Bot-S: The Origin of Species

So utterly helpless are the
masters, that when Huber shut up thirty of them without a slave,
but with plenty of the food which they like best, and with their own
larva: and pupae to stimulate them to work, they did nothing; they
could not even feed themselves, and many perished of hunger.
Huber then introduced a single slave (F. fusca), and she instantly
set to work, fed and saved the survivors; made some cells and tended
the larvae, and put all to rights. What can be more extraordinary
than these well-ascertained facts? If we had not known of any
other slave-making ant, it would have been hopeless to speculate
how so wonderful an instinct could have been perfected.

Another species, Formica sanguinea, was likewise first discovered
by P. Huber to be a slave-making ant. This species is found in the
southern parts of England, and its habits have been attended to by
Mr. F. Smith, of the British Museum, to whom I am much indebted
for information on this and other subjects. Although fully trusting
to the statements of Huber and Mr. Smith, I tried to approach the
subject in a sceptical frame of mind, as any one may well be excused
for doubting the existence of so extraordinary an instinct as that of
making slaves. Hence, I will give the observations which I made in
some little detail. I opened fourteen nests of F. sanguinea, and found
a few slaves in all. Males and fertile females of the slave species (F.
fusca) are found only in their own proper communities, and have
never been observed in the nests of F. sanguinea. The slaves

SLAVE-MAKING INSTINCT 265

are black and not above half the size of their red masters, so that
the contrast in their appearance is great. When the nest is slightly
disturbed, the slaves occasionally come out, and like their masters
are much agitated and defend the nest: when the nest is much
disturbed, and the larvae and pup^ are exposed, the slaves work
energetically together with their masters in carrying them away to
a place of safety. Hence, it is clear, that the slaves feel quite at home.
During the months of June and July, on three successive years, I
watched for many hours several nests in Surrey and Sussex, and
never saw a slave either leave or enter a nest. As, during these
months, the slaves are very few in number, I thought that they might
behave differently when more numerous; but Mr. Smith informs me
that he has watched the nests at various hours during May, June, and
August, both in Surrey and Hampshire, and has never seen the
slaves, though present in large numbers in August, either leave or
enter the nest. Hence, he considers them as strictly household slaves.
The masters, on the other hand, may be constantly seen bringing in
materials for the nest, and food of all kinds. During the year i860,
however, in the month of July, I came across a community with an
unusually large stock of slaves, and I observed a few slaves mingled
with their masters leaving the nest, and marching along the same
road to a tall Scotch-fir tree, twenty-five yards distant, which they
ascended together, probably in search of aphides or cocci. According
to Huber, who had ample opportunities for observation, the slaves
in Switzerland habitually work with their masters in making the
nest, and they alone open and close the doors in the morning and
evening; and, as Huber expressly states, their principal office is to
search for aphides. This difference in the usual habits of the masters
and slaves in the two countries, probably depends merely on the
slaves being captured in greater numbers in Switzerland than in
England.

One day I fortunately witnessed a migration of F. sanguinea from
one nest to another, and it was a most interesting spectacle to behold
the masters carefully carrying their slaves in their jaws instead of
being carried by them, as in the case of F. rufescens. Another day
my attention was struck by about a score of the slave-makers haunt-
ing the same spot, and evidently not in search of food; they ap-

ORIGIN OF SPECIES

266

proached and were vigorously repulsed by an independent com-
munity of the slave-species (F. f usca) ; sometimes as many as three
of these ants clinging to the legs of the slave-making F. sanguinea.
The latter ruthlessly killed their small opponents, and carried their
dead bodies as food to their nest, twenty-nine yards distant; but
they were prevented from getting any pup^ to rear as slaves. I then
dug up a small parcel of the pups of F. fusca from another nest,
and put them down on a bare spot near the place of combat; they
were eagerly seized and carried off by the tyrants, who perhaps
fancied that, after all, they had been victorious in their late com-
bat.

At the same time I laid on the same place a small parcel of the
pups of another species, F. flava, with a few of these litde yellow ants
still clinging to the fragments of their nest. This species is some-
times, though rarely, made into slaves, as has been described by
Mr. Smith. Although so small a species, it is very courageous, and I
have seen it ferociously attack other ants. In one instance I found
to my surprise an independent community of F. flava under a stone
beneath a nest of the slave-making F. sanguinea; and when I had
accidentally disturbed both nests, the little ants attacked their big
neighbours with surprising courage. Now I was curious to ascertain
whether F. sanguinea could distinguish the pup^ of F. fusca, which
they habitually make into slaves, from those of the little and furious
F. flava, which they rarely capture, and it was evident that they did
at once distinguish them; for we have seen that they eagerly and
instantly seized the pupse of F. fusca, whereas they were much terri-
fied when they came across the pupae, or even the earth from the nest,
of F. flava, and quickly ran away; but in about a quarter of an
hour, shortly after all the little yellow ants had crawled away, they
took heart and carried off the pupae.

One evening I visited another community of F. sanguinea, and
found a number of these ants returning home and entering their
nests, carrying the dead bodies of F. fusca (showing that it was not
a migration) and numerous pupae. I traced a long file of ants
burthened with booty, for about forty yards back, to a very thick
clump of heath, whence I saw the last individual of F. sanguinea
emerge, carrying a pupa; but I was not able to find the desolated nest
in the thick heath. The nest, however, must have been close at hand,

SLAVE-MAKING INSTINCT 267

for two or three individuals of F. fusca were rushing about in the
greatest agitation, and one was perched motionless with its own pupa
in its mouth on the top of a spray of heath, an image of despair over
its ravaged home.

Such are the facts, though they did not need confirmation by me,
in regard to the wonderful instinct of making slaves. Let it be
observed what a contrast the instinctive habits of F. sanguinea present
witli those of the continental F. rufescens. The latter does not build
its own nest, does not determine its own migrations, does not collect
food for itself or its young, and cannot even feed itself: it is absolutely
dependent on its numerous slaves. Formica sanguinea, on the other
hand, possesses much fewer slaves, and in the early part of the
summer extremely few: the masters determine when and where a
new nest shall be formed, and when they migrate, the masters carry
the slaves. Both in Switzerland and England the slaves seem to have
the exclusive care of the larvse, and the masters alone go on slave-
making expeditions. In Switzerland the slaves and masters work
together, making and bringing materials for the nest; both, but
chiefly che slaves, tend, and milk, as it may be called, their aphides;
and thus both collect food for 'the community. In England the
masters alone usually leave the nest to collect building materials
and food for themselves, their slaves and larvsc. So that the masters
in this country receive much less service from their slaves than they
do in Switzerland.

By what steps the instinct of F. sanguinea originated I will not
pretend to conjecture. But as ants which are not slave-makers will,
as I have seen, carry off the pupae of other species, if scattered near
their nests, it is possible that such pupae originally stored as food
might become developed; and the foreign ants thus unintentionally
reared would then follow their proper instincts, and do what work
they could. If their presence proved useful to the species which had
seized them — ^if it were more advantageous to this species to capture
workers than to procreate them — ^the habit of collecting pupae, origi-
nally for food, might by natural selection be strengthened and ren-
dered permanent for the very different purpose of raising slaves.
When the instinct was once acquired, if carried out to a much less ex-
tent even than in our British F. sanguinea, which, as we have seen, is
less aided by its slaves than the same species in Switzerland, natural

ORIGIN OF SPECIES

268

selection might increase and modify the instinct— always supposing
each modification to be of use to the species— until an ant was
formed as abjectly dependent on its slaves as is the Formica rufescens.

CELL-MAKIXG INSTrSXT OF THE HI\X BEE

I will not here enter on minute details on this subject, but will
merely give an outline of the conclusions at which I have arrived.
He must be a dull man who can examine the exquisite structure
of a comb, so beautifully adapted to its end, without enthusiastic
admiration. We hear from mathematicians that bees have practically
solved a recondite problem, and have made their cells of the proper
shape to hold the greatest possible amount of honey, with the least
possible consumption of precious wax in their construction. It has
been remarked that a skilful workman with fitting tools and
measures, would find it very difficult to make cells of wax of the true
form, though this is effected by a crowd of bees working in a dark
hive. Granting whatever instincts you please, it seems at first quite
inconceivable how they can make all the necessary angles and planes,
or even perceive when they are correctly made. But the difficulty is
not nearly so great as it at first appears: all this beautiful work can
be shown, I think, to follow from a few simple instincts.

I was led to investigate this subject by Mr. Waterhouse, who has
shown that the form of the cell stands in close relation to the presence
of adjoining cells; and the following view may, perhaps, be con-
sidered only as a modification of his theory. Let us look to the great
principle of gradation, and see whether Nature does not reveal to
us her method of work. At one end of a short series we have humble-
bees, which use their old cocoons to hold honey, sometimes adding
to them short tubes of wax, and likewise making separate and very
irregular rounded cells of wax. At the other end of the series we have
the cells of the hive bee, placed in a double layer: each cell, as is
well known, is an hexagonal prism, with the basal edges of its six
sides bevelled so as to join an inverted pyramid, of three rhombs.
These rhombs have certain angles, and the three which form the
pyramidal base of a single cell dn one side of the comb enter into
the composition of the bases of three adjoining cells on the opposite
side. In the series between the extreme perfection of the cells of the

CELL-MAKING INSTINCT 269

hive-bee and the simplicity o£ those o£ the humble-bee we have the
cells of the Mexican Melipona domestica, carefully described and
figured by Pierre Huber. The Melipona itself is intermediate in
structure between the hive and humble-bee, but more nearly related
to the latter; it forms a nearly regular waxen comb of cylindrical
cells, in which the young are hatched, and, in addition, some large
cells of wax for holding honey. These latter cells are nearly spherical
and of nearly equal sizes, and are aggregated into an irregular mass.
But the important point to notice is, that these cells are always made
at that degree of nearness to each other that they would have inter-
sected or broken into each other if the spheres had been completed;
but this is never permitted, the bees building perfectly flat walls of
wax between the spheres which thus tend to intersect. Hence, each
cell consists of an outer spherical portion, and of two, three, or more
flat surfaces, according as the cell adjoins two, three, or more other
cells. When one cell rests on three other cells, which, from the
spheres being nearly of the same size, is very frequendy and neces-
sarily the case, the three flat surfaces are united into a pyramid; and
this pyramid, as Huber has remarked, is manifestly a gross imitation
of the three-sided pyramidal base of the cell of the hive-bee. As in
the cells of the hive-bee, so here, the three plane surfaces in any one
cell necessarily enter into 'the construction of three adjoining cells.
It is obvious that the Melipona saves wax, and what is more impor-
tant, labour, by this manner of building; for the flat walls between
the adjoining cells are not double, but are of the same thickness as
the outer spherical portions, and yet each flat portion forms a part of
two cells.

Reflecting on this case, it occurred to me that if the Melipona had
made its spheres at some given distance from each other, and had
made them of equal sizes and had arranged them symmetrically in
a double layer, the resulting structure would have been as perfect as
the comb of the hive-bee. Accordingly I wrote to Professor Miller
of Cambridge, and this geometer has kindly read over the following
statement, drawn up from his information, and tells me that it is
stricdy correct : —

If a number of equal spheres be described with their centres
placed in two parallel layers; with the centre of each sphere at the

270 ORIGIN OF SPECIES

distance o£ radius X Va, or radius X 1.41421 (or at some lesser
distance), from the centres of the six surrounding spheres in the
same layer; and at the same distance from the centres of the adjoin-
ing spheres in the other and parallel layer; then, if planes of inter-
section between the several spheres in both layers be formed, there
will result a double layer of hexagonal prisms united together by
pyramidal bases formed of three rhombs; and the rhombs and the
sides of the hexagonal prisms will have every angle identically the
same with the best measurements which have been made of the cells
of the hive-bee. But I hear from Professor Wyman, who has made
numerous careful measurements, that the accuracy of the workman-
ship of the bee has been gready exaggerated; so much so, that what-
ever the typical form of the cell may be, it is rarely, if ever, realised.

Hence, we may safely conclude that, if we could slightly modify
the instincts already possessed by the Melipona, and in themselves
not very wonderful, this bee would make a structure as wonderfully
perfect as that of the hive-bee. We must suppose the Melipona to
have the power of forming her cells truly spherical, and of equal
sizes; and this would not be very surprising, seeing that she already
does so to a certain extent, and seeing what perfectly cylindrical
burrows many insects make in wood, apparently by turning round
on a fixed point. We must suppose the Melipona to arrange her cells
in level layers, as she already does her cyhndrical cells; and we must
further suppose, and this is the greatest difficulty, that she can some-
how judge accurately at what distance to stand from her fellow-
labourers when several are making their spheres; but she is already
so far enabled to judge of distance, that she always describes her
spheres so as to intersect to a certain extent; and then she unites
the points of intersection by perfecdy flat surfaces. By such modi-
fications of instincts which in themselves are not very wonder-
ful— hardly more wonderful than those which guide a bird to make
its nest,— I believe that the hive-bee has acquired, through natural
selection, her inimitable architectural powers.

But this theory can be tested by experiment. Following the
example of Mr. Tegetmeier, I separated two combs, and put between
them a long, thick, rectangular strip of wax: the bees instantly began
to excavate minute circular pits in it; and as they deepened these

CELL-MAKING INSTINCT 27I

little pits, they made them wider and wider until they were converted
into shallow basins, appearing to the eye perfectly true or parts of
a sphere, and of about the diameter of a cell. It was most interesting
to observe that, wherever several bees had begun to excavate these
basins near together, they had begun their work at such a distance
from each other, that by the time the basins had acquired the above-
stated width (/, e. about the width of an ordinary cell), and were in
depth about one-sixth of the diameter of the sphere of which they
formed a part, the rims of the basins intersected or broke into each
other. As soon as this occurred, the bees ceased to excavate, and
began to build up flat walls of wax on the lines of intersection
between the basins, so that each hexagonal prism was built upon the
scalloped edge of a smooth basin, instead of on the straight edges
of a three-sided pyramid as in the case of ordinary cells.

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 they had been a little deepened, came to
have flat bases; and these flat bases, formed by thin little plates of
the vermilion wax left ungnawed, were situated, as far as the eye
could judge, exactly along the planes of imaginary intersection be-
tween 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 deepening 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 inter-
section.

Considering how flexible thin wax is, I do not see that there is
any difflculty in the bees, whilst at work on the two sides of a strip

272 ORIGIN OF SPECIES

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-completed rhombs at the base of a just commenced cell, which
were slighdy concave on one side, where I suppose that the bees
had excavated too quickly, 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 was 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 oppo-
site 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 excavating at the same rate, and by
endeavouring to make equal spherical hollows, but never allowing
the spheres to break into each other. Now bees, as may be clearly
seen by examining the edge of a growing comb, do make a rough,
circumferential 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 cele-
brated elder Huber, but I am convinced of their accuracy; and if
I had space, I could show that they are conformable with my theory.

Huber’s statement, that the very first cell is excavated out of a litde
parallel-sided wall of wax, is not, as far as I have seen, strictly cor-
rect; the first commencement having always been a little hood of

CELL-MAKING INSTINCT 273

wax; but I will not here enter on details. 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 inter-
section between two adjoining spheres. I have several specimens
showing clearly that they can do this. Even in the rude circum-
ferential rim or wall of wax round a growing comb, flexures may
sometimes be observed, corresponding in position 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 ulti-
mately 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 by 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 comb
without injuring the delicate hexagonal walls. These walls, as Profes-
sor Miller has kindly ascertained for me, vary greatly in thickness;
being, on an average of twelve measurements made near the border
of the comb, sirs of an inch in thickness; whereas the basal rhom-
boidal plates are thicker, nearly in the proportion of three to two,
having a mean thickness, from twenty-one measurements, of -sm
of an inch. By the above singular manner of building, strength is
continually given to the comb, with the utmost ultimate economy
of wax.

It seems at first to add to the dijficulty 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 individuals work even at the commence-
ment 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

274 ORIGIN OF SPECIES

extreme margin of the circumferential rim of a growing comb, with
an extremely thin layer of melted vermilion wax; and I invariably
found that the colour was most delicately diffused 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 intersec-
tion 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 inter-
mediate between two adjoining spheres; but, as far as I 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. Nor 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 inside
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

CELL-MAKING INSTINCT 275

just begun, sweeping spheres or cylinders, and building up inter-
mediate planes.

As natural selection acts only by the accumulation of slight modi-
fications of structure or instinct, each profitable to the individual
under its conditions of life, it may reasonably be asked, how a long
and graduated succession of modified architectural instincts, all
tending towards the present perfect plan of construction, could have
profited the progenitors 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 materials
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 experi-
mentally 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 collecting 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 country;
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 modification in her instincts led her to make
her waxen cells near together, so as to intersect a little; for a wall in
common even to two adjoining cells would save some little labour
and wax. Hence it would continually be more and more advan-

ORIGIN OF SPECIES

276

tageous to our humble-bees, i£ they were to make their cells more
and more regular, nearer together, and aggregated 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 present;
for then, as we have seen, the spherical surfaces would wholly dis-
appear 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
economising labour and wax.

Thus, as I believe, the most wonderful of all known instincts, 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 know-
ing that they swept their spheres at one particular distance 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 in-
stincts to new swarms, which in their turn will have had the best
chance of succeeding in the struggle for existence.

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

OBJECTIONS TO THE THEORY 277

in the one without an immediate corresponding 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 hammers with
its beak till it gets at the kernel. Now what special difficulty would
there be in natural selection preserving all the slight individual vari-
ations 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 compulsion, 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, subsequently
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 in-
stincts 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 agglutinated 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 mate-
rials, and to make its nest exclusively of inspissated 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 inter-
mediate gradations are known to exist; cases of instincts of such

278 ORIGIN OF SPECIES

trifling importance, that they could hardly have been acted on by
natural selection; cases of instincts almost identically the same in
animals so remote in the scale of nature, that we cannot account for
their similarity 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 in-
capable 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 been an
ordinary animal, I should have unhesitatingly assumed that all its
characters had been slowly acquired through natural selection;
namely, by individuals having been born with slight profitable modi-
fications, which were inherited by the offspring; and that these again
varied and again were selected, and so onwards. But with the work-
ing ant we have an insect differing 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

OBJECTIONS TO THE THEORY 279

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 instances,
both in our domestic productions and in those in a state of nature, of
all sorts of differences of inherited structure which are correlated with
certain ages, and with either sex. We have differences correlated
not only with one sex, but with that short period when the reproduc-
tive 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 cor-
related with the sterile condition of certain members of insect-
communities: the difficulty lies in understanding how such cor-
related 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 selection may be
applied to the family, as well as to the individual, and may thus
gain the desired end. Breeders of catde 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
selecdon, that a breed of catde, always yielding oxen with extraor-
dinarily long horns, could, it is probable, be formed by carefully
watching which individual bulls and cows, when matched, produce
oxen 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 variedes 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 com-
pared with the fertile male and female ants, and the double sterile
plants with the neuters of the same community. As with the varieties

28 o origin of species

of the stockj so with social insects, selection has been applied to the
family, and not to the individual, for the sake of gaining a service-
able end. Hence, we may conclude that slight modifications of struc-
ture or of instinct, correlated with the sterile condition of certain
members of the community, have proved advantageous: consequendy
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 repeated 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 difficulty;
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 wonderful sort
of shield on their heads, the use of which is quite unknown: in 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 abdomen which secretes a sort of honey, sup-
plying the place of that excreted by the aphides, or the domestic cattle
as they may be called, which our European ants guard and imprison.

It will indeed be thought that I have an overweening confidence
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 different from the fertile males and
females through natural selection, we may conclude from the analogy
of ordinary variations, that the successive, slight, profitable modifica-
tions 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 advantageous modi-

OBJECTIONS TO THE THEORY 281

fication, all the neuters ultimately came to be thus characterized.
According to this view we ought occasionally to find in the same
nest neuter insects, presenting gradations o£ 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 hap-
pens that the larger or the smaller sized workers are the most numer-
ous; 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 rudimentary 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 intermediate con-
dition. 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 condition.
I may digress by adding, that if the smaller workers had been the
most useful to the community, 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 between 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 per-

ORIGIN OF SPECIES

282

haps best appreciate the amount o£ difference in these workers by
my giving not the actual measurements, but a strictly accurate illus-
tration: the difference was the same as if we were to see a set of
workmen 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 insensibly into each
other, as does the widely-different structure 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 ‘Natural-
ist on the Amazons,’ has described analogous cases.

With these facts before me, I believe that natural selection, 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
simultaneously 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 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 appear-
ing under two or even three distinct female forms; and by Fritz
Muller, of certain Brazilian crustaceans likewise appearing under
two widely distinct male forms. But this subject need not here be
discussed.

I have now explained how, 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

SUMMARY 283

to a social community of ants, on the same principle that the division
of labour is useful to civilised man. Ants, however, work by in-
herited 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, therefore, 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
difficulty 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, with-
out 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.

SUMMARY

I have endeavoured in this chapter briefly to show 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 will 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 produced for the good of other animals, though animals take

ORIGIN OF SPECIES

284

advantage o£ 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 fore-
going views, but is otherwise inexplicable, — ail 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 instance, 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 satisfactory
to look at such instincts as the young cuckoo ejecting its foster-
brothers,-— ants making slaves,— the larvae of ichneumonidae feeding
within the live bodies of caterpillars,— not as specially endowed or
created instincts, but as small consequences 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 hyWrids — Stenlity
various in degree, not universal, affected by close interbreeding,
removed by domestication — ^Laws governing the sterility of hybrids
— Sterility not a special endowment, but incidental on other differ-
ences, not accumulated by natural selection — Causes of the sterility
of first crosses and of hybrids — ^Parallelism between the effects of
changed conditions of life and of crossing — Dimorphism and Tri-
morphism — Fertility of varieties when crossed, and of their mongrel
offspring not universal — Hybrids and mongrels cornpared independ-
ently of their fertility — Summary.

T he view commonly entertained by naturalists is that species,
when intercrossed, have been specially endowed with ste-
rility, in order to prevent their confusion. This view certainly
seems at first highly probable, for species living together could hardly
have been kept distinct 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 off-
spring, cannot have been acquired, as I shall show, by the preserva-
tion of successive profitable degrees of sterility. It is an incidental
result of differences in the reproductive systems of the parent-species.

In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded; namely,
the sterility of species when first crossed, and the sterihty 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 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

:^85

ORIGIN OF SPECIES

286

developed, or are imperfectly developed. This distinction is impor-
tant, when the cause of the sterility, which is common to the two
cases, has to be considered. The distinction probably 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 conscientious 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 com-
pares 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 impor-
tant, must be secluded in order to prevent pollen being brought to it
by insects from other plants. Nearly aU the plants experimented on
by Gartner were potted, and were kept in a chamber in his house.
That these processes are often injurious to the fertility of a plant
cannot be doubted; for Gartner gives in his table about a score of
cases of plants which he castrated, and artificially fertilised wdth their
own pollen, and (excluding all cases such as the Leguminosae, in
which there is an acknowledged di£5culty in the manipulation) half

DEGREES OF STERILITY 287

o£ 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 cceulea),
which the best botanists rank as varieties, and found them absolutely
sterile, we may doubt whether many species are really so sterile,
when intercrossed, 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 insen-
sibly, and, on the other hand, that the fertility of pure species is so
easily affected by various circumstances, that for all practical pur-
poses it is most difEcult to say where perfect fertility ends and ste-
rility begins. I think no better evidence of this can be required than
that the two most experienced 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 instructive
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 experiments made during different years. It
can thus be shown that neither sterility nor fertility affords any cer-
tain distinction 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 consdtutional and structural
differences.

In regard to the sterility of hybrids in successive generations;
though Gartner was enabled to rear some hybrids, carefully 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 fer-
tility never increases, but generally 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 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

288 ORIGIN OF SPECIES

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 experimentalists
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 injuri-
ous 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 fer-
tility, notwithstanding the frequent ill effects from manipulation,
sometimes decidedly increases, and goes on increasing. Now, in the
process of artificial fertilisation, pollen is as often taken by chance
(as I know from my own experience) from the anthers of another
flower, as from the anthers of the flower itself which is to be fer-
tilised; so that a cross between two flowers, though probably often
on the same plant, would be thus effected. Moreover, whenever
complicated 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 spontaneously 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 experi-
enced hybridiser, namely, the Hon. and Rev. W. Herbert. He is as
emphatic in his conclusion that some hybrids are perfectly fertile —
as fertile as the pure parent-species — as are Kdlreuter 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

DEGREES OF STERILITY 289

having hot-houses at his command. Of his many important state-
ments 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
commonly 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, Ver-
bascum and Passiflora, can easily be fertilised by pollen from a dis-
tinct 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 Hildebrand, in various orchids as shown by Mr. Scott and
Fritz Muller, 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 pro-
duced four flowers; three were fertilised 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 impregnated
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.

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 Pelargonium, Fuchsia, Calceo-
laria, Petunia, Rhododendron, etc., 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

290 ORIGIN OF SPECIES

been a natural species from the mountains of Chili.” I have taken
some pains to ascertain the degree of fertility of some 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 be-
lieved 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 influ-
ence of close interbreeding is thus prevented. Any one may readily
convince himself of the efficiency 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 arrangements 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 confine-
ment, we have no right to expect that the first crosses between them
and the canary, 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 effiects of close inter-
breeding. 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

DEGREES OF STERILITY 29I

all surprising that the inherent sterility in the hybrids should have
gone on increasing.

Although I know of hardly any thoroughly well-authenticated
cases of perfectly fertile hybrid animals, I have reason to believe that
the hybrids from Cervulus vaginalis and Reevesii, and from Phasi-
anus colchicus with P. torquatus, are perfectly fertile. M. Quatre-
fages states that the hybrids from two moths (Bombyx cynthia and
arrindia) were proved in Paris to be fertile inter se for eight genera-
tions. It has lately been asserted that two such distinct species as the
hare and rabbit, when they can be got to breed together, produce
offspring which are highly fertile when crossed with one of the
parent-species. The hybrids from the common and Chinese 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 instance 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 geese)
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 Captain 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 first produced perfectly fertile
hybrids, or that the hybrids subsequently reared under domestica-
tion 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

292 ORIGIN OF SPECIES

together and have produced quite fertile hybrids. So again I have
lately acquired decisive evidence 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 differences
in habits, voice, constitution, etc., these two forms must be regarded
as good and distinct species. The same remarks 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 character-
istic, but as one capable of being removed by domestication.

Finally, considering all the ascertained facts on the intercrossing 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 consid-
ered as absolutely universal.

LAWS GOV’lERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS

We will now consider a little more in detail the laws governing
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 confusion. The following conclusions
are drawn up chiefly from Gartner’s admirable work on the hybridi-
sation of plants. I have taken much pains to ascertain how far they
apply to animals, and, considering how scanty our knowledge is in
regard 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
different species applied to the stigma of some one species of the
same genus, yields a perfect gradation in the number of seeds pro-

LAWS GOVERNING THE STERILITY 293

duced, up to nearly complete or even quite complete futility; 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 fertility 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 sign of incipient fertilisation. From this
extreme degree of sterility we have self-fertilised hybrids producing
a greater and greater number of seeds up to perfect fertiUty.

The hybrids raised from two species which are very difficult to
cross, and which rarely produce any offspring, are generally very
sterile; but; the parallelism between the difficulty of making a first
cross, and the sterility of the hybrids thus produced~two classes of
facts which are generally confounded 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.

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 innately variable; for it is not
always the same in degree when the same two species are crossed
under the same circumstances; it depends in part upon the constitu-
tion of the individuals which happen to have been chosen for the
experiment. So it is with hybrids, for their degree of fertility is often
found to differ gready 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 resemblance
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 afiinity. This, is clearly shown by

ORIGIN OF SPECIES

294

hybrids never having been raised between species ranked by system-
atists 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 persevering 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 differ-
ence; 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 sufficient to prevent two
species crossing. It can be shown that plants most widely different
in habit and general appearance, and having strongly marked differ-
ences in every part of the flower, even in the pollen, in the fruit, and
in the cotyledons, can be crossed. Annual and perennial plants,
deciduous and evergreen trees, plants irihabiting different stations
and fitted for extremely different climates, 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 completely 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 recipro-
cal crosses between the same two species was long ago observed by

LAWS GOVERNING THE STERILITY 295

Kolreuter. To give an instance: Mirabilis jalapa can easily be fer-
tilised by the pollen of M. longiflora^ and the hybrids thus produced
are sufBciently fertile; but Kolreuter tried more than two hundred
times, during eight following years, to fertilise reciprocally M. longi-
flora 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 nnany bot-
anists 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 crossing 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 necessarily go together. There are
certain hybrids which, instead 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 independent of its external resemblance to either pure parent.

Considering the several rules now given, which govern the fer-
tility 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 fertility, or even to fertility
under certain conditions in excess; that their fertility, besides being

296 ORIGIN OF SPECIES

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 afiinity 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 suppose it would be equally important to keep
from blending^ together? Why should the degree of sterility be
innately variable 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 differences in their
reproductive systems; the differences being of so peculiar and limited
a nature, that, in reciprocal crosses between the same two species, the
male sexual element of the one will often freely act on the female
sexual element of the other, but not in a reversed direction. It will

LAWS GOVERNING THE STERILITY 297

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 some-
times 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, etc.; but in a multitude
of cases we can assign no reason -whatever. Great diversity in the
size of two plants, one being woody and the other herbaceous, one
being evergreen and the other deciduous, an adaptation to widely
different climates, do not always prevent the two grafting together.
As in hybridisation, so with grafting, the capacity is limited by sys-
tematic affinity, for no one has been able to graft together trees
belonging 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 capacity, as in hybridisation,
is by no means absolutely governed by systematic afiinity. 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 different vari-
eties of the apricot and peach on certain varieties of the plum.

As Gartner found that there was sometimes an innate difference
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, can-
not 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 repro-
ductive organs in an imperfect condition, is a different case from

298 ORIGIN OF SPECIES

the difficulty of uniting two pure species, which have their repro-
ductive organs perfect; yet these two distinct classes of cases run to a
large extent parallel. Something 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 extraordinary cases of Hip-
peastrum, Passiflora, etc., which seed much more freely when fer-
tilised with the pollen of a distinct species, than when fertilised with
pollen from the same plant.

We thus see, that, although there is a clear and great difference
between the mere adhesion of grafted stocks, and the union of the
male and female elements in the act of reproduction, yet that there
is a rude degree of parallelism in the results of grafting and of
crossing distinct species. And as we must look at the curious and
complex laws governing the facility with which trees can be grafted
on each other as incidental on imknown differences in their vegeta-
tive 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 various species has been a special endowment; although
in the case of crossing, the difficulty is as important for the endurance
and stability of specific forms, as in the case of grafting it is unim-
portant for their welfare.

ORIGIlSr 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

CAUSES OF THE STERILITY 299

another variety. For it would clearly be advantageous to two vari-
eties 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 distinct
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 rendered 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 reproductive 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 dif-
ficulty will be found to lie in the existence of many graduated steps
from slightly lessened fertility to absolute sterility. It may be admit-
ted 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 degree 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 selec-
tion. 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 infertility, and which thus

300 ORIGIN OF SPECIES

approached by one small step towards absolute sterility? Yet an
advance o£ this kind, if the theory of natural selection be brought
to bear, must have incessandy occurred with many species, for a
muldtude 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, from an
advantage having been thus indirectly given to the community to
which they belonged over other communities of the same species;
but an individual animal not belonging to a social community, if
rendered slighdy sterile when crossed with some other variety, would
not thus itself gain any advantage or indirectly give any advantage
to the other individuals of the same variety, thus leading to their
preservation.

But it would be superfluous to discuss this question in detail; 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 alfected by the pollen of
certain other species, for the germen swells. It is here manifestly
impossible to select the more sterile individuals, which have already
ceased to yield seeds; so that this acme of sterility, 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 steriHty in first crosses and
in hybrids. In the case of first crosses, the greater or less difficulty
in effecting an union and in obtaining 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

CAUSES OF THE STERILITY 3OI

of one species is placed on the stigma of a distantly allied species,
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 Thuret’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 devel-
oped, and then perish at an early period. This latter alternative has
not been sujfficiently attended to; but I believe, from observations
communicated to me by Mr. Hewitt, who has had great experience
in hybridising pheasants and fowls, that the early death of the
embryo is a very frequent cause of sterility in first crosses. Mr. Salter
has recendy given the results of an examination of about 500 eggs
produced from various crosses between three species of Gallus and
their hybrids; the majority of these eggs had been fertilised; and in
the majority of the fertilised eggs, the embryos had either been par-
tially developed and had then perished, or had become nearly
mature, 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, “without any
obvious cause, apparently from mere inability to live”; so that from
the 500 eggs only twelve chickens were reared. With plants, hy-
bridised 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
circumstanced before and after birth; when born and living in a
country where their two parents Uve, they are generally placed under
suitable conditions of life. But a hybrid partakes of only halE of the

ORIGIN OF SPECIES

302

nature and constitution o£ its mother; it may therefore before birth,
as long as it is nourished within its mother’s womb, or within the
egg or seed produced by the mother, be exposed to conditions in
some degree 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 after all,
the cause more probably lies in some imperfection in the original
act of impregnation, causing the embryo to be imperfectly devel-
oped, rather than in the conditions to which it is subsequently ex-
posed.

In regard to the sterility of hybrids, in which the sexual elements
are imperfectly developed, the case is somewhat different. 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. Between the sterility thus superinduced and that of hybrids,
there are many points of similarity. In both cases the sterility is
independent of general health, and is often accompanied by excess
of size or great luxuriance. In both cases the sterility occurs in vari-
ous 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 affinity, for
whole groups of animals and plants are rendered impotent by the
same unnatural conditions; and whole groups of species tend to
produce sterile hybrids. On the other hand, one species in a group
will sometimes resist great changes of conditions with unimpaired
fertility; and certain species in a group will produce unusually fer-
tile 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 condi-
tions 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 ffian when sterility
ensues. So it is with hybrids, for their offspring in successive genera-

CAUSES OF THE STERILITY 303

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 un-
natural crossing of two species, the reproductive system, independ-
ently of the general state of health, is affected in a very similar
manner. In the one case, the conditions 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
distinct structures and constitutions, including of course the repro-
ductive systems, having been blended into one. For it is scarcely
possible that two organisations should be compounded into one,
without some disturbance occurring in the development, or peri-
odical 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 hybrids 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 hybrids produced from
reciprocal crosses; or the increased sterility 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 explanation is offered why an organism, when placed
under natural conditions, is rendered sterile. All that I have at-
tempted 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 different

304 ORIGIN OF SPECIES

class of facts. It is an old and almost universal belief 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, eta, from one soil or climate to
another, and back again. During the convalescence of animals, great
benefit is derived from almost any 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 species, which differ to
a certain extent, gives vigour and fertility 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 condi-
tions 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 condi-
tions, or which have slightly varied, give vigour and fertility to the
offspring. But, as we have seen, organic beings long habituated to
certain uniform conditions under a state of nature, when subjected,
as under confinement, to a considerable change in their conditions,
very frequendy 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 always 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 generally 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 conditions, 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,

DIMORPHISM AND TRIMORPHISM 305

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 tendency is slightly
disturbed by any change, the vital forces gain in power.

RECIPROCAL DIMORPHISM AND TRIMORPHISM

This subject may be here briefly discussed, and will be found to
throw some light on hybridism. Several plants belonging to distinct
orders present two forms, which exist in about equal numbers and
which differ in no respect except 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 differing 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 stamens, 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 corresponding 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 illegitimate,
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 dis-
tinct species be placed on the stigma of a flower, and its own pollen

3o6 origin of species

be afterwards, even after a considerable interval of time, placed on
the 5ame stigma, its action is so strongly prepotent that it generally
annihilates the effect of the foreign 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 illegitimately, and twenty-four hours afterwards legitimately,
with pollen taken from a peculiarly coloured variety, and all the seed-
lings were similarly coloured; this shows that the legitimate pollen,
though applied twenty-four hours subsequently, had wholly de-
stroyed 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 fertilised with
the greatest ease by pollen from the longer stamens 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 be-
have 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 illegitimate unions. The chief result
is that these illegidmate 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 during 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 com-
pared with that of hybrids when crossed inter se. If, on the other
hand, a hybrid is crossed with either pure parent-species, the sterility

DIMORPHISM AND TRIMORPHISM 307

is usually much lessened; and so it is when an illegitimate plant is
fertilised by a legitimate plant. In the same manner 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
hybrids raised from the same seed-capsule the degree of sterility is
innately variable, so it is in a marked manner with illegitimate
plants. Lastly, many hybrids are profuse and persistent flowerets,
whilst other and more sterile hybrids produce few flowers, and are
weak, miserable dwarfs; exactly similar cases occur with the illegiti-
mate 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 be-
tween so-called distinct 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 crossing 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 speci-
fied respects as if they had been two distinct species. But to make
the case sure, he would raise plants from his supposed hybridized
seed, and he would find that the seedlings' Were miserably dwarfed
and utterly sterile, and that they behaved in all other respects like
ordinary hybrids. He might then maintain 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 completely mistaken.

The facts now given on dimorphic and trimorphic plants are
important, because they show us, first, that the physiological test of
lessened fertility, both in first crosses and in hybrids, is no safe

3o8 origin of species

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 constitution, 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 &st sight exacdy the reverse of what occurs, in the ordinary unions
of the individuals of the same species and with crosses between dis-
tinct 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 distinct species
when crossed and of their hybrid progeny, depends exclusively on
the nature of their sexual elements, and not on any difference in
their structure or general constitution. 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 united with great difficulty,
with the female of a second species, whilst the converse cross can be
effected with perfect facility. That excellent observer, Gartner, like-
wise concluded that species when crossed are sterile owing to differ-
ences 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 varieties, 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 exceptions, presently to be given, I fully
admit that this is the rule. But the sub j ect is surrounded- by difficul-
ties^ for, looking to varieties produced Under nature, if two forms

FERTILITY OF VARIETIES 309

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 consequently 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 pro-
duced, 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 resembling
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 difference 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 sexual constitution. Now
the varying conditions to which domesticated animals and cultivated
plants have been subjected, have had so little tendency towards modi-
fying the reproductive system in a manner leading to mutual
sterility, that we have good grounds for admitting the directly; oppo-
site doctrine of Pallas, namely, that such conditions generally elim-
inate 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 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 Paltasian

ORIGIN OF SPECIES

310

doctrine of the elimination of sterility through long-continued domes-
tication be admitted, and it can hardly be rejected, it becomes in
the highest degree improbable that similar conditions long con-
tinued should likewise induce this tendency; though in certain cases,
with species having a peculiar constitution, sterility might occasion-
ally 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, imme-
diately to be given, have been observed.

The real difEcuity in our present subject is not, as it appears 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 surprising, seeing how profoundly
ignorant we are in regard to the normal and abnormal action of the
reproductive system. But we can see that species, owing to their
struggle for existence with numerous competitors, will have been ex-
posed during long periods of time to more uniform conditions, than
have domestic varieties; and this may well make a wide difference in
the result. For we know how commonly 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 unnatural 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 of crossing with other varieties which had orig-
inated 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 impossible 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

FERTILITY OF VARIETIES 3II

at least as good as that from which we believe in the sterility of a
multitude of species. 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 several years
a dwarf kind of maize with yellow seeds, and 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 pro-
duced 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 distinct species; and it
is important to notice that the hybrid plants thus raised were them-
selves perfectly fertile; so that even Gartner did not venture to con-
sider the two varieties as specifically distinct.

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 differences 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 classification 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 number of experi-
ments made during many years on nine species of Verbascum, by
so good an observer and so hostile a witness as Gartner: namely
that the yellow and white varieties when crossed produce less
seed than the similarly coloured varieties of the same species.
Moreover, he asserts that when yellow and white varieties of one
species are crossed 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 pro-
portion of eighty-six to 100, than the similarly coloured varieties.

ORIGIN OF SPECIES

312

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 ferdle 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 varieties
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 infertile 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 varie-
ties not having been exposed for very long periods to uniform con-
ditions 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 en-
dowment, but as incidental on changes of an unknown nature in
their sexual elements.

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 differences between the so-called hybrid

HYBRIDS AND MONGRELS COMPARED 313

offspring of species, and the so-called mongrel offspring of varieties.
An4 on the other hand, they agree most closely in many important
respects.

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 off-
spring in both cases is notorious; but some few instances 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 mosdy domestic varieties (very few experiments 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, fails under these circumstances to per-
form its proper function 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 reproduc-
tive 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

ORIGIN OF SPECIES

314

parent-form; but this, if it be true, is certainly only a difference in
degree* Moreover, Gartner expressly 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
difference 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 experi-
mented 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 ex-
periment; and seems directly opposed to the results of several ex-
periments 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 pro-
duced 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 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 animals; but
the subject is here much complicated, partly owing to the existence
of secondary sexual characters; but more especially owing to prepo-
tency in transmitting likeness running more strongly in one sex than
in the other, both when one species is crossed with another, and

HYBRIDS AND MONGRELS COMPARED 315

when one variety is crossed with another variety. For instance, I
think those authors are right who maintain that the ass has a pre-
potent 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 supposed 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 hybrids, 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 selec-
tion. A tendency to sudden reversions to the perfect character of
either parent would, also, be much more likely to occur with mon-
grels, which are descended from varieties often suddenly produced
and semi-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 con-
clusion that the laws of resemblance of the child to its parents are
the same, whether the two parents 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 similarity in the off-
spring of crossed species, and of crossed varieties. 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 harmonises perfectly with the view that there
is no essential distinction between species and varieties.

3i6

ORIGIN OF SPECIES

SUMMARY OF CHAPTER

First crosses between forms, suflBciently 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 individuals of the same species, and is eminendy sus-
ceptible 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 sometimes widely different in reciprocal crosses be-
tween 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 vegetative systems, so in crossing, the
greater or less facility of one species to unite with another is inci-
dental 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 prevent their inarching in our forests.

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 circumstances; 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 dis-
turbed 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 hy-
brids. 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 fertility of all organic beings; and secondly,
that the crossing of forms, which have been exposed to slightly

SUMMARY 317

different conditions o£ 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 trimorphic plants and
of their illegitimate progeny, perhaps render 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 modified, leading to their
mutual infertility, we do not know; but it seems to stand in some
close relation to species having 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 cor-
respond, 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 circumstances — should all run, to a certain extent,
parallel with the systematic affinity of the forms subjected to experi-
ment; for systematic affinity includes resemblances of all kinds.

First crosses between forms known to be varieties, or sufficiendy
alike to be considered as varieties, and their mongrel 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. Independendy of the question of fertility, in all other
respects there is the closest general resemblance between hybrids and

ORIGIN OF SPECIES

^ - -

mongrels, — ^in their variability, in their power of absorbing each
other by repeated crosses, and in their inheritance of characters from
both parent'forms. Finally, then, although we are as ignorant 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

Ox 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 deposi-
tion — On the lapse of time as estimated by years — On the poorness
of our paiseontological collections — On the intermittence of geologi-
cal 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-^n their sudden appearance in the
lowest known fossiiiferous strata — ^Antiquity of the habitable earth.

I N the sixth chapter I enumerated the chief objections which
might be justly urged against the views maintained in this
volume. Most of them have now been discussed. One, namely
the distinctness of specific forms, and their not being blended to-
gether 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 apparendy most favourable for
their presence, namely on an extensive and continuous area with
graduated 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, there-
fore, that the really governing conditions of life do not graduate
away quite insensibly like heat or moisture. I endeavoured, also, to
show that intermediate varieties, 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 im-
provement. 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 varieties
continually take the places of and supplant their parent-forms. But
just in proportion as this process of extermination has acted on an
enormous scale, so must the number of intermediate varieties, which
have formerly existed, be truly enormous. Why then is not every

319

320 ORIGIN OF SPECIES

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 intermediate between them. But
this is a wholly false view; we should always look for forms inter-
mediate between each species and a common but unknown progeni-
tor; 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 in-
termediate between the fantail and pouter; none, for instance, com-
bining 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 his-
torical 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
descended 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 other. Hence, in all such cases, we should be unable to
recognise the parent-form of any two or more species, even if we
closely compared the structure of the parent with that of its modified

THE LAPSE OF TIME 321

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 imply that one form had remained
for a very long period unaltered, whilst its descendants had under-
gone 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 con-
nected with the parent-species of each genus, by differences 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 INFEEJRED FROM THE RATE OF
DEPOSITION AND EXTENT OF DENUDATION

Independently of our not finding fossil remains of such infinitely
numerous connecting links, it may be objected that time cannot
have sujEced 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 not admit how vast have been the
past periods of time, may at once close this volume. Not that it
suiSSces 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

ORIGIN OF SPECIES

322

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 deposited. As Lyell has well remarked, the extent
and thickness of our sedimentary formations are the result and the
measure of the denudation which the earth’s crust has elsewhere
' 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 com-
prehend something about the duration of past time, the monuments
of which we see all around us.

It is good to wander along the coast, when formed of moderately
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 frag-
ments 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 pro-
ductions, showing how litde 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
surface and the vegetation show that elsewhere years have elapsed
since the waters washed their base.

We have, however, recendy learnt from the observations of Ram-
say, in the van of many excellent observers— of Jukes, Geikie, Croll,
and others, that subaerial degradation is a 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 during heavy rain, and to a greater extent than might

THE LAPSE OF TIME 323

be supposed, 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 degrada-
tion 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 dis-
trict 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 various 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 com-
posed having resisted subaerial denudation better than the surround-
ing surface; this surface 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 removed over many extensive
areas, and on the other hand the thickness of our sedimentary
formations. I remember having been much struck when viewing
volcanic islands, which have been worn by the waves and pared all
round into perpendicular 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 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 difference whether the upheaval was sudden, or,
as most geologists now believe, was slow and effected by many
starts, the surface of the land has been so completely planed down

324 ORIGIN OF SPECIES

that no trace of these vast dislocations is externally visible. The
Craven fault, for instance, extends for upwards of thirty miles, and
along this line the vertical displacement of the strata varies from
600 to 3000 feet. Professor Ramsay has published an account of a
downthrow in Anglesea of 2,300 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 sedi-
mentary strata are of wonderful thickness. In the Cordillera I
estimated one mass of conglomerate at ten thousand feet; and al-
though 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
measurement in most cases, of the successive formations in di-gerent
parts of Great Britain; and this is the result:—

Feet

Palseozoic strata (not including igneous beds) 57 jI 54

Secondary strata

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 thick-
ness 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 rocks in Britain gives
but an inadequate idea of the time which has elapsed during their
accumulation. The consideration of these various facts impresses
the mind almost in the same manner as does the vain endeavour to
grapple with the idea of eternity.

Nevertheless this impression is pardy false. Mr. Croll, in an in-
teresting 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

THE LAPSE OF TIME

325

phenomena, and then at the figures representing several million
years, the two produce a totally dififerent effect on the mind, and the
figures are at once pronounced 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 1,000 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 considerations 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 illustra-
tion: Take a narrow strip of paper, eighty-three feet four 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 in-
significant 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 succes-
sion. It is not to be supposed that species in a state of nature ever
change so quickly as domestic animals under the guidance of
methodical selection. The comparison would be in every way fairer
with the effects which follow from unconscious selection, that is
the preservation of 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 slow-
ness follows from all the inhabitants of the same country being
already so well adapted to each other, that new places in the polity

326 ORIGIN OF SPECIES

of nature do not occur until after long intervals, due to the occur-
rence 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
determining, 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 PALAEONTOLOGICAL 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 admirable palaeontologist,
Edward Forbes, should never be forgotten, 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 portion of the surface of the earth has been geologically
explored, and no part with sufficient care, as the important dis-
coveries 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 enormously large, proportion of the ocean, the bright
blue tint of the water bespeaks its purity. The many cases on record
of a formation conformably covered, after an immense interval of
time, by another and later formation, without the underlying 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 percolation 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
instance, the several species of the Chthamalinae (a sub-family of

PALiEONTOLOGICAL COLLECTIONS 327

sessile cirripedes) coat the rocks all over the world in infinite num-
bers; they are all strictly littoral, with the exception o£ 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 existed during the Chalk period. Lasdy, many great
deposits requiring a vast length of time for their accumulation, are
entirely destitute of organic remains, without our being able to as-
sign 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 ex-
tending for at least 300 miles from Vienna to Switzerland; and
although this great mass has been most carefully searched, no fos-
sils. except a few vegetable 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 degree. 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 have been found in the lias. In regard to mam-
miferous 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 not a cave or true lacustrine bed is
known belonging to the age of our secondary or palaeozoic forma-
tions.

But the imperfection in the geological record largely results from
another and more important cause than any of the foregoing;
namely, from the several formations being separated from each
other by wide intervals of time. This doctrine 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 diflicult to avoid believing that they are closely con-

328 ORIGIN OB SPECIES

secutive. 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 peculiar 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
consecutive formations, we may infer that this could nowhere be
ascertained. The frequent and great changes in the mineralogical
composition of consecutive formations, generally implying 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 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 degrada-
tion of the coast-rocks and from muddy streams entering the sea.
The explanation, no doubt, is, that the littoral and sub-littoral de-
posits 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 accumulated in
extremely thick, solid, or extensive masses, in order to withstand

PALi^lONTOLOGICAL COLLECTIONS 329

the incessant action o£ the waves, when first upraised and during
successive oscillations of level, as well as the subsequent subaerial
degradation. Such thick and extensive 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 thickness and extent over a
shallow bottom, if it continue slowly to subside. In this latter case,
as long as the rate of subsidence 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 denudation,
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 formation, 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 suf-
fered, but which will hardly last to a distant geological age, was
deposited during a downward oscillation of level, and thus gained
considerable thickness.

All geological facts tell us plainly that each area has undergone
numerous slow oscillations of level, and apparently these oscillations
have affected wide spaces. Consequently, formations rich in fossils
jand 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, thicl(^ deposits cannot have been accumulated in
the shallow parts, which are the most favourable to life. Still less

33^ ORIGIN OF SPECIES

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 destroyed by being upraised and brought
within the limits of the coast-action.

These remarks apply chiefly to littoral and sub-littoral deposits.
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
thirty or forty to sixty 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 forma-
tions, so that it would run a good chance of being worn away by
atmospheric degradation and by the action of the sea during sub-
sequent oscillations of level. It has, however, been suggested
by Mr. Hopkins, that if one part of the area, after rising and be-
fore being denuded, subsided, the deposit formed during the
rising movement, though not thick, might afterwards become pro-
tected 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 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 pro-
found depths of the ocean, the former protecting mantle of rock
may not have been very thick. Admitting then that gneiss, mica-
schist, granite, diorite, etc., 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 sub-
sequently been completely denuded of all overlying, strata? That
such extensive areas do exist. cannot be doubted; the granitic region
of Parime is described by Humboldt as being at least nineteen times

PALiEONTOLOGICAL COLLECTIONS 331

as large as Switzerland. South o£ 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, aU con-
joined. This region has not been carefully explored, but from the
concurrent testimony of travellers, the granitic area is very large;
thus. Von Eschwege gives a detailed section of these rocks, stretch-
ing 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. Numerous specimens, collected
along the whole coast from near Rio Janeiro to the mouth of the
Plata, a distance of 1,100 geographical 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 weU-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 I
find that the metamorphic (excluding “the semi-metamorphic”)
and granitic rocks exceed, in the proportion of 19 to 12.5, the whole
of the newer Palaeozoic formations. In many regions the meta-
morphic and granitic rocks would be found much more widely
extended than they appear to be, if all the sedimentary beds were
removed which rest unconformably on them, and which could 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 denuded, 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 explained, for the forma-
tion 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 in-
habitants will decrease (excepting on the shores of a continent when
first broken up into an archipelago), and consequently, during sub-

ORIGIN OF SPECIES

332

sidence, though there will be much extinction, few new varieties or
species will be formed; and it is during these very periods of sub-
sidence 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 formation, 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 Ammonites; and Hilgendorf has described a most curious case
of ten graduated forms of Planorbis multiformis in the successive
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 com-
monly include a graduated series of links between the species which
lived at its commencement and close; but I cannot assign due pro-
portional weight to the following considerations.

Although each formation may mark a very long lapse of years,
each probably is short compared with the period requisite to change
one species into another. I am aware that two palaeontologists, whose
opinions are worthy of much deference, namely Bronn and Wood-
ward, 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 com-
ing 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 com-
pared with the rest of the world; nor have the several stages of the

ABSENCE OF INTERMEDIATE VARIETIES 333

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 immigrated 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 apparendy 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 deposit,
but have become extinct in the immediately surrounding 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 ascertained amount of migration of the inhabitants
of Europe during 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 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 during
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 prob-
ably first appear and disappear at different levels, owing to the
migrations of species and to geographical changes. And in the dis-
tant 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.

334 ORIGIN OF SPECIES

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, sujOEcient for
the slow process of modification; hence the deposit must be a very
thick one; and the species undergoing 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 approximately 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 subsidence. 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 continues. 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 palaontologist, 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 intermittent in its
accumulation. When we see, as is so often the case, a formation
composed of beds of widely difierent mineralogical 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 ignorant 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 forma-
tion having been upraised, 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 accumulation.
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

ABSENCE OB INTERMEDIATE VARIETIES 335

not have been suspected, had not the trees been preserved: thus Sir
C. Lyell and Dr. Dawson found carboniferous beds 1,400 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 reappeared, 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 geological formation, a section would not in-
clude all the fine intermediate gradations which must, on our theory,
have existed, 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 between any two forms, they rank botli
as species, unless they are enabled to connect them together by the
closest intermediate gradations; and this, from the reasons just as-
signed, we can seldom hope to effect in any one geological section.
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 inter-
mediate 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 be-
tween 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 substages of the
same formation. Some experienced conchologists 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

33^ ORIGIN OF SPECIES

change which on the theory we ought to find. Look again at the
later tertiary deposits, which include many shells believed by the
majority of naturalists to be identical with existing species; but some
excellent naturalists, as Agassiz and Pictet, maintain that all these
tertiary species are specifically distinct, though the distinction is ad-
mitted 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 representatives, or unless we admit, in opposition
to the judgment of most naturalists, that these tertiary species are all
truly distinct 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 specifically different, yet are far more
closely related to each other than are the species found in more widely
separated formations; so that here again we have undoubted evidence
of 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 wan-
der 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 perfected in some considerable degree. Ac-
cording 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 varieties; so that, with shells and
other marine animals, it is probable that those which had the widest
range, far exceeding 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 gready 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

ABSENCE OF INTERMEDIATE VARIETIES 337

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 undergoing 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 spe-
cies this can rarely be done. We shall, perhaps, best perceive the im-
probability of our being enabled to connect species by numerous,
fine, intermediate, 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 conchologists as distinct species from their European
representatives, 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 discovering in a fossil
state numerous intermediate gradations; and such success is improb-
able 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 errone-
ous. As Sir J. Lubbock has remarked, ‘‘Every species is a link between
other allied forms.” If we take a genus having a score of species,
recent and extinct, 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 revealed, is the former existence of in-
finitely numerous gradations, as fine as existing varieties, connecting
together nearly all existing and extinct species. But this ought not
to be expected; yet this has been repeatedly advanced as a most seri-
ous 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 imagi-

338 ORIGIN OF SPECIES

nary illustration. The Malay Archipelago is about the size of Europe
from the North Cape to the Mediterranean, and from Britain to Rus-
sia; 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 condition of the Malay Archipelago, with its numerous large
islands separated by wide and shallow seas, probably represents the
former state of Europe, whilst most of our formations were accumu-
lating. 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 imperfecdy would they represent the natural
history of the world!

But we have every reason to beHeve that the terrestrial productions
of the archipelago would be preserved in an extremely imperfect
manner in the formations which we suppose to be there accumulat-
ing. 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
sediment 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 secondary formations lie
in the past, would generally be formed in the archipelago only dur-
ing 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 ex-
tensive and shallow seas within the archipelago, sedimentary beds
•could hardly be accumulated of great thickness during the periods of
elevation, or become capped and protected 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 varia-
tion, but the geological record would then be less perfect.

ABSENCE OF INTERMEDIATE VARIETIES 339

It may be doubted whether the duration of any one great period
of subsidence over the whole or part of the archipelago, together with
a contemporaneous accumulation of sediment, would exceed
average duration of the same specific forms; and these contingencies
are indispensable for the preservation of all the transitional gradations
between any two or more species. If such gradations were not all
fully preserved, 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 oscillations 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 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 analogy 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
slighdy different sub-stages of the same formation, they would, ac-
cording to the principles followed by many paleontologists, be ranked
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 infiinite num-
ber of those fine transitional forms which, on our theory, have con-
nected 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 palseontologists, 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

ORIGIN OF SPECIES

340

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 suddenly
appear in certain formations, has been urged by several palaeontolo-
gists—for instance, by Agassiz, Pictet, and Sedgwick—as a fatal ob-
jection to the belief in the transmutation of species. If numerous
species, belonging to the same genera 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 progenitors must
have lived long before their modified descendants. But we continu-
ally 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 palaeontological evidence may be implicitly trusted; nega-
tive evidence is worthless, as experience has so often shown. We con-
tinually forget how large the world is, compared with the area over
which our geological formations have been carefully examined; we
forget that groups of species may elsewhere have long existed, and
have slowly multiplied, before they invaded the ancient archipela-
goes of Europe and the United States. We do not make due allow-
ance for the intervals of time which have elapsed between our con-
secutive 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 par-
ent-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 re-
quire 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 con-
sequently that the transitional forms would often long remain con-
fined to some one region; but that, when this adaptation had once

APPEARANCE OF WHOLE GROUPS 34I

been effected, and a few species had thus acquired a great advantage
over other organisms, 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 re-
view of this work, in commenting on early transitional forms, and
taking birds as an illustration, cannot see how the successive modifi-
cations of the anterior limbs of a supposed prototype could possibly
have been of any advantage. But look at the penguins of the South-
ern Ocean; have not these birds their front limbs in this precise in-
termediate 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 suppose that we
here see the real transitional grades through which the wings of
birds have passed; but what special difficulty is there in believing that
it might profit the modified descendants of the penguin, first to be-
come 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, published in 1844-46 and 1853-57,
elusions on the first appearance 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 abrupdy come in at the commence-
ment of the tertiary series. And now one of the richest known accu-
mulations of fossil mammals belongs to the middle of the secondary
series; and true mammals have been discovered in the new red sand-
stone 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

ORIGIN OF SPECIES

342

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, palaeontologists maintained that
the whole class of birds came suddenly into existence during the
eocene period; but now we know, on the authority of Professor
Owen, that a bird certainly lived during the deposition of the upper
greensand; and still more recently, that strange bird, the Arche-
opteryx, with a long lizard-like tail, bearing a pair of feathers on
each joint, and with its wings furnished with two free claws, has
been iscovered in the oolitic slates of Solenhofen. Hardly any recent
discovery shows more forcibly than this, how litde 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 ex-
tinct tertiary species; from the extraordinary abundance of the indi-
viduals 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 fifty 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 cir-
cumstances, 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 de-
veloped at the commencement 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 unmistakeable 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.

APPEARANCE OF WHOLE GROUPS 343

The case most frequently insisted on by palaeontologists of the ap-
parendy sudden appearance of a whole group of species, is that of
the teleostean fishes, low down, according to Agassiz, in the Chalk
period. This group includes the large majority of existing species.
But certain Jurassic and Triassic 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 difEculty, 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 formations in Europe, Some few families of fish now
have a confined range; the teleostean fishes might formerly
have had a similarly confined range, and after having been largely
developed 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 present. Even at this day,
if the Malay Archipelago were converted 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 multi-
plied; and here they would remain confined, imtil 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 geology 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 succession 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.

344

ORIGIN OF SPECIES

ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE
LOWEST KNOW^^ 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 suddenly appear in the lowest
known fossiliferous rocks. Most of the arguments which have con-
' vinced me that ail the existing species of the same group are de-
scended from a single progenitor, apply with equal force to the earli-
est 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 Cambrian age,
and which probably differed gready from any known animal. Some
of the most ancient animals, as the Naudlus, Lingula, etc., do not
differ much from living species; and it cannot on our theory be sup-
posed, that these old species 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 periods 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 for-
midable objection; for it seems doubtful whether the earth, in a fit
state for the habitation of living creatures, has lasted long enough.
Sir W. Thompson concludes that the consolidadon of the crust can
hardly have occurred less than twenty or more than 400 million
years ago, but probably not less than ninety-eight 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 sixty 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 dme for the many and great mutadons of
life, which have certainly occurred since the Cambrian formadon;
and the previous 140 million years can hardly be considered as suffi-
cient for the development of the varied forms of life which already

SUDDEI^ APPEARANCE OF GROUPS 345

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 deposits
belonging to these assumed earliest periods prior to the Cambrian
system, I can give no . satisfactory answer. Several eminent geolo-
gists, with Sir R. Murchison at their head, were until recently con-
vinced 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 tri-
lobites, and containing various molluscs and annelids. The presence
of phosphatic nodules and bituminous matter, even in some of the
lowest azoic rocks, probably 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 possi-
bly far surpass that of all the succeeding rocks, from the base of the
palseozoic series to the present time. We are thus carried back to a
period so remote that the appearance 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
aU classes of animals, but is highly organised for its class; it existed in
countless numbers, and, as Dr. Dawson has remarked, certainly
preyed on other minute organic beings, which must have lived in
great numbers. Thus the words, which I wrote in 1859, about the ex-
istence of living beings long before the Cambrian period, and which
are almost the same with those since used by Sir W. Logan, have
proved true. Nevertheless, the difEculty of assigning any good reason

346 ORIGIN OF SPECIES

for the absence of vast piles of strata rich in fossils beneath the Cam-
brian system is very great. It does not seem probable that the most
ancient beds have been quite worn 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 formations next succeeding them in age, and these would always
have existed in a partially metamorphosed condition. But the descrip-
tions which we possess of the Silurian deposits over immense ter-
ritories 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 metamorphism.

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 for-
mations 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 extensive 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 rem-
nant of any palaeozoic and secondary formation. Hence, we may per-
haps infer that during the palaeozoic and secondary periods, neither
continents nor continental islands existed where our oceans now
extend; for had they existed, palaeozoic and secondary formations
would in aU probability have been accumulated from sediment de-
rived from their wear and tear; and these would have been at least

SUDDEN APPEARANCE OF GROUPS 347

partially upheaved by the oscillations of level, which must have inter-
vened 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 continents now
exist, large tracts of land have existed, subjected no doubt to great
oscillations of level, since the Cambrian period. The colored map ap-
pended to my volume on Coral Reefs, led me to conclude that the
great oceans are still mainly areas of subsidence, the great archipela-
goes still areas of oscillations of level, and the continents areas of ele-
vation. But we have no reason to assume that things have thus re-
mained 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 ante-
cedent to the Cambrian epoch, continents may have existed where
oceans are now spread out; and clear and open oceans may have ex-
isted 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 forma-
tions in a recognisable condition older than the Cambrian strata, sup-
posing such to have been formerly deposited; for it might well hap-
pen 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
superincumbent water, might have undergone far more meta-
morphic 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 re-
quire some special explanation; and we may perhaps believe that we
see in these large areas, the many formations long anterior to the
Cambrian 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 between the species
which now exist and which formerly existed, we do not find infinitely
numerous fine transitional forms closely joining them all together; —

348 ORIGIN OF SPECIES

the sudden manner 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 ail undoubtedly of the most serious nature. We see this in the fact
that the most eminent palseontologists, namely, Cuvier, Agassiz, Bar-
rande, Pictet, Falconer, E. Forbes, etc., and all our greatest geologists,
as Lyell, Murchison, Sedgwick, etc., have unanimously, often vehe-
mently, maintained the immutability of species. But Sir Charles Lyell
now gives the support of his high authority to the other side; and
most geologists and paleontologists are much shaken in their for-
mer belief. Those who believe that the geological record is in any
degree perfect, will undoubtedly at once reject the theory. For my
part, following out Lyell’s metaphor, I look at the geological record
as a history of the world imperfecdy kept, and written in a changing
dialect; of this history we possess the last volume alone, relating
only to two or three countries. Of this volume, only here and there
a short chapter has been preserved; and of each page, only here and
there a few lines. Each word of the slowly-changing language, more
or less difierent in the successive chapters, may represent the forms of
life, which are entombed in our consecutive formations, and which
falsely appear to have been abrupdy introduced. On this view, the
difficulties above discussed are gready 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 appearance and dis-
appearance 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.

I ET us now see whether the several facts and laws relating 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 modification, 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 proportion 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 appearance 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 given a striking instance of a similar
fact, for an existing crocodile is associated with many lost mammals

349

ORIGIN OF SPECIES

350

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
gready. 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 formadon. 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 disappeared 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 preexisting fauna to reappear; but Ly ell’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 inhabitants of an area to
change abrupdy, 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 indi-
vidual 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 competition. Hence it is by no means surprising that one species
should retain the same identical form much longer than others; or,
if changing, should change in a less degree. We find similar relations
between the existing inhabitants of distinct countries; for instance,
the land shells and coleopterous insects of Madeira have come to

GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 35 1

differ considerably from their nearest allies on the continent of
Europe, whereas the marine shells and birds have remained un-
altered. We can perhaps understand the apparently quicker rate
of change in terrestrial and in more highly organised productions
compared with marine and lower productions, by the more complex
relations of the higher beings to their organic and inorganic condi-
tions 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 organism 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 sediment being deposited on subsiding
areas, our formations have been almost necessarily accumulated at
wide and irregularly intermittent intervals of time; consequently
the amount of orgardc change exhibited by the fossils embedded in
consecutive 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 supplant it; yet the two forms — the old and the new
— ^would not be identically the same; for both would almost certainly
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

ORIGIN OF SPECIES

352

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 off-
spring, it is incredible that a fantaii, 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 succes-
sive variations would almost certainly be in some degree different,
and the newly-formed variety would probably inherit from its pro-
genitor some characteristic 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 disappeared, never reappears;
that is, its existence, as long as it lasts, is continuous. I am aware
that there are some apparent exceptions to this rule, but the excep-
tions are surprisingly few, so few that E. Forbes, Pictet, and Wood-
ward (though all strongly opposed to such views as I maintain)
admit its truth; and the rule stricdy 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 com-
mon 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 generations, from the lowest Silurian stratum
to the present day.

We have seen in the last chapter that whole groups of species some-
times falsely appear to have been abruptly developed; 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 maximum, 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 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

EXTINCTION

353

beds, marking the decrease and final extinction o£ the species. This
gradual increase in number o£ the species of a group is strictly con-
formable 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 production 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 disappearance of
species and of groups of species. On the theory of natural selection,
the extinction of old forms and the production of new and improved
forms are intimately connected 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, etc., whose general views
would naturally lead them to this conclusion. On the contrary, we
have every reason to believe, from the study of the tertiary forma-
tions, 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
inhabitants 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 la§t for very unequal periods;
some groups, as we have seen, have endured from the earliest known
dawn of life to the present day; 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
endures. 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 extermina^

ORIGIN OF SPECIES

354

tion, 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 monsters, 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 Spaniards into South America,
has run wild over the whole country and has increased in numbers
at an unparalleled rate, I asked myself what could so recendy have
exterminated the former horse under conditions of life apparendy
so favourable. 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 sdll 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
condidons 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 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 unfavour-
able conditions were which checked its increase, 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 cer-
tainly have become rarer and rarer, and finally extinct;— its place
being seized on by some more successful competitor.

EXTINCTION

355

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 litde is this subject understood,
that I have heard surprise repeatedly expressed at such great mon-
sters as the Mastodon and the more ancient Dinosaurians having
become extinct; as if mere bodily strength gave victory in the batde
of life. Mere size, on the contrary, would in some cases determine,
as has been remarked by Owen, quicker exterminadon from the
greater amount of requisite food. Before man inhabited India or
Africa, some cause must have checked the condnued increase of the
exisdng elephant. A highly capable judge, Dr, Falconer, believes
that it is chiefly insects which, from incessantly harassing and weak-
ening the elephant in India, check its increase; and this was Bruce’s
conclusion with respect to the African elephant in Abyssinia. It is
certain that insects and blood-sucking bats determine the existence
of the larger naturalized quadrupeds in several parts of South
America.

We see in many cases in the more recent terdary formadons, that
rarity precedes exdnction; 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 of a species, and yet to marvel gready 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 surprise 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 main-
tained by having some advantage over those with which it comes
into competition; and the consequent 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
neighbourhood; when much improved it is transported far and near,

35^ ORIGIN OF SPECIES

like our short-horn catde, and takes the place of other breeds in other
countries. Thus the appearance of new forms and the disappearance
of old forms, both those naturally 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 species have
not gone on indefinitely increasing, at least during the later geologi-
cal 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 generally cause the extermi-
nation 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 extermina-
tion. Thus, as I believe, a number of new species descended from
one species, that is a new genus, comes to supplant an old genus,
belonging 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 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 inferiority in common. But whether it be species belong-
ing 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 peculiar
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 forma-
tions, 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.

FORMS OF LIFE CHANGING 357

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 secondary period, we must
remember what has been already said on the probable wide intervals
of time between our consecutive formations; and in these intervals
there may have been much slow extermination. Moreover, when, by
sudden 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 partake 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 extinction; 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 species 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 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 SIMULTANEOUSLY
THROUGHOUT THE WORLD

Scarcely any palaeontological discovery is more striking than the
fact that the forms of life change almost simultaneously 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 present an

358 ORIGIN OF SPECIES

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 formations either above or below, occur in the same order at
these distant points of the world. In the several successive palaeozoic
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
parallelism in the successive forms of life, in the palaeozoic and
tertiary stages, would still be manifest, and the several forma-
tions could be easily correlated.

These observations, however, relate to the marine inhabitants of
the world: we have not sufficient data to judge whether the pro-
ductions 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 Toxo-
don had been brought to Europe from La Plata, without any infor-
mation 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, including 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

FORMS OF LIFE CHANGING 359

the southern hemisphere. So, again, several highly competent
observers maintain that the existing productions 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 containing fossil remains in some
degree allied, and from not including those forms which are found
only in the older underlying deposits, would be correcdy ranked as
simultaneous 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 refer-
ring 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
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 general 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 currents, climate, or other physical conditions, as
the cause of these great rriutations in the forms of life throughout the
world, under the most different climates. We must, as Barrande 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 already dominant, or have some advantage

ORIGIN OF SPECIES

360

over the other forms in their own country, give birth to the greatest
number of new varieties or incipient species. We have distinct evi-
dence 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, vary-
ing, 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
geographical 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 inhabitants of the distinct continents than with the marine
inhabitants of the continuous sea. We might therefore expect to
find, as we do find, a less strict degree of parallelism in the 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 advantage over their already dominant parents, as
well as over other species, and again spreading, varying, and pro-
ducing new forms. The old forms which are beaten and which
yield their places to the new and victorious forms, will generally be
allied in groups, from inheriting some inferiority in common; and
rherefore, 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 disappearance.

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

FORMS OF LIFE CHANGING 361

are concerned, occurred during 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
inhabitants 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 reason to believe that
large areas are affected by the same movement, 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 exacdy, the same period, we
should find in both, from the causes explained in the foregoing
paragraphs, the same general succession in the forms of life; but
the species would not exactly correspond; for there will have been
a litde more dme in the one region than in the other for modifica-
tion, 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 parallelism between the suc-
cessive 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 diffi-
cult to account for considering the proximity of the two areas, —
unless, indeed, it be assumed that an isthmus separated two seas
inhabited by distinct, but contemporaneous, faunas. Lyell has made
similar observations on some of the later tertiary formations. Bar-
rande, also, shows that there is a striking general parallelism in the
successive Silurian deposits of Bohemia and Scandinavia; neverthe-
less he finds a surprising amount of difference in the species. If the
several formations in these regions have not been deposited during
the same exact periods, — a formation in one region often corre-
sponding with a blank interval in the other,— and if in both regions
the species have gone on slowly chfnging during the accumulation

362 ORIGIN OF SPECIES

of the several formations and during the long intervals of time
between them; in this case the several 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 into 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 differs from living forms. But, as
Buckland long ago remarked, extinct species can all be classed either
in still existing groups, or between them. That the extinct forms of
life help to fill up 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, and to give some instances. If we confine our attention
either to the living or to the extinct species of the same class, did
series is far less perfect than 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 ani-
mals; and in the writings of Agassiz, of prophetic or synthetic 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 inter-
vals 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 pachyderms 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 South
America connects to a certain extent these two grand divisions. No

AFFINITIES OF EXTINCT SPECIES 363

one will deny that the Hipparion is intermediate between the
existing horse and certain older ungulate forms. What a wonderful
connecting link in the chain of mammals is the Typotherium from
South America, as the name given to it by Professor Gervais
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 Halitherium 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 quad-
rupeds, to which the Sirenia are in other respects allied. The ceta-
ceans 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 themselves, are considered by Profes-
sor Huxley to be undoubtedly cetaceans, “and to constitute con-
necting links with the aquatic carnivora.”

Even the wide interval 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 which includes the most gigantic of
all terrestrial reptiles. Turning to the Invertebrata, Barrande asserts
(a higher authority 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
between genera belonging to distinct families. The most common
case, especially with respect to very distinct groups, such as fish and
reptiles, seems to be, that, supposing them to be distinguished at the

364 ORIGIN OF SPECIES

present day by a score o£ characters, the ancient members are sep-
arated 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 characters 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 ages; and it would be difficult to prove the
truth of the proposition, for every now and then a living animal,
as the Lepidosiren, is discovered having affinities directed towards
very distinct groups. Yet if we compare the older reptiles and batra-
chians, the older fish, the older cephalopods, and the eocene mam-
mals, 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 accord 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 con-
sidered as extinct. The three existing genera will form

a small family; and a closely allied family or sub-family; and
rn^^y a third family. These three families, together with the
many extinct genera on the several 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 prin-
ciple 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 necessary contingency; it depends solely
on the descendants from a species being thus enabled to seize on

AFFINITIES OF EXTINCT SPECIES 365

many and different places in the economy of nature. Therefore it is
quite possible, 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

All the many forms, extinct and recent, descended from (A),
make, as before remarked, one order; and this order, from the con-
tinued effects of extinction and divergence of character, has become
divided into several sub-families and families, 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 instance, the genera ct, i, f, m®, were

disinterred, these three families would be so closely linked together
that they probably would have to be united into one great family, in
nearly the same manner as has occurred with ruminants and certain
pachyderms. Yet he who objected to consider 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, etc,, and etc.)
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 to on the uppermost line, be sup-
posed to differ from each other by half-a-dozen important char-
acters, then the families which existed at the period marked VI.
would certainly have differed from each other by a less number of
characters; for they would at this early stage of descent have diverged
in a less degree from their common progenitor. Thus it comes that

366 ORIGIN OF SPECIES

ancient and extinct genera are often in a greater or less degree inter-
mediate 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 unequal lengths
of time, and will have been modified in various degrees. As we
possess only the last volume of the geological 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 periods,
undergone much modification, should in the older formations make
some shght approach to each other; so that the older members should
dijGFer 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 pafeontologists 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 man-
ner. 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 intermediate 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
modified 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 forma-
tions. Subject to these allowances, the fauna of each geological
period undoubtedly is intermediate in character, between the pre-
ceding and succeeding faunas. I need give only one instance, namely,
the manner in which the fossils of the Devonian system, when this

AFFINITIES OF EXTINCT SPECIES 367

system was first discovered, were at once recognised by palasontoio-
gists as intermediate in character between those of the overlying
carboniferous, and underlying Silurian systems. But each fauna is
not necessarily exactly intermediate, as unequal intervals of time
have elapsed between consecutive 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 excep-
tions to the rule. For instance, the species of mastodons and ele-
phants, when arranged by Dr. Falconer in two series, — in the first
place according to their mutual afiSnities, and in the second place
according to their periods of existence, — do not accord in arrange-
ment. The species extreme in character are not the oldest or the
most recent; nor are those which are intermediate in character, inter-
mediate 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 occa-
sionally have lasted much longer than a form elsewhere subsequently
produced, especially in the case of terrestrial 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 remains
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 fossils from two remote formations. Pictet gives
as a well-known instance, the general resemblance of the organic
remains from the several stages of the Chalk formation, though the

ORIGIN OF SPECIES

368

species are distinct in each stage. This fact alone, from its generality,
seems to have shaken Professor Pictet in his belief in the immuta-
bility of species. He who is acquainted with the distribution of
existing species over the globe, will not attempt to account for the
close resemblance of distinct species in closely consecutive forma-
tions, by the physical conditions of the ancient areas having remained
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 cli-
mates 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 interrupted, 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
alHed forms, or, as they have been called by some authors, repre-
sentative 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 differentia-
tion 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 advantage to each being, so natural selection will tend
to render the organisation of each being more specialised and per-
fect, and in this sense higher; not but that it may leave many crea-

STATE OF DEVELOPMENT COMPARED 369

tures with simple and unimproved structures fitted for simple
conditions of life, and in some cases will even degrade 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 paleozoic 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 paleontologists would answer
in the affifmative; 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 extremely remote geological
epoch; and that certain land and fresh-water shells have remaii^ed
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 insisted 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 Protozoa? Such objec-
tions 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 Foraminifera, for instance, could be proved to have
first come into existence during the Laurentian epoch, or the above
Brachiopods during the Cambrian formation; for in this case, there
would not have been time sufficient for the development of these
organisms up to the standard which they 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 will, during each successive age, have to be slightly modified,
so as to hold their places in relation to slight changes in their condi-

ORIGIN OF SPECIES

370

dons. 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 advanced 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 wtithin the known history of the world
organisation has largely advanced. 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 formerly 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 organisation. To
attempt to compare members of distinct types in the scale of high-
ness seems hopeless; who will decide whether a cuttle-fish be higher
than a bee — that insect which the great Von 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 difBculties in deciding which forms are the most advanced
in organisation, we ought not solely to compare the highest mem-
bers 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 to compare all the members, high and low, at two periods.
At an ancient epoch the highest and lowest molluscoidal animals,
namely, cephalopods and brachiopods, swarmed in numbers; at the
present time both groups are greatly reduced, whilst others, inter-
mediate in organisation, have largely increased; consequently some
jaaturalists maintain that molluscs were formerly more highly
developed than at present; but a stronger case can be made out on

STATE OF DEVELOPMENT COMPARED J7I

the opposite 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 representa-
tives. We ought also to compare the relative proportional 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 lower
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 extraordinary 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 multi-
tude of British forms would in the course of time become thoroughly
naturalised there, and would exterminate many of the natives. On
the other hand, from the fact that hardly a single inhabitant 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 considerable number would be enabled
to seize on places now occupied 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 New Zealand. Yet the most
skilful 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 embryos of recent
animals belonging to the same classes; and that the geological suc-
cession of extinct forms is nearly parallel 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 super-

ORIGIN OF SPECIES

372

vened at a not early age, and having been inherited at a correspond-
ing age. This process, whilst it leaves the embryo almost unaltered,
continually adds, in the course of successive generations, 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 degree less distinct
from each other than are the typical members of the same groups at
the present day, it would be vain to look for animals having the
common embryological character of the Vertebrata, until beds rich
in fossils are discovered 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 mani-
fest, 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
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 wonderful 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 relation between the extinct and living land shells

SUCCESSION OF SAME TYPES 373

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 Australia and of parts of
South America^ under the same latitude, would attempt to account,
on the one hand through dissimilar physical conditions, for the dis-
similarity 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 marsupials; and I have shown in the publica-
tions 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 relation to the distribution of marine
animals.

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 conti-
nent 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 intermigration, the feebler
will yield to the more dominant forms, and there will be nothing
immutable in the distribution of organic beings.

ORIGIN OF SPECIES

374

It may be asked in ridicule, whether I suppose that the mega-
therium 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 char-
acters 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 forma-
tion, and in a succeeding formation there be six other allied or repre-
sentative genera each with the same number of species, then we may
conclude that generally only one species of each of the older genera
has left modified descendants, which constitute the new genera con-
taining 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 species 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
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 extremely
imperfect; that only a small portion of the globe has been geo-
logically explored with care; that only certain classes of organic
beings have been largely preserved in a fossil state; that the number
both of specimens and of species, preserved in our museums, is abso-
lutely as nothing compared 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 many kinds, and thick enough to oudast
future degradation, great intervals of time must have elapsed between
most of our successive formations; that there has probably been more

SUMMARY 375

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 often-
est given rise to new species; that varieties have at first been local;
and lastly, although each species must have passed through numer-
ous 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 with the periods during
which each remained in an unchanged condition. These causes,
taken conjointly, will to a large extent 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 perfecdy restored, as a new 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 geological
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 dis-
believe in the immense intervals of time which must have elapsed
between our consecutive formations; he may overlook how impor-
tant 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 existed 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

376 ORIGIN* OF SPECIES

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 He still buried under the ocean.

Passing from these difficulties, the other great leading facts in
palaeontology agree admirably with the theory of descent with modi-
fication through variation and natural selection. We can thus under-
stand how it is that new species come in slowly and successively; how
species of different 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 increase in numbers slowly, and
endure for unequal periods of time; for the process of modification is
necessarily slow, 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 vigorous groups,
from their inferiority inherited from a common 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, lingering 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 gener-
ally succeed in displacing the groups which are their inferiors in the
struggle for existence. Hence, after long intervals of time, the pro-
ductions 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, W.e can understand,
from the continued tendency to divergence of character, why, the

SUMMARY

377

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 inter-
mediate 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 forma-
tions are closely allied; for they are closely linked together by genera-
tion. We can clearly see why the remains of an intermediate
formation are intermediate in character.

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 structure has generally become more
specialised; and this may account for the common belief held by so
many palaeontologists, 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 according 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 believe,
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 disappear. 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 supplanted 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 physical
conditions — ^Importance of barriers — ^Affinity of the productions of
the same continent — Centres of creation — ^Means of dispersal, by
changes of climate and of the level of the land, and by occasion^
means — Dispersal during the Glacial period — ^Alternate Glacial
periods in the North and South.

I N 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 conclusion. 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
fundamental divisions in geographical distribution is that between
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 cannot 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
conditions are peculiar in only a slight degree. Notwithstanding
this general parallelism in the conditions of the Old and New
Worlds, how widely different are their living productions!

In the southern hemisphere, if we compare large tracts of land in
Au^ralia^ South Africa, and western South America, between lati*:

378

GEOGRAPHICAL DISTRIBUTION 379

tudes 25° and 35°, we shaU 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 America south of latitude 35° with those north
of 25°, which consequently 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 vnth respect to the inhabi-
tants 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 differences between the produc-
tions of various regions. We see this in the great differences in nearly
all the terrestrial productions 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 migration
for the northern temperate forms, as -there now is for the stricdy
arctic productions. We see the same fact in the great difference be-
tween 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, deserts, etc., are not as
impassable, or likely to have endured so long, as the oceans separat-
ing 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, Crustacea, or echinodermata in common;
but Dr. Gunther has recently shown that about thirty per cent, of the
fishes are the same on the opposite sides of the isthmus of Panama;
and this fact has led naturalists to believe that the isthmus was for-
merly open. Westward of the shores of America, a wide space of
open ocean extends, with not an island as a halting-place for emi-
grants; here we have a barrier of another kind, and as soon as this

380 ORIGIN OF SPECIES

is passed we meet in die eastern islands o£ 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 die other hand, proceeding 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 Eakern and Western America and the
eastern Pacific islands, yet many 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 exacdy
opposite meridians of longitude.

A third great fact, pardy included in the foregoing statement, is
the aifinity of the productions of the same continent or of the same
sea, though the species themselves are distinct at diiferent points and
stations. It is a law of the widest generality, and every continent
oders innumerable instances. Nevertheless, the naturalist, in travel-
ling, 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 nc^ 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 north-
ward the plains of La Plata by another species of the same genus;
and not by a true ostrich or emu, like those inhabiting Africa 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 belonging 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

GEOGRAPHICAL DISTRIBUTION 38 1

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 South
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 Ameri-
can 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 regions may be attributed to modification
through variation and natural selection, and probably in a sub-
ordinate degree to the definite influence of different physical con-
ditions. 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 former immigrants;
— and on the action of the inhabitants on each other in leading to the
preservation of different modifications; the relation of organism to
organism in the struggle for life being, as I have already often re-
marked, the most important of all relations. Thus the high im-
portance of barriers comes into play by checking migration; as does
time for the slow process of modification through natural selection.
Widely ranging species, abounding in individuals, which have
already triumphed over many .competitors in their own widely ex-
tended .homes, will have the best chance of seizing on new places,
when they spread out into new countries. In their new homes they
will be exposed to new conditions, and will frequently undergo
further modification and improvement; and thus they will become
still further victorious, and will produce groups of modified descend-
ants. On this principle of inheritance with modification we can

ORIGIN OF SPECIES

382

understand how it is that sections o£ 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 ad-
vantage of by natural selection, only so far as it profits each indi-
vidual in its complex struggle for life, so the amount of modifica-
tion in different species will be no uniform quantity. If a number
of species, after having 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 any thing. These prin-
ciples 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 distant 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 undergone during whole geological periods little modifi-
cation, there is not much difficulty in believing that they have mi-
grated from the same region; for during the vast geographical 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 comparatively recent times, there is great diffi-
culty on this head. It 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 individuals
identically the same should have been produced from parents spe-
cifically distinct.

CENTRES OF SUPPOSED CREATION

383

SINGLE CENTRES OF SUPPOSED CREATION

We are thus brought to the question which has been largely dis-
cussed 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 continuous; 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 per-
haps with any other organic beings; and, accordingly, we find no
inexplicable instances of the same mammals inhabiting distant
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 pro-
duced at two separate points, why do we not find a single mammal
common to Europe and Australia or South America.? The condi-
tions of life are nearly the same, so that a multitude of European
animals and plants have become naturalised in America and Aus-
tralia; and some of the aboriginal plants are identically the same at
these distant points of the northern and southern hemispheres. The
answer, as I believe, is, that mammals have not been able to mi-
grate, whereas some plants, from their varied means of dispersal,
have migrated across the wide and broken interspaces. The great
and striking influence of barriers 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 confined to a single region;

ORIGIN OF SPECIES

384

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 k would be, if a directly opposite rule were to prevail,
when we go down one step lower in the series, namely, to the indi-
viduals 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 per-
mitted, is the most probable. Undoubtedly 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
die exceptions to continuity of range are so numerous and of so
grave a nature, that we ought to give up the belief, rendered prob-
able 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 explanadon could be offered of many in-
stances. 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 aiitarctic regions; and secondly (in the
following chapter), the wide distribution of fresh-water produc-
tions; and thirdly, the occurrence of the same terrestrial species on
islands and on the nearest mainland, 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
explained on the view of each species having migrated from a single
birthplace; then, considering our ignorance with respect to former
climatal and geographical changes and to the various occasional

CENTRES OF SUPPOSED CREATION 385

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 modi-
fication during their migration, from some one area. If, when most
of the species inhabiting one region are different from those of
another region, though closely alHed 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 modi-
fication. A volcanic island, for instance, upheaved and formed at
the 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 in-
heritance to the inhabitants of that continent. Cases of this nature
are common, and are, as we shall hereafter see, inexplicable on the
theory of independent 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
preexisting closely allied species.” And it is now well known that
he attributes this coincidence to descent with modification.

The question of single or multiple centres of creation difiers from
another though allied question, — ^namely, whether all the indi-
viduals of the same species are descended from a single pair, or
single hermaphrodite, or whether, as some authors suppose, from
many individuals simultaneously created. With organic beings 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 organ-
isms which habitually unite for each birth, or which occasionally
intercross, the individuals of the same species inhabiting the same

386 ORIGIN OF SPECIES

area will be kept nearly uniform by intercrossing; so that many indi-
viduals will go on simultaneously 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 subject. I
can give here only the briefest abstract of the more important facts.
Change of climate must have had a powerful influence on migration.
A region now impassable to certain organisms from the nature of
its climate, might have been a high road for migration, when the
climate was different. 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 terrestrial productions to pass from
one to the other. No geologist disputes that great mutations of level
have occurred within the period of existing organisms. Edward
Forbes insisted that all the islands in the Adantic must have been
recently connected with Europe or Africa, and Europe likewise with
America. Other authors have thus hypothedcally 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 recendy 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

MEANS OF DISPERSAL 387

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 continents, 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 pro-
ceeded 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 separate,
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 continent, — the close relation of
the tertiary inhabitants of several 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 ocean, —
these and other such facts are opposed to the admission of such pro-
digious geographical revolutions within the recent period, as are
necessary on the view advanced by Forbes and admitted by his
followers. The nature and relative 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
composition of such islands favour the admission that they are the
wrecks of sunken continents; — if they had originally existed as con-
tinental mountain ranges, some at least of the islands would have
been formed, like other mountain summits, of granite, metamorphic
schists, old fossiliferous and other rocks, instead of consisting of
mere piles of volcanic matter.

388 ORIGIN OF SPECIES

I must now say a few words on what are called accidental means,
but which more properly should be called occasional means of dis-
tribution. I shall here confine myself to plants. In botanical works,
this or that plant is often stated to be ill adapted for wide dissemi-
nation; but the greater or less facilities 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 surprise
I found that out of eighty-seven kinds, sixty-four germinated after
an immersion of twenty-eight days, and a few survived an immer-
sion of 137 days. It deserves notice that certain orders were far more
injured than others: nine Leguminosx were tried, and, with one
exception, they resisted the salt water badly; seven species of the
allied orders, Hydrophyllaceas and Polemoniace^, were all killed by
a month’s immersion. For convenience’ sake I chiefly tried small
seeds without the capsule 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. Afterwards I
tried some larger fruits, capsules, etc., and some of these floated for
a long time. It is well known what a difference there is in the buoy-
ancy of green and seasoned timber; 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 dry the stems
and branches of ninety-four 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 hazelnuts sank immediately, but when dried they
floated for ninety days, and afterwards when planted germinated;
an asparagus-plant with ripe berries floated for twenty-three days,
when dried it floated for eighty-five days, and the seeds afterwards
germinated; the ripe seeds of Helosciadium sank in two days, when
dried they floated for above ninety days, and afterwards germinated.
Altogether, out of the ninety-four dried plants, eighteen floated for
above twenty-eight days; and some of the eighteen floated for a very
much longer period. So that as ff- kinds of seeds germinated after
an immersion of twenty-eight days; and as if distinct species with
ripe fruit (but not all the same species as in the foregoing experi-

MEANS OF DISPERSAL

389

ment) floated, after being dried, for above twenty-eight days, we
may conclude, as far as anything can be inferred from these scanty
facts, that the seeds of kinds of plants of any country might be
floated by sea currents during twenty-eight days, and would retain
their power of germination. In Johnston’s Physical Adas, the average
rate of the several Atlantic currents is thirty-three miles per diem
(some currents running at the rate of sixty miles per diem) ; on this
average, the seeds of xw 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 favourable spot, would germinate,

Subsequendy 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 floadng plants. He tried ninety-eight seeds, mosdy dif-
ferent 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
previously dry the plants or branches with the fruit; and this, as we
have seen, would have caused some of them to have floated much
longer. The result was that if of his seeds of different kinds floated
for forty-two days, and were then capable of germination. But I do
not doubt that plants exposed to the waves would float for a less
time than those protected from violent movement as in our experi-
ments. 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

390 ORIGIN OF SPECIES

earth are frequently 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 dicoty-
ledonous 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 instance, are killed by even a
few days’ immersion in sea water; but some taken out of the crop
of a pigeon, which had floated on artificial sea water for thirty days,
to my surprise nearly ail 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 circum-
stances their rate of flight would often be thirty-five 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
digestive organs of a turkey. In the course of two months, I picked
up in my garden twelve kinds of seeds, out of the excrement 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 devoured 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 five hundred 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, millet,
canary, hemp, clover, and beet germinated after having been from
twelve to twenty-one hours in the stomachs of different birds of

MEANS OF DISPERSAL

391

prey; and two seeds of beet grew after having been thns 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
visited 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 appeared, and have
not since visited the island. Now, in parts of Natal it is believed
by some farmers, though on insufficient 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 belief 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

392 ORIGIN OF SPECIES

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 (Saxicolae), 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 eighty-
two plants sprung from it: these consisted of twelve monocoty-
ledons, including the common oat, and at least one kind of grass,
and of seventy 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 migrate— for
instance, the millions of quails across the Mediterranean— 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 from one part to another
of the arctic and antarctic regions; and during the Glacial period
from one part of the now temperate 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 comparison
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

MEANS OF DISPERSAL 393

the archipelago. 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 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 western 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 Atlantic 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

394 ORIGIN OF SPECIES

hundred kinds o£ seeds or animals transported to an island, even if
far less well-stocked than Britain, perhaps not more than one would
be so weir fitted to its new home, as to become naturalised. But this
is no valid argument against what would be effected by occasional
means of transport, 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 germinate 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 living at distant points, without the
apparent possibility of their having migrated from one point to the
other. It is indeed 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 in-
dependently created at many distinct points; and we might have
remained in this same belief, had not Agassiz and others called
vivid attention 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 inorganic, 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

DISPERSAL DURING GLACIAL PERIOD 395

by the vine and maize. Throughout a large part o£ the United
States, erratic boulders and scored rocks plainly reveal a former cold
period.

The former influence of the glacial climate on the distribution
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 became 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 further 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 inhabitants
would descend to the plains. By the time that the cold 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 likewise 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 north-
ward, closely followed up in their retreat by the productions 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 ascending, 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 eadb 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

396 ORIGIN OF SPECIES

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 remarked by Ramond,
are more especially allied to the plants of northern Scandinavia;
those of the United States, to Labrador; 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 the same
species on distant mountain summits, we may almost 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 back-
wards 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 migrated 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 productions, left isolated from the moment of the returning
warmth, first at the bases and ultimately on the summits of the
mountains, the case will 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 probability, have become
mingled with ancient Alpine species, which must have existed on
the mountains before the commencement of the Glacial epoch, and
which during the coldest period will have been temporarily driven
down to the plains; they will, also, have been subsequently exposed
to somewhat different climatal influences. Their mutual relations
will thus have been in some degree disturbed; consequently they
will have been liable to modification; and they have been modified ^

DISPERSAL DURING GLACIAL PERIOD 397

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 doubtful forms or sub-species, and some as dis-
tinct yet closely allied species representing each other on the several
ranges.

In the foregoing illustration I have assumed that at the com-
mencement of our imaginary Glacial period, the arctic productions
'were as uniform round the polar regions as they are at the present
day. But 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 asked how I account for this degree of uniformity in the
sub-arctic and temperate forms round the world, at the commence-
ment 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 cHmate 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 latitude 6o°, lived during the Pliocene period, farther
north under the Polar Circle, in latitude 66°-6y°; 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 America. And this continuity
of the circumpolar land, with the consequent freedom under a more
favourable climate for intermigration, will account for the supposed

398 ORIGIN OF SPECIES

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 continents
have long remained in nearly the same relative position, 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 con-
tinuous 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,
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 litde 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
Adantic Ocean. We can further understand the singular fact re-
marked on by several observers that the productions of Europe and
America during 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 productions 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. Con-
sequently 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-

ALTERNATE GLACIAL PERIODS 399

ranges and on the arctic lands of Europe and North America. Hence,
it has come, that when we compare the now living productions of
the temperate 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 naturalists rank as geo-
graphical 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 southern migra-
tion of a marine fauna, which, during the Pliocene or even a some-
what earlier period, was nearly uniform along 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 western 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 animals,
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 inexplicable on the theory of crea-
tion. 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 con-
vinced that Forbes’ view may be largely extended. In Europe we
meet with the plainest evidence of the Glacial period, from the

ORIGIN OF SPECIES

400

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 4,000 feet down the
valleys. The same observer has recently found great moraines at a
low level on the Atlas range in North 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 Asiatic continent,
on the opposite side of the equator, we know, from 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 com-
municated 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 fragments
of rock have been observed on the eastern side of the continent, as
far south as latitude 36 and on the shores of the Pacific,
where the climate is now so different, as far south as lat. 46°.
Erratic boulders have, also, been noticed on the Rocky Mountains.
In the Cordillera of South America, nearly under the equator,
glaciers once extended far below 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 latitude 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 transported far from their parent source.

ALTERNATE GLACIAL PERIODS 4OI

From these several facts, namely from the glacial action having
extended all round the northern and southern hemispheres — from
the period having been in a geological sense recent in both hemi-
spheres — ^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 recendy 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 attempted
to show that a glacial condition of climate is the result of various
physical causes, brought into operation by an increase in the eccen-
tricity 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. Croll, 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 convinced 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 hemisphere
passes through a cold period the temperature of the southern hemi-
sphere is actually raised, with the winters 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 in-
clined 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 inconsiderable part of its scanty

ORIGIN OF SPECIES

402

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 moun-
tains 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 Silk of Caraccas, the
illustrious Humboldt long ago found species belonging to genera
characteristic of the Cordillera.

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 representative European forms are
found, which have not been discovered 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 astonish-
ing 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 pic-
ture 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, extend along
the heights of the peninsula of Malacca, and are thinly scattered on

ALTERNATE GLACIAL PERIODS 403

the one hand over India, and on the other hand as far north as
Japan.

On the southern mountains of Australia, Dr. F. Muller has dis-
covered several European species; other species, not introduced by
man, occur on the lowlands; and a long Hst 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 Intro-
duction to the Flora of New Zealand,’ by Dr. Hooker, analogous
and striking facts are 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 tem-
perate plains of the north and south, are either the same species or
varieties of the same species. It should, however, be observed 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 in-
habiting 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 animals. In marine pro-
ductions, similar cases likewise occur; as an example, I may quote
a statement by the highest authority. Professor 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
reappearance on the shores of New Zealand, Tasmania, etc., of
northern forms of fish. Dr. Hooker informs me that twenty-five
species of Alg^ 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 temperate 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 low-
lands of these great continents were everywhere tenanted under the

404 ORIGIN OF SPECIES

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 latitude, 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, like that described by Hooker as growing
luxuriantiy at the height of from four to five thousand feet on the
lower slopes of the Himalayas, but with perhaps a still greater pre-
ponderance 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. Seemann 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 actually warmer, throws
any clear light on the present apparently inexplicable distribution
of various organisms in the temperate 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 temperate 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 equatorial lowlands. The inhabitants of these hot low-
lands 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 de
stroyed, being replaced by the equatorial forms returning from the

ALTERNATE GLACIAL PERIODS 405

south. Some, however, of the northern temperate forms would
almost certainly have ascended any adjoining high land, where, if
sufficiendy lofty, they would have long survived like the arcdc forms
on the mountains of Europe. They might have survived, even if
the climate was not perfecdy fitted for them, for the change of
temperature must have been very slow, and plants undoubtedly
possess a certain capacity for acclimadsation, as shown by their
transmitting to their offspring different constitutional 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 lowlands. The northern forms
which had before been left on the mountains would now descend
and mingle with the southern 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 Hable 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 hemi-
spheres of former Glacial periods; for these will account, in accord-
ance 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 intermediate 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 species 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

ORIGIN OF SPECIES

406

north to the south is due to the greater extent o£ 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 perfection, 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 migrate southward with the southern forms; but not
so the southern in regard to the northern forms. In the same man-
ner 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 naturalised 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 inter-
tropical mountains were stocked with endemic Alpine forms; but
these have almost everywhere yielded to the more dominant forms
generated in the larger areas and more efScient workshops of the
north. In many islands the native productions are nearly equalled,
or even outnumbered, by those which have become naturalised; and
this is the first stage towards their extinction. Mountains 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

ALTERNATE GLACIAL PERIODS 407

once be able to migrate southward, by keeping to the cooler currents,
whilst others might remain and survive in the colder depths until
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 inhabited 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 con-
cerned 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 sup-
pose that there has been time since the commencement of the last
Glacial period for their migration and subsequent 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 antarctic 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

ORIGIN OF SPECIES

408

to various points o£ the southern hemisphere by occasional means of
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 language
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. CroU’s conclusion that successive Glacial
periods in the one hemisphere coincide with warmer periods in the
opposite hemisphere, together with the admission of the slow modi-
fication of species, 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 direction, 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 moun-
tain summits, in a line gently rising from the arctic lowlands to
a great altitude under the equator. The various beings thus left
stranded may be compared with savage races of man, driven up anc
surviving in the mountain fastnesses of almost every land, whicl
serves as a record, full of interest to us, of the former inhabitants 0:
the surrounding lowlands.

CHAPTER XIII

Geographical Distribution — continued

Distribution o£ fresh-water productions — On the inhabitants of oceanic
islands — Absence of batrachians and of terrestrial mammals — On the
relation of the inhabitants of islands to those of the nearest mainland
— On colonisation from the nearest source with subsequent modifica-
tion — Summary of the last and present chapter.

FRESH-WATER PRODUCTIONS

AS lakes and river systems are separated from each other by bar-
/ \ riers of land, it might have been thought that fresh-water
JL jL 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 reverse. 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, etc., and
at the dissimilarity of the surrounding 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 become 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 coun-
tries; and liability to wide dispersal would follow from this capacity
as an almost necessary 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. Gun-
ther has lately shown that the Galaxias attenuatus inhabits Tasmania,
New Zealand, the Falkland Islands, and the mainland of South
America. This is a wonderful case, and probably indicates dispersal

409

ORIGIN OF SPECIES

414

on the truth o£ the two theories o£ independent creation 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 Wollaston for insects. New
Zealand, for instance, with its lofty mountains and diversified sta-
tions, 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 num-
ber with the species which swarm over equal areas in southwestern
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 introduced plants are included in these num-
bers, and the comparison in some other respects is not quite fair. We
have evidence that the barren island of Ascension aboriginally pos-
sessed 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 naturalised plants and animals have nearly
or quite exterminated 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 animals were
not created for oceanic islands; for man has unintentionally stocked
them far more fully and perfectly than did nature.

Although in oceanic islands the species are few in number, the pro-
portion of endemic kinds 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 com-
pare 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 in-
tervals 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

INHABITANTS OF OCEANIC ISLANDS 415

means follows that, because 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 difference seems to depend partly
on the species which are not modified having immigrated 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 cer-
tainly gain in vigour; so that even an occasional cross would produce
more effect than might have been anticipated. I will give a few illus-
trations of the foregoing remarks : in the Galapagos Islands there are
twenty-six land birds; of these twenty-one (or perhaps twenty-three)
are peculiar, whereas of the eleven marine birds only two are pecu-
liar; 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 Amer-
ica 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 frequendy
visit this island. Almost every year, as I am informed by Mr. E. V.
Har court, many European and African birds are blown to Madeira;
this island is inhabited by ninety-nine 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 condnents 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 conse-
quently have been but little liable to modification. Any tendency to
modification will also have been checked by intercrossing with the
unmodified immigrants, 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 dispersed, yet we
can see that their eggs or larvx, perhaps attached to seaweed or float-

ORIGIN OF SPECIES

416

ing 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 different orders of insects inhabiting Madeira present
nearly parallel cases.

Oceanic islands are sometimes deficient in animals of certain 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
separated from Australia by a profoundly deep sea; from its geologi-
cal character and the direction of its mountain ranges, the Rev. W. B.
Clarke has lately maintained that this island, as well as New Cale-
donia, should be considered as appurtenances of Australia. Turning
to plants, Dr. Hooker has shown that in the Galapagos Islands the
proportional numbers 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 generally
accounted for by supposed differences in the physical conditions of the
islands; but this explanation is not a litde doubtful. Facility of immi-
gration seems to have been fully as important as the nature of the
conditions.

Many remarkable little facts could be given with respect to the
inhabitants of oceanic islands. For instance, in certain islands
not tenanted by a single mammal, some of the endemic plants
have beautifully hooked seeds; yet few relations are more mani-
fest than that hooks serve for the transportal 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 modi-
fied would form an endemic species, still retaining its hooks,
which would form a useless appendage like the shrivelled wings
under the soldered wingcovers 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, confined ranges. Hence trees would be little likely to reach
distant oceanic islands; and an herbaceous plant, which had no chance

ABSENCE OF BATRACHIANS 417

of successf ally competing with the many fully developed trees grow-
ing 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 or 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 asser-
tion, and have found it true, with the exception of New Zealand,
New Caledonia, the Andaman Islands, and perhaps the Solomon
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 stiU more doubtful with respect
to the Andaman and Solomon groups and the Seychelles. This gen-
eral 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 introduced 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
difBculty in their transportal across the sea, and therefore we can see
why they do not exist on stricdy 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

ORIGIN OF SPECIES

418

this group cannot be considered as oceanic, as it lies on a bank in con-
nection with the mainland at the distance of about 280 miles; more-
over, icebergs formerly brought boulders to its western shores, and
they may have formerly transported foxes, as now frequently hap-
pens 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 quadrupeds
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 volcanic islands are sufficiently an-
cient, as shown by the stupendous degradation which they have suf-
fered, and by their tertiary strata: there has also been time for the
production of endemic species belonging to other classes; and on con-
tinents it is known that new species of mammals appear and disap-
pear at a quicker rate than other and lower animals. Although
terrestrial mammals do not occur on oceanic islands, aerial mammals
do occur on almost every island. New Zealand possesses two bats
found nowhere else in the world: Norfolk Island, the Viti archipel-
ago, the Bonin Islands, the Caroline and Marianne archipelagoes,
and Mauritius, 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 transported across a wide space of
sea, but bats can fly across. Bats have been seen wandering by day
far over the Adantic 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 terrestrial mammals.

Another interesting relation exists, namely between the depth of
the sea separating islands from each other or , from the nearest conti-
nent, and the degree of affinity of their mammalian inhabitants. Mr.

ABSENCE OF BATRACHIANS

419

Windsor Earl has made some striking observations on this head,
since greatly extended by Mr. Wallace’s admirable researches, in re-
gard 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 moderately
shallow submarine bank, and these islands are inhabited 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 shal-
low 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 sub-
merged bank, nearly 1000 fathoms in depth, and here we find Ameri-
can forms, but the species and even the genera are quite distinct. As
the amount of modification which, animals 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 under-
stand how it is that a relation exists between the depth of the sea
separating two mammalian faunas, and the degree of their afiinity, —
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 of 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, notwithstanding the presence of aerial bats, — ^the singular
proportions of certain orders of plants, — ^herbaceous forms having
been developed into trees, etc., — ^seem to me co accord better with the
belief in the efiBciency pf occasional means of transport, carried on
during a long course of time, than with the belief in the former con-
nection of all oceanic islands with the nearest continent; for on this
latter view it is probable that the various classes would have immi-
grated more uniformly, and from the species having entered in a body
their mutual relations would not have been much disturbed, and

ORIGIN OF SPECIES

420

consequently they would either not have been modified, or all the
species in a more equable manner.

I do not deny that there are many and serious difficulties in under-
standing how many of the inhabitants of the more remote islands,
whether still retaining the same specific form or subsequently modi-
fied, 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 inhabited by land shells, generally by endemic species, but some-
times 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, sink in 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
occurred 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 perfecdy 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, belonging 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 revived. 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

INHABITANTS OF ISLANDS

421

Helix tried by Aucapitaine, recovered. It is, however, not at all prob-
able 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, with-
out being actually the same. Numerous instances could be given.
The Galapagos archipelago, situated under the equator, lies at the
distance of between 500 and 600 miles from the shores of South
America. Here 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 commonly be assumed to
have been here created: yet the close affinity of most of these birds to
American species is manifest in every character, in their habits, ges-
tures, 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 archi-
pelago, and nowhere else, bear so plainly the stamp of affinity to
those created in America? There is notiiing 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 associated to-
gether, 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 nature 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 difierence 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 crea-

ORIGIN OF SPECIES

422

tion; 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 almost uni-
versal rule that the endemic productions of islands are related to
those of the nearest continent, or of the nearest large island. The
exceptions are few, and most of them can be explained. Thus, al-
though 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 expected; but it is also plainly
related to South America, which, although the next nearest conti-
nent, is so enormously remote, that the fact becomes an anomaly.
But this difficulty partially 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 vegetation, during
a warmer tertiary period, before the commencement of the last Gla-
cial 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 between the
inhabitants of islands and the nearest mainland, is sometimes dis-
played on a small scale, but in a most interesting manner, within the
limits of the same archipelago. Thus each separate island of the
Galapagos archipelago is tenanted, 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 Ameri-

INHABITANTS OF ISLANDS 423

can 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 immi-
grants have been differently modified, though only in a small degree,
in islands situated within sight of each other, having the same
geological nature, the same height, climate, etc.? This long appeared
to 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 disputed 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 archi-
pelago, 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 expeaed if the islands have been stocked
by occasional means of transport— a seed, for instance, 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 undoubtedly be exposed to different conditions in the
different islands, for it would have to compete with a different set
of organisms; 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 Archipel-
ago, 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

ORIGIN OF SPECIES

426

The relation between the power and extent of migration in certain
species,- either at the present or at some former period, 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 species has an immense range;
but, if variation were to be carried 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 certain
powerfully winged birds, will necessarily range widely; for we should
never forget that to range widely implies not only the power of cross-
ing 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 dis-
tributed to the most remote points of the world, are descended from
a single progenitor, 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; conse-
quently they will have had a better chance of ranging widely and of
still retaining the same specific character. This fact, together with

SUMMARY

427

that of the seeds 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 generally related to those which live
on the surrounding low lands and dry lands,— the striking relation-
ship between the inhabitants of islands and those of the nearest main-
land — ^the still closer relationship of the distinct inhabitants of the
islands in the same archipelago — ^are inexplicable on the ordinary
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 insuperable in
believing that all the individuals of the same species, wherever found,
are descended from common parents. And we are led to this con-
clusion, which has been arrived at by many naturalists under the
designation of single centres of creation, by various general consider-
ations, more especially from the importance of barriers of all kinds,
and from the analogical distribution of sub-genera, genera, and
famihes.

With respect to distinct species belonging to the same 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

ORIGIN OF SPECIES

428

some forms of life have changed very slowly, enormous periods of
time having been thus granted for their migration, the difficulties
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 distribution, I
have attempted to show how important a part the last Glacial period
has played, which affected even the equatorial 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 discussed at some little length the means of dispersal of fresh-
water productions.

If the difficulties 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 geo-
graphical distribution are explicable 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 rela-
tion of organism to organism is of the highest importance, we can
see why two areas having nearly the same physical conditions 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; according to the nature of the communication
which allowed certain forms and not others to enter, either in greater
or lesser numbers; according or not, as' those which entered hap-
pened to come into more or less direct competition with each other
and with the aborigines: and according as the immigrants were

SUMMARY

429

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 slighdy modified;
some developed in great force, some existing in scanty numbers — and
this we do find in the several great geographical provinces 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 pecuHar, 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
organisms, 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 alUed or repre-
sentative 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 striking
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 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 excep-
tions to the rule are so few, that they may fairly be attributed to our
not having as yet discovered in an intermediate deposit certain forms
which are absent in it, but which occur both above and below: so in

ORIGIN OF SPECIES

430

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 excep-
tions, which are not rare, may, as I have attempted to show, be
accounted for by former migrations under different circuipstances,
or through occasional means of transport, or by the species having
become extinct in the intermediate tracts. Both in time and space
species and groups of species have their points of maximum develop-
ment. 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 suc-
cession 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 clasSj 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 generally change less than
the highly organised; but there are in both cases marked exceptions
to the rule. According to our theory, these several relations through-
out time and space are intelligible; for whether we look to the allied
forms of hfe 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 ordi-
nary generation; in both cases the laws of variation have been the
same, and modificadons have been accumulated by the same means
of natural selection.

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 separates 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 correspond-
ing age — Rudimentary organs; their origin explained — Summary.

classification

F rom 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 in-
habit 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 Variation
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 pro-
duced, ultimately become 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, tend to
go on increasing in size. I further attempted to show that from the
varying descendants of each species trying to occupy as many and

431

ORIGIN OF SPECIES

432

as different places as possible in the economy of nature, they con-
stantly tend to diverge in character. This latter conclusion is sup-
ported by observing the great diversity of forms which, in any small
area, come into the closest competition, and by certain facts in
naturalisation.

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 illus-
trating the action, as formerly explained, of these several principles;
and he will see that the inevitable result is, that the modified de-
scendants proceeding from one progenitor become broken up into
groups subordinate 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 sufficiently
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, that minerals and the elemental
substances can be thus arranged. In this case there is of course no
relation to genealogical succession, and no cause can at present be
assigned for their falling into groups. But with organic beings the

CLASSIFICATION

433

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 arti-
ficial method of enunciating, as briefly as possible, general proposi-
tions,— 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 man)^
naturalists think that something 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
famous 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 classifications than mere resem-
blance. 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 observed by various degrees of modification,
is partially revealed 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
L either gives some unknown plan of creation, or is simply a scheme
rfor 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 those parts of the structure which deter-
mined the habits of life, and the general place of each being in the
economy of nature, would be of very high importance in classifica-
tion. Nothing can be more false. No one regards the external simi-
larity of a mouse to a shrew, of a dugong to a whale, of a whale to

434 ORIGIN OF SPECIES

a fish, as of any importance. These resemblances, though so inti-
mately connected with the whole life of the being, are ranked as
merely “adaptive or analogical characters”; 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 gener-
ative organs, being those which are most remotely related 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 character.” 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 morphological characters which
are not functionally important, we have seen that they are often of
the highest service in classification. This depends on their con-
stancy throughout many allied groups; and their constancy chiefly
depends on any slight deviations not having been preserved and
accumulated 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 physiological 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 Proteace^, says their generic importance, “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 sestivation. Any
one or these characters singly is frequently of more than generic

CLASSIFICATION

435

importance, though here even vi^hen all taken together they appear
insufficient to separate Cnestis from Connarus.” To give an example
amongst insects: in one great division of the Hymenoptera, the
antenna, 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 organs are
of high physiological or vital importance; yet, undoubtedly, 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 exhibiting the close affinity between ruminants and
pachyderms. Robert Brown has strongly insisted on the fact that the
position 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 physiological im-
portance, but which are universally admitted as highly serviceable
in the definition of whole groups. For instance, 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 inflection of the angle of the lower jaw in Mar-
supials — ^the manner in which the wings of insects are folded — ^mere
colour in certain Algae — ^mere pubescence on parts of the flower
in grasses — ^the nature of the dermal covering, as hair or feath-
ers, in the Vertebrata. If the Ornithorhynchus had 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 affiinity 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 char-

436 ORIGIN OF SPECIES

acters 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 physiological 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 Malpighiaceae, bear perfect and degraded flowers; in the latter,
as A. de Jussieu has remarked, “The greater number of the charac-
ters 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 im-
portant 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 Malpighiaceae. 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 characters 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 use it as of subordinate value.
This principle has been broadly confessed by some naturalists to
be the true one; and by none more clearly than by that excellent
botanist, Aug. St. Hilaire. If several trifling characters 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, important organs, such as those for pro-
pelling the blood, or for aerating it, or -those for propagating the
race, are found nearly uniform, they are considered as highly service-
able in classification; but in some groups all these, the most impor-

CLASSIFICATION

437

tant vital organs, are found to offer characters of quite subordinate
values. Thus, as Fritz Muller 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 Cypridina has well-developed branchiae, 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 ob-
vious, 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. Never-
theless, their importance has sometimes been exaggerated, owing to
the adaptive characters of larv^ not having been excluded; in order
to show this, Fritz Muller 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 divisions of
flowering plants are founded on differences in the embryo,— on the
number and position of the cotyledons, and on the mode of develop-
ment 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.

Our classifications are often plainly influenced by chains of ajSini-
ties. 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 oppo-
site 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, arid 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

ORIGIN OF SPECIES

438

groups o£ closely allied forms. Temminck insists on the utility or
even necessity of this practice in certain groups of birds; and it has
been followed by several entomologists and botanists.

Finally, with respect to the comparative value of the various 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 arbitrary 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 research has detected
important structural differences, at first overlooked, but because
numerous allied species with slightly different grades of difference,
have been subsequendy discovered.

All the foregoing rules and aids and difficulties in classification
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 naturaHsts 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 community 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.

But I must explain my meaning more fully. I believe that the
arrangement of the groups within each class, in due subordination
and relation to each other, must be strictly genealogical in order to
be natural; but that the amount of difference 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 represent 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 trans-

CLASSIFICATION

439

mitted modified descendants to the present day, represented by the
fifteen genera to on the uppermost horizontal line. Now all
these modified descendants from a single species are related in blood
or descent in the sanie degree;- they may metaphorically 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 L But the
existing genus may be supposed to have been but slightly modi-
fied; and it will then rank with the parent-genus F; just as some
few still living organisms belong 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 arrange-
ment remains stricdy true, not only at the present time, but at each
successive period of descent. All the modified descendants from A
will have inherited something in common from their common parent,
as will all the descendants 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 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 diagram 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 afBnities which we discover in nature amongst the beings of the
same group. Thus, the natural system is genealogical in its arrange-
ment, like a pedigree: but the amount of modification which the
different groups have undergone has to be expressed by ranking

440 ORIGIN OF SPECIES

them under different so-called genera, sub-families, families, sections,
orders, and classes.

It may be worth while to illustrate this view of classification, by
taking the case of languages. 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 inter-
mediate and slowly changing 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 genealogical; and this would be strictly natural, as
it would connect together all languages, extinct and recent, by the
closest affinities, and would give die filiation and origin of each
tongue.

In confirmation of this view, let us glance at the classification 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
followed 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 pineapple 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,
etc.; whereas with sheep the horns are much less serviceable, because
•less constant. In classing varieties, I apprehend that if we had a

CLASSIFICATION

441

real pedigree, a genealogical classification would be universally pre-
ferred; 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, Monachan-
thus, Myanthus, and Catasetum, which had previously been ranked
as three distinct genera, were known to be sometimes produced on
the same plant, they were immediately 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 Steenstrup, 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 because 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
larv^ are sometimes extremely different; and as it has been used
in classing varieties which have undergone a certain, and sometimes
a considerable, amount of modification, may not this same element
of descent have been unconsciously 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

442 ORIGIN OF SPECIES

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 characters 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 struc-
tures 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 throughout many and differ-
ent 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 different 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,
concur throughout a large group of beings having different habits,
we may feel almost sure, on the theory of descent, that these char-
acters have been inherited from a common ancestor; and we know
that such aggregated characters have especial value in classifica-
tion.

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, betray the hidden bond of community of de-
scent. Let two forms have not a single character in common, yet, if
these extreme forms are connected together by a chain of interme-
diate 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 existence — are generally the most con-
stant, 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 impor-
tance. Geographical distribution may sometimes be brought usefully

ANALOGICAL RESEMBLANCES 443

into play in classing large genera, because all the species of the same
genus, inhabiting any distinct and isolated region, are in all proba-
bility descended from the same parents.

ANALOGICAL RESE\£BLANCES

We can understand, on the above views, the very important dis-
tinction between real affinities and analogical or adaptive resem-
blances. Lamarck first called attention to this subject, and he has
been ably followed by Macleay and others. The resemblance in the
shape of the body and in the fin-like anterior limbs between dugongs
and whales, and between these two orders of mammals 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
resemblances may be accounted for, as it seems to me, by adaptation
for similarly active movements through thickets and herbage, to-
gether with concealment from enemies.

Amongst insects there are innumerable similar instances; thus
Linnasus, misled by external appearances, actually classed an homop-
terous 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
resemblance 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 classifica-
tion, only in so far as they reveal descent, we4:an clearly understand
why analogical or adaptive characters, although of the utmost im-
portance 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 sitfiilar conditions, and thus
have assumed a close external ne^mblance; but such resemblances
will not reveal — ^will rather teaij to conceal their blood-relationship.
We can thus also understand jj^ apparent paradox, that the very

444 ORIGIN OF SPECIES

same characters are analogical when one group is compared with
another, but give true affinities when the members o£ the same group
are compared together: thus, the shape o£ the body and fin-like Hmbs
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-Hke Hmbs offer characters exhibiting true affinity;
for as these parts are so nearly similar throughout the whole family,
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 resemblance of the jaws of the dog and Tasmanian wolf or
Thylacinus, — ^animals which are widely sundered in the natural
system. But this resemblance 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 difierent insects
possessing luminous organs, — and of orchids and asclepiads having
pollen-masses with viscid discs, come under this same head of ana-
logical resemblances. But these cases are. so wonderful that they were
introduced as difficulties 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 detected. The
end gained is the same,; but the rntpajas, though appearing superficially

ANALOGICAL RESEMBLANCES 445

to be the same, are essentially different. 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 com-
mon in their constitution, that they are apt to vary under similar
exciting causes in a similar manner; and this would obviously aid in
the acquirement through natural selection of parts or organs, strik-
ingly like each other, independendy 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 cir-
cumstances, — ^to inhabit, for instance, the three elements of land, air,
and water, — ^we can perhaps understand how it is that a numerical
parallelism has sometimes been observed between the sub-groups of
istinct classes. A naturalist, struck with a parallelism of this nature,
by arbitrarily 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 similar habits of life,
but has been gained for the sake of protection. I allude to the won-
derful 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 South America, where, for in-
stance, 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 instances, it might have been passed over as a strange coinci-
dence. But, if we proceed from a district where one Leptalis imi^

446 ORIGIN OF SPECIES

tates 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 in-
clude species that imitate other butterflies. The mockers and mocked
always inhabit the same region; we never find an imitator Uving re-
mote 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 Lepidoptera mimick-
ing 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-
icked forms, can be shown by a graduated series to be merely 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 alHes.

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 ex-
tent, otherwise they could not exist in such swarms; and a large
amount of evidence has now been collected, showing that they are
distasteful to birds and other insect-devouring animals. The mocking
forms, on the other hand, that inhabit the same district, are com-
paratively 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 practised eyes of an
entomologist, it would often deceive predaceous birds and insects,

ANALOGICAL RESEMBLANCES 447

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 resemble 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 occurred, and
of these one alone resembled to a certain extent, the common
Ithomia 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 butterfly
inhabiting the same district, this variety, from its resemblance to a
flourishing and litde 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
propagate their kind.” So that we have an excellent illustration of
natural selection.

Messrs. Wallace and Trimen have likewise described several
equally striking cases of imitation in the Lepidoptera of the Malay
Archipelago and Africa, and with some other insects. 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 in-
stance 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,
hke most weak creatures, to trickery and dissimulation.

It should be observed that the process of imitation probably never
commenced between forms widely dissimilar in colour. But starting
with species already somewhat like each other, the closest re-
semblance, if beneficial, could readily be gained by the above means;
and if the imitated form was subsequendy 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-

ORIGIN OF SPECIES

448

mately assume an appearance or colouring wholly unlike that o£ the
other members of the family to which it belonged. There is, how-
ever, some difficulty on this head, for it is necessary to suppose in
some cases that ancient members belonging to several distinct groups,
before they had diverged to their present extent, accidentally resem-
bled 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 BEINGS

As the modified descendants of dominant species, belonging to
the larger genera, tend to inherit the advantages 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 nature. The larger and more dominant
groups within each class thus tend to go on increasing in size; and
they consequendy supplant many smaller and feebler groups. Thus
we can account for the fact that all organisms, recent and extinct, 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 dis-
covery 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 process 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 intermediate 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 extermi-
nated and utterly lost. And we have some evidence of aberrant
groups having suffered severely from extinction, for they are almost
always represented by extremely few species; and such species as do
occur are generally very distinct from each other, which again implies

AFFINITIES CONNECTING ORGANIC BEINGS 449

extinction. The genera Ornithorhynchus and Lepidosiren, for ex-
ample, 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 favourable conditions.

Mr. Waterhouse has remarked that, when a member belonging to
one group of animals exhibits an aflEnity to a quite distinct group,
this affinity in most cases is general and not special; thus, accord-
ing to Mr. Waterhouse, of all rodents, 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 accordance
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
much moification in divergent directions. On either view we must
suppose that the bizcacha has retained, by inheritance, more of the
characters of its ancient progenitor ithan 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 aU marsupials, as Mr.
Waterhouse 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 resemblance 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 divergence in
character of the species descended from a common progenitor, to-

ORIGIN OF SPECIES

450

gether with their retention by inheritance of some characters in com-
mon, we can understand the excessively complex and radiating
affinities by which all the members of the same family or higher
group are connected together. 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 conse-
quently be related to each other by circuitous lines of affinity of vari-
ous 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 the aid of a genealogical tree, and almost im-
possible to do so without this aid, we can understand the extraordi-
nary difficulty which naturalists have experienced in describing,
without the aid of a diagram, the various affinities which they per-
ceive 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 intervals between the
several groups in each class. We may thus account for the distinct-
ness 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 utterly 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 di-
verse forms are still linked together 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 hved
on this earth were suddenly to reappear, though it would be quite
impossible to give definitions by which each group could be dis-
tinguished, still a natural classification, or at least a natural arrange-
ment, 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

AFFINITIES CONNECTING ORGANIC BEINGS 45 1

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 immedi-
ate parents and descendants. Yet the arrangement in the diagram
would still hold good and would 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 to-
gether. We could not, as I have said, define the several groups; but
we could pick out types, or forms, representing most of the char-
acters 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 col-
lection: 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 no-t 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 ex-
tinction and divergence of character in the descendants from any one
parent-species, explains that great and universal feature in the afiin-
ities 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 classing ac-
knowledged varieties, however different they may be from their par-
ents; 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 perfected, genealogical in its arrangement, with the
grades of difference expressed by the terms genera, families, orders,
etc., we can understand the rules which we are compelled to follow
in our classification. We can understand why we value certain resem-
blances far more than others; why we use rudimentary and useless

ORIGIN OF SPECIES

452

organs, or others of trifling physiological importance; why, in finding
the relations between one group and another, we summarily reject
analo^cal or adaptive characters, and yet use these same characters
within the limits of the same group. We can clearly see how it is that
all living and extinct forms can be grouped together within a few
great classes; and how the several members of each class are con-
nected together by the most complex and radiating lines of affinities.
We shall never, probably, disentangle the inextricable web of the
affinities between the members 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 recendy brought his great knowledge and abifities 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 em-
bryological characters, but receives aid from homologous and rudi-
mentary organs, as well as from the successive periods at which the
various forms of life are believed to have first appeared in our geologi-
cal formations. 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, independendy 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 differ-
ent 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 subor^
dinate 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

MORPHOLOGY

453

branches of trees,— those of the ground-dwelling, insect or root-eat-
ing, bandicoots, — and those of 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 pat-
tern, it is obvious that the hind feet of these several animals are used
for as widely different purposes as it is possible 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 rela-
tives, having feet constructed on the ordinary plan. Professor Flower,
from whom these statements are taken, remarks in conclusion: “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 relationship, of inheritance from a
common ancestor?”

Geoffroy St. Hilaire has strongly insisted on the high importance
of relative position or connexion in homologous parts; they may
differ to almost any extent in form and size, and yet remain con-
nected together in the same invariable order. We never find, for in-
stance, 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 different
than the immensely long spiral proboscis of a sphinx-moth, the curi-
ous 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, mandi-
bles, and two pairs of maxillae. The same law governs the construc-
tion 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 ‘Na-
ture 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

ORIGIN OF SPECIES

454

Creator 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 Hmb 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 mem-
brane 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 connexion 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 pat-
tern, for whatever purpose they served, we can at once perceive the
plain signification of the homologous construction of the limbs
throughout the class. So with the mouths of insects, we have only to
suppose that their common progenitor had an upper lip, mandibles,
and two pairs of maxillse, these 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. Nevertheless,
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 subject;
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 are homologous — ^that is, correspond in
number and in relative connexion — ^with the elemental parts of a

MORPHOLOGY

455

certain number of vertebra. 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 relative 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 possibility of one organ being transformed into another; and
we can actually see, during the early or embryonic stages of develop-
ment in flowers, as well as in crustaceans and many other animals,
that organs, which when mature become extremely 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 enclosed in a box com-
posed of such numerous and such extraordinarily shaped pieces of
bone, apparently representing vertebrae? 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 con-
struction 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 com-
plex mouth formed of many parts, consequendy 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, an-
swer 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 or-
gans, for such questions are almost beyond investigation. It is, how-
ever, probable that some serial structures are the result of cells multi-
plying by division, entailing the multiplication of the parts 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 remarked, of all low or little specialised

456 ORIGIN OF SPECIES

forms; therefore the unknown progenitor of the Vertebrata prob-
ably possessed many vertebrae; the unknown progenitor of the Articu-
lata, many segments; and the unknown progenitor of flowering
plants, many leaves arranged in one or more spires. We have also for-
merly 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 considerable numbers, and being highly
variable, would naturally afford the materials for adaptation to the
most different purposes; yet they would generally retain, through the
force of inheritance, plain traces of their original or fundamental re-
semblance. 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 conditions. Such parts, whether more or less
modified, unless their common origin became wholly obscure, would
be serially 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 homologous with another part in the same indi-
vidual. And we can understand this fact; for in molluscs, even in the
lowest members of the class, we do not find nearly so much indefinite
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 ap-
pears, as has lately been well shown in a remarkable paper by Mr. E.
Ray Lankester, who has drawn an important distinction between cer-
tain classes of cases which have all been equally ranked by natural-
ists as homologous. He proposes to call the structures which resemble
each other in distinct animals, owing to their descent from a com-
mon progenitor with subsequent modification, homogenous; and the
resemblances which cannot thus be accounted for, he proposes 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 four cavities of the heart in
the two classes are homoplastic, — ^that is, have been independendy de-

DEVELOPMENT AND EMBRYOLOGY 457

veloped. Mr. Lankester also adduces the close resemblance of the
parts on the right and left sides of the body, and in the successive seg-
ments of the same individual animal; and here we have parts com-
monly called homologous, which bear no relation to the descent of
distinct species from a common progenitor. Homoplastic structures
are the same with those which I have classed, though in a very imper-
fect manner, as analogous modifications or resemblances. Their for-
mation 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 or function, of which many instances have
been given.

Naturalists frequendy speak of the skull as formed of metamor-
phosed vertebrae; the jaws of crabs as metamorphosed legs; the sta-
mens and pistils in flowers as metamorphosed 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, etc., as having been
metamorphosed, not one from the other, as they now exist, but from
some common and simpler element. Most naturalists, 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 actu-
ally been converted into skulls or jaws. Yet so strong is the appear-
ance of this having occurred, that naturalists can hardly avoid em-
ploying 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 inherit-
ance, if they had really been metamorphosed from true though
extremely simple legs, is in part explained.

DEVELOPMENT AND EMBRYOLOGY

This is one of the most important subjects in the whole round of
natural history. The metamorphoses of insects, with which every one
is familiar, are generally effected abruptly by a few stages; but the
transformations are in reality numerous and gradual, though con-
cealed. A certain ephemerous insect (Chloeon) during its develop-

458 ORIGIN OF SPECIES

merit, moults, as shown by Sir J. Lubbock, 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 primary and gradual
manner. Many insects, and especially certain crustaceans, show us
what wonderful changes of structure can be effected during develop-
ment. Such changes, however, reach their acme in the so-called
alternate generations of some of the lower animals. It is, for in-
stance, an astonishing fact that a delicate branching coralline, studded
with polypi and attached to a submarine rock, should produce, first
by budding and then by transverse division, a host of huge floating
jelly-fishes; and that these should produce eggs, from which are
hatched swimming animalcules, which attach themselves to rocks
and become developed into branching corallines; and so on in an end-
less cycle. The belief in the essential identity of the process of alter-
nate 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 larvae, 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 remarkable discovery
was first announced, I was asked how was it possible to account for
the larvae of this fly having acquired the power of asexual reproduc-
tion. As long as the case remained unique no answer could be given.
But already Grimm has shown that another fly, a Chironomus, re-
produces 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 of the Cecidomyia with the
parthenogenesis of the Coccidae,” the term parthenogenesis implying
that the mature females of the Coccidae are capable of producing fer-
tile 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 accel-
erate parthenogenetic reproduction by gradual steps to an earlier and
earlier age,— Chironomus showing us an almost exactly intermedi-
ate stage, viz., that of the pupa — ^and we can perhaps account for the
marvellous case of the Cecidomyia,

DEVELOPMENT AND EMBRYOLOGY 459

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 different purposes in the adult
state. So again it has been shown that generally the embryos of the
most distinct species belonging 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 an-
other, 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 ex-
isted in the earliest stage of their development we should learn noth-
ing, 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 development, closely resemble each other, however different
the adults may become; and so it is with very many other animals.
A trace of the law of embryonic resemblance occasionally lasts till a
rather late age: thus birds of the same genus, and of allied genera,
often resemble each other in their immature plumage; as we see in
the spotted feathers in 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; the
womb of its mother, in the egg of the bird which is hatched in a nest,
and in the spawn 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. Lub-
bock in his remarks on the close similarity of the larvse of some in-
sects belonging to very different orders, and on the dissimilarity of
the larvae of other insects within the same order, according 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 even be given of the larv^ of allied species, or groups of species,
differing more from each other than do the adults. In most cases,
however, the larv^, though active, still obey, more or less closely, the
law of common embryonic resemblance. Cirripedes afford a good in-
stance of this; even the illustrious Cuvier did not perceive that a bar-
nacle was a crustacean; but a glance at the larva shows this in an
unmistakable manner. So again the two main divisions of cirripedes,
the pedunculated and sessile, though differing widely in external
have larvae in all their stages barely distinguishable.

The embryo in the course of development generally rises in organi-
sation; 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 ani-

DEVELOPMENT AND EMBRYOLOGY 461

mal must be considered 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 sim-
ple 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 antennas; 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 become 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 struc-
ture, and into what I have called complemental males; and in the
latter the development has assuredly been 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 differ-
ence as in some necessary manner contingent on growth. But there
is no reason why, for instance, the wing of a bat, or the fin of a por-
poise, 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 differ widely from the adult :
thus Owen has remarked in regard to cuttlefish, “there is no meta-
morphosis; the cephalopodic character is manifested long before the
parts of the embryo are completed.” 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

462 ORIGIN OF SPECIES

and often great changes during their development. Spiders, again,
barely undergo any metamorphosis. The larvse of most insects pass
through a v^orm-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 sim-
ple 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 crustaceans belong, no other member is as yet known to be
first developed under the nauplius-fofm, though many appear as
zoeas; nevertheless Muller 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 embryology,—
namely, the very general, though not universal, difference in struc-
ture between the embryo and the adult; the various parts in the same
individual embryo, which ultimately 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 embryo often
retaining whilst within the egg or womb, structures 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 per-
fectly 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 affecting 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

X>£VELOPMENT AND EMBRYOLOGY 463

other way; for it is notorious that breeders of catde, horses, and vari-
ous 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 variation 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 generation. 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 only appear at corre-
sponding 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 mean-
ing 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 generally ap-
pear at a not very early period of life, and are inherited at a corre-
sponding 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, maintain that the j^reyhound 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 measuring the old dogs and their six-days-old pup-

ORIGIN OF SPECIES

464

pies, 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 carefully measured the propor-
tions (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, 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 speci-
fied 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, etc., for breeding, when nearly
grown up: they are indifferent whether the desired qualities are ac-
quired 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 characteristic differences which have been accumulated by
man’s selection, and which give value to his breeds, do not generally
appear at a very early period of life, and are inherited at a correspond-
ing 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

DEVELOPMENT AND EMBRYOLOGY 465

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 correspond-
ing 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 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 gready in the adult state. What-
ever influence long-continued use or disuse may have had in modify-
ing 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 modified
only in a slight degree, through the effects of the increased 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 cuttlefish, land shells, fresh-water crus-
taceans, 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 contingencies; namely, from the young having to pro-

466 ORIGIN OF SPECIES

Yide 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 Muller 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 simpH-
fied 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 habits of life, would commonly be
found unoccupied or ill-occupied by other organisms. In this case
the gradual acquirement 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 different plan, or if it
profited a larva already different from its parent to change still fur-
ther, then, on the principle of inheritance at corresponding ages, the
young or the larvae might be rendered by natural selection more and
more different from their parents to any conceivable extent. Differ-
ences in the larva might, also, become correlated with successive
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, etc., would be
useless; and in this case the metamorphosis would be retrograde.

From the remarks just made we can see how by changes of struc-
ture in the young, in conformity with changed habits of life, to-
gether 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

DEVELOPMENT AND EMBRYOLOGY 467

Sitaris — a beetle which passes through certain unusual stages o£ de-
velopment — 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. Afterwards they undergo a
complete change; their eyes disappear; 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 existing 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 malacostraca, appear at first as larvae under the nauplius-
form; 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 as-
signed by Fritz Muller, it is probable that at some very remote period
an independent adult animal, resembling the NaupHus, existed, and
subsequently produced, along several divergent lines of descent, the
above-named great crustacean 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.

ORIGIN OF SPECIES

468

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
s^radations, the best, and, if our collections were nearly perfect, the
)nly possible arrangement, would be genealogical; descent being the
lidden bond of connexion which naturalists have been seeking under
;he term of the Natural System. On this view we can understand
how it is that, in the eyes of most naturalists, the structure of the
embryo is even more important for classification than ithat of the
adult. In two or more groups of animals, however much they may
differ from each other in struaure 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 re-
veals community of descent; but dissimilarity in embryonic develop-
ment 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 cirri-
pedes, -though externally so like shellfish, are at once known by their
larv^ to belong to the great class of crustaceans. As the embryo often
shows 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 believes 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 appeared. It should also be
borne in mind, that the law may be true, but yet, owing to the geolog-
ical 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

RUDIMENTARY ORGANS

469

adapted in its larval state to some special line of life, and transmitted
the same larval state to a whole group of descendants; for such larvae
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 the principle of varia-
tions 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 corresponding period. Embryology rises greatly in in-
terest, when we look at the embryo as a picture, more or less obscured,
of the progenitor, either in its adult or larval state, cf 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 na-
ture. 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 jflight. What can be more curious than the presence of teeth in
fatal 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 meaning 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. Rudimen-
tary organs sometimes retain their potentiality: this occasionally oc-
curs with the mammae 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 rudi-
mentary teats; but the latter in our domestic cows sometimes become
well developed and yield milk. In regard to plants the petals are

ORIGIN OF SPECIES

470

sometimes rudimentary, and sometimes well-developed in the indi-
viduals of the same spcies. In certain plants having separated sexes
Kolreuter found that by crossing a species, in which the male flowers
included a rudiment of a pistil, with an hermaphrodite species, hav-
ing of course a well-developed pistil, the rudiment in the hybrid off-
spring was much increased in size; and this clearly shows that the
rudimentary and perfect pistils are essentially alike in nature. An
animal may possess 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
feathered 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 rudimentary 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 ova-
rium. 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 con-
joined 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 giv-
ing 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, unless we
have reason to suppose that they were formerly more highly de-
veloped, ought not to be considered as rudimentary. They may be

RUDIMENTARY ORGANS 47I

in a nascent condition, and in progress towards further development.
Rudimentary organs, on the other hand, 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 pro-
duced 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 diiSicult to distinguish between rudimen-
tary 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 per-
fect state, and consequendy will have become long ago extinct. The
wing of the penguin is of high service, acting as a fin; it may, there-
fore, represent 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 use-
less, and is truly rudimentary. Owen considers the simple filamentary
limbs of the Lepidosiren as the “beginnings of organs which attain
full functional development in higher vertebrates”; but, according to
the view lately advocated by Dr. Gunther, they are probably rem-
nants, consisting of the persistent axis of a fin, with the lateral rays
or branches aborted. The mammary glands of the Ornithorhynchus
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 re-
spects. In closely allied species, also, the extent to which the same or-
gan 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 entirely ab-

ORIGIN OF SPECIES

472

sent which analogy would lead us to expect to find in them, and
which are occasionally found in monstrous individuals. Thus in most
of the Scrophulariaceas 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 becomes
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 un-
derstand 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 uni-
versal 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 rudimentary
organs. In reflecting on them, every one must be struck with aston-
ishment; 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 im-
perfect and useless. In works on natural history, rudimentary or-
gans are generally said to have been created “for the sake of sym-
metry,” 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 con-
sistent 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 Weis-
mann 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 ellip-
tic courses round their planets “for the sake of symmetry”; because
the planets thus revolve round the sun? An eminent physiologist
accounts for the presence of rudimentary organs, by supposing that

RUDIMENTARY ORGANS

473

they serve to excrete matter in excess, or matter injurious to the sys-
tem; but can we suppose that the minute papilla, which often repre-
sents 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 growing em-
bryonic calf by removing matter so precious as phosphate of lime?
When a man’s fingers have been amputated, imperfect nails have
been known to appear on the stumps, and I could as soon believe that
these vestiges of nails are developed in order to excrete horny mat-
ter, as that the rudimentary nails on the fin of the manatee have been
developed for this same purpose.

On the view of descent with modification, the origin of rudimen-
tary organs is comparatively simple; and we can understand to a
large extent the laws governing their imperfect development. We
have plenty of cases of rudimentary organs in our domestic produc-
tions, 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
hornless 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 render-
ing 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 ulti-
mately 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.

474 ORIGIN OF SPECIES

Aay change in structure and function, which can be effected by
small stages, is within the power of natural selection; so that an or-
gan rendered, through changed habits of life, useless 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 ren-
dered 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 ks 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 k in the embryo. Thus we can understand the greater size of
rudimentary organs in the embryo relatively to the adjoining 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 re-
duced 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 reduced, how can
it be still further reduced in size until the merest vestige is left; and
how can it be finally quite obliterated It is scarcely possible that dis-
use 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 which has become use-
less would be rendered, independently of the effects of disuse, rudi-
mentary, and would at last be wholly suppressed; for the variations
towards diminished size would no longer be checked by natural se-
lection. The principle of the economy of growth, explained in a
former chapter, by which the materials, forming any part, if not use-

SUMMAHY

4 /^

ful to the possessor, are saved as far as is possible, will perhaps come
into play in rendering a useless part rudimentary. But -this principle
will almost necessarily be confined to the earlier stages of the process
of reduction; for we cannot suppose that a minute papilla, for in-
stance, representing in a male flower the pistil of the female flower,
and formed merely of cellular tissue, could be further reduced or
absorbed 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 rudimen-
tary parts as useful as, or even sometimes more useful than, parts
of high physiological importance. Rudimentary organs may be com-
pared with 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 con-
clude that the existence of organs in rudimentary, imperfect, and use-
less condition, or quite aborted, far from presenting a strange diffi-
culty, as they assuredly do on the old doctrine of creation, might even
have been anticipated in accordance with the views here explained,

SUMMARY

In this chapter I have attempted to show, that the arrangement of
all organic beings throughout all time in groups under groups — ^that
the nature of the relationships by which all living and extinct or-
ganisms are united by complex, radiating, and circuitous lines of
affinities into a few grand classes, — ^the rules followed and the diffi-
culties encountered 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 between analogical or
adaptive characters, and characters of true affinity; and other such
rules; — all naturally follow if we admit the common parentage of al-
lied forms, together with their modification through variation and

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 extended — Effects of its
adoption on the study of Natural History — Concluding remarks.

34S this whole Yolume is one long argument, it may be conven-
/ % ient to the reader to have the leading facts and inferences
JL Vr briefly recapitulated.

That many and serious objections may be advanced against the the-
ory of descent with modif icat ioj^ through Variation and Natural ^e-
Ie ction,_l do not deny. I have endeavoured to give to them their full
force. Nothing at first can appear more diflicult to believe than that
the more complex organs and instincts have been perfected, not by
means superior to, though analogous with, human reason, but b yjthe
accum ulatio n of innumerable s light variadons, each good forjhe Jn-
dividual possessor. N evertheleis7 fhTs~Hffficulty , thougBT appearing to
our imagination insuperably great, cannot be considered real if we
admit the following prOposidons, namely, that all parts of the organ-
isadon and instincts offer, at least, individual differences — ^that th^j^
is a_ struggl e for existence leadir^jtq^ t^ preservation of profitable
deviadons 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 proposidons cannot, I think, be disputed.

It is, no doubt, extremely diflicult even to conjecture by what grada-
tions 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 extremely 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 th e theo ry o f Natu ral Selection; and

478

RECAPITULATION AND CONCLUSION 479

one o£ the most cur ious of these is the existence in the same com-
munity of two or thr^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 uni-
versal 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 conclusively to show that this sterility is no more
a special endowment 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
consideration 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 func-
tions.

Although the fertility of varieties when intercrossed and of their
mongrel offspring has been asserted by so many authors to be uni-
versal, 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 experimented on have been produced under domes-
tication; 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 intercrossed, we
ought not to expect that domestication would likewise induce sterihty
in their modified descendants when crossed- This elimination of
sterility apparently follows from the same cause which allows our
domestic animals to breed freely under diversified circumstances; and
this again apparently follows from their having been gradually
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 off-

ORIGIN OF SPECIES

482

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 lat-
ter, from existing in greater numbers, would generally be modified
and improved at a quicker rate than the intermediate varieties, which
existed in lesser numbers; so that the intermediate varieties would,
in the long run, be supplanted and exterminated.

On this doctrine of the extermination of an infinitude of con-
necting 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 collection of fossil remains afford
plain evidence of the gradation 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 together, it does not yield the infinitely many fine grada-
tions between past and present species required on the theory; and
this is the most obvious of the many objections which may be 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? Although we now know that
organic beings appeared on this globe, at a period incalculably remote,
long before the lowest 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 certainly existed. The parent-form of any two or
more species would not be in all its characters directly intermediate
between its modified offspring, any more than the rock pigeon is

RECAPITULATION AND CONCLUSION 483

directly intermediate in crop and tail between its descendants, 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 naturahsts as so many new species, more
especially if found in diiferent geological sub-stages, let their differ-
ences 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 Hnks 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
become 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 frequendy
and vary most, and varieties are often at first local — ^both causes
rendering the discovery of intermediate links in any one formation
less likely. Local varieties 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 intermittent in their
accumulation; and their duration has probably 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 fossiliferous formations thick enough to resist
future degradation can as a general rule be accumulated only where
much sediment is deposited on the subsiding bed of the sea. During
the alternate periods of elevation and of stationary level the record
will generally be blank. During these latter periods there will prob-

ORIGIN OF SPECIES

484

ably 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 continents 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; consequendy formations much older than any now known
may lie buried beneath 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, firsdy, 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 universe and of the interior of our globe to
speculate with safety on its past duration.

That the geological record is imperfect, aU 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 manner. We clearly see this in the fossil remains from
consecutive formations invariably being much more closely related
to each other, than are the fossils from widely separated formations.

Such is the sum of the several chief objections and difficulties
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 important objections relate to questions on which we
are confessedly ignorant; nor do we know how ignorant we are.
We do not know all the possible transitional gradations between 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 imperfect is the Geological Record.
Serious .as these several objections are, in my judgment they are by

RECAPITULATION ANB CONCLUSION 485

no means sufficient to overthrow the theory of descent with subse-
quent modification.

Now let us turn to the other side of the argument. Under domesti-
cation 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, compensa-
tion, the increased use and disuse of parts, and the definite action
of the surrounding conditions. There is much difficulty in ascertain-
ing how largely our domestic productions have been modified; but
we may safely infer that the amount has been large, and that modi-
fication 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 occa-
sionally produced by our oldest domesticated productions.

Variability is not actually caused by man; he only unintentionally
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 educated eye. This uncon-
scious 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 effi-

486 ORIGIN OF SPECIES

ciently 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 geometrical 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 survive. A grain in the balance
may determine which individuals 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 during any season, over those with which
they come into competition, 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 generally 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 unaccountable 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 quantity. Man, though acting on external characters alone

RECAPITULATION AND CONCLUSION 487

and often capriciously, can produce within a short period a great
result by adding up mere individual differences in his domestic
productions; and every one admits that species present individual
differences. But, besides such differences, all naturalists admit that
natural varieties exist, which are considered sufficiently distinct to be
worthy of record in systematic works. No one has drawn any clear
distinction between individual 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 continent
when divided by barriers of any kind, and on outlying islands, what
a multitude of forms exist, which some experienced 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 accumulated through
natural selection, or the survival of the fittest? If man can by
patience select variations useful to him, why, 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 scruti-
nising the whole constitution, structure, and habits of each creature,
— favouring 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 crea-
tion, 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 where many species of a genus have been produced,

ORIGIN OF SPECIES

488

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 incipi-
ent 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 ajSinities
they are clustered in litde groups round other species— in both
respects resembling varieties. These are strange relations on the
view that each species was independently created, but are intelligible
if each existed first as a variety.

As each species tends by its geometrical rate of reproduction to
increase inordinately in number; and as the modified descendants of
each species will be enabled to increase by as 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 varieties 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 exterminate the
older, less improved, and intermediate varieties; 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 con-
tingency 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

RECAPITULATIOISI AND CONCLUSION 489

the grouping of all organic beings under what is called the Natural
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 modifica-
tions; it can act only by short and slow steps. Hence, the canon of
“Natura non facit saltum,” which every fresh addition to our knowl-
edge 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 inherited, 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 inno-
vation. 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 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 thrushdike 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 selection 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 anticipated.

We can to a certain extent understand how it is that there is so
much beauty throughout namre; for this may be largely attributed
to the agency of selection. That beauty, according 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 resemblance 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 rendered the
voice of the male musical to the female, as well as to our ears.
Flowers and fruit have been rendered conspicuous by brilliant

490 ORIGIN OF SPECIES

colours in contrast with the green foliage, 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-inhabi-
tants; 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 productions from another land. Nor
ought we to marvel if all the contrivances in nature be not, as far as
we can judge, 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 ichneumonidx 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 production 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 produced some direct and definite
effect, but how much we cannot 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 loggerheaded duck, which has wings incapable of flight, in
nearly the same condition as in the domestic duck; or when we look
at the burrowing tucu tucu, which is occasionally blind, and then at

RECAPITULATION AND CONCLUSION 49 1

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 appearance 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!

On the ordinary view of each species having been independently
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 differendy coloured flowers, than
if all possessed the same coloured flowers? If species are only well-
marked varieties, of which the characters have become 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 inherited
without change for an immense period. It is inexplicable 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
expect 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

ORIGIN OF SPECIES

492

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
dilEculty 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 ncf indispensable, as we see in the case of 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 having inherited much in
common, we can understand how it is that allied species, when
placed under widely different conditions of life, yet follow nearly
the same instincts; 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 resemblance 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 inde-
pendently created and varieties had been produced through sec-
ondary 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 successive intervals; and the amount of
change, after equal intervals 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

RECAPITULATION AND CONCLUSION 493

world, almost inevitably follows from the principle of natural selec-
tion; 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 inter-
mediate in character between the fossils in the formations above and
below, is simply explained by their intermediate 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 existing 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 func-
tions. This fact is perfecdy compatible with numerous beings still
retaining simple and but little improved structures, fitted for simple
conditions of life; it is likewise compatible 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 law of the long endurance of allied forms on the same
continent, — of marsupials in Australia, of edentata in America, and
other such cases,— is intelligible, for within the same country 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 migration from one
part of the world to another, owing to former climatal and geo-
graphical changes and to the many occasional and unknown means
of ispersal, then we can understand, on the theory of descent with
modification, most of the great leading facts in Distribution. We

ORIGIN OF SPECIES

494

can see why there should be so striking a parallelism in the distribu-
tion o£ organic beings throughout space, and in their geological suc-
cession throughout time; for in both cases the beings have been con-
nected by the bond of ordinary generation, and the means of modifi-
cation have been the same. We see the full meaning of the wonderful
fact, which has struck every traveller, namely, that on the same conti-
nent, under the most diverse conditions, under heat and cold, on
mountain and lowland, on .deserts and marshes, most of the inhabit-
ants 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 mountains, 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 different 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.

RECAPITULATION AND CONCLUSION 495

The existence of closely allied or representative species in any
two areas, implies, on the theory of descent with modification, 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
varieties 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 immigrants might have
derived. We see this in the striking relation of nearly all the plants
and animals of the Galapagos archipelago, of Juan Fernandez, and
of the other American islands, to the plants and animals of the
neighbouring 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 explana-
tion on the theory of creation.

The fact, as we have seen, that all past and present organic 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 extinction and divergence of character. On these
same principles 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 classifica-
tion; why adaptive characters, though of paramount importance to
the beings, are of hardly any importance in classification; why
characters derived from rudimentary parts, though of no service to
the beings, are often of high classificatory value; and why embryo-
logical characters are often the most valuable of all. The real affini-
ties of all organic beings, in contradistinction to their adaptive
resemblances, are due to inheritance or community of descent. The
Natural System is a genealogical arrangement, with the acquired
grades of ffifference, marked by the terms, varieties, species, genera,
families, etc.; and we have to discover the lines of descent by the
most permanent characters whatever they may be and of however
slight vital importance.

The similar framework of bones in the hand of a man, wing of

496 ORIGIN OF SPECIES

a bat, fin o£ a porpoise, and leg of the horse,— the same number of
vertebrae forming the neck of the giraffe and of the elephant, — and
innumerable other such facts, at once explain themselves on the
theory of descent with slow and slight successive modifications. The
similarity of pattern in the wing and in the leg of a bat, though used
for such different 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 modification of parts or
organs, which were aboriginally alike in an early progenitor in each
of these classes. On the principle 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 run-
ning in loops, like those of a fish which has to breathe the air dis-
solved 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 Hfe; hence the organ will not be reduced
or rendered rudimentary at this early age. The calf, for instance,
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 with-
out 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 ail 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 soldered wing-covers of many beetles,

RECAPITULATION AND CONCLUSION 497

should so frequently occur. Nature may be said to have taken pains
to reveal her scheme of modification, by means of rudimentary
organs, 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 varia-
tions; aided in an important manner by the inherited effects of the
use and disuse of parts; and in an unimportant manner, that is in
relation 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 appears that I formerly
underrated the frequency and value of these latter forms of variation,
as leading to permanent modifications of structure independently
of natural selection. But as my conclusions have lately been much
misrepresented, and it has been stated that I attribute the modifica-
tion of species exclusively to natural selection, I may be permitted
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: ‘1 am convinced that natural
selection has been the main but not the exclusive means of modi-
fication.” This has been of no avail. Great is the power of steady
misrepresentation; but the history of science shows that fortunately
this power does not long endure.

It can hardly be supposed that a false theory would explain, in so
satisfactory a manner as does the theory of natural selection, the
several large classes of facts above specified. 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 revolution of the earth
on its own axis was until lately supported by hardly any direct evi-
dence. 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 results consequent on this unknown

ORIGIN OF 5PECIES

498

element of attraction; notwithstanding that Leibnitz formerly
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 satisfactory, 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 attacked by Leibnitz, “as subversive of natural, and
inferentially 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-development 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 muta-
bility 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
intercrossed are invariably sterile, and varieties invariably fertile; or
that sterility is a special endowment and sign of creation. The belief
that species were immutable productions 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 mind cannot possibly grasp the full meaning of
the term of even a million years; it cannot add up and perceive

RECAPITULATION AND CONCLUSION 499

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,” etc., and to think that we give an explanation when we
only restate 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 immutability of species, may be influ-
enced 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 question 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 belief 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 conclusion to arrive at. They admit that
a multitude of forms, which till lately they themselves thought were
special creations, and which are still thus looked at by the majority
of naturalists, and which consequently have all the external char-
acteristic 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. Nevertheless they do not pretend
that they can define, or even conjecture, 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 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

500 ORIGIN OF SPECIES

birth. But do they really believe that at innumerable 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 questions cannot
be answered by those who beUeve in the appearance 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
avinm “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 plair.,
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 naturahsts 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 natural-
ists on the subject of evolution, and never once met with any sympa-
thetic agreement. It is probable that some did then believe in
evolution, but they were either silent, or expressed themselves so
ambiguously 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
tbink that species have suddenly given birth, through quite unex-
plained means, to new and totally different forms: but, as I have
attempted to show, weighty evidence can be opposed to the admis-
sion of great and abrupt modifications. Under a scientific point of
view, and as leading to further investigation, but litde 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 adced how far I extend the doctrine of the modification

RECAPITULATION AND CONCLUSION 5OI

of species. The question is difficult to answer, because 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 existing orders.

Organs in a rudimentary condition plainly show that an early
progenitor had the organ in a fully developed condition; and this
in some cases impUes 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. Nevertheless all Hving things
have much in common, in their chemical composition, their cellular
structure, their laws of growth, and their Hability to injurious influ-
ences. We see this even in so trifling a fact as that the same poison
often similarly affects plants and animals; or that the poison secreted
by the gallfly 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 start from a common origin. If we look even
to the two main divisions — namely, to the animal and vegetable
kingdoms — 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 dgx may claim to
have first a characteristically animal, and then an unequivocal
vegetable, existence.” Therefore, on the principle of natural selection

ORIGIN OF SPECIES

502

with divergence of character, 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 admit
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 Verte-
brata, Articulata, etc., we have distinct evidence in their embryo-
logical, 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 species are gen-
erally admitted, we can dimly foresee that there will be a consider-
able revolution in natural history. Systematists 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 relief. 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 consideration than it is at present; for differences,
however slight, between any two forms, if not blended by inter-
mediate gradations, are looked at by most naturalists as sufficient
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 con-
nected. Hence, without rejecting the consideration of the present
existence of intermediate gradations between any two forms, we

RECAPITULATION AND CONCLUSION 5O3

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 gready in interest. The terms used by naturalists, of affinity,
relationship, community of type, paternity, morphology, adaptive
characters, rudimentary and aborted organs, etc., 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 some-
thing wholly beyond 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 possessor, 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 numer-
ous 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 opened,
on the causes and laws of variation, on correlation, on the effects
of use and disuse, on the direct action of external conditions, and
so forth. The study of domestic productions 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 diverging lines of descent

ORIGIN OF SPECIES

504

in our natural genealogies, by characters of any kind which have
long been inherited. Rudimentary 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
between the inhabitants of the sea on the opposite sides of a con-
tinent, 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 im-
perfection 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 accumu-
lation of each great fossiliferous formation will be recognised as
having depended on an unusual concurrence of favourable circum-
stances, and the blank intervals 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
preceding and succeeding organic forms. We must be cautious in
attempting to correlate as stricdy contemporaneous two formations,
which do not include many identical species, by the general succes-
sion of the forms of life. As species are produced and exterminated
by slowly acting and still existing 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
suddenly altered physical conditions, namely, the mutual relation

RECAPITULATION AND CONCLUSION 505

of organism to organism, — the improvement of one organism entail-
ing 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 researches.
Psychology will be securely based on the foundation 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 created. To my
mind it accords better with what we know of the laws impressed
on matter by the Creator, that the production 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 lineal descendants of some few beings which lived long before
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 trans-
mit 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
Cambrian epoch, we may feel certain that the ordinary succession

ORIGIN OF SPECIES

506

by generation has never once been broken, and that no cataclysm
has desolated the whole world. Hence we may look with some con-
fidence to a secure future o£ 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 interesting 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;
Variability 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 im-
proved 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 this view of life, with its several powers, having
been originally 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 endless 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 o£ animals or plants which deviate in important
characters from their nearest allies, so as not to b6 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 characteristic
of the species have not been produced in the skin and its appendages.
Albinism is the state of being an albino.

Alga: — K class of plants including the ordinary seaweeds and the filamentous fresh-
water weeds.

Alternation of Generations — ^This term is applied to a peculiar mode of reproduction
which 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 — h group of fossil, spiral, chambered shells, allied to the existing pearly
Nautilus, but having the partitions between the chambers waved in compli-
cated 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 are said to be analogous,
and to be analogues of each other.

Animalcule — k 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 provided with appendages
for locomotion and with gills. It includes the ordinary marine worms, the
earthworms, and the leeches.

Antennce — ^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 fertili 2 ing
dust is produced.

Aplacentalia, Aplacentata or Aplacental Mammals — See Mammalia.

♦ 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 endeavoured to give the explanations
of the terms in as popular a form as possible.

507

GLOSSARY

508

Archetypal— Oi or belonging to tbe Archetype, or ideal primitive form upon which
all the beings of a group seem to be organized.

Articulata~k 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 — ^Arrested in development at a very early stage.

Balanus — The genus including the common Acorn shells which live in abundance
on the rocks of the seacoast.

Batrachians—k 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.)

Botdders—Lzxgt transported blocks of stone generally embedded in clays or gravels.

Brachiopoda — k 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 frmged arms, by the
action of which food is carried to the mouth.

Branchice — Gills or organs for respiration in water.

Pertaining to gills or branchiae.

Cambrian System — k series of very ancient Pakozoic rocks, between the Laurentian
and the Silurian. Until recently these were regarded as the oldest fossiliferous
rocks.

Canidce — ^The Dog-family, including the Dog, Wolf, Fox, Jackal, etc.

Carapace — ^The shell enveloping the anterior part of the body in Crustaceans gen-
erally; 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, or Palaeozoic, system
of formations.

Caudal— Ot or belonging to the tail.

Cephalo pods— 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, etc., having the
form of the body fish-like, the skin naked, and only the fore limbs developed.

Chelonia — ^An order of Reptiles, including the Turtles, Tortoises, etc.

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 represent the limbs.

Cocctis — ^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 — k cak 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.”

Coelospermous — ^A term applied to those fruits of the Umbelliferae 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 su-aight line down the middle of the back.

GLOSSARY 509

Column — peculiar organ in the flowers o£ Orchids, in which the stamens, style and
stigma (or the reproductive parts) are united.

Composite or Compositous Plants — Plants in which the inflorescence consists of
numerous small flowers (florets) brought together into a dense head, the base
of which is enclosed by a common envelope. {Examples, the Daisy, Dande-
lion, etc.)

Conferva — ^The filamentous weeds of fresh water.

Conglomerate — h 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, etc., 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.

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, etc.)

Curculio — ^The old generic term for the Beetles known as Weevils, characterized by
their four jointed feet, and by the head being produced into a sort of beak
upon the sides of which the antennas 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 or less united.

Dimorphic — ^Having two distinct forms. — ^Dimorphism is the condition of the appear-
ance 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 of at least
the middle incisor (front) teeth in both jaws. {Examples, the Sloths and
Armadillos.)

The hardened fore-wings of Beetles, serving as sheaths for the membranous
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, gills attached to the feet or organs of the mouth, and the feet
fringed with fine hairs. They are generally of small size.

510 GLOSSARY

Eocene — ^The earliest o£ the three divisions of the Tertiary epoch of geologists. 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 animals naturally inhabiting a certain country or regioUj
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 imperfecdy developed in some respects, and collected into a dense
spike or head, as in the Grasses, the Dandelion, etc.

Foetal — Of or belonging to the fcetus, or embryo in course of development.

Foraminifera — class of animals of very low organization and generally of small size,
having a jelly-like body, from the surface of which delicate filaments can be
given ojfl 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.

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 — 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 develop-
ment 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 glacial periods have occurred repeat-
edly 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 oesophagus or gullet.

Gneiss — A rock approaching granite in composition, but more or less laminated, and
really produced by the alteration of a sedimentary deposit after its consolidation.

Grallatores — ^The so-called Wading-birds (Storks, Cranes, Snipes, etc.), which are
generally furnished with long legs, bare of feathers above the heel, and have no
membranes between the toes.

orranite — A rock consisting essentially of crystals of felspar and mica in a mass of
quartz.

Haoitat — ^The locality in which a plant or animal naturally lives.

Hcmiptera — ^An order or sub-order of Insects, characterized by the possession 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.

GLOSSARY 51 1

Hermaphrodite — ^Possessing the organs of both sexes.

Homology — ^That relation between parts which results from their development from
corresponding embryonic parts, either in different animals, as in the case of the
arm of rnan, 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, etc., 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 — hxi order or sub-order of Insects having (like the Hemiptera) a jointed
beak, but in which the fore-wings are either wholly membranous or wholly
leathery. The Cicadce, Frog-hoppers, and Aphides, are well-known examples.

Hybrid — The offspring of the union of two distinct species.

Hymenoptera — ^An order of Insects possessing biting jaws and usually four mem-
branous wings in which there are a few veins. Bees and Wasps are familiar
examples of this group.

Hypertrophied — ^Excessively developed.

Ichneumonidce — K. 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.

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 consist of a gelatinous
material enclosed in a delicate membrane, the whole or part of which is fur-
nished with short vibrating hairs (called cilia), by means of which the animal-
cules swim through the water or convey the minute particles of their food to
the orifice of the mouth.

Insectivorous — ^Feeding on Insects.

Invertebrata, or Invei'tebrate Animals — Those animals which do not possess a back-
bone or spinal column.

Lacuna — Spaces left among the tissues in some of the lower animals, and serving m
place of vessels for the circulation of the fluids of the body.

Lamellated — ^Furnished with lamella 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 — K group of greatly altered and very ancient rocks, which is greatly devel-
oped 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.

Leguminosce — ^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).

Lemuridce — A group of four-handed animals, distinct from the Monkeys and ap-
proaching the Insectivorous Quadrupeds in some of their characters 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 pro-
boscis, and of four large more or less scaly wings. It includes the well-known
Butterflies and Moths.

GLOSSARY

512

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, etc., together with the Woodlice and Sandhoppers.

Mammalia — ^The highest class of animals, including the ordinary hairy quadrupeds,
the Whales, and Man, and charactermed by the production of living young
which are nourished ^ter birth by milk from the teats {Mammce, Mammary
glands) of the mother. A striking difference 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, includ-
ing the greater part of the class, are called Placental mammals; the latter,
or Aplacentai mammals, include the Marsupials and Monotremes {Ornitho-
rhynchus).

Mammiferous — ^Having mammae 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 ven-
tral pouch (marsupium), such as the Kangaroos, Opossums, etc. (see Mam-
malia).

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
m the skin and its appendages.

Metamorphic Roc\s — 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 general 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 con-
secutive 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, etc.)

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) belonging to a slightly
lower group.

Nascent — Commencing development.

Natatory — Adapted for the purpose of swimming-

Nauplius-form — ^The earliest stage in the development of many Crustacea, especially
belonging to the lower groups. In this stage the animal has a short body,
with indistinct indications of a division into segments, 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.

GLOSSARY

513

Vetiration — ^The arrangement o£ 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 Membrane — 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, etc., 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 — K 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 egg4ike calcareous
bodies.

Operculum — 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 — h. term applied to those fruits of the Umbellifera^ 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 converted into the fruit.

O vigorous — ^Egg-bearing.

Ovules (of Plants) — ^The seeds in the earliest condition.

Pachyderms — K group of Mammalia, .so called from their thick skins, and including
the Elephant, Rhinoceros, Hippopotamus, etc.

Palceozoic — ^The oldest system of fossiliferous rocks.

Palpi — ^Jointed appendages to some of the organs of the mouth in Insects and
Crustacea.

Papilionacecs — ^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 — ^The 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-cells.

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.

5i6 glossary

Tridactyle — Three-fingered, or composed of three movable parts attached to

- 1

group of rustaceans, somewhat resembling the

Woodlice in external form, and, like some of them, capable of rolling them-
selves up into a ball. Their remains are found only in the Palaeozoic rocks,
and most abundantly in those of Silurian age.

Trimorphic — Presenting three distinct forms.

Umbellijerce — 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.)

Ungulata — Hoofed quadrupeds.

Unicellular — Consisting of a single cell.

Vascular — Containing blood-vessels.

Vei'miform — ^Like a worm.

Vertehrata; or Vertebrate Animals — The highest division of the animal kingdom,
led from the presence in most cases of a backbone composed of numerous
*,firtphra^. which constitutes the centre of the skeleton and at the
entral parts of the nervous system.

Whorls — circles or spiral lines in which the parts of plants are arranged upon
the axis of growth.

Workers — See Neuters.

Zoea-stage — ^The earliest stage in the development of many of the higher Crustacea,
so called from the name of Zoea, applied to these young animals when they
were supposed to constitute a peculiar genus,

— ^In many of the lower animals (such as the Corals, Medusa, etc.) 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 different from that of the egg. The individuality of the species
is represented by the whole of the form produced between two sexual
reproductions; and these forms, which are apparently individual animals, have
been called zooids.

INDEX

Aberrant groups, 448.

Abyssinia, plants of, 405.

Acclimatisation, 144.

Adoxa, 215.

Affinities of extinct species, 362.

, of organic beings, 448.

Agassiz, on Amblyopsis, 144.

on groups of species suddenly ap-
pearing, 348.

on prophetic forms, 362.

, on embryological succession, 371.

on the Glacial period, 394.

, on embryological characters, 437.

on the latest tertiary forms, 336.

■ , on parallelism of embryological

development and geological succession,
468.

Alex., on pedicellarise, 236.

Algse of New Zealand, 403.

Alligators, males, fighting, 95.

Alternate generations, 458.

Amblyopsis, blind fish, 144.

America, North, productions allied to
those of Europe, 398.

, , boulders and glaciers of, 400.

, South, no modern formations on

west coast, 328.

Ammonites, sudden extinction of, 357*
Anagallis, sterility of, 287.

Analogy of variations, 161.

Ancylus, 41 1-

Andaman Islands inhabited by a toad, 417,
Animals, not domesticated from being
variable, 32.

domestic, descended from several

stocks, 33.

, , acclimatisation of, 146.

Animals of Australia, 119.

with thicker fur in cold climates,

139-

, blind, in caves, 142.

extinct, of Australia, 372.

Anomma, 281.

Antarctic islands, ancient flora of, 422.
Antechinus, 443,

Ants attending aphides, 254.

Ants, slave-making instinct, 264.

neuters, structure of, 280.

Apes, not having acquired intellectual
powers, 224.

Aphides, attended by ants, 254.

Aphis, development of, 462.

Apteryx, 177.

Arab horses, 46.

Aralo'Caspian Sea, 373.

Archeopteryx, 342.

Archiac, M. de, on the succession of
species, 359.

Artichoke, Jerusalem, 147.

Ascension, plants of, 414.

Asclepias, pollen of, 190.

Asparagus, 388.

Aspicarpa, 436.

Asses, striped, 162.

improved by selection, 52.

Ateuchus, 141.

Aucapitaine, on land-shells, 420-
Audubon, on habits of frigate-bird, 183.

on variation in birds’ nests, 254.

on heron eating seeds, 412.

Australia, animals of, 119,

^,dogs of, 258.

extinct animals of, 372.

European plants in, 403.

glaciers of, 400.

Azara, on flies destroying cattle, 81.
Azores, flora of, 392.

Babington, Mr., on British plants, 59.
Baer, Von, standard of Highness, 129.

comparison of bee and fish, 370.

-, embryonic similarity of the Verte-

brata, 459.

Baker, Sir S., on the giraffe, 221,
Balancement of growth, 150.

Baleen, 226.

Barberry, flowers of, 104.

Barrande, M., on Silurian colonies, 350,

on the succession of species, 359.

on parallelism of palaeozoic forma-
tions, 361.

on affinities of ancient species, 363.

517

INDEX

Barriers, importance o£, 379 *

Bates, Mr., on mimetic buttertlies, 445

446.

Batrachians on islands, 4 ^ 7 *

Bats, how structure acquired, i 77 *

, distribution of, 418.

Bear, catching water-insects, 178.

Beauty, how acquired, 199, 489 -
Bee, sting of, 204. _

, queen, killing rivals, 204.

Australian, extermination or, 04

Bees fertilising flowers, 81.

,hive, not sucking the red cloven

Brent, Mr., on house-tumblers, 257.
Britain, mammals of, 419.

Broca, Prof., on Natural Selection, 21 1.
Bronn, Prof., on duration of specific
forms, 332.

, various objections by, 21 1.

Brown, Robert, on classification, 434.

S^quard, on inherited mutilations,
141.

Busk, Mr., on the Polyzoa, 237.
Butterflies, mimetic, 445. 446 . _
Buzareingues, on sterility of varieties, 31 1.

lOI.

, Ligurian, 102. _ ^

j hive, cell-making instmct, 200.

variation in habits, 254*

, parasitic, 263.

, humble, cells of, 269.

Beetles, wingless, in Madeira, 141.

with deficient tarsi, 14^*

Bentham, Mr., on British plants, 39 -

■, on classification, 438.

Berkeley, Mr., on seeds in salt water, 38b.
Bermuda, birds of, 415 -
Birds acquiring fear, 255.

, beauty of, 202. ^

annually cross the Atlantic, 392 -

, colour of, on continents, 139.

footsteps, and remains of, m sec-
ondary rocks, 34 ^-

fossil, in caves of Brazil, 372 *

,o£ Madeira, Bermuda, and Gala-
pagos, 415 -

Birds, song of males, 96-

—transporting seeds, 39 ^*

— waders, 41 1.

, wingless, 140, I 77 > ^ 78 *

Bizcacha, 380.

, affinities of, 449 -

Bladder for swimming, in fish, 186.
Blindness of cave animals, 142-
Blyth, Mr., on distincmess of Indian

cattle, 33 - . .

, on striped hemionus, 103.

, on crossed geese, 291.

Borrow, Mr., on the Spanish pointer, 46.
Bory St. Vincent, on Batrachians, 417 -
Bosquet, M., on fossil Chthamalus, 342.
Boulders, erratic, on the Azores, 392.
Branchiae, 186, 187.

- — of crustaceans, 191.

Braun, Prof., on the seeds of Fuma-
riaceae, 215.

Cabbage, varieties of, crossed, 105.
Calceolaria, 289.

Canary-birds, sterility of hybrids, 290.
Cape de Verde islands, productions of,
421.

, plants of, on mountains, 402.

Cape of Good Hope, plants of, 134, 414 -
Carpenter, Dr., on foraminifera, 369.
Carthamus, 215.

Catasetum, 195, 44 1 -
Cats, with blue eyes, deaf, 27.

-, variation in habits of, 256.

-curling tail when going to spring,
203.

Cattle destroying fir-trees, 80.

— destroyed by flies in Paraguay, 81.
— , breeds of, locally extinct, 114.

, fertility of Indian and European

breeds, 291.

— Indian, 33, 292.

Cave, inhabitants of, blind, 142.
Cecidomyia, 458. ^

Celts, proving antiquity of man, 32.
Centres of Creation, 383.

Cephalopodae, structures of eyes, 190.

, development of, 461.

Cercopithecus, tail of, 232.

Ceroxylus laceratus, 225.

Cervulus, 291.

Cetacea, teeth and hair, 149.

, development of the whalebone, 226.

Cetaceans, 226.

Ceylon, plants of, 402.

Chalk formation, 357 -
Characters, divergence of, ii 5 -
sexual, variable, 153, I 57 -
adaptive or analogical, 443-
Charlock, 84.

Checks to increase, 77.

, mutual, 79 -

INDEX

5^9

Chelae of Crustaceans, 237.

Chickens, instinctive tameness of, 258.
Chironomus, its asexual reproduction,
458.

Chthamalina, 326.

Chthamalus, cretacean species of, 342.
Circumstances favourable to selection of
domestic products, 50.

to natural selection, 107.

Cirripedes capable of crossing, 107.

, carapace aborted, 151.

, their ovigerous frena, 187.

, fossil, 342.

larvae of, 461.

ClaparMe, Prof., on the hair-claspers of
the Acaridae, 192.

Clarke, Rev. W. B., on old glaciers in
Australia, 400.

Classification, 431.

Clift, Mr., on the succession of types,
372.

Climate, effects of, in checking increase
of beings, 77.

, adaptation of, to organisms, 145.

Climbing plants, i 85 .

developments of, 241.

Clover visited by bees, 104.

Cobites, intestine of, 185.

Cockroach, 84.

Collections, palaeontological, poor, 326.
Colour, influenced by climate, 139.

, in relation to attack by flies, 199.

Columba livia, parent of domestic pig-
eons, 36.

Colymbetes, 41 1.

Compensation of growth, 150.
Compositae, flowers and seeds of, 149.

, outer and inner florets of, 215.

, male flowers of, 470.

Conclusion, general, 497
Conditions, slight changes in, favourable
to fertility, 304.

Convergence of genera, 133.

Coot, 180.

Cope, Prof., on the acceleration or re-
tardation of the period of reproduction,
188.

Coral-islands, seeds drifted to, 389-90.

reefs, indicating movements of

earth, 387.

Corncrake, 181.

Correlated variation in domestic produc-
tions, 27.

Coryanthes, 194.

Creation, single centres of, 383.

Crinum, 289.

Croll, Mr., on subaerial denudation, 325.
3 on the age of our oldest forma-
tions, 344.

on alternate Glacial periods in the

North and South, 401.

Crosses, reciprocal, 293.

Crossing of domestic animals, importance
in altering breeds, 33.

advantages of, 103.

unfavourable to selection, 108.

Criiger, Dr., on Coryanthes, 194.
Crustacea of New Zealand, 403.
Crustacean, blind, 142.

air-breathers, 192.

Crustaceans, their chelae, 237-8.
Cryptocerus, 280.

Ctenomys, blind, 142.

Cuckoo, instinct of, 251, 259.
Cunningham, Mr., on the flight of the
logger-headed duck, 140,

Currants, grafts of, 297-
Currents of sea, rate of, 389.

Cuvier, on fossil monkeys, 341.

^jFred., on instinct, 251.

Cyclostoma, resisting salt water, 420,

Dana, Prof., on blind cave-animals, 144.

, on relations of crustaceans of

Japan, 399.

, on crustaceans of New Zealand,

403*

Dawson, Dr., on eozoon, 345.

De Candolle, Aug. Pyr., on struggle for
existence, 72.

■, on umbeliiferae, 150.

— — , on general affinities, 449.

Alph., on the variability of oaks,

^jOn low plants, widely dispersed,

427. ^ , .

on widely-ranging plants bemg va-
riable, 65.

, on naturalisation, ir8.

, on winged seeds, 150.

on Alpine species suddenly becom-
ing rare, 171.

on distribution of plants with large

seeds, 389.

, on vegetation of Australia, 405.

^,on fresh-water plants, 412.

, on insular plants, 414.

INDEX

522

Gouid, on distribution of genera of birds,
426.

Gourds, crossed, 311.

Graba, on the Uria lacrymas, 99.

Grafting, capacity of, 297.

Granite, areas of denuded, 330-1.

Grasses, varieties of, 116-7.

Gray, Dr. Asa, on the variability of oaks,
62.

, on man not causing variability, 87.

, on sexes of the holly, 10 1.

, on trees of the United States, 106.

, on naturalised plants in the United

States, 1 1 8.

, on sestivation, 216.

, on Alpine plants, 394.

— . — , on rarity of intermediate varieties:
X72.

— Dr. J, E., on striped mule, 163.
Grebe, 179.

Grimm, on asexual reproduction, 458.
Groups, aberrant, 448.

Grouse, colours of, 92.

, red, a doubtful species, 60.

Growth, compensation of, 150.

Gunther, Dr., on flat-fish, 231.

, on prehensile tails, 233.

on the fishes of Panama, 379*

, on the range of fresh-water fishes,

409,

-, on the limbs of Lepidosiren, 471.

Haast, Dr., on glaciers of New Zealand;
400.

Habit, effect of, under domestication, 27

effect of, under nature, 140-1.

, diversified, of same species, 178-

Hackel, Prof,, on classification and the
lines of descent, 452.

Hair and teeth, correlated, 148.
Halitherium, 363.

Harcourt, Mr. E. V., on the birds of Ma-
deira, 415.

Hartung, M., on boulders in the Azores,
392-3-

Hazel nuts, 388.

Hearne, on habits of bears, 178.

Heath, changes in vegetation, 79-80.
Hector, Dr., on glaciers of New Zealand,
400.

Heer, Oswald, on ancient cultivated
plants, 32.

, on plants of Madeira, iii,

Helianthemum, 216.

Helix pomatia, 420.

, resisting salt water, 420,

Helmholtz, M., on the imperfection of
the human eye, 203-4.

Helosciadium, 388.

Hemionus, striped, 165.

Hensen, Dr., on the eyes of Cephalopods,
190.

Herbert, W., on struggle for existence, 72.

, on sterility of hybrids, 288-9.

Hermaphrodites crossing, 103.

Heron eating seed, 412.

Heron, Sir R., on peacocks, 96.
Heusinger, on white animals poisoned by
certain plants, 27.

Hewitt, Mr., on sterility of first crosses,
301.

Hildebrand, Prof., on the self-sterility of
Corydalis, 289.

Hilgendorf, on intermediate varieties
332.

Himalaya, glaciers of, 400.

, plants of, 402.

Hippeastrum, 289.

Hippicampus, 233.

Hofmeister, Prof., on the movements ol
plants, 242-3.

Holly-trees, sexes of, 100.

Hooker, Dr., on man not causing varia-
bility, 87.

on trees of New Zealand, 106.

, on acclimatisation of Himalayan

trees, 145.

on flowers of umbelliferae, 149.

, on the position of ovules, 214.

, on glaciers of Himalaya, 400.

^5 on glaciers of the Lebanon, 400.

^,on plants of Tierra del Fuego,

401-2.

^5 on plants of mountains of Fernando

Po, 402.

, on algae of New Zealand, 403.

on Australian plants, 403, 422.

on vegetation at the base of the

Himalaya, 404.

, on relations of flora of America,

405

on flora of the Antarctic lands, 407,

422.

^,on the plants of the Galapagos,

416, 421.

Hooks on palms, 198.

on seeds, on islands, 416.

Hopkins, Mr., on denudation, 330.

INDEX 523

Hornbill, remarkable instinct of, 284.
Horns, rudimentary, 473.

Horse, fossil, in La Plata, 354.

, proportions of, when young, 464.

Horses, destroyed by flies in Paraguay,
80-1.

, striped, 163.

Horticulturists, selection applied by, 43-4.
Huber, on cells of bees, 272.

, P., on reason blended with instinct,

251.

, on habitual nature of instincts, 252.

, on slave-making ants, 264.

, on Melipona domestica, 269.

Hudson, Mr., on the Ground-Wood-
pecker of La Plata, 179.

, on the Molothrus, 262.

Humble-bees, cells of, 268.

Hunter, J., on secondary sexual charac-
ters, 153.

Hutton, Captain, on crossed geese, 291.
Huxley, Prof., on structure of hermaphro-
dites, 106-7.

, on the affinities of the Sirenia, 363.

, on forms connecting birds and rep-
tiles, 364.

on homologous organs, 457.

, on the development of aphis, 462.

Hybrids and mongrels compared, 312.
Hybridism, 285.

Hydra, structure of, 185.

Hymenoptera, fighting, 95.
Hymenopterous insects, diving, 180.
Hyoseris, 215.

Ibla, 15 1.

Icebergs transporting seeds, 392.
Increase, rate of, 73.

Individuals, large numbers of favourable
to selection, 107.

, many, whether simultaneously cre-
ated, 385.

Inheritance, laws of, 29.

, at corresponding ages, 29, 96.

Insects, colour of, fitted for their sta-
tions, 92.

, sea-side, colours of, 139.

, blind, in caves, 143.

, luminous, 190.

, their resemblance to certain objects,

224.

neuter, 279-80

Instinct, 251.

Instinct, not varying simultaneously with
structure, 278.

Instincts, domestic, 255.

Intercrossing, advantages of, 103, 304.
Islands, oceanic, 413.

Isolation favourable to selection, 109.

Japan, productions of, 399.

Java, plants of, 402.

Jones, Mr. J. M., on the birds of Bermuda,
415.

Jourdain, M., on the eye-spots of star-
fishes, 182.

Jukes, Prof., on subaerial denudation,
322.

Jussieu, on classification, 436.

Kentucky, caves of, 142-3-
Kerguelen Land, flora of, 407, 422.
Kidney bean, acclimatisarion of, 147.
Kidneys of birds, 148.

Kirby, on tarsi deficient in beetles,
141.

Knight, Andrew, on cause of variation,

33-

Kolreuter, on Intercrossing, 103.

, on the barberry, 104.

on sterility of hybrids, 286-7/.

^,on reciprocal crosses, 294-5.

, on crossed varieties of nicotiana,

312-

^,on crossing male and hermaphro-
dite flowers, 470.

Lamarck, on adaptive characters, 443.
Lancelet, 13 1-
eyes of, 183.

Landois, on the development of the wings
of insects, 187.

Land shells, distribution of, 421.

^jof Madeira, naturalised, 424-5.

, resisting salt water, 420.

Languages, classification of, 440.

Lankester, Mr. E. Ray, on Longevity,
210.

^,on homolo^es, 457.

Lapse, great, of time, 321.

Lanrae, 460.

Laurel, nectar secreted by the leaves, 99-
Laurentian formation, 345.

IlSnDEX

Laws of variation, 138.

Leech, varieties of, 83.

Leguminosse, nectar secreted by glands,
99-

Leibnitz’ attack on Newton, 490-
Lepidosiren, 364*

, limbs in a nascent condition, 47^ *

Lewes, Mr. G. H., on species not having
changed in Egypt, 210.

, on the Salamandra atra, 470*

on many forms of life having been

at first evolved, 502.

Life, struggle for, 73.

Lingula, Silurian, 344.

Linnseus, aphorism of, 433.

Lion, name of, 95.

, young of, striped, 459.

Lobelia fulgens, 81, 104-5.

, sterility of crosses, 289.

Lockwood, Mr., on the ova of the Hip-
pocampus, 233.

Locusts transporting seeds, 391.

Logan, Sir W-, on Laurentian formation,

345*

Lowe, Rev. R. T., on locusts visitmg
Madeira, 391.

Lowness of structure connected with
variability, 152.

, related to wide distribution, 427.

Lubbock, Sir J., on the nerves of Coc-
cus, 56.

, on secondary sexual characters, 159.

, on a diving hymenopterous insect,

180-

, on affinities, 337.

, on metamorphoses, 460.

Lucas, Dr. P., on inheritance, 28.

, on resemblance of child to parent,

315.

Lund and Clausen, on fossils of Brazil,
372.

Lyell, Sir C., on the struggle for ex-
istence, 72.

on modern changes of the earth,

102.

, on terrestrial animals not having

been developed on islands, 223.

, on a carboniferous land shell, 327.

, on strata beneath Silurian system,

345.

, on the imperfection of the geologi-
cal record, 348.

Lyell, Sir C., on tertiary formations of
Europe and North America, 358.

— , on parallelism of tertiary forma-
tions, 361.

on transport of seeds by icebergs,
392.

on great alterations of climate, 408.

on. the distribution of fresh-water
shells, 41 1-

, on land shells of Madeira, 424.

Lyell and Dawson, on fossilized trees in
Nova Scotia, 334-5-

Lythmm salicaria, trimorphic, 307.

Macleay, on analogical characters, 443.
Macrauchenia, 362.

McDonnell, Dr., on electric organs, 189.
Madeira, plants of, iii.

beetles of, wingless, 141.
fossil land shells of, 372-3.

.birds of, 415.

Magpie tame in Norway, 255.

Males fighting, 95.

Maize, crossed, 31 1.

Malay Archipelago compared with Eu-
rope, 338.

mammals of, 419.

Malm, on flatfish, 230.

Malpighiaceae, small imperfect flowers of,

214-

^,436. ^

Mammse, their development, 233.

• , rudimentary, 469.

Mammals, fossil, in secondary formation,

— , insular, 416.

Man, origin of, 505,

Manatee, rudimentary nails of, 473.
Marsupials of Australia, 119.

, structure of their feet, 452--3.

, fossil species of, 372.

Martens, M., experiment on seeds, 389.
Martin, Mr. W. C., on striped mules, 164.
Masters, Dr., on Saponaria, 216-
Matteucci, on the electric organs of rays,
188-9.

Matthiola, reciprocal crosses of, 295.
Maurandia, 242.

Means of dispersal, 386.

Melipona domestica, 269.

MprreU. Dr., on the American cuckoo,

, on the appearance of species, 349. 259.

■ , on Barrande’s colonies, 350 Metamorphism of oldest rocks, 345.

INDEX

Mice destroying bees, 82.

, acclimatisation of, 146.

tails of, 232-3.

Miller, Prof., on the cells of bees, 269-70,
273 -

Mirabilis, crosses of, 295.

Missel -thrush, 84.

Mistletoe, complex relations of, 20.
Mivart, Mr., on the relation of hair and
teeth, 148-9.

, on the eyes of cephalopods, 190.

, various objections to natural selec-
tion, 219.

, on abrupt modifications, 246-7.

, on the resemblance of the mouse

and Antechinus, 443.

Mocking-thrush of the Galapagos, 424.
Modification of species not abrupt, 500.
Moles, blind, 142.

Molothrus, habits of, 262.

Mongrels, fertility and sterility of, 308.

and hybrids compared, 312.

Monkeys, fossil, 341.

Monachanthus, 441.

Mons, Van, on the origin of fruit-trees.
Monstrosities, 54.

Moquin-Tandon, on sea-side plants, 139.
Morphology, 452.

Morren, on the leaves of Oxalis, 242-
Moths, hybrid, 291.

Mozart, musical powers of, 252.

Mud, seeds in, 412.

Mules, striped, 164.

Muller, Adolf, on the instincts of the
cuckoo, 260.

Muller, Dr. Ferdinand, on Alpine Aus-
tralian plants, 403.

Muller, Fritz, on dimorphic crustaceans,
57, 282.

^,on the lancelet, 13 1.

, on air-breathing crustaceans, 192.

on climbing plants, 242.

on the self-sterility of orchids, 289-

, on embryology in relation to classi-
fication, 437.

, on the metamorphoses of crusta-
ceans, 462, 467.

on terrestrial and fresh-water or-
ganisms not undergoing any meta-
morphosis, 466.

Multiplication of species not indefinite,
I33“4-

Murchison, Sir R., on the formations of
Russia, 328.

525

Murchison, on azoic formations, 345.

on extinction, 353.

Murie, Dr., on the modification of the
skull in old age, 188.

Murray, Mr. A., on cave-insects, 144.
Mustek vision, 175.

Myanthus, 441.

Myrmecocystus, 280.

Myrmica, eyes of, 281.

Nageli, on morphological characters, 212,
Nails, rudimentary, 473,

Nathusius, Von, on pigs, 199.

Natural history, future progress of, 503.

selection, 87.

system, 433.

Naturalisation of forms distinct from
the indigenous species, 118.
Naturalisation in New Zealand, 203.
Naudin, on analogous variations in
gourds, 160.

, on hybrid gourds, 311.

^,on reversion, 314.

Nautilus, Silurian, 344.

Nectar of plants, 99—100.

Nectaries, how formed, 99.

Nelumbium luteum, 412.

Nests, variations in, 254, 277, 284,

Neuter insects, 280-1.

Newman, CoL, on bumblebees, 82.

New Zealand, productions of, not per-
fect, 203.

naturalised products of, 371.

fossil birds of, 372.

glaciers of, 400.

crustaceans of, 403.

, algae of, 403.

number of plants of, 414.

flora of, 422.

Newton, Sir L, attacked for irreligion,
498.

^jProf., on earth attached to a part-
ridge’s foot, 392.

Nicotiana, crossed varieties of, 312.

, certain species very sterile, 294.

Nitsche, Dr., on the Polyzoa, 237.

Noble, Mr., on fertility of Rhododendron,
290.

Nodules, phosphatic, in azoic rocks, 345.
O

Oaks, variability of, 62-3-
Onites apelles, 141.

INDEX

526

Ononis, small imperfect flowers of, 214.
Orchids, fertilisation of, 194-5.

the development of their flowers,

239-40.

, forms of, 441.

Orchis, pollen of, 190.

Organisation, tendency to advance, 129-

30*

Organs of extreme perfection, 181.

, electric, of fishes, 190.

, of little importance, 196.

•, homologous, 454~5-

, rudiments of, and nascent, 469.

Ornithorhy nchus, 1 1 1 , 435.

, mammae of, 234.

Ostrich not capable of flight, 223,

, habit of laying eggs togedier, 263.

, American, two species of, 380.

Otter, habits of, how acquired, 175.
Ouzel, water, 179.

Owen, Prof., on birds not flying, 140.

^jon vegetative repetition, 152.

, on variability of unusually de-
veloped parts, 153.

, on the eyes of fishes, 1 83.

, on the swim bladder of fishes, 186,

on fossil horse of La Plata, 354.

, on generalised form, 362.

, on relation of Ruminants and

Pachyderms, 362.

on fossil birds of New Zealand, 372.

f on succession of types, 372.

, on afiSnities of the dugong, 434.

on homologous organs, 453.

on the metamorphosis of cephalo-

pods, 461.

Pacific Ocean, faunas of, 380.

Pacini, on electric organs, 189-90.

Paley, on no organ formed to give pain,
203.

Pallas, on the fertility of the domesti-
cated descendants of wild stocks, 291.
Palm with hooks, 198.

Papaver bracteatum, 216.

Paraguay, cattle destroyed by flies, 80-1.
Parasites, 262-3.

Partridge, with ball of earth attached to
foot, 391.

Parts greatly developed, variable, 153.
Parus major, 178,

Passiflora, 289-

Peaches in United States, 92.

Pear, grafts of, 297.

Pedicellarise, 236.

Pelargonium, flowers of, 149.

sterility of, 289,

Pelvis of women, 148-
Peloria, 149.

Period, Glacial, 394.

Petrels, habits of, 179.

Phasianus, fertility of hybrids, 291.
Pheasant, young, wild, 258.

Pictet, Prof., on groups of species sud-
denly appearing, 341.

on rate of organic change, 350.

on continuous succession of genera,

352.

on change in latest tertiary forms,

336.

, on close alliance of fossils in con-
secutive formations, 367—8.

on early transitional links, 341.

Pierce, Mr., on varieties of wolves, 97,
Pigeons with feathered feet and skin be-
tween toes, 28.

breeds described, and origin of, 34.

, breeds of, how produced, 49.

tumbler, not being able to get out

of egg, 93.

reverting to blue colour, 162.

instinct of tumbling, 257.

young of, 464.

Pigs, black, not affected by the paint-
root, 27.

modified by want of exercise, 199.

Pistil, rudimentary, 470.

Plants, poisonous, not affecting certain
coloured animals, 27.

selection applied to, 47.

, gradual improvement of, 48.

, not improved in barbarous coun-
tries, 48.

, dimorphic, 57.

f destroyed by insects, 78-9.

in midst of range, have to strug-
gle with other plants, 85.

, nectar of, 99.

, fleshy, on sea-shores, 139.

climbing, 186, 241.

fresh- water, distribution of, 41 1.

low in scale, widely distributed,

427-

PleuronectidjE, their structure, 229.
Plumage, laws of change in sexes of birds,

96.

Plums in the United States, 92.

Pointer dog, origin of, 46.

INDEX

Pointer, habits o£, 257.

Poison not affecting certain coloured ani-
mals, 27.

, similar effect of, on animals and

plants, 501.

Pollen of fir-trees, 204.

transported by various means, 193—4,

201.

Pollinia, their development, 239.

Polyzoa, their avicularia, 237.

Poole, Col., on striped hemionus, 165.
Potamogeton, 412.

Pouchet, on the colours of flatfish, 232.
Prestwich, Mr., on English and French
eocene formations, 361.

Proctotrupes, 180.

Proteolepas, 15 1.

Proteus, 144.

Psychology, future progress of, 505.
Prygoma, found in the chalk, 342.

Quagga, striped, 166.

Quatrefages, M., on hybrid moths, 291.
Quercus, variability of, 62.

Quince, grafts of, 297.

Rabbits, disposition of young, 257,
Races, domestic, characters of, 30.

Race horses, Arab, 46.

, English, 386.

Radcliffe, Dr., the electrical organs of
the torpedo, 1 89.

Ramond, on plants of Pyrenees, 396.
Ramsay, Prof., on subaerial denudation,

323-

, on thickness of the British forma-
tions, 324.

, on faults, 324.

Mr., on instincts of cuckoo, 261.

Ratio of increase, 73.

Rats supplanting each other, 84.

, acclimatisation of, 146.

, blind, in cave, 143,

Rattlesnake, 202.

Reason and instinct, 251.

Recapitulation, general, 478.

Reciprocity of crosses, 294*

Record, geological, imperfect, 319.
Rengger, on flies destroying cattle, 80-1.
Reproduction, rate of, 73-4.
Resemblance, protective of insects, 224-

to parents in mongrels and hybrids,

314-5.

527

Reversion, law of inheritance, 29.

in pigeons, to blue colour, 162.

Rhododendron, sterility of, 290.

Richard, Prof., on Aspicarpa, 436.

Richardson, Sir J., on structure of squir-
rels, 176.

on fishes of the southern hemis-
phere, 403-

Robinia, grafts of, 298.

Rodents, blind, 142.

Rogers, Prof., Map of N. America, 331.

Rudimentary organs, 470.

Rudiments important for classification,
435 *

Riitimeyer, on Indian cattle, 33, 292.

Salamandra atra, 470.

Saliva used in nests, 277.

Salvin, Mr., on the beaks of ducks,
227-

Sageret, on grafts, 297.

Salmons, males fighting, and hooked
jaws of, 95.

Salt water, how far injurious to seeds,
388-9.

not destructive to landshells, 420.

Salter, Mr., on early death of hybrid em-
brjfos, 301.

Saurophagus sulphuratus, 178.

Schacht, Prof., on phyllotaxy, 215.

Schiodte, on blind insects, 143.

on flatfish, 230.

Schlegel, on snakes, 148.

Schobi, Dr., on the ears of mice, 213.

Scott, J., Mr., on the self-sterility of or-
chids, 289.

, on the crossing of varieties of Ver-

bascum, 311.

Seawater, how far injurious to seeds,
388-9.

not destructive to landshells, 420.

Sebright, Sir J,, on crossed animals, 34.

Sedgwick, Prof., on groups of species
suddenly appearing, 340.

Seedlings destroyed by insects, 77.

Seeds, nutriment in, 85.

— — , winged, 150.

means of dissemination, 193, 20 x,

390-

power of resisting salt water, 389.

in crops and intestines of birds,

390.

f eaten by fish, 391, 412.

f in mud, 412.

INDEX

528

Seeds, hooked, on islands, 416.

Selection o£ domestic products, 40.

principle not of recent origin, 45.

unconscious, 45.

, natural, 87.

, sexual, 94.

objections to term, 88.

natural, has not induced sterility,

298-9.

Sexes, relations of, 95-
Sexual characters variable, 157.

selection, 94.

Sheep, merino, their selection, 43.

■ two sub -breeds, unintentionally

produced, 46.

■ mountain varieties of, 83-

Shells, colours of, 139.

, hinges of, 193.

, littoral, seldom embedded, 327.

, fresh-water, long retain the same

forms, 369.

^ dispersal of, 410-

, of Madeira, 414.

, land, distribution of, 421.

resisting salt water, 420.

Shrew-mouse, 443.

Silene, infertility of crosses, 293.

Siiliman, Prof., on blind rat, 143,

Sirenia, their affinities, 363.

Sitaris, metamorphosis of, 468-
Skulls of young mammals, 198, 455.
Slave-making instinct, 264.

Smith, Col. Hamilton, on striped horses,
164-

^,Mr. Fred., on slave-making ants,

264.

— on neuter ants, 281.

Smitt, Dr., on the Polyzoa, 237.

Snake with tooth for cutting through
egg-shell, 262.

Somerville, Lord, on selection of sheep,
43.

Sorbus, grafts of, 298.

Sorex, 443.

Spaniel, King Charles’s breed, 46.
Specialisation of organs, 129-30.

Species, polymorphic, 56.

, dominant, 66.

common, variable, 65.

in large genera variable, 66.

, groups of, suddenly appearing,

340, 344.

beneath Silurian formations, 345,

Species successively appearing, 349.

Species changing simultaneously through-
out the world, 357.

Spencer, Lord, on increase in size of cat-
tie, 46,

, Herbert, Mr., on the first steps in

differentiation, 132.

, on the tendency to an equilibrium

in all forces, 304—5.

Sphex, parasitic, 263.

Spiders, development of, 462.

Sports in plants, 26.

Sprengel, C. C., on crossing, 103.

, on ray-florets, 149.

Squalodon, 363.

Squirrels, gradations in structure, 176.
Staffordshire, heath, changes in, 79.

Stag beetles, fighting, 95.

Star-fishes, eyes of, 182.

, their pedicellarise, 236.

Sterility from changed conditions of life,
25.

of hybrids, 287.

laws of, 292.

causes of, 298.

, from unfavourable conditions, 303.

not induced through natural selec-
tion, 299.

St. Helena, productions of, 414.

St. Hilaire, Aug., on variability of cer-
tain plants, 216.

^jon classification, 436.

St. John, Mr., on habits of cats, 256.
Sting of bee, 204.

Stocks, aboriginal, of domestic animals,

3 ^* .

Strata, thickness of, in Britain, 324.

Stripes on horses, 1 64.

Structure, degrees of utility of, 199.
Struggle for existence, 71.

Succession, geological, 349.

of types in same areas, 372.

Swallow, one species supplanting an-
other, 84-

Swaysland, Mr., on earth adhering to the
feet of migratory birds, 392.

Swifts, nests of, 277.

Swim bladder, 186.

Switzerland, lake habitations of, 32.
System, natural, 433.

Tail of giraffe, 196.

of aquatic animals, 197.

— prehensile, 233.

INDEX

Tail, rudimentaiy, 473.

Tanais, dimorphic, 57.

Tarsi, deficient, 141.

Tausch, Dr., on Umbelliferae, 215.

Teeth and hair correlated, 148.

, rudimentary, embryonic calf, 469,

496.

Tegetmeier, Mr., on cells of bees, 270,

275*

Temminck, on distribution aiding classi-
fication, 437-8.

Tendrils, their development, 241.
Thompson, Sir W., on the age of the
habitable world, 345.

, on the consolidation of the crust

of the earth, 484.

Thouin, on grafts, 298.

Thrush, aquatic species of, 180.

, mocldng, of the Galapagos, 424,

, young of, spotted, 459.

, nest of, 284.

Thwaites, Mr., on acclimatisation, 145.
Thylacinus, 444.

Tierra del Fuego, dogs of, 258.

Timber, drift, 389.

Time, lapse of, 321.

by itself not causing modification,

107.

Titmouse, 178.

Toads on islands, 417.

Tobacco, crossed varieties of, 312.

Tomes, Mr,, on the distribution of bats,
418.

Transitions in varieties rare, 171.
Traquair, Dr., on flatfish, 231.
Trautschold, on intermediate varieties,

332.

Trees on islands belong to peculiar orders,
416,

with separated sexes, 106.

Trifolium pratense, 81, 10 1.

incarnatum, loi.

Trigonia, 356.

Trilobites, 345,

, sudden extinction of, 357.

Trimen, Mr., on imitating-insects, 447.
Trimorphism in plants, 57, 307.
Troglodytes, 284.

Tucu tucu, blind, 142.

Tumbler-pigeons, habits of, hereditary,
257 -

Tumbler, young of, 464.

Turkey cock, tuft of hair on breast, 96.

, naked skin on head, 198.

529

Turkey, young of, instinctively wild, 258.
Turnip and cabbage, analogous variations
of, 160.

Type, unity of, 207.

Types, succession of, in same areas, 37Z
Typotherium, 363.

Udders enlarged by use, 27.

, rudimentary, 469.

Ulex, young leaves of, 459.

Umbelliferae, flowers and seeds of, 150.

, outer and inner florets of, 2x5.

Unity of type, 207.

Uria lacrymans, 99.

Use, effects of, under domestication, 27.

, effects of, in a state of nature, 140.

Utility, how far important in the con-
struction of each part, 199.

Valenciennes, on fresh-water fish, 410.
Variability of mongrels and hybrids,
3 .^ 3 ;

Variation under domestication, 23.

caused by reproductive system be-
ing affected by conditions of life, 24.

under namre, 54.

^,laws of, 138.

correlated, 27, 147, 199.

Variations appear at corresponding ages,
29 . 93 -

analogous in distinct species, 159.

Varieties, natural, 54.

struggle between, 84.

domestic, extinction of, 114.

transitional, rarity of, 171.

when crossed, fertile, 312.

, sterile, 310.

classification of, 440.

Verbascum, sterility of, 289.

^.varieties of crossed, 31 1.

Verlot, M., on double stocks, 279.
Verneuil, M. de, on the succession of
species, 359.

Vibracula of the Polyzoa, 238.

Viola, small imperfect flowers of, 214,
tricolor, 81.

Virchow, on the structure of the crystal-
line lens, 1 83-4.

Virginia, pigs of, 92.

Volcanic islands, denudation of, 323.
Vulture, naked skin on head, 198.

530

INDEX

Wading-birds, ^

Wagner, Dr., on Cecidomyia, 45 o*

, Moritz, on the importance of isola

tion, 109. .

Wallace, Mr., on origin of species, i 9 -
, on the limit of variation under do-
mestication, 52.

, on dimorphic lepidoptera, 57 » 282.

on races in the Malay Archipelago,

on the improvement of the eye

183. ...

on the walking stick insect, 225.

-,on laws of geographical distribu

tion, 3 ^ 5 *

, on the Malay Archipelago, 4^9

, on mimetic animals, 447 - .

Walsh, Mr. B. D., on Phytophagic forms,

-jOn equal variability, 160.

Water, fresh, productions of, 409-
Water-hen, 180.

Waterhouse, Mr., on Australian mar-
supials, 119* , , .

on greatly developed parts bemg

variable, I 53 «

j on the cells of bees, 200.

•, on general affinities, 449* . .

Watson, Mr. H. C., on range o£ varieties
of British plants, 58 -- 9 , 69.

- — , on acclimatisation, 145.

on flora of Azores, 392.

on Alpine plants, 396.^ ^ ^

^ on rarity of intermediate varieties,

172.

on convergence, 132.

,on the indefinite multiplicaUon of

species, 133* . ,

Weale, Mr., on locusts transporting seeds,

White Mountains, flora of, 394.

Whitaker, Mr., on lines of escarpment,
323.

Wichura, Max, on hybrids, $01^ 3 ^ 4 *

Wings, reduction of size, 140.

of insects homologous with branchiae,

186, 187. . . ,

rudimentary, in insects, 409.

Wolf crossed with dog, 257,

of Falkland Isles, 417.

Wollaston, Mr., on varieties of insects, 60.
-,on fossil varieties of shells in Ma-
deira, 64.

on colours of insects on seashore,

^ 39 - , , ,

on wingless beetles, 141.
on rarity of intermediate varieties,

172. .

on insular msects, 414*
on land shells of Madeira natural-
ised, 424.

Wolves, varieties of, 97 -
Woodcock with earth attached to leg,
391-2.

Woodpecker, habits of, 179.

, green colour of, 197-8.

Woodward, Mr., on the duration
specific forms, 332.

-,on Pyrgoma, 342.
on the continuous succession
genera, 352.

, on the succession of types, 372.

World, species changing simultaneously
throughout, 359.

Wrens, nest of, 284. ,

Wright, Mr. Chauncey, on the girafte,

221.

on abrupt modifications, 249.
Wyman, Prof., on correlation of colour
and effects of poison, 27.

1 11 £ ..T.*

Web of feet in water-birds, 181.

Weismann, Prof., on the causes of van-
ability, 24. _

, on rudimentary organs, 472.

West Indian Islands, mammals of, 419.

Westwood, on species in large genera be-
ing closely allied to others, 68.

, on the tarsi of Engidae, 158.

, on the antenna of Hymenopterous

insects, 435.

Whales, 225-^.

Wheat, varieties of, ii 7 *

Youatt, Mr., on selection, 43 -

, on sub-breeds of sheep, 46.

, on rudimentary horns in young cat-
tle, 473-

Zanthoxylon, 216.

Zebra, stripes on, 162-3,
Zeuglodon, 3 ^ 3 -

The Origin of Species - Part 2