-
ing manner, within the limits of the same archipelago. Thus
each separate island of the Galapagos Archipelago is ten-
anted, and the fact is a marvellous one, by many distinct
species ; but these species are related to each other in a very
much closer manner than to the inhabitants of the American
continent, or of any other quarter of the world. This is
what might have been expected, for islands situated so near
to each other would almost necessarily receive immigrants
from the same original source, and from each other. But
how is it that many of the immigrants have been differently
modified, though only in a small degree, in islands situated
within sight of each other, having the same geological na-
ture, the same height, climate, &c. ? This long appeared to
442 ORIGIN OF SPECIES
me a great difficulty: but it arises in chief part from the
deeply-seated error of considering the physical conditions of
a country as the most important : whereas it cannot be dis-
puted that the nature of the other species with which each
has to compete, is at least as important, and generally a far
more important element of success. Now if we look to the
species which inhabit the Galapagos Archipelago, and are
likewise found in other parts of the world, we find that they
differ considerably in the several islands. This difference
might indeed have been expected if the islands have been
stocked by occasional means of transport — a seed, for in-
stance, of one plant having been brought to one island, and
that of another plant to another island, though all proceeding
from the same general source. Hence, when in former times
an immigrant first settled on one of the islands, or when it
subsequently spread from one to another, it would undoubt-
edly be exposed to different conditions in the different islands,
for it would have to compete with a different set of organ-
isms ; a plant, for instance, would find the ground best fitted
for it occupied by somewhat different species in the different
islands, and would be exposed to the attacks of somewhat
different enemies. If then it varied, natural selection would
probably favour different varieties in the different islands.
Some species, however, might spread and yet retain the same
character throughout the group, just as we see some species
spreading widely throughout a continent and remaining the
same.
The really surprising fact in this case of the Galapagos
Archipelago, and in a lesser degree in some analogous cases,
is that each new species after being formed in any one island,
did not spread quickly to the other islands. But the islands,
though in sight of each other, are separated by deep arms of
the sea, in most cases wider than the British Channel, and
there is no reason to suppose that they have at any former
period been continuously united. The currents of the sea are
rapid and sweep between the islands, and gales of wind are
extraordinarily rare ; so that the islands are far more effect-
ually separated from each other than they appear on a map.
Nevertheless some of the species, both of those found in other
parts of the world and of those confined to the archipelago,
INHABITANTS OF ISLANDS 443
are common to the several islands ; and we may infer from
their present manner of distribution, that they have spread
from one island to the others. But we often take, I think, an
erroneous view of the probability of closely-allied species in-
vading each other's territory, when put into tree intercom-
munication. Undoubtedly, if one species has any advantage
over another, it will in a very brief time wholly or in part
supplant it; but if both are equally well fitted for their own
places, both will probably hold their separate places for al-
most any length of time. Being familiar with the fact that
many species, naturalised through man's agency, have
spread with astonishing rapidity over wide areas, we are apt
to infer that most species would thus spread; but we should
remember that the species which become naturalised in new
countries are not generally closely allied to the aboriginal
inhabitants, but are very distinct forms, belonging in a large
proportion of cases, as shown by Alph. de Candolle, to dis-
tinct genera. In the Galapagos Archipelago, many even ot
the birds, though so well adapted for flying from island to
island, dilTer on the different islands; thus there are three
closely-allied species of mocking-thrush, each confined to its
own island. Now let us suppose the mocking-thrush of Chat-
ham Island to be blown to Charles Island, which has its own
mocking-thrush ; why should it succeed in establishing itself
there? We may safely infer that Charles Island is well
stocked with its own species, for annually more eggs are laid
and young birds hatched, than can possibly be reared; and
we may infer that the mocking-thrush peculiar to Charles
Island is at least as well fitted for its home as is the species
peculiar to Chatham Island. Sir C. Lyell and Mr. Wollaston
have communicated to me a remarkable fact bearing on this
subject; namely, that Madeira and the adjoining islet of
Porto Santo possess many distinct but representative species
of land-shells, some of which live in crevices of stone; and
although large quantities of stone are annually transported
from Porto Santo to Madeira, yet this latter island has not
become colonised by the Porto Santo species ; nevertheless
both islands have been colonised by European land-shells,
which no doubt had some advantage over the indigenous spe-
cies. From these considerations I think we need not greatly
444 ORIGIN OF SPECIES
marvel at the endemic species which inhabit the several
islands of the Galapagos Archipelago, not having all spread
from island to island. On the same continent, also, preoccu-
pation has probably played an important part in checking
the commingling of the species which inhabit different dis-
tricts with nearly the same physical conditions. Thus, the
south-east and south-west corners of Australia have nearly
the same physical conditions, and are united by continuous
land, yet they are inhabited by a vast number of distinct
mammals, birds, and plants ; so it is, according to Mr. Bates,
with the butterflies and other animals inhabiting the great,
open, and continuous valley of the Amazons.
The same principle which governs the general character of
the inhabitants of oceanic islands, namely, the relation to the
source whence colonists could have been most easily derived,
together with their subsequent modification, is of the widest
application throughout nature. We see this on every moun-
tain-summit, in every lake and marsh. For Alpine species,
excepting in as far as the same species have become widely
spread during the Glacial epoch, are related to those of the
surrounding lowlands ; thus we have in South America, Al-
pine humming-birds, Alpine rodents, Alpine plants, &c., all
strictly belonging to American forms ; and it is obvious that
a mountain, as it became slowly upheaved, would be colonised
from the surrounding lowlands. So it is with the inhabitants
of lakes and marshes, excepting in so far as great facility of
transport has allowed the same forms to prevail throughout
large portions of the world. We see this same principle in
the character of most of the blind animals inhabiting the
caves of America and of Europe. Other analogous facts
could be given. It will, I believe, be found universally true,
that wherever in two regions, let them be ever so distant,
many closely allied or representative species occur, there \,.'A
likewise be found some identical species ; and wherever many
closely-allied species occur, there will be found many forms
which some naturalists rank as distinct species, and others as
mere varieties ; these doubtful forms showing us the steps in
the progress of modification.
The relation between the power and extent of migration in
certain species, either at the present or at some former pe-
INHABITANTS OF ISLANDS 44S
riod, and the existence at remote points of the world of
closely-allied species, is shown in another and more general
way. Mr. Gould remarked to me long ago, that in those
genera of birds which range over the world, many of the
species have very wide ranges. I can hardly doubt that this
rule is generally true, though difficult of proof. Amongst
mammals, we see it strikingly displayed in Bats, and in a
lesser degree in the Felidse and Canidse. We see the same
rule in the distribution of butterflies and beetles. So it is
with most of the inhabitants of fresh water, for many of the
genera in the most distinct classes range over the world, and
many of the species have enormous ranges. It is not meant
that all, but that some of the species have very wide ranges
in the genera which range very widely. Nor is it meant, that
the species in such genera have on an average a very wide
range ; for this will largely depend on how far the process
of modification has gone ; for instance, two varieties of the
same species inhabit America and Europe, and thus the spe-
cies has an immense range; but, if variation were to be car-
ried a little further, the two varieties would be ranked as
distinct species, and their range would be greatly reduced.
Still less is it meant, that species which have the capacity of
crossing barriers and ranging widely, as in the case of cer-
tain powerfully-winged birds, will necessarily range widely;
for we should never forget that to range widely implies not
only the power of crossing barriers, but the more important
power of being victorious in distant lands in the struggle for
life with foreign associates. But according to the view that
all the species of a genus, though distributed to the most
remote points of the world, are descended from a single pro-
genitor, we ought to find, and I believe as a general rule we
do find, that some at least of the species range very widely.
We should bear in mind that many genera in all classes are
of ancient origin, and the species in this case will have had
ample time for dispersal and subsequent modification. There
is also reason to believe from geological evidence that within
each great class the lower organisms change at a slower rate
than the higher ; consequently they will have had a better
chance of ranging widely and of still retaining the same spe-
cific character. This fact, together with that of the seeds
446 ORIGIN OF SPECIES
and eggs of most lowly organised forms being very minute
and better fitted for distant transportal, probably accounts for
a law which has long been observed, and which has lately
been discussed by Alph. de Candolle in regard to plants,
namely, that the lower any group of organisms stands the
more widely it ranges.
The relations just discussed, — namely, lower organisms
ranging more widely than the higher, — some of the species of
widely-ranging genera themselves ranging widely, — such
facts, as alpine, lacustrine, and marsh productions being gen-
erally related to those which live on the surrounding low
lands and dry lands, — the striking relationship between the
inhabitants of islands and those of the nearest mainland —
the still closer relationship of the distinct inhabitants of the
islands in the same archipelago — are inexplicable on the ordi-
nary view of the independent creation of each species, but
are explicable if we admit colonisation from the nearest or
readiest source, together with the subsequent adaptation of
the colonists to their new homes
SUMMARY OF THE LAST AND PRESENT CHAPTERS
In these chapters I have endeavoured to show, that if we
make due allowance for our ignorance of the full effects of
changes of climate and of the level of the land, which have
certainly occurred within the recent period, and of other
changes which have probably occurred, — if we remember
how ignorant we are with respect to the many curious means
of occasional transport, — if we bear in mind, and this is a
very important consideration, how often a species may have
ranged continuously over a wide area, and then have become
extinct in the intermediate tracts, — the difficulty is not insu-
perable in believing that all the individuals of the sam.e
species, wherever found, are descended from common par
ents. And we are led to this conclusion, which has been ar-
rivedatbymany naturalists under the designation of single cen-
tres of creation, by various general considerations, more espe-
cially from the importance of barriers of all kinds, and from
the analogical distribution of sub-genera, genera, and families.
With respect to distinct species belonging to the same
SUMMARY 447
genus, which on our theory have spread from one parent-
source; if we make the same allowances as before for our
ignorance, and remember that some forms of life have
changed very slowly, enormous periods of time having been
thus granted for their migration, the difificulties are far from
insuperable; though in this case, as in that of the individuals
of the same species, they are often great.
As exemplifying the effects of climatal changes on distribu-
tion, I have attempted to show how important a part the last
Glacial period has played, which affected even the equa-
torial regions, and which, during the alternations of the cold
in the north and south, allowed the productions of opposite
hemispheres to mingle, and left some of them stranded on the
mountain-summits in all parts of the world. As showing how
diversified are the means of occasional transport, I have dis-
cussed at some little length the means of dispersal of fresh-
water productions.
If the difificulties be not insuperable in admitting that in the
long course of time all the individuals of the same species,
and likewise of the several species belonging to the same
genus, have proceeded from some one source ; then all the
grand leading facts of geographical distribution are explic-
able on the theory of migration, together with subsequent
modification and the multiplication of new forms. We can
thus understand the high importance of barriers, whether
of land or water, in not only separating, but in apparently
forming the several zoological and botanical provinces. We
can thus understand the concentration of related species within
the same areas ; and how it is that under different latitudes,
for instance in South America, the inhabitants of the plains
and mountains, of the forests, marshes, and deserts, are
linked together in so mysterious a manner, and are likewise
linked to the extinct beings which formerly inhabited the
same continent. Bearing in mind that the mutual relation
of organism to organism is of the highest importance, we can
see why two areas having nearly the same physical condi-
tions should often be inhabited by very different forms of
life ; for according to the length of time which has elapsed
since the colonists entered one of the regions, or both ; ac-
cording to the nature of the communication which allowed
448 ORIGIN OF SPECIES
certain forms and not others to enter, either in greater or
lesser numbers ; according or not, as those which entered
happened to come into more or less direct competition with
each other and with the aborigines : and according as the im-
migrants were capable of varying more or less rapidly, there
would ensue in the two or more regions, independently of
their physical conditions, infinitely diversified conditions of
life, — there would be an almost endless amount of organic
action and reaction, — and we should find some groups of
beings greatly, and some only slightly modified, — some de-
veloped in great force, some existing in scanty numbers —
and this we do find in the several great geographical prov-
inces of the world.
On these same principles we can understand, as I have
endeavoured to show, why oceanic islands should have few
inhabitants, but that of these, a large proportion should be
endemic or peculiar; and why, in relation to the means of
migration, one group of beings should have all its species pe-
culiar, and another group, even within the same class, should
have all its species the same with those in an adjoining
quarter of the world. We can see why whole groups of or-
ganisms, as batrachians and terrestrial mammals, should be
absent from oceanic islands, whilst the most isolated islands
should possess their own peculiar species of aerial mammals
or bats. We can see why, in islands, there should be some
relation between the presence of mammals, in a more or less
modified condition, and the depth of the sea between such
islands and the mainland. We can clearly see why all the
inhabitants of an archipelago, though specifically distinct on
the several islets, should be closely related to each other;
and should likewise be related, but less closely, to those of
the nearest continent, or other source whence immigrants
might have been derived. We can see why, if there exist
very closely allied or representative species in two areas,
however distant from each other, some identical species will
almost always there be found.
As the late Edward Forbes often insisted, there is a strik-
ing parallelism in the laws of life throughout time and
space; the laws governing the succession of forms in past
times being nearly the same with those governing at the
SUMMARY 449
present time the differences in different areas. We see this
in many facts. The endurance of each species and group of
species is continuous in time ; for the apparent exceptions to
the rule are so few, that they may fairly be attributed to our \
not having as yet discovered in an intermediate deposit cer-
tain forms which are absent in it, but which occur both
above and below: so in space, it certainly is the general rule
that the area inhabited by a single species, or by a group of
species, is continuous, and the exceptions, which are not rare,
may, as I have attempted to show, be accounted for by
former migrations under dift'erent circumstances, or through
occasional means of transport, or by the species having be-
come extinct in the intermediate tracts. Both in time and
space species and groups of species have their points of maxi-
mum development. Groups of species, living during the
same period of time, or living within the same area, are often
characterised by trifling features in common, as of sculpture
or colour. In looking to the long succession of past ages, as
in looking to distant provinces throughout the world, we find
that species in certain classes differ little from each other,
whilst those in another class, or only in a different section of
the same order, differ greatly from each other. In both time
and space the lowly organised members of each class gen-
erally change less than the highly organised; but there are
in both cases marked exceptions to the rule. According to
our theory, these several relations throughout time and
space are intelligible ; for whether we look to the allied forms
of life which have changed during successive ages, or to
those which have changed after having migrated into distant
quarters, in both cases they are connected by the same bond '
of ordinary generation; in both cases the laws of variation
have been the same, and modifications have been accumulated
by the same means of natural selection.
— HCXI
CHAPTER XIV
Mutual Affinities of Organic Beings: Morphology:
Embryology: Rudimentary Organs
Classification, groups subordinate to groups — Natural system— Rules
and difficulties in classification, explained on the theory of
descent with modification — Classification of varieties — Descent
always used in classification — Analogical or adaptive characters
— Affinities, general, complex, and radiating — Extinction sepa-
rates and defines groups — Morphology, between members of
the same class, between parts of the same individual —
Embryology, laws of, explained by variations not supervening
at an early age, and being inherited at a corresponding age —
Rudimentary organs ; their origin explained — Summary.
classification
FROM the most remote period in the history of the world
organic beings have been found to resemble each other
in descending degrees, so that they can be classed in
groups under groups. This classification is not arbitrary like
the grouping of the stars in constellations. The existence of
groups would have been of simple significance, if one group
had been exclusively fitted to inhabit the land, and another
the water; one to feed on flesh, another on vegetable matter,
and so on; but the case is widely different, for it is notorious
how commonly members of even the same sub-group have
different habits. In the second and fourth chapters, on Vari-
ation and on Natural Selection, I have attempted to show
that within each country it is the widely ranging, the much
diffused and common, that is the dominant species, belonging
to the larger genera in each class, which vary most. The
varieties, or incipient species, thus produced, ultimately be-
come converted into new and distinct species ; and these, on
the principle of inheritance, tend to produce other new and
dominant species. Consequently the groups which are now
large, and which generally include many dominant species,
450
CLASSIFICATION 451
tend to go on increasing in size. I furlher attempted to show
that from the varying descendants of each species trying to
occupy as many and as different places as possible in the
economy of nature, they constantly tend to diverge in char-
acter. This latter conclusion is supported by observing the
great diversity of forms which, in any small area, come into
the closest competition, and by certain facts in natural-
isation.
I attempted also to show that there is a steady tendency in
the forms which are increasing in number and diverging in
character, to supplant and exterminate the preceding, less
divergent and less improved forms. I request the reader to
turn to the diagram illustrating the action, as formerly ex-
plained, of these several principles ; and he will see that the
inevitable result is, that the modified descendants proceeding
from one progenitor become broken up into groups subordi-
nate to groups. In the diagram each letter on the uppermost
line may represent a genus including several species ; and the
whole of the genera along this upper line form together one
class, for all are descended from one ancient parent, and,
consequently, have inherited something in common. But the
three genera on the left hand have, on this same principle,
much in common, and form a sub-family, distinct from that
containing the next two genera on the right hand, which
diverged from a common parent at the fifth stage of descent.
These five genera have also much in common, though less
than when grouped in sub- families ; and they form a family
distinct from that containing the three genera still farther to
the right hand, which diverged at an earlier period. And all
these genera, descended from (A), form an order distinct
from the genera descended from (I). So that we here have
many species descended from a single progenitor grouped
into genera ; and the genera into sub-families, families, and
orders, all under one great class. The grand fact of the
natural subordination of organic beings in groups under
groups, which, from its familiarity, does not always suffi-
ciently strike us, is in my judgment thus explained. No
doubt organic beings, like all other objects, can be classed in
many ways, either artificially by single characters, or more
naturally by a number of characters. We know, for instance.
452 ORIGIN OF SPECIES
that minerals and the elemental substances can be thus ar-
ranged. In this case there is of course no relation to gene-
alogical succession, and no cause can at present be assigned
for their falling into groups. But with organic beings the
case is different, and the view above given accords with their
natural arrangement in group under group; and no other
explanation has ever been attempted.
Naturalists, as we have seen, try to arrange the species,
genera, and families in each class, on what is called the
Natural System. But what is meant by this system? Some
authors look at it merely as a scheme for arranging together
those living objects which are most alike, and for separating
those which are most unlike ; or as an artificial method of
enunciating, as briefly as possible, general propositions, —
that is, by one sentence to give the characters common, for
instance, to all mammals, by another those common to all
carnivora, by another those common to the dog-genus, and
then, by adding a single sentence, a full description is given
of each kind of dog. The ingenuity and utility of this system
are indisputable. But many naturalists think that som.ething
more is meant by the Natural System; they believe that it
reveals the plan of the Creator; that unless it be specified
whether order in time or space, or both, or what else is meant
by the plan of the Creator, it seems to me that nothing is
thus added to our knowledge. Expressions such as that fa-
mous one by Linnaeus, which we often meet with in a more
or less concealed form, namely, that the characters do not
make the genus, but that the genus gives the characters, seem
to imply that some deeper bond is included in our classifica-
tions than mere resemblance. I believe that this is the case,
and that community of descent — the one known cause of close
similarity in organic beings — is the bond, which though ob-
served by various degrees of modification, is partially re-
vealed to us by our classifications
Let us now consider the rules followed in classification,
and the difficulties which are encountered on the view that
classification either gives some unknown plan of creation, or
is simply a scheme for enunciating general propositions and
of placing together the forms most like each other. It might
have been thought (and was in ancient times thought) that
CLASSIFICATION 453
those parts of the structure which determined the habits of
Hfe, and the general place of each being in the economy of
nature, would be of very high importance in classification.
Nothing can be more false. No one regards the external
similarity of a mouse to a shrew, of a dugong to a whale, of
a whale to a fish, as of any importance. These resemblances,
though so intimately connected with the whole life of the
being, are ranked as merely "adaptive or analogical charac-
ters ;" but to the consideration of these resemblances we
shall recur. It may even be given as a general rule, that the
less any part of the organisation is concerned with special
habits, the more important it becomes for classification. As
an instance : Owen, in speaking of the dugong, says, "The
generative organs, being those which are most remotely re-
lated to the habits and food of an animal, I have always
regarded as affording very clear indications of its true
affinities. We are least likely in the modifications of these
organs to mistake a merely adaptive for an essential char-
acter." With plants how remarkable it is that the organs
of vegetation, on which their nutrition and life depend, are
of little signification ; whereas the organs of reproduction,
with their product the seed and embryo, are of paramount
importance ! So again in formerly discussing certain mor-
phological characters which are not functionally important,
we have seen that they are often of the highest service in
classification. This depends on their constancy throughout
many allied groups ; and their constancy chiefly depends on
any slight deviations not having been preserved and accumu-
lated by natural selection, which acts only on serviceable
characters.
That the mere physiological importance of an organ does
not determine its classificatory value, is almost proved by the
fact, that in allied groups, in which the same organ, as we
have every reason to suppose, has nearly the same physiolog-
ical value, its classificatory value is widely different. No
naturalist can have worked long at any group without being
struck with this fact; and it has been fully acknowledged in
the writings of almost every author. It will suffice to quote
the highest authority, Robert' Brown, who, in speaking of
certain organs in the Proteaceae, says their generic impor-
454 ORIGIN OF SPECIES
tance, "like that of all their parts, not only in this, but, as I
apprehend, in every natural family, is very unequal^ and in
some cases seems to be entirely lost." Again, in another
work he says, the genera of the Connaraceae "differ in having
one or more ovaria, in the existence or absence of albumen,
in the imbricate or valvular aestivation. Any one of these
characters singly is frequently of more than generic impor-
tance, though here even when all taken together they appear
insufficient to separate Cnestis from Connarus." To give an
example amongst insects : in one great division of the Hy-
menoptera, the antennae, as Westwood has remarked, are
most constant in structure ; in another division they differ
much, and the differences are of quite subordinate value in
classification; yet no one will say that the antennae in these
two divisions of the same order are of unequal physiological
importance. Any number of instances could be given of the
varying importance for classification of the same important
organ within the same group of beings.
Again, no one will say that rudimentary or atrophied or-
gans are of high physiological or vital importance ; yet, un-
doubtedly, organs in this condition are often of much value
in classification. No one will dispute that the rudimentary
teeth in the upper jaws of young ruminants, and certain
rudimentary bones of the leg, are highly serviceable in ex-
hibiting the close affinity between ruminants and pachyderms,
Robert Brown has strongly insisted on the fact that the posi-
tion of the rudimentary florets is of the highest importance
in the classification of the grasses.
Numerous instances could be given of characters derived
from parts which must be considered of very trifling physio-
logical importance, but which are universally admitted as
highly serviceable in the definition of whole groups. For in-
stance, whether or not there is an open passage from the
nostrils to the mouth, the only character, according to Owen,
which absolutely distinguishes fishes and reptiles — the inflec-
tion of the angle of the lower jaw in Marsupials — the man-
ner in which the wings of insects are folded — mere colour in
certain Algse — mere pubescence on parts of the flower in
grasses — the nature of the dermal covering, as hair or
feathers, in the Vertebrata. If the Ornithorhynchus had
CLASSIFICATION 455
been covered with feathers instead of hair, this external and
trifling character would have been considered by naturalists
as an important aid in determining the degree of affinity of
this strange creature to birds.
The importance, for classification, of trifling characters,
mainly depends on their being correlated with many other
characters of more or less importance. The value indeed of
an aggregate of characters is very evident in natural history.
Hence, as has often been remarked, a species may depart
from its allies in several characters, both of high physiologi-
cal importance, and of almost universal prevalence, and yet
leave us in no doubt where it should be ranked. Hence, also,
it has been found that a classification founded on any single
character, however important that may be, has always failed;
for no part of the organisation is invariably constant The
importance of an aggregate of characters, even when none
are important, alone explains the aphorism enunciated by
Linnaeus, namely, that the characters do not give the genus,
but the genus gives the characters ; for this seems founded
on the appreciation of many trifling points of resemblance, too
slight to be defined. Certain plants, belonging to the Mal-
pighiacese, bear perfect and degraded flowers; in the latter,
as A. de Jussieu has remarked, "the greater number of the
characters proper to the species, to the genus, to the family,
to the class, disappear, and thus laugh at our classification."
When Aspicarpa produced in France, during several years,
only these degraded flowers, departing so wonderfully in a
number of the most important points of structure from the
proper type of the order, yet M. Richard sagaciously saw,
as Jussieu observes, that this genus should still be retained
amongst the Malpighiacese. This case well illustrates the
spirit of our classifications.
Practically, when naturalists are at work, they do not
trouble themselves about the physiological value of the char-
acters which they use in defining a group or in allocating any
particular species. If they find a character nearly uniform,
and common to a great number of forms, and not common
to others, they use it as one of high value ; if common to
some lesser number, they useit as of subordinate value. This
principle has been broadly confessed by some naturalists to
456 ORIGIN OF SPECIES
be the true one ; and by none more clearly than by that ex-
cellent botanist, Aug. St. Hilaire. If several trifling char-
acters are always found in combination, though no apparent
bond of connection can be discovered between them, especial
value is set on them. As in most groups of animals, impor-
tant organs, such as those for propelling the blood, or for
aerating it, or those for propagating the race, are found
nearly uniform, they are considered as highly serviceable in
classification ; but in some groups all these, the most impor-
tant vital organs, are found to offer characters of quite sub-
ordinate values. Thus, as Fritz Miiller has lately remarked,
in the same group of crustaceans, Cypridina is furnished with
a heart, whilst in too closely allied genera, namely Cypris
and Cytherea, there is no such organ ; one species of Cypri-
dina has well-developed branchis, whilst another species is
destitute of them.
We can see why characters derived from the embryo
should be of equal importance with those derived from the
adult, for a natural classification of course includes all ages.
But it is by no means obvious, on the ordinary view, why
the structure of the embryo should be more important for
this purpose than that of the adult, which alone plays its full
part in the economy of nature. Yet it has been strongly
urged by those great naturalists, Milne Edwards and Agassiz,
that embryological characters are the most important of all;
and this doctrine has very generally been admitted as true.
Nevertheless, their importance has sometimes been exag-
gerated, owing to the adaptive characters of larvae not hav-
ing been excluded; in order to show this, Fritz Miiller
arranged by the aid of such characters alone the great class
of crustaceans, and the arrangement did not prove a natural
one. But there can be no doubt that embryonic, excluding
larval characters, are of the highest value for classification,
not only with animals but with plants. Thus the main di-
visions of flowering plants are founded on differences in the
embryo, — on the number and position of the cotyledons, and
on the mode of development of the plumule and radicle. We
shall immediately see why these characters possess so high a
value in classification, namely, from the natural system being
genealogical in its arrangement.
CLASSIFICATION 457
Our classifications are often plainly influenced by chains
of afiinities. Nothing can be easier than to define a number
of characters common to all birds ; but with crustaceans, any
such definition has hitherto been found impossible. There
are crustaceans at the opposite ends of the series, which have
hardly a character in common; yet the species at both ends,
from being plainly allied to others, and these to others,
and so onwards, can be recognised as unequivocally belonging
to this, and to no other class of the Articulata.
Geographical distribution has often been used, though
perhaps not quite logically, in classification, more especially
in very large groups of closely allied forms. Temminck in-
sists on the utility or even necessity of this practice in certain
groups of birds; and it has been followed by several ento-
mologists and botanists.
Finally, with respect to the comparative value of the vari-
ous groups of species, such as orders, sub-orders, families,
sub-families, and genera, they seem to be, at least at present,
almost arbitrary. Several of the best botanists, such as Mr.
Bentham and others, have strongly insisted on their arbi-
trary value. Instances could be given amongst plants and
insects, of a group first ranked by practised naturalists
as only a genus, and then raised to the rank of a sub-family
or family; and this has been done, not because further re-
search has detected important structural differences, at
first overlooked, but because numerous allied species with
slightly different grades of difference, have been subse-
quently discovered.
All the foregoing rules and aids and difficulties in classifi-
cation may be explained, if I do not greatly deceive myself,
on the view that the Natural System is founded on descent
with modification ; — that the characters which naturalists
consider as showing true affinity between any two or more
species, are those which have been inherited from a common
parent, all true classification being genealogical ; — that com-
munity of descent is the hidden bond which naturalists have
been unconsciously seeking, and not some unknown plan of
creation, or the enunciation of general propositions, and the
mere putting together and separating objects more or less
alike.
458 ORIGIN OF SPECIES
But I must explain my meaning more fully. I believe that
the arrangement of the groups within each class, in due sub-
ordination and relation to each other, must be strictly genea-
logical in order to be natural ; but that the amount of differ-
ence in the several branches or groups, though allied in the
same degree in blood to their common progenitor, may differ
greatly, being due to the different degrees of modification
which they have undergone ; and this is expressed by the
forms being ranked under different genera, families, sections,
or orders. The reader will best understand what is meant,
if he will take the trouble to refer to the diagram in the
fourth chapter. We will suppose the letters A to L to repre-
sent allied genera existing during the Silurian epoch, and
descended from some still earlier form. In three of these
genera (A, F, and I), a species has transmitted modified de-
scendants to the present day, represented by the fifteen
genera (a" to 2") on the uppermost horizontal line. Now
all these modified descendants from a single species, are re-
lated in blood or descent in the same degree ; they may meta-
phorically be called cousins to the same millionth degree ;
yet they differ widely and in different degrees from each
other. The forms descended from A, now broken up into
two or three families, constitute a distinct order from those
descended from I, also broken up into two families. Nor
can the existing species, descended from A, be ranked in the
same genus with the parent A; or those from I, with the
parent I. But the existing genus F^* may be supposed to have
been but slightly modified; and it will then rank with the
parent-genus F; just as some few still living organisms be-
long to Silurian genera. So that the comparative value of
the differences between these organic beings, which are all
related to each other in the same degree in blood, has come
to be widely different. Nevertheless their genealogical
arrangement remains strictly true, not only at the present
time, but at each successive period of descent. All the modi-
fied descendants from A will have inherited something in
common from their common parent, as will all the descend-
ants from I ; so will it be with each subordinate branch of
descendants, at each successive stage. If, however, we sup-
pose any descendant of A, or of I, to have become so much
CLASSIFICATION 459
modified as to have lost all traces of its parentage, in this
case, its place in the natural system will be lost, as seems
to have occurred with some few existing organisms. All
the descendants of the genus F, along its whole line of
descent, are supposed to have been but little modified, and
they form a single genus. But this genus, though much
isolated, will still occupy its proper intermediate position.
The representation of the groups, as here given in the dia-
gram on a flat surface, is much too simple. The branches
ought to have diverged in all directions. If the names of
the groups had been simply written down in a linear series,
the representation would have been still less natural ; and it
is notoriously not possible to represent in a series, on a flat
surface, the affinities which we discover in nature amongst
the beings of the same group. Thus, the natural system is
genealogical in its arrangement, like a pedigree : but the
amount of modification which the different groups have
undergone has to be expressed by ranking them under differ-
ent so-called genera, sub-families, families, sections, orders,
and classes.
It may be worth while to illustrate this view of classifica-
tion, by taking the case of Tanguages. If we possessed a
perfect pedigree of mankind, a genealogical arrangement of
the races of man would afford the best classification of the
various languages now spoken throughout the world ; and if
all extinct languages, and all intermediate and slowly chang-
ing dialects, were to be included, such an arrangement would
be the only possible one. Yet it might be that some ancient
languages had altered very little and had given rise to few
new languages, whilst others had altered much owing to the
spreading, isolation, and state of civilisation of the several
co-descended races, and had thus given rise to many new
dialects and languages. The various degrees of difference
between the languages of the same stock, would have to be
expressed by groups subordinate to groups ; but the proper
or even the only possible arrangement would still be genea-
logical; and this would be strictly natural, as it would con-
nect together all languages, extinct and recent, by the closest
affinities, and would give_ the filiation and origin of each
tongue.
460 ORIGIN OF SPECIES
In confirmation of this view, let us glance at the classifi-
cation of varieties, which are known or believed to be
descended from a single species. These are grouped under
the species, with the sub-varieties under the varieties; and
in some cases, as with the domestic pigeon, with several
other grades of difference. Nearly the same rules are fol-
lowed as in classifying species. Authors have insisted on
the necessity of arranging varieties on a natural instead of
an artificial system; we are cautioned, for instance, not to
class two varieties of the pine-apple together, merely because
their fruit, though the most important part, happens to be
nearly identical ; no one puts the Swedish and common turnip
together, though the esculent and thickened stems are so
similar. Whatever part is found to be most constant, is used
in classing varieties; thus the great agriculturist Marshall
says the horns are very useful for this purpose with cattle,
because they are less variable than the shape or colour of the
body, &c. ; whereas with sheep the horns are much less serv-
iceable, because less constant. In classing varieties, I
apprehend that if we had a real pedigree, a genealogical
classification would be universally preferred; and it has been
attempted in some cases. For we might feel sure, whether
there had been more or less modification, that the principle
of inheritance would keep the forms together which were
allied in the greatest number of points. In tumbler pigeons,
though some of the sub-varieties differ in the important
character of the length of the beak, yet all are kept together
from having the common habit of tumbling; but the short-
faced breed has nearly or quite lost this habit; nevertheless,
without any thought on the subject, these tumblers are kept
in the same group, because allied in blood and alike in some
other respects.
With species in a state of nature, every naturalist has in
fact brought descent into his classification; for he includes
in his lowest grade, that of species, the two sexes ; and how
enormously these sometimes differ in the most important
characters, is known to every naturalist : scarcely a single
fact can be predicated in common of the adult males and
hermaphrodites of certain cirripedes, and yet no one dreams
of separating them. As soon as the three Orchidean forms.
CLASSIFICATION 461
Monachanthus, Myanthus, and Catasetum, which had previ-
ously been ranked as three distinct genera, were known to be
sometimes produced on the same plant, they were immedi-
ately considered as varieties ; and now I have been able to
show that they are the male, female, and hermaphrodite
forms of the same species. The naturalist includes as one
species the various larval stages of the same individual, how-
ever much they may differ from each other and from the
adult, as well as the so-called alternate generations of Steen-
strup, which can only in a technical sense be considered as
the same individual. He includes monsters and varieties, not
from their partial resemblance to the parent-form, but be-
cause they are descended from it.
As descent has universally been used in classing together
the individuals of the same species, though the males and
females and larvae are sometimes extremely different; and as
it has been used in classing varieties which have undergone
a certain, and sometimes a considerable amount of modifica-
tion, may not this same element of descent have been uncon-
sciously used in grouping species under genera, and genera
under higher groups, all under the so-called natural system?
I believe it has been unconsciously used ; and thus only can
I understand the several rules and guides which have been
followed by our best systematists. As we have no written
pedigrees, we are forced to trace community of descent by
resemblances of any kind. Therefore we choose those char-
acters which are the least likely to have been modified, in
relation to the conditions of life to which each species has
been recently exposed. Rudimentary structures on this view
are as good as, or even sometimes better than, other parts
of the organisation. We care not how trifling a character
may be — let it be the mere inflection of the angle of the jaw,
the manner in which an insect's wing is folded, whether the
skin be covered by hair or feathers — if it prevail through-
out many and different species, especially those having very
different habits of life, it assumes high value ; for we can
account for its presence in so many forms with such differ-
ent habits, only by inheritance from a common parent. We
may err in this respect in regard to single points of structure,
but when several characters, let them be ever so trifling.
462 ORIGIN OF SPECIES
concur throughout a large group of beings having different
habits, we may feel almost sure^ on the theory of descent,
that these characters have been inherited from a common
ancestor ; and we know that such aggregated characters have
especial value in classification.
We can understand why a species or a group of species
may depart from its allies, in several of its most important
characteristics, and yet be safely classed with them. This
may be safely done, and is often done, as long as a sufficient
number of characters, let them be ever so unimportant, be-
trays the hidden bond of community of descent. Let two
forms have not a single character in common, yet, if these
extreme forms are connected together by a chain of inter-
mediate groups, we may at once infer their community of
descent, and we put them all into the same class. As we find
organs of high physiological importance — those which serve
to preserve life under the most diverse conditions of exist-
ence — are generally the most constant, we attach especial
value to them ; but if these same organs, in another group
or section of a group, are found to differ much, we at once
value them less in our classification. We shall presently see
why embryological characters are of such high classificatory
importance. Geographical distribution may sometimes be
brought usefully into play in classing large genera, because
all the species of the same genus, inhabiting any distinct and
isolated region, are in all probability descended from the
same parents.
Analogical Resemblances. — We can understand, on the
above views, the very important distinction between real
affinities and analogical or adaptive resemblances. Lamarck
first called attention to this subject, and he has been ably fol-
lowed by Macleay and others. The resemblance in the shape
of the body and in the fin-like anterior limbs between du-
gongs and whales, and between these two orders of mam-
mals and fishes are analogical. So is the resemblance
between a mouse and a shrew-mouse (Sorex), which belong
to different orders; and the still closer resemblance, insisted
on by Mr. Mivart, between the mouse and a small marsupial
animal (Antechinus) of Australia. These latter resem-
blances may be accounted for, as it seems to me, by adapta-
ANALOGICAL RESEMBLANCES 463
tion for similarly active movements through thickets and
b.erbage. together vi'ith concealment from enemies.
Amongst insects there are innumerable similar instances ;
thus Linnaeus, misled by external appearances, actually
classed an homopterous insect as a moth. We see something
of the same kind even with our domestic varieties, as in the
strikingly similar shape of the body in the improved breeds
of the Chinese and common pig, which are descended from
distinct species; and in the similarly thickened stems of the
common and specifically distinct Swedish turnip. The re-
semblance between the greyhound and the racehorse is hardly
more fanciful than the analogies which have been drawn by
some authors between widely different animals.
On the view of characters being of real importance for
classification, only in so far as they reveal descent, we can
clearly understand why analogical or adaptive characters,
although of the utmost importance to the welfare of the
being, are almost valueless to the systematist. For animals,
belonging to two most distinct lines of descent, may have
become adapted to similar conditions, and thus have assumed
a close external resemblance ; but such resemblances will not
reveal — will rather tend to conceal their blood-relationship.
We can thus also understand the apparent paradox, that the
very same characters are analogical when one group is com-
pared with another, but give true affinities when the members
of the same group are compared together: thus, the shape of
the body and fin-like limbs are only analogical when whales
are compared with fishes, being adaptations in both classes
for swimming through the water ; but between the several
members of the whale family, the shape of the body and the
fin-like limbs offer characters exhibiting true affinity; for as
these parts are so nearly similar throughout the whole fam-
ily, we cannot doubt that they have been inherited from a
common ancestor. So it is with fishes.
Numerous cases could be given of striking resemblances
in quite distinct beings between single parts or organs, which
have been adapted for the same functions. A good instance
is afforded by the close reserhblance of the jaws of the dog
and Tasmanian wolf or Thylacinus, — animals which are
widely sundered in the natural system. But this resemblance
464 ORIGIN OF SPECIES
IS confined to general appearance, as in the prominence of
the canines, and in the cutting shape of the molar teeth. For
the teeth really differ much: thus the dog has on each side
of the upper jaw four pre-molars and only two molars;
whilst the Thylacinus has three pre-molars and four molars.
The molars also differ much in the two animals in relative
size and structure. The adult dentition is preceded by a
widely different milk dentition. Any one may of course deny
that the teeth in either case have been adapted for tearing
flesh, through the natural selection of successive variations;
but if this be admitted in the one case, it is unintelligible to
me that it should be denied in the other. I am glad to find
that so high an authority as Professor Flower has come to
this same conclusion.
The extraordinary cases given in a former chapter, of
widely different fishes possessing electric organs, — of widely
different insects possessing luminous organs, — and of orchids
and asclepiads having pollen-masses with viscid discs, come
under this same head of analogical resemblances. But these
cases are so wonderful that they were introduced as diffi-
culties or objections to our theory. In all such cases some
fundamental difference in the growth or development of the
parts, and generally in their matured structure, can be de-
tected. The end gained is the same, but the means, though
appearing superficially to be the same, are essentially differ-
ent. The principle formerly alluded to under the term of
analogical variation has probably in these cases often come
into play; that is, the members of the same class, although
only distantly allied, have inherited so much in common in
their constitution, that they are apt to vary under similar
exciting causes in a similar manner; and this would obvi-
ously aid in the acquirement through natural selection of
parts or organs, strikingly like each other, independently of
their direct inheritance from a common progenitor.
As species belonging to distinct classes have often been
adapted by successive slight modifications to live under
nearly similar circumstances, — to inhabit, for instance, the
three elements of land, air, and water, — we can perhaps un-
derstand how it is that a numerical parallelism has sometimes
been observed between the sub-groups of distinct classes. A
ANALOGICAL RESEMBLANCES 465
naturalist, struck with a parallelism of this nature, by arbi-
trarily raising or sinking the value of the groups in several
classes (and all our experience shows that their valuation
is as yet arbitrary), could easily extend the parallelism over
a wide range; and thus the septenary, quinary, quaternary
and ternary classifications have probably arisen.
There is another and curious class of cases in which close
external resemblance does not depend on adaptation to sim-
ilar habits of life, but has been gained for the sake of pro-
tection. I allude to the wonderful manner in which certain
butterflies imitate, as first described by Mr. Bates, other and
quite distinct species. This excellent observer has shown
that in some districts of S. America, where, for instance, an
Ithomia abounds in gaudy swarms, another butterfly, namely,
a Leptalis, is often found mingled in the same flock; and the
latter so closely resembles the Ithomia in every shade and
stripe of colour and even in the shape of its wings, that Mr.
Bates, with his eyes sharpened by collecting during eleven
years, was, though always on his guard, continually deceived.
When the mockers and the mocked are caught and compared,
they are found to be very different in essential structure, and
to belong not only to distinct genera, but often to distinct
families. Had this mimicry occurred in only one or two in-
stances, it might have been passed over as a strange coinci-
dence. But, if we proceed from a district where one Leptalis
imitates an Ithomia, another mocking and mocked species
belonging to the same two genera, equally close in their
resemblance, may be found. Altogether no less than ten
genera are enumerated, which include species that imitate
other butterflies. The mockers and mocked always inhabit
the same region; we never find an imitator living remote
from the form which it imitates. The mockers are almost
invariably rare insects; the mocked in almost every case
abound in swarms. In the same district in which a species
of Leptalis closely imitates an Ithomia, there are sometimes
other Lepidotcra mimicking the same Ithomia : so that in the
same place, species of three genera of butterflies and even a
moth are found all closely resembling a butterfly belonging
to a fourth genus. It deserves especial notice that many of
the mimicking forms of -the Leptalis, as well as of the mim-
466 ORIGIN OF SPECIES
icked forms, can be shown by a graduated series to be mereTv
varieties of the same species ; whilst others are undoubtedly
distinct species. But why, it may be asked, are certain forms
treated as the mimicked and others as the mimickers? Mr.
Bates satisfactorily answers this question, by showing that
the form which is imitated keeps the usual dress of the group
to which it belongs, whilst the counterfeiters have changed
their dress and do not resemble their nearest allies.
We are next led to inquire what reason can be assigned
for certain butterflies and moths so often assuming the dress
of another and quite distinct form; why, to the perplexity of
naturalists, has nature condescended to the tricks of the
stage? Mr. Bates has, no doubt, hit on the true explanation.
The mocked forms, which always abound in numbers, must
habitually escape destruction to a large extent, otherwise
they could not exist in such swarms; and a large amount of
evidence has now been collected, showing that they are dis-
tasteful to birds and other insect-devouring animals. The
mocking forms, on the other hand, that inhabit the same dis-
trict, are comparatively rare, and belong to rare groups;
hence they must suffer habitually from some danger, for
otherwise, from the number of eggs laid by all butterflies,
they would in three or four generations swarm over the
whole country. Now if a member of one of these perse-
cuted and rare groups were to assume a dress so like that of
a well-protected species that it continually deceived the prac-
tised eyes of an entomologist, it would often deceive preda-
ceous birds and insects, and thus often escape destruction.
Mr. Bates may almost be said to have actually witnessed the
process by which the mimickers have come so closely to re-
semble the mimicked; for he found that some of the forms
of Leptalis which mimic so many other butterflies, varied in
an extreme degree. In one district several varieties oc-
curred, and of these one alone resembled to a certain ex'
tent, the common Tthomia of the same district. In another
district there were two or three varieties, one of which was
much commoner than the others, and this closely mocked
another form of Ithomia. From facts of this nature, Mr.
Bates concludes that the Leptalis first varies; and when a
variety happens to resemble in some degree any common
AFFINITIES CONNECTING ORGANIC BEINGS 467
butterfly inhabiting the same district, this variety, from its
resemblance to a flourishing and Httle-persecuted kind, has
a better chance of escaping destruction from predaceous
birds and insects, and is consequently oftener preserved;—
"the less perfect degrees of resemblance being generation
after generation eliminated, and only the others left to pro-
pagate their kind." So that we have an excellent illus-
tration of natural selection.
Messrs. Wallace and Trimen have likewise described sev-
eral equally striking cases of imitation in the Lepidoptera of
the Malay Archipelago and Africa, and with some other in-
sects. Mr. Wallace has also detected one such case with
birds, but we have none with the larger quadrupeds. The
much greater frequency of imitation with insects than with
other animals, is probably the consequence of their small
size ; insects cannot defend themselves, excepting indeed the
kinds furnished with a sting, and I have never heard of an
instance of such kinds mocking other insects, though they
are mocked; insects cannot easily escape by flight from the
larger animals which prey on them; therefore, speaking
metaphorically, they are reduced, like most weak creatures,
to trickery and dissimulation.
It should be observed that the process of imitation prob-
ably never commenced between forms widely dissimilar in
colour. But starting with species already somewhat like
each other, the closest resemblance, if beneficial, could read-
ily be gained by the above means; and if the imitated form
was subsequently and gradually modified through any agency,
the imitating form would be led along the same track, and
thus be altered to almost any extent, so that it might ulti-
mately assume an appearance or colouring wholly unlike that
of the other members of the family to which it belonged.
There is, however, some difficulty on this head, for it is nec-
essary to suppose in some cases that ancient members belong-
ing to several distinct groups, before they had diverged to
their present extent, accidentally resembled a member of
another and protected group in a sufficient degree to afford
some slight protection ; this having given the basis for the
subsequent acquisition of the most perfect resemblance.
On the Nature of the Affinities connecting Organic
468 ORIGIN OF SPECIES
Beings. — As the modified descendants of dominant species,
belonging to the larger genera, tend to inherit the advan-
tages which made the groups to which they belong large and
their parents dominant, they are almost sure to spread widely,
and to seize on more and more places in the economy of na-
ture. The larger and more dominant groups within each
class thus tend to go on increasing in size ; and they conse-
quently supplant many smaller and feebler groups. Thus we
can account for the fact that all organisms, recent and ex-
tinct, are included under a few great orders, and under still
fewer classes. As showing how few the higher groups are
in number, and how widely they are spread throughout the
world, the fact is striking that the discovery of Australia
has not added an insect belonging to a new class ; and that
in the vegetable kingdom, as I learn from Dr. Hooker, it
has added only two or three families of small size.
In the chapter on Geological Succession I attempted to
show, on the principle of each group having generally
diverged much in character during the long-continued proc-
ess of modification, how it is that the more ancient forms of
life often present characters in some degree intermediate
between existing groups. As some few of the old and in-
termediate forms have transmitted to the present day de-
scendants but little modified, these constitute our so-called
osculant or aberrant species. The more aberrant any form
is, the greater must be the number of connecting forms which
have been exterminated and utterly lost. And we have some
evidence of aberrant groups having suffered severely from
extinction, for they are almost always represented by ex-
tremely few species ; and such species as do occur are gen-
erally very distinct from each other, which again implies
extinction. The genera Ornithorhynchus and Lepidosiren,
for example, would not have been less aberrant had each
been represented by a dozen species, instead of as at present
by a single one, or by two or three. We can, I think, account
for this fact only by looking at aberrant groups as forms
which have been conquered by more successful competitors,
with a few members still preserved under unusually favour-
able conditions.
Mr. Waterhouse has remarked that, when a member be-
AFFINITIES CONNECTING ORGANIC BEINGS 469
longing to one group of animals exhibits an affinity to a
quite distinct group, this affinity in most cases is general and
not special ; thus, according to Mr. Waterhouse, of all Ro-
dents, the bizcacha is most nearly related to Marsupials ; but
in the points in which it approaches this order, its relations
are general, that is, not to any one marsupial species more
than to another. As these points of affinity are believed to
be real and not merely adaptive, they must be due in accord-
ance with our view to inheritance from a common progenitor.
Therefore we must suppose either that all Rodents, including
the bizcacha, branched off from some ancient Marsupial,
which will naturally have been more or less intermediate in
character with respect to all existing Marsupials; or that
both Rodents and Marsupials branched off from a common
progenitor, and that both groups have since undergone mwch
modification in divergent directions. On either view we
must suppose that the bizcacha has retained, by inheritance,
more of the characters of its ancient progenitor than have
other Rodents ; and therefore it will not be specially related
to any one existing Marsupial, but indirectly to all or nearly
all Marsupials, from having partially retained the character
of their common progenitor, or of some early member of the
group. On the other hand, of all Marsupials, as Mr. Water-
house has remarked, the Phascolomys resembles most nearly,
not any one species, but the general order of Rodents. In
this case, however, it may be strongly suspected that the re-
semblance is only analogical, owing to the Phascolomys
having become adapted to habits like those of a Rodent. The
elder De Candolle has made nearly similar observations on the
general nature of the affinities of distinct families of plants.
On the principle of the multiplication and gradual diver-
gence in char cter of the species descended from a common
progenitor, together with their retention by inheritance of
some characters in common, we can understand the exces-
sively complex and radiating affinities by which all the mem-
bers of the same family or higher group are connected to-
gether. For the common progenitor of a whole family, now
broken up by extinction into distinct groups and sub-groups,
will have transmitted some of its characters, modified in
various ways and degrees, to all the species; and they will
470 ORIGIN OF SPECIES
consequently be related to each other by circuitous lines of
affinity of various lengths (as may be seen in the diagram so
often referred to), mounting up through many predecessors.
As it is difficult to show the blood relationship between the
numerous kindred of any ancient and noble family even by
(he aid of a genealogical tree, and almost impossible to do so
without this aid, we can understand the extraodinary diffi-
culty which naturalists have experienced in describing, with-
out the aid of a diagram, the various affinities which they
perceive between the many living and extinct members of
the same great natural class.
Extinction, as we have seen in the fourth chapter, has
played an important part in defining and widening the inter-
vals between the several groups in each class. We may thus
account for the distinctness of whole classes from each other
— for instance, of birds from all other vertebrate animals — ■
by the belief that many ancient forms of life have been ut-
terly lost, through which the early progenitors of birds were
formerly connected with the early progenitors of the other
and at that time less differentiated vertebrate classes. There
has been much less extinction of the forms of life which once
connected fishes with batrachians. There has been still less
within some whole classes, for instance the Crustacea, for
here the most wonderfully diverse forms are still linked to-
gether by a long and only partially broken chain of affinities.
Extinction has only defined the groups: it has by no means
made them; for if every form which has ever lived on this
earth were suddenly to reappear, though it would be quite
impossible to give definitions by which each group could be
distinguished, still a natural classification, or at least a natu-
ral arrangement, would be possible. We shall see this by
turning to the diagram; the letters, A to L, may represent
eleven Silurian genera, some of which have produced large
groups of modified descendants, with every link in each
branch and sub-branch still alive ; and the links not greater
than those between existing varieties. In this case it would
be quite impossible to give definitions by which the several
members of the several groups could be distinguished from
their more immediate parents and descendants. Yet the
arrangement in the diagram would still hold good and would
AFFINITIES CONNECTING ORGANIC BEINGS 471
be natural ; for, on the principle of inheritance, all the forms
descended, for instance, from A, would have something in
common. In a tree we can distinguish this or that branch,
though at the actual fork the two unite and blend together.
We could not, as I have said, define the several groups ; bat
we could pick out types, or forms, representing most of the
characters of each group, whether large or small, and thus
give a general idea of the value of the differences between
them. This is what we should be driven to, if we were ever
to succeed in collecting all the forms in any one class which
have lived throughout all time and space. Assuredly we shall
never succeed in making so perfect a collection : nevertheless,
in certain classes, we are tending towards this end ; and
Milne Edwards has lately insisted, in an able paper, on the
high importance of looking to types, whether or not we can
separate and define the groups to which such types belong.
Finally, we have seen that natural selection, which follows
from the struggle for existence, and which almost inevitably
leads to extinction and divergence of character in the de-
scendants from any one parent-species, explains that great
and universal feature in the affinities of all organic beings,
namely, their subordination in group under group. We use
the element of descent in classing the individuals of both
sexes and of all ages under one species, although they may
have but few characters in common ; we use descent in class-
ing acknowledged varieties, however different they may be
from their parents; and I believe that this element of descent
is the hidden bond of connexion which naturalists have
sought under the term of the Natural System. On this idea
of the natural system being, in so far as it is has been per-
fected, genealogical in its arrangement, with the grades of
difference expressed by the terms genera, families, orders,
&c., we can understand the rules which we are compelled to
follow in our classification. We can understand why we value
certain resemblances far more than others ; why we use rudi-
mentary and useless organs, or others of trifling physio-
logical importance; why, in findine the relations between one
group and another, we summarily reject analogical or adap-
tive characters, and yet use these same characters within the
limits of the same groups- We can clearly see how it is that
472 ORIGIN OF SPECIES
all living and extinct forms can be grouped together within
a few great classes; and how the several members of each
class are connected together by the most complex and radi-
ating lines of affinities. We shall never, probably, disen-
tangle the inextricable web of the affinities between the mem-
bers of any one class; but when we have a distinct object in
view, and do not look to some unknown plan of creation, we
may hope to make sure but slow progress.
Professor Hackel in his 'Generelle Morphologie' and in
other works, has recently brought his great knowledge and
abilities to bear on what he calls phylogeny, or the lines of
descent of all organic beings. In drawing up the several
series he trusts chiefly to embryological characters, but re-
ceives aid from homologous and rudimentary organs, as well
as from the successive periods at which the various forms of
life are believed to have first appeared in our geological for-
mations. He has thus boldly made a great beginning, and
shows us how classification will in the future be treated.
MORPHOLOGY
We have seen that the members of the same class, inde-
pendently of their habits of life, resemble each other in the
general plan of their organisation. This resemblance is often
expressed by the term "unity of type;" or by saying that the
several parts and organs in the different species of the class
are homologous. The whole subject is included under the
general term of Morphology. This is one of the most inter-
esting departments of natural history, and may almost be
said to be its very soul. What can be more curious than
that the hand of a man, formed for grasping, that of a mole
for digging, the leg of the horse, the paddle of the porpoise,
and the wing of the bat, should all be constructed on the
same pattern, and should include similar bones, in the same
relative positions? How curious it is, to give a subordinate
though striking instance, that the hind-feet of the kangaroo,
which are so well fitted for bounding over the open plains,
— those of the climbing, leaf-eating koala, equally well fitted
for grasping the branches of trees, — those of the ground-
dwelling, insect or root-eating, bandicoots, — and those of
MORPHOLOGY 473
some other Australian marsupials, — should all be constructed
on the same extraordinary type, namely with the bones of
the second and third digits extremely slender and enveloped
within the same skin, so that they appear like a single toe
furnished with two claws. Notwithstanding this similarity
of pattern, it is obvious that the hind feet of these several
animals are used for as widely different purposes as it is pos-
sible to conceive. The case is rendered all the more striking
by the American opossums, which follow nearly the same
habits of life as some of their Australian relatives, having
feet constructed on the ordinary plan. Professor Flower,
from whom these statements are taken, remarks in conclu-
sion : '"We may call this conformity to type, without getting
much nearer to an explanation of the phenomenon ;" and he
then adds, "but is it not powerfully suggestive of true rela-
tionship, of inheritance from a common ancestor?"
GeoiTroy St. Hilaire has strongly insisted on the high im-
portance of relative position or connexion in homologous
parts ; they may differ to almost any extent in form and size,
and yet remain connected together in the same invariable
order. We never find, for instance, the bones of the arm
and fore-arm, or of the thigh and leg, transposed. Hence
the same names can be given to the homologous bones in
widely different animals. We see the same great law in the
construction of the mouths of insects : what can be more dif-
ferent than the immensely long spiral proboscis of a sphinx-
moth, the curious folded one of a bee or bug, and the great
jaws of a beetle? — yet all these organs, serving for such
widely different purposes, are formed by infinitely numerous
modifications of an upper lip, mandibles, and two pairs of
maxillae. The same law governs the construction of the
mouths and limbs of crustaceans. So it is with the flowers
of plants.
Nothing can be more hopeless than to attempt to explain
this similarity of pattern in members of the same class, by
utility or by the doctrine of final causes. The hopelessness
of the attempt has been expressly admitted by Owen in his
most interesting work on the 'Nature of Limbs.' On the
ordinary view of the independent creation of each being, we
can only say that so it is; — that it has pleased the Creator
474 ORIGIN OF SPECIES
to construct all the animals and plants in each great class on
a uniform plan; but this is not a scientific explanation.
The explanation is to a large extent simple on the theory
of the selection of successive slight modifications, — each
modification being profitable in some way to the modified
form, but often affecting by correlation other parts of the
organisation. In changes of this nature, there will be little
or no tendency to alter the original pattern, or to transpose
the parts The bones of a limb might be shortened and flat-
tened to any extent, becoming at the same time enveloped in
thick membrane, so as to serve as a fin ; or a webbed hand
might have all its bones, or certain bones, lengthened to any
extent, with the membrane connecting them increased, so as
to serve as a wing; yet all these modifications would not
tend to alter the framework of the bones or the relative con-
nexion of the parts If we suppose that an early progenitor
— the archetype as it may be called — of all mammals, birds,
and reptiles, had its limbs constructed on the existing general
pattern, for whatever purpose they served, we can at once
perceive the plain signification of the homologous construc-
tion of the limbs throughout the class. So with the mouths
of insects, we have only to suppose that their common pro-
genitor had an upper lip, mandibles, and two pairs of max-
illae, thase parts being perhaps very simple in form; and then
natural selection will account for the infinite diversity in the
structure and functions of the mouths of insects. Never-
theless, it is conceivable that the general pattern of an organ
might become so much obscured as to be finally lost, by the
reduction and ultimately by the complete abortion of certain
parts, by the fusion of other parts, and by the doubling or
multiplication of others, — variations which we know to be
within the limits of possibility. In the paddles of the gigantic
extinct sea-lizards, and in the mouths of certain suctorial
crustaceans, the general pattern seems thus to have become
partially obscured.
There is another and equally curious branch of our sub-
ject ; namely, serial homologies, or the comparison of the
different parts or organs in the same individual, and not of
the same parts or organs in different members of the same
class. Most physiologists believe that the bones of the skull
MORPHOLOGY 475
are homologous — that is, correspond in number and in rela-
tive connexion — with the elemental parts of a certain number
of vertebric. The anterior and posterior limbs in all the
higher vertebrate classes are plainly homologous. So it is
with the wonderfully complex jaws and legs of crustaceans.
It is familiar to almost every one, that in a flower the rela-
tive position of the sepals, petals, stamens, and pistils, as well
as their intimate structure, are intelligible on the view that
they consist of metamorphosed leaves, arranged in a spire.
In monstrous plants, we often get direct evidence of the pos-
sibility of one organ being transformed into another ; and
we can actually see, during the early or embryonic stages of
development in flowers, as well as in crustaceans and many
other animals, that organs, which when mature become ex-
tremely different are at first exactly alike.
How inexplicable are the cases of serial homologies on the
ordinary view of creation ! Why should the brain be en-
closed in a box composed of such numerous and such extra-
ordinarily shaped pieces of bone, apparently representing ver-
tebrae? As Owen has remarked, the benefit derived from the
yielding of the separate pieces in the act of parturition by
mammals, will by no means explain the same construction in
the skulls of birds and reptiles. Why should similar bones
have been created to form the wing and the leg of a bat,
used as they are for such totally different purposes, namely
flying and walking? Why should one crustacean, which has
an extremely complex mouth formed of many parts, conse-
quently always have fewer legs ; or conversely, those with
many legs have simpler mouths? Why should the sepals,
petals, stamens, and pistils, in each flower, though fitted for
such distinct purposes, be all constructed on the same
pattern ?
On the theory of natural selection, we can, to a certain
extent, answer these questions. We need not here consider
how the bodies of some animals first became divided into a
series of segments, or how they became divided into right
and left sides, with corresponding organs, for such questions
are almost beyond investigation. It is, however, probable
that some serial structures arc the result of cells multiplv-
ing by division, entailing the multiplication of the parts
476 ORIGIN OF SPECIES
developed from such cells. It must suffice for our purpose
to bear in mind that an indefinite repetition of the same part
or organ is the common characteristic, as Owen has re-
marked, of all low or little specialised forms ; therefore the
unknown progenitor of the Vertebrata probably possessed
many vertebrae; the unknown progenitor of the Articulata,
many segments ; and the unknown progenitor of flowering
plants, many leaves arranged in one or more spires. We
have also formerly seen that parts many times repeated are
eminently liable to vary, not only in number, but in form.
Consequently such parts, being already present in consider-
able numbers, and being highly variable, would naturally
afford the materials for adaptation to the most different pur-
poses; yet they would generally retain, through the force
of inheritance, plain traces of their original or fundamental
resemblance. They would retain this resemblance all the
more, as the variations, which afforded the basis for their
subsequent modification through natural selection, would tend
from the first to be similar; the parts being at an early stage
of growth alike, and being subjected to nearly the same con-
ditions. Such parts, whether more or less modified, unless
their common origin became wholly obscure, would be se-
rially homologous.
In the great class of molluscs, though the parts in distinct
species can be shown to be homologous, only a few serial
homologies, such as the valves of Chitons, can be indicated;
that is, we are seldom enabled to say that one part is homol-
ogous with another part in the same individual. And we
can understand this fact; for in molluscs, even in the lowest
members of the class, we do not find nearly so much indefi-
nite repetition of any one part as we find in the other great
classes of the animal and vegetable kingdoms.
But morphology is a much more complex subject than it at
first appears, as has lately been well shown in a remarkable
paper by Mr. E. Ray Lankester, who has drawn an important
distinction between certain classes of cases which have all
been equally ranked by naturalists as homologous. He pro-
poses to call the structures which resemble each other in
distinct animals, owing to their descent from a common pro-
genitor with subsequent modification, homogenous; and the
MORPHOLOGY 477
resemblances which cannot thus be accounted for, he pro-
poses to call homoplastic. For instance, he believes that the
hearts of birds and mammals are as a whole homogenous, —
that is, have been derived from a common progenitor; but
that the foui cavities of the heart in the two classes are
homoplastic, — that is, have been independently developed.
Mr. Lankester also adduces the close resemblance of the
parts on the right and left sides of the body, and in the suc-
cessive segments of the same individual animal ; and here we
have parts commonly called homologous, which bear no rela-
tion to the descent of distinct species from a common pro-
genitor. Homoplastic structures are the same with those
which I have classed, though in a very imperfect manner,
as analogous modifications oi resemblances. Their forma-
tion may be attributed in part to distinct organisms, or to
distinct parts of the same organism, having varied in an
analogous manner; and in part to similar modifications,
having been preserved for the same general purpose oi func-
tion, — of which many instances have been given.
Naturalists frequently speak of the skull as formed of
metamorphosed vertebrae; the jaws of crabs as metamor-
phosed legs ; the stamens and pistils in flowers as meta-
morphosed leaves; but it would in most cases be more
correct, as Professor Huxley has remarked, to speak of
both skull and vertebrae, jaws and legs, &c., as having been
metamorphosed, not one from the other, as they now exist,
but from some common and simpler element. Most natu-
ralists, however, use such language only in a metaphorical
sense; they are far from meaning that during a long course
of descent, primordial organs of any kind — vertebrae in the
one case and legs in the other — have actually been converted
into skulls or jaws. Yet so strong is the appearance of this
having occurred, that naturalists can hardly avoid employing
language having this plain signification. According to the
views here maintained, such language may be used literally;
and the wonderful fact of the jaws, for instance, of a crab
retaining numerous characters, which they probably would
have retained through inheritance, if they had really been
metamorphosed from true though extremely simple legs, is
in part explained.
478 ORIGIN OF SPECIES
DEVELOPMENT AND EMBKYOLOGY
This is one of the most important subjects in the whole
round of natural history. The metamorphoses of insects,
with which every one is famiHar, are generally effected ab-
ruptly by a few stages ; but the transformations are in reality
numerous and gradual, though concealed. A certain ephem-
erous insect (Chlocon) during its development, moults, as
shown by Sir J. I.ubbock, above twenty times, and each
time undergoes a certain amount of change ; and in this
case we see the act of metamorphosis performed in a pri-
mary and gradual manner. Many insects, and especially cer-
tain crustaceans, show us what wonderful changes of struc-
ture can be effected during development Such changes,
however, reach their acme in the so-called alternate genera-
tions of some of the lower animals. It is, for instance, an
astonishing fact that a delicate branching coralline, studded
with polypi and attached to a submarine rock, should pro-
duce, first by budding and then by transverse division, a
host of huge floating jelly-fishes; and that these should pro-
duce eggs, from which are hatched swunming animalcules,
which attach themselves to rocks and become developed into
branching corallines ; and so on in an endless cycle. The
belief in the essential identity of the process of alternate
generation and of ordinary metamorphosis has been greatly
strengthened by Wagner's discovery of the larva or maggot
of a fly, namely the Cecidomyia, producing asexually other
larviE, and these others, which finally are developed into
mature males and females, propagating their kind in the
ordinary manner by eggs.
It may be worth notice that when Wagner's remarkabld
discovery was first announced, I was asked how was il
possible to account for the larvae of this fly having acquired
the power of asexual reproduction. As long as the case
remained unique no answer could be given. But already
Grimm has shown that another fly, a Chironomus, reproduces
itself in nearly the same manner, and he believes that this
occurs frequently in the Order. It is the pupa, and not the
larva, of the Chironomus which has this power; and Grimm
further shows that this case, to a certain extent, "unites that
DEVELOPMENT AND EMBRYOLOGY 479
of the Cecidomyia with the parthenogenesis of the Coc-
cidae;" — the term parthenogenesis implying that the mature
females of the Coccidse are capable of producing fertile eggs
without the concourse of the male. Certain animals belong-
ing to several classes are now known to have the power of
ordinary reproduction at an unusually early age; and we
have only to accelerate parthenogenetic reproduction by
gradual steps to an earlier and earlier age, — Chironomus
showing us an almost exactly intermediate stage, viz., that of
the pupa — and we can perhaps account for the marvellous
case of the Cecidomyia
It has already been stated that various parts in the same
individual which are exactly alike during an early embryonic
period, become widely different and serve for widely differ-
ent purposes in the adult state. So again it has been shown
that generally the embryos of the most distinct species be-
longing to the same class are closely similar, but become,
when fully developed, widely dissimilar. A better proof of
this latter fact cannot be given than the statement of Von
Baer that "the embryos of mammalia, of birds, lizards, and
"snakes, probably also of chelonia, are in their earliest states
"exceedingly like one another, both as a whole and in the
"mode of development of their parts; so much so, in fact,
"that we can often distinguish the embryos only by their
"size. In my possession are two little embryos in spirit,
"whose names I have omitted to attach, and at present I am
"quite unable to say to what class they belong. They may
"be lizards or small birds, or very young mammalia, so
"complete is the similarity in the mode of formation of the
"head and trunk in these animals. The extremities, however,
"are still absent in these embryos. But even if they had
"existed in the»earliest stage of their development we should
"learn nothing, for the feet of lizards and mammals, the
"wings and feet of birds, no less than the hands and feet
"of man, all arise from the same fundamental form." The
larvae of most crustaceans, at corresponding stages of devel-
opment, closely resemble each other, however different the
adults may become ; and so it is with very many other ani-
mals. A trace of the law Cf embryonic resemblance occa-
sionally lasts till a rather late age : thus birds of the same
480 ORIGIN OF SPECIES
genus, and of allied genera, often resemble each other in
their immature plumage ; as we see in the spotted feathers
Ml the young of the thrush group. In the cat tribe, most
of the species when adult are striped or spotted in lines; and
stripes or spots can be plainly distinguished in the whelp of
the lion and the puma. We occasionally though rarely see
something of the same kind in plants; thus the first leaves
of the ulex or furze, and the first leaves of the phyllodineous
acacias, are pinnate or divided like the ordinary leaves of the
leguminosae.
The points of structure, in which the embryos of widely
different animals within the same class resemble each other,
often have no direct relation to their condition of existence.
We cannot, for instance, suppose that in the embryos of the
vertebrata the peculiar loop-like courses of the arteries near
the branchial slits are related to similar conditions, — in the
young mammal which is nourished in the womb of its mother,
in the egg of the bird which is hatched in a nest, and in the
^•■pawn of a frog under water. We have no more reason to
believe in such a relation, than we have to believe that the
similar bones in the hand of a man, wing of a bat, and fin
of a porpoise, are related to similar conditions of life. No
one supposes that the stripes on the whelp of a lion, or the
spots on the young blackbird, are of any use to these animals.
The case, however, is different when an animal during any
part of its embryonic career is active, and has to provide for
itself. The period of activity may come on earlier or later
in life; but whenever it comes on, the adaptation of the larva
to its conditions of life is just as perfect and as beautiful as
in the adult animal. In how important a manner this has
acted, has recently been well shown by Sir J. Lubbock in his
remarks on the close similarity of the larvse of some insects
belonging to very different orders, and on the dissimilarity
of the larvse of other insects within the same order, accord-
ing to their habits of life. Owing to such adaptations, the
similarity of the larvae of allied animals is sometimes greatly
obscured; especially when there is a division of labour during
the different stages of development, as when the same larva
has during one stage to search for food, and during another
stage has to search for a place of attachment. Cases can
DEVELOPMENT AND EMBRYOLOGY 481
even be given of the larvae of allied species, or groups of
species, differing more from each other than do the adults.
In most cases, however, the larvae, though active, still obey,
more or less closely, the law of common embryonic resem-
blance. Cirripedes aft"ord a good instance of this; even the
illustrious Cuvier did not perceive that a barnacle was a
crustacean: but a glance at the larva shows this in an un-
mistakable manner. So again the two main divisions of
cirripedes, the pedunculated and sessile, though differing
widely in external appearance, have larvae in all their stages
barely distinguishable.
The embryo in the course of development generally rises
in organisation; I use this expression, though I am aware
that it is hardly possible to define clearly what is meant by
the organisation being higher or lower. But no one probably
will dispute that the butterfly is higher than the caterpillar.
In some cases, however, the mature animal must be consid-
ered as lower in the scale than the larva, as with certain
parasitic crustaceans. To refer once again to cirripedes : the
larvae in the first stage have three pairs of locomotive organs,
a simple single eye, and a probosciformed mouth, with which
they feed largely, for they increase much in size. In the
second stage, answering to the chrysalis stage of butterflies,
they have six pairs of beautifully constructed natatory legs,
a pair of magnificent compound eyes, and extremely complex
antennae; but they have a closed and imperfect mouth, and
cannot feed : their function at this stage is, to search out by
their well-developed organs of sense, and to reach by their
active powers of swimming, a proper place on which to be-
come attached and to undergo their final metamorphosis.
When this is completed they are fixed for life: their legs are
now converted into prehensile organs ; they again obtain a
well-constructed mouth ; but they have no antennae, and their
two eyes are now reconverted into a minute, single, simple
eye-spot. In this last and complete state, cirripedes may be
considered as either more highly or more lowly organised
than they were in the larval condition. But in some genera
the larvae become developed. into hermaphrodites having the
ordinary structure, and into what I have called complemental
males; and in the latter the development has assuredly been
482 ORIGIN OF SPECIES
retrograde, for the male is a mere sack, which lives for a
short time and is destitute of mouth, stomach, and every
other organ of importance, excepting those for reproduction.
We are so much accustomed to see a difference in structure
between the embryo and the adult, that we are tempted to
look at this difference as in some necessary manner contin-
gent on growth. But there is no reason why, for instance,
the wing of a bat, or the fin of a porpoise, should not have
been sketched out with all their parts in proper proportion,
as soon as any part became visible. In some whole groups
of animals and in certain members of other groups this is
the case, and the embryo does not at any period dift'er widely
from the adult: thus Owen has remarked in regard to cuttle-
fish, ''there is no metamorphosis ; the cephalopodic character
is manifested long before the parts of the embryo are com-
pleted." Land-shells and fresh-water crustaceans are born
having their proper forms, whilst the marine members of the
same two great classes pass through considerable and often
great changes during their development. Spiders, again,
barely undergo any metamorphosis. The larvae of most in-
sects pass through a worm-like stage, whether they are active
and adapted to diversified habits, or are inactive from being
placed in the midst of proper nutriment or from being fed
by their parents ; but in some few cases, as in that of Aphis,
if we look to the admirable drawings of the development of
this insect, by Professor Huxley, we see hardly any trace
of the vermiform stage.
Sometimes it is only the earlier developmental stages which
fail. Thus Fritz Muller has made the remarkable discovery
that certain shrimp-like crustaceans (allied to Penoeus) first
appear under the simple nauplius-form, and after passing
(through two or more zoea-stages, and then through the
mysis-stage, finally acquire their mature structure : now in
the whole great malacostracan order, to which these crusta-
ceans belong, no other member is as yet known to be first
developed under the nauplius-form, though many appear as
zoeas; nevertheless Miiller assigns reasons for his belief,
that if there had been no suppression of development, all
these crustaceans would have appeared as nauplii.
How, then, can we explain these several facts in embry-
DEVELOPMENT AND EMBRYOLOGY 483
ology, — namely, the very general, though not universal, dif-
ference in structure between the embryo and the adult ; —
the various parts in the same individual embryo, which ulti-
mately become very unlike and serve for diverse purposes,
being at an early period of growth alike; — the common, but
not invariable, resemblance between the embryos or larvae
of the most distinct species in the same class; — the em-
bryo often retaining whilst within the egg or womb, struc-
tures which are of no service to it, either at that or at a
later period of life; on the other hand larvae, which have to
provide for their own wants, being perfectly adapted to the
surrounding conditions; — and lastly the fact of certain larvae
standing higher in the scale of organisation than the mature
animal into which they are developed? I believe that all
these facts can be explained, as follows.
It is commonly assumed, perhaps from monstrosities affect-
ing the embryo at a very early period, that slight variations
or individual differences necessarily appear at an equally
early period. We have little evidence on this head, but what
we have certainly points the other way; for it is notorious
that breeders of cattle, horses, and various fancy animals,
cannot positively tell, until some time after birth, what will
be the merits or demerits of their young animals. We see
this plainly in our own children ; we cannot tell whether a
child will be tall or short, or what its precise features will
be. The question is not, at what period of life each varia-
tion may have been caused, but at what period the effects are
displayed. The cause may have acted, and I believe often
has acted, on one or both parents before the act of genera-
tion. It deserves notice that it is of no importance to a very
young animal, as long as it remains in its mother's womb or
in the egg, or as long as it is nourished and protected by its
parent, whether most of its characters are acquired a little
earlier or later in life. It would not signify, for instance, to
a bird which obtained its food by having a much-curved
beak whether or not whilst young it possessed a beak of this
shape, as long as it was fed by its parents.
I have stated in the first chapter, that at whatever age a
variation first appears in the parent, it tends to reappear at
a corresponding age in the offspring. Certain variations can
484 ORIGIN OF SPECIES
only appear at corresponding ages ; for instance, peculiarities
in the caterpillar, cocoon, or imago states of the silk-moth :
or, again, in the full-grown horns of cattle. But variations,
which, for all that we can see might have first appeared
either earlier or later in life, likewise tend to reappear at a
corresponding age in the offspring and parent. I am far
from meaning that this is invariably the case, and I could
give several exceptional cases of variations (taking the word
in the largest sense) which have supervened at an earlier
age in the child than in the parent.
These two principles, namely, that slight variations gen-
erally appear at a not very early period of life, and are in-
herited at a corresponding not early period, explain, as I
believe, all the above specified leading facts in embryology.
But first let us look to a few analogous cases in our domestic
varieties. Some authors who have written on Dogs, main-
tain that the greyhound and bulldog, though so different,
are really closely allied varieties, descended from the same
wild stock; hence I was curious to see how far their puppies
differed from each other : I was told by breeders that they
differed just as much as their parents, and this, judging by
the eye, seemed almost to be the case ; but on actually meas-
uring the old dogs and their six-days-old puppies, I found
that the puppies had not acquired nearly their full amount of
proportional difference. So, again, I was told that the foals
of cart and race-horses — breeds which have been almost
wholly formed by selection under domestication — differed as
much as the full-grown animals; but having had careful
measurements made of the dams and of the three-days-old
colts of race and heavy cart-horses, I find that this is by no
means the case.
As we have conclusive evidence that the breeds of the
Pigeon are descended from a single wild species, I compared
the young within twelve hours after being hatched ; I care-
fully measured the proportions (but will not here give the
details) of the beak, width of mouth, length of nostril and
of eyelid, size of feet and length of leg, in the wild parent-
species, in pouters, fantails, runts, barbs, dragons, carriers,
and tumblers. Now some of these birds, when mature, differ
in so extraordinary a manner in the length and form of beak,
DEVELOPMENT AND EMBRYOLOGY 485
and in other characters, that they would certainly have been
ranked as distinct genera if found in a state of nature. But
when the nestling birds of these several breeds were placed
in a row, though most of them could just be distinguished,
the proportional differences in the above specified points
were incomparably less than in the full-grown birds. Some
characteristic points of difference — for instance, that of the
width of mouth — could hardly be detected in the young. But
there was one remarkable exception to this rule, for the
young of the short-faced tumbler differed from the young
of the wild rock-pigeon and of the other breeds, in almost
exactly the same proportions as in the adult state.
These facts are explained by the above two principles.
Fanciers select their dogs, horses, pigeons, &c., for breeding,
when nearly grown up : they are indifferent whether the de-
sired qualities are acquired earlier or later in life, if the
full-grown animal possesses them. And the cases just given,
more especially that of the pigeons, show that the charac-
teristic differences which have been accumulated by man's
selection, and which give value to his breeds, do not gen-
erally appear at a very early period of life, and are inherited
at a corresponding not early period. But the case of the
short-faced tumbler, which when twelve hours old possessed
its proper characters, proves that this is not the universal
rule; for here the characteristic differences must either have
appeared at an earlier period than usual, or, if not so, the
differences must have been inherited, not at a corresponding,
but at an earlier age.
Now let us apply these two principles to species in a state
of nature. Let us take a group of birds, descended from
some ancient form and modified through natural selection
for different habits. Then, from the many slight successive
variations having supervened in the several species at a not
early age, and having been inherited at a corresponding age,
the young will have been but little modified, and they will still
resemble each other much more closely than do the adults, —
just as we have seen with the breeds of the pigeon. We may
extend this view to widely distinct structures and to whole
classes. The fore-limbs, for instance, which once served
as legs to a remote progenitor, may have become, through
486 ORIGIN OF SPECIES
a long course of modification, adapted in one descendant to
act as hands, in another as paddles, in another as wings;
but on the above two principles the fore-limbs will not have
been much modified in the embryos of these several forms ;
although in each form the fore-limb will differ greatly in the
adult state. Whatever influence long-continued use or disuse
may have had in modifying the limbs or other parts of any
species, this will chiefly or solely have affected it when nearly
mature, when it was compelled to use its full powers to gain
its own living; and the effects thus produced will have been
transmitted to the offspring at a corresponding nearly mature
age. Thus the young will not be modified, or will be modi-
fied only in a slight degree, through the effects of the in-
creased use or disuse of parts.
With some animals the successive variations may have
supervened at a very early period of. life, or the steps may
have been inherited at an earlier age than that at which they
first occurred. In either of these cases, the young or embryo
will closely resemble the mature parent-form, as we have
seen with the short-faced tumbler. And this is the rule of
development in certain whole groups, or in certain sub-
groups alone, as with cuttle-fish, land-shells, fresh-water
crustaceans, spiders, and some members of the great class of
insects. With respect to the final cause of the young in
such groups not passing through any metamorphosis, we
can see that this would follow from the following contin-
gencies ; namely, from the young having to provide at a very
early age for their own wants, and from their following the
same habits of life with their parents; for in this case, it
would be indispensable for their existence that they should
be modified in the same manner as their parents. Again, with
respect to the singular fact that many terrestrial and fresh-
water animals do not undergo any metamorphosis, whilst
marine members of the same groups pass through various
transformations, Fritz Miiller has suggested that the process
of slowly modifying and adapting an animal to live on the
land or in fresh water, instead of in the sea, would be greatly
simplified by its not passing through any larval stage; for
it is not probable that places well adapted for both the larval
and mature stages, under such new and greatly changed
DEVELOPMENT AND EMBRYOLOGY 487
habits of life, would commonly be found unoccupied or ill-
occupied by other organisms. In this case the gradual ac-
quirement at an earlier and earlier age of the adult structure
would be favoured by natural selection; and all traces of
former metamorphoses would finally be lost.
If, on the other hand, it profited the young of an animal
to follow habits of life slightly different from those of the
parent-form, and consequently to be constructed on a slightly
ditterent plan, or if it profited a larva already different from
its parent to change still further, then, on the principle of
inheritance at corresponding ages, the young or the larvae
might be rendered by natural selection more and more dif-
ferent from their parents to any conceivable extent. Differ-
ences in the larva might, also, become correlated with suc-
cessive stages of its development; so that the larva, in the
first stage, might come to differ greatly from the larva in the
second stage, as is the case with many animals. The adult
might also become fitted for sites or habits, in which organs
of locomotion or of the senses, &c., would be useless; and
in this case the metamorphosis would be retrograde.
From the remarks just made we can see how by changes
of structure in the young, in conformity with changed habits
of life, together with inheritance at corresponding ages,
animals might come to pass through stages of development,
perfectly distinct from the primordial condition of their
adult progenitors. Most of our best authorities are now
convinced that the various larval and pupal stages of insects
have thus been acquired through adaptation, and not through
inheritance from some ancient form. The curious case of
Sitaris — a beetle which passes through certain unusual stages
of development — will illustrate how this might occur. The
first larval form is described by M. Fabre, as an active,
minute insect, furnished with six legs, two long antennae, and
four eyes. These larvae are hatched in the nests of bees;
and when the male-bees emerge from their burrows, in the
spring, which they do before the females, the larvae spring
on them, and afterwards crawl on to the females whilst
paired with the males. As soon as the female bee deposits
her eggs on the surface of the honey stored in the cells, the
larvae of the Sitaris leap on the eggs and devour them.
488 ORIGIN OF SPECIES
Afterwards they undergo a complete change; their eyes dis-
appear ; their legs and antennae become rudimentary, and
they feed on honey ; so that they now more closely resemble
the ordinary larvae of insects ; ultimately they undergo a
further transformation, and finally emerge as the perfect
beetle. Now, if an insect, undergoing transformations like
those of the Sitaris, were to become the progenitor of a
whole new class of insects, the course of development of the
new class would be widely different from that of our exist-
ing insects; and the first larval stage certainly would not
represent the former condition of any adult and ancient
form.
On the other hand it is highly probable that with many
animals the embryonic or larval stages show us, more or less
completely, the condition of the progenitor of the whole
group in its adult state. In the great class of the Crustacea,
forms wonderfully distinct from each other, namely, suctorial
parasites, cirripedes, entomostraca, and even the malacos-
traca, appear at first as larvae under the nauplius-f orm ; and
as these larvae live and feed in the open sea, and are not
adapted for any peculiar habits of life, and from other
reasons assigned by Fritz Miiller, it is probable that at some
very remote period an independent adult animal, resembling
the Nauplius, existed, and subsequently produced, along sev-
eral divergent lines of descent, the above-named great Crus-
tacean groups. So again it is probable, from what we know
of the embryos of mammals, birds, fishes, and reptiles, that
these animals are the modified descendants of some ancient
progenitor, which was furnished in its adult state with
branchiae, a swim-bladder, four fin-like limbs, and a long tail,
all fitted for an aquatic life.
As all the organic beings, extinct and recent, which have
ever lived, can be arranged within a few great classes ; and
as all within each class have, according to our theory, been
connected together by fine gradations, the best, and, if our
collections were nearly perfect, the only possible arrange-
ment, would be genealogical; descent being the hidden bond
of connexion which naturalists have been seeking under the
term of the Natural System. On this view we can under-
stand how it is that, in the eyes of most naturalists, the
DEVELOPMENT AND EMBRYOLOGY 489
Structure of the embryo is even more important for classi-
fication than that of the adult. In two or more groups of
animals, however much they may differ from each other in
structure and habits in their adult condition, if they pass
through closely similar embryonic stages, we may feel assured
that they all are descended from one parent-form, and are
therefore closely related. Thus, community in embryonic
structure reveals community of descent; but dissimilarity in
embryonic development does not prove discommunity of
descent, for in one of two groups the developmental stages
may have been suppressed, or may have been so greatly
modified through adaptation to new habits of life, as to be
no longer recognisable. Even in groups, in which the adults
have been modified to an extreme degree, community of
origin is often revealed by the structure of the larvae; we
have seen, for instance, that cirripedes, though externally so
like shell-fish, are at once known by their larvae to belong to
the great class of crustaceans. As the embryo often show3
us more or less plainly the structure of the less modified
and ancient progenitor of the group, we can see why ancient
and extinct forms so often resemble in their adult state the
embryos of existing species of the same class. Agassiz be-
lieves this to be a universal law of nature ; and we may hope
hereafter to see the law proved true. It can, however, be
proved true only in those cases in which the ancient state of
the progenitor of the group has not been wholly obliterated,
either by successive variations having supervened at a very
early period of growth, or by such variations having been
inherited at an earlier age than that at which they first ap-
peared. It should also be borne in mind, that the law may
be true, but yet, owing to the geological record not extending
far enough back in time, may remain for a long period, or
for ever, incapable of demonstration. The law will not
strictly hold good in those cases in which an ancient form
became adapted in its larvae state to some special line of life,
and transmitted the same larval state to a whole group of
descendants; for such larval will not resemble any still more
ancient form in its adult state.
Thus, as it seems to me,' the leading facts in embryology,
which are second to none in importance, are explained on
490 ORIGIN OF SPECIES
the principle of variations in the many descendants from
some one ancient progenitor, having appeared at a not very
early period of life, and having been inherited at a cor-
responding period. Embryology rises greatly in interest,
when we look at the embryo as a picture, more or less ob-
scured, of the progenitor, either in its adult or larval state,
of all the members of the same great class.
RUDIMENTARY, ATROPHIED, AND ABORTED ORGANS
Organs or parts in this strange condition, bearing the plain
stamp of inutility, are extremely common, or even general,
throughout nature. It would be impossible to name one of
the higher animals in which some part or other is not in a
rudimentary condition. In the mammalia, for instance, the
males possess rudimentary mammae; in snakes one lobe of the
lungs is rudimentary; in birds the "bastard-wing" may safely
be considered as a rudimentary digit, and in some species the
whole wing is so far rudimentary that it cannot be used for
flight. What can be more curious than the presence of teeth
in fcetal whales, which when grown up have not a tooth in
their heads ; or the teeth, which never cut through the gums,
in the upper jaws of unborn calves?
Rudimentary organs plainly declare their origin and mean-
ing in various ways. There are beetles belonging to closely
allied species, or even to the same identical species, which
have either full-sized and perfect wings, or mere rudiments
of membrane, which not rarely lie under wing-covers firmly
soldered together ; and in these cases it is impossible to
doubt, that the rudiments represent wings. Rudimentary
organs sometimes retain their potentiality: this occasionally
occurs with the mammne of male mammals, which have been
known to become well developed and to secrete milk. So
again in the udders in the genus Bos, there are normally four
developed and two rudimentary teats; but the latter in our
domestic cows sometimes become well developed and yield
milk. In regard to plants the petals are sometimes rudimen-
tary, and sometimes well-developed in the individuals of
the same species. In certain plants having separated sexes
Kolreuter found that by crossing a species, in which the male
RUDIMENTARY ORGANS 401
flowers included a rudiment of a pistil, with an hermaphro-
dite species, having of course a well-developed pistil, the
rudiment in the hybrid offspring was much increased in size;
and this clearly shows that the rudimentary and perfect
pistils are essentially alike in nature. An animal may pos-
sess various parts in a perfect state, and yet they may in one
sense be rudimentary, for they are useless: thus the tadpole
of the common Salamander or Water-newt, as Mr. G. H.
Lewes remarks, "has gills, and passes its existence in the
"water ; but the Salamandra atra, which lives high up among
"the mountains, brings forth its young full-formed. This
"animal never lives in the water. Yet if we open a gravid
"female, we find tadpoles inside her with exquisitely feath-
"ered gills ; and when placed in water they swim about like
"the tadpoles of the water-newt. Obviously this aquatic
"organisation has no reference to the future life of the
"animal, nor has it any adaptation to its embryonic condition ;
"it has solely reference to ancestral adaptations, it repeats
"a phase in the development of its progenitors."
An organ, serving for two purposes, may become rudimen-
tary or utterly aborted for one, even the more important
purpose, and remain perfectly efficient for the other. Thus
in plants, the office of the pistil is to allow the pollen-tubes
to reach the ovules within the ovarium. The pistil consists
of a stigma supported on a style ; but in some Compositae,
the male florets, which of course cannot be fecundated, have
a rudimentary pistil, for it is not crowned with a stigma; but
the style remains well developed and is clothed in the usual
manner with hairs, which serve to brush the pollen out of
the surrounding and conjoined anthers. Again, an organ
may become rudimentary for its proper purpose, and be used
for a distinct one : in certain fishes the swim-bladder seems
to be rudimentary for its proper function of giving buoyancy,
but has become converted into a nascent breathing organ or
lung. Many similar instances could be given.
Useful organs, however little they may be developed, un-
less we have reason to suppose that they were formerly more
highly developed, ought not to be considered as rudimentary.
They may be in a nascent condition, and in progress towards
further development. Rudimentary organs, on the other hand.
492 ORIGIN OF SPECIES
are either quite useless, such as teeth which never cut through
the gums, or almost useless, such as the wings of an ostrich,
which serve merely as sails. As organs in this condition
would formerly, when still less developed, have been of even
less use, than at present, they cannot formerly have been
produced through variation and natural selection, which acts
solely by the preservation of useful modifications. They
have been partially retained by the power of inheritance, and
relate to a former state of things. It is, however, often
difficult to distinguish between rudimentary and nascent
organs; for we can judge only by analogy whether a part is
capable of further development, in which case alone it de-
serves to be called nascent. Organs in this condition will
always be somewhat rare; for beings thus provided will
commonly have been supplanted by their successors with the
same organ in a more perfect state, and consequently will
have become long ago extinct. The wing of the penguin
is of high service, acting as a fin; it may, therefore, repre-
sent the nascent state of the wing: not that I believe this to
be the case; it is more probably a reduced organ, modified
for a new function: the wing of the Apteryx, on the other
hand, is quite useless, and is truly rudimentary. Owen con-
siders the simple filamentary limbs of the Lepidosiren as the
"beginnings of organs which attain full functional develop-
ment in higher vertebrates ;" but, according to the view lately
advocated by Dr. Giinther, they are probably remnants, con-
sisting of the persistent axis of a fin, with the lateral rays or
branches aborted. The mammary glands of the Ornitho-
rhynchus may be considered, in comparison with the udders
of a cow, as in a nascent condition. The ovigerous frena of
certain cirripedes, which have ceased to give attachment to
the ova and are feebly developed, are nascent branchiae.
Rudimentary organs in the individuals of the same species
are very liable to vary in the degree of their development
and in other respects. In closely allied species, also, the
extent to which the same organ has been reduced occasionally
differs much. This latter fact is well exemplified in the
state of the wings of female moths belonging to the same
family. Rudimentary organs may be utterly aborted; and
this implies, that in certain animals or plants, parts are en-
RUDIMENTARY ORGANS 493
tirely absent which analogy would lead us to expect to find in
them, and which are occasionally found in monstrous indi-
viduals. Thus in most of the Scrophulariaceae the fifth
stamen is utterly aborted; yet we may conclude that a fifth
stamen once existed, for a rudiment of it is found in many
species of the family, and this rudiment occasionally be-
comes perfectly developed, as may sometimes be seen in the
common snap-dragon. In tracing the homologies of any
part in different members of the same class, nothing is more
common, or, in order fully to understand the relations of the
parts, more useful than the discovery of rudiments. This is
well shown in the drawings given by Owen of the leg-bones
of the horse, ox, and rhinoceros.
It is an important fact that rudimentary organs, such as
teeth in the upper jaws of whales and ruminants, can often
be detected in the embryo, but afterwards wholly disappear.
It is also, I believe, a universal rule, that a rudimentary part
is of greater size in the embryo relatively to the adjoining
parts, than in the adult ; so that the organ at this early age is
less rudimentary, or even cannot be said to be in any degree
rudimentary. Hence rudimentary organs in the adult are
often said to have retained their embryonic condition.
I have now given the leading facts with respect to rudi-
mentary organs. In reflecting on them, every one must be
struck with astonishment; for the same reasoning power
which tells us that most parts and organs are exquisitely
adapted for certain purposes, tells us with equal plainness
that these rudimentary or atrophied organs are imperfect and
useless. In works on natural history, rudimentary organs
are generally said to have been created "for the sake of
symmetry," or in order "to complete the scheme of nature."
But this is not an explanation, merely a re-statement of the
fact. Nor is it consistent with itself: thus the boa-constrictor
has rudiments of hind-limbs and of a pelvis, and if it be
said that these bones have been retained "to complete the
scheme of nature," why, as Professor Weismann asks, have
they not been retained by other snakes, which do not possess
even a vestige of these same bones ? What would be thought
of an astronomer who maintained that the satellites revolve
in elliptic courses round their planets " for the sake of sym-
494 ORIGIN OF SPECIES
metry," because the planets thus revolve round the sun? An
eminent physiologist accounts for the presence of rudimen-
tary organs, by supposing that they serve to excrete matter
in excess, or matter injurious to the system; but can we sup-
pose that the minute papilla, which often represents the pistil
in male flowers, and which is formed of mere cellular tissue,
can thus act? Can we suppose that rudimentary teeth, which
are subsequently absorbed, are beneficial to the rapidly grow-
ing embryonic calf by removing matter so precious as phos-
phate of lime? When a man's fingers have been amputated,
imperfect nails have been known to appear on the stumps,
and T could as soon believe that these vestiges of nails are
developed in order to excrete horny matter, as that the rudi-
mentary nails on the fin of the manatee have been developed
for this same purpose.
On the view of descent with modification, the origin of
rudimentary organs is comparatively simple ; and we can
understand to a large extent the laws governing their imper-
fect development. We have plenty of cases of rudimentary
organs in our domestic productions, — as the stump of a tail
in tailless breeds, — the vestige of an ear in earless breeds of
sheep, — the reappearance of minute dangling horns in horn-
less breeds of cattle, more especially, according to Youatt, in
young animals, — and the state of the whole flower in the
cauliflower. We often see rudiments of various parts in
monsters; but I doubt whether any of these cases throw
light on the origin of rudimentary organs in a state of nature,
further than by showing that rudiments can be produced ; for
the balance of evidence clearly indicates that species under
nature do not undergo great and abrupt changes. But we
learn from the study of our domestic productions that the
disuse of parts leads to their reduced size; and that the result
is inherited.
It appears probable that disuse has been the main agent in
rendering organs rudimentary. It would at first lead by slow
steps to the more and more complete reduction of a part,
until at last it became rudimentary, — as in the case of the
eyes of animals inhabiting dark caverns, and of the wings
of birds inhabiting oceanic islands, which have seldom been
forced by beasts of prey' to take flight, and have ultimately
RUDIMENTARY ORGANS 495
lost the power of flying. Again, an organ, useful under cer-
tain conditions, might become injurious under others, as with
the wings of beetles living on small and exposed islands;
and in this case natural selection will have aided in reducing
the organ, until it was rendered harmless and rudimentary.
Any change in structure and function, which can be effected
by small stages, is within the power of natural selection ; so
that an organ rendered, through changed habits of life, use-
less or injurious for one purpose, might be modified and used
for another purpose. An organ might, also, be retained for
one alone of its former functions. Organs, originally formed
by the aid of natural selection, when rendered useless may
well be variable, for their variations can no longer be checked
by natural selection. All this agrees well with what we see
under nature. Moreover, at whatever period of life either
disuse or selection reduces an organ, and this will generally
be when the being has come to maturity and has to exert
its full powers of action, the principle of inheritance at
corresponding ages will tend to reproduce the organ in its
reduced state at the same mature age, but will seldom affect
it in the embryo. Thus we can understand the greater size
of rudimentary organs in the embryo relatively to the ad-
joining parts, and their lesser relative size in the adult. If,
for instance, the digit of an adult animal was used less and
less during many generations, owing to some change of
habits, or if an organ or gland was less and less functionally
exercised, we may infer that it would become reduced in size
in the adult descendants of this animal, but would retain
nearly its original standard of development in the embryo.
There remains, however, this difficulty. After an organ has
ceased being used, and has become in consequence much re-
duced, how can it be still further reduced in size until the
merest vestige is left; and how can it be finally quite obliter-
ated? It is scarcely possible that disuse can go on producing
any further effect after the organ has once been rendered
functionless. Some additional explanation is here requisite
which I cannot give. If, for instance, it could be proved
that every part of the organisation tends to vary in a greater
degree towards diminution than towards augmentation of
size, then we should be. able to understand how an organ
496 ORIGIN OF SPECIES
which has become useless would be rendered, independently
of the effects of disuse, rudimentary and would at last be
wholly suppressed; for the variations towards diminished
size would no longer be checked by natural selection. The
principle of the economy of growth, explained in a former
chapter, by which the materials forming any part, if not
useful to the possessor, are saved as far as is possible, will
perhaps come into play in rendering a useless part rudimen-
tary. But this principle will almost necessarily be confined
to the earlier stages of the process of reduction; for we can-
not suppose that a minute papilla, for instance, representing
in a male flower the pistil of the female flower, and formed
merely of cellular tissue, could be further reduced or ab-
sorbed for the sake of economising nutriment.
Finally, as rudimentary organs, by whatever steps they
may have been degraded into their present useless condition,
are the record of a former state of things, and have been
retained solely through the power of inheritance, — we can
understand, on the genealogical view of classification, how it
is that systematists, in placing organisms in their proper
places in the natural system, have often found rudimentary
parts as useful as, or even sometimes more useful than, parts
of high physiological importance. Rudimentary organs may
be compared witfe the letters in a word, still retained in the
spelling, but become useless in the pronunciation, but which
serve as a clue for its derivation. On the view of descent
with modification, we may conclude that the existence of
organs in rudimentary, imperfect, and useless condition, or
quite aborted, far from presenting a strange difficulty, as
they assuredly do on the old doctrine of creation, might even
have been anticipated in accordance with the yiews here
explained.
SUMMARY
In this chapter I have attempted to show, that the arrange-
ment of all organic beings throughout all time in groups
under groups — that the nature of the relationships by which
all living and extinct organisms are united by complex, radi-
ating, and circuitous lines of affinities into a few grand
classes, — the rules followed and the difficulties encountered
SUMMARY 497
by naturalists in their classifications, — the value set upon
characters, if constant and prevalent, whether of high or
of the most trifling importance, or, as with rudimentary
organs, of no importance, — the wide opposition in value be-
tween analogical or adaptive characters, and characters of
true affinity; and other such rules; — all naturally follow if
we admit the common parentage of allied forms, together
with their modification through variation and natural selec-
tion, with the contingencies of extinction and divergence of
character. In considering this view of classification, it
should be borne in mind that the element of descent has been
universally used in ranking together the sexes, ages, dimor-
phic forms, and acknowledged varieties of the same species,
however much they may differ from each other in structure.
If we extend the use of this element of descent, — the one
certainly known cause of similarity in organic beings, — we
shall understand what is meant by the Natural System: it is
genealogical in its attempted arrangement, with the grades
of acquired difference marked by the terms, varieties, species,
genera, families, orders, and classes.
On this same view of descent with modification, most of
the great facts in Morphology become intelligible, — whether
we look to the same pattern displayed by the different species
of the same class in their homologous organs, to whatever
purpose applied; or to the serial and lateral homologies in
each individual animal and plant.
On the principle of successive slight variations, not neces-
sarily or generally supervening at a very early period of life,
and being inherited at a corresponding period, we can under-
stand the leading facts in Embryology ; namely, the close
resemblance in the individual embryo of the parts which are
homologous, and which when matured become widely dif-
ferent in structure and function ; and the resemblance of the
homologous parts or organs in allied though distinct species,
though fitted in the adult state for habits as different as is
possible. Larvae are active embryos, which have been spe-
cially modified in a greater or less degree in relation to their
habits of life, with their modifications inherited at a corre-
sponding early age. On these same principles, — and bearing
in mind that when organs are reduced in size, either from
498 SUMMARY
disuse or through natural selection, it will generally be at
that period of life when the being has to provide for its own
wants, and bearing in mind how strong is the force of in-
heritance — the occurrence of rudimentary organs might even
have been anticipated. The importance of embryological
characters and of rudimentary organs in classification is
intelligible, on the view that a natural arrangement must be
genealogical.
Finally, the several classes of facts which have been con-
sidered in this chapter, seem to me to proclaim so plainly,
that the innumerable species, genera and families, with which
this world is peopled, are all descended, each within its own
class or group, from common parents, and have all been
modified in the course of descent, that I should without hesi-
tation adopt this view, even if it were unsupported by other
facts or arguments.
CHAPTER XV
Recapitulation and Conclusion
Recapitulation of the objections to the theory of Natural Selection —
Recapitulation of the general and special circumstances in its
favour — Causes of the general belief in the immutability of
species — How far the theory of Natural Selection may be ex-
tended^ — Effects of its adoption on the study of Natural History
• — Concluding remarks.
AS this whole volume is one long argument, it may be
convenient to the reader to have the leading facts
- and inferences briefly recapitulated.
That many and serious objections may be advanced against
the theory of descent with modification through variation
and natural selection, I do not deny. I have endeavoured to
give to them their full force. Nothing at first can appear
more difficult to believe than that the more complex organs
and instincts have been perfected, not by means superior to,
though analogous with, human reason, but by the accumu-
lation of innumerable slight variations, each good for the
individual possessor. Nevertheless, this difficulty, though
appearing to our imagination insuperably great, cannot be
considered real if we admit the following propositions,
namely, that all parts of the organisation and instincts offer,
at least, individual differences — that there is a struggle for
existence leading to the preservation of profitable deviations
of structure or instinct — and, lastly, that gradations in the
state of perfection of each organ may have existed, each good
of its kind. The truth of these propositions cannot, I think,
be disputed.
It is, no doubt, extremely difficult even to conjecture by
what gradations many structures have been perfected, more
especially amongst broken and failing groups of organic
beings, which have suffered much extinction ; but we see so
many strange gradations in nature, that we ought to be ex-
. 499
500 ORIGIN OF SPECIES
tremely cautious in saying that any organ or instinct, or any
whole structure, could not have arrived at its present state by
many graduated steps. There are, it must be admitted, cases
of special difficulty opposed to the theory of natural selec-
tion ; and one of the most curious of these is the existence in
the same community of two or three defined castes of workers
or sterile female ants ; but I have attempted to show how
these difficulties can be mastered.
With respect to the almost universal sterility of species
when first crossed, which forms so remarkable a contrast with
the almost universal fertility of varieties when crossed, I
must refer the reader to the recapitulation of the facts given
at the end of the ninth chapter, which seem to me conclu-
sively to show that this sterility is no more a special endow-
ment than is the incapacity of two distinct kinds of trees to
be grafted together ; but that it is incidental on differences
confined to the reproductive systems of the intercrossed
species. We see the truth of this conclusion in the vast
difference in the results of crossing the same two species
reciprocally, — that is, when one species is first used as the
father and then as the mother. Analogy from the consider-
ation of dimorphic and trimorphic plants clearly leads to the
same conclusion, for when the forms are illegitimately united,
they yield few or no seed, and their offspring are more or
less sterile ; and these forms belong to the same undoubted
species, and differ from each other in no respect except in
their reproductive organs and functions.
Although the fertility of varieties when intercrossed and
of their mongrel offspring has been asserted by so many
authors to be universal, this cannot be considered as quite
correct after the facts given on the high authority of Gartner
and Kolreuter. Most of the varieties which have been ex-
perimented on have been produced under domestication; and
as domestication (I do not mean mere confinement) almost
certainly tends to eliminate that sterility which, judging from
analogy, would have affected the parent-species if inter-
crossed, we ought not to expect that domestication would
likewise induce sterility in their modified descendants when
crossed. This elimination of sterility apparently fol-
lows from the same cause which allows our domestic
RECAPITULATION AND CONCLUSION 501
animals to breed freely under diversified circumstances ; and
this again apparently follows from their having been grad-
ually accustomed to frequent changes in their conditions
of life.
A double and parallel series of facts seems to throw much
light on the sterility of species, when first crossed, and of
their hybrid offspring. On the one side, there is good reason
to believe that slight changes in the conditions of life give
vigour and fertility to all organic beings. We know also
that a cross between the distinct individuals of the same
variety, and between distinct varieties, increases the number
of their offspring, and certainly gives to them increased size
and vigour. This is chiefly owing to the forms which are
crossed having been exposed to somewhat different con-
ditions of life; for I have ascertained by a laborious series of
experiments that if all the individuals of the same variety be
subjected during several generations to the same conditions,
the good derived from crossing is often much diminished or
wholly disappears. This is one side of the case. On the
other side, we know that species which have long been ex-
posed to nearly uniform conditions, when they are subjected
under confinement to new and greatly changed conditions,
either perish, or if they survive, are rendered sterile, though
retaining perfect health. This does not occur, or only in a
very slight degree, with our domesticated productions, which
have long been exposed to fluctuating conditions. Hence
when we find that hybrids produced by a cross between two
distinct species are few in number, owing to their perishing
soon after conception or at a very early age, or if surviving
that they are rendered more or less sterile, it seems highly
probable that this result is due to their having been in fact
sabjected to a great change in their conditions of life, from
being compounded of two distinct organisations. He who
will explain in a definite manner why, for instance, an ele-
phant or a fox will not breed under confinement in its native
country, whilst the domestic pig or dog will breed freely under
the most diversified conditions, will at the same time be able
to give a definite answer to the question why two distinct
species, when crossed, as well as their hybrid offspring, are
generally rendered more or less sterile, whilst two domesti-
502 ORIGIN OF SPECIES
cated varieties when crossed and their mongrel offspring are
perfectly fertile.
Turning to geographical distribution, the difficulties en-
countered on the theory of descent with modification are
serious enough. All the individuals of the same species, and
all the species of the same genus, or even higher group, are
descended from common parents ; and therefore, in however
distant and isolated parts of the world they may now be found,
they must in the course of successive generations have
travelled from some one point to all the others. We are
often wholly unable even to conjecture how this could have
been effected. Yet, as we have reason to believe that some
species have retained the same specific form for very long
periods of time, immensely long as measured by years, too
much stress ought not to be laid on the occasional wide dif-
fusion of the same species ; for during very long periods
there will always have been a good chance for wide migra-
tion by many means. A broken or interrupted range may
often be accounted for by the extinction of the species in the
intermediate regions. It cannot be denied that we are as yet
very ignorant as to the full extent of the various climatal
and geographical changes which have affected the earth dur-
ing modern periods; and such changes will often have facili-
tated migration. As an example, I have attempted to show
how potent has been the influence of the Glacial period on
the distribution of the same and of allied species throughout
the world. We are as yet profoundly ignorant of the many
occasional means of transport. With respect to distinct
species of the sam.e genus inhabiting distant and isolated
regions, as the process of modification has necessarily been
slow, all the means of migration will have been possible dur-
ing a very long period ; and consequently the difficulty of the
wide diffusion of the species of the same genus is in some
degree lessened.
As according to the theory of natural selection an inter-
minable number of intermediate forms must have existed,
linking together all the species in each group by gradations
as fine as are our existing varieties, it may be asked. Why do
we not see these linking forms all around ns ? Why are not all
organic beings blended together in an inextricable chaos?
RECAPITULATION AND CONCLUSION 503
With respect to existing forms, we should remember that we
have no right to expect (excepting in rare cases) to discover
directly connecting Hnks between them, but only between
each and some extinct and supplanted form. Even on a wide
area, which has during a long period remained continuous,
and of which the climatic and other conditions of life change
insensibly in proceeding from a district occupied by one
species into another district occupied by a closely allied
species, we have no just right to expect often to find inter-
mediate varieties in the intermediate zones. For we have
reason to believe that only a few species of a genus ever
undergo change ; the other species becoming utterly extinct
and leaving no modified progeny. Of the species which do
change, only a few within the same country change at the
same time ; and all modifications are slowly effected. I have
also shown that the intermediate varieties which probably at
first existed in the intermediate zones, would be liable to be
supplanted by the allied forms on either hand; for the latter,
from existing in greater numbers, would generally be modi-
fied and improved at a quicker rate than the intermediate vari-
eties, which existed in lesser numbers ; so that the inter-
mediate varieties would, in the long run, be supplanted and
exterminated.
On this doctrine of the extermination of an infinitude of
■connecting links, between the living and extinct inhabitants
of the world, and at each successive period between the
extinct and still older species, why is not every geological
formation charged with such links ? Why does not every col-
lection of fossil remains afford plain evidence of the grada-
tion and mutation of the forms of life ? Although geological
research has undoubtedly revealed the former existence of
many links, bringing numerous forms of life much closer to-
gether, it does not yield the infinitely many fine gradations
l)etween past and present species required on the theory; and
this is the most obvious of the many objections which may
he urged against it. Why, again, do whole groups of allied
species appear, though this appearance is often false, to have
come in suddenly on the successive geological stages? Al-
though we now know that organic beings appeared on this
globe, at a period incalculably remote, long before the lowest
504 ORIGIN OF SPECIES
bed of the Cambrian system was deposited, why do we not
find beneath this system great piles of strata stored with the
remains of the progenitors of the Cambrian fossils? For on
the theory, such strata must somewhere have been deposited
at these ancient and utterly unknown epochs of the world's
history.
I can answer these questions and objections only on the
supposition that the geological record is far more imperfect
than most geologists believe. The number of specimens in
all our museums is absolutely as nothing compared with the
countless generations of countless species which have cer-
tainly existed. The parent-form of any two or more species
would not be in all its characters directly intermediate be-
tween its modified offspring, any more than the rock-pigeon
is directly intermediate in crop and tail between its descend-
ants, the pouter and fantail pigeons. We should not be able
to recognise a species as the parent of another and modified
species, if we were to examine the two ever so closely, unless
we possessed most of the intermediate links; and owing to
the imperfection of the geological record, we have no just
right to expect to find so many links, If two or three, or
even more linking forms were discovered, they would simply
be ranked by many naturalists as so many new species, more
especially if found in different geological sub-stages, let their
differences be ever so slight. Numerous existing doubtful
forms could be named which are probably varieties ; but who
will pretend that in future ages so many fossil links will be
discovered, that naturalists will be able to decide whether or
not these doubtful forms ought to be called varieties? Only
a small portion of the world has been geologically explored.
Only organic beings of certain classes can be preserved in a
fossil condition, at least in any great number. Many species
when once formed never undergo any further change but be-
come extinct without leaving modified descendants; and the
periods, during which species have undergone modification,
though long as measured by years, have probably been short
in comparison with the periods during which they retained
the same form. It is the dominant and widely ranging species
which vary most frequently and vary most, and varieties are
often at first local — both causes rendering the discovery of
RECAPITULATION AND CONCLUSION 505
intermediate links in any one formation less likely. Local
A'arieties will not spread into other and distant regions until
they are considerably modified and improved ; and when they
have spread, and are discovered in a geological formation,
they appear as if suddenly created there, and will be simply
classed as new species. Most formations have been inter-
mittent in their accumulation; and their duration has prob-
ably been shorter than the average duration of specific forms.
Successive formations are in most cases separated from each
other by blank intervals of time of great length; for fos-
siliferous formations thick enough to resist future degrada-
tion can as a general rule be accumulated only where much
sediment is deposited on the subsiding bed of the sea. Dur-
ing the alternate periods of elevation and of stationary level
the record will generally be blank. During these latter
periods there will probably be more variability in the forms
of life ; during periods of subsidence, more extinction.
With respect to the absence of strata rich in fossils beneath
the Cambrian formation, I can recur only to the hypothesis
given in the tenth chapter; namely, that though our conti-
nents and oceans have endured for an enormous period in
nearly their present relative positions, we have no reason to
assume that this has always been the case ; consequently for-
mations much older than any now known may lie buried be-
neath the great oceans. With respect to the lapse of time
not having been sufficient since our planet was consolidated
for the assumed amount of organic change, and this objection,
as urged by Sir William Thompson, is probably one of the
gravest as yet advanced, I can only say, firstly, that we do
not know at what rate species change as measured by years,
and secondly, that many philosophers are not as yet willing
to admit that we know enough of the constitution of the uni-
verse and of the interior of our globe to speculate with safety
on its past duration.
That the geological record is imperfect all will admit ; but
that it is imperfect to the degree required by our theory, few
will be inclined to admit. If we look to long enough intervals
of time, geology plainly declares that species have all
changed ; and they have changed in the manner required by
the theory, for they have changed slowly and in a graduated
806 ORIGIN OF SPECIES
manner. We clearly see this in the fossil remains from con-
secutive formations invariably being much more closely re-
lated to each other, than are the fossils from v^idely separated
formations.
Such is the sum of the several chief objections and diffi-
culties which may be justly urged against the theory; and I
have now briefly recapitulated the answers and explanations
which, as far as I can see, may be given. I have felt these
difficulties far too heavily during many years to doubt their
weight. But it deserves especial notice that the more im-
portant objections relate to questions on which we are con-
fessedly ignorant; nor do we know how ignorant we are.
We do not know all the possible transitional gradations be-
tween the simplest and the most perfect organs ; it cannot be
pretended that we know all the varied means of Distribution
during the long lapse of years, or that we know how imper-
fect is the Geological Record. Serious as these several ob-
jections are, in njy judgment they are by no means sufficient to
overthrow the theory of descent with subsequent modification.
Now let us turn to the other side of the argument. Under
domestication we see much variability, caused, or at least
excited, by changed conditions of life ; but often in so obscure
a manner, that we are tempted to consider the variations as
spontaneous. Variability is governed by many complex laws,
— by correlated growth, compensation, the increased use and
disuse of parts, and the definite action of the surrounding
conditions. There is much difficulty in ascertaining how
largely our domestic productions have been modified; but we
may safely infer that the amount has been large, and that
modification can be inherited for long periods. As long as
the conditions of life remain the same, we have reason to
believe that a modification, which has already been inherited
for many generations, may continue to be inherited for an
almost infinite number of generations. On the other hand,
we have evidence that variability when it has once come into
play, does not cease under domestication for a very long
period ; nor do we know that it ever ceases, for new varieties
are still occasionally produced by our oldest domesticated
productions.
RECAPITULATION AND CONCLUSION 507
Variability is not actually caused by man ; he only uninten-
tionally exposes organic beings to new conditions of life, and
then nature acts on the organisation and causes it to vary.
But man can and does select the variations given to him by
nature, and thus accumulates them in any desired manner.
He thus adapts animals and plants for his own benefit or
pleasure. He may do this methodically, or he may do it
unconsciously by preserving the individuals most useful or
pleasing to him without any intention of altering the breed.
It is certain that he can largely influence the character of a
breed by selecting, in each successive generation, individual
differences so slight as to be inappreciable except by an edu-
cated eye. This unconscious process of selection has been
the great agency in the formation of the most distinct and
useful domestic breeds. That many breeds produced by man
have to a large extent the character of natural species, is
shown by the inextricable doubts whether many of them are
varieties or aboriginally distinct species.
There is no reason why the principles which have acted so
efficiently under domestication should not have acted under
nature. In the survival of favoured individuals and races,
during the constantly-recurrent Struggle for Existence, we
see a powerful and ever-acting form of Selection. The
struggle for existence inevitably follows from the high geo-
metrical ratio of increase which is common to all organic
beings. This high rate of increase is proved by calculation, —
by the rapid increase of many animals and plants during a
succession of peculiar seasons, and when naturalised in new
countries. More individuals are born than can possibly sur-
vive. A grain in the balance may determine which indi-
viduals shall live and which shall die, — which variety or
species shall increase in number, and which shall decrease,
or finally become extinct. As the individuals of the same
species come in all respects into the closest competition with
each other, the struggle will generally be most severe between
them ; it will be almost equally severe between the varieties
of the same species, and next in severity between the species
of the same genus. On the other hand the struggle will often
be severe between beings remote in the scale of nature. The
slightest advantage in certain individuals, at any age or dur-
508 ORIGIN OF SPECIES
ing any season^ over those with which they come into com-
petition, or better adaptation in however slight a degree to
the surrounding physical conditions, will, in the long run,
turn the balance.
With animals having separated sexes, there will be in most
cases a struggle between the males for the possession of the
females. The most vigorous males, or those which have most
successfully struggled with their conditions of life, will gen-
erally leave most progeny. But success will often depend on
the males having special weapons, or means of defence, or
charms ; and a slight advantage will lead to victory.
As geology plainly proclaims that each land has undergone
great physical changes, we might have expected to find that
organic beings have varied under nature, in the same way as
they have varied under domestication. And if there has been
any variability under nature, it would be an unaccovmtable
fact if natural selection had not come into play. It has often
been asserted, but the assertion is incapable of proof, that the
amount of variation under nature is a strictly limited quan-
tity. Man, though acting on external characters alone and
often capriciously, can produce within a short period a great
result by adding up mere individual differences in his domes-
tic productions; and every one admits that species present
individual differences. But, besides such differences, all nat-
uralists admit that natural varieties exist, which are consid-
ered sufficiently distinct to be worthy of record in systematic
works. No one has drawn any clear distinction between in-
dividual differences and slight varieties ; or between more
plainly marked varieties and sub-species, and species. On
separate continents, and on different parts of the same con-
tinent when divided by barriers of any kind, and on outlying
islands, what a multitude of forms exist, which some experi-
enced naturalists rank as varieties, others as geographical
races or sub-species, and others as distinct, though closely
allied species !
If then, animals and plants do vary, let it be ever so slightly
or slowly, why should not variations or individual differences,
which are in any way beneficial, be preserved and accumu-
lated through natural selection, or the survival of the fittest?
If man can by patience select variations useful to him, why,
RECAPITULATION AND CONCLUSION 509
under changing and complex conditions of life, should not
variations useful to nature's living products often arise, and
be preserved or selected ? What limit can be put to this power,
acting during long ages and rigidly scrutinising the whole
constitution, structure, and habits of each creature, — favour-
ing the good and rejecting the bad? I can see no limit to
this power, in slowly and beautifully adapting each form to
the most complex relations of life. The theory of natural
selection, even if we look no farther than this, seems to be
in the highest degree probable. I have already recapitulated,
as fairly as I could, the opposed difficulties and objections:
now let us turn to the special facts and arguments in favour
of the theory.
On the view that species are only strongly marked and
permanent varieties, and that each species first existed as a
variety, we can see why it is that no line of demarcation can
be drawn between species, commonly supposed to have been
produced by special acts of creation, and varieties which are
acknowledged to have been produced by secondary laws. On
this same view we can understand how it is that in a region
v/here many species of a genus have been produced, and
where they now flourish, these same species should present
many varieties ; for where the manufactory of species has
been active, we might expect, as a general rule, to find it still
in action; and this is the case if varieties be incipient species.
Moreover, the species of the larger genera, which afford the
greater number of varieties or incipient species, retain to a
certain degree the character of varieties ; for they differ from
each other by a less amount of difference than do the species
of smaller genera. The closely allied species also of the
larger genera apparently have restricted ranges, and in their
affinities they are clustered in little groups round other
species — in both respects resembling varieties. These are
strange relations on the view that each species was inde-
pendently created, but are intelligible if each existed first as
a variety.
As each species tends by its geometrical rate of reproduc-
tion to increase inordinately in number; and as the modified
descendants of each species "will be enabled to increase by as
510 ORIGIN OF SPECIES
much as they become more diversified in habits and structure,
so as to be able to seize on many and widely different places
in the economy of nature, there will be a constant tendency
in natural selection to preserve the most divergent offspring
of any one species. Hence, during a long-continued course
of modification, the slight differences characteristic of varie-
ties of the same species, tend to be augmented into the greater
differences characteristic of the species of the same genus.
New and improved varieties will inevitably supplant and ex-
terminate the older, less improved, and intermediate vari-
eties ; and thus species are rendered to a large extent defined
and distinct objects. Dominant species belonging to the
larger groups within each class tend to give birth to new and
dominant forms ; so that each large group tends to become
still larger, and at the same time more divergent in char-
acter. But as all groups cannot thus go on increasing in
size, for the world would not hold them, the more dominant
groups beat the less dominant. This tendency in the large
groups to go on increasing in size and diverging in character,
together with the inevitable contingency of much extinction,
explains the arrangement of all the forms of life in groups
subordinate to groups, all within a few great classes, which
has prevailed throughout all time. This grand fact of the
grouping of all organic beings under what is called the Nat-
ural System, is utterly inexplicable on the theory of creation.
As natural selection acts solely by accumulating slight,
successive, favourable variations, it can produce no great or
sudden modifications ; it can act only by short and slow steps.
Hence, the canon of "Natura non facit saltum," which every
fresh addition to our knowledge tends to confirm, is on this
theory intelligible. We can see why throughout nature the
same general end is gained by an almost infinite diversity of
means, for every peculiarity when once acquired is long in-
herited, and structures already modified in many different
ways have to be adapted for the same general purpose. We
can, in short, see why nature is prodigal in variety, though
niggard in innovation. But why this should be a law of
nature if each species has been independently created no man
can explain.
Many other facts are, as it seems to me, explicable on
RECAPITULATION AND CONCLUSION 511
this theory. How strange it is that a bird, under the form
of a woodpecker, should prey on insects on the ground; that
upland geese which rarely or never swim, should possess
webbed feet; that a thrush-like bird should dive and feed
on sub-aquatic insects; and that a petrel should have the
habits and structure fitting it for the life of an auk ! and so
in endless other cases. But on the view of each species
constantly trying to increase in number, with natural selec-
tion always ready to adapt the slowly varying descendants of
each to any unoccupied or ill-occupied place in nature, these
facts cease to be strange, or might even have been antici-
pated.
We can to a certain extent understand how it is that there
is so much beauty throughout nature; for this may be largely
attributed to the agency of selection. That beauty, accord-
ing to our sense of it, is not universal, must be admitted by
every one who will look at some venomous snakes, at some
fishes, and at certain hideous bats with a distorted resem-
blance to the human face. Sexual selection has given the
most brilliant colours, elegant patterns, and other ornaments
to the males, and sometimes to both sexes of many birds,
butterflies, and other animals. With birds it has often ren-
dered the voice of the male musical to the female, as well as
to our ears. Flowers and fruit have been rendered con-
spicuous by brilliant colours in contrast with the green foli-
age, in order that the flowers may be easily seen, visited,
and fertilised by insects, and the seeds disseminated by birds.
How it comes that certain colours, sounds, and forms should
give pleasure to man and the lower animals, — that is, how
the sense of beauty in its simplest form was first acquired, —
we do not know any more than how certain odours and
flavours were first rendered agreeable.
As natural selection acts by competition, it adapts and
improves the inhabitants of each country only in relation to
their co-inhabitants ; so that we need feel no surprise at the
species of any one country, although on the ordinary view
supposed to have been created and specially adapted for that
country, being beaten and supplanted by the naturalised pro-
ductions from another land. Nor ought we to marvel if all
the contrivances in nature -be not, as far as we can judge,
S12 ORIGIN OF SPECIES
absolutely perfect, as in the case even of the human eye; or
if some of them be abhorrent to our ideas of fitness. We
need not marvel at the sting of the bee, when used against
an enemy, causing the bee's own death; at drones being
produced in such great numbers for one single act, and being
then slaughtered by their sterile sisters ; at the astonishing
waste of pollen by our fir-trees; at the instinctive hatred of
the queen-bee for her own fertile daughters; at ichneumon-
idae feeding within the living bodies of caterpillars ; or at
other such cases. The wonder indeed is, on the theory of
natural selection^ that more cases of the want of absolute
perfection have not been detected.
The complex and little known laws governing the produc-
tion of varieties are the same, as far as we can judge, with
the laws which have governed the production of distinct
species. In both cases physical conditions seem to have pro-
duced some direct and definite effect, but how much we can-
not say Thus, when varieties enter any new station, they
occasionally assume some of the characters proper to the
species of that station. With both varieties and species, use
and disuse seem to have produced a considerable effect; for
it is impossible to resist this conclusion when we look, for
instance, at the logger-headed duck, which has wings in-
capable of flight; in nearly the same condition as in the do-
mestic duck ; or when we look at the burrowing tucu-tucu,
which is occasionally blind, and then at certain moles, which
are habitually blind and have their eyes covered with skin ;
or when we look at the blind animals inhabiting the dark
caves of America and Europe. With varieties and species,
correlated variation seems to have played an important part,
so that when one part has been modified other parts have
been necessarily modified. With both varieties and species,
reversions to long-lost characters occasionally occur. How
inexplicable on the theory of creation is the occasional ap-
pearance of stripes on the shoulders and legs of the several
species of the horse-genus and of their hybrids ! How simply
is this fact explained if we believe that these species are all
descended from a striped progenitor, in the same manner as
the several domestic breeds of the pigeon are descended
from the blue and barred rock-pigeon !
RECAPITULATION AND CONCLUSION 513
On the ordinary view of each species having been inde-
pendently created, why should specific characters, or those
by which the species of the same genus differ from each
other, be more variable than generic characters in which
they all agree? Why, for instance, should the colour of a
flower be more likely to vary in any one species of a genus,
if the other species possess differently coloured flowers, than
if all possessed the same coloured flowers? If species are
only well-marked varieties, of which the characters have be-
come in a high degree permanent, we can understand this
fact; for they have already varied since they branched off
from a common progenitor in certain characters, by which
they have come to be specifically distinct from each other ;
therefore these same characters would be more likely again
to vary than the generic characters which have been in-
herited without change for an immense period. It is inex-
plicable on the theory of creation why a part developed in a
very unusual manner in one species alone of a genus, and
therefore, as we may naturally infer, of great importance to
that species, should be eminently liable to variation; but, on
our view, this part has undergone, since the several species
branched off from a common progenitor, an unusual amount
of variability and modification, and therefore we might ex-
pect the part generally to be still variable. But a part may
be developed in the most unusual manner, like the wing of a
bat, and yet not be more variable than any other structure,
if the part be common to many subordinate forms, that is, if
it has been inherited for a very long period; for in this case
it will have been rendered constant by long-continued natural
selection.
Glancing at instincts, marvellous as some are, they offer
no greater difficulty than do corporeal structures on the
theory of the natural selection of successive, slight, but
profitable modifications. We can thus understand why nature
moves by graduated steps in endowing different animals of
the same class with their several instincts. I have attempted
to show how much light the principle of gradation throws
on the admirable architectural powers of the hive-bee. Habit
no doubt often comes into play in modifying instincts ; but
it certainly is not indispensable, as we see in the case of
Q—Ur ■
514 ORIGIN OF SPECIES
neuter insects, which leave no progeny to inherit the effects
of long-continued habit. On the view of all the species of
the same genus having descended from a common parent,
and havmg inherited much in common, we can understand
how it is that allied species, when placed under widely dif-
ferent conditions of life, yet follow nearly the same in-
stincts; why the thrushes of tropical and temperate South
America, for instance, line their nests with mud like our
British species. On the view of instincts having been slowly
acquired through natural selection, we need not marvel at
some instincts being not perfect and liable to mistakes, and
at many instincts causing other animals to suffer.
If species be only well-marked and permanent varieties,
we can at once see why their crossed offspring should follow
the same complex laws in their degrees and kinds of resem-
blance to their parents, — in being absorbed into each other
by successive crosses, and in other such points, — as do the
crossed offspring of acknowledged varieties. This similarity
would be a strange fact, if species had been independently
created and varieties had been produced through secondary
laws.
If we admit that the geological record is imperfect to an
extreme degree, then the facts, which the record does give,
strongly support the theory of descent with modification.
New species have come on the stage slowly and at succes-
sive intervals, and the amount of change, after equal inter-
vals of time, is widely different in different groups. The
extinction of species and of whole groups of species, which
has played so conspicuous a part in the history of the organic
world, almost inevitably follows from the principle of nat-
ural selection; for old forms are supplanted by new and
improved forms. Neither single species nor groups of
species reappear when the chain of ordinary generation is
once broken. The gradual diffusion of dominant forms, with
the slow modification of their descendants, causes the forms
of life, after long intervals of time, to appear as if they had
changed simultaneously throughout the world. The fact of
the fossil remains of each formation being in some degree
intermediate in character between the fossils in the forma-
tions above and below, is simply explained by their inter-
RECAPITULATION AND CONCLUSION 515
mediate position in the chain of descent. The grand fact
that all extinct beings can be classed with all recent beings,
naturally follows from the living and the extinct being the
offspring of common parents. As species have generally
diverged in character during their long course of descent
and modification, we can understand why it is that the more
ancient forms, or early progenitors of each group, so often
occupy a position in some degree intermediate between ex-
isting groups. Recent forms are generally looked upon as
being, on the whole, higher in the scale of organisation than
ancient forms; and they must be higher, in so far as the
later and more improved forms have conquered the older and
less improved forms in the struggle for life; they have also
generally had their organs more specialised for different
functions. This fact is perfectly compatible with numerous
beings still retaining simple and but little improved struc-
tures, fitted for simple conditioi of life ; it is likewise com-
patible with some forms having retrograded in organisation,
by having become at each stage of descent better fitted for
new and degraded habits of life. Lastly, the wonderful lav^
of the long endurance of allied forms on the same conti-
nent, — of marsupials in Australia, of edentata in America,
and other such cases, is intelligible, for within the same coun-
try the existing and the extinct will be closely allied by
descent.
Looking to geographical distribution, if we admit that
there has been during the long course of ages much migra-
tion from one part of the world to another, owing to former
climatal and geographical changes and to the many occa-
sional and unknown means of dispersal, then we can under-
stand, on the theory of descent with modification, most of
the great leading facts in Distribution. We can see why
there should be so striking a parallelism in the distribution
of organic beings throughout space, and in their geological
succession throughout time ; for in both cases the beings
have been connected by the bond of ordinary generation, and
the means of modification have been the same. We see the
full meaning of the wonderful fact, which has struck every
traveller, namely, that on the same continent, under the most
diverse conditions, under* heat and cold, on mountain and
516 ORIGIN OF SPECIES
lowland, on deserts and marshes, most of the inhabitants
within each great class are plainly related ; for they are the
descendants of the same progenitors and early colonists.
On this same principle of former migration, combined in
most cases with modification, we can understand, by the aid
of the Glacial period, the identity of some few plants, and
the close alliance of many others, on the most distant moun-
tains, and in the northern and southern temperate zones;
and likewise the close alliance of some of the inhabitants
of the sea in the northern and southern temperate latitudes,
though separated by the whole intertropical ocean. Al-
though two countries may present physical conditions as
closely similar as the same species ever require, we need feel
no surprise at their inhabitants being widely different, if
they have been for a long period completely sundered from
each other; for as the relation of organism to organism is
the most important of all relations, and as the two countries
will have received colonists at various periods and in differ-
ent proportions, from some other country or from each other,
the course of modification in the two areas will inevitably
have been different.
On this view of migration, with subsequent modification, we
see why oceanic islands are inhabited by only few species,
but of these, why many are peculiar or endemic forms. We
clearly see why species belonging to those groups of animals
which cannot cross wide spaces of the ocean, as frogs and
terrestrial mammals, do not inhabit oceanic islands; and
why, on the other hand, new and peculiar species of bats,
animals which can traverse the ocean, are often found on
islands far distant from any continent. Such cases as the
presence of peculiar species of bats on oceanic islands and
the absence of all other terrestrial mammals, are facts utterly
inexplicable on the theory of independent acts of creation.
The existence of closely allied or representative species
in any two areas, implies, on the theory of descent with modi-
fication, that the same parent-forms formerly inhabited both
areas : and we almost invariably find that wherever many
closely allied species inhabit two areas, some identical
species are still common to both. Wherever many closely
allied yet distinct species occur, doubtful forms and vari-
RECAPITULATION AND CONCLUSION 517
eties belonging to the same groups likewise occur. It is a
rule of high generality that the inhabitants of each area are
related to the inhabitants of the nearest source whence im-
migrants might have derived. We see this in the striking
relation of nearly all the plants and animals of the Gala-
pagos archipelago, of Juan Fernandez, and of the other
A.merican islands, to the plants and animals of the neigh-
bouring American mainland ; and of those of the Cape de
Verde archipelago, and of the other African islands to the
African mainland. It must be admitted that these facts
receive no explanation on the theory of creation.
The fact, as we have seen, that all past and present or-
ganic beings can be arranged within a few great classes, in
groups subordinate to groups, and with the extinct groups
often falling in between the recent groups, is intelligible on
the theory of natural selection with its contingencies of ex-
tinction and divergence of character. On these same prin-
ciples we see how it is, that the mutual affinities of the forms
within each class are so complex and circuitous. We see
why certain characters are far more serviceable than others
for classification ; — why adaptive characters, though of para-
mount importance to the beings, are of hardly any impor-
tance in classification ; why characters derived from rudi-
mentary parts, though of no service to the beings, are often
of high classificatory value ; and why embryological charac-
ters are often the most valuable of all. The real affinities
of all organic beings, in contradistinction to their adaptive
resemblances, are due to inheritance or community of de-
scent. The Natural System is a genealogical arrangement,
with the acquired grades of difference, marked by the terms,
varieties, species, genera, families, &c. ; and we have to dis-
cover the lines of descent by the most permanent characters
whatever they may be and of however slight vital impor-
tance.
The similar framework of bones in the hand of a man,
wing of a bat, fin of a porpoise, and leg of the horse, — the
same number of vertebra? forming the neck of the giraffe
and of the elephant, — and inaumerable other such facts, at
once explain themselves on the theory of descent with slow
and slight successive modifications. The similarity of pat-
518 ORIGIN OF SPECIES
tern in the wing and in the leg of a bat, though used for
such dififerent purpose, — in the jaws and legs of a crab, —
in the petals, stamens, and pistils of a flower, is likewise, to
a large extent, intelligible on the view of the gradual modi-
fication of parts or organs, which were aboriginally alike in
an early progenitor in each of these classes. On the prin-
ciple of successive variations not always supervening at an
early age, and being inherited at a corresponding not early
period of life, we clearly see why the embryos of mammals,
birds, reptiles, and fishes should be so closely similar, and so
unlike the adult forms. We may cease marvelling at the
embryo of an air-breathing mammal or bird having branchial
slits and arteries running in loops, like those of a fish which
has to breathe the air dissolved in water by the aid of well-
developed branchiae.
Disuse, aided sometimes by natural selection, will often
have reduced organs when rendered useless under changed
habits or conditions of life; and we can understand on this
view the meaning of rudimentary organs. But disuse and
selection will generally act on each creature, when it has
come to maturity and has to play its full part in the struggle
for existence, and will thus have little power on an organ
during early life ; hence the organ will not be reduced or
rendered rudimentary at this early age. The calf, for in-
stance, has inherited teeth, which never cut through the
gums of the upper jaw, from an early progenitor having well-
developed teeth; and we may believe, that the teeth in the
mature animal were formerly reduced by disuse, owing to
the tongue and palate, or lips, having become excellently
fitted through natural selection to browse without their aid ;
whereas in the calf, the teeth have been left unaffected, and
on the principle of inheritance at- corresponding ages have
been inherited from a remote period to the present day. On
the view of each organism with all its separate parts having
been specially created, how utterly inexplicable is it that
organs bearing the plain stamp of inutility, such as the teeth
in the embryonic calf or the shrivelled wings under the sol-
dered wing-covers of many beetles, should so frequently
occur. Nature may be said to have taken pains to reveal
her scheme of modification, by means of rudimentary organs,
RECAPITULATION AND CONCLUSION 519
of embryological and homologous structures, but we are too
blind to understand her meaning.
I have now recapitulated the facts and considerations
which have thoroughly convinced me that species have been
modified, during a long course of descent. This has been
effected chiefly through the natural selection of numerous
successive, slight, favourable variations; aided in an im-
portant manner by the inherited effects of the use and dis-
use of parts ; and in an unimportant manner, that is in rela-
tion to adaptive structures, whether past or present, by the
direct action of external conditions, and by variations which
seem to us in our ignorance to arise spontaneously. It ap-
pears that I formerly underrated the frequency and value of
these latter forms of variation, as leading to permanent modi-
fications of structure independently of natural selection.
But as my conclusions have lately been much misrepre-
sented, and it has been stated that I attribute the modifica-
tion of species exclusively to natural selection, I may be per-
mitted to remark that in the first edition of this work, and
subsequently, I placed in a most conspicuous position —
namely, at the close of the Introduction the following words:
"I am convinced that natural selection has been the main
but not the exclusive means of modification." This has been
of no avail. Great is the power of steady misrepresenta-
tion; but the history of science shows that fortunately this
power does not long endure.
It can hardly be supposed that a false theory would ex-
plain, in so satisfactory a manner as does the theory of nat-
ural selection, the several large classes of facts above speci-
fied. It has recently been objected that this is an unsafe
method of arguing; but it is a method used in judging of
the common events of life, and has often been used by the
greatest natural philosophers. The undulatory theory of
light has thus been arrived at; and the belief in the revolu-
tion of the earth on its own axis was until lately supported
by hardly any direct evidence. It is no valid objection that
science as yet throws no light on the far higher problem of
the essence or origin of life.. Who can explain what is the
essence of the attraction of gravity? No one now objects
to following out the resoalts consequent on this unknown
520 ORIGIN OF SPECIES
element of attraction; notwithstanding that Leibnitz for-
merly accused Newton of introducing "occult qualities and
miracles into philosophy."
I see no good reason why the views given in this volume
should shock the religious feelings of any one. It is satis-
factory, as showing how transient such impressions are, to
remember that the greatest discovery ever made by man,
namely, the law of the attraction of gravity, was also at-
tacked by Leibnitz_, "as subversive of natural, and inferen-
tially of revealed, religion." A celebrated author and divine
has written to me that "he has gradually learnt to see that
it is just as noble a conception of the Deity to believe that
He created a few original forms capable of self-develop-
ment into other and needful forms, as to believe that He
required a fresh act of creation to supply the voids caused
by the action of His laws."
Why, it may be asked, until recently did nearly all the
most eminent living naturalists and geologists disbelieve in
the mutability of species. It cannot be asserted that organic
beings in a state of nature are subject to no variation; it
cannot be proved that the amount of variation in the course
of long ages is a limited quantity; no clear distinction has
been, or can be, drawn between species and well-marked
varieties. It cannot be maintained that species when inter-
crossed are invariably sterile, and varieties invariably
fertile; or that sterility is a special endowment and sign of
creation. The belief that species were immutable produc-
tions was almost unavoidable as long as the history of the
world was thought to be of short duration; and now that we
have acquired some idea of the lapse of time, we are too apt
to assume, without proof, that the geological record is so
perfect that it would have afforded us plain evidence of the
mutation of species, if they had undergone mutation.
But the chief cause of our natural unwillingness to admit
that one species has given birth to other and distinct species,
is that we are always slow in admitting great changes of
which we do not see the steps. The difficulty is the same as
that felt by so many geologists, when Lyell first insisted that
long lines of inland cliffs had been formed, and great valleys
excavated, by the agencies which we see still at work. The
RECAPITULATION AND CONCLUSION 521
mind cannot possibly grasp the full meaning of the term of
even a million years; it cannot add up and perceive the full
effects of many slight variations, accumulated during an
almost infinite number of generations.
Although I am fully convinced of the truth of the views
given in this volume under the form of an abstract, I by no
means expect to convince experienced naturalists whose
minds are stocked with a multitude of facts all viewed,
during a long course of years, from a point of view directly
opposite to mine. It is so easy to hide our ignorance under
such expressions as the "plan of creation," "unity of design,"
&c., and to think that we give an explanation when we only
re-state a fact. Any one whose disposition leads him to
attach more weight to unexplained difficulties than to the
explanation of a certain number of facts will certainly reject
the theory. A few naturalists, endowed with much flexibility
of mind, and who have already begun to doubt the immu-
tability of species, may be influenced by this volume ; but I
look with confidence to the future, — to young and rising
naturalists, who will be able to view both sides of the ques-
tion with impartiality. Whoever is led to believe that species
are mutable will do good service by conscientiously express-
ing his conviction; for thus only can the load of prejudice by
which this subject is overwhelmed be removed.
Several eminent naturalists have of late published their be-
lief that a multitude of reputed species in each genus are
not real species ; but that other species are real, that is, have
been independently created. This seems to me a strange con-
clusion to arrive at. They admit that a multitude of forms,
which till lately they themselves thought were special crea-
tions, and which are still thus looked at by the majority of
naturalists, and which consequently have all the external
characteristic features of true species, — they admit that these
have been produced by variation, but they refuse to extend
the same view to other and slightly different forms. Never-
theless they do not pretend that they can define, or even con-
jecture, which are the created forms of life, and which are
those produced by secondary, laws. They admit variation as
a vera causa in one case, they arbitrarily reject it in another,
without assigning any distinction in the two cases. The day
522 ORIGIN OF SPECIES
will come when this will be given as a curious illustration of
the blindness of preconceived opinion. These authors seem
no more startled at a miraculous act of creation than at an
ordinary birth. But do they really believe that at innu-
merable periods in the earth's history certain elemental atoms
have been commanded suddenly to flash into living tissues?
Do they believe that at each supposed act of creation one
individual or many were produced? Were all the infinitely
numerous kinds of animals and plants created as eggs or
seed, or as full grown? and in the case of mammals, were
they created bearing the false marks of nourishment from
the mother's womb? Undoubtedly some of these same ques-
tions cannot be answered by those who believe in the appear-
ance or creation of only a few forms of life, or of some
one form alone. It has been maintained by several authors
that it is as easy to believe in the creation of a million beings
as of one; but Maupertuis' philosophical axiom "of least
action" leads the mind more willingly to admit the smaller
number ; and certainly we ought not to believe that innu-
merable beings within each great class have been created
with plain, but deceptive, marks of descent from a single
parent.
As a record of a former state of things, I have retained in
the foregoing paragraphs, and elsewhere, several sentences
which imply that naturalists believe in the separate creation
of each species ; and I have been much censured for having
thus expressed myself. But undoubtedly this was the general
belief when the first edition of the present work appeared.
I formerly spoke to very many naturalists on the subject of
evolution, and never once met with any sympathetic agree-
ment. It is probable that some did then believe in evolution,
but they were either silent, or expressed themselves so am-
biguously that it was not easy to understand their meaning.
Now things are wholly changed, and almost every naturalist
admits the great principle of evolution. There are, however,
some who still think that species have suddenly given birth,
through quite unexplained means, to new and totally differ-
ent forms : but, as I have attempted to show, weighty evi-
dence can be opposed to the admission of great and abrupt
modifications. Under a scientific point of view, and as lead-
RECAPITULATION AND CONCLUSION 523
ing to further investigation, but little advantage is gained
by believing that new forms are suddenly developed in an
inexplicable manner from old and widely different forms,
over the old belief in the creation of species from the dust of
the earth.
It may be asked how far I extend the doctrine of the modi-
fication of species. The question is difficult to answer, be-
cause the more distinct the forms are which we consider, by
so much the arguments in favour of community of descent
become fewer in number and less in force. But some
arguments of the greatest weight extend very far. All
the members of whole classes are connected together by
a chain of affinities, and all can be classed on the same
principle, in groups subordinate to groups. Fossil remains
sometimes tend to fill up very wide intervals between exist-
ing orders.
Organs in a rudimentary condition plainly show that an
early progenitor had the organ in a fully developed condi-
tion ; and this in some cases implies an enormous amount of
modification in the descendants. Throughout whole classes
various structures are formed on the same pattern, and at a
very early age the embryos closely resemble each other.
Therefore I cannot doubt that the theory of descent with
modification embraces all the members of the same great
class or kingdom. I believe that animals are descended from
at most only four or five progenitors, and plants from an
equal or lesser number.
Analogy would lead me one step farther, namely, to the
belief that all animals and plants are descended from some
one prototype. But analogy may be a deceitful guide. Never-
theless all living things have much in common, in their
chemical composition, their cellular structure, their laws of
growth, and their liability to injurious influences. We see
this even in so trifling a fact as that the same poison often
similarly affects plants and animals; or that the poison se-
creted by the gall-fly produces monstrous growths on the
wild rose or oak-tree. With all organic beings, excepting
perhaps some of the very lowest, sexual reproduction seems
to be essentially similar. With all, as far as is at present
known, the germinal vesicle is the same ; so that all organisms
524 ORIGIN OF SPECIES
start from a common origin. If we look even to the two
main divisions — namely, to the animal and vegetable king-
doms — certain low forms are so far intermediate in character
that naturalists have disputed to which kingdom they should
be referred. As Professor Asa Gray has remarked, "the
spores and other reproductive bodies of many of the lower
algae may claim to have first a characteristically animal,
and then an unequivocal vegetable existence." Therefore,
on the principle of natural selection with divergence of char-
acter, it does not seem incredible that, from some such low
and intermediate form, both animals and plants may have
been developed; and, if we admit this, we must likewise ad-
mit that all the organic beings which have ever lived on this
earth may be descended from some one primordial form.
But this inference is chiefly grounded on analogy, and it is
immaterial whether or not it be accepted. No doubt it is
possible, as Mr. G. H. Lewes has urged, that at the first
commencement of life many different forms were evolved;
but if so, we may conclude that only a very few have left
modified descendants. For, as I have recently remarked in
regard to the members of each great kingdom, such as the
Vertebrata, Articulata, &c., we have distinct evidence in
their embryological, homologous, and rudimentary structures,
that within each kingdom all the members are descended
from a single progenitor.
When the views advanced by me in this volume, and by
Mr. Wallace, or when analogous views on the origin of spe-
cies are generally admitted, we can dimly foresee that there
will be a considerable revolution in natural history. Sys-
tematists will be able to pursue their labours as at present;
but they will not be incessantly haunted by the shadowy
doubt whether this or that form be a true species. This, I
feel sure and I speak after experience, will be no slight re-
lief. The endless disputes whether or not some fifty species
of British brambles are good species will cease. Systematists
will have only to decide (not that this will be easy) whether
any form be sufficiently constant and distinct from other
forms, to be capable of definition ; and if definable, whether
the differences be sufficiently important to deserve a specific
name. This latter point will become a far more essential
RECAPITULATION AND CONCLUSION S25
consideration than it is at present; for differences, however
slight, between any two forms, if not blended by interme-
diate gradations, are looked at by most naturalists as suffi-
cient to raise both forms to the rank of species.
Hereafter we shall be compelled to acknowledge that the
only distinction between species and well-marked varieties is,
that the latter are known, or believed, to be connected at the
present day by intermediate gradations whereas species were
formerly thus connected. Hence, without rejecting the con-
sideration of the present existence of intermediate grada-
tions between any two forms, we shall be led to weigh more
carefully and to value higher the actual amount of difference
between them. It is quite possible that forms now generally
acknowledged to be merely varieties may hereafter be
thought worthy of specific names; and in this case scientific
and common language will come into accordance. In short,
we shall have to treat species in the same manner as those
naturalists treat genera, who admit that genera are merely
artificial combinations made for convenience. This may not
be a cheering prospect; but we shall at least be freed from
the vain search for the undiscovered and undiscoverable
essence of the term species.
The other and more general departments of natural history
will rise greatly in interest. The terms used by naturalists,
of affinity, relationship, community of type, paternity, mor-
phology, adaptive characters, rudimentary and aborted
organs, &c., will cease to be metaphorical, and will have a
plain signification. When we no longer look at an organic
being as a savage looks at a ship, as something wholly be-
yond his comprehension; when we regard every production
of nature as one which has had a long history; when we
contemplate every complex structure and instinct as the
summing up of many contrivances, each useful to the pos-
sessor, in the same way as any great mechanical invention
is the summing up of the labour, the experience, the reason,
and even the blunders of numerous workmen ; when we
thus view each organic being, how far more interesting — I
speak from experience — does the study of natural history
become !
A grand and almost -untrodden field of inquiry will be
526 ORIGIN OF SPECIES
opened, on the causes and laws of variation, on correlation,
on the effects of use and disuse, on the direct action of ex-
ternal conditions, and so forth. The study of domestic pro-
ductions will rise immensely in value. A new variety raised
by man will be a more important and interesting subject for
study than one more species added to the infinitude of already
recorded species. Our classifications will come to be, as far
as they can be so made, genealogies ; and will then truly give
what may be called the plan of creation. The rules for
classifying will no doubt become simpler when we have a
definite object in view. We possess no pedigrees or armorial
bearings ; and we have to discover and trace the many di-
verging lines of descent in our natural genealogies, by char-
acters of any kind which have long been inherited. Rudi-
mentary organs will speak infallibly with respect to the
nature of long-lost structures. Species and groups of species
which are called aberrant, and which may fancifully be
called living fossils, will aid us in forming a picture of the
ancient forms of life. Embryology will often reveal to us
the structure, in some degree obscured, of the prototypes of
each great class.
When we can feel assured that all the individuals of the
same species, and all the closely allied species of most genera,
have within a not very remote period descended from one
parent, and have migrated from some one birth-place; and
when we better know the many means of migration, then, by
the light which geology now throws, and will continue to
throw, on former changes of climate and of the level of the
land, we shall surely be enabled to trace in an admirable
manner the former migrations of the inhabitants of the whole
world. Even at present, by comparing the differences be-
tween the inhabitants of the sea on the opposite sides of a
continent, and the nature of the various inhabitants on that
continent in relation to their apparent means of immigration,
some light can be thrown on ancient geography.
The noble science of Geology loses glory from the extreme
imperfection of the record. The crust of the earth with its
imbedded remains must not be looked at as a well-filled
museum, but as a poor collection made at hazard and at rare
intervals. The accumulation of each great fossiliferous for-
RECAPITULATION AND CONCLUSION 527
mation will be recognised as having depended on an unusual
concurrence of favourable circumstances, and the blank in-
tervals between the successive stages as having been of vast
duration. But we shall be able to gauge with some security
the duration of these intervals by a comparison of the pre-
ceding and succeeding organic forms. We must be cautious
in attempting to correlate as strictly contemporaneous two
formations, which do not include many identical species, by
the general succession of the forms of life. As species are
produced and exterminated by slowly acting and still exist-
ing causes, and not by miraculous acts of creation; and as
the most important of all causes of organic change is one
which is almost independent of altered and perhaps sud-
denly altered physical conditions, namely, the mutual rela-
tion of organism to organism, — the improvement of one
organism entailing the improvement or the extermination
of others ; it follows, that the amount of organic change in
the fossils of consecutive formations probably serves as a
fair measure of the relative, though not actual lapse of
time. A number of species, however, keeping in a body
might remain for a long period unchanged, whilst within
the same period, several of these species by migrating into
new countries and coming into competition with foreign
associates, might become modified; so that we must not
overrate the accuracy of organic change as a measure of
time.
In the future I see open fields for far more important re-
searches. Psychology will be securely based on the founda-
tion already well laid by Mr. Herbert Spencer, that of the
necessary acquirement of each mental power and capacity by
gradation. Much light will be thrown on the origin of man
and his history.
Authors of the highest eminence seem to be fully satisfied
with the view that each species has been independently cre-
ated. To my mind it accords better with what we know of
the laws impressed on matter by the Creator, that the pro-
duction and extinction of the past and present inhabitants
of the world should have been due to secondary causes, like
those determining the birth and death of the individual.
When I view all beings, not as special creations, but as the
528 ORIGIN OF SPECIES
lineal descendants of some few beings which lived long be-
fore the first bed of the Cambrian system was deposited,
they seem to me to become ennobled. Judging from the past,
we may safely infer that not one living species will transmit
its unaltered likeness to a distant futurity. And of the
species now living very few will transmit progeny of any
kind to a far distant futurity; for the manner in which all
organic beings are grouped, shows that the greater number
of species in each genus, and all the species in many genera,
have left no descendants, but have become utterly extinct.
We can so far take a prophetic glance into futurity as to
foretell that it will be the common and widely-spread species,
belonging to the larger and dominant groups within each
class, which will ultimately prevail and procreate new and
dominant species. As all the living forms of life are the
lineal descendants of those which lived long before the Cam-
brian epoch, we may feel certain that the ordinary' succes-
sion by generation has never once been broken, and that no
cataclysm has desolated the whole world. Hence we may
look with some confidence to a secure future of great length
And as natural selection works solely by and for the good
of each being, all corporeal and mental endowments will tend
to progress towards perfection.
It is mteresting to contemplate a tangled bank, clothed
with many plants of many kinds, with birds singing on the
bushes, with various insects flitting about, and with worms
crawling through the damp earth, and to reflect that these
elaborately constructed forms, so different from each other,
and dependent upon each other in so complex a manner, have
all been produced by laws acting around us. These laws,
taken in the largest sense, being Growth with Reproduction ;
Inheritance which is almost implied by reproduction ; Varia-
bility from the indirect and direct action of the conditions of
life, and from use and disuse : a Ratio of Increase so high as
to lead to a Struggle for Life, and as a consequence to
Natural Selection, entailing Divergence of Character and the
Extinction of less-improved forms. Thus, from the war of
nature, from famine and death, the most exalted object
which we are capable of conceiving, namely, the production
of the higher animals, directly follows. There is grandeur in
RECAPITULATION AND CONCLUSION 529
this view of life, with its several powers, having been origi-
nally breathed by the Creator into a few forms or into one ;
and that, whilst this planet has gone cycling on according
to the fixed law of gravity, from so simple a beginning end-
less forms most beautiful and most wonderful have been, and
are being evolved.
GLOSSARY
OF THE
PRINCIPAL SCIENTIFIC TERMS USED IN THE
PRESENT VOLUME*
Aberrant — Forms or groups of animals or plants which deviate in important
characters from their nearest allies, so as not to be easily included in
the same group with them, are said to be aberrant.
Aberration (in Optics) — In the refraction of light by a convex lens the rays
passing through different parts of the lens are brought to a focus at
slightly different distances — this is called spherical aberration ; at the
same time the coloured rays are separated by the prismatic action of
the lens and likewise brought to a focus at different distances — this
is chromatic aberration.
Abnormal — Contrary to the general rule.
Aborted — An organ is said to be aborted when its development has been
arrested at a very early stage.
Albinism — Albinos are animals in which the usual colouring matters char-
acteristic of the species have not been produced in the skin and its
appendages. Albinism is the state of being an albino.
Alga — A class of plants including the ordinary sea-weeds and the filamentous
fresh-water weeds.
Alternation of Generations — This term is applied to a peculiar mode of
reproduction wliich prevails among many of the lower animals, in
which the egg produces a living form quite different from its parent,
but from which the parent-form is reproduced by a process of budding,
or by the division of the substance of the first product of the egg.
Ammonites — A group of fossil, spiral, chambered shells, allied to the exist-
ing pearly Nautilus, but having the jjartitions between the chambers
waved in complicated patterns at their junction with the outer wall
of the shell.
Analogy — 'The resemblance of structures which depends upon similarity of
function, as in the wings of insects and birds. Such structures am
said to be analogous, and to be analogues of each other.
Animalcule— A minute animal: generally applied to those visible only by
the microscope.
Annelids — A class of worms in which the surface of the body exhibits a
more or less distinct division into rings or segments, generally pro-
vided with appendages for locomotion and with gills. It includes the
ordinary marine worms, the earthworms, and the leeches.
Antenna — Jointed organs appended to the head in Insects, Crustacea, and
Centipedes, and not belonging to the mouth.
Anthers — ^The summits of the stamens of flowers, in which the pollen or
fertilizing dust is produced.
Aplaccntalia, Aplacentata or Aplaccntal Mammals — See Mammalia.
Archetypal — Of or belonging to the Archetype, or ideal primitive form upon
which all the beings of a group seem to be organized.
Articulata — A great division of the Animal Kingdom characterized generally
by having the surface of the body divided into rings called segments,
a greater or less number of which are furnished with jointed legs
(such as Insects, Crustaceans, and Centipedes).
Asymmetrical — Having the two sides unlike.
Atrophied — Attested in development at a very early stage.
• I am indebted to the kindness of Mr. W. S. Dallas for this Glossary,
which has been given because several readers have complained to me that
some of the terms used were unintelligible to them. Mr. Dallas has endeav-
oured to give the explanations of the terms in as popular a form as possible.
" 531
532 GLOSSARY
Balanus — 'The genus including the common Acorn-shells which live in
abundance on the rocks of the sea-coast.
Batrachians—A class of animals allied to the Reptiles, but undergoing a
peculiar metamorphosis, in which the young animal is generally
aquatic and breathes by gills. (Examples, Frogs, Toads, and Newts.)
Boulders — Large transported blocks of stone generally embedded in clays or
gravels.
Brachiopoda — A class of marine Mollusca, or soft-bodied animals, furnished
with a bivalve shell, attached to submarine objects by a stalk which
passes through an aperture in one of the valves, and furnished with
fringed arms, by the action of which food is carried to the mouth.
BranchicE — Gills or organs for respiration in water.
Branchial — Pertaining to gills or branchiae.
Cambrian System — A series of very ancient Palaeozoic rocks, between the
Laurentian and the Silurian. Until recently these were regarded as
the oldest fossiliferous rocks.
Canida: — The Dog-family, including the Dog, Wolf, Fox, Jackal, &c.
Carapace— Tht shell enveloping the anterior part of the body in Crustaceans
generally; applied also to the hard shelly pieces of the Cirripedes.
Carboniferous — This term is applied to the great formation which includes,
among other rocks, the coal measures. It belongs to the oldest, ar
Palaeozoic, system of formations.
Caudal — Of or belonging to the tail.
Cephalopods — 'The highest class of the Mollusca, or soft-bodied animals,
characterized by having the mouth surrounded by a greater or less
number of fleshy arms or tentacles, which, in most living species, are
furnished with sucking-cups. {Examples, Cuttle-fish, Nautilus.)
Cetacea — An order of Mammalia, including the Whales, Dolphins, &c.,
havirig the form of the body fish-like, the skin naked, and only the
forelimbs developed.
Chelonia — An order of Reptiles, including the Turtles, Tortoises, &c.
Cirripedes — An order of Crustaceans including the Barnacles and Acorn-
shells. Their young resemble those of many other Crustaceans in
form; but when mature they are always attached to other objects,
either directly or by means of a stalk, and their bodies are enclosed
by a calcareous shell composed of several pieces, two of which can
open to give issue to a bunch of curled, jointed tentacles, which rep-
resent the limbs.
Coccus- — ^The genus of Insects including the Cochineal. In these the male
is a minute, winged fly, and the female generally a motionless, berry-
like mass.
Cocoon— A case usually of silky material, in which insects are frequently
enveloped during the second or resting stage (pupa) of their existence.
The term " cocoon-stage " is here used as equivalent to " pupa-stage."
Ccelospermoiis — A term applied to those fruits of the Umbelhferae which
have the seed hollowed on the inner face.
Coleoptera — Beetles, an order of Insects, having a biting mouth and the first
pair of wings more or less horny, forming sheaths for the second pair,
and usually meeting in a straight line down the middle of the back.
Column — A peculiar organ in the flowers of Orchids, in which the stamens,
style and stigma (or the reproductive parts) are united.
CompositcB or Compositous Plants — Plants in which the inflorescence con-
sists of numerous small flowers (florets) brought together into a dense
head, the base of which is enclosed by a common envelope. (Examples,
the Daisy, Dandelion, &c.)
Conjerz>(e — 'The filamentous weeds of fresh water.
Conglomerate — A rock made up of fragments of rock or pebbles, cemented
together by some other material.
Corolla — 'The second envelope of a flower, usually composed of coloured,
leaf-like organs (petals), which may be united by their edges either
in the basal part or throughout.
Correlation — The normal coincidence of one phenomenon, character, &c.,
with another.
Corymb — A bunch of flowers in which those springing from the lower part
of the flower stalk are supported on long stalks so as to be nearly on
a level with the upper ones.
GLOSSARY 533
Cotyledons — The first or seed-leaves of plants.
Crustaceans — ^A class of articulated animals, having the skin of the body
generally more or less hardened by the deposition of calcareous matter,
breathing by means of gills. (Examples, Crab, Lobster, Shrimp, &c.)
Curculio — The old generic term for the Beetles known as Weevils, charac-
terized by their four-jointed feet, and by the head being produced into
a sort of beak, upon the sides of which the antennae are inserted.
Cutaneous — Of or belonging to the skin.
Degradation — The wearing down of land by the action of the sea or of
meteoric agencies.
Denudation — The wearing away of the surface of the land by water.
Devonian System or Formation — A series of Palaeozoic rocks, including the
Old Red Sandstone.
Dicotyledons or Dicotyledonous Plants — ^A class of plants characterized by
having two seed leaves, by the formation of new wood between the
bark and the old wood (exogenous growth), and by the reticulation
of the veins of the leaves. The parts of the flowers are generally in
multiples of five.
Differentiation — The separation or discrimination of parts or organs which
in simpler forms of life are more orless united.
Dimorphic — Having two distinct forms. — Dimorphism is the condition of the
appearance of the same species under two dissimilar forms.
Dioecious — Having the organs of the sexes upon distinct individuals.
Diorite—A peculiar form of Greenstone.
Dorsal — Of or belonging to the back
Edentata — A peculiar order of Quadrupeds, characterized by the absence ot
at least the middle incisor (front) teeth in both jaws (Examples, the
Sloths and Armadillos.)
Elytra — The hardened fore-wings of Beetles, serving as sheaths for the mem-
branous hind-wings, which constitute the true organs of flight.
Embryo — The young animal undergoing development within the egg or
womb.
Embryology — The study of the development of the embryo.
Endemic — Peculiar to a given locality.
Entomostraca — A division of the class Crustacea, having all the segments
of the body usually distinct, giJls attached to the feet or organs of
the mouth, and the feet fringed with fine hairs. They are generally
of smal'. size.
Eocene — 'The earliest of the three divisions of the Tertiary epoch of geolo-
gists. Rocks of this age contain a small proportion of shells identical
with species now living.
Ephemerous Insects — Insects allied to the May-fly.
Fauna — The totality of the anirnals naturally inhabiting a certain country
or region, or which have lived during a given geological period
FelidcE — The Cat-family.
Feral — Having become wild from a state of cultivation or domestication.
Flora — The totality of the plants growing naturally in a country, or during
a given geological period.
Florets — Flowers imperfectly developed in some respects, and collected into
a dense spike or head, as in the Grasses, the Dandelion, &c.
Fwtal — Of or belonging to the fcetus, or embryo in course of development.
Foraminifera — -A class of animals of very low organization, and generally
of small size, having a jelly-like body, from the surface of which deli-
cate filaments can be given off and retracted for the prehension of
external objects, and having a calcareous or sandy shell, usually
divided into chambers, and perforated with small apertures.
Fossiliferous — Containing fossils.
Fossorial — Having a faculty of digging. The Fossorial Hymenoptera are a
group of Wasp-like Insects, which burrow in sandy soil to make nests
for their young.
Frenum (pi. Frena) — A small •band or fold of skin
534 GLOSSARY
Fungi (sing. Fungus) — A class of cellular plants, of which Mushrooms,
Toadstools, and Moulds are familiar examples.
Furcula—The forked bone formed by the union of the collar-bones in many
birds, such as the common Fowl.
Gallinaceous Birds — An order of Birds of which the common Fowl, Turkey,
and Pheasant are well-known examples.
Callus — The genus of birds which includes the common Fowl.
Ganglion — A swelling or knot from which nerves are given off as from a
centre.
Ganoid Fishes — Fishes covered with peculiar enamelled bony scales. Most
of them are extinct.
Germinal Vesicle — A minute vesicle in the eggs of animals, from which the
development of the embryo proceeds.
Glacial Period — A period of great cold and of enormous extension of ice
upon the surface of the earth. It is believed that glacia! periods have
occurred repeatedly during the geological history of the earth, but
the term is generally applied to the close of the Tertiary epoch, when
nearly the whole of Europe was subjected to an arctic "climate.
Gland — An organ which secretes or separates some peculiar product from
the blood or sap of animals or plants.
Glottis — The opening of the windpipe into the cesophagus or gullet.
Gneiss — A rock approaching granite in composition, but more or less lami-
nated, and really produced by the alteration of a sedimentary deposit
after its consolidation.
Grallatores — The so-called Wading-birds (Storks, Cranes, Snipes, &c.), which
are generally furnished with long legs, bare of feathers above the
heel, and have no membranes between the toes.
Granite — A rock consisting essentially of crystals of felspar and mica in a
mass of quartz.
Habitat — The locality in which a plant or animal naturally lives.
Hemiptera— An order or sub-order cf Insects, characterized by the posses-
sion of a jointed beak or rostrum, and by having the fore-wings
horny in the basal portion and membranous at the extremity, where
they cross each other. This group includes the various species
of Bugs. *
Hermaphrodite — Possessing the organs of both sexes.
Homology — That relation between parts which results from their develop-
ment from corresponding embryonic parts, either in different animals,
as in the case of the arm of man, the fore-leg of a quadruped, and
the wing of a bird; or in the same individual, as in the case of the
fore and hind legs in quadrupeds, and the segments or rings and
their appendages of which the body of a worm, a centipede, &c., is
composed. The latter is called serial homology. The parts which
stand in such a relation to each other are said to be homologous,
and one such part or organ is called the homologue of the other. In
different plants the parts of the flower are homologous, and in general
these parts are regarded as homologous with leaves.
Homoptera — ^An order or sub-order of Insects having (like the Hemiptera)
a jointed beak, but in which the fore-wings are either wholly mem-
branous or wholly leathery. The Cicada:, Frog-hoppers, and Aphides,
are well-known examples.
Hybrid — The offspring of the union of two distinct species.
H'ymenoptera — An order of Insects possessing biting jaws and usually four
membranous wings in which there are a few veins. Bees and Wasps
are familiar examples of this group.
Hypertrophied — Excessively developed.
Ichneumonidcc — A family of Hymenopterous insects, the members of which
lay their eggs in the bodies or eggs of other insects.
Imago — The perfect (generally winged) reproductive state of an insect.
Indigens — The aboriginal animal or vegetable inhabitants of a country or
region.
GLOSSARY S3S
Inflorescence — The mode of arrangement of the flowers of plants.
Infusoria — A class of microscopic Animalcules, so called from their having
originally been observed in infusions of vegetable matters. They con-
sist of a gelatinous material enclosed in a delicate membrane, the
■whole or part of which is furnished with short vibrating hairs (called
cilia), by means of which the animalcules swim through the water or
convey the minute particles of their food to the orifice of the mouth.
Insectivorous — Feeding on Insects.
Invertebrata, or Invertebrate Animals— Those animals which do not possess
a backbone or spinal column.
Lacuner — Spaces left among the tissues in some of the lower animals, and
serving in place of vessels for the circulation of the fluids of ihe body.
LameUatcd — Furnished with lamellse or little plates.
Larva (pi. Larva) — The first condition of an insect at its issuing from the
egg, when it is usually in the form of a grub, caterpillar, or maggot.
Larynx — The upper part of the windpipe opening into the gullet.
Laurentian — A group of greatly altered and very ancient rocks, which is
greatly developed along the course of the St. Laurence, whence the
name. It is in these that the earliest known traces of organic bodies
have been found.
Leguminoscr — An order of plants represented by the common Peas and
Beans, having an irregular flower in which one petal stands up like
a wing, and the stamens and pistil are enclosed in a sheath formed
by two other petals. The fruit is a pod (or legume).
LcmuridiT — 'A group of four-handed animals, distinct from the Monkeys
and approaching the Insectivorous Quadrupeds in some of their char-
acters and habits. Its members have the nostrils curved or twisted,
and a claw instead of a nail upon the first finger of the hind hands.
Lepidoptera — An order of Insects, characterized by the possession of a
spiral proboscis, and of four large more or less scaly wings. It
includes the well-known Butterflies and Moths.
Littoral — Inhabiting the seashore.
Loess — A marly deposit of recent (Post-Tertiary) date, which occupies a
great part of the valley of the Rhine.
Malacostraca — The higher division of the Crustacea, including the ordinary
Crabs, Lobsters, Shrimps, &c., together with the Woodlice and Sand-
hoppers.
Mammalia — The highest class of animals, including the ordinary hairy
quadrupeds, the Whales, and Man, and characterized by the produc-
tion of living young which are nourished after birth by milk from
the teats (Mammcr, Mammary glands) of the mother. A striking dif-
ference in embryonic development has led to the division of this
class into two great groups, in one of these, when the embryo has
attained a certain stage, a vascular connection, called the placenta,
is formed between the embryo and the mother; in the other this is
wanting, and the young are produced in a very incomplete state.
The former, including the greater part of the class, are called
Placental mammals; the latter, or Aplacental mammals, include the
Marsupials and Monotremes (Ornithorhynclius).
Mammiferous — Having mammw or teats (see Mammalia).
Mandibles, in Insects — The first or uppermost pair of jaws,_ which are
generally solid, horny, biting organs. In Birds the term is applied to
both jaws with their horny coverings. In Quadrupeds the mandible is
properly the lower jaw.
Marsupials — An order of Mammalia in which the young are born in a very
incomplete state of development, and carried by the mother, while
sucking, in a ventral pouch (marsupium), such as the Kangaroos,
Opossums, &c. (see Mammalia).
Maxilla:, in Insects — The second or lower pair of jaws, which are composed
of several joints and furnished with peculiar jointed appendages called
palpi, or feelers.
Melanism — The opposite of albinism; an undue development of colouring
material in the skin and ite appendages.
536 GLOSSARY
Metamorphic Rocks — Sedimentary rocks which have undergone alteration,
generally by the action of heat, subsequently to their deposition and
consolidation.
Mollusca — ^One of the great divisions of the Animal Kingdom, including
those animals which have a soft body, usually furnished with a shell,
and in which the nervous ganglia, or centres, present no definite gen-
eral arrangement. They are generally known under the denomination
of "shell-fish"; the cuttle-fish, and the common snails, whelks,
oysters, mussels, and cockles, may serve as examples of them.
Monocotyledons, or Monocotyledonous Plants — Plants in which the seed
sends up only a single seed-leaf (or cotyledon) ; characterized by the
absence of consecutive layers of wood in the stem (endogenous
growth), by the veins of the leaves being generally straight, and
by the parts of the flowers being generally in multiples of three.
(Examples, Grasses, _ Lilies, Orchids, Palms, &c.)
Moraines— The accumulations of fragments of rock brought down by
glaciers.
Morphology — The law of form or structure independent of function.
Mysis-stage — A stage in the development of certain Crustaceans (Prawns),
in which they closely resemble the adults of a genus {Mysis) belong-
ing to a slightly lower group.
Nascent — Commencing development.
Natatory — Adapted for the purpose of swimming.
Nauplius-form — The earliest stage In the development 9f many Crustacea,
especially belonging to the lower groups. In this stage the animal
has a short body, with indistinct indications of a division into seg-
ments, and three pairs of fringed limbs. This form of the common
fresh-water Cyclops was described as a distinct genus under the name
of Nauplius.
Neuration — The arrangement of the veins or nervures in the wings of
Insects.
Neuters — Imperfectly developed females of certain social insects (such as
Ants and Bees), which perform all the labours of the community.
Hence they are also called workers.
Nictitating M embrane — A semi-transparent membrane, which can be drawn
across the eye in Birds and Reptiles, either to moderate the effects
of a strong light or to sweep particles of dust, &c., from the surface
of the eye.
Ocelli — The simple eyes or stemmata of Insects, usually situated on the
crown of the head between the great compound eyes.
(Esophagus — The gullet.
Oolitic— A great series of secondary rocks, so called from the texture of
some of its members, which appear to be made up of a mass of small
egg-like calcareous bodies.
Operculum — A calcareous plate employed by many Mollusca to close the
aperture of their shell. The opercular valves of Cirripedes are those
which close the aperture of the shell.
Orbit— The bony cavity for the reception of the eye.
Organism — An organized being, whether plant or animal.
Orthospermous — ^A term applied to those fruits of the Umbelliferse which
have the seed straight.
Osculant — Forms or groups apparently intermediate between and connecting
other groups are said to be osculant.
Ova — Eggs.
Ovarium or Ovary {in Plants) — The lower part of the pistil or female organ
of the flower, containing the ovules or incipient seeds; by growth after
the other organs of the flower have fallen, it usually becomes con-
verted into the fruit.
Ovigerous — Egg-bearing.
Ovules {of Plants) — ^The seeds in the earliest condition.
Pachyderms — 'A group of Mammalia, so called from their thick skins, and
including the Elephant, Rhinoceros, Hippopotamus, &c.
PalcEosoic — ^The oldest system of fossiliferous rocks.
GLOSSARY 537
Palpi — Jointed appendages to some of the organs of the mouth in Insects
and Crustacea.
Papilionacea — An order of Plants (see Leguminos.*:). The flowers of
these plants are called papilionaceous, or butterfly-like, from the
fancied resemblance of the expanded superior petals to the wings of
a butterfly.
Parasite — An animal or plant living upon or in, and at the expense of,
another organism.
Parthenogenesis— Th^ production of living organisms from unimpregnated
eggs or seeds.
Pedunculated — Supported upon a stem or stalk. The pedunculated oak has
its acorns borne upon a footstool.
Peloria or Pelorism — The appearance of regularity of structure in the
flowers of plants which normally bear irregular flowers.
Pelvis — The bony arch to which the hind limbs of vertebrate animals are
articulated.
Petals — The leaves of the corolla, or second circle of organs in a flower.
They are usually of delicate texture and brightly coloured.
Phyllodineous — Having flattened, leaf-like twigs or leafstalks instead of
true leaves.
Pigment — The colouring material produced generally in the superficial parts
of animals. The cells secreting it are called pigment-celts.
Pinnate — 'Bearing leaflets on each side of a central stalk.
Pistils — The female organs of a flower, which occupy a position in the centre
of the other floral organs. The pistil is generally divisible into the
ovary or germen, the style and the stigma.
Placentalia, Placentata, or Placental Mammals — See Mammalia.
Plantigrades — Quadrupeds which walk upon the whole sole of the foot, like
the Bears.
Plastic Period — The latest portion of the Tertiary epoch.
Plumule {in Plants) — The minute bud between the seed-leaves of newly-
germinated plants.
Plutonic Rocks — Rocks supposed to have been produced by igneous action
in the depths of the earth.
Pollen — The male element in flowering plants; usually a fine dust produced
by the anthers, which, by contact with the stigma, effects the fecunda-
tion of the seeds. This impregnation is brought about by means of
tubes (pollen-tubes) which issue from the pollen-grains adhering to
the stigma, and penetrate through the tissues until they reach the
ovary.
Polyandrous {Flowers) — Flowers having many stamens.
Polygamous Plants — Plants in which some flowers are unisexual and others
hermaphrodite. The unisexual (male and female) flowers may be
on the same or on different plants.
Polymorphic — Presenting many forms.
Polyzoary — The common structure formed by the cells of the Polyzoa, such
as the well-known Sea-mats.
Prehensile — Capable of grasping.
Prepotent — Having a superiority of power.
Primaries — The feathers forming the tip of the wing of a bird, and inserted
upon that part which represents the hand of man.
Processes — Projecting portions of bones, usually for the attachment of
muscles, ligaments, &c.
Propolis — ^A resinous material collected by the Hive-Bees from the opening
buds of various trees.
Protean — Exceedingly variable.
Protozoa — -The lowest great division of the Animal Kingdom. These animals
are composed of a gelatinous material, and show scarcely any trace of
distinct organs. The Infusoria, Foraminifera, and Sponges, with some
other forms, belong to this division.
Pypa {pi. Pupa) — The second stage in the development of an Insect, from
which it emerges in the perfect (winged) reproductive form. In most
insects the pupal stage is passed in perfect repose. The chrysalis is
the pupal state of butterflies. .
Radicle — ^The minute root of aiv embryo plant.
538 GLOSSARY
Ramus— One-ha\{ of the lower jaw in the Mammalia. The portion which
rises to articulate with the skull is called the ascending ramw:.
Range — The extent of country over which a plant or animal is naturally
spread. Range in time expresses the distribution of a species or
group through the fossiliferous beds of the earth's crust.
Retina — The delicate inner coat of the eye, formed by nervous filaments
spreading from the optic nerve, and serving for the perception of the
impressions produced by light.
Retrogressioti— Backward development. When an animal, as it approaches
maturity, becomes less perfectly organized than might be expected
from its early stages and known relationships, it is said to undergo
a retrograde development or metamorphosis.
Rhizopods — -A class of lowly organized animals (Protozoa), having a gelat-
inovis body, the surface of which can be protruded in the form of
root-like processes or filaments, which serve for locomotion and
the prehension of food. The most important order is that of the
Foraminifera.
Rodents — 'The gnawing Mammalia, such as the Rats, Rabbits, and Squirrels.
They are especially characterized by the possession of a single pair
of chisel-like cutting teeth in each jaw, between which and the
grinding teeth there is a great gap.
Rubus — The Bramble Genu3.
Rudimentary — V^ery imperfectly developed.
Rutyiinants — The group of Quadrupeds which ruminate or chew the cud,
such as Oxen, Sheep, and Deer. They have divided hoofs, and are
destitute of front teeth in the upper jaw.
Sacral — Belonging to the sacrum, or the bone composed usually of two or
more united vertebrae to which the sides of the pelvis in vertebrate
animals are attached.
Sarcode — The gelatinous material of which the bodies of the lowest animals
(Protozoa) are composed.
Scutella — ^The horny plates with which the feet of birds are generally more
or less covered, especially in front.
Sedimentary Formations — Rocks deposited as sediments from water.
Segments— The traverse rings of which the body of an articulate animal
or Annelid is composed.
Sepals — The leaves or segments of the calyx, or outermost envelope of an
ordinary flower. They are usually green, but sometimes brightly
coloured.
Serratures — Teeth like those of a saw.
Sessile — Not supported on a stem or footstalk.
Silurian System— A very ancient system of fossiliferous rocks belonging to
the earlier part of the PaL-eozoic series.
Specialisation — The setting apart of a particular organ for the performance
of a particular function.
Spinal Chord — The central portion of the nervous system in the Vertebrata,
which descends from the brain through the arches of the vertebrae,
and gives off nearly all the nerves to the various organs of the body.
Stamens — The male organs of flowering plants, standing in a circle within
the petals. They usually consist of a filament and an anther, the
anther being the essential part in which the pollen, or fecundating
dust, is formed.
Sternum — ^The breast-bone.
Stigma — The apical portion of the pistil in flowering plants.
Stipules — Small leafy organs placed at the base of the footstalks of the
leaves in many plants.
Style — The middle portion of the perfect pistil, which rises like a column
from the ovary and supports the stigma at its summit.
Subcutaneous — Situated beneath the skin.
Suctorial — Adapted for sucking.
Sutures (in the skull) — The lines of junction of the bones of which the
skull is composed.
Tarsus (pi. Tarsi) — The pointed feet of articulate animals, such as Insects.
Teleostean Fishes — Fishes of the kind familiar to us in the present day,
having the skeleton usually completely ossified and the scales horny.
GLOSSARY 539
Tentacula or Tentacles — Delicate fleshy organs of prehension or touch pos-
sessed by many of the lower animali.
Tertiary — ^The latest geological epoch, immediately preceding the establish-
ment of the present order of things.
Trachea — The windpipe or passage for the admission of air to the lungs.
Tridactyle — Three-fingered, or composed of three movable parts attached
to a common base.
Trilobites — A peculiar group of extinct Crustaceans, somtwhat resembling
the Woodlice in external form, and, like some of them; capable of
rolling themselves up into a ball. Their remains are found only in
the Palasozoic rocks, and most abundantly in those of Silurian age.
Trimorphic — Presenting three distinct forms.
Utnbellifera — An order of plants in which the flowers, which contain five
stamens and a pistil with two styles, are supported upon footstalks
which spring from the top of the flower stem and spread out like
the wires of an umbrella, so as to bring all the flowers in the same
head (umbel) nearly to the same level. {Examples, Parsley and
Carrot.)
Ungulaia — Hoofed quadrupeds.
Unicellular — Consisting of a single cell.
Vascular — Containing blood-vessels.
Vermiform — Like a worm.
Vertebrata; or Vertebrate Animals — The highest division of the animal
kingdom, so called from the presence in most cases of a backbone
composed of numerous joints or vertcbrcc, which constitutes the cen-
tre of the skeleton and at the same time supports and protects the
central parts of the nervous system.
IVhorls — The circles or spiral lines in which the parts of plants are
arranged upon the axis of growth.
Workers — See Neuters.
Zo'ea-stage — The earliest stage in the development of many of the higher
Crustacea, so called from the name of Zo'ea, applied to these young
animals when they were supposed to constitute a peculiar genus.
Zooids — In many of the lower animals (such as the Corals, Medusa, &c.)
reproduction takes place in two ways, namely, by means of eggs and
by a process of budding with or without separation from the parent
of the product of the latter, which is often very dilTerent from that
of the egg. The individuality of the species is repiesented by the
whole of the form produced between two sexual reproductions; and
these forms, which are apparently individual animals, have been
called xooids.
INDEX
Aberrant groups, 468.
Abyssinia, plants of, 423.
Acclimatisation, 152.
Adoxa_, 225.
Affinities of extinct species, 377.
of organic beings, 467.
Agassiz, on Amblyopsis, 152.
, on groups of species suddenly
appearing, 363.
, on prophetic forms, 378.
, on embryological succession,
388.
, on the Glacial period, 412.
— ' — , on embryological characters,
456.
— - — , on the latest tertiary forms,
350.
, on parallelism of embryologi-
cal development and geological
succession, 489.
, Alex., on pedicellariae, 247.
Algse of New Zealand, 421.
Alligators, males, fighting, loi.
Alternate generations, 478.
Amblyopsis, blind fish, 152.
America, North, productions allied
to those of Europe, 416.
, , boulders and glaciers
of, 418.
, South, no modern formations
on west coast, 343.
Ammonites, sudden extinction of,
. 372.
Anagallis, sterility of, 300.
Analogy of variations, 170.
Ancylus, 429.
Andaman Islands inhabited by a
toad, 435.
Animals, not domesticated from
being variable, 35.
, domestic, descended from
several stocks, 36.
, , acclimatisation of, 154.
Animals of Australia, 126.
with thicker fur in cold cli-
mates, 146.
, blind, in caves, 150.
extinct, of Australia, 389.
Anomma, 294.
Antarctic islands, ancient flora of,
441.
Antechinus, 462.
Ants attending aphides, 265.
, s-lave-making instinct, 275.
!■ , neuters, structure of, 292.
Apes, not having acquired intel-
lectual powers, 234.
Aphides, attended by ants, 265.
Aphis, development of, 482.
Apteryx, 186.
Arab horses, 50.
Aralo-Caspian Sea, 389.
Archeopteryx, 356.
Archiac, M. de, on the succession
of species, 374.
Artichoke, Jerusalem, 154.
Ascension, plants of, 432.
Asclepias, pollen of, 200.
Asparagus, 406.
Aspicarpa, 455.
Asses, striped, 171.
, improved by selection, 55.
Ateuchus, 148.
Aucapitaine, on land-sh'ells, 439.
Audubon, on habits of frigate-bird,
189.
, on variation in birds' lests,
266.
, on heron eating seeds, 431.
Australia, animals of, 126.
, dogs of, 269.
, extinct animals of, 388.
, European plants in, 420.
, glaciers of, 418.
Azara, on flies destroying cattle,
86.
Azores, flora of, 410.
B
Babington, Mr., on British plants,
^63.
Baer, Von, standard of Highness,
135.
, comparison of bee and fish,
386.
, embryonic similarity of the
Vertebrata, 479.
Baker, Sir S., on the giraffe, 231.
Balancement of growth, 158.
Baleen, 237.
Barberry, flowers of, iii.
Barrande, M., on Silurian colonies,
36s.
, on the succession of species,
,V5.
— , on parallelism of palaeozoic
formations, 377.
, on affinities of ancient species,
379.
Barriers, importance of, 396.
540
INDEX
541
Bates, Mr., on mimetic butterflies,
465, 466.
Batrachians on islands, 435.
Bats, how structure acquired, 186.
, distribution of, 437.
Bear, catching water-insects, 188.
Beauty, how acquired, 209, 511.
Bee, sting of, 214.
, queen, killing rivals, 214.
, Australian, extermination of,
90.
Bees fertilising flowers, 88.
, hive, not sucking the red
clover, 108.
, Ligurian, 108.
, hive, cell-making instinct, 279
, variation in habits, 266.
, parasitic, 275.
, humble, cells of, 280.
Beetles, wingless, in Maderia, 148.
with deficient tarsi, 148.
Bentham, Mr., on British plants, 63.
, on classification, 457.
Berkeley, Mr., on seeds in salt water,
Bermuda, birds of, 433.
Birds acquiring fear, 266.
, beauty of, 212.
annually cross the Atlantic,
410.
, colour of, on continents, 146.
, footsteps, and remains of, in
secondary rocks, 357.
, fossil, in caves of Brazil,
388.
, of Madeira, Bermuda, and
Galapagos, 433.
Birds, song of males, 102.
transporting seeds, 409.
, waders, 430.
, wingless, 147, 186, 187.
Bizcacba, 398.
, affinities of, 469,
Bladder for swimming, in fish, 195.
Blindness of cave animals, 150.
Blyth, Mr., on distinctness of Indian
cattle, 35.
, on striped hemionus, 171
, on crossed geese, 304.
Borrow, Mr., on the Spanish pointer,
49.
Bory St. Vincent, on Batrachians,
.„43S-
Bosquet, M., on fossil Chthamalus,
357-
Boulders, erratic, on the Azores, 308.
Branchiae, 196, 197.
of crustaceans, 201.
Braun, Prof., on the seeds of Fuma-
riaceae, 226.
Brent, Mr., on house-tumblers, 268.
Britain, mammals of, 437.
Broca, Prof., on Natural Selection,
222.
Bronn, Prof., on duration of specific
forms, 347.
, various objections by, 22^:
Brown, Robert, on classification, 453.
, Sequard, on inherited mutila-
tions, 148.
Busk, Mr., on the Polyzoa, 248.
Butterflies, mimetic, 465, 466.
Buzareingues, on sterility of varie-
ties, 325.
Cabbage, varieties of, crossed, 112.
Calceolaria, 303.
Canary-birds, sterility of hybrids,
^ 303-
Cape de V erde islands, productions
of, 440.
, plants of, on mountains, 420.
Cape of Good Hope, plants of, 140,
432.
Carpenter, Dr.. on foraminifera, 385.
Carthamus, 225.
Catasetum, 204, 461.
Cats, with blue eyes, deaf, 29.
, variation in habits of, 267.
curling tail when going to
spring, 213.
Cattle destroying fir-trees, 86.
destroyed by flies in Paraguay,
86.
, breeds of, locally extinct, 121.
, fertility of Indian and Euro-
pean breeds, 304.
, Indian, 35, 305.
Cave, inhabitants of, blind, 150.
Cecidomyia, 478.
Celts, proving antiquity of man, 35.
Centres of Creation, 400.
Cephalopoda?, structures of eyes, 200.
, development of, 482.
Cercopithecus, tail of, 243.
Ceroxylus laceratus, 236.
Cervulus, 304.
Cetacea, teeth and hair, 156.
, development of the whalebone,
236.
Cetaceans, 236.
Ceylon, plants of, 420.
Chalk formation, 373.
Characters, divergence of, 121.
, sexual, variable, 161, 165.
, adaptive or analogical, 462.
Charlock. 50.
Checks to increase, 83.
, mutual, 85.
Chelae of Crustaceans, 248.
Chickens, instinctive tameness of
269.
Chironomus, its asexual reproduction,
478.
Chthamalinae, 341.
Chthamalus, cretacean species of,
357-
Circumstances favourable to selec-
tion of domestic products, 53.
to natural selection, 114.
Cirripedes capable of crossing, 113,
, carapace aborted, 159.
542
INDEX
Cirripedes, their ovigerous frena,
196.
■ , fossil, 357.
, larvae of, 481.
Claparede, Prof., on the hair-clasp-
ers of the Acaridse, 202.
Clarke, Rev. W. B., on old glaciers
in Australia, 418.
Classification, 450.
Clift, Mr., on the succession of
types, 388.
Climate, effects of, in checking in-
crease of beings, 84.
, adaptation of, to organisms,
^ 'S3-
Climbing plants, 195.
, developments of, 252.
Clover visited by bees, no, in.
Cobites, intestine of, 194.
Cockroach, 90.
Collections, palsontological, poor,
340-
Colour, influenced by climate, 146.
, in relation to attack by flies,
209.
Columba livia, parent of domestic
pigeons, 39.
Colymbetes, 429.
Compensation of growth, 15S.
Composite, flowers and seeds of, 157.
■ , outer and inner florets of, 225.
^^ , male flowers of, 491.
Conclusion, general, 519.
Conditions, slight changes in, favour-
able to fertility, 317.
Convergence of genera, 139.
Coot, 189.
Cope, Prof., on the acceleration or
retardation of the period of repro-
duction, 197.
Coral-islands, seeds drifted to, 406.
reefs, indicating movements of
earth, 406.
Corn-crake, 190.
Correlated variation in domestic
productions, 29.
Coryanthes, 203.
Creation, single centres of, 400.
Crinum, 302.
Croll, Mr., on subaerial denudation,
339.
, on the age of our oldest for-
mations, 359.
• , on alternate Glacial periods in
the North and South, 418.
Crosses, reciprocal, 306.
Crossing of domestic animals, im-
portance in altering breeds, 36.
, advantages of, no.
, unfavourable to selection, 113,
114.
Criiger, Dr., on Coryanthes, 204.
Crustacea of New Zealand, 421.
Crustacean, blind, 150.
air-breathers, 202.
Crustaceans, their chelae, 249.
Cryptocerus, 292.
Ctenomys, blind, 150.
Cuckoo, instinct of, 262, 270.
Cunningham, Mr., on the flight of
the logger-headed duck, 147.
Currants, grafts of, 311.
Currents of sea, rate of, 406.
Cuvier, on conditions of existence,
262, 263.
, on fossil monkeys, 356.
, Fred., on instinct, 262, 263.
Cyclostoma, resisting salt water, 439.
D
Dana, Prof., on blind cave-animals,
'51-
, on relations of crustaceans of
Japan, 417.
, on crustaceans of New Zea-
land, 421.
Dawson, Dr., on eozoon, 360.
De Candolle, Aug. Pyr., on struggle
for existence, 77.
, on umbelliferre, 157.
, on general affinities, 469.
, Alph., on the variability of
oaks, 67.
•, on low plants, widely dis-
persed, 446.
, on widely-ranging plants being
variable, 6g.
, on naturalisation, 125.
, on winged seeds, 158.
, on Alpine species suddenly be-
coming rare, 180.
, on distribution of plants with
large seeds, 407.
— ■ — , on vegetation of Australia,
423, 424.
, on fresh-water plants, 430.
, on insular plants, 432.
Degradation of rocks, 336.
Denudation, rate of, 337.
■ of oldest rocks, 360.
■ of granitic areas, 345.
Development of ancient forms, 384.
Devonian system, 382.
Dianthus, fertility of crosses, 306,
307-
Dimorphism in plants, 61, 319.
Dirt on feet of birds, 409.
Dispersal, means of, 403.
during Glacial period, 411.
Distribution, geographical, 395.
, meanc of, 403.
Disuse, effect of, under nature, 147.
Divergence of character, 122.
Diversification of means for same
general purpose, 203.
Division, physiological, of labour, 125.
Dog, resemblance of jaw to that of
the Thylacinus, 463.
Dogs, hairless, with imperfect teeth,
30.
descended from several wild
stocks, 35.
, domestic instincts of, s68.
INDEX
543
Dogs, inherited civilisation of, 268.
, fertility of breeds together,
305.
, of crosses, 322.
, proportions of body in differ-
ent breeds, when young, 484.
Domestication, variation under, 25.
Double flowers, 292.
Downing, Mr., on fruit-trees in
America, 98.
Dragon-flies, intestines of, 194.
Drift-timber, 407.
Driver ant, 294.
Drones killed by other bees, 214.
Duck, domestic, wings of, reduced,2g.
, beak of, 237.
, logger-headed, 186.
Duckweed, 429.
Dugong, affinities of, 453.
Dung-beetlei with deficient tarsi,
148.
Dytiscus, 429.
E
Earl, Mr. W., on the INIalay Archi-
pelago, 437.
Ears, drooping, in domestic animals,
29.
, rudimentary, 494.
Earth, seeds in roots of trees, 407.
charged with seeds, 409.
Echinodermata, their pedicellari:e,
246.
Eciton, 292.
Economy of organisation, 159.
Edentata, teeth and hair, 156.
, fossil species of, 515.
Edvyards, Milne, on physiological
division of labour, 126.
, on gradations of structure,
205.
, on embryological characters,
456.
Eggs, young birds escaping from,
100.
Egypt, productions of, not modified,
220.
E'ectric organs, 198.
Elephant, rate of increase, 79.
, of Glacial period, 154.
Embryology. 478.
Eozoon Canadense, 360.
Existence, struggle for, 76.
-, condition of, 218.
Extinction, as bearing on natural
selection, 134.
of domestic varieties, 130.
. 368.
Eye, structure of, 191.
, correction for aberration, 216.
Eyes, reduced in moles, 149.
Fabre, M., on hymenoptera fighting,'
102.
, on parasitic sphex, 275.
, on Sitaris, 487, 488.
FaIconer,_ Dr., on naturalisation of
plants in India, 80.
on elephants and mastodons,
383. ^
and Cautley, on mammals of
sub-Himalayan beds, 389.
Falkland Islands, wolf of, 436.
Faults, 338.
Faunas, marine, 397.
Fear, instinctive, in birds, 269.
Feet of birds, young molluscs ad-
hering to, 429.
Fertilisation variously efTected, 203,
21 1.
Fertility of hybrids, 302.
, from slight changes in con-
ditions, 318.
of crossed varieties, 322.
Fir-trees destroyed by cattle, 86.
— , pollen of, 215.
Fish, flying, 187.
, teleostean, sudden appearance
of, 357:
, eating seeds, 408, 430.
, fresh-water, distribution of,
427, 428.
Fishes, ganoid, now confined to
fresh water, 118.
, electric organs of, 198.
, ganoid, living in fresh water,
372-
, of southern hemisphere, 421.
Flat-fish, their structure, 240.
Flight, powers of, how acquired,
186, 187.
Flint-tools, proving antiquity of
man, 35.
Flower, Prof., on the larynx, 246.
, on Halitherium, 378.
, on the resemblance between
the jaws of the dog and Thylaci-
nus, 464.
, on the homology of the feet
of certain marsupials, 473.
Flowers, structure of, in relation to
crossing, 106.
, of compositK and umbelliferae,
157. 225.
, beauty of, 211.
, doub'e, 292.
Flysch formation, destitute of or-
ganic remains, 341.
Forbes, Mr. D., on glacial action in
the Andes, 418.
— — , E., on colours of shells, 146.
, on abrupt range of shells in
depth, 181.
, on poorness of palxontological
collections, 340.
, on continuous succession of
genera, 367.
, on continental extensions, 404,
405.
■ , on distribution during Glacial
period, 412.
, on parallelism in time and
space, 448.
544
INDEX
Forests, changes in, in America, 88.
Formation, Devonian, 382.
, Cambrian, 360.
Formations, thickness of, in Britain,
338..
, intermittent, 348.
Formica, rufescens, 2TT.
, sanguinea, 276.
, flava, neuter of, 293.
Forms, lowly organised, long endur-
ing, 136, 137.
Frena, ovigerous, of cirripedes, 196.
Fresh-water productions, dispersal
of, 427.
Fries, on species in large genera
being closely allied to other spe-
cies, 73.
Frigate-bird, 189.
Frogs on islands, 436.
Fruit-trees, gradual improvement of,
51-
in United States, 98.
, varieties of, acclimatised in
United States, 134.
Fuci, crossed, 308, 314.
Fur, thicker in cold climates, 146.
Galapagos Archipelago, birds of,
434 ^ .
, productions of, 439, 440
Galaxias, its wide range, 427.
Galeopithecus, 185.
Game, increase of, checked by ver-
min, 84.
Gartner, on sterility of hybrids, 299,
300. 305-
, on reciprocal crosses, 308.
, on crossed maize and verbas-
cum, 325
, on comparison of hybrids and
mongrels, 7.2T, 328.
Gaudry, Prof._, on intermediate gen-
era of fossil mammals in Attica,
378.
Geese, fertility when crossed, 304.
, upland, 189.
Geikie, Mr., on subaerial denuda-
tion, 336.
Genealogy, important in classifica-
tion, 457.
Generations, alternate, 478.
Geoifroy St. Hilaire, on balance-
ment, 158.
, on homologous organs, 473.
, Isidore, on variability of re-
peated parts, 160
, on correlation, in monstrosi-
ties, 29.
, on correlation, 156.
, on variable parts being often
monstrous, 165.
Geographical distribution, 395.
Geology, future progress of, 527.
, imperfection of the record,
527-
Gervais, Prof., on Typotherium, 378.
Giraffe, tail of, 206.
— ■ — _, structure of, 230,
Glacial period, 411.
, affecting the North and South,
415-
Glands, mammary, 244.
Gmelin, on distribution, 412.
Godwin-Austen, Mr., on the Malay
Archipelago, 352.
Goethe, on compensation of growth,
158.
Gomphia, 22-].
Gooseberry, grafts of, 311.
Gould, Dr. Aug. A., on land-shells,
438.
— — , Mr., on colours of birds, 146.
, on instincts of cuckoo, 273.
, on distribution of genera of
birds, 445.
Gourds, crossed, 325.
Graba, on the Uria lacrymas, 105.
Grafting, capacity of, 310, 311.
Granite, areas of denuded, 345.
Grasses, varieties of, 124.
Gray, Dr. Asa, on the variability of
oaks, 66.
, on man not causing variabil-
ity, 93-
•, on sexes of the holly, 107.
, on trees of the United States,
113-
, on naturalised plants in the
United States, 125.
, on aestivation, 226.
, on Alpine plants, 412.
, on rarity of intermediate va-
rieties, 182.
•, Dr. J. E., on striped mule,
171.
Grebe, 189.
Grimm, on asexual reproduction,
478.
Groups, aberrant, 46S
Grouse, colours of, 98.
, red, a doubtful species, 64.
Growth, compensation of, 158.
Giinther, Dr., on flat-fish, 244.
, on prehensile tails, 244.
, on the fishes of Panama, 396.
, on the range of fresh-water
fishes, 427.
, on the limbs of Lepidosiren,
492.
H
Haast, Dr., on glaciers of New Zea-
land, 418.
Habit, effect of, under domestica-
tion, 29.
, effect of, under nature, 148.
, diversified, of same species,
187.
Hackel, Prof., on classification and
the lines of descent, 472.
Hair and teeth, correlated, 156.
Halitherium, 378.
INDEX
545
Harcourt, Mr. E. V., on the birds
of Madeira, 433.
Hartung, M., on boulders in the
Azores, 410.
Hazel-nuts, 406.
Hearne, on habits of bears, 188.
Heath, changes in vegetation, 85.
Hector, Dr., on glaciers of New-
Zealand, 418.
Heer, Oswald, on ancient cultivated
plants, 35.
— ■ — , on plants of Madeira, 118.
Helianthemum, 227.
Helix pomatia, 439.
, resisting salt water, 439.
Helmholtz, M., on the imperfection
of the human eye, 214.
Helosciadium, 406.
Hemionus, striped, 173.
Hensen, Dr., on the eyes of Cepha-
lopods, 200.
Herbert, W., on struggle for exist-
ence, 77.
, on sterility of hybrids, 302.
Hermaphrodites crossing, 109, 110.
Heron eating seed, 431.
Heron, Sir R., on peacocks, 102.
Heusinger, on white animals poi-
soned by certain plants, 30.
Hewitt, Mr., on sterility of first
crosses, 314.
Hildebrand, Prof., on the self-steril-
ity of Corydalis, 302.
Hilgendorf, on intermediate varie-
ties, 346.
Himalaya, glaciers of, 417.
, plants of, 420.
Hippeastrum, 302.
Hippicampus, 245.
Hofmeister, Prof., on the move-
ments of plants, 254.
Holly-trees, sexes of, 107.
Hooker, Dr., on trees of New Zea-
land, 113.
, on acclimatisation of Hima-
layan trees, 153.
, on flowers of umbelliferae, 157.
, on the position of ovules,
224.
, on glaciers of Himalaya, 417,
418.
, on algae of New Zealand, 420.
-^ — , on vegetation at the base of
the Himalaya, 421.
, on plants of Tierra del Fuego,
419.
, on Australian plants, 420, 441.
, on relations of^ flora of Amer-
ica, 423.
, on flora of the Antarctic lands,
425. 441-
• , on the plants of the Galapagos,
434. 440.
■ , on glaciers of the Lebanon^
417.
, on man not causing variabil-
ity, 9.^-
Hooker, on plants of mountains of
Fernando Po, 420.
Hooks on palms, 208.
on seeds, on islands, 435.
Hopkins, Mr., on denudation, 344.
Hornbill, remarkable instinct of,
295.
Horns, rudimentary, 494.
Horse, fossil, in La Plata, 369.
, proportions of, when young,
484.
Horses destroyed by flies in Para-
guay, 86.
, striped, 171.
Horticulturiits, selection applied by,
47-
Huber, on cells of bees, 284.
, P., oil reason blended with
instinct, 262.
, on habitual nature of instincts,
263.
, on slave-making ants, 277.
, on Meiipona domestica, 281.
Hudson, Mr., on the Ground- Wood-
pecker of La Plata, 188.
, on the Molothrus, 273, 274.
Humble-bees, cells of, 280.
Hunter, J., on secondary sexual
characters, 161.
Hutton, Captain, on crossed geese,
304.
Huxley, Prof., on structure of her.
maphrodiles, 113.
, on the affinities of the Sirenia,
378. 379- . u- J J.
, on foims connecting birds an**
reptiles, 379.
, on homologous organs, 477.
, on the development of aphis,
482.
Hybrids and mongrels compared,
^ 327.
Hybridism, 298.
Hydra, structure of, 194.
Hymenoptera, fighting, 102.
Hymenopterous insects, diving, 199.
Hyoseris, 225.
Ibla, 159.
Icebergs transporting seeds, 410.
Increase, rate of, 79.
Individuals, numbers favourable to
selection, 114.
, many, whether simultaneously
created, 402.
Inheritance, laws of, 31.
, at corresponding ages, 31,
lOI.
Insects, colour of, fitted for their
stations, 100.
, sea-side, colours of, 146.
, blind, in caves, 149, I50-
, luminous, 199.
, their resemblance to certain
objects, 235.
, neuter. 292.
-HC XI
546
INDEX
Instinct, 262.
, not varying simultaneously
with structure, 290.
Instincts, domestic, 267.
Intercrossing, advantages of, 110,
318.
Islands, oceanic, 431.
Isolation favourable to selection,
116.
Japan, productions of, 417.
Java, plants of, 420.
Jonts, Mr. J. M., on the birds of
Berrnuda, 433.
Jourdain, M., on the eye-spots of
star-fishes, 191.
Jukes, Prof., on subaerial denuda-
tion, 336.
Jussieu, on classification, 455.
K
Kentucky, caves of, 150.
Kerguelen-land, flora of, 425, 441.
Kidney-bean, acclimatisation of, 155.
Kidneys of birds, 156.
Kirby, on tarsi deficient in beetles,
148.
Knight, Andrew, on cause of varia-
tion, 25.
Kolreuter, on Intercrossing, 109.
, on the barberry, iii.
, on sterility of hybrids, 299,
300.
, on reciprocal crosses, 308.
, on crossed varieties of nico-
tiana, 326.
, on crossing male and her-
maphrodite flowers, 490.
Lamarck, on adaptive characters,
462.
Lancelet, 137.
, eyes of, 193.
Landois, on the development of the
wings of insects, iq6.
Land-shells, distribution of, 438.
, of Madeira, naturalised, 443.
, resisting salt water, 439.
Languages, classification of, 459.
Lankester, Mr. E. Ray, on Longev-
ity, 229.
, on homologies, 477.
Lapse, great, of time, 335.
Larva, 480.
Laurel, nectar secreted by the
leaves, 106.
Laurentian formation, 360.
Lav/s of variation, 145.
Leech, varieties of, 89.
Leguminosx, nectar secreted by
glands, 106.
Leibnitz' attack on Newton, 520.
Lepidosiren, 118, 380.
Lcpidosiren, limbs in a nascent con-
dition, 492.
Lewes, Mr. G. H., on species not
having changed in Egypt, 220.
, on the Salamandra atra, 491.
, on many forms of life having
been at first evolved, 524.
Life, struggle for, 78.
Lingula, Silurian, 359.
Linnxus, aphorism of, 452.
Lion, mane of, 102.
, young of, striped, 480.
Lobelia fulgens, 87, :ii.
, sterility of crosses, 302.
Lockwood, Mr., on the ova of the
Hippocampus, 244.
Locusts transporting seeds, 408.
Logan, Sir W., on Laurentian for-
mation, 360.
Lowe, Rev. R. T., on locusts visit-
ing Madeira, 408.
Lowness of structure connected with
variability, 160.
•, related to wide distribution,
446.
Lubbock, Sir J., on the nerves of
coccus, 60.
, on secondary sexual charac-
ters, 167.
, on a diving hymenopterous
insect, 189.
, on afhnitics, 352.
, on mctamorpnoses, 480.
Lucas, Dr. P., on inheritance, 30.
, on resemblance of child to
parent, 329.
Lund and Clausen, on fossils of
Brazil, 38S.
Lyell, Sir C, on the struggle for
existence, 77.
, on modern changes of the
earth, 109.
, Sir C., on terrestrial animals
not having been developed on
islands, 233.
, on a carboniferous land-shell,
341-
■, on strata beneath Silurian
system, 360.
, on the imperfection of the geo-
logical record, 363.
, on the appearance of species,
363.
, on Barrande's colonies, 364.
— J-, on tertiary formations of
Europe and North America, 373.
, on parallelism of tertiary for-
mations, 377.
, on transport of seeds by ice-
bergs, 410.
, on great alterations of climate,
426.
, on the distribution of fresh-
water shells, 420.
, on land-shells of Madeira, 443.
Lycll and Dawson, on fossilized
trees in Xova Scotia, 349.
Lythrum salicaria, trimorphic, 321.
INDEX
547
M
Macleay, on analogical characters,
46--.
Macrauchenia, 378.
M'Donnell, Dr., on electric organs,
198.
Madeira, plants of, 118.
, beetles of, wingless, 148.
, fossil land-shells of, 3S9.
, birds of, 433.
Magpie tame in Korway,
266.
with
Males fighting, 102.
Maize, crossed, 325.
Malay Archipelago compared
Europe, 352.
, mammals of, 437.
Malm, on flat-fish, 241.
Malpighiacese, small imperfect flow-
ers of, 225.
. 455-
Mammae, their development, 244.
, rudimentary, 490.
Mammals, fossil, in secondary for-
mation, 356.
, insular, 435.
Man, origin of, 527.
Manatee, rudimentary nails of, 494.
Marsupials of Australia, 126.
■, structure of their feet, 473.
, fossil species of, 388.
Martens, M., experiment on seeds,
406.
Martin, Mr. W. C, on striped mules,
173-
Masters, Dr.. on Saponaria, 227.
Matteucci, on the electric organs of
rays, 198.
Matthiola, reciprocal crosses of, 308.
Maurandia, 253.
Means of dispersal, 403.
Melipona domestica, 280.
Merrell, Dr., on the American
cuckoo, 270.
Metamorphism of oldest rocks, 360.
Mice destroying bees, 88.
, acclimatisation of, 153.
, tails of, 244.
Miller, Prof., on the cells of bees,
281, 285.
Mirabilis, crosses of, 308.
Missel-thrush, 90.
Mistletoe, complex relations of, 22.
Mivart, Mr., on the relation of hair
and teeth, 156.
, on the eyes of cephalopods,
200.
, various objections to Natural
Selection, 229.
, on abrupt modifications, 258,
259-
, on the resemblance of the
mouse and antechinus, 462.
Mocking-thrush of the Galapagos,
443.
Modification of species not abrupt,
523.
Moles, blind, 151.
Molothrus, habits of, 273.
Mongrels, fertility and sterility of,
332-
and hybrids compared, 327.
Monkeys, fossil, 356.
Monachanthus, 461.
Mons, Van, on the origin of fruit-
trees, 44.
Monstrosities, 58.
Moquin-Tandon, on sea-side plants,
146.
Morphology, 472.
Morren, on the leaves of Oxalis, 254.
Moths, hybrid, 304.
Mozart, musical powers of, 263.
Mud, seeds in, 429.
Mules, striped, 173.
Miillcr, Adolf, on the instincts of
the cuckco, 271.
Miiller, Dr. Ferdinand, on Alpine
Australian plants, 420.
Muller, Fritz, on dimorphic crus-
taceans, 61, 295.
, on the lancelet, 137.
, on air-breathing crustaceans,
201.
, on climbing plants, 253.
, on the self-sterility of orchids,
302.
, on embryology in relation to
classification, 456.
, on the metamorphoses of crus-
taceans, 482, 488.
, on terrestrial and fresh-water
organisms not undergoing any
metamorphosis, 486.
Multiplication of species not indefi-
nite, 140.
Murchison. Sir R., on the forma-
tions of Russia, 342.
, on azoic formations, 360.
, on extinction, 368.
Murie, Dr., on the modification of
the skull in old age, 197.
Murray, Mr. A., on cave-insects,
152.
Mustela vision, 184.
Myanthus, 461.
Myrmecocystus, 292.
Myrmica, eyes of, 294.
N
Nageli, on morphological characters,
222.
Nails, rudimentary, 494.
Nathusius, Von, on pigs, 209.
Natural history, future progress of,
525.
selection, 93.
system, 452.
Naturalisation of forms distinct
from the indigenous species, 125.
Naturalisation in New Zealand, 213.
Naudin, on analogous variations in
gourds, 168.
548
INDEX
Naudin, on hybrid gourds, 325.
, on reversion, 328.
Nautilus, Silurian, 359.
Nectar of plants, 106.
Nectaries, how formed, 106.
Nelumbium luteum, 430.
Nests, variations in, 266, 289, 296.
Neuter insects, 293, 294.
Newman, Col., on humble-bees, 88.
New Zealand, productions of, not
perfect, 213.
, naturalised products of, 387.
, fossil birds of, 389.
, glaciers of, 418.
, crustaceans of, 420.
, algae of, 421.
, number of plants of, 432.
— , flora of, 441.
Newton, Sir I., attacked for irre-
ligion, 520.
, Prof., on earth attached to a
partridge's foot, 409.
Nicotiana, crossed varieties of, 326.
, certain species very sterile,
Nitsche, Dr., on the Polyzoa, 248.
i\oble, Mr., on fertility of Rhodo-
dendron, 303.
Nodules, phosphatic, in azoic rocks,
360.
O
Oaks, variability of, 67.
Onites apelles, 148.
Ononis, small imperfect flowers of,
224.
Orchids, fertilisation of, 204, 205.
, the development of their
flowers, 251.
', forms of, 461.
Orchis, pollen of, 200.
Organisation, tendency to advance,
^ '35.
Organs of extreme perfection, 190.
, electric, of fishes, 199.
■ — ■ — , of little importance, 205.
, homologous, 474.
, rudiments of, and nascent,
490.
Ornithorhynchus, 118, 454.
— ■ — , mammK of, 245.
Ostrich not capable of flight, 233
, habit of laying eggs together
274.
, American, two species of, 397
Otter, habits of, how acquired, 184,
Ouzel, water, 189.
Owen, Prof., on birds not flying
147-
, on vegetative repetition, 160
, on variability of unusually
developed parts, 161.
, on the eyes of fishes, 193.
, on the swim-bladder of fishes,
196.
, on fossil horse of La Plata,
369-
Owen, Prof., on generalized form,
378.
, on relation of ruminants and
pachyderms, 378.
, on fossil birds of New Zea-
land, 388.
, on succession of types, 389.
, on affinities of the dugong,
453-
, on homologous organs, 473.
, on the metamorphosis of
cephalopods, 482.
Pacific Ocean, faunas of, 397.
Pacini, on electric organs, 199.
Pa!ey, on no organ formed to give
pain, 213.
Pallas, on the fertility of the do-
mesticated descendants of wild
stocks, 305.
Palm with hooks, 208.
Papaver bracteatum, 226.
Paraguay, cattle destroyed by flies,
86.
Parasites, 274, 275.
Partridge, with ball of earth at-
tached to foot, 409.
Parts greatly developed, variable,
161.
Parus major, 188.
Passiflora, 302.
Peaches in United States, 98.
Pear, grafts of, 311.
Pedicellarix, 247.
Pelargonium, flowers of, 157.
, sterility of, 303.
Pelvis of women, 156.
Peloria, 157.
Period, Glacial, 411.
Petrels, habits of, 189.
Phasianus, fertility of hybrids, 304.
Pheasant, young, wild, 269. •
Pictet, Prof., on groups of species
suddenly appearing, 356.
, on rate of organic change,
365-
, on continuous succession of
genera, 367.
; on change in latest tertiary
forms, 350.
, on close alliance of fossils in
consecutive formations, 383.
, on early transitional links, 355.
Pierce, Mr., on varieties of wolves,
103.
Pigeons with feathered feet and
skin between toes, 30.
, breeds described, and origin
of, 37-
-, breeds of, how produced, 52,
53-
, tumbler, not being able to get
out of egg, 100.
, reverting to blue colour, 170.
, instinct of tumbling, 268.
INDEX
549
Pigeons, young of, 485-
Pigs, black, not affected by the
paint-root, 30.
— , modified by want of exercise,
209.
Pistil, rudimentary, 491-
Plants, poisonous, not affecting cer-
tain coloured animals, 30.
, selection applied to, 51.
, gradual improvement of, 51.
, not improved in barbarous
countries, 51.
, dimorphic, 61.
, destroyed by insects, 84.
■ , in midst of range, have to
struggle with other plants, 97.
, nectar of, 106.
, fleshy, on sea-shores, 146.
, climbing, 196, 252.
' , fresh-water, distribution of,
429.
, low in scale, widely distrib-
uted, 446.
Pleuronectidae, their structure, 240.
Plumage, laws of change in sexes
of birds, 102.
Plums in the United States, 98.
Pointer dog, origin of, 49-
, habits of, 269.
Poison not affecting certain col-
oured animals, 30.
, similar effect of, on animals
and plants, 523.
Pollen of fir-trees, 215.
transported by various means,
203, 204, 211.
Pollinia, their development, 251.
Polyzoa, their avicularia, 249.
Poole, Col., on striped hemionus,
173-
Potamogeton, 430.
Pouchet, on the colours of flat-fish,
243-
Prestwich, Mr., on English and
French eocene formations, 377.
Proctotrupes, 189.
Proteolepas, 159.
Proteus, 152.
Psychology, future progress of, 527.
Prygoma, found in the chalk, 357.
Q
Quagga, striped, 174.
Quatrefages, M., on hybrid moths,
~ 304-
Quercus, variability of, 67.
Quince, grafts of, 311.
Rabbits, disposition of young, 269.
Races, domestic, characters of, 33.
Race-horses, Arab, 50.
, English, 403.
Radcliffe, Dr., the electrical organs
of the torpedo, 198.
Ramond, on plants of Pyrenees,
413-
Ramsay, Prof., on subaerial denu-
dation, 337.
, on thickness of the British
formations, 338.
, on faults, 338.
, Mr., on instincts of cuckoo,
272.
Ratio of increase, 79.
Rats supplanting each other, 90.
, acclimatisation of, 153.
— , blind, in cave, 150.
Rattle-snake, 213.
Reason and instinct, 262.
Recapitulation, general, 499.
Reciprocity of crosses, 308.
Record, geological, imperfect, 333.
Rengger, on flies destroying cattle,
86.
Reproduction, rate of, 80.
Resemblance, protective, of insects,
-35- . , .
to parents in mongrels and
hybrids, 329.
Reversion, law of inheritance, 32.
, in pigeons, to blue colour,
170.
Rhododendron, sterility of, 303, 304.
Richard, Prof., on Aspicarpa, 455-
Richardson, Sir J., on structure of
squirrels, 185.
, on fishes of the southern
hemisphere, 421.
Robinia, grafts of, 311.
Rodents, blind, 149.
Rogers, Prof., Map of N. America,
345-
Rudimentary organs, 491.
Rudiments important for classifica-
tion, 435- ^ ,. , ,
Riitimeyer, on Indian cattle, 36,
305-
S
Salamandra atra, 491.
Saliva used in nests, 289.
Salvin, Mr., on the beaks of ducks,
238.
Sageret, on grafts, 311.
Salmons, males fighting, and hooked
jaws of, loi.
Salt water, how far injurious to
seeds, 405, 406.
• not destructive to land-shells,
438, 439- , , , , ,
Salter, Mr., on early death of hy-
brid embryos. 315.
Saurophagus sulphuratu?, 187.
Schacht, Prof., on Phyllotaxy, 225.
Schiodte, on blind insects, 150.
, on flat-fish, 240.
Schlegel, on snakes, 156.
Schfibl, Dr., on the ears of mice,
223.
Scott, J., Mr., on the self-sterility
of orchids, 302.
550
INDEX
Scott, J., Mr., on the crossing of
varieties of verbascum, 326.
Sea-water, how far injurious to
seeds, 405, 406.
not destructive to land-shells,
438, 439-
Sebright, Sir J., on crossed animals,
Z7-
Sedgwick, Prof., on groups of spe-
cies suddenly appearing, 354.
Seedlings destroyed by insects, 82.
Seeds, nutriment in, 91.
, winged, 158.
• , means of dissemination, 203,
212, 407, 408.
, power of resisting salt water,
406.
-, in crops and intestines of
birds, 407, 408.
— ■ — , eaten by fish, 408, 430.
, in mud, 428.
, hooked, on islands, 435.
Selection of domestic products, 43,
44.
, principle not of recent origin,
48.
, unconscious, 49.
■ , natural, 93.
, sexual, loi.
■ — ■ — , objections to term, 94.
' natural, has not induced steril-
ity, 312.
Sexes, relations of, 101.
Sexual characters variable, 166.
selection, loi.
Sheep, Merino, their selection, 46.
, two sub-breeds, unintention-
ally produced, 50.
, mountain varieties of, 89.
Shells, colours of, 146.
, hinges of, 202.
, littoral, seldom embedded, 341.
, fresh-water, long retain the
same forms, 385.
, , dispersal of, 428.
, of Madeira, 433.
, land, distribution of, 433.
. , resisting salt water,
438, 439-
Shrew-mouse, 462.
Silene, infertility of crosses, 307.
Silliman, Prof., on blind rat, 150.
Sirenia, their affinities, 378.
Sitaris, metamorphosis of, 488.
Skulls of young mammals, 208, 475.
Slave-making instinct, 275.
Smith, Col. Hamilton, on striped
horses, 172.
, Mr. Fred., on slave-making
ants, 276.
, on neuter ants, 294.
Smitt, Dr., on the Polyzoa, 248.
Snake with tooth for cutting
through egg-shell, ^73-
Somerville, Lord, on selection of
sheep, 46.
Sorbus, grafts of, 311.
Sorex, 462.
Spaniel, King Charles's breed, 49.
Specialisation of organs, 135.
Species, polymorphic, 60.
, dominant, 71. ■
, common, variable, 69.
• in large genera variable, 75.
, groups of, suddenly appearing,
354. 359-
beneath Silurian formations,
369.
Species successively appearing, 364.
changing simultaneously
throughout the world, ^y},.
Spencer, Lord, on increase in size
of cattle, 50.
, Herbert, Mr., on the first
steps in differentiation, 138.
, on the tendency to an equilib-
rium in all forces, 318.
Sphex, parasitic, 275.
Spiders, development of, 482.
Sports in plants, 28.
Sprengel, C. C, on crossing, 1C.9.
— — , on ray-florets, 157.
Squalodon, 379.
Squirrels, gradations in structure,
185.
Staffordshire, heath, changes in,
85-
Stag-beetles, fighting, loi.
Star-fishes, eyes of, 191.
;, their pedicellariae, 247.
Sterility from changed conditions of
life, z-j.
of hybrids, 300.
, laws of, 305.
• , causes of, 312.
— • — , from unfavourable conditions,
316.
not induced through natural
selection, 313.
St. Helena, productions of, 433.
St. Hilaire, Aug., on variability of
certain plants, 226, 227.
, on classification, 456.
St. John, Mr., on habits of cats,
267.
Sting of bee, 214.
Stocks, aboriginal, of domestic ani-
mals, 35.
Strata, thickness of, in Britain, 339.
Stripes on horses, 172.
Structure, degrees of utility of,
209.
Struggle for existence, 76.
Succession, geological, 364.
of types in same areas, .388.
Swallow, one species supplanting
another, go.
Svvaysland, Mr., on earth adhering
to the feet of migratory birds,
409.
Swifts, nests of, 2S9.
Swim-bladder, 195.
Switzerland, lake-habitations of, 35.
System, natural, 452.
INDEX
551
Tail of giraffe, 206.
of aquatic animals, 207.
, prehensile, 244.
, rudimentary, 494.
Tanais, dimorphic, 61.
Tarsi, deficient, 148.
Tausch, Dr., on umbelliferae, 226.
Teeth and hair correlated, 156.
, rudimentary, embryonic calf,
490, 518.
Tegetmeier, Mr., on cells of bees,
282, 287.
Temminck, on distribution aiding
classification, 457.
Tendrils, their development, 252.
Thompson, Sir \V., on the age of
the habitable world, 360.
— — , on the consolidation of the
crust of the earth, 505.
Thouin, on grafts, 311.
Thrush, aquatic species of, 189.
, mocking, of the Galapagos,
443- ^ , „
, young of, spotted, 480.
•, nest of, 296.
Thwaites, Mr., on acclimatisation,
153-
Thylacinus, 463.
•Tierra del Fuego, dogs of, 269.
Timber, drift, 407.
Time, lapse of, 335.
by itself not causing modifica-
tion, 114.
Titmouse, 188.
Toads on islands, 436.
Tobacco, crossed varieties of, 326.
Tomes, Mr., on the distribution of
bats, 437.
Transitions in varieties rare, 180.
Traquair, Dr., on flat-fish, 242.
Trautschold, on intermediate varie-
ties, 346.
Trees on islands belong to peculiar
orders, 435.
with separated sexes, 113.
Trifolium pratense, 87, 108.
incarnatum, 108.
Trigonia, 372.
Trilobites, 360.
, sudden extinction of, 372.
Trimen, Mr., on imitating-insects,
Trimorphism in plants, 62, 321.
Troglodytes, 296.
Tuco-tuco, blind, 149.
Tumbler-pigeons, habits of, heredi-
tary, 268.
Tumbler, young of, 484.
Turkey-cock, tuft of hair on breast,
103.
, naked skin on head, 208.
, young of, instinctively wild,
269.
Turnip and cabbage, analogous vari-
ations of, 168.
Type, unity of, 218.
Types, succession of, in same areas,
388.
Typotherium, 378.
U
Udders enlarged by use, 29.
r, rudimentary, 490.
Ulex, young leaves of, 480.
Umbelliferae, flowers and seeds of,
'57-
, outer and inner florets of,
225.
Unity of type, 218.
Uria lacrymans, 105.
Use, effects of, under domestica-
tion, 29.
, effects of, in a state of na-
ture, 147.
Utility, how far important in the
construction of each part, 209.
\'alenciennes, on fresh-water fish,
428.
\ ariability of mongrels and hy-
brids, 326.
\'ariation under domestication, 25.
caused by reproductive system
being affected by conditions of
life, 26, 27.
■ under nature, 58.
, laws of, 145.
•, correlated, 29, 155, 209.
Variations appear at corresponding
ages, 30, 99.
' analogous in distinct species,
168.
Varieties, natural, 58.
, struggle between, 90.
-, domestic, extinction of, 121.
, transitional, rarity of, 180.
, when crossed, fertile, 326.
, , sterile, 325.
, classification of, 460.
V'erbascum, sterility of, 302.
, varieties of crossed, 325, 326.
V'erlot, M.. on double stocks, 292.
Verneuil, M. de, on the succession
of species, 374.
Vibracula of the Polyzoa, 249.
Viola, small imperfect flowers of,
224.
, tricolor, 87.
Virchow, on the structure of the
crystalline lens, 193.
Virginia, pigs of, 98.
Volcanic islands, denudation of,
337-
Vulture, naked skin on head, 208.
W
Wading-birds, 430.
Wagner, Dr., on Cecidomyia, 478.
552
INDEX
Wagner, Moritz, on the importance
01 isolation, ii6
Wallace, Mr., on origin of species,
21.
, on the limit of variation un-
der domestication, 56.
, on dimorphic lepidoptera, 61,
295-
■ , on races in the Malay Archi-
pelago, 63.
, on the improvement of the
eye, 193-
■ , on the walking-stick insect,
236.
— , on laws of geographical dis-
tribution, 402.
, on the Malay Archipelago,
437-
, on mimetic animals, 467.
Walsh, Mr. B. D., on phytophagic
forms, 64.
, on equal variability, 168.
Water, fresh, productions of, 427.
Water-hen, 189.
Waterhouse, Mr., on Australian
marsupials, 126.
, on greatly developed parts
being variable, 161.
, on the cells of bees, 280.
— ■ — , on general affinities, 468.
Watson, Mr. H. C, on range of
varieties of British plants, 63, 74.
, on acclimatisation, 153.
, on flora of Azores, 410.
, on Alpine plants, 413.
, on rarity of intermediate va-
rieties, 182.
, on convergence, 138.
, on the indefinite multiplica-
tion of species, 139.
Weale, Mr., on locusts transporting
seeds, 409.
Web of feet in water-birds, 190.
Weismann, Prof., on the causes of
variability, 26.
, on rudimentary organs, 493.
West Indian Islands, mammals of,
437-
Westwood, on species in large
genera being closely allied to
others, 73.
, on the tarsi of Engidas, 166.
, on the antenns of hymenop-
terous insects, 454.
Whales, 236.
Wheat, varieties of, 124.
White Mountains, flora of, 412.
Whitaker, Mr., on lines of escarp-
ment, 337.
Wichura, Max, on hybrids, 315, 328.
Wings, reduction of size, 148.
of insects homologous with
branchia;, 195.
, rudimentary, in insects, 490.
Wolf crossed with dog, 268.
— ■ — • of Falkland Isles, 436.
VVollaston, Mr., on varieties of in-
sects, 64.
, on fossil varieties of shells in
Madeira, 69.
•, on colours of insects on sea-
shore, 146.
, on wingless beetles, 148, 149.
, on rarity of intermediate va-
rieties, 182.
, on insular insects, 432.
, on land-shells of Madeira
naturalised, 443.
Wolves, varieties of, 103.
Woodcock with earth attached to
leg, 409.
Woodpecker, habits of, 188.
-, green colour of, 207.
Woodward, Mr., on the duration of
specific forms, 347.
, on Pyrgoma, 357.
, on the continuous succession
of genera, 367.
, on the succession of types,
389.
World, species changing simultane-
ously throughout, 374.
Wrens, nest of, 296.
Wright, Mr. Chauncey, on the
giraffe, 231.
, on abrupt modifications, 260.
Wyman, Prof., on correlation of-
colour and effects of poison, 30.
, on the cells of the bee, 28*.
Youatt, Mr., on selection, 46.
, on sub-breeds of sheep, 50.
, on rudimentary horns
young cattle. 494.
Zanthoxylon, 227.
Zebra, stripes on, 171,
Zeuglodon, 379.
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