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The Origin of Species
THE HARVARD CLASSICS
EDITED BY CHARLES W. ELIOT^, LL.D.
The Origin of Species
By Charles Darwin
Wkh \ntroductions and l:^otes
Yolume ii
P. F. Collier & Son Corporation
NEW YORK
Copyright, 1937
By P. F, Coluer & Son Corporation
Copyright, 1909
By P. F. Collier & Son
Fifty-Third Printing, 1960
Manufactured in U. S. A.
'‘But with, regard to the material worlds we can at least go so
far as this — we can perceive that events are brought about not
by insulated interpositions of Divine power, exerted in each
particular case, but by the establishment of general laws.”
Whewell: Bridgewater Treatise.
“The only distinct meaning of the word ‘natural’ is stated,
fixed, or settled; since what is natural as much requires and pre-
supposes an intelligent agent to render it so, i.e., to effect it
continually or at stated times, as what is supernatural or miracu-
lous does to effect it for once.”
Butler: Analogy of Repealed Religion.
“To conclude, therefore, let no man out of a weak conceit of
sobriety, or an ill-applied moderation, think or maintain, that
a man can search too far or be too well studied in the book of
God’s word, or in the book of God’s works; divinity or philoso-
phy; but rather let men endeavour an endless progress or pro-
ficience in both.”
Bacon: Advancement of Learning.
Down, Bec\enham, Kent,
First Edition, November 2.4th, iS^g.
Sixth Edition, January, i8y2.
CONTENTS
PAGE
EDITOR’S INTRODUCTION 5
AN HISTORICAL SKETCH
Of the Progress of Opinion on the Origin of Species .... 9
INTRODUCTION 19
CHAPTER I
Variation under Domestication 23
CHAPTER II
Variation under Nature 54
CHAPTER III
Struggle for Existence 71
CHAPTER IV
Natural Selection; or the Survival of the Fittest 87
CHAPTER V
Laws of Variation 138
CHAPTER VI
Difficulties of the Theory 169
CHAPTER VII
Miscellaneous Objections to the Theory of Natural Selection 209
CHAPTER VIII
Instinct 251
CHAPTER IX
Hybridism 285
3
4
CONTENTS
CHAPTER X
On the Imperfection of the Geological Record 3^9
CHAPTER XI
On the Geological Succession of Organic Beings ...... 349
CHAPTER XII
Geographical Distribution 37^
CHAPTER XIII
Geographical Distribution — continued 4^9
CHAPTER XIV
Mutual Affinities of Organic Beings; Morphology — Embry-
ology — Rudimentary Organs 43 ^
CHAPTER XV
Recapitulation and Conclusion 47^
Glossary
Index
INTRODUCTORY NOTE
Charles Robert Darwin, born at Shrewsbury, England, on February
12, 1809, came of a family of remarkable intellectual distinction which
is still sustained in the present generation. His father was a successful
physician with remarkable powers of observation, and his grandfather
was Erasmus Darwin, the well-known author of “The Botanic Garden.”
He went to school at Shrewsbury, where he failed to profit from the
strict classical curriculum there in force; nor did the regular professional
courses at Edinburgh University, where he spent two years studying
medicine, succeed in rousing his interest. In 1827 he was entered at
Christ’s College, Cambridge, to study for the B. A. degree, preparatory
to entering the Church; but while there his friendship with Henslow,
the professor of botany, led to his enlarging his general scientific knowl-
edge and finally to his joining the expedition of the “Beagle” in the
capacity of naturalist. From this Darwin returned after a voyage of five
years with a vast first-hand knowledge of geology and zoology, a reputa-
tion as a successful collector, and, most important of all, with the ger-
minal ideas of his theory of evolution. The next few years were spent in
working up the materials he had collected; but his health gave signs of
breaking, and for the rest of his life he suffered constantly, but without
complaint- With extraordinary courage and endurance he took up a life
of seclusion and methodical regularity, and accomplished his colossal
results in spite of the most severe physical handicap. He had married in
1839, three years later he withdrew from London to the little village
of Down, about sixteen miles out, where he spent the rest of his life. His
custom, which was almost a method, was to work till he was on the verge
of complete collapse, and then to take a holiday just sufficient to restore
him to working condition.
As early as 1842 Darwin had thrown into rough form the outlines of
his theory of evolution, but the enormous extent of the investigations he
engaged in for the purpose of testing it led to a constant postponing of
publication. Finally in June, 1858, A. R. Wallace sent him a manuscript
containing a statement of an identical theory of the origin of species,
which had been arrived at entirely independently. On the advice of Lyell,
the geologist, and Hooker, the botanist, Wallace’s paper and a letter of
Darwin’s of the previous year, in which he had outlined his theory to
Asa Gray, were read together on July i, 1858, and published by the
Linnsean Society. In November of the following year “The Origin of
5
6 INTRODUCTORY NOTE
Species” was published, and the great battle was begun between the old
science and the new. This work was followed in 1868 by his ‘‘Variation
of Animals and Plants under Domestication,” that in turn by the
“Descent of Man” in 1871, and that again by “The Expression of the
Emotions in Man and Animals.” Each of these books was the elaboration
or complement of a section of its predecessor. The later years of Darwin’s
life were chiefly devoted to botanical research, and resulted in a series
of treatises of the highest scientific value. He died at Down on April 19,
1882, and is buried in Westminster Abbey.
The idea of the evolution of organisms, so far from originating with
Darwin, is a very old one. Glimpses of it appear in the ancient Greek
philosophers, especially Empedocles and Aristotle; modern philosophy
from Bacon onward shows an increasing definiteness in its grasp of the
conception; and in the age preceding Darwin’s, Buff on, Erasmus Darwin,
and Lamarck had given it a fairly concrete expression. As we approach
the date of the publication of “The Origin of Species” adherence to the
doctrine not only by naturalists but by poets, such as Goethe, becomes
comparatively frequent; and in the six years before the joint announce-
ment of Darwin and Wallace, Herbert Spencer had been supporting and
applying it vigorously in the field of psychology. '
To these partial anticipations, however, Darwin owed little. When he
became interested in the problem, the doctrine of the fixity of species
was still generally held; and his solution occurred to him mainly as the
result of his own observation and thinking. Speaking of the voyage of
the “Beagle,” he says, “On my return home in the autumn of 1836 I
immediately began to prepare my journal for publication, and then saw
how many facts indicated the common descent of species. ... In July
(1837) I opened my first note-book for facts in relation to the Origin of
Species, about which I had long reflected, and never ceased working for
the next twenty years, . . . Had been greatly struck from about the
month of previous March on character of South American fossils, and
species on Galapagos Archipelago. These facts (especially latter) origin
of all my views.” Again, “In October (1838), that is fifteen months after
I had begun my systematic inquiry, I happened to read for amusement
‘Malthus on Population,’ and being well prepared to appreciate the strug-
gle for existence which everywhere goes on from long-continued observa-
tion of the habits of animals and plants, it at once struck me that under
these circumstances favorable variations would tend to be preserved, and
unfavorable ones to be destroyed. The result of this would be the forma-
tion of new species. Here then I had at last got a theory by which to
'Work.”
INTRODUCTORY NOTE 7
From these statements by Darwin himself we can see how far it is
from being the case that he merely gathered the ripe fruit of the labors
of his predecessors. All progress is continuous, and Darwin, like other
men, built on the foundations laid by others; but to say this is not to
deny him originality in the only vital sense of that word. And the impor-
tance of his contribution — in verifying the doctrine of descent, in inter-
preting and applying it, and in revealing its bearings on all departments
of the investigation of nature — ^is proved by the fact that his work opened
a new epoch in science and philosophy. As Huxley said, “Whatever be
the ultimate verdict of posterity upon this or that opinion which Mr.
Darwin has propounded; whatever adumbrations or anticipations of his
doctrines may be found in the writings of his predecessors; the broad
fact remains that, since the publication and by reason of the publication
of ‘The Origin of Species’ the fundamental conceptions and the aims of
the students of living Nature have been completely changed.”
The present year (1909) has seen the celebration of the hundredth
anniversary of Darwin’s birth and the fiftieth anniversary of the publica-
tion of his great work. Among the numerous expressions of honor and
gratitude which the world of science has poured upon his memory, none
is more significant than the volume on “Darwin and Modern Science”
which has been issued by the press of his old University of Cambridge.
In this are collected nearly thirty papers by the leaders of modern science
dealing with the influence of Darwin upon various fields of thought and
research, and with the later developments and modifications of his con-
clusions, Biology, in many different departments. Anthropology, Geology,
Psychology, Philosophy, Sociology, Religion, Language, History, and
Astronomy are all represented, and the mere enumeration suggests the
colossal nature of his achievement and its results.
Yet the spirit of the man was almost as wonderful as his work. His
disinterestedness, his modesty, and his absolute fairness were not only
beautiful in themselves, but remain as a proof of the importance of
character in intellectual labor. Here is his own frank and candid sum-
ming up of his abilities: “My success as a man of science, whatever this
may have amounted to, has been determined, as far as I can judge, by
complex and diversified mental qualities and conditions. Of these, the
most important have been — ^the love of science — ^unbounded patience in
long reflecting over any subject — ^industry in observing and collecting
facts — ^and a fair share of invention as well as of common sense. With
such moderate abilities as I possess, it is truly surprising that I should
have influenced to a considerable extent the belief of scientific men on
some important points.”
AN HISTORICAL SKETCH
OF THE PROGRESS OF OPINION ON
THE ORIGIN OF SPECIES,
PREVIOUSLY TO THE PUBLICATION OF
THE FIRST EDITION OF THIS WORK.
I WILL here give a brief sketch of the progress of opinion on the Origin
of Species. Until recently the great majority of naturalists believed that
species were immutable productions, and had been separately created.
This view has been ably maintained by many authors. Some few natural-
ists, on the other hand, have believed that species undergo modification,
and that the existing forms of life are the descendants by true generation
of pre-existing forms. Passing over allusions to the subject in the classical
writers,^ the first author who in modern times has treated it in a scientific
spirit was Bufifon, But as his opinions fluctuated greatly at difierent
periods, and as he does not enter on the causes or means of the trans-
formation of species, I need not here enter on details.
Lamarck was the first man whose conclusions on the subject excited
much attention. This justly-celebrated naturalist first published his views
in i8oi; he much enlarged them in 1809 in his Thilosophie Zoologique,'
^Aristotle, in his ‘Physicae Auscultationes’ (lib. 2, cap. 8 , s. 2), after remarking
that rain does not fall in order to make the corn grow, any more than it falls
to spoil the farmer’s corn when threshed out of doors, applies the same argument
to organisation; and adds (as translated by Mr. Clair Grece, who first pointed out
the passage to me), “So what hinders the different parts [of the body] from having
this merely accidental relation in nature? as the teeth, for example, grow by necessity,
the front ones sharp, adapted for dividing, and the grinders flat, and serviceable for
masticating the food; since they were not made for the sake of this, but it was the
result of accident. And in like manner as to the other parts in which there appears
to exist an adaptation to an end. Wheresoever, therefore, all things together (that
is, all the parts of one whole) happened like as if they were made for the sake
of something, these were preserved, having been appropriately constituted by an
internal spontaneity; and whatsoever things were not thus constituted, perished, and
still perish.” We here see the principle of natural selection shadowed forth, but how
little Aristotle fully comprehended the principle, is shown by his remarks on the
formation of the teeth.
9
10
HISTORICAL SKETCH
and subsequently, in 1815, in the Introduction to his ‘Hist. Nat. des
Animaux sans Vertebres.’ In these works he upholds the doctrine that
species, including man, are descended from other species. He first did
the eminent service of arousing attention to the probability of all change
in the organic, as well as in the inorganic world, being the result of law,
and not of miraculous interposition. Lamarck seems to have been chiefly
led to his conclusion on the gradual change of species, by the difficulty of
distinguishing species and varieties, by the almost perfect gradation of
forms in certain groups, and by the analogy of domestic productions.
With respect to the means of modification, he attributed something to
the direct action of the physical conditions of life, something to the
crossing of already existing forms, and much to use and disuse, that is,
to the effects of habit. To this latter agency he seems to attribute all the
beautiful adaptations in nature; such as the long neck of the giraffe for
browsing on the branches of trees. But he likewise believed in a law
of progressive development; and as all the forms of life thus tend to
progress, in order to account for the existence at the present day of simple
productions, he maintains that such forms are now spontaneously gener-
ated.^
Geoffroy Saint-Hilaire, as is stated in his ‘Life,’ written by his son,
suspected, as early as 1795, that what we call species are various degenera-
tions of the same type. It was not until 1828 that he published his con-
viction that the same forms have not been perpetuated since the origin
of all things. Geoffroy seems to have relied chiefly on the condition of
life, or the “monde amhiant'' as the cause of change. He was cautious
in drawing conclusions, and did not believe that existing species are now
undergoing modification; and, as his son adds, “C’est done un probleme
a reserver entierement a Tavenir, suppose meme que Tavenir doive avoir
prise sur lui.”
2 1 have taken the date of the first publication of Lamarck from Isidore Geoffroy
Saint-Hilaire’s (‘Hist. Nat. G^n^rale,’ tom. ii. p. 405, 1859) excellent history of
opinion on tfiis subject. In this work a full account is given of Buffon’s conclusions
on the same subject. It is curious how largely my grandfather, Dr. Erasmus Darwin,
anticipated the views and erroneous grounds of opinion of Lamarck in his ‘Zoonomia’
(vol. i. pp. 500-510), published in 1794. According to Isid. Geoffroy there is no
doubt that Goethe was an extreme partisan of similar views, as shown in the
Introduction to a work written in 1794 and 1795, but not published till long
afterwards; he has pointedly remarked (‘Goethe als Naturforscher,’ von Dr. Karl
Meding, s. 34) that the future question for naturalists will be how, for instance,
cattie got their horns, and not for what they are used. It is rather a singular instance
of the manner in which similar views arise at about the same time, that Goethe in
Germany, Dr. Darwin in England, and Geoffroy Saint-Hilaire (as we shall immediately
see) in France, came to the same conclusion on the origin of species, in the years
I794-5-
II
HISTORICAL. SKETCH
In 1813, Dr. W. C. Wells read before the Royal Society ‘An Account of
a White Female, part of whose skin resembles that of a Negro’; but his
paper was not published until his famous ‘Two Essays upon Dew and
Single Vision’ appeared in 1818. In this paper he distincdy recognises the
principle of natural selection, and this is the first recognition which has
been indicated; but he applies it only to the races of man, and to certain
characters alone. After remarking that negroes and mulattoes enjoy an
immunity from certain tropical diseases, he observes, firstly, that all ani-
mals tend to vary in some degree, and, secondly, that agriculturists im-
prove their domesticated animals by selection; and then, he adds, but
what is done in this latter case “by art, seems to be done with equal
efficacy, though more slowly, by nature, in the formation of varieties of
mankind, fitted for the country which they inhabit. Of the accidental
varieties of man, which would occur among the first few and scattered
inhabitants of the middle regions of Africa, some one would be better
fitted than the others to bear the diseases of the country. This race would
consequendy multiply, while the others would decrease; not only from
their inability to sustain the attacks of disease, but from their incapacity
of contending with their more vigorous neighbours. The colour of this
vigorous race I take for granted, from what has been already said, would
be dark. But the same disposition to form varieties still existing, a darker
and a darker race would in the course of time occur: and as the darkest
would be the best fitted for the climate, this would at length become the
most prevalent, if not the only race, in the particular country in which
it had originated.” He then extends these same views to the white
inhabitants of colder climates. I am indebted to Mr. Rowley, of the
United States, for having called my attention, through Mr. Brace, to the
above passage in Dr. Wells’s work.
The Hon. and Rev. W. Herbert, afterwards Dean of Manchester, in
the fourth volume of the ‘Horticultural Transactions,’ 1822, and in his
work of the ‘Amaryllidaceae’ (1837, pp. 19, 339), declares that “horti-
cultural experiments have established, beyond the possibility of refuta-
tion, that botanical species are only a higher and more permanent class
of varieties.” He extends the same view to animals. The Dean believes
that single species of each genus were created in an originally highly
plastic condition, and that these have produced, chiefly by intercrossing,
but likewise by variation, all our existing species.
In 1826 Professor Grant, in the concluding paragraph in his well-
known paper (‘Edinburgh Philosophical Journal,’ vol. xiv. p. 283) on the
Spongilla, clearly declares his belief that species are descended from other
12
HISTORICAL SKETCH
species, and that they become improved in the course of modification.
This same view was given in his Fifty-fifth Lecture, published in the
‘Lancet’ in 1834.
In 1831 Mr. Patrick Matthew published his work on ‘Naval Timber
and Arboriculture,’ in which he gives precisely the same view on ‘the
origin of species as that (presendy to be alluded to) propounded by Mr.
Wallace and myself in the ‘Linnean Journal,’ and as that enlarged in the
present volume. Unfortunately the view was given by Mr. Matthew very
briefly in scattered passages in an appendix to a work on a different sub-
ject, so that it remained unnoticed until Mr. Matthew himself drew
attention to it in the ‘Gardener’s Chronicle,’ on April 7th, i860. The
differences of Mr. Matthew’s view from mine are not of much impor-
tance: he seems to consider that the world was nearly depopulated at
successive periods, and then re-stocked; and he gives as an alternative,
that new forms may be generated “without the presence of any mould
or germ of former aggregates.” I am not sure that I understand some
passages; but it seems that he attributes much influence to the direct
action of the conditions of life. He clearly saw, however, the full force
of the principle of natural selection.
The celebrated geologist and naturalist, Von Buch, in his excellent
‘Description Physique des Isles Canaries’ (1836, p. 147), clearly expresses
his belief that varieties slowly become changed into permanent species,
which are no longer capable of intercrossing.
Rafinesque, in his ‘New Flora of North America,’ published in 1836,
wrote (p. 6) as follows: — “All species might have been varieties once, and
many varieties are gradually becoming species by assuming constant and
peculiar characters”; but farther on (p. 18) he adds, “except the original
types or ancestors of the genus.”
In 1843-44 Professor Haldeman (‘Boston Journal of Nat. Hist. U.
States,’ vol. iv. p. 468) has ably given the arguments for and against the
hypothesis of the development and modification of species: he seems to
lean towards the side of change.
The ‘Vestiges of Creation’ appeared in 1844. tenth and much
improved edition (1853) the anonymous author says (p. 155): — “The
proposition determined on after much consideration is, that the several
series of animated beings, from the simplest and oldest up to the highest
and most recent, are, under the providence of God, the results, first, of an
impulse which has been imparted to the forms of life, advancing them,
in definite times, by generation, through grades of organisation terminat-
ing in the highest dicotyledons and vertebrata, these grades being few in
HISTORICAL SKETCH I3
number, and generally marked by intervals of organic character, which
we find to be a practical difficulty in ascertaining affinities; second, of
another impulse connected with the vital forces, tending, in the course of
generations, to modify organic structures in accordance with external
circumstances,! as food, the nature of the habitat, and the meteoric
agencies, these being the adaptations’ of the natural theologian.” The
author apparendy believes that organisation progresses by sudden leaps,
but that the effects produced by the conditions of life are gradual. He
argues with much force on general grounds that species are not immu-
table productions. But I cannot see how the two supposed '‘impulses”
account in a scientific sense for the numerous and beautiful co-adaptations
which we see throughout nature; I cannot see that we thus gain any
insight how, for instance, a woodpecker has become adapted to its peculiar
habits of life. The work, from its powerful and brilliant style, though
displaying in the earlier editions little accurate knowledge and a great
want of scientific caution, immediately had a very wide circulation. In
my opinion it has done excellent service in this country in calling attention
to the subject, in removing prejudice, and in thus preparing the ground
for the reception of analogous views.
In 1846 the veteran geologist M. J. d’Omalius d’Halloy published in an
excellent though short paper ('Bulletins de I’Acad. Roy. Bruxelles,’ tom.
xiii. p. 581) his opinion that it is more probable that new species have
been produced by descent with modification than that they have been
separately created: the author first promulgated this opinion in 1831.
Professor Owen, in 1849 (‘Nature of Limbs,’ p. 86), wrote as follows:
"The archetypal idea was manifested in the flesh under diverse such
modifications, upon this planet, long prior to the existence of those
animal species that actually exemplify it. To what natural laws or
secondary causes the orderly succession and progression of such organic
phenomena may have been committed, we, as yet, are ignorant.” In his
address to the British Association, in 1858, he speaks (p. li.) of "the
axiom of the continuous operation of creative power, or of the ordained
becoming of living things.” Farther on (p. xc.), after referring to
geographical distribution, he adds, "These phenomena shake our confi-
dence in the conclusion that the Apteryx of New Zealand and the Red
Grouse of England were distinct creations in and for those islands
respectively. Always, also, it may be well to bear in mind that by the
word 'creation’ the zoologist means 'a process he knows not what.’ ” He
amplifies this idea by adding that when such cases as that of the Red
Grouse are "enumerated by the zoologist as evidence of distinct creation
14 HISTORICAL SKETCH
of the bird in and for such islands, he chiefly expresses that he knows not
how the Red Grouse came to be there, and there exclusively; signifying
also, by this mode of expressing such ignorance, his belief that both the
bird and the islands owed their origin to a great first Creative Cause.”
If we interpret these sentences given in the same address, one by the
other, it appears that this eminent philosopher felt in 1858 his confidence
shaken that the Apteryx and the Red Grouse first appeared in their
respective homes, “he knew not how,” or by some process “he knew not
what.”
This address was delivered after the papers by Mr. Wallace and myself
on the Origin of Species, presendy to be referred to, had been read
before the Linnean Society. When the first edition of this work was
published, I was so completely deceived, as were many others, by such
expressions as “the continuous operation of creative power,” that I
included Professor Owen with other palaeontologists as being firmly con-
vinced of the immutability of species; but it appears (‘Anat. of Verte-
brates,’ vol. iii. p. 796) that this was on my part a preposterous error.
In the last edition of this work I inferred, and the inference still seems
to me perfectly just, from a passage beginning with the words “no
doubt the type-form,” etc. (Ibid., vol. i. p. xxxv.), that Professor Owen
admitted that natural selection may have done something in the forma-
tion of a new species; but this it appears (Ibid., vol. iii. p. 798) is inac-
curate and without evidence. I also gave some extracts from a corre-
spondence between Professor Owen and the Editor of the ‘London
Review,’ from which it appeared manifest to the Editor as well as to
myself, that Professor Owen claimed to have promulgated the theory of
natural selection before I had done so; and I expressed my surprise and
satisfaction at this announcement; but as far as it is possible to under-
stand certain recently published passages (Ibid., vol. iii. p. 798) I have
either partially or wholly again fallen into error. It is consolatory to me
that others find Professor Owen’s controversial writings as dijfScult to
understand and to reconcile with each other, as I do. As far as the
mere enunciation of the principle of natural selection is concerned, it is
quite immaterial whether or not Professor Owen preceded me, for both
of us, as shown in this historical sketch, were long ago preceded by Dr.
Wells and Mr. Matthews.
M. Isidore Geoffrey Saint-Hilaire, in his lectures delivered in 1850
(of which a resume appeared in the ‘Revue et Mag. de Zoolog.,’ Jan.
1851), briefly gives his reason for believing that specific characters “sont
fix&, pour chaque espke, tant qu’elle se perp^tue au milieu des memes
HISTORICAL SKETCH 1 5
circonstances: ils se modifient, si les circonstances ambiantes viennent a
changer.” “En r&um^ V observation des animaux sauvages demontre
deja la variabilite limitee des especes, Les experiences sur les animaux
sauvages devenus domestiques, et sur les animaux domestiques redevenus
sauvages, la demontrent plus clairement encore. Ces memes experiences
prouvent, de plus, que les differences produites peuvent ^tre de valeur
generique.” In his ‘Hist. Nat. Generale’ (tom ii. p. 340, 1859) he ampli-
fies analogous conclusions.
From a circular lately issued it appears that Dr. Freke, in 1851
(‘Dublin Medical Press,' p. 322), propounded the doctrine that all organic
beings have descended from one primordial form. His grounds of belief
and treatment of the subject are wholly different from mine; but as Dr.
Freke has now (1861) published his Essay on the ‘Origin of Species by
means of Organic Affinity,’ the difficult attempt to give any idea of his
views would be superfluous on my part.
Mr, Herbert Spencer, in an essay (originally published in the ‘Leader,’
March, 1852, and republished in his ‘Essays,’ in 1858), has contrasted the
theories of the Creation and the Development of organic beings with
remarkable skill and force. He argues from the analogy of domestic
productions, from the changes which the embryos of many species
undergo, from the difficulty of distinguishing species and varieties,
and from the principle of general gradation, that species have been
modified; and he attributes the modification to the change of circum-
stances. The author (1855) has also treated Psychology on the principle
of the necessary acquirement of each mental power and capacity by
gradation.
In 1852 M. Naudin, a distinguished botanist, expressly stated, in an
admirable paper on the Origin of Species (‘Revue Horticole,’ p. 102;
since partly republished in the ‘Nouvelles Archives du Museum,’ tom. i,
p. 171), his belief that species are formed in an analogous manner as
varieties are under cultivation; and the latter process he attributes to
man’s power of selection. But he does not show how selection acts under
nature. He believes, like Dean Herbert, that species, when nascent, were
more plastic than at present. He lays weight on what he calls the prin-
ciple of finality, “puissance mysteriuse, indetermin^e; fatalite pour les
uns; pour les autres, volonte providentielle, dont Taction incessante sur
les etres vivants determine, a toutes les epoques de Texistence du monde,
la forme, le volume, et la duree de chacun d’eux en raison de sa destin^e
dans Tordre de choses dont il fait partie. C’est cette puissance qui
harmonise chaque membre a Tensemble, en Tappropriant a la function
HISTORICAL SKETCH
l6
qu’il doit remplir dans Torganisme gen^rale de la nature, fonction qux
est pour lui sa raison d’etre/’ ^
In 1853 a celebrated geologist, Count Keyserling (‘Bulletin de la Soc,
Geolog.,’ 2nd ser., tom. x. p. 357), suggested that as new diseases, sup-
posed to have been caused by some miasma, have arisen and spread
over the world, so at certain periods the germs of existing species may
have been chemically affected by circumambient molecules of a par-
ticular nature, and thus have given rise to new forms.
In this same year, 1853, Dr. Schaaffhausen published an excellent
pamphlet (‘Verhand. des Naturhist. Vereins der Preuss. Rheinlands,’
etc.), in which he maintains the development of organic forms on the
earth. He infers that many species have kept true for long periods,
whereas a few have become modified. The distinction of species he
explains by the destruction of intermediate graduated forms. “Thus living
plants and animals are not separated from the extinct by new creations,
but are to be regarded as their descendants through continued repro-
duction.”
A well-known French botanist, M. Lecoq, writes in 1854 (Ttudes sur
Geograph. Bot.,’ tom. i. p. 250), “On voit que nos recherches sur la
fixite ou la variation de I’espke, nous conduisent directement aiix idees
emises, par deux hommes justement celebres, Geofiroy Saint-Hilaire et
Goethe.” Some other passages scattered through M. Lecoq’s large work,
make it a little doubtful how far he extends his views on the modification
of species.
The ‘Philosophy of Creation’ has been treated in a masterly manner
by the Rev. Baden Powell, in his ‘Essays on the Unity of Worlds,’ 1855.
Nothing can be more striking than the manner in which he shows that
the introduction of new species is “a regular, not a casual phenomenon,”
or, as Sir John Herschel expresses it, “a natural in contradistinction to
a miraculous process.”
The third volume of the ‘Journal of the Linnean Society’ contains
papers, read July ist, 1858, by Mr. Wallace and myself, in which, as
3 From references in Bronn’s ‘Untersuchungen iiber die Entwickelungs-Gesetze,’
it appears that the celebrated botanist and palaeontologist, Unger, published, in 1852,
his belief that species undergo development and modification. Dalton, likewise, in
Pander and Dalton’s work on Fossil Sloths, expressed, in 1821, a similar belief.
Similar views have, as is well known, been maintained by Oken in his mystical
*Natur-Philosophie.’ From other references in Godron’s work *Sur I’Espbce,’ it seems
that Bory St. Vincent, Burdach, Poiret, and Fries, have all admitted that new species
are continually being produced.
I may add, that of the thirty-four authors named in this Historical Sketch, who
believe in the modification of species, or at least disbelieve in separate acts of creation,
twenty-seven have written on special branches of natural history or geology.
HISTORICAL SKETCH 1 7
stated in the introductory remarks to this volume, the theory of Natural
Selection is promulgated by Mr. Wallace with admirable force and
clearness.
Von Baer, towards whom all zoologists feel so profound a respect,
expressed about the year 1859 Prof. Rudolph Wagner, ‘Zoologisch-
Anthropologische Untersuchungen,’ 1861, s. 51) his conviction, chiefly
grounded on the laws of geographical distribution, that forms now
perfectly distinct have descended from a single parent-form.
In June, 1859, Professor Huxley gave a lecture before the Royal Institu-
tion on the ‘Persistent Types of Animal Life.’ Referring to such cases,
he remarks, “It is difficult to comprehend the meaning of such facts as
these, if we suppose that each species of animal and plant, or each great
type of organisation, was formed and placed upon the surface of the
globe at long intervals by a distinct act of creative power; and it is well
to recollect that such an assumption is as unsupported by tradition or
revelation as it is opposed to the general analogy of nature. If, on the
other hand, we view ‘Persistent Types’ in relation to that hypothesis
which supposes the species living at any time to be the result of the
gradual modification of pre-existing species, a hypothesis, which, though
unproven, and sadly damaged by some of its supporters, is yet the only
one to which physiology lends any countenance; their existence would
seem to show that the amount of modification which living beings have
undergone during geological time is but very small in relation to the
whole series of changes which they have suffered.”
In December, 1859, Dr. Hooker published his ‘Introduction to the
Australian Flora.’ In the first part of this great work he admits the
truth of the descent and modification of species, and supports this doc-
trine by many original observations.
The first edition of this work was published on November 24th, 1859,
and the second edition on January 7th, i860.
INTRODUCTION
When on board H.M.S. ‘Beagle,’ as naturalist, I was much struck with
certain facts in the distribution of the organic beings inhabiting South
America, and in the geological relations of the present to the past inhabi-
tants of that continent. These facts, as will be seen in the latter chapters
of this volume, seemed to throw some light on the origin of species —
that mystery of mysteries, as it has been called by one of our greatest
philosophers. On my return home, it occurred to me, in 1837, that some-
thing might perhaps be made out on this question by patiently accumu-
lating and reflecting on all sorts of facts which could possibly have any
bearing on it. After five years’ work I allowed myself to speculate on the
subject, and drew up some short notes; these I enlarged in 1844 into a
sketch of the conclusions, which then seemed to me probable; from that
period to the present day I have steadily pursued the same object. I hope
that I may be excused for entering on these personal details, as I give
them to show that I have not been hasty in coming to a decision.
My work is now (1859) nearly finished; but as it will take me many
more years to complete it, and as my health is far from strong, I have
been urged to publish this Abstract. I have more especially been induced
to do this, as Mr. Wallace, who is now studying the natural history of
the Malay Archipelago, has arrived at almost exactiy the same general
conclusions that I have on the origin of species. In 1858 he sent me a
memoir on this subject, with a request that I would forward it to Sir
Charles Lyell, who sent it to the Linnean Society, and it is published in
the third volume of the Journal of that Society. Sir C. Lyell and Dr.
Hooker, who both knew of my work — ^the latter having read my sketch
of 1844 — ^honoured me by thinking it advisable to publish, with Mr.
Wallace’s excellent memoir, some brief extracts from my manuscripts.
This Abstract, which I now publish, must necessarily be imperfect. I
cannot here give references and authorities for my several statements; and
I must trust to the reader reposing some confidence in my accuracy. No
doubt errors will have crept in, though I hope I have always been cautious
in trusting to good authorities alone. I can here give only the general
conclusions at which I have arrived, with a few facts in illustration, but
which, I hope, in most cases will suffice. No one can feel more sensible
than I do of the necessity of hereafter publishing in detail all the facts,
19
20
INTRODUCTION
with references, on which my conclusions have been grounded; and I
hope in a future work to do this. For I am well aware that scarcely a
single point is discussed in this volume on which facts cannot be adduced,
often apparently leading to conclusions directly opposite to those at which
I have arrived. A fair result can be obtained only by fully stating and
balancing the facts and arguments on both sides of each question; and
this is here impossible.
I much regret that want of space prevents my having the satisfaction of
acknowledging the generous assistance which I have received from very
many naturalists, some of them personally unknown to me. I cannot,
however, let this opportunity pass without expressing my deep obliga-
tions to Dr, Hooker, who, for the last fifteen years,, has aided me in
every possible way by his large stores of knowledge and his excellent
judgment.
In considering the origin of species, it is quite conceivable that a
naturalist, reflecting on the mutual aiEnities of organic beings, on their
embryological relations, their geographical distribution, geological succes-
sion, and other such facts, might come to the conclusion that species have
not been independently created, but had descended, like varieties, from
other species. Nevertheless, such a conclusion, even if well founded,
would be unsatisfactory, until it could be shown how the innumerable
species inhabiting this world have been modified, so as to acquire that
perfection of structure and coadaptation which justly excites our admira-
tion. Naturalists continually refer to external conditions, such as climate,
food, etc., as the only possible cause of variation. In one limited sense, as
we shall hereafter see, this may be true; but it is preposterous to attribute
to mere external conditions, the stn^g^e, for instance, of the wood-
pecker, with its feet, tail, beak, and' tongue, so admirably adapted to
catch insects under the bark of trees. In the case of the mistletoe, which
draws its nourishment from certain trees, which has seeds that must be
transported by certain birds, and which has flowers with separate sexes
absolutely requiring the agency of certain insects to bring pollen from one
flower to the other, it is equally preposterous to account for the structure
of this parasite, with its relations to several distinct organic beings, by
the effects of external conditions, or of habit, or of the volition of the
plant itself.
It is, therefore, of the highest importance to gain a clear insight into the
means of modification and coadaptation. At the commencement of my
observations it seemed to me probable that a careful study of domesticated
animals and of cultivated plants would offer the best chance of making
out this obscure problem. Nor have I been disappointed; in this and in
INTRODUCTION 21
all other perplexing cases I have invariably found that our knowledge,
imperfect though it be, of variation under domestication, afforded the
best and safest clue. I may venture to express my conviction of the high
value of such studies, although they have been very commonly neglected
by naturalists.
From these considerations, I shall devote the first chapter of this
Abstract to Variation under Domestication. We shall thus see that a
large amount of hereditary modification is at least possible; and, what is
equally or more important, we shall see how great is the power of man in
accumulating by his Selection successive slight variations. I will then pass
on the variability of species in a state of nature; but I shall, unfortunately,
be compelled to treat this subject far too briefly, as it can be treated
properly only by giving long catalogues of facts. We shall, however, be
enabled to discuss what circumstances are most favourable to variation.
In the next chapter the Struggle for Existence amongst all organic beings
throughout the world, which inevitably follows from the high geometri-
cal ratio of their increase, will be considered. This is the doctrine of
Malthus, applied to the whole animal and vegetable kingdoms. As many
more individuals of each species are born than can possibly survive; and
as, consequently, there is a frequently recurrent struggle for existence, it
follows that any being, if it vary however slightly in any manner profit-
able to itself, under the complex and sometimes varying conditions of
life, will have a better chance of surviving, and thus be naturally selected.
From the strong principle of inheritance, any selected variety will tend
to propagate its new and modified form.
This fundamental subject of Natural Selection will be treated at some
length in the fourth chapter; and we shall then see how Natural Selec-
tion almost inevitably causes much Extinction of the less improved forms
of life, and leads to what I have called Divergence of Character. In the
next chapter I shall discuss the complex and little known laws of varia-
tion. In the five succeeding chapters, the most apparent and gravest
difflculties in accepting the theory will be given: namely, first, the diffi-
culties of transitions, or how a simple being or a simple organ can be
changed and perfected into a highly developed being or into an elab-
orately constructed organ; secondly, the subject of Instinct, or the mental
powers of animals; thirdly, Hybridism, or the infertility of species and
the fertility of varieties when intercrossed; and fourthly, the imperfection
of the Geological Record. In the next chapter I shall consider the geologi-
cal succession of organic beings throughout time; in the twelfth and
thirteenth, their geographical distribution throughout space; in the four-
teenth, their classification or mutual affinities, both when mature and in
22
INTRODUCTION
an embryonic condition. In the last chapter I shall give a brief recapitu-
lation of the whole work, and a few concluding remarks.
No one ought to feel surprise at much remaining as yet unexplained
in regard to the origin of species and varieties, if he make due allowance
for our profound ignorance in regard to the mutual relations of the many
beings which live around us. Who can explain why one species ranges
widely and is very numerous, and why another allied species has a narrow
range and is rare? Yet these relations are of the highest importance, for
they determine the present welfare, and, as I believe, the future success
and modification of every inhabitant of this world. Still less do we know
of the mutual relations of the innumerable inhabitants of the world dur-
ing the many past geological epochs in its history. Although much
remains obscure, and will long remain obscure, I can entertain no doubt,
after the most deliberate study and dispassionate judgment of which I am
capable, that the view which most naturalists until recentiy entertained,
and which I formerly entertained — ^i^amely, that each species has been
independendy created — is erroneous. ' I am fully convinced that species
are not immutable; but that those belonging to what are called the same
genera are lineal descendants of some other and generally extinct species,
in the same manner as the acknowledged varieties of any one species are
the descendants of that species. Furthermore, I am convinced that
Natural Selection has been the most important, but not the exclusive,
means of modification/
ORIGIN OF SPECIES
CHAPTER I
Variation under Domestication
Causes of variability — ^EfiFects of habit and the use or disuse of parts — ■
Correlated variation — ^Inheritance — ^Character of domestic varieties
— ^Difficulty of distinguishing between varieties and species — Origin
of domestic varieties from one or more species — Domestic pigeons,
their differences and origin — ^Principles of selection, anciently fol-
lowed, their effects — ^Methodical and unconscious selection — Un-
known origin of our domestic productions — Circumstances fav-
ourable to man’s power of selection.
CAUSES OF variability
W HEN we compare the individuals of the same variety or
sub-variety of our older cultivated plants and animals, one
of the first points which strikes us is, that they.generally dif-
fer more from each other than do the individuals of any one species
or variety in a state of nature. And if we reflect on the vast diversity
of the plants and animals which have been cultivated, and which have
varied during all ages under the most different climates and treat-
ment, we are driven to conclude that this great variability is due to
our domestic productions having been raised under conditions of life
not so uniform as, and somewhat different from, those to which the
parent species had been exposed under nature. There is, also, some
probability in the view propounded by Andrew Knight, that this
variability may be partly connected with excess of food. It seems
clear that organic beings must be exposed during several generations
to new conditions to cause- any great amount of variation; and that,
when the organisation has once begun to vary, it generally continues
varying for many generations. No case is on record of a variable or-
ganism ceasing to vary under cultivation. Our oldest cultivated
plants, such as wheat, still yield new varieties: our oldest domesticated
animals are still capable of rapid improvement or modification.
23
ORIGIN OF SPECIES
24
As far as I am able to judge, after long attending to the subject,
the conditions of life appear to act in two ways, — directly on the
whole organisation or on certain parts alone, and indirectly by affect-
ing the reproductive system- With respect to the direct action, we
must bear in mind that in every case, as Professor Weismann has
lately insisted, and as I have incidentally shown in my work on
“‘Variation under Domestication,’ there are two factors: namely, the
nature of the organism, and the nature of the conditions. The former
seems to be much the more important; for nearly similar variations
sometimes arise under, as far as we can judge, dissimilar conditions;
and, on the other hand, dissimilar variations arise under conditions
which appear to be nearly uniform. The effects on the offspring are
either definite or indefinite. They may be considered as definite when
all or nearly all the offspring of individuals exposed to certain con-
ditions during several generations are modified in the same manner.
It is extremely difficult to come to any conclusion in regard to the
extent of the changes which have been thus definitely induced. There
can, however, be little doubt about many slight changes, such as size
from the amount of food, colour from the nature of the food, thick-
ness of the skin and hair from climate, etc. Each of the endless varia-
tions which we see in the plumage of our fowls must have had some
efficient cause; and if the same cause were to act uniformly during a
long series of generations on many individuals, all probably would be
modified in the same manner. Such facts as the complex and extra-
ordinary outgrowths which variably follow from the insertion of a
minute drop of poison by a gall-producing insect, show us what
singular modifications might result in the case of plants from a
chemical change in the nature of the sap.
Indefinite variability is a much more common result of changed
conditions than definite variability, and has probably played a more
important part in the formation of our domestic races. We see in-
definite variability in the endless slight peculiarities which distinguish
the individuals of the same species, and which cannot be accounted
for by inheritance from either parent or from some more remote an-
xestor. Even strongly marked differences occasionally appear in the
young of the same litter, and in seedlings from the same seed capsule.
At long intervals of time, out of millions of individuals reared in the
VARIATION UNDER DOMESTICATION 2 ^
same country and fed on nearly the same food, deviations of struc-
ture so strongly pronounced as to deserve to be called monstrosities
arise; but monstrosities cannot be separated by any distinct line from
slighter variations. All such changes of structure, whether extremely
slight or strongly marked, which appear amongst many individuals
living together, may be considered as the indefinite effects of the con- .
ditions of life on each individual organism, in nearly the same man-
ner as the chill affects different men in an indefinite manner, accord-
ing to their state of body or constitution, causinj^ coughs or colds,
rheumatism or inflammation of various organs.
With respect to what I have called the indirect action of changed
conditions, namely, through the reproductive system of being
affected, we may infer that variability is thus induced, partly from
the fact of this system being extremely sensitive to any change in
the conditions, and partly from the similarity, as Kolreuter and others
have remarked, between the variability which follows from the
crossing of distinct species, and that which may be observed with
plants and animals when reared under new or unnatural conditions.
Many facts clearly show how eminently susceptible the reproductive
system is to very slight changes in the surrounding conditions. Noth-
ing is more easy than to tame an animal, and few things more diffi-
cult than to get it to breed freely under confinement, even when the
male and female unite. How many animals there are which will
not breed, though kept in an almost free state in their native coun-
try! This is generally, but erroneously, attributed to vitiated in-
stincts. Many cultivated plants display the utmost vigour, and yet
rarely or never seed! In some few cases it has been discovered that a
very trifling change, such as a little more or less water at some par-
ticular period of growth, will determine whether or not a plant will
produce seeds. I cannot here give the details which I have collected
and elsewhere published on this curious subject; but to show how
singular the laws are which determine the reproduction of animals
under confinement, I may mention that carnivorous animals,
even from the tropics, breed in this country pretty freely under
confinement, with the exception of the plantigrades or bear fam-^^
ily, which seldom produce young; whereas carnivorous birds, with
the rarest exceptions, hardly ever lay fertile eggs. Many exotic
ORIGIN OF SPECIES
26
plants have pollen utterly worthless, in the same condition as in
the most sterile hybrids. When, on the one hand, we see domesti-
cated animals and plants, though often weak and sickly, breeding
freely under confinement; and when, on the other hand, we see indi-
viduals, though taken young from a state of nature perfectly tamed,
long-lived and healthy (of which I could, give numerous instances),
yet having their reproductive system so seriously affected by unper-
ceived causes as to fail to act, we need not be surprised at this system,
when it does act under confinement, acting irregularly, and produc-
ing offspring somewhat unlike their parents. I may add, that as
some organisms breed freely under the most unnatural conditions
(for instance, rabbits and ferrets kept in hutches), showing that their
reproductive organs are not easily affected; so will some animals
and plants withstand domestication or cultivation, and vary very
slightly—perhaps hardly more than in a state of nature.
Some naturalists have maintained that all variations are connected
with the act of sexual reproduction; but this is certainly an error; for
I have given in another work a long list of “sporting plants,” as they
are called by gardeners; that is, of plants which have suddenly pro-
duced a single bud with a new and sometimes widely different char-
acter from that of the other buds on the same plant. These bud varia-
tions, as they may be named, can be propagated by grafts, offsets,
etc., and sometimes by seed. They occur rarely under nature, but
are far from rare under culture. As a single bud out of the many
thousands, produced year after year on the same tree under uniform
conditions, has been known suddenly to assume a new charac-
ter; and as buds on distinct trees, growing under different condi-
tions, have sometimes yielded nearly the same variety — for in-
stance, buds on peach-trees producing nectarines, and buds on
common roses producing moss roses— we clearly see that the nature
of the condition is of subordinate importance in comparison with
the nature of the organism in determining each particular form of
variation; perhaps of not more importance than the nature of the
spark, by which a mass of combustible matter is ignited, has in
determining the nature of the flames.
VARIATION UNDER DOMESTICATION
27
EFFECTS OF HABIT AN0 OF THE USE OR DISUSE OF PARTS;
CORRELATED VARIATION; INHERITANCE
Changed habits produce an inherited effect, as in the period of the
flowering of plants when transported from one climate to another.
With animals the increased use or disuse of parts has had a more
marked influence; thus I find in the domestic duck that the bones of
the wing weigh less and the bones of the leg more, in proportion to
the whole skeleton, than do the same bones in the wild duck; and this
change may be safely attributed to the domestic duck flying much
less, and walking more, than its wild parents. The great and in-
herited development of the udders in cows and goats in countries
where they are habitually milked, in comparison with these organs
in other countries, is probably another instance of the effects of use.
Not one of our domestic animals can be named which has not in
some country drooping ears; and the view which has been suggested
that the drooping is due to disuse of the muscles of the ear, from the
animals being seldom much alarmed, seems probable.
Many laws regulate variation, some few of which can be dimly
seen, and will hereafter be briefly discussed. I will here only allude
to what may be called correlated variation. Important, changes in the
embryo or larva will probably entail changes in the mature animal.
In monstrosities, the correlations between quite distinct parts are
very curious; and many instances are given in Isidore Geoffroy St.
Hilaire’s great work on this subject. Breeders believe that long limbs
are almost always accompanied by an elongated head. Some instances
of correlation are quite whimsical : thus cats which are entirely white
and have blue eyes are generally deaf; but it has been lately stated
by Mr. Tait that this is confined to the males. Colour and constitu-
tional peculiarities go together, of which many remarkable cases
could be given amongst animals and plants. From facts collected by
Heusinger, it appears that white sheep and pigs are injured by cer-
tain plants, whilst dark-coloured individuals escape: Professor
Wyman has recently communicated to me a good illustration of this
fact; on asking some farmers in Virginia how it was that all their
pigs were black, they informed him that the pigs ate the paintroot
(Lachnanthes), which colored their bones pink, and which caused the
hoofs of all but the black varieties to drop off; and one of the
ORIGIN OF SPECIES
28
“crackers” Virginia squatters) added, “We select the black
members of a litter for raising, as they alone have a good chance of
living.” Hairless dogs have imperfect teeth; long-haired and coarse-
haired animals are apt to have, as is asserted, long or many horns;
pigeons ^vith feathered feet have skin between their outer toes;
pigeons with short beaks have small feet, and those with long beaks
large feet. Hence if man goes on selecting, and thus augmenting, any
peculiarity, he will almost certainly modify unintentionally other
parts of the structure, owing to the mysterious laws of correlation.
The results of the various, unknown, or but dimly understood laws
of variation are infinitely complex and diversified. It is well worth
while carefully to study the several treatises on some of our old culti*
vated plants, as on the hyacinth, potato, even the dahlia, etc.; and it is
really surprising to note the endless points of structure and consti-
tution in which the varieties and sub-varieties differ slightly from
each other. The whole organisation seems to have become plastic,
and departs in a slight degree from that of the parental type.
Any variation which is not inherited is unimportant for us. But
the number and diversity of inheritable deviations of structure, both
those of slight and those of considerable physiological importance,
are endless. Dr. Prosper Lucas’s treatise, in two large volumes, is
the fullest and the best on this subject. No breeder doubts how strong
is the tendency to inheritance; that like produces like, is his funda-
mental belief: doubts have been thrown on this principle only by
theoretical writers. When any deviation of structure often appears,
and we see it in the father and child, we cannot tell whether it may
not be due to the same cause having acted on both; but when amongst
individuals, apparently exposed to the same conditions, any very rare
deviation, due to some extraordinary combination of circumstances,
appears in the parent — say, once amongst several million individuals
— and it reappears in the child, the mere doctrine of chances almost
compels us to attribute its reappearance to inheritance. Every one
must have heard of cases of albinism, prickly skin, hairy bodies, etc.,
appearing in several members of the same family. If strange and
rare deviations of structure are really inherited, less strange and com-
moner deviations may be freely admitted to be inheritable. Per-
haps the correct way of viewing the whole subject would be, to look
VARIATION UNDER DOMESTICATION 29
at the inheritance of every character whatever as the rule, and non-
inheritance as the anomaly.
The laws governing inheritance are for the most part unknown.
No one can say why the same peculiarity in different individuals of
the same species, or in different species, is sometimes inherited and
sometimes not so; why the child o&en reverts in certain characters to
its grandfather or grandmother or more remote ancestor; why a
peculiarity is often transmitted from one sex to both sexes, or to one
sex alone, more commonly but not exclusively to the like sex. It is
a fact of some importance to us, that peculiarities appearing in the
males of our domestic breeds are often transmitted, either exclusively
or in a much greater degree, to the males alone. A much more impor-
tant rule, which I think may be trusted, is that, at whatever period
of life a peculiarity first appears, it tends to reappear in the off-
spring at a corresponding age, -though sometimes earlier. In many
cases this could not be otherwise; thus the inherited peculiarities in
the horns of cattle could appear only in the offspring when nearly
mature; peculiarities in the silkworm are known to appear at the cor-
responding caterpillar or cocoon stage. But hereditary diseases and
some other facts make me believe that the rule has a wider extension,
and that, when there is no apparent reason why a peculiarity should
appear at any particular age, yet that it does tend to appear in the
offspring at the same period at which it first appeared in the parent.
I believe this rule to be of the highest importance in explaining the
laws of embryology. These remarks are of course confined to the first
appearance of the peculiarity, and not to the primary cause which
may have acted on the ovules or on the male element; in nearly the
same manner as the increased length of the horns in the offspring
from a short-horned cow by a long-horned bull, though appearing late
in life, is clearly due to the male element.
Having alluded to the subject of reversion, I may here refer to a
statement often made by naturalists— namely, that our domestic vari-
eties, when run wild, gradually but invariably revert in character to
their aboriginal stocks. Hence it has been argued that no deductions
can be drawn from domestic races to species in a state of nature. I
have in vain endeavoured to discover on what decisive facts the above
statement has so often and so boldly been made. There would be
30 ORIGIN OF SPECIES
great difficulty in proving its truth: we may safely conclude that very
many of the most strongly marked domestic varieties could not pos-
sibly live in a wild state. In many cases we do not know what the
aboriginal stock was, and so could not tell whether or not nearly per-
fect reversion had ensued. It would be necessary, in order to prevent
the effects of intercrossing, that only a single variety should have
been turned loose in its new home. Nevertheless, as our varieties
certainly do occasionally revert in some of their characters to ancestral
forms, it seems to me not improbable that if we could succeed in
naturalising, or were to cultivate, during many generations, the sev-
eral races, for instance, of the cabbage, in very poor soil (in which
case, however, some effect would have to be attributed to the definite
action of the poor soil), that they would, to a large extent, or even
wholly, revert to the wild aboriginal stock. Whether or not the ex-
periment would succeed, is not of great importance for our line of
argument; for by the experiment itself the conditions of life are
changed. If it could be shown that our domestic varieties mani-
fested a strong tendency to reversion, — ^that is, to lose their acquired
characters, whilst kept under the same conditions, and whilst kept in
a considerable body, so that free intercrossing might check, by blend-
ing together, any slight deviations in their structure, in such case, I
grant that we could deduce nothing from domestic varieties in regard-
to species. But there is not a shadow of evidence in favour of this
view : to assert that we could not breed our cart and race horses, long
and short horned cattle, and poultry of various breeds, and esculent
vegetables, for an unlimited number of generations, would be opposed
to all experience.
CHARACTER OF DOMESTIC VARIETIES; DIFFICULTY OF DISTINGUISHING BE-
TWEEN VARIETIES AND SPECIES; ORIGIN OF DOMESTIC VARIETIES FROM
ONE OR MORE SPECIES
When we look to the hereditary varieties or races of our domestic
animals and plants, and compare them with closely allied species, we
generally perceive in each domestic race, as already remarked, less
uniformity of character than in true species. Domestic races often
have a somewhat monstrous character; by which I mean, that,
although differing from each other, and from other species of die
CHARACTER OF DOMESTIC VARIETIES 3 1
same genus, in several trifling respects, they often differ in an extreme
degree in some one part, both when compared one with another,
and more especially when compared with the species under nature
to which they are nearest allied. With these exceptions (and with
that of the perfect fertility of varieties when crossed, — a subject here-
after to be discussed), domestic races of the same species differ from
each other in the same manner as do the closely allied species of the
same genus in a state of nature, but the differences in most cases are
less in degree. This must be admitted as true, for the domestic races
of many animals and plants have been ranked by some competent
judges as the descendants of aboriginally distinct species, and by other
competent judges as mere varieties. If any well-marked distinction
existed between a domestic race and a species, this source of doubt
would not so perpetually recur. It has often been stated that domestic
races do not differ from each other in characters of generic value. It
can be shown that this statement is not correct; but naturalists diffef
much in determining what characters are of generic value; all such
valuations being at present empirical. When it is explained how
genera originate under nature, it will be seen that we have no right
to expect often to find a generic amount of difference in our domesti-
cated races.
In attempting to estimate the amount of structural difference be-
tween allied domestic races, we are soon involved in doubt, from not
knowing whether they are descended from one or several parent
species. This point, if it could be cleared up, would be interesting; if,
for instance, it could be shown that the greyhound, bloodhound, ter-
rier, spaniel, and bulldog, which we all know propagate their kind
truly, were the offspring of any single species, then such facts would
have great weight in making us doubt about the immutability of the
many closely allied natural species — ^for instance, of the many foxes
— ^inhabiting different quarters of the world. I do not believe, as we
shall presently see, that the whole amount of difference between the
several breeds of the dog has been produced under domestication; I
believe that a small part of the diflFerence is due to their being de-
scended from distinct species. In the case of strongly marked races of
some other domesticated species, there is presumptive or even strong
evidence, that all are descended from a single wild stock.
32 ORIGIN OF SPECIES
It has often been assumed that man has chosen for domestication
animals and plants having an extraordinary inherent tendency to
vary, and likewise to withstand diverse climates. I do not dispute
that these capacities have added largely to. the value of most of our
domesticated productions; but how could a savage possibly know,
when he first tamed an animal, whether it would vary in succeeding
generations, and whether it would endure other climates? Has the
little variability of the ass and goose, or the small power of endurance
of warmth by the reindeer, or of cold by the common camel, pre-
vented their domestication? I cannot doubt that if other animals and
plants, equal in number to our domesticated productions, and be-
longing to equally diverse classes and countries, were taken from a
state of nature, and could be made to breed for an equal number of
generations under domestication, they would on an average vary as
largely as the parent species of our existing domesticated productions
have varied.
In the case of most of our anciently domesticated animals and
plants, it is not possible to come to any definite conclusion, whether
they are descended from one or several wild species. The argument
mainly relied on by those who believe in the multiple origin of our
domestic animals is, that we find in the most ancient times, on the
monuments of Egypt, and in the lake habitations of Switzerland,
much diversity in the breeds; and that some of these ancient breeds
closely resemble, or are even identical with, those still existing. But
this only throws far backwards the history of civilisation, and shows
that animals were domesticated at a much earlier period than has
hitherto been supposed. The lake inhabitants, of Switzerland culti-
vated several kinds of wheat and barley, the pea, the poppy for oil,
and flax; and they possessed several domesticated animals. They also
carried on commerce with other nations. All this clearly shows, as
Heer has remarked, that they had at this early age progressed con-
siderably in civilisation; and this again implies a long continued
previous period of less advanced civilisation, during which the domes-
ticated anirhals, kept by different tribes in different districts, might
have varied and given rise to distinct races. Since the discovery of
flint tools in the superficial formations of many parts of the world,
all geologists believe that barbarian man existed at an enormously
CHARACTER OF DOMESTIC VARIETIES 33
remote period and we know that at the present day there is hardly
a tribe so barbarous, as not to have domesticated at least the dog.
The origin of most of our domestic animals will probably for ever
remain vague. But I may here state, that, looking to the domestic
dogs of the whole world, I have, after a laborious collection of all
known facts, come to the conclusion that several wild species of
Canids have been tamed, and that their blood, in some cases mingled
together, flows in the veins of our domestic breeds. In regard to
sheep and goats I can form no decided opinion. From facts com-
municated to me by Mr. Blyth, on the habits, voice, constitution, and
structure of the humped Indian cattle, it is almost certain that they
are descended from a different aboriginal stock from our European
cattle and some competent judges believe that these latter have had
two or three wild progenitors, — ^whether or not these deserve to be
called species. This conclusion, as well as that of the specific distinc-
tion between the humped and common cattle, may, indeed, be looked
upon as established by the admirable researches of Professor Riiti-
meyer. With respect to horses, from reasons which I cannot here
give, I am doubtfully inclined to believe, in opposition to several au-
thors, that all the races belong to the same species. Having kept
nearly all the English breeds of the fowl alive, having bred and
crossed them, and examined their skeletons, it appears to me almost
certain that all are the descendants of the wild Indian fowl, Gallus
bankiva; and this is the conclusion of Mr. Blyth, and of others who
have studied this bird in India. In regard to ducks and rabbits, some
breeds of which differ much from each other, the evidence is clear
that they are all descended from the common wild duck and rabbit.
The doctrine of the origin of our several domestic races from sev-
eral aboriginal stocks, has been carried to an absurd extreme by some
authors. They believe that every race which breeds true, let the dis-
tinctive characters be ever so slight, has had its wild prototype. At
this rate there must have existed at least a score of species of wild
cattle, as many sheep, and several goats, in Europe alone, and several
even within Great Britain. One author believes that there formerly
existed eleven wild species of sheep peculiar to Great Britain! When
we bear in mind that Britain has now not one peculiar mammal, and
France but few distinct from those of Germany,, and so with Hun-
34 ORIGIN OF SPECIES
gary, Spain, etc., but that each of these kingdoms possesses several
peculiar breeds of cattle, sheep, etc., we must admit that many do-
mestic breeds must have originated in Europe; from whence other-
wise could they have been derived? So it is in India. Even in the
case of the breeds of the domestic dog throughout the world, which
I admit are descended from several wild species, it cannot be doubted
that there has been an immense amount of inherited variation; for
who will believe that animals closely resembling the Italian grey-
hound, the bloodhound, the bulldog, pugdog, or Blenheim spaniel,
etc. — so unlike all wild Canida — ever existed in a state of nature? It
has often been loosely said that all our races of dogs have been pro-
duced by the crossing of a few aboriginal species; but by crossing we
can only get forms in some degree intermediate between their par-
ents; and if we account for our several domestic races by this process,
we must admit the former existence of the most extreme forms, as
the Italian greyhound, bloodhound, bulldog, etc., in the wild state.
Moreover, the possibility of making distinct races by crossing has
been greatly exaggerated Many cases are on record, showing that a
race may be modified by occasional crosses, if aided by the careful se-
lection of the individuals which present the desired character; but to
obtain a race intermediate between two quite distinct races, would
be very difficult. Sir J. Sebright expressly experimented with this
object and failed. The offspring from the first cross between two pure
breeds is tolerably and sometimes (as I have found with pigeons)
quite uniform in character, and everything seems simple enough;
but when these mongrels are crossed one with another for several
generations, hardly two of them are alike, and then the difficulty of
the task becomes manifest.
BREEDS OF THE DOMESTIC PIGEON, THEIR DIFFERENCES AND ORIGIN
Believing that k is always best to study some special group, I have,
after deliberation, taken up domestic pigeons. I have kept every
breed which I could purchase or obtain, and have been most kindly
favoured with skins from several quarters of the world, more espe-
cially by the Hon. W. Elliot, from India, and by the Hon. C. Murray,
from Persia. Many treatises in different languages have been pub-
lished on pigeons, and some of them are very important, as being of
DOMESTIC PIGEONS 35
considerable antiquity. I have associated with several eminent fan-
ciers, and have been permitted to join two of the London Pigeon
Clubs. The diversity of the breeds is something astonishing. Com-
pare the English carrier and the short-faced tumbler, and see the won-
derful difference in their beaks, entailing corresponding differences
in their skulls. The carrier, more especially the male bird, is also re-
markable from the wonderful development of the carunculated skin
about the head; and this is accompanied by greatly elongated eye-
lids, very large external orifices to the nostrils, and a wide gape of
mouth. The short-faced tumbler has a beak in outline almost like that
of a finch; and the common tumbler has the singular inherited habit
of flying at a great height in a compact flock, and tumbling in the
air head over heels. The runt is a bird of great size, with long mas-
sive beak and large feet; some of the sub-breeds of runts have very
long necks, others very long wings and tails, others singularly short
tails. The barb is allied to the carrier, but, instead of a long beak,
has a very short and broad one. The pouter has a much elongated
body, wings, and legs; and its enormously developed crop, which it
glories in inflating, may well excite astonishment and even laughter.
The turbit has a short and conical beak, with a line of reversed
feathers down the breast; and it has the habit of continually expand-
ing, slightly, the upper part of the oesophagus. The Jacobin has the
feathers so much reversed along the back of the neck that they form
a hood; and it has, proportionally to its size, elongated wing and
tail feathers. The trumpeter and laugher, as their names express,
utter a very different coo from the other breeds. The fantail has
thirty or even forty tail-feathers, instead of twelve or fourteen — the
normal number in all the members of the great pigeon family : these
feathers are kept expanded, and are carried so erect, that in good
birds the head and tail touch: the- oil-gland is quite aborted. Several
other less distinct breeds might be specified.
In the skeletons of the several breeds, the development of the
bones of the face in length and breadth and curvature differs enor-
mously. The shape, as well as the breadth and length of the ramus
of the lower jaw, varies in a highly remarkable manner. The caudal
and sacral vertebrae vary in number; as does the number of the ribs,
together with their relative breadth and the presence of processes.
ORIGIN OF SPECIES
36
The size and shape of the apertures in the sternum are highly
variable; so is the degree of divergence and relative size of the two
arms of the furcula. The proportional width of the gape of mouth,
the proportional length of the eyelids, of the orifice of the nostrils,
of the tongue (not always in strict correlation with the length of
beak), the size of the crop and of the upper part of the oesophagus;
the development and abortion of the oil-gland; the number of the
primary wing and caudal feathers; the relative length of the wing
and tail to each other and to the body; the relative length of the
leg and foot; ‘the number of scutellae on the toes, the development
of skin between the toes, are all points of structure which are vari-
able. The period at which the perfect plumage is acquired varies,
as does the state of the down with which the nestling birds are
clothed when hatched. The shape and size of the eggs vary. The
manner of flight, and in some breeds the voice and disposition, differs
remarkably. Lastly, in certain breeds, the males and females have
come to differ in a slight degree from each other.
Altogether at least a score of pigeons might be chosen, which, if
shown to an ornithologist, and he were told that they were wild
birds, would certainly be ranked by him as well-defined species.
Moreover, I do not believe that any ornithologist would in this case
place the English carrier, the short-faced tumbler, the runt, the barb,
pouter, and fantail in the same genus; more especially as in each of
these breeds several truly inherited sub-breeds, or species, as he would
call them, could be shown him.
Great as are the differences between the breeds of the pigeon, I
am fully convinced that the common opinion of naturalists is correct,
namely, that all are descended from the rock pigeon (Columba livia),
including under this term several geographical races or sub-species,
which differ from each other in the most trifling respects. As several
of the reasons which have led me to this belief are in some degree
applicable in other cases, I will here briefly give them. If the several
breeds are not varieties, and have not proceeded from the rock
pigeon, they must have descended from at least seven or eight aborigi-
nal stocks; for it is impossible to make the present domestic breeds
by the crossing of any lesser number: how, for instance, could a
pouter be produced by crossing two breeds unless one of the parent-
DOMESTIC PIGEONS 37
stocks possessed the characteristic enormous crop? The supposed
aboriginal stocks must all have been rock pigeons, that is, they did
not breed or willingly perch on trees. But besides C. livia, with its
geographical sub-species, only two or three other species of rock
pigeons are known and these have not any of the characters of the
domestic breeds. Hence the supposed aboriginal stocks must either
still exist in the countries where they were originally domesticated,
and yet be unknown to ornithologists; and this, considering their
size, habits, and remarkable characters, seems improbable; or they
must have become extinct in the wild state. But birds breeding on
precipices, and good fliers, are unlikely to be exterminated; and the
common rock pigeon, which has the same habits with the domestic
breeds, has not been exterminated even on several of the smaller
British islets, or on the shores of the Mediterranean. Hence the
supposed extermination of so many species having similar habits
with the rock pigeon seems a very rash assumption. Moreover, the
several above-named domesticated breeds have been transported to
all parts of the world, and, therefore, some of them must have been
carried back again into their native country; but not one has become
wild or feral, though the dovecot pigeon, which is the rock pigeon
in a very slightly altered state, has become feral in several places.
Again, all recent experience shows that it is difficult to get wild ani-
mals to breed freely under domestication; yet on the hypothesis of
the multiple origin of our pigeons, it must be assumed that at least
seven or eight species were so thoroughly domesticated in ancient
times by half-civilised man, as to be quite prolific under confinement.
An argument of great weight, and applicable in several other
cases, is, that the above-specified breeds, though agreeing generally
with the wild rock pigeon in constitution, habits, voice, colouring,
and in most parts of their structure, yet are certainly highly abnormal
in other parts; we may look in vain through the whole great family
of Columbidae for a beak like that of the English carrier, or that of
the short-faced tumbler, or barb; for reversed feathers like those of
the Jacobin; for a crop like that of the pouter; for tail-feathers like
those of the fantail. Hence it must be assumed not only that half-
civilised man succeeded in thoroughly domesticating several species,
but that he intentionally or by chance picked out extraordinarily
ORIGIN OF SPECIES
38
abnormal species; and further, that these very species have since all
become extinct or unknown. So many strange contingencies are
improbable in the highest degree.
Some facts in regard to the colouring of pigeons well deserve
consideration. The rock pigeon is of a slaty-blue, with white loins;
but the Indian sub-species, C. intermedia of Strickland, has this
part bluish. The tail has a terminal dark bar, with the outer feathers
externally edged at the base with white. The wings have two black
bars. Some semi-domestic breeds, and some truly wild breeds, have,
besides the two black bars, the wings chequered with black. These
several marks do not occur together in any other species of the whole
family. Now, in every one of the domestic breeds, taking thoroughly
well-bred birds, all the above marks, even to the white edging of
the outer tail-feathers, sometimes concur perfectly developed. More-
over, when birds belonging to two or more distinct breeds are
crossed, none of which are blue or have any of the above-specified
marks, the mongrel offspring are very apt suddenly to acquire these
characters. To give one instance out of several which I have ob-
served: — I crossed some white fantails, which breed very true, with
some black barbs — and k so happens that blue varieties of barbs are
so rare that I never heard of an instance in England; and the mon-
grels were black, brown, and mottled. I also crossed a barb with a
spot, which is a white bird with a red tail and red spot on the fore-
head, and which notoriously breeds very true; the mongrels were
dusky and mottled. I then crossed one of the mongrel barb-fantails
with a mongrel barb-spot, and they produced a bird of as beautiful
a blue colour, with the white loins, double black wing-bar, and barred
and white-edged tail-feathers, as any wild rock pigeon! We can
understand these facts, on the well-known principle of reversion to
ancestral characters, if all the domestic breeds are descended from
the rock pigeon. But if we deny this, we must make one of the two
following highly improbable suppositions. Either, first, that all the
several imagined aboriginal stocks were coloured and marked like
the rock pigeon, although no other existing species is thus coloured
and marked, so that in each separate breed there might be a tendency
to revert to the very same colours and markings. Or, secondly, that
each breed, even the purest, has within a dozen, or at most within a
DOMESTIC PIGEONS 39
score, of generations, been crossed by the rock pigeon; I say within
a dozen or twenty generations, for no instance is known of crossed
descendants reverting to an ancestor of foreign blood, removed by
a greater number of generations. In a breed which has been crossed
only once, the tendency to revert to any character derived from such
a cross will naturally become less and less, as in each succeeding gen-
eration there will be less of the foreign blood; but when there has
been no cross, and there is a tendency in the breed to revert to a
character which was lost during some former generation, this tend-
ency, for all that we can see to the contrary, may be transmitted
undiminished for an indefinite number of generations. These two
distinct cases of reversion are often confounded together by those
who have written on inheritance.
Lastly, the hybrids or mongrels from between all the breeds of the
pigeon are perfectly fertile, as I can state from my own observations,
purposely made, on the most distinct breeds. Now, hardly any cases
have been ascertained with certainty of hybrids from two quite
distinct species of animals being perfectly fertile. Some authors
believe that ‘long-continued domestication eliminates this strong
tendency to sterility in species. From the history of the dog, and of
some other domestic animals, this conclusion is probably quite
correct, if applied to species closely related to each other. But to
extend it so far as to suppose that species, aboriginally as distinct as
carriers, tumblers, pouters, and fantails now are, should yield off-
spring perfectly fertile inter se, would be rash in the extreme.
From these several reasons, namely, — the improbability of man
having formerly made seven or eight supposed species of pigeons to
breed freely under domestication; — ^these supposed species being
quite unknown in a wild state, and their not having become any-
where feral; — these species presenting certain very abnormal char-
acters, as compared with all other Columbid^, though so like the
rock pigeon in most respects ;“the occasional re-appearance of the
blue colour and various black marks in all the breeds, both when
kept pure and when crossed;— and lastly, the mongrel offspring being
perfectly fertile;— from these several reasons, taken together, we may
safely conclude that all our domestic breeds are descended from the
rock pigeon or Columba livia with its geographical sub-species.
40 ORIGIN OF SPECIES
In favour of this view, I may add, firstly, that the wild C. livia
has been found capable of domestication in Europe and India; and
that it agrees in habits and in a great number of points of structure
with all the domestic breeds. Secondly, that, although an English
carrier or a short-faced tumbler differs immensely in certain char-
acters from the rock pigeon, yet that, by comparing the several sub-
breeds of these two races, more especially those brought from distant
countries, we can make, between them and the rock pigeon, an
almost perfect series; so we can in some other cases, but not with
all the breeds. Thirdly, those characters which are mainly distinctive
of each breed are in each eminently variable, for instance the wattle
and length of beak of the carrier, the shortness of that of the tumbler,
and the number of tail-feathers in the fantail; and the explanation
of this fact will be obvious when we treat of Selection. Fourthly,
pigeons have been watched and tended with the utmost care, and
loved by many people. They have been domesticated for thousands
of years in several quarters of the world; the earliest known record
of pigeons is in the fifth ^Egyptian dynasty, about 3000 b.c., as was
pointed out to me by Professor Lepsius; but Mr. Birch informs me
that pigeons are given in a bill of fare in the previous dynasty. In
the time of the Romans, as we hear from Pliny, immense prices
were given for pigeons; “nay, they are come to this pass, that they
can reckon up their pedigree and race,” Pigeons were much valued
by Akber Khan, in India, about the year 1600; never less than 20,000
pigeons were taken with the court. “The monarchs of Iran and
Turan sent him some very rare birds”; and, continues the courtly
historian, “His Majesty, by crossing the breeds, which method was
never practised before, has improved them astonishingly.” About
this same period the Dutch were as eager about pigeons as were the
old Romans. The paramount importance of these considerations in
explaining the immense amount of variation which pigeons have
undergone, will likewise be obvious when we treat of Selection. We
shall then, also, see how it is that the several breeds so often have a
somewhat monstrous character. It is also a most favourable circum-
stance for the production of distinct breeds, that male and female
pigeons can be easily mated for life; and thus different breeds can
be kept together in the same aviary.
SELECTION BY MAN 4 1
I have discussed the probable origin of domestic pigeons at some,
yet quite insufficient, length; because when I first kept pigeons and
watched the several kinds, well knowing how truly they breed, I
felt fully as much difficulty in believing that since they had been
domesticated they had all proceeded from a common parent, as any
naturalist could in coming to a similar conclusion in regard to the
many species of finches, or other groups of birds, in nature. One
circumstance has struck me much; namely, that nearly all the
breeders of the various domestic animals and the cultivators of plants,
with whom I have conversed, or whose treatises I have read, are
firmly convinced that the several breeds to which each has attended,
are descended from so many aboriginally distinct species. Ask, as
I have asked, a celebrated raiser of Hereford cattle, whether his cattle
might not have descended from Longhorns, or both from a common
parent-stock, and he will laugh you to scorn. I have never met a
pigeon, or poultry, or duck, or rabbit fancier, who was not fully
convinced that each main breed was descended from a distinct
species. Van Mons, in his treatise on pears and apples, shows how
utterly he disbelieves that the several sorts, for instance a Ribston-
pippin or Codlin-apple, could ever have proceeded from the seeds
of the same tree. Innumerable other examples could be given. The
explanation, I think, is simple: from long-continued study they are
strongly impressed with the differences between the several races;
and though they well know that each race varies slightly, for they
win their prizes by selecting such slight differences, yet they ignore
all general arguments, and refuse to sum up in their minds slight
differences accumulated during many successive generations. May
not those naturalists who, knowing far less of the laws of inheritance
than does the breeder, and knowing no more than he does of the
intermediate links in the long lines of descent, yet admit that many
of our domestic races are descended from the same parents — ^may
they not learn a lesson of caution, when they deride the idea of species
in a state of nature being lineal descendants of other species ?
PRINCIPLES OF SELECTION ANCIENTLY FOLLOWED, AND THEIR EFFECTS
Let us now briefly consider the steps by which domestic races have
been produced, either from one or from several allied species. Some
42 ORIGIN OF SPECIES
effect may be attributed to the direct and definite action of the
external conditions of life, and some to habit; but he would be a
bold man who would account by such agencies for the differences
between a dray and race horse, a greyhound and bloodhound, a
carrier and tumbler pigeon. One of the most remarkable features
in our domesticated races is that we see in them adaptation, not
indeed to the animal’s or plant’s own good, but to man’s use or
fancy. Some variations useful to him have probably arisen suddenly,
or by one step; many botanists, for instance, believe that the fuller’s
teasel, with its hooks, which cannot be rivalled by any mechanical
contrivance, is only a variety of the wild Dipsacus; and this amount
of change may have suddenly arisen in a seedhng. So it has probably
been with the turnspit dog; and this is known to have been the
case with the ancon sheep. But when we compare the dray horse
and race horse, the dromedary and camel, the various breeds of
sheep fitted either for cultivated land or mountain pasture, with the
wool of one breed good for one purpose, and that of another breed
for another purpose; when we compare the many breeds of dogs,
each good for man in different ways; when we compare the game
cock, so pertinacious in battle, with other breeds so little quarrelsome,
with “everlasting layers” which never desire to sit, and with the
bantam so small and elegant; when we compare the host of agricul-
tural, culinary, orchard, and flower garden races of plants, most
useful to man at different seasons and for different purposes, or so
beautiful in his eyes, we must, I think, look further than to mere
variability. We cannot suppose that all the breeds were suddenly
produced as perfect and as useful as we now see them; indeed,
in many cases, we know that this has not been their history. The
key is man’s power of accumulative selection: nature gives successive
variations; man adds them up in certain directions useful to him.
In this sense he may be said to have made for himself useful breeds.
The great power of this principle of selection is not hypothetical.
It is certain that several of our eminent breeders have, even within
a single lifetime, modified to a large extent their breeds of cattle and
sheep. In order fully to realise what they have done, it is almost
necessary to read several of the many treatises devoted to this subject,
and to. inspect the animals. Breeders habitually speak of an animal’s
SELECTION BY MAN 43
organisation as something plastic, which they can model almost as
they please. If I had space I could quote numerous passages to this
effect from highly competent authorities. Youatt, who was probably
better acquainted with the works of agriculturists than almost any
other individual, and who was himself a very good judge of animals,
speaks of the principle of selection as “that which enables the agri-
culturist, not only to modify the character of his flock, but to change
it altogether. It is the magician’s wand, by means of which he
may summon into life whatever form and mould he pleases.” Lord
Somerville, speaking of what breeders have done for sheep, says: —
“It would seem as if they had chalked out upon a wall a form perfect
in itself, and then had given it existence.” In Saxony the importance
of the principle of selection in regard to merino sheep is so fully
recognised, that men follow it as a trade; the sheep are placed on a
•table and are studied, like a picture by a connoisseur; this is done
three times at intervals of months, and the sheep are each time
marked and classed, so that the very best may ultimately be selected
for breeding.
What English breeders have actually effected is proved by the
enormous prices given for animals with a good pedigree; and these
have been exported to almost every quarter of the world. The im-
provement is by no means generally due to crossing different breeds;
all the best breeders are strongly opposed to this practice, except some-
times amongst closely allied sub-breeds. And when a cross has been
made, the closest selection is far more indispensable even than in
ordinary cases. If selection consisted merely in separating some very
distinct variety, and breeding from it, the principle would be so
obvious as hardly to be worth notice; but its importance consists in
the great effect produced by the accumulation in one direction,
during successive generations, of differences absolutely inappreciable
by an uneducated eye— differences which I for one have vainly
attempted to appreciate. Not one man in a thousand has accuracy
of eye and judgment sufficient to become an eminent breeder. If
gifted with these qualities, and he studies his subject for years, and
devotes his lifetime to it with indomitable perseverance, he will
succeed, and may make great improvements; if he wants any of
these qualities, he will assuredly fail. Few would readily believe
ORIGIN OF SPECIES
44
in 'the natural capacity and years of practice requisite to become even
a skilful pigeon fancier.
The same principles are followed by horticulturists; but the
variations are here often more abrupt. No one supposes that our
choicest productions have been produced by a single variation from
the aboriginal stock. We have proofs that this has not been so in
several cases in which exact records have been kept; thus, to give
a very trifling instance, the steadily increasing size of the common
gooseberry may be quoted. We see an astonishing improvement in
many florists’ flowers, when the flowers of the present day are com-
pared with drawings made only twenty or thirty years ago. When
a race of plants is once pretty well established, the seed-raisers do not
pick out the best plants, but merely go over their seed-beds, and pull
up the “rogues,” as they call the plants that deviate from the proper
standard. With animals this kind of selection is, in fact, likewise
followed; for hardly any one is so careless as to breed from his
worst animals.
In regard to plants, there is another means of observing the ac-
cumulated effects of selection — namely, by comparing the diversity
of flowers in the different varieties of the same species in the flower
garden; the diversity of leaves, pods, or tubers, or whatever part is
valued, in the kitchen garden, in comparison with the flowers of the
same varieties; and the diversity of fruit of the same species in the
orchard, in comparison with the leaves and flowers of the same set
of varieties. See how different the leaves of the cabbage are, and
how extremely alike the flowers; how unlike the flowers of the
heartsease are, and how alike the leaves; how much the fruit of the
different kinds of gooseberries differ in size, colour, shape, and hairi-
ness, and yet the flowers present very slight differences. It is not that
the varieties which differ largely in some one point do not differ at
all in other points; this is hardly ever, — I speak after careful observa-
tion, — perhaps never, the case. The law of correlated variation, the
importance of which should never be overlooked, will ensure some
differences; but, as a general rule, it cannot be doubted that the con-
tinued selection of slight variations, either in the leaves, the flowers,
or the fruit, will produce races differing from each other chiefly in
these characters.
UNCONSCIOUS SELECTION 45
It may be objected that the principle of selection has been reduced
to methodical practice for scarcely more than three-quarters of a
century; it has certainly been more attended to of late years, and
many treatises have been published on the subject; and the result
has been, in a corresponding degree, rapid and important. But it is
very far from true that the principle is a modern discovery. I could
give several references to works of high antiquity, in which the full
importance of the principle is acknowledged. In rude and barbarous
periods of English history choice animals were often imported, and
laws were passed to prevent their exportation: the destruction of
horses under a certain size was ordered, and this may be compared
to the “roguing” of plants by nurserymen. The principle of selection
I find distinctly given in an ancient Chinese encyclopaedia. Explicit
rules are laid down by some of the Roman classical writers. From
passages in Genesis, it is clear that the colour of domesticated animals
was at that early period attended to. Savages now sometimes cross
their dogs with wild canine animals, to improve the breed, and they
formerly did so, as is attested by passages in Pliny. The savages in
South Africa match their draught cattle by colour, as do some of
the Esquimaux their teams of dogs. Livingstone states that good
domestic breeds are highly valued by the negroes in the interior of
Africa who have not associated with Europeans. Some of these facts
do not show actual selection, but they show that the breeding of
domestic animals was carefully attended to in ancient times, and is
now attended to by the lowest savages. It would, indeed, have been
a strange fact, had attention not been paid to breeding, for the
inheritance of good and bad qualities is so obvious.
UNCONSCIOUS SELECTION
At the present time, eminent breeders try by methodical selection,
with a distinct object in view, to make a new strain or sub-breed,
superior to anything of the kind in the country. But, for our purpose,
a form of Selection, which may be called Unconscious, and which
results from every one trying to possess and breed from the best
individual animals, is more important. Thus, a man who intends
keeping pointers naturally tries to get as good dogs as he can, and
afterwards breeds from his own best dogs, but he has no wish or
46 ORIGIN OF SPECIES
expectation of permanently altering the breed. Nevertheless ^ve may
infer that this process, continued during centuries, would improve
and modify any breed, in the same way as Bakewell, Collins, etc., by
this very same process, only carried on more methodically, did greatly
modify, even during their lifetimes, the forms and qualities of their
cattle. Slow and insensible changes of this kind can never be recog-
nised unless actual measurements or careful drawings of the breeds in
question have been made long ago, which may serve for comparison.
In some cases, however, unchanged, or but little changed individuals
of the same breed exist in less civilised districts, where the breed has
been less improved. There is reason to believe that King Charles’
spaniel has been unconsciously modified to a large extent since the
time of that monarch. Some highly competent authorities are con-
vinced that the setter is directly derived from the spaniel, and has
probably been slowly altered from it. It is known that the English
pointer has been greatly changed within the last century, and in this
case the change has, it is believed, been chiefly effected by crosses
with the foxhound; but what concerns us is, that the change has
been effected unconsciously and gradually, and yet so effectually, that,
though the old Spanish pointer certainly came from Spain, Mr. Bor-
row has not seen, as I am informed by him, any native dog in Spain
like our pointer. By a simple process of selection, and by careful
training, English race horses have come to surpass in fleetness and
size the parent Arabs, so that the latter, by the regulations for the
Goodwood Races, are favoured in the weights which they carry.
Lord Spencer and others have shown how the cattle of England have
increased in weight and in early maturity, compared with the stock
formerly kept in this country. By comparing the accounts given in
various old treatises of the former and present state of carrier and
tumbler pigeons in Britain, India, and Persia, we can trace the
stages through which they have insensibly passed, and come to differ
so greatly from the rock pigeon.
Youatt gives an excellent illustration of the effects of a course of
selection, which may be considered as unconscious, in so far that
the breeders could never have expected, or even wished, to produce
the result which ensued—namely, the production of two distinct
strains. The two flocks of Leicester sheep kept by Mr. Buckley and
UNCONSCIOUS SELECTION 47
Mr. Burgess, as Mr. Youatt remarks, “have been purely bred from
the original stock of Mr. Bakewell for upwards of fifty years. There
is not a suspicion existing in the mind of any one at all acquainted
with the subject, that the owner of either of them has deviated in
any one instance from the pure blood of Mr. Bakewell’s flock, and
yet the difference between the sheep possessed by these two gentle-
men is so great that they have the appearance of being quite different
varieties.”
If there exist savages so barbarous as never to think of the in-
herited character of the offspring of their domestic animals, yet any
one animal particularly useful to them, for any special purpose, would
be carefully preserved during famines and other accidents, to which
savages are so liable, and such choice animals would thus generally
leave more offspring than the inferior ones; so that in this case there
would be a kind of unconscious selection going on. We see the
value set on animals even by the barbarians of Tierra del Fuego, by
their killing and devouring their old women, in times of dearth, as of
less value than -their dogs.
In plants the same gradual process of improvement, through the
occasional preservation of the best individuals, whether or not suffi-
ciently distinct to be ranked at their first appearance as distinct
varieties, and whether or not two or more species or races have
become blended together by crossing, may plainly be recognised in
the increased size and beauty which we now see in the varieties of
the heartsease, rose, pelargonium, dahlia, and other plants, when
compared with the older varieties or with their parent-stocks. No
one would ever expect to get a first-rate heartsease or dahlia from
the seed of a wild plant. No one would expect to raise a first-rate
melting pear from the seed of the wild pear, though he might succeed
from a poor seedling growing wild, if it had come from a garden-
stock. The pear, though cultivated in classical times, appears, from
Pliny’s description, to have been a . fruit of very inferior quality. I
have seen great surprise expressed in horticultural works at the
wonderful skill of gardeners, in having produced such splendid
results from such poor materials; but the art has been simple, and,
as far as the final result is concerned, has been followed almost un-
consciously. It has consisted in always cultivating the best known
4b ORIGIN OF SPECIES
variety, sowing its seeds, and, when a slightly better variety chanced
to appear, selecting it, and so onwards. But the gardeners o£ the
classical period, who cultivated the best pears which they could pro-
cure, never thought what splendid fruit we should eat; though we
owe our excellent fruit in some small degree to their having naturally
chosen and preserved the best varieties they could anywhere find.
A large amount of change, thus slowly and unconsciously accumu-
lated, explains, as I believe, the well-known fact, that in a number
of cases we cannot recognise, and therefore do not know, the wild
parent-stocks of the plants which have been longest cultivated in our
flower and kitchen gardens. If it has taken centuries or thousands
of years to improve or modify most of our plants up to their present
standard of usefulness to man, we can understand how it is that
neither Australia, the Cape of Good Hope, nor any other region
inhabited by quite uncivilised man, has afforded us a single plant
worth culture. It is not that these countries, so rich in species, do not
by a strange chance possess the aboriginal stocks of any useful plants,
but that the native plants have not been improved by continued
selection up to a standard of perfection comparable with that
acquired by the plants in countries anciently civilised.
In regard to the domestic animals kept by uncivilised man, it
should not be overlooked that they almost always have to struggle
for their own food, at least during certain seasons. And in two
countries very differently circumstanced, individuals of the same
species, having slightly different constitutions or structure, would
often succeed better in the one country than in the other; and thus
by a process of “natural selection,” as will hereafter be more fully
explained, two sub-breeds might be formed. This, perhaps, partly
explains why the varieties kept by savages, as has been remarked by
some authors, have more of the character of true species than the
varieties kept in civilised countries.
On the view here given of the important part which selection by
man has played, it becomes at once obvious, how it is that our
domestic races show adaptation in their structure or in their habits
to man’s wants or fancies. We can, I think, further understand the
frequently abnormal character of our domestic races, and likewise
their differences being so great in external characters, and relatively
UNCONSCIOUS SELECTION 49
SO slight in internal parts or organs* Man can hardly select, or only
with much difficulty, any deviation of structure excepting such as is
externally visible; and indeed he rarely cares for what is internal.
He can never act by selection, excepting on variations which are first
given to him in some slight degree by nature. No man would ever
try to make a fantail till he saw a pigeon with a tail developed in
some slight degree in an unusual manner, or a pouter till he saw a
pigeon with a crop of somewhat unusual size; and the more ab-
normal or unusual any character was when it first appeared, the
more likely it would be to catch his attention. But to use such an
expression as trying to make a fantail, is, I have no doubt, in most
cases, utterly incorrect. The man who first selected a pigeon with
a slightly larger tail, never dreamed what the descendants of that
pigeon would become through long-continued, partly unconscious
and partly methodical, selection. Perhaps the parent-bird of all fan-
tails had only fourteen tail-feathers somewhat expanded, like the
present Java fantail, or like individuals of other and distinct breeds,
in which as many as seventeen tail-feathers have been counted.
Perhaps the first pouter pigeon did not inflate its crop much more
than the turbit now does the upper part of its oesophagus, — a habit
which is disregarded by all fanciers, as it is not one of the points
of the breed.
Nor let it be thought that some great deviation of structure would
be necessary to catch the fancier’s eye: he perceives extremely small
differences, and it is in human nature to fancy any novelty, however
slight, in one’s own possession. Nor must the value which would
formerly have been set on any slight differences in the individuals of
the same species, be judged of by the value which is now set on
them, after several breeds have fairly been established. It is known
that with pigeons many slight variations now occasionally appear,
but these are rejected as faults or deviations from the standard of
perfection in each breed. The common goose has not given rise to
any marked varieties; hence the Toulouse and the common breed,
which differ only in colour, that most fleeting of characters, have
lately been exhibited as distinct at our poultry shows.
These views appear to explain what has sometimes been noticed,
namely, that we know hardly anything about the origin or history
ORIGIN OF SPECIES
50
of any of our domestic breeds. But, in fact, a breed, like a dialect
of a language, can hardly be said to have a distinct origin. A man
preserves and breeds from an individual with some slight deviation
of structure, or takes more care than usual in matching his best
animals, and thus improves them, and the improved animals slowly
spread in the immediate neighbourhood. But they will as yet hardly
have a distinct name, and from being only slightly valued, their
history will have been disregarded. When further improved by the
same slow and gradual process, they will spread more widely, and
will be recognised as something distinct and valuable, and will then
probably first receive a provincial name. In semi-civilised countries,
with little free communication, the spreading of a new sub-breed
would be a slow process. As soon as the points of value are once
acknowledged, the principle, as I have called it, of unconscious
selection will always tend, — ^perhaps more at one period than at an-
other, as the breed rises or falls in fashion,— perhaps more in one
district than in another, according to the state of civilisation of the
inhabitants, — slowly to add to the characteristic features of the
breed, whatever they may be. But the chance will be infinitely small
of any record having been preserved of such slow, varying, and
insensible changes.
CIRCUMSTANCES FAVOURABLE TO MAN’s POWER OF SELECTION
I will now say a few words on the circumstances, favourable, or
the reverse, to man’s power of selection. A high degree of variability
is obviously favourable, as freely giving the materials for selection
to work on; not that mere individual differences are not amply suffi-
cient, with extreme care, to allow of the accumulation of a large
amount of modification in almost any desired direction. But as
variations manifestly useful or pleasing to man appear only occasion-
ally, the chance of their appearance will be much increased by a large
number of individuals being kept. Hence, number is of the highest
importance for success. On this principle Marshall formerly re-
marked, with respect to the sheep of parts of Yorkshire, “as they
generally belong to poor people, and are mostly in small lots, they
never can be improved.” On the other hand, nurserymen, from
keeping large stocks of the same plant, are generally far more success-
CIRCUMSTANCES FAVOURABLE TO SELECTION 5 1
fill than amateurs in raising new and valuable varieties. A large
number of individuals of an animal or plant can be reared only
where the conditions for its propagation are favourable. When the
individuals are scanty, all will be allowed to breed, whatever their
quality may be, and this will effectually prevent selection. But
probably the most important element is that the animal or plant
should be so highly valued by man, that the closest attention is paid
to even the slightest deviations in its qualities or structure. Unless
such attention be paid nothing can be effected. I have seen it gravely
remarked, that it was most fortunate that the strawberry began to
vary just when gardeners began to attend to this plant. No doubt
the strawberry had always varied since it was cultivated, but the
slight varieties had been neglected. As soon, however, as gardeners
picked out individual plants with slightly larger, earlier, or better
fruit, and raised seedlings from them, and again picked out the best
seedlings and bred from them, then (with some aid by crossing
distinct species) those many admirable varieties of the strawberry
were raised which have appeared during the last half-century.
With animals, facility in preventing crosses is an important ele-
ment in the formation of new races, — at least, in a country which is
already stocked with other races. In this respect enclosure of the
land plays a part. Wandering savages or the inhabitants of open
plains rarely possess more than one breed of the same species. Pi-
geons can be mated for life, and this is a great convenience to the
fancier, for thus many races may be improved and kept true, though
mingled in the same aviary; and this circumstance must have largely
favoured the formation of new breeds. Pigeons, I may add, can be
propagated in great numbers and at a very quick rate, and inferior
birds may be freely rejected, as when killed they serve for food. On
the other hand, cats, from their nocturnal rambling habits, cannot
be easily matched, and, although so much valued by women and
children, we rarely see a distinct breed long kept up; such breeds as
we do sometimes see are almost always imported from some other
country. Although I do not doubt that some domestic animals vary
less than others, yet the rarity or absence of distinct breeds of the
cat, the donkey, peacock, goose, etc., may be attributed in main part
to selection not having been brought into play: in cats, from the
52 ORIGIN OF SPECIES
difficulty in pairing them; in donkeys, from only a few being kept
by poor people, and little attention paid to their breeding; for recently
in certain parts of Spain and of the United States this animal has
been surprisingly modified and improved by careful selection; in pea-
cocks, from not being very easily reared and a large stock not kept;
in geese, from being valuable only for two purposes, food and
feathers^ and more especially from no pleasure having been felt in
the display of distinct breeds; but the goose, under the conditions
to which it is exposed when domesticated, seems to have a singularly
inflexible organisation, though it has varied to a slight extent, as I
have elsewhere described.
Some authors have maintained that the amount of variation in
our domestic productions is soon reached, and can never afterwards
be exceeded. It would be somewhat rash to assert that the limit has
been attained in any one case; for almost all our animals and plants
have been greatly improved in many ways within a recent period;
and this implies variation. It would be equally rash to assert that
characters now increased to their usual limit, could not, after remain-
ing fixed for many centuries, again vary under new conditions of
life. No doubt, as Mr. Wallace has remarked with much truth, a
limit will be at last reached. For instance, there must be a limit to
the fleetness of any terrestrial animal, as this will be determined by
the friction to be overcome, the weight of body to be carried, and the
power of contraction in the muscular fibres. But what concerns us
is that the domestic varieties of the same species differ from each
other in almost every character, which man has attended to and
selected, more than do the distinct species of the same genera. Isidore
Geoffrey St. Hilaire has proved this in regard to size, and so it is
with colour and probably with the length of hair. With respect to
fleetness, which depends on many bodily characters, Eclipse was far
fleeter, and a dray horse is incomparably stronger than any two
natural species belonging to the same genus. So with plants, the
seeds of the different varieties of the bean or maize probably differ
more in size, than do the seeds of the distinct species in any one
genus in the same two families. The same remark holds good in
regard to the fruit of the several varieties of the plum, and still more
strongly with the melon, as well as in many other analogous cases.
CIRCUMSTANCES FAVOURABLE TO SELECTION 53
To sum up on the origin o£ our domestic races of animals and
plants. Changed conditions of life are of the highest importance in
causing variability, both by acting directly on the organisation, and
indirectly by affecting the reproductive system. It is not probable
that variability is an inherent and necessary contingent, under all
circumstances. The greater or less force of inheritance and reversion
determine whether variations shall endure. Variability is governed
by many unknown laws, of which correlated growth is probably the
most important. Something, but how much we do not know, may
be attributed to the definite action of the conditions of life. Some,
perhaps a great, effect may be attributed to the increased use or dis-
use of parts. The final result is thus rendered infinitely complex.
In some cases the intercrossing of aboriginally distinct species appears
to have played an important part in the origin of our breeds. When
several breeds have once been formed in any country, their occasional
intercrossing, with the aid of selection, has, no doubt, largely aided
in the formation of new sub-breeds; but the importance of crossing
has been much exaggerated, both in regard to animals and to those
plants which are propagated by seed. With plants which are tempo-
rarily propagated by cuttings, buds, etc., the importance of crossing
is immense; for the cultivator may here disregard the extreme varia-
bility both of hybrids and of mongrels, and the sterility of hybrids;
but plants not propagated by seed are of little importance to us, for
their endurance is only temporary. Over all these causes of change,
the accumulative action of Selection, whether applied methodically
and quickly, or unconsciously and slowly, but more efficiently, seems
to have been the predominant power.
CHAPTER II
Variation Under Nature
Variability — Individual differences — Doubtful species — Wide ranging,
much diffused, and common species, vary most — Species of the
larger genera in each country vary more frequently than the species
of the smaller genera — Many of the species of the larger genera
resemble varieties in being very closely, but unequally, related to each
other, and in having restricted ranges.
E FORE applying the principles arrived at in the last chapter
to organic beings in a state of nature, we must briefly discuss
whether these latter are subject to any variation. To treat this
subject properly, a long catalogue of dry facts ought to be given; but
these I shall reserve for a future work. Nor shall I here discuss the
various definitions which have been given of the term species. No
one definition has satisfied all naturalists; yet every naturalist knows
vaguely what he means when he speaks of a species. Generally the
term includes the unknown element of a distinct act of creation.
The term “variety” is almost equally difficult to define; but here
community of descent is almost universally implied, though it can
rarely be proved. We have also what are called monstrosities; but
they graduate into varieties. By a monstrosity I presume is meant
some considerable deviation of structure, generally injurious, or not
useful to the species. Some authors use the term “variation” in a
technical sense, as implying a modification directly due to the
physical conditions of life; and “variations” in this sense are sup-
posed not to be inherited; but who can say that the dwarfed condi-
tion of shells in the brackish waters of the Baltic, or dwarfed plants
on Alpine summits, or the thicker fur of an animal from far north-
wards, would not in some cases be inherited for at least a few genera-
tions? And in this case I presume that the form would be called a
variety.
It may be doubted whether sudden and considerable deviations of
structure such as we occasionally see in our domestic productions,
54
INDIVIDUAL DIFFERENCES 55
more especially with plants, are ever permanently propagated in a
state of nature. Almost every part of every organic being is so beau-
tifully related to its complex conditions of life that it seems as im-
probable that any part should have been suddenly produced perfect,
as that a complex machine should have been invented by man in a
perfect state. Under domestication monstrosities sometimes occur
which resemble normal structures in widely different animals. Thus
pigs have occasionally been born with a sort of proboscis, and if any
wild species of the same genus had naturally possessed a proboscis,
it might have been argued that this had appeared as a monstrosity;
but I have as yet failed to find, after diligent search, cases of mon-
strosities resembling normal structures in nearly allied forms, and
these alone bear on the question. If monstrous forms of this kind
ever do appear in a state of nature and are capable of reproduction
(which is not always the case), as they occur rarely and singly, their
preservation would depend on unusually favourable circumstances.
They would, also, during the first and succeeding generations cross
with the ordinary form, and thus their abnormal character would
almost inevitably be lost. But I shall have to return in a future
chapter to the preservation and perpetuation of single or occasional
variations.
INDIVIDUAL DIFFERENCES
The many slight differences which appear in the offspring from
the same parents, or which it may be presumed have thus arisen,
from being observed in the individuals of the same species inhabiting
the same confined locality, may be called individual differences.
No one supposes that all the individuals of the same species are cast
in the same actual mould. These individual differences are of the
highest importance for us, for they are often inherited, as must be
familiar to every one; and they thus afford materials for natural
selection to act on and accumulate, in the same manner as man
accumulates in any given direction individual differences in his
domesticated productions. These individual differences generally
affect what naturalists consider unimportant parts; but I could
show, by a long catalogue of facts, that parts which must be called
important, whether viewed under a physiological or classificatory
56 ORIGIN OF SPECIES
point of view, sometimes vary in the individuals of the same species.
I am convinced that the most experienced naturalist would be sur-
prised at the number of the cases of variability, even in important
parts of structure, which he could collect on good authority, as I
have collected, during a course of years. It should be remembered
that systematists are far from being pleased at finding variability in
important characters, and that there are not many men who will
laboriously examine internal and important organs, and compare
them in many specimens of the same species. It would never have
been expected that the branching of the main nerves close to the
great central ganglion of an insect would have been variable in the
same species; it might have been thought that changes of this nature
could have been effected only by slow degrees; yet Sir J. Lubbock
has shown a degree of variability in these main nerves in Coccus,
which may almost be compared to the irregular branching of the
stem of a tree. This philosophical naturalist, I may add, has also
shown that the muscles in the larvae of certain insects are far from
uniform. Authors sometimes argue in a circle when they state that
important organs never vary; for these same authors practically
rank those parts as important (as some few naturalists have hon-
estly confessed) which do not vary; and, under this point of view,
no instance will ever be found of an important part varying; but
under any other point of view many instances assuredly can be
given.
There is one point connected with individual differences, which
is extremely perplexing: I refer to those genera which have been
called “protean” or “polymorphic,” in which the species present an
inordinate amount of variation. With respect to many of these
forms, hardly two naturalists agree whether to rank them as species
or as varieties. We may instance Rubus, Rosa, and Hieracium
amongst plants, several genera of insects and of Brachiopod shells.
In most polymorphic genera some of the species have fixed and
definite characters. Genera which are polymorphic in one country,
seem to be, with a few exceptions, polymorphic in other countries,
and likewise, judging from Brachiopod shells, at former periods of
time. These facts are very perplexing, for they seem to show that
this kind of variability is independent of the conditions of life. I
INDIVIDUAL DIFFERENCES 57
am inclined to suspect that we see, at least in some of these poly-
morphic genera, variations which are of no service or disservice to
the species, and which consequently have not been seized on and
rendered definite by natural selection, as hereafter to be explained.
Individuals of the same species often present, as is known to
every one, great differences of structure, independently of varia-
tion, as in the two sexes of various animals, in the two or three
castes of sterile female or workers amongst insects, and in the
immature and larval states of many of the lower animals.
There are, also, cases of dimorphism and trimorphism, both
with animals and plants. Thus, Mr. Wallace, who has lately called
attention to the subject, has shown that the females of certain species
of butterflies, in the Malayan Archipelago, regularly appeared under
two or even three conspicuously distinct forms, not connected by
intermediate varieties. Fritz Muller has described analogous but
more extraordinary cases with the males of certain Brazilian crus-
taceans: thus, the male of a Tanais regularly occurs under two
distinct forms; one of these has strong and differently shaped pincers,
and the other has antenna much more abundantly furnished with
smelling-hairs. Although in most of these cases, the two or three
forms, both with animals and plants, are not now connected by
intermediate gradations, it is probable that they were once thus
connected. Mr. Wallace, for instance, describes a certain butterfly
which presents in the same island a great range of varieties con-
nected by intermediate links, and the extreme links of the chain
closely resemble the two forms of an allied dimorphic species in-
habiting another part of the Malay Archipelago. Thus also with
ants, the several worker-castes are generally quite distinct; but in
some, cases, as we shall hereafter see, the castes are connected to-
gether by finely graduated varieties. So it is, as I have myself
observed, with some dimorphic plants. It certainly at first appears
a highly remarkable fact that the same female butterfly should have
the power of producing at the same time three distinct female forms
and a male; and that an hermaphrodite plant should produce from
the same seed-capsule three distinct hermaphrodite forms, bearing
three different kinds of females and three or even six different kinds
of males. Nevertheless these cases are only exaggerations of the
ORIGIN OF SPECIES
58
common fact that the female produces offspring of two sexes which
sometimes differ from each other in a wonderful manner,
DOUBTFUL SPECIES
The forms which possess in some considerable degree the char-
acter of species, but which are so closely similar to other forms, or
are so closely linked to them by intermediate gradations, that natu-
ralists do not like to rank them as distinct species, are in several
respects the most important for us. We have every reason to believe
that many of these doubtful and closely allied forms have perma-
nently retained their characters for a long time; for as long, as far
as we know, as have good and true species. Practically, when a
naturalist can unite by means of intermediate links any two forms,
he treats the one as a variety of the other; ranking the most common,
but sometimes the one first described, as the species, and the other
as the variety. But cases of great difficulty, which I will not here
enumerate, sometimes arise in deciding whether or not to rank one
form as a variety of another, even when they are closely connected
by intermediate links; nor will the commonly assumed hybrid
nature of the intermediate forms always remove the difficulty. In
very many cases, however, one form is ranked as a variety of another,
not because the intermediate links have actually been found, but
because analogy leads the observer to suppose either that they do
now somewhere exist, or may formerly have existed; and here a
wide door for the entry of doubt and conjecture is opened.
Hence, in determining whether a form should be ranked as a
species or a variety, the opinion of naturalists having sound judg-
ment and wide experience seems the only guide to follow- We
must, however, in many cases, decide by a majority of natural-
ists, for few well-marked and well-known varieties can be named
which have not been ranked as species by at least some competent
judges.
That varieties of this doubtful nature are far from uncommon,
cannot be disputed. Compare the several floras of Great Britain, of
France, or of the United States, drawn up by different botanists,
and see what a surprising number of forms have been ranked by one
botanist as good species^ and by another as mere varieties. Mr. H. C.
DOUBTFUL SPECIES 59
Watson, to whom I lie under deep obligation for assistance of all
kinds, has marked for me 182 British plants, which are generally
considered as varieties, but which have all been ranked by botanists
as species; and in making this list he has omitted many trifling
varieties, but which nevertheless have been ranked by some botanists
as species, and he has entirely omitted several highly polymorphic
genera. Under genera, including the most polymorphic forms, Mr.
Babington gives 251 species, whereas Mr. Bentham gives only 112, —
a difference of 139 doubtful forms! Amongst animals which unite
for each birth, and which are highly locomotive, doubtful forms,
ranked by one zoologist as a species and by another as a variety, can
rarely be found within the same country, but are common in sepa-
rated areas. How many of the birds and insects in North America
and Europe, which differ very slightly from each other, have been
ranked by one eminent naturalist as undoubted species, and by
another as varieties, or, as they are often called, geographical races!
Mr. Wallace, in several valuable papers on the various animals,
especially on the Lepidoptera, inhabiting the islands of the great
Malayan Archipelago, show that they may be classed under four
heads, namely, as variable forms, as local forms, as geographical
races or sub-species, and as true representative species. The first or
variable forms vary much within the limits of the same island. The
local forms are moderately constant and distinct in each separate
island; but when all from the several islands are compared together,
the differences are seen to be so slight .and graduated, that it is
impossible to define or describe them, though at the same time the
extreme forms are sufficiently distinct. The geographical races or
sub-species are local forms completely fixed and isolated; but as
they do not differ from each other by strongly marked and impor-
tant characters, “There is no possible test but individual opinion to
determine which of them shall be considered as species and which
as varieties.” Lastly, representative species fill the same place in the
natural economy of each island as do the local forms and sub-
species; but as they are distinguished from each other by a greater
amount of difference than that between the local forms and sub-
species, they are almost universffily ranked by naturalists as true
species. Nevertheless, no certain criterion can possibly be given by
ORIGIN OF SPECIES
6o
which variable forms, local forms, sub-species, and representative
species can be recognised.
Many years ago, when comparing, and seeing others compare, the
birds from the closely neighbouring islands of the Galapagos
archipelago, one with another, and with those from the American
mainland, I was* much struck how entirely vague and arbitrary is the
distinction between species and varieties. On the islets of the little
Madeira group there are many insects which are characterised as
varieties in Mr. Wollaston's admirable work, but which would cer-
tainly be ranked as distinct species by many entomologists. Even
Ireland has a few animals, now generally regarded as varieties, but
which have been ranked as species by some zoologists. Several ex-
perienced ornithologists consider our British red grouse as only a
strongly marked race of Norwegian species, whereas the greater
number rank it as an undoubted species peculiar to Great Britain.
A wide distance between the homes of two doubtful forms leads
many naturalists to rank them as distinct species; but what distance,
it has been well asked, will suffice; if that between America and
Europe is ample, will that between Europe and the Azores, or
Madeira, or the Canaries, or between the several islets of these small
archipelagos, be sufficient?
Mr. B. D. Walsh, a distinguished entomologist of the United
States, has described what he calls Phytophagic varieties and Phyto-
phagic species. Most vegetable-feeding insects live on one kind of
plant or on one group of plants; some feed indiscriminately on many
kinds, but do not in consequence vary. In several cases, however,
insects found living on different plants, have been observed by Mr.
Walsh to present in their larval or mature state, or in both states,
slight, though constant differences in colour, size, or in the nature of
their secretions. In some instances the males alone, in other instances
both males and females, have been observed thus to differ in a slight
degree. When the differences are rather more strongly marked, and
when both sexes and all ages are affected, the forms are ranked by
all entomologists as good species. But no observer can determine
for another, even if he can do so for himself, which of these Phyto-
phagic forms ought to be called species and which varieties. Mr,
Walsh ranks the forms which it may be supposed would freely inter
DOUBTFUL SPECIES 6 1
cross, as varieties; and those which appear to have lost this power,
as species. As the differences depend on the insects having long fed
on distinct plants, it cannot be expected that intermediate links
connecting the several forms should now be found. The naturalist
thus loses his best guide in determining whether to rank doubtful
forms as varieties or species. This hkewise necessarily occurs with
closely allied organisms, which inhabit distinct continents or islands.
When, on the other hand, an animal or plant ranges over the same
continent, or inhabits many islands in the same archipelago, and
presents different forms in the different areas, there is always a good
chance that intermediate forms will be discovered which will link
together the extreme states; and these are then degraded to the rank
of varieties.
Some few naturalists maintain that animals never present varie-
ties; but then these same naturalists rank the slightest difference as
of specific value; and when the same identical form is met with in
two distinct countries, or in two geological formations, they believe
that two distinct species are hidden under the same dress. The term
species thus comes to be a mere useless abstraction, implying and as-,
suming a separate act of creation. It is certain that many forms, con-
sidered by highly competent judges to be varieties, resemble species so
completely in character, that they have been thus ranked by other
highly competent judges. But to discuss whether they ought to be
called species or varieties, before any definition of these terms has
been generally accepted, is vainly to beat the air.
Many of the cases of strongly-marked varieties or doubtful species
well deserve consideration; for several interesting lines of argu-
ment, from geographical distribution, analogical variation, hybridism,
etc., have been brought to bear in the attempt to determine their
rank; but space does not here permit me to discuss them. Close in-
vestigation, in many cases, will no doubt bring naturalists to agree
how to rank doubtful forms. Yet it must be confessed that it is in
the best known countries that we find the greatest number of them.
I have been struck with the fact, that if any animal or plant in a state
of nature be highly useful to man, or from any cause closely attracts
his attention, varieties of it will almost universally be found recorded.
These varieties, moreover, will often be ranked by some authors as
62 ORIGIN OF SPECIES
species. Look at the common oak, how closely it has been studied;
yet a German author makes more than a dozen species out of forms,
which are almost universally considered by other botanists to be varie-
ties; and in this country the highest botanical authorities and prac-
tical men can be quoted to show that the sessile and pedunculated
oaks are either good and distinct species or mere varieties.
I may here allude to a remarkable memoir lately published by
A. de Candolle, on the oaks of the whole world. No one ever had
more ample materials for the discrimination of the species, or could
have worked on them with more zeal and sagacity. He first gives
in detail all the many points of structure which vary in the several
species, and estimates numerically the relative frequency of the
variations. He specifies above a dozen characters which may be
found varying even on the same branch, sometimes according to
age or development, sometimes without any assignable reason. Such
characters are not of course of specific value, but they are, as Asa
Gray has remarked in commenting on this memoir, such as gen-
erally enter into specific definitions. De Candolle then goes on to
say that he gives the rank of species to the forms that differ by
characters never varying on the same tree, and never found con-
nected by intermediate states. After this discussion, the result of so
much labour, he emphatically remarks: “They are mistaken, who
repeat that the greater part of our species are clearly limited, and
that the doubtful species are in a feeble minority. This seemed to
be true, so long as a genus was imperfectly known, and its species
were founded upon a few specimens, that is to say, were provisional.
Just as we come to know them better, intermediate forms flow in,
and doubts as to specific limits augment.” He also adds that it is
the best known species which present the greatest number of spon-
taneous varieties and sub-varieties. The Quercus robur has twenty-
eight varieties, all of which, excepting six, are clustered round three
sub-species, namely, Q. pedunculata, sessiliflora, and pubescens. The
forms which connect these three sub-species are comparatively rare;
and, as Asa Gray again remarks, if these connecting forms which
are now rare, were to become wholly extinct, the three sub-species
would hold exacdy the same relation to each other, as do the four
or five provisionally admitted species which closely surround the
DOUBTFUL SPECIES 63
typical Quercus robur. Finally, De Candolle admits that out of the
300 species, which will be enumerated in his Prodromus as belong-
ing to the oak family, at least two-thirds are provisional species, that
is, are not known stricdy to fulfil the definition above given of a
true species. It should be added that De Candolle no longer believes
that species are immutable creations, but concludes that the derivative
theory is the most natural one, “and the most accordant with the
known facts in palaeontology, geographical botany, and zoology, of
anatomical structure and classification.”
When a young naturalist commences the study of a group of
organisms quite unknown to him, he is at first much perplexed in
determining what differences to consider as specific, and what as
varietal; for he knows nothing of the amount and kind of variation
to which the group is subject; and this shows, at least, how very
generally there is some variation. But if he confine his attention to
one class within one country, he will soon make up his mind how to
rank most of the doubtful forms. His general tendency will be to
make many species, for he will become impressed, just like the
pigeon or poultry fancier before alluded to, with the amount of
difference in the forms which he is continually studying; and he
has little general knowledge of analogical variation in other groups
and in other countries, by which to correct his first impressions. As
he extends the range of his observations, he will meet with more
cases of difficulty; for he will encounter a greater number of closely
allied forms. But if his observations be widely extended, he will in
the end generally be able to make up his own mind; but he will
succeed in this at the expense of admitting much variation, and the
truth of this admission will often be disputed by other naturalists.
When he comes to study allied forms brought from countries not
now continuous, in which case he cannot hope to find intermediate
links, he will be compelled to trust almost entirely to analogy, and
his difficulties will rise to a climax.
Certainly no clear line of demarcation has as yet been drawn be-
tween species and sub-species — ^that is, the forms which in the opinion
of some naturalists come very near to, but do not quite arrive at, the
rank of species: or, again, between sub-species and well-marked
varieties, or between lesser varieties and individual differences.
64 ORIGIN OF SPECIES
These differences blend into each other by an insensible series; and
a series impresses the mind with the idea of an actual passage.
Hence I look at individual differences, though of small interest
to the systematist, as of the highest importance for us, as being the
first steps towards such slight varieties as are barely thought worth
recording in works on natural history. And I look at varieties which
are in any degree more distinct and permanent, as steps towards
more strongly-marked and permanent varieties; and at the latter, as
leading to sub-species, and then to species. The passage from one
stage of difference to another may, in many cases, be the simple re-
sult of the nature of the organism and of the different physical con-
ditions to which it has long been exposed; but with respect to the
more important and adaptive characters, the passage from one stage
of difference to another, may be safely attributed to the cumulative
action of natural selection, hereafter to be explained, and to the
effects of the increased use or disuse of parts. A well-marked variety
may therefore be called an incipient species; but whether this belief
is justifiable must be judged by the weight of the various facts and
considerations to be given throughout this work.
It need not be supposed that all varieties or incipient species attain
the rank of species. They may become extinct, or they may endure
as varieties for very long periods, as has been shown to be the case
by Mr. Wollaston with the varieties of certain fossil land shells in
Madeira, and with plants by Gaston de Saporta. If a variety were
to flourish so as to exceed in numbers the parent species, it would
then rank as the species, and the species as the variety; or it might
come to supplant and exterminate the parent species; or both might
co-exist, and both rank as independent species. But we shall here-
after return to this subject.
From these remarks it will be seen that I look at the term species
as one arbitrarily given, for the sake of convenience, to a set of
individuals closely resembling each other, and that it does not es-
sentially differ from the term variety, which is given to less distinct
and more fluctuating forms. The term variety, again, in comparison
with mere individual differences, is also applied arbitrarily, for con-
venience’ sake.
■DOMINANT SPECIES VARY MOST
65
WIDE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES VARY MOST
Guided by theoretical considerations, I thought that some inter-
esting results might be obtained in regard to the nature and rela-
tions of the species which vary most, by tabulating all the varieties
in several well-worked floras. At first this seemed a simple task;
but Mr. H. C. Watson, to whom I am much indebted for valuable
advice and assistance on this subject, soon convinced me that there
were many difficulties as did subsequently Dr. Hooker, even in
stronger terms. I shall reserve for a future work the discussion of
these difficulties, and the tables of the proportional numbers of the
varying species. Dr. Hooker permits me to add that after having
carefully read my manuscript, and examined the tables, he thinks
that the following statements are fairly well established. The whole
subject, however, treated as it necessarily here is with much brevity,
is rather perplexing, and allusions cannot be avoided to the “struggle
for existence,” “divergence of character,” and other questions, here-
after to be discussed.
Alphonse de Candolle and others have shown that plants which
have very wide ranges generally present varieties; and this might
have been expected, as they are exposed to diverse physical condi-
tions, and as they come into competition (which, as we shall here-
after see, is an equally or more important circumstance) with dif-
ferent sets of organic beings. But my tables further show, that, in
any limited country, the species which are the most common, that
is, abound most in individuals, and the species which are most
widely diffused within their own country (and this is a different
consideration from wide range, and to a certain extent from com-
monness), oftenest give rise to varieties sufficiently well-marked to
have been recorded in botanical works. Hence it is the most flourish-
ing, or, as they may be called, the dominant species, — those which
range widely, are the most diffused in their own country, and are
the most numerous in individuals, — which oftenest produce well-
marked varieties, or, as I consider them, incipient species. And this,
perhaps, might have been anticipated; for, as varieties, in order to
become in any degree permanent, necessarily have to struggle with
the other inhabitants of the country, the species which are already
66 ORIGIN OF SPECIES
dominant will be the most likely to yield offspring, which, though
in some slight degree modified, still inherit those advantages that
enabled their parents to become dominant over their compatriots.
In these remarks on predominance, it should be understood that
reference is made only to the forms which come into competition
with each other, and more especially to the members of the same
genus or class having nearly similar habits of life. With respect to
the number of individuals, or commonness of species, the comparison
of course relates only to the members of the same group. One of
the higher plants may be said to be dominant if it be more numerous
in individuals and more widely diffused than the other plants of the
same country, which live under nearly the same conditions. A plant
of this kind is not the less dominant because some conferva inhabit-
ing the water or some parasitic fungus is infinitely more numerous
in individuals, and more widely diffused. But if the conferva or
parasitic fungus exceeds its allies in the above respects, it will then
be dominant within its own class.
SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY MORE FREQUENTLY
THAN THE SPECIES OF THE SMALLER GENERA
If the plants inhabiting a country, as described in any Flora, be
divided into two equal masses, all those in the larger genera
those including many species) being placed on one side, and all those
in the smaller genera on the other side, the former will be found to
include a somewhat larger number of the very common and much
diffused or dominant species. This might have been anticipated; for
the mere fact of many species of the same genus inhabiting any
country, shows that there is something in the organic or inorganic
conditions of that country favourable to the genus; and, consequently,
we might have expected to have found in the larger genera, or those
including many species, a larger proportional number of dominant
species. But so many causes tend to obscure this result, that I am
surprised that my tables show even a small majority on the side of
the larger genera. I will here allude to only two causes of obscurity.
Fresh-water and salt-loving plants generally have very wide ranges
and are much diffused, but this seems to be connected with the
nature of the stations inhabited by them, and has little or no relation
SPECIES OF LARGER GENERA VARIABLE 67
to the size of the genera to which the species belong. Again, plants
low in the scale of organisation are generally much more widely
diffused than plants higher in the scale; and here again there is no
close relation to the size of the genera. The cause of lowly-organised
plants ranging widely will be discussed in our chapter on Geographi-
cal Distribution.
From looldng at species as only strongly marked and well-defined
varieties, I was led to anticipate that the species of the larger genera
in each country would oftener present varieties, than the species of
the smaller genera; for wherever many closely related species (/.(?.,
species of the same genus) have been formed, many varieties or
incipient species ought, as a general rule, to be now forming. Where
many large trees grow, we expect to find saplings. Where many
species of a genus have been formed through variation, circumstances
have been favourable for variation; and hence we might expect that
the circumstances would generally be still favourable to variation.
On the other hand, if we look at each species as a special act of
creation, there is no apparent reason why more varieties should occur
in a group having many species, than in one having few.
To test the truth of this anticipation I have arranged the plants of
twelve countries, and the coleopterous insects of two districts, into
two nearly equal masses, the species of the larger genera on one side,
and those of the smaller genera on the other side, and it has invariably
proved to be the case that a larger proportion of the species on the
side of the larger genera presented varieties, than on the side of the
smaller genera. Moreover, the species of the large genera which
present any varieties, invariably present a larger average number of
varieties than do the species of the small genera. Both these results
follow when another division is made, and when all the least genera,
with from only one to four species, are altogether excluded from the
tables. These facts are of plain signification on the view that species
are only strongly-marked and permanent varieties; for wherever
many species of the same genus have been formed, or where, if we
may use the expression, the manufactory of species has been active,
we ought generally to find the manufactory still in action, more
especially as we have every reason to believe the process of manu-
facturing new species to be a slow one. And this certainly holds true,
6g ORIGIN OF SPECIES
if varieties be looked at as incipient species; for my tables clearly
show as a general rule that, wherever many species of a genus have
been formed, the species of that genus present a number of varieties,
that is, of incipient species, beyond the average. It is not that all large
genera are now varying much, and are thus increasing in the number
of their species, or that no small genera are now varying and increas-
ing; for if this had been so, it would have been fatal to my theory;
inasmuch as geology plainly tells us that small genera have in the
lapse of time often increased gready in size; and that l^rge genera
have often come to their maxima, decline, and disappeared, t^ll that
we want to show is, that, where many species of a genus have been
formed, on an average many are still forming; and this certainly
holds good.
l^IANY OF THE SPECIES INCLUDED WITHIN THE LARGER GENERA RESEMBLE
VARIETIES IN BEING VERY CLOSELY, BUT UNEQUALLY, RELATED TO EACH
OTHER, AND IN HAVING RESTRICTED RANGES
There are other relations between the species of large genera and
their recorded varieties which deserve notice. We have seen that
there is no infallible criterion by which to distinguish species and
well-marked varieties; and when intermediate links have not been
found between doubtful forms, naturalists are compelled to come to
a determination by the amount of difference between them, judging
by analogy whether or not the amount suffices to raise one or both
to the rank of species. Hence the amount of difference is one very
important criterion in settling whether two forms should be ranked
as species or varieties. Now Fries has remarked in regard to plants,
and Westwood in regard to insects, that in large genera the amount
of difference between the species is often exceedingly small. I have
endeavoured to test this numerically by averages, and, as far as my
imperfect results go, they confirm the view. I have also consulted
some sagacious and experienced observers, and, after deliberation,
they concur in this view.* In this respect, therefore, the species of the
larger genera resemble varieties, more than do the species of the
smaller genera. Or the case may be put in another way, and it may be
said, that in the larger genera, in which a number of varieties or incip-
ient species greater than the average are now manufacturing, many of
RESEMBLE VARIETIES 69
the species already manufactured still to a certain extent resemble
varieties, for they differ from each other by less than the usual amount
of difference.
Moreover, the species of the larger genera are related to each other
in the same manner as the varieties of any one species are related to
each other. No naturalist pretends -that all the species of a genus are
equally distinct from each other; they may generally be divided
into sub-genera, or sections, or lesser groups. As Fries has well
remarked, litde groups of species are generally clustered like satel-
lites around other species. And what are varieties but groups of
forms, unequally related to each other, and clustered round certain
forms — ^that is, round their parent-species? Undoubtedly there is one
most important point of difference between varieties and species;
namely, that the amount of difference between varieties, when com-
pared with each other or with their parent-species, is much less than
that between the species of the same genus. But when we come to dis-
cuss the principle, as I call it, of Divergence of Character, we shall see
how this may be explained, and how the lesser differences between
varieties tend to increase into the greater differences between
species.
There is one other point which is worth notice. Varieties generally
have much restricted ranges : this statement is indeed scarcely more
than a truism, for, if a variety were found to have a wider range than
that of its supposed parent-species, their denominations would be
reversed. But there is reason to believe that the species which are
very closely allied to other species, and in so far resemble varieties,
often have much restricted ranges. For instance, Mr. H, C. Watson
has marked for me in the well-sifted London Catalogue of Plants
(4th edition) sixty-three plants which are therein ranked as species,
but which he considers as so closely allied to other species as to be
of doubtful value: these sixty-three reputed species range on an
average over 6.9 of the provinces into which Mr. Watson has divided
Great Britain. Now, in this same catalogue, fifty-three acknowledged
varieties are recorded, and these range over 7.7 provinces; whereas,
the species to which these varieties belong range over 14.3 provinces.
So that the acknowledged varieties have nearly the same restricted
average range, as have the closely allied forms, marked for me by
ORIGIN OF SPECIES
70
Mr. Watson as doubtful species, but which are almost universally
ranked by British botanists as good and true species.
SUMMARY
Finally, varieties cannot be distinguished from species,-— except,
first, by the discovery of intermediate linking forms; and, secondly,
by a certain indefinite amount of difference between them; for two
forms, if differing very little, are generally ranked as varieties, not-
withstanding that they cannot be closely connected; but the amount
of difference considered necessary to give to any two forms the rank
of species cannot be defined. In genera having more than the average
number of species in any country, the species of these genera have
more than the average number of varieties. In large genera the
species are apt to be closely, but unequally, allied together, forming
little clusters round other species. Species very closely allied to other
species apparendy have restricted ranges. In all these respects the
species of large genera present a strong analogy with varieties. And
we can clearly understand these analogies, if species once existed as
varieties, and thus originated; whereas, these analogies are utterly
inexplicable if species are independent creations.
We have, also, seen that it is the most flourishing or dominant
secies of the larger genera within each class which on an average
yield the greatest number of varieties; and varieties, as we shall
hereafter see, tend to become converted into new and distinct species.
Thus the larger genera tend to become larger; and throughout nature
the forms of life which are now dominant tend to become still more
dominant by leaving many modified and dominant descendants.
But by steps hereafter to be explained, the larger genera also tend to
break up into smaller genera. 'And thus, the forms of life throughout
the universe become divided into groups subordinate to groups.
CHAPTER III
Struggle for Existence
Its bearing on natural selection — ^The term used in a wide sense — Geo-
metrical ratio of increase — Rapid increase of naturalized animals and
plants — ^Nature of the checks to increase — Competition universal —
Effects of climate — Protection from the number of individuals —
Complex relations of all animals and plants throughout nature —
Struggle for life most severe between individuals and varieties of the
same species: often severe between species of the same genus — ^The
relation of organism to organism the most important of all relations.
E FORE entering on the subject of this chapter, I must make
a few preliminary remarks, to show how the struggle for
existence bears on Natural Selection. It has been seen in the
last chapter that amongst organic beings in a state of nature there is
some individual variability : indeed I am not aware that this has ever
been disputed. It is immaterial for us whether a multitude of doubt-
ful forms be called species or sub-species or varieties; what rank,
for instance, the two or three hundred doubtful forms of British
plants are entided to hold, if the existence of any well-marked vari-
edes be admitted. But the mere existence of individual variability
and of some few well-marked varieties, though necessary as the
foundation for the work, helps us but little in understanding how
species arise in nature. How have all those exquisite adaptations of
one part of the organisation to another part, and to the conditions
of life, and of one organic being to another being, been perfected?
We see these beautiful co-adaptations most plainly in the woodpecker
and the mistletoe; and only a little less plainly in the humblest para-
site which clings to the hairs of a quadruped or feathers of a bird: in
the structure of the beetle which dives through the water: in
the plumed seed which is wafted by the gentlest breeze; in short, we
see beautiful adaptations everywhere and in every part of the organic
world.
Again, it may be asked, how is it that varieties, which I have called
^2 ORIGIN OF SPECIES
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more
than do the varieties of the same species ? How do those groups of
species, which constitute what are called distinct genera, and which
differ from each other more than do the species of the same genus,
arise? All these results, as we shall more fully see in the next chapter,
follow from the struggle for life. Owing to this struggle, variations,
however slight and from whatever cause proceeding, if they be in
any degree profitable to the individuals of a species, in their infinitely
complex relations to other organic beings and to their physical condi-
tions of life, will tend to the preservation of such individuals, and
will generally be inherited by the offspring. The offspring, also, will
thus have a better chance of surviving, for, of the many individuals
of any species which are periodically born, but a small number can
survive. I have called this principle, by which each slight variation,
if useful, is preserved, by the term Natural Selection, in order to mark
its relation to man’s power of selection. But the expression often
used by Mr. Herbert Spencer, of the Survival of the Fittest, is more
accurate, and is sometimes equally convenient. We have seen that
man by selection can certainly produce great results, and can adapt
organic beings to his own uses, through the accumulation of shght
but useful variations, given to him by the hand of Nature. But
Natural Selection, as we shall hereafter see, is a power incessantly
ready for action, and is as immeasurably superior to man’s feeble
efforts, as the works of Nature are to those of Art.
We will now discuss in a licde more detail the struggle for ex-
istence. In my future work this subject will be treated, as it well
deserves, at greater length. The elder De Candolle and Lyell have
largely and philosophically shown that all organic beings are exposed
to severe competition. In regard to plants, no one has treated this
subject with more spirit and ability than W. Herbert, Dean of Man-
chester, evidendy the result of his great horticultural knowledge.
Nothing is easier than to admit in words the truth of the universal
struggle for life, or more difficult— at least, I have found it so— than
constantly to bear this conclusion in mind. Yet unless it be thor-
oughly engrained in the mind, the whole economy of nature, with
every fact oh distributiom rarity, abundance, extinction, and vari-
STRUGGLE FOR EXISTENCE 73
ation, will be dimly seen or quite misunderstood. We behold the
face of nature bright with gladness, we often see superabundance
of food; we do not see, or we forget, that the birds which are idly
singing round us mostly live on insects or seeds, and are thus con-
stantly destroying life; or we forget how largely these songsters, or
their eggs, or their nestlings, are destroyed by birds and beasts of
prey; we do not always bear in mind, that, though food may be now
superabundant, it is not so at all seasons of each recurring year.
THE TERM, STRUGGLE FOR EXISTENCE, USED IN A LARGE SENSE
I should premise that I use this term in a large and metaphorical |
sense including dependence of one being on another, and including
(which is more important) not only the life of the individual, but
success in leaving progeny. Two canine animals, in a time of dearth,
may be truly said to struggle with each other which shall get food
and live. But a plant on the edge of a desert is said to struggle for life
against the drought, though more properly it should be said to be
dependent on the moisture. A plant which annually produces a
thousand seeds, of which only one of an average comes to maturity,
may be more truly said to struggle with the plants of the same and
other kinds which already clothe the ground. The mistletoe is
dependent on the apple and a few other trees, but can only in a far-
fetched sense be said to struggle with these trees, for, if too many of
these parasites grow on the same tree, it languishes and dies. But
several seedling mistletoes, growing close together on the same
branch, may more truly be said to struggle with each other. As the
mistletoe is disseminated by birds, its existence depends on them;
and it may metaphorically be said to struggle with other fruit-bearing
plants, in tempting the birds to devour and thus disseminate its seeds.
In these several senses, which pass into each other, I use for con-
venience’ sake the general term of Struggle for Existence.
GEOMETRICAL RATIO OF INCREASE
A Struggle for existence inevitably follows from the high rate at
which all organic beings tend to increase. Every being, which during
its natural lifetime produces several eggs or seeds, must suffer
destruction during some period of its life, and during some season
ORIGIN OF SPECIES
74
or occasional year; otherwise, on the principle of geometrical increase,
its numbers would quickly become so inordinately great that no
country could support the product. Hence, as more individuals are
produced than can possibly survive, there must in every case be a
struggle for existence, either one individual with another of the same
species, or with the individuals of distinct species, or with the physical
conditions of life. It is the doctrine of Malthus applied with mani-
fold force to the whole animal and vegetable kingdoms; for in this
case there can be no artificial increase of food, and no prudential
restraint from marriage. Although some species may be now increas-
ing, more or less rapidly, in numbers, all cannot do so, for the world
would not hold them.
There is no exception to the rule that every organic being naturally
increases at so high a rate, that, if not destroyed, the earth would soon
be covered by the progeny of a single pair. Even slow-breeding man
has doubled in twenty-five years, and at this rate in less than a thou-
sand years, there would Hterally not be standing-roomi for his pro-
geny. Linnaeus has calculated that if an annual plant produced only
two seeds—and there is no plant so unproductive as this— and their
seedlings next year produced two, and so on, then in twenty years
there would be a million plants. The elephant is reckoned the slow-
est breeder of all known animals, and I have taken some pains to
estimate its probable minimum rate of natural increase; it will be
safest to assume that it begins breeding when thirty years old, and
goes on breeding till ninety years old, bringing forth six young in
the interval, and surviving till one hundred years old; if this be so,
afmr a period of from 740 to 750 years there would be nearly nineteen
million elephants alive, descended from the first pair.
But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonish-
ingly rapid increase of various animals in a state of nature, when
circumstances have been favourable . to them during two or three
Mowing seasons. Still more striking is the evidence from our
domestic animals of many kinds which have run wild in several
ms of the world; if the statements of the rate of increase of slow-
br^ding cattle and horses in South America, and latterly in Australia,
had not been weU authenticated, they would have been incredible
GEOMETRICAL RATIO OF INCREASE 75
So it is with plants; cases could be given of introduced plants which
have become common throughout whole islands in a . period of less
than ten years. Several of the plants, such as the cardoon and a tall
thistle, which are now the commonest over the wide plains of La
Plata, clothing square leagues of surface almost to the exclusion of
every other plant, have been introduced from Europe; and there are
plants which now range in India, as I hear from Dr. Falconer, from
Cape Comorin to the Himalaya, which have been imported from
America since its discovery. In such cases, and endless others could
be given, no one supposes that the fertility of the animals or plants
has been suddenly and temporarily increased in any sensible degree.
The obvious explanation is that the conditions of life have been
highly favourable, and that there has constantly been less destruction
of the old and young, and that nearly all the young have been enabled
to breed. Their geometrical ratio of increase, the result of which
never fails to be surprising, simply explains their extraordinarily
rapid increase and wide diffusion in their new homes.
In a state of nature almost every full-grown plant annually pro-
duces seed, and amongst animals there are very few which do not
annually pair. Hence we may confidently assert, that all plants and
animals are tending to increase at a geometrical ratio, — that all would
rapidly stock every station in which they could anyhow exist, — and
that this geometrical tendency to increase must be checked by
destruction at some period of life. Our familiarity with the larger
domestic animals tends, I think, to mislead us: we see no great de-
struction falling on them, but we do not keep in mind that thousands
are annually slaughtered for food, and that in a state of nature an
equal number would have somehow to be disposed of.
The only difference between organisms which annually produce
eggs or seeds by the thousand, and those which produce extremely
few, is, that the slow-breeders would require a few more years to
people, under favourable conditions, a whole district, let it be ever
so large. The condor lays a couple of eggs and the ostrich a score,
and yet in the same country the condor may be the more numerous
of the two; the Fulmar petrel lays but one egg, yet it is believed to
be the most numerous bird in the world. One fly deposits hundreds
eggs, and another, like the hippobosca, a single one; but this dif-
76 ORIGIN OF SPECIES
ference does not determine how many individuals o£ the two species
can be supported in a district, A large number of eggs is of some
importance to those species which depend on a fluctuating amount
of food, for it allows them rapidly to increase in number. But the
real importance of a large number of eggs or seeds is to make up for
much destruction at some period of life; and this period in the great
majority of cases is an early one. If an animal can in any way protect
its own eggs or young, a small number may be produced, and yet the
average stock be fully kept up; but if many eggs or young are
destroyed, many must be produced, or the species will become
extinct. It would suffice to keep up the full number of a tree, which
lived on an average for a thousand years, if a single seed were pro-
duced once in a thousand years, supposing that this seed were never
destroyed, and could be ensured to germinate in a fitting place. So
that, in all cases, the average number of any animal or plant depends
only indirectly on the number of its eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind— never to forget that every single
organic being may be said to be striving to the utmost to increase in
numbers; that each lives by a struggle at some period of its life;
that heavy destruction inevitably falls either on the young or old,
during each generation or at recurrent intervals. Lighten any check,
mitigate the destruction ever so little, and die number of the species
will almost instantaneously increase to any amount.
NATURE OF THE CHECKS TO INCREASE
The causes which check the natural tendency of each species to
increase are most obscure. Look at the most vigorous species; by as
much as it swarms in numbers, by so much will it tend to increase
still further. We know not exactly what the checks are even in a
single instance. Nor will this surprise any one who reflects how
ignorant we are on this head, even in regard to mankind, although
so incomparably better known than any other animal. This subject
of the checks to increase has been ably treated by several authors,
and I hope in a future work to discuss it at considerable length, more
especially in regard to the feral animals of South America. Here I
will make only a few remarks, just to recall to the reader’s mind
NATURE OF THE CHECKS TO INCREASE 77
some of the chief points. Eggs or very young animals seem generally
to suffer most, but this is not invariably the case. With plants there
is a vast destruction of seeds, but, from some observations which I
have made, it appears that the seedlings suffer most from germi-
nating in ground already thickly stocked with other plants. Seed-
lings, also, are destroyed in vast numbers by various enemies; for
instance, on a piece of ground three feet long and two wide, dug and
cleared, and where there could be no choking from other plants, I
marked all the seedlings of our native weeds as they came up, and
out of 357 no less than 295 were destroyed, chiefly by slugs and
insects. If turf which has long been mown (and the case would be
the same with turf closely browsed by quadrupeds) be let to grow,
the more vigorous plants gradually kill the less vigorous, though
fully grown plants; thus out of twenty species growing on a little
plot of mown turf (three feet by four) nine species, perished, from
the other species being allowed to grow up freely.
The amount of food for each species of course gives the extreme
limit to which each can increase; but very frequently it is not the
obtaining food, but the serving as prey to other animals, which
determines the average numbers of a species. Thus, there seems to
be little doubt that the stock of partridges, grouse and hares on any
large estate depends chiefly on the destruction of vermin. If not one
head of game were shot during the next twenty years in England,
and, at the same time, if no vermin were destroyed, there would, in
all probability, be less game than at present, although hundreds of
thousands of game animals are now annually shot. On the other
hand, in some cases, as with the elephant, none are destroyed by
beasts of prey; for even the tiger in India most rarely dares to attack
a young elephant protected by its dam.
Climate plays an important part in determining the average num-
bers of a species, and periodical seasons of extreme cold or drought
seem to be the most effective of all checks. I estimated (chiefly from
the greatly reduced numbers of nests in the spring) that the winter
of 1854-5 destroyed four-fifths of the birds in my own grounds; and
this is a tremendous destruction, when we remember that ten per
cent, is an extraordinarily severe mortality from epidemics with man.
The action of climate seems at first sight to be quite independent of
ORIGIN OF SPECIES
78
the struggle for existence; but in so far as climate chiefly acts in
reducing food, it brings on the most severe struggle between the
individuals, whether of the same or of distinct species, which subsist
on the same kind of food. Even when climate, for instance, extreme
cold, acts directly, it will be the least vigorous individuals, or those
which have got least food through the advancing winter, which will
sujSer the most. When we travel from south to north, or from a
damp region to a dry, we invariably see some species gradually get-
ting rarer and rarer, and finally disappearing; and the change of
climate being conspicuous, we are tempted to attribute the whole
effect to its direct action. But this is a false view; we forget that each
species, even where it most abounds, is constantly suffering enormous
destruction at some period of its life, from enemies or from competi-
tors for the same place and food; and if these enemies or competitors
be in the least degree favoured by any slight change of climate, they
will increase in numbers; and as each area is already fully stocked
with inhabitants, the other species must decrease. When we travel
southward and see a species decreasing in numbers, we may feel sure
that the cause lies quite as much in other species being favoured, as
in this one being hurt. So it is when we travel northward, but in a
somewhat lesser degree, for the number of species of all kinds, and
therefore of competitors, decreases northwards; hence in going north-
wards, or in ascending a mountain, we far oftener meet with stunted
forms, due to the directly injurious action of climate, than we do in
proceeding southwards or in descending a mountain. When we
reach the Arctic regions, or snow-capped summits, or absolute deserts,
the struggle for life is almost exclusively with the elements.
That climate acts in main part indirectly by favouring other species,
we clearly see in the prodigious number of plants which in our
prdens can perfectly well endure our climate, but which never
became naturalised, for they cannot compete with our native plants
nor resist destruction by our native animals.
_ When a species, owing to highly favoured circumstances, increases
inordinately in numbers m a small tract, epidemics— at least, this
seems generally to occur with our game animals— often ensue; and
here we have a hmiting check independent of the struggle for life.
But even some of these so-called epidemics appear to be due to para-
STRUGGLE FOR EXISTENCE 79
sitic worms, which have from some cause, possibly in part through
facility of diffusion amongst the crowded animals, been dispropor-
tionally favoured: and here comes in a sort of struggle between the
parasite and its prey.
On the other hand, in many cases, a large stock of individuals of/
the same species, relatively to the numbers of its enemies, is absolutely
necessary for its preservation. Thus we can easily raise plenty of
corn and rape-seed, etc., in our fields, because the seeds are in great
excess, compared with the number of birds which feed on them; nor
can the birds, though having a superabundance of food at this one
season, increase in number proportionally to the supply of seed, as
their numbers are checked during winter; but any one who has tried,
knows how troublesome it is to get seed from a few wheat or other
such plants in a garden: I have in this case lost every single seed. This
view of the necessity of a large stock of the same species for its pres^
ervation, explains, I believe, some singular facts in nature such as that
of very rare plants being sometimes extremely abundant, in the few
spots where they do exist; and that of some social plants being social,
that is, abounding in individuals, even on the extreme verge of their
range. For in such cases, we may believe, that a plant could exist
only where the conditions of its life were so favourable that many
could exist together, and thus save the species from utter destruction.
I should add that the good effects of intercrossing, and the ill effects
of close interbreeding, no doubt come into play in many of these
cases; but I will not here enlarge on this subject.
COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO EACH OTHER IN THE
STRUGGLE FOR EXISTENCE
Many cases are on record showing how complex and unexpected
are the checks and relations between organic beings, which have to
struggle together in the same country. I will give only a single
instance, which, though a simple one, interested me. In Stafford-
shire, on the estate of a relation, where I had ample means of inves-
tigation, there was a large and extremely barren heath, which had
never been touched by the hand of man; but several acres of exactly
the same nature had been enclosed twenty-five years previously and
planted with Scotch fir. The change in the native vegetation of the
8o ORIGIN OF SPECIES
planted part o£ the heath was most remarkable, more than is gen-
erally seen in passing from one quite different soil to another: not
only the proportional numbers of the heath plants were wholly
changed, but twelve species of plants (not counting grasses and
carices) flourished in the plantations, which could not be found on
the heath. The effect on die insects must have been still greater, for
six insectivorous birds were very common in the plantations, which
were not to be seen on the heath; and the heath was frequented by
two or three distinct insectivorous birds. Here we see how potent
has been the effect of the introduction of a single tree, nothing what-
ever else having been done, with the exception of the land having
been enclosed, so that cattie could not enter. But how important an
element enclosure is, I plainly saw near Farnham, in Surrey. Here
there are extensive heaths, with a few clumps of old Scotch firs on
the distant hilltops: within the last ten years large spaces have been
enclosed, and self-sown firs are now springing up in multitudes, so
close together that all cannot live. When I ascertained that these
young trees had not been sown or planted, I was so much surprised
at their numbers that I went to several points of view, whence I could
examine hundreds of acres of the unenclosed heath, and literally I
could not see a single Scotch fir, except the old planted clumps. But
on looking closely between the stems of the heath, I found a multi-
tude of seedlings and little trees which had been perpetually browsed
down by the cattle. In one square yard, at a point some hundred
yards distant from one of the old clumps, I counted thirty-two little
trees; and one of them, with twenty-six rings of growth, had, during
many years, tried to raise its head above the stems of the heath, and
had failed. No wonder that, as soon as the land was enclosed, it
became thickly clothed with vigorously growing young firs. Yet the
heath was so extremely barren and so extensive that no one would
ever have imagined that cattle would have so closely and effectually
searched it for food.
Here we see that cattle absolutely determine the existence of the
Scotch fir; but in several parts of the world insects determine the
existence of cattle. Perhaps Paraguay offers the most curious instance
of this; for here neither cattle nor horses nor dogs have ever run
wild, though 'they swarm southward and northward in a feral state;
STRUGGLE FOR EXISTENCE 8 1
and Azara and Rengger have shown that this is caused by the greater
number in Paraguay o£ a certain fly, which lays its eggs in the navels
of these animals when first born. The increase of these flies,
numerous as they are, must be habitually checked by some means,
probably by other parasitic insects. Hence, if certain insectivorous
birds were to decrease in Paraguay, 'the parasitic insects would prob-
ably increase; and this would lessen the number of the navel-
frequenting flies — then cattle and horses would become feral, and this
would certainly greatly alter (as indeed I have observed in parts of
South America) the vegetation: this again would largely affect the
insects; and this, as we have just seen in Staffordshire, the insectiv-
orous birds, and so onwards in ever-increasing circles of complexity.
Not ‘that under nature the relations will ever be as simple as this.
Battle within battle must be continually recurring with varying suc-
cess; and yet in the long-run the forces are so nicely balanced, that
the face of nature remains for long periods of time uniform, though
assuredly the merest trifle would give the victory to one organic being
over another. Nevertheless, so profound is our ignorance, and so
high our presumption, that we marvel when we hear of the extinc-
tion of an organic being; and as we do not see the cause, we invoke
cataclysms to desolate the world, or invent laws on the duration of
the forms of life!
I am 'tempted to give one more instance showing how plants and
animals, remote in the scale of nature, are bound together by a web
of complex relations. I shall hereafter have occasion to show that
the exotic Lobelia fulgens is never visited in my garden by insects,
and consequently, from its peculiar structure, never sets a seed.
Nearly all our orchidaceous pl«mts absolutely require the visits of
insects to remove their pollen-masses and thus to fertilise them. I find
from experiments that bumblebees are almost indispensable to the
fertilisation of the heartsease (Viola tricolor), for other bees do not
visit this flower. I have also found that the visits of bees are necessary
for the fertilisation of some kinds of clover; for instance, twenty
heads of Dutch clover (Trifolium repens) yielded 2,290 seeds, but
twenty other heads protected from bees produced not one. Again,
one hundred heads of red clover (T. pratense) produced 2,700 seeds,
but the same number of protected heads produced not a single seed.
82 ORIGIN OF SPECIES
Humblebees alone visit red clover, as other bees cannot reach the
nectar. It has been suggested that moths may fertilise the clovers j
but I doubt whether they could do so in the case of the red clover,
from their weight not being sufiBcient to depress the wing petals.
Hence we may infer as highly probable that, if the whole genus of
^bumblebees became extinct or very rare in England, the heartsease
and red clover would become very rare, or wholly disappear. The
number of humblebees in any district depends in a great measure
upon the number of field mice, which destroy their combs and nests;
and Colonel Newman, who has long attended to the habits of hum-
blebees, believes that “more than two-thirds of them are thus de-
stroyed all over England.’’ Now the number of mice is largely
dependent, as every one knows, on the number of cats; and Colonel
Newman says, “Near villages and small towns I have found the
nests of humblebees more numerous than elsewhere, which I attrib-
ute to the number of cats that destroy the mice.” Hence it is quite
credible that the presence of a fehne animal in large numbers in a
district might determine, through the intervention first of mice and
then of bees, the frequency of certain flowers in that district!
In the case of every species, many different checks, acting at
different periods of life, and during different seasons or years, prob-
ably come into play; some one check or some few being generally
the most potent; but all will concur in determining the average
number or even the existence of the species. In some cases it can
be shown that widely different checks act on the same species in
different districts. When we look at 'the plants and bushes clothing
an entangled bank, we are tempted to attribute their proportional
numbers and kinds to what we call chance. But how false a view is
this! Every one has heard that when an American forest is cut down,
a very different vegetation springs up; but it has been observed that
ancient Indian ruins in the southern United States, which must for-
merly have been cleared of trees, now display the same beautiful
diversity and proportion of kinds as in the surrounding virgin forest.
What a struggle must have gone on during long centuries between
the several kinds of trees, each annually scattering its seeds by the
thousand; what war between insect and insect — ^between insects,
snails, and other animals with birds and beasts of prey — all striving
STRUGGLE FOR EXISTENCE 83
to increase, all feeding on each other, or on the trees, their seeds and
seedlings, or on the other plants which first clothed the ground and
thus checked the growth of the trees! Throw up a handful of
feathers, and all fall to the ground according to definite laws; but
how simple is the problem where each shall fall compared to that of
the action and reaction of the innumerable plants and animals which
have determined, in the course of centuries, the proportional num-
bers and kinds of trees now growing on the old Indian ruins!
The dependency of one organic being on another, as of a parasite
on its prey, lies generally between beings remote in the scale of
nature. This is likewise sometimes the case with those which may
be strictly said to struggle with each other for existence, as in the
case of locusts and grass-feeding quadrupeds. But the struggle will
almost invariably be most severe between the individuals of the same
species, for they frequent the same districts, require the same food,
and are exposed to the same dangers. In the case of varieties of the
same species, the struggle will generally be almost equally severe,
and we sometimes see the contest soon decided: for instance, if
several varieties of wheat be sown together, and the mixed seed be
resown, some of the varieties which best suit the soil or climate, or
are naturally the most fertile, will beat the others and so yield more
seed, and will consequently in a few years supplant the other vari-
eties. To keep up a mixed stock of even such extremely close varieties
as the variously-coloured sweet peas, they must be each year harvested
separately, and the seed then mixed in due proportion, otherwise the
weaker kinds will steadily decrease in number and disappear. So
again with the varieties of sheep; it has been asserted that certain
mountain varieties will starve out other mountain varieties, so that
they cannot be kept together. The same result has followed from
keeping together different varieties of the medicinal leech. It may
even be doubted whether the varieties of any of our domestic plants
or animals have so exactly the same strength, habits, and constitution,
that the original proportions of a mixed stock (crossing being pre-
vented) could be kept up for half-a-dozen generations, if they were
allowed to struggle together, in the same manner as beings in a state
of nature, and if the seed or young were not annually preserved in
due proportion.
ORIGIN OF SPECIES
STRUGGLE FOR LIFE MOST SEVERE BETWEEN INDIVIDUALS AND VARIETIES OF
THE SAME SPECIES
As the species of the same genus usually have, though by no means
invariably, much similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between them,
if they come into competition with each other, than between the
species of distinct genera. We see this in the recent extension over
parts of the United States of one species of swallow having caused
the decrease of another species. The recent increase of the missel
thrush in parts of Scotland has caused the decrease of the song thrush.
How frequently we hear of one species of rat taking the place of
another species under the most different climates! In Russia the
small Asiatic cockroach has everywhere driven before it its great
congener. In Australia the imported hive bee is rapidly extermi-
nating the small, stingless native bee. One species of charlock has
been known to supplant another species; and so in other cases.
We can dimly see why the competition should be most severe be-
tween allied forms, which fill nearly the same place in the economy
of nature; but probably in no one case could we precisely say why
one species has been victorious over another in the great battle
of life.
A corollary of the highest importance may be deduced from the
foregoing remarks, namely, that the structure of every organic being
is related, in the most essential yet often hidden manner, to that of all
the other organic beings, with which it comes into competition for
food or residence, or from which k has to escape, or on which it
preys. This is obvious in the structure of the teeth and talons of the
tiger; and in that of the legs and claws of the parasite which clings
to the hair on the tiger’s body. But in the beautifully plumed seed
of the dandelion, and in the flattened and fringed legs of the water
beede, -the relation seems at first confined to the elements of air and
water. Yet the advantage of plumed seeds no doubt stands in the
closest relation to the land being already thickly clothed with other
plants; so that the seeds may be widely distributed and fall on unoc-
cupied ground. In the water beede, the structure of its legs, so well
adapted for diving, allows it to compete with other aquatic insects.
STRUGGLE FOR EXISTENCE 85
to hunt for its own prey, and to escape serving as prey to other
animals.
The store of nutriment laid up within the seeds of many plants
seems at first sight to have no sort of relation to other plants. But
from the strong growth of young plants produced from such seeds,
as peas and beans, when sown in the midst of long grass, it may be
suspected that the chief use of the nutriment in the seed is to favour
the growth of the seedlings, whilst struggling with other plants
growing vigorously all around.
Look at a plant in the midst of its range! Why does it not double
or quadruple its numbers? We know that it can perfectly well with-
stand a little more heat or cold, dampness or dryness, for elsewhere it
ranges into slightly hotter or colder, damper or drier districts. In this
case we can clearly see that if we wish in imagination to give the
plant the power of increasing in number, we should have to give it
some advantage over its competitors, or over the animals which prey
on it. On the confines of its geographical range, a change of consti-
tution with respect to climate would clearly be an advantage to our
plant; but we have reason to believe that only a few plants or animals
range so far, that they are destroyed exclusively by the rigour of the
climate. Not until we reach the extreme confines of life, in the
Arctic regions or on the borders of an utter desert, will competition
cease. The land may be extremely cold or dry, yet there will be
competition between some few species, or between the individuals
of the same species, for the warmest or dampest spots.
Hence we can see that when a plant or animal is placed in a new
country amongst new competitors, the conditions of its life will gen-
erally be changed in an essential manner, although the climate may
be exactly the same as in its former home. If its average numbers
are to increase in its new home, we should have to modify it in a
different way to what we should have had to do in its native country;
for we should have to give it some advantage over a different set of
competitors or enemies.
It is good thus to try in imagination to give to any one species an
advantage over another. Probably in no single instance should we
know what to do. This ought to convince us of our ignorance on
the mutual relations of all organic beings; a conviction as necessary,
ORIGIN OF SPECIES
86
as it is difficult to acquire. All that we can do, is to keep steadily in
mind that each organic being is striving to increase in a geometrical
ratio; that each at some period of its life, during some season of the
year, during each generation, or at intervals, has to struggle for life
and to suffer great destruction. When we reflect on this struggle, we
may console ourselves with the full belief, that the war of nature is
not incessant, that no fear is felt, that death is generally prompt, and
that the vigorous, the healthy, and the happy survive and multiply.
CHAPTER IV
Natural Selection; or the Survival of the Fittest
Natural Selection — ^its power compared with man’s selection — ^its power
on characters of trifling importance — its power at all ages and on
both sexes — Sexual selection — On the generality of intercrosses
between individuals of the same species— Circumstances favourable
and unfavourable to the results of Natural Selection, namely, inter-
crossing, isolation, number of individuals — Slow action — ^Extinction
caused by Natural Selection — ^Divergence of Character, related to the
diversity of inhabitants of any small area, and to naturalisation —
Action of Natural Selection, through divergence of Character and
Extinction, on the descendants from a common parent — ^Explains the
grouping of all organic beings — Advance in organisation — ^Low
forms preserved — Convergence of Character — Indefinite multipli-
cation of species — Summary.
H OW will the struggle for existence, briefly discussed in the
last chapter, act in regard to variation ? Can the principle
of selection, which we have seen is so potent in the hands
of man, apply under nature? I think we shall see that it can act
most efficiently. Let the endless number of slight variations and
individual differences occurring in our domestic productions, and,
in a lesser degree, in those under nature, be borne in mind; as well
as the strength of the hereditary tendency. Under domestication, it
may be truly said that the whole organisation becomes in some de-
gree plastic. But the variability, which we almost universally meet
with in our domestic productions, is npt directly produced, as
Hooker and Asa Gray have well remarked, by man; he can neither
originate varieties, nor prevent their occurrence; he can only pre-
serve and accumulate such as do occur. Unintentionally he exposes
organic beings to new and changing conditions of life, and variabil-
ity ensues; but similar changes of conditions might and do occur
under nature. Let it also be borne in mind how infinitely complex
and close-fitting are the mutual relations of all organic beings to
each other and to their physical conditions of life; and consequently
87
ORIGIN OF SPECIES
88
what infinitely varied diversities of structure might be of use to
each being under changing conditions of life. Can it, then, be
thought improbable, seeing that variations useful to man have um
doubtedly occurred, that other variations useful in some way to each
being in the great and complex battle of life, should occur in the
course of many successive generations? If such do occur, can we
doubt (remembering that many more individuals are born than can
possibly survive) that individuals having any advantage, however
slight, over others, would have the best chance of surviving and of
procreating their kind? On the other hand, we may feel sure that
any variation in the least degree injurious would be rigidly destroyed.
This preservation of favourable individual differences and variations,
and the destruction of those which are injurious, I have called
Natural Selection, or the Survival of the Fittest. Variations neither
useful nor injurious would not be affected by natural selection, and
would be left either a fluctuating element, as perhaps we see in cer-
tain polymorphic species, or would ultimately become fixed, owing
to the nature of the organism and the nature of the conditions.
Several writers have misapprehended or objected to the term
Natural Selection. Some have even imagined that natural selection
induces variability, whereas it implies only the preservation of such
variations as arise and are beneficial to the being under its conditions
of life. No one objects to agriculturists speaking of the potent effects
of man’s selection; and in this case the individual differences given
by nature, which man for some object selects, must of necessity first
occur. Others have objected that the term selection implies con-
scious choice in the animals which become modified; and it has
even been urged that, as plants have no volition, natural selection is
not applicable to them! In the literal sense of the word, no doubt,
natural selection is a false term; but who ever objected to chemists
speaking of the elective affinities of the various elements ? — and yet
an acid cannot strictly be said to elect the base with which it in
preference combines. It has been said that I speak of natural selection
as an active power or deity; but who objects to an author speaking
of the attraction of gravity as ruling the movements of the planets?
Every one knows what is meant and is implied by such metaphorical
expressions; and they are almost necessary for brevity. So again it
NATURAL SELECTION
»9
is difficult to avoid personifying the word Nature; but I mean by
Nature, only the aggregate action and product of many natural
laws, and by laws the sequence of events as ascertained by us. With
a little familiarity such superficial objections will be forgotten.
We shall best understand the probable course of natural selection
by taking the case of a country undergoing some slight physical
change, for instance, of climate. The proportional numbers of its
inhabitants will almost immediately undergo a chang4 and some
species will probably become extinct. We may conclude, from what
we have seen of the intimate and complex manner in which the
inhabitants of each country are bound together, that any change in
the numerical proportions of the inhabitants, independently of the
change of climate itself, would seriously affect the others. If the
country were open on its borders, new forms would certainly im-
migrate, and this would likewise seriously disturb the relations of
some of the former inhabitants. Let it be remembered how power-
ful the influence of a single introduced tree or mammal has been
shown to be. But in the case of an island, or of a country partly
surrounded by barriers, into which new and better adapted forms
could not freely enter, we should then have places in the economy
of nature which would assuredly be better filled up, if some of the
original inhabitants were in some manner modified; for, had the
area been open to immigration, these same places would have been
seized on by intruders. In such cases, slight modifications, which in
any way favoured the individuals of any species, by better adapting
them to their altered conditions, would tend to be preserved; and
natural selection would have free scope for the work of improvement.
We have good reason to believe, as shown in the first chapter,
that changes in the conditions of life give a tendency to increased
variability; and in the foregoing cases the conditions have changed,
and this would manifestly be favourable to natural selection, by
affording a better chance of the occurrence of profitable variations.
Unless such occur, natural selection can do nothing. Under the term
of “variations,” it must never be forgotten that mere individual
differences are included. As man can produce a great result with
his domestic animals and plants by adding up in any given direc-
tion individual differences, so could natural selection, but far more
90 ORIGIN OF SPECIES
easily from having incomparably longer time for action. Nor do I
believe that any great physical change, as of climate, or any unusual
degree of isolation, to check immigration, is necessary in order that
new and unoccupied places should be left, for natural selection to
fill up by improving some of the varying inhabitants. For as all
the inhabitants of each country are struggling together with nicely
balanced forces, extremely slight modifications in the structure or
habits of one species would often give it an advantage over others;
and still further modifications of the same kind would often still
further increase the advantage, as long as the species continued under
the same conditions of life and profited by similar means of sub-
sistence and defence. No country can be named in which all the
native inhabitants are now so perfectly adapted to each other and
to the physical conditions under which they live, that none of them
could be still better adapted or improved; for in all countries, the
natives have been so far conquered by naturalised productions, that
they have allowed some foreigners to take firm possession of the
land. And as foreigners have thus in every country beaten some of
the natives, we may safely conclude that the natives might have been
modified with advantage, so as to have better resisted the intruders.
As man can produce, and certainly has produced, a great result
by his methodical and unconscious means of selection, what may
not natural selection effect? Man can act only on external and
visible characters: Nature, if I may be allowed to personify the
natural preservation or survival of the fittest, cares nothing for
appearances, except in so far as they are useful to any being. She
can act on every internal organ, on every shade of constitutional
difference, on the whole machinery of life. Man selects only for his
own good: Nature only for that of the being which she tends. Every
selected character is fully exercised by her, as is implied by the fact
of their selection. Man keeps the natives of many climates in the
same country; he seldom exercises each selected character in some
peculiar and fitting manner; he feeds a long and a short beaked
pigeon on the same food; he does not exercise a long-backed or
long-legged quadruped in any peculiar manner; he exposes sheep
with long and short wool to the same climate. He does not allow
the most vigorous males to struggle for the females. He does not
NATURAL SELECTION
9 ^
rigidly destroy all inferior animals, but protects during each varying
season, as far as lies in his power, all his productions. He often
begins his selection by some half-monstrous form; or at least by
some modification prominent enough to catch the eye or to be plainly
useful to him. Under nature, the slightest differences of structure or
constitution may well turn the nicely balanced scale in the struggle
for life, and so be preserved. How fleeting are the wishes and efforts
of man! how short his time! and consequently how poor will be
his results, compared with those accumulated by Nature during
whole geological periods? Can we wonder, then, that Nature’s pro-
ductions should be far “truer” in character than man’s productions;
that they should be infinitely better adapted to the most complex
conditions of life, and should plainly bear the stamp of far higher
workmanship?
It may metaphorically be said that natural selection is daily and
hourly scrutinising, throughout the world, the slightest variations;
rejecting those that are bad, preserving and adding up all that are
good; silently and insensibly working, whenever and wherever op-
portunity offers, at the improvement of each organic being in rela-
tion to its organic and inorganic conditions of life. We see nothing
of these slow changes in progress, until the hand of time has marked
the lapse of ages, and then so imperfect is our view into long-past
geological ages, that we see only that the forms of Hfe are now
different from what they formerly were.
In order that any great amount of modification should be effected
in a species, a variety, when once formed, must again, perhaps after
a long interval of time, vary or present individual differences of the
same favourable nature as before; and these must be again pre-
served, and so onwards step by step. Seeing that individual differ-
ences of the same kind perpetually recur, this can hardly be con-
sidered as an unwarrantable assumption. But whether it is true, we
can judge only by seeing how far the hypothesis accords with and
explains the general phenomena of nature. On the other hand, the
ordinary belief that the amount of possible variation is a strictly
limited quantity is likewise a simple assumption.
Although natural selection can act only through and for the good
of each being, yet ' characters and structures, which we are apt to
92 ORIGIN OF SPECIES
consider as of very trifling importance, may thus be acted on. When
we see leaf-eating insects green, and bark-feeders mottled-grey; the
alpine ptarmigan white in winter, the red grouse the colour of
heather, we must believe that these tints are of service to these birds
and insects in preserving them from danger. Grouse, if not destroyed
at some period of their lives, would increase in countless numbers;
they are known to suffer largely from birds of prey; and hawks are
guided by eyesight to their prey — ^so much so, that on parts of the
Continent persons are warned not to keep white pigeons, as being
the most liable to destruction. Hence natural selection might be
effective in giving the proper colour to each kind of grouse, and in
keeping that colour, when once acquired, true and constant. Nor
ought we to think that the occasional destruction of an animal of
any particular colour would produce little effect: we should remem-
ber how essential it is in a flock of white sheep to destroy a lamb
with the faintest trace of black. We have seen how the colour of
the hogs, which feed on the “paint root” in Virginia, determines
whether they shall live or die. In plants, the down on the fruit and
the colour of the flesh are considered by botanists as characters of
the most trifling importance: yet we hear from an excellent horti-
culturist, Downing, that in the United States smooth-skinned fruits
suffer far more from a beetle, a Curculio, than those with down;
that purple plums suffer far more from a certain disease than yellow
plums, whereas another disease attacks yellow-fleshed peaches far
more than those with other coloured flesh. If, with all the aids of
art, these slight differences make a great difference in cultivating
the several varieties, assuredly, in a state of nature, where the trees
would have to struggle with other trees and with a host of enemies,
such differences would effectually settle which variety, whether a
smooth or downy, a yellow or purple fleshed fruit, should succeed.
In looking at many small points of difference between species,
which, as far as our ignorance permits us to judge, seem quite un-
important, we must not forget that climate, food, etc., have no doubt
produced some direct effect. It is also necessary to bear in mind
that, owing to the law of correlation, when one part varies, and the
variations are accumulated through natural selection, other modifi-
cations, often of the most unexpected nature, will ensue.
NATURAL SELECTION 93
As we see that those variations which, under domestication, ap-
pear at any particular period of life, tend to reappear in the offspring
at the same period; for instance, in the shape, size, and flavour of
the seeds of the many varieties of our culinary and agricultural
plants; in the caterpillar and cocoon stages of the varieties of the
silkworm; in the eggs of poultry, and in the colour of the down of
their chickens; in the horns of our sheep and cattle when nearly
adult; so in a state of nature natural selection will be enabled to act
on and modify organic beings at any age, by the accumulation of
variations profitable at that age, and by their inheritance at a cor-
responding age. If it profit a plant to have its seeds more and more
widely disseminated by the wind, I can see no greater difficulty in
this being effected through natural selection, than in the cotton
planter increasing and improving by selection the down in the pods
on his cotton trees. Natural selection may modify and adapt the
larva of an insect to a score of contingencies, wholly different from
those which concern the mature insect; and these modifications may
effect, through correlation, the structure of the adult. So, conversely,
modifications in the adult may affect the structure of the larva; but
in all cases natural selection will ensure that they shall not be in-
jurious: for if they were so, the species would become extinct.
Natural selection will modify the structure of the young in rela-
tion to the parent, and of the parent in relation to the young. In
social animals it will adapt the structure of each individual for the
benefit of the whole community; if the community profits by the
selected change. What natural selection cannot do, is to modify the
structure of one species, without giving it any advantage, for the
good of another species; and though statements to this effect may
be found in works of natural history, I cannot find one case which
will bear investigation. A structure used only once in an animaPs
life, if of high importance to it, might be modified to any extent by
natural selection; for instance, the great jaws possessed by certain
insects, used exclusively for opening the cocoon — or the hard tip to
the beak of unhatched birds, used for breaking the egg. It has been
asserted, that of the best short-beaked tumbler pigeons a greater
number perish in the egg than are able to get out of it; so that
fanciers assist in the act of hatching. Now, if nature had to make
94 ORIGIN OF SPECIES
the beak o£ a full-grown pigeon very short for the bird’s own ad-
vantage, the process of modification would be very slow, and there
would be simultaneously the most rigorous selection of all the young
birds within the egg, which had the most powerful and hardest
beaks, for all with weak beaks would inevitably perish; or, more
delicate and more easily broken shells might be selected, the thick-
ness of the shell being known to vary like every other structure.
It may be well here to remark that with all beings there must be
much fortuitous destruction, which can have little or no influence
on the course of natural selection. For instance a vast number of
eggs or seeds are annually devoured, and these could be modified
through natural selection only if they varied in some manner which
protected them from their enemies. Yet many of these eggs or
seeds would perhaps, if not destroyed, have yielded individuals bet-
ter adapted to their conditions of life than any of those which hap-
pened to survive. So again a vast number of mature animals and
plants, whether or not they be the best adapted to their conditions,
must be annually destroyed by accidental causes, which would not
be in the least degree mitigated by certain changes of structure or
constitution which would in other ways be beneficial to the species.
But let the destruction of the adults be ever so heavy, if the number
which can exist in any district be not wholly kept down by such
causes,— or again let the destruction of eggs or seeds be so great that
only a hundredth or a thousandth part are developed,— yet of those
which do survive, the best adapted individuals, supposing that there
is any variability in a favourable direction, will tend , to propagate
their kind in larger numbers than the less well adapted. If the
numbers be wholly kept down by the causes just indicated, as will
often have been the case, natural selection will be powerless in cer-
tain beneficial directions; but this is no valid objection to its ejffl-
ciency at other times and in other ways; for we are far from having
any reason to suppose that many species ever undergo modification
and improvement at the same time in the same area.
SEXUAL SELECTION
Inasmuch as peculiarities often appear under domestication in one
sex and become hereditarily attached to that sex, so no doubt it will
SEXUAL SELECTION
95
be under nature. Thus it is rendered possible for the two sexes to
be modified through natural selection in relation to different habits
of life, as is sometimes the case; or for one sex to be modified in
relation to the other sex, as commonly occurs. This leads me to say
a few words on what I have called Sexual Selection. This form of
selection depends, not on a struggle for existence in relation to other
organic beings or to external conditions, but on a struggle between
the individuals of one sex, generally the males, for the possession of
the other sex. The result is not death to the unsuccessful competitor,
but few or no offspring. Sexual selection is, therefore, less rigorous
than natural selection. Generally, the most vigorous males, those
which are best fitted for their places in nature, will leave most
progeny. But in many cases, victory depends not so much on gen-
eral vigour, as on having special weapons, confined to the male sex.
A hornless stag or spurless cock would have a poor chance of leav-
ing numerous offspring. Sexual selection, by always allowing the
victor to breed, might surely give indomitable courage, length to
the spur, and strength to the wing to strike in the spurred leg, in
nearly the same manner as does the brutal cockfighter by the care-
ful selection of his best cocks. How low in the scale of nature the
law of battle descends, I know not; male alligators have been de-
scribed as fighting, bellowing, and whirling round, like Indians in
a war dance, for the possession of the females; male salmons have
been observed fighting all day long; male stag beetles sometimes
bear wounds from the huge mandibles of other males; the males of
certain hymenopterous insects have been frequently seen by that
inimitable observer M. Fabre, fighting for a particular female who
sits by, an apparently unconcerned beholder of the struggle, and
then retires with the conqueror. The war is, perhaps, severest be-
tween the males of polygamous animals, and these seem oftenest
provided with special weapons. The males of carnivorous animals
are already well armed; though to them and to others, special means
of defence may be given through means of sexual selection, as the
mane of the lion, and the hooked jaw to the male salmon; for the
shield may be as important for victory, as the sword or spear.
Amongst birds, the contest is often of a more peaceful character.
All those who have attended to the subject, believe that there is the
96 ORIGIN OF SPECIES
severest rivalry between the males of many species to attract, by
singing, the females. The rock thrush of Guiana, birds of paradise,
and some others, congregate; and successive males display with the
most elaborate care, and show off in the best manner, their gorgeous
plumage; they likewise perform strange antics before the females,
which, standing by as spectators, at last choose the most attractive
partner. Those who have closely attended to birds in confinement
well know that they often take individual preferences and dislikes;
thus Sir R. Heron has described how a pied peacock was eminently
attractive to all his hen birds. I cannot here enter on the necessary
details; but if man can in a short time give beauty and an elegant
carriage to his bantams, according to his standard of beauty, I can
see no good reason to doubt that female birds, by selecting, during
thousands of generations, the most melodious or beautiful males,
according to their standard of beauty, might produce a marked
effect. Some well-known laws, with respect to the plumage of male
and female birds, in comparison with the plumage of the young,
can pardy be explained through the action of sexual selection on
variations occurring at different ages, and transmitted to the males
alone or to both sexes at corresponding ages; but I have not space
here to enter on this subject.
Thus it is, as I believe, that when the males and females of any
animal have the same general habits of Hfe, but differ in structure,
colour, or ornament, such differences have been mainly caused by
sexual selection: that is, by individual males having had, in succes-
sive generations, some slight advantage over other males, in their
weapons, means of defence, or charms, which they have transmitted
to their male offspring alone. Yet, I would not wish to attribute all
sexual differences to this agency: for we see in our domestic animals
peculiarities arising and becoming attached to the male sex, which
apparently have not been augmented through selection by man. The
tuft of hair on the breast of the wild turkey cock cannot be of any
use, and it is doubtful whether it can be ornamental in the eyes of
the female bird; indeed, had the tuft appeared under domestication,
it would have been called a monstrosity.
ACTION OF NATURAL SELECTION
97
ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR THE
SURVIVAL OF THE FITTEST
In order to make it clear how, as I believe, natural selection acts,
I must beg permission to give one or two imaginary illustrations.
Let us take the case of a wolf, which preys on various animals,
securing some by craft, some by strength, and some by fleetness;
and let us suppose that the fleetest prey, a deer for instance, had
from any change in the country increased in numbers, or that other
prey had decreased in numbers, during that season of the year when
the wolf was hardest pressed for food. Under such circumstances
the swiftest and slimmest wolves would have the best chance of
surviving and so be preserved or selected, — ^provided always that
they retained strength to master their prey at this or some other
period of the year, when they were compelled to prey on other
animals. I can see no more reason to doubt that this would be the
result, than that man should be able to improve the fleetness of his
greyhounds by careful and methodical selection, or by that kind of
unconscious selection which follows from each man trying to keep
the best dogs without any thought of modifying the breed. I may
add, that, according to Mr. Pierce, there are two varieties of the wolf
inhabiting the Catskill Mountains, in the United States, one with a
light greyhound-like form, which pursues deer, and the other more
bulky, with shorter legs, which more frequently attacks the shep-
herd’s flocks.
It should be observed that, in the above illustration, I speak of
the slimmest individual wolves, and not of any single strongly-
marked variation having been preserved. In former editions of this
work I sometimes spoke as if this latter alternative had frequendy
occurred. I saw the great importance of individual differences, and
this led me fully to discuss the results of unconscious selection by
man, which depends on the preservation of all the more or less
valuable individuals, and on the destruction of the worst. I saw,
also, that the preservation in a state of nature of any occasional
deviation of structure, such as a monstrosity, would be a rare event;
and that, if at first preserved, it would generally be lost by subsequent
intercrossing with ordinary individuals. Nevertheless, until reading
98 ORIGIN OF SPECIES
an able and valuable article in the ‘North British Review’ (1867),
I did not appreciate how rarely single variations, whether slight or
strongly-marked, could be perpetuated. The author takes the case
of a pair of animals, producing during their lifetime two hundred
offspring, of which, from various causes of destruction, only two on
an average survive to procreate their kind. This is rather an extreme
estimate for most of the higher animals, but by no means so for
many of the lower organisms. He then shows that if a single in-
dividual were born, which varied in some manner, giving it twice
as good a chance of life as that of the other individuals, yet the
chances would be strongly against its survival. Supposing it to sur-
vive and to breed, and that half its young inherited the favourable
variation; still, as the reviewer goes on to show, the young would
have only a slighdy better chance of surviving and breeding; and
this chance would go on decreasing in the succeeding generations.
The justice of these remarks cannot, I think, be disputed. If, for in-
stance, a bird of some kind could procure its food more easily by
having its beak curved, and if one were born with its beak strongly
curved, and which consequently flourished, nevertheless there would
be a very poor chance of this one individual perpetuating its kind
to the exclusion of the common form; but there can hardly be a
doubt, judging by what we see taking place under domestication,
that this result would follow from the preservation during many
generations of a large number of individuals with more or less
strongly curved beaks, and from the destruction of a still larger
number with the straightest beaks.
It should not, however, be overlooked that certain rather strongly
marked variations, which no one would rank as mere individual
differences, frequently recur owing to a similar organisation being
similarly acted on — of which fact numerous instances could be given
with our domestic productions. In such cases, if the varying in-
dividual did not actually transmit to its offspring its newly acquired
character, it would undoubtedly transmit to them, as long as the
existing conditions remained the same, a still stronger tendency to<
vary in the same manner. There can also be little doubt that the
tendency to vary in the same manner has often been so strong that
all the individuals of the same species have been similarly modified
ACTION OF NATURAL SELECTION 99
without the aid o£ any form of selection. Or only a third, fifth, or
tenth part of the individuals may have been thus affected, of which
fact several instances could be given. Thus Graba estimates that
about one-fifth of the guillemots in the Faroe Islands consist of a
variety so well marked, that it was formerly ranked as a distinct
species under the name of Uria lacrymans. In cases of this kind, if
the variation were of a beneficial nature, the original form would
soon be supplanted by the modified form, through the survival of
the fittest.
To the effects of intercrossing in eliminating variations of all
kinds, I shall have to recur; but it may be here remarked that most
animals and plants keep to their proper homes, and do not need-
lessly wander about; we see this even with migratory birds, which
almost always return to the same spot. Consequently each newly-
formed variety would generally be at first local, as seems to be the
common rule with varieties in a state of nature; so that similarly
modified individuals would soon exist in a small body together, and
would often breed together. If the new variety were successful in
its battle for life, it would slowly spread from a central district,
competing with and conquering the unchanged individuals on the
margins of an ever-increasing circle.
It may be worth while to give another and more complex illustra-
tion of the action of natural selection. Certain plants excrete sweet
juice, apparently for the sake of eliminating something injurious
from the sap: this is effected, for instance, by glands at the base of
the stipules in some Leguminosse, and at the backs of the leaves of
the common laurel. This juice, though small in quantity, is greedily
sought by insects; but their visits do not in any way benefit the
plant. Now, let us suppose that the juice or nectar was excreted
from the inside of the flowers of a certain number of plants of any
species. Insects in seeking the nectar would get dusted with pollen,
and would often transport it from one flower to another. The flow-
ers of two distinct individuals of the same species would thus get
crossed; and the act of crossing, as can be fully proved, gives rise to
vigorous seedlings, which consequendy would have the best chance
of flourishing and surviving. The plants which produced flowers
with the largest glands or nectaries, excreting most nectar, would
100 ORIGIN OF SPECIES
oftenest be visited by insects, and would oftenest be crossed; and so
in the long run would gain the upper hand and form a local variety.
The flowers, also, which had their stamens and pistils placed, in
relation to the size and habits of the particular insect which visited
them, so as to favour in any degree the transportal of the pollen,
would likewise be favoured. We might have taken the case of in-
sects visiting flowers for the sake of collecting pollen instead of nec-
tar; and as pollen is formed for the sole purpose of fertilisation, its
destruction appears to be a simple loss to the plant; yet if a little
pollen were carried, at first occasionally and then habitually, by the
pollen-devouring insects from flower to flower, and a cross thus
effected, although nine-tenths of the pollen were destroyed, it might
still be a great gain to the plant to be thus robbed; and the in-
dividuals which produced more and more pollen, and had larger
anthers, would be selected.
When our plant, by the above process long continued, had been
rendered highly attractive to insects, they would, unintentionally on
their part, regularly carry pollen from flower to flower; and that
they do this effectually, I could easily show by many striking facts.
I will give only one, as likewise illustrating one step in the separation
of the sexes of plants. Some holly trees bear only male flowers, which
have four stamens producing a rather small quantity of pollen, and
a rudimentary pistil; other holly trees bear only female flowers;
these have a fuU-sized pistil, and four stamens with shrivelled
anthers, in which not a grain of pollen can be detected. Having
found a female tree exactly sixty yards from a male tree, I put the
stigmas of twenty flowers, taken from different branches, under the
microscope, and on all, without exception, there were a few pollen
grains, and on some a profusion. As the wind had set for several
days from the female to the male tree, the pollen could not thus
have been carried. The weather had been cold and boisterous, and
therefore not favourable to bees, nevertheless every female flower
which I examined had been effectually fertilised by the bees, which
had flown from tree to tree in search of nectar. But to return to
our imaginary case: as soon as the plant had been rendered so highly
attractive to insects that pollen was regularly carried from flower to
flower, another process might commence. No naturalist doubts the
ACTION OF NATURAL SELECTION lOI
advantage of what has been called the “physiological division of
labour”; hence we may believe that it would be advantageous to a
plant to produce stamens alone in one flower or on one whole plant,
and pistils alone in another flower or on another plant. In plants
under culture and placed under new conditions of life, sometimes
the male organs and sometimes the female organs become more or
less impotent; now if we suppose this to occur in ever so slight a
degree under nature, then, as pollen is already carried regularly
from flower to flower, and as a more complete separation of the
sexes of our plant would be advantageous on the principle of the
division of labour, individuals with this tendency more and more
increased, would be continually favoured or selected, until at last
a complete separation of the sexes might be effected. It would take
up too much space to show the various steps, through dimorphism
and other means, by which the separation of the sexes in plants of
various kinds is apparently now in progress; but I may add that
some of the species of holly in North America, are, according to
Asa Gray, in an exactly intermediate condition, or, as he expresses
it, are more or less diceciously polygamous.
Let us now turn to the nectar-feeding insects; we may suppose
the plant, of which we have been slowly increasing the nectar by
continued selection, to be a common plant; and that certain insects
depended in main part on its nectar for food. I could give many
facts showing how anxious bees are to save time: for instance, their
habit of cutting holes and sucking the nectar at the bases of certain
flowers, which with a very litde more trouble, they can enter by the
mouth. Bearing such facts in mind, it may be believed that under
certain circumstances individual differences in the curvature or
length of the proboscis, etc., too slight to be appreciated by us, might
profit a bee or other insect, so that certain individuals would be able
to obtain their food more quickly than others; and thus the com-
munities to which they belonged would flourish and throw off many
swarms inheriting the same peculiarities. The tubes of the corolla
of the common red and incarnate clovers (Trifolium pratense and
incar natum) do not on a hasty glance appear to differ in length;
yet the hive bee can easily suck the nectar out of the incarnate
clover, but not out of the common red clover, which is visited by
102 ORIGIN OF SPECIES
humble bees alone; so that whole fields of the red ciover offer in
vain an abundant supply of precious nectar to the hive bee. That
this nectar is much liked by the hive bee is certain; for I have re-
peatedly seen, but only in the autumn, many hive bees sucking the
flowers through holes bitten in the base of the tube by humble bees.
The difference in the length of the corolla in the two kinds of clover,
which determines the visits of the hive bee, must be very trifling;
for I have been assured that when red clover has been mown, the
flowers of the second crop are somewhat smaller, and that these are
visited by many hive bees. I do not know whether this statement is
accurate; nor whether another published statement can be trusted,
namely, that the Ligurian bee, which is generally considered a mere
variety of the common hive bee, and which freely crosses with it, is
able to reach and suck the nectar of the red clover. Thus, in a coun-
try where this kind of clover abounded, it might be a great advan-
tage to the hive bee to have a slightly longer or differently constructed
proboscis. On the other hand, as the fertility of this clover absolutely
depends on bees visiting the flowers, if humble bees were to become
rare in any country, it might be a great advantage to the plant to
have a shorter or more deeply divided corolla, so that the hive bees
should be enabled to suck its flowers. Thus I can understand how a
flower and a bee might slowly become, either simultaneously or one
after the other, modified and adapted to each other in the most per-
fect manner, by the continued preservation of all the individuals
which presented slight deviations of structure mutually favourable
to each other.
I am well aware that this doctrine of natural selection, exempli-
fied in the above imaginary instances, is open to the same objections
which were first urged against Sir Charles Lyell’s noble views on
“the modern changes of the earth, as illustrative of geology”; but
we now seldom hear the agencies which we see still at work, spoken
of as trifling or insignificant, when used in explaining the excavation
of the deepest valleys or the formation of long lines of inland cliffs.
Natural selection acts only by the preservation and accumulation of
small inherited modifications, each profitable to the preserved being;
and as modern geology has almost banished such views as the ex-
cavation of a great valley by a single diluvial wave, so will natural
ON THE INTERCROSSING OF INDIVIDUALS IO3
selection banish the belief of the continued creation of new organic
beings, or of any great and sudden modification in their structure.
ON THE INTERCROSSING OF INDIVIDUALS
I must here introduce a short digression. In the case of animals
and plants with separated sexes, it is of course obvious that two in-
dividuals must always (with the exception of the curious and not
well understood cases of parthenogenesis) unite for each birth; but
in the case of hermaphrodites this is far from obvious. Nevertheless
there is reason to believe that with all hermaphrodites two individ-
uals, either occasionally or habitually, concur for the reproduction
of their kind. This view was long ago doubtfully suggested by
Sprengel, Knight, and Kolreuter. We shall presently see its im-
portance; but I must here treat the subject with extreme brevity,
though I have the materials prepared for an ample discussion. All
vertebrate animals, all insects, and some other large groups of
animals, pair for each birth. Modern research has much diminished
the number of supposed hermaphrodites, and of real hermaphrodites
a large number pair; that is, two individuals regularly unite for re-
production, which is all that concerns us. But still there are many
hermaphrodite animals which certainly do not habitually pair, and
a vast majority of plants are hermaphrodites. What reason, it may
be asked, is there for supposing in these cases that two individuals
ever concur in reproduction? As it is impossible here to enter on
details, I must trust to some general considerations alone.
In the first place, I have collected so large a body of facts, and
made so many experiments, showing, in accordance with the almost
universal belief of breeders, that with animals and plants a cross
between different varieties, or between individuals of the same va-
riety but of another strain, gives vigour and fertility to the offspring;
and on the other hand, that close interbreeding diminishes vigour
and fertility; that these facts alone incline me to believe that it is a
general law of nature that no organic being fertilises itself for a
perpetuity of generations; but that a cross with another individual
is occasionally— perhaps at long intervals of time— indispensable.
On the belief that this is a law of nature, we can, I think, under-
stand several large classes of facts, such as the following, which on
ORIGIN OF SPECIES
104
any other view are inexplicable. Every hybridizer knows how un-
favourable exposure to wet is to the fertilisation of a flower, yet what
a multitude of flowers have their anthers and stigmas fully exposed
to the weather! If an occasional cross be indispensable, notwith-
standing that the plant’s own anthers and pistil stand so near each
other as almost to insure self-fertilisation, the fullest freedom for
the entrance of pollen from another individual will explain the above
state of exposure of the organs. Many flowers, on the other hand,
have their organs of fructification closely enclosed, as in the great
papilionaceous or pea-family; but these almost invariably present
beautiful and curious adaptations in relation to the visits of insects.
So necessary are the visits of bees to many papilionaceous flowers,
that their fertility is gready diminished if these visits be prevented.
Now, it is scarcely possible for insects to fly from flower to flower,
and not to carry pollen from one to the other, to the great good of
the plant. Insects act like a camel-hair pencil, and it is sufficient, to
ensure fertilisation, just to touch with the same brush the anthers of
one flower and then the stigma of another; but it must not be sup-
posed that bees would thus produce a multitude of hybrids between
distinct species; for if a plant’s own pollen and that from another
species are placed on the same stigma, the former is so prepotent that
it invariably and completely destroys, as has been shown by Gartner,
the influence of the foreign pollen.
When the stamens of a flower suddenly spring towards the pistil,
or slowly move one after the other towards it, the contrivance seems
adapted solely to ensure self-fertilisation; and no doubt it is useful
for this end: but the agency of insects is often required to cause the
stamens to spring forward, as Kolreuter has shown to be the case
with the barberry; and in this very genus, which seems to have a
special contrivance for self-fertilisation, it is well known that, if
closely-allied forms or varieties are planted near each other, it is
hardly possible to raise pure seedlings, so largely do they naturally
cross. In numerous other cases, far from self-fertilisation being
favoured, there are special contrivances which effectually prevent the
stigma receiving pollen from its own flower, as I could show from
the works of Sprengel and others, as well as from my own observa-
tions: for instance, in Lobelia fulgens, there is a really beautiful and
ON THE INTERCROSSING OF INDIVIDUALS IO5
elaborate contrivance by which all the infinitely numerous pollen-
granules are swept out of the conjoined anthers of each flower, before
the stigma of that individual flower is ready to receive them; and as
this flower is never visited, at least in my garden, by insects, it never
sets a seed, though by placing pollen from one flower on the stigma
of another, I raise plenty of seedlings. Another species of Lobelia,
which is visited by bees, seeds freely in my garden. In very many
other cases, though there is no special mechanical contrivance to
prevent the stigma receiving pollen from the same flower, yet^ as
Sprengel, and more recently Hildebrand, and others, have shown,
and as I can confirm, either the anthers burst before the stigma is
ready for fertilisation, or the stigma is ready before the pollen of that
flower is ready, so that these so-named dichogamous plants have in
fact separated sexes, and must habitually be crossed. So it is with
the reciprocally dimorphic and trimorphic plants previously alluded
to. How strange are these facts! How strange that the pollen and
stigmatic surface of the same flower, though placed so close together,
as if for the very purpose of self-fertilisation, should be in so many
cases mutually useless to each other How simply are these facts
explained on the view of an occasional cross with a distinct indi-
vidual being advantageous or indispensable!
If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority of the
seedlings thus raised turn out, as I have found, mongrels: for in-
stance, I raised 233 seedling cabbages from some plants of different
varieties growing near each other, and of these only seventy-eight
were true to their kind, and some even of these were not perfectly
true. Yet the pistil of each cabbage-flower is surrounded not only by
its own six stamens but by those of the many other flowers on the
same plant; and the pollen of each flower readily gets on its own
stigma without insect agency; for I have found that plants carefully
protected from insects produce the full number of pods. How, then,
comes it that such a vast number of the seedlings are mongrelized ?
It must arise from the pollen of a distinct variety having a prepotent
effect over the flower’s own pollen; and that this is part of the general
law of good being derived from the intercrossing of distinct indi-
viduals of the same species. When distinct species are crossed the
I06 ORIGIN OF SPECIES
case is reversed, for a plant’s own pollen is almost always prepotent
over foreign pollen; but to this subject we shall return in a future
chapter.
In the case of a large tree covered with innumerable flowers, it
may be objected that pollen could seldom be carried from tree to
tree, and at most only from flower to flower on the same tree; and
flowers on the same tree can be considered as distinct individuals
only in a limited sense. I believe this objection to be valid, but that
nature has largely provided against it by giving to trees a strong
tendency to bear flowers with separated sexes. When the sexes are
separated, although the male and female flowers may be produced
on the same tree, pollen must be regularly carried from flower to
flower; and this will give a better chance of pollen being occasionally
carried from tree to tree. That trees belonging to all orders have
their sexes more often separated than other plants, I find to be the
case in this country; and at my request Dr. Hooker tabulated the
trees of New Zealand, and Dr. Asa Gray those of the United States,
and the result was as I anticipated. On the other hand, Dr. Hooker
informs me that the rule does not hold good in Australia: but if
most of the Australian trees are dichogamous, the same result would
follow as if they bore flowers with separated sexes. I have made
these few remarks on trees simply to call attention to the subject.
Turning for a brief space to animals: various terrestrial species
are hermaphrodites, such as the land Mollusca and earthworms;
but these all pair. As yet I have not found a single terrestrial animal
which can fertilise itself. This remarkable fact, which offers so
strong a contrast with terrestrial plants, is intelligible on the view
of an occasional cross being indispensable; for owing to the nature
of the fertilising element there are no means, analogous to the action
of insects and of the wind with plants, by which an occasional cross
could be effected with terrestrial animals without the concurrence
of two individuals. Of aquatic animals, there are many self-fertilis-
ing hermaphrodites; but here the currents of water offer an obvious
means for an occasional cross. As in the case of flowers, I have as
yet failed, after consultation with one of the highest authorities,
namely, Professor Huxley, to discover a single hermaphrodite animal
with the organs of reproduction so perfectly enclosed that access
PRODUCTION OF NEW FORMS IO7
from without, and the occasional influence of a distinct individual,
can be shown to be physically impossible. Cirripedes long appeared
to me to present, under this point of view, a case of great difficulty;
but I have been enabled, by a fortunate chance, to prove that two
individuals, though both are self-fertilising hermaphrodites, do
sometimes cross.
It must have struck most naturalists as a strange anomaly that,
both with animals and plants, some species of the same family and
even of the same genus, though agreeing closely with each other in
their whole organisation, are hermaphrodites, and some unisexual.
But if, in fact, all hermaphrodites do occasionally intercross, the
difference between them and unisexual species is, as far as function
is concerned, very small.
From these several considerations and from the many special facts
which I have collected, but which I am unable here to give, it appears
that with animals and plants an occasional intercross between dis-
tinct individuals is a very general, if not universal, law of nature.
CIRCUMSTANCES FAVOURABLE FOR THE PRODUCTION OF NEW
FORMS THROUGH NATURAL SELECTION
This is an extremely intricate subject. A great amount of variabil-
ity, under which term individual differences are always included,
will evidently be favourable. A large number of individuals, by
giving a better chance within any given period for the appearance
of profitable variations, will compensate for a lesser amount of
variability in each individual and is, I believe, a highly important
element of success. Though Nature grants long periods of time for
the work of natural selection, she does not grant an indefinite period;
for as all organic beings are striving to seize on each place in the
economy of nature, if any one species does not become modified and
improved in a corresponding degree with its competitors, it will be
exterminated. Unless favourable variations be inherited by some at
least of the offspring, nothing can be effected by natural selection.
The tendency to reversion may often check or prevent the work;
but as this tendency has not prevented man from forming by selec-
tion numerous domestic races, why should it prevail against natural
selection?
I08 ORIGIN OF SPECIES
In the case o£ methodical selection, a breeder selects for some
definite object, and if the individuals be allowed freely to intercross,
his work will completely fail. But when many men, without in-
tending to alter the breed, have a nearly common standard of per-
fection, and all try to procure and breed from the best animals,
improvement surely but slowly follows from this unconscious proc-
ess of selection, notwithstanding that there is no separation of se-
lected individuals. Thus it will be under nature; for within a con-
fined area, with some place in the natural polity not perfectly occu-
pied, all the individuals varying in the right direction, though in
different degrees, will tend to be preserved. But if the area be large,
its several districts will almost certainly present different conditions
of life; and then, if the same species undergoes modification in dif-
ferent districts, the newly-formed varieties will intercross on the
confines of each. But we shall see in the sixth chapter that inter-
mediate varieties, inhabiting intermediate districts, will in the long
run generally be supplanted by one of the adjoining varieties. Inter-
crossing will chiefly affect those animals which unite for each birth
and wander much, and which do not breed at a very quick rate.
Hence with animals of this nature, for instance, birds, varieties will
generally be confined to separated countries; and this I find to be
the case. With hermaphrodite organisms which cross only occa-
sionally, and likewise with animals which unite for each birth, but
which wander litde and can increase at a rapid rate, a new and im-
proved variety might be quickly formed on any one spot, and might
there maintain itself in a body and afterwards spread, so that the
individuals of the new variety would chiefly cross together. On this
principle, nurserymen always prefer saving seed from a large body
of plants, as the chance of intercrossing is thus lessened.
Even with animals which unite for each birth, and which do not
propagate rapidly, we must not assume that free intercrossing would
always eliminate the effects of natural selection; for I can bring
forward a considerable body of facts showing that within the same
area, two varieties of the same animal may long remain distinct,
from haunting different stations, from breeding at slightly different
seasons, or from the individuals of each variety preferring to pair
together.
PRODUCTION OF NEW FORMS IO9
Intercrossing plays a very important part in nature by keeping
the individuals of the same species, or of the same variety, true and
uniform in character. It will obviously thus act far more efficiently
with those animals which unite for each birth; but, as already stated,
we have reason to believe that occasional intercrosses take place with
all animals and plants. Even if these take place only at long intervals
of time, the young thus produced will gain so much in vigour and
fertility over the offspring from long-continued self-fertilisation,
that they will have a better chance of surviving and propagating
their kind; and thus in the long run the influence of crosses, even
at rare intervals, will be great. With respect to organic beings ex-
tremely low in the scale, which do not propagate sexually, nor con-
jugate, and which cannot possibly intercross, uniformity of character
can be retained by them under the same conditions of life, only
through the principle of inheritance, and through natural selection
which will destroy any individuals departing from the proper type.
If the conditions of life change and the form undergoes modification,
uniformity of character can be given to the modified offspring, solely
by natural selection preserving similar favourable variations.
Isolation, also, is an important element in the modification of
species through natural selection. In a confined or isolated area, if
not very large, the organic and inorganic conditions of life will
generally be almost uniform; so that natural selection will tend to
modify all the varying individuals of the same species in the same
manner. Intercrossing with the inhabitants of the surrounding dis-
tricts will, also, be thus prevented. Moritz Wagner has lately pub-
lished an interesting essay on this subject, and has shown that the
service rendered by isolation in preventing crosses between newly-
formed varieties is probably greater even than I supposed. But from
reasons already assigned I can by no means agree with this naturalist,
that migration an(d isolation are necessary elements for the formation
of new species. The importance of isolation is likewise great in pre-
venting, after any physical change in the conditions such as of cli-
mate, elevation of the land, etc., the immigration of better adapted
organisms; and thus new places in the natural economy of the dis-
trict will be left open to be filled up by the modification of the old
inhabitants. Lastly, isolation will give time for a new variety to be
no ORIGIN OF SPECIES
improved at a slow rate; and this may sometimes be of much im-
portance. If, however, an isolated area be very small, either from
Mng surrounded by barriers, or from having very peculiar physi-
cal conditions, the total number of the inhabitants will be small;
and this will retard the production of new species through natural
selection, by decreasing the chances of favourable variations aris-
ing.
The mere lapse of time by itself does nothing, either for or against
natural selection. I state this because it has been erroneously asserted
that the element of time has been assumed by me to play an all-
important part in modifying species, as if all the forms of life were
necessarily undergoing change through some innate law. Lapse of
time is only so far important, and its importance in this respect is
great, that it gives a better chance of beneficial variations arising
and of their being selected, accumulated, and fixed. It likewise tends
to increase the direct action of the physical conditions of life, in re^
lation to the constitution of each organism.
If we turn to nature to test the truth of these remarks, and look
at any small isolated area, such as an oceanic island, although the
number of species inhabiting it is small, as we shall see in our chapter
on Geographical Distribution; yet of these species a very large pro-
portion are endemic,— that is, have been produced there and nowhere
else in the world. Hence an oceanic island at first sight seems to
have been highly favourable for the production of new species. But
we may thus deceive ourselves, for to ascertain whether a small
isolated area, or a large open area like a continent, has been most
favourable for the production of new organic forms, we ought to
make the comparison within equal times; and this we are incapable
of doing.
Although isolation is of great importance in the production of new
species, on the whole I am inclined to believe that largeness of area
is still more important, especially for the production of species which
shall prove capable of enduring for a long period, and of spreading
widely. Throughout a great and open area, not only will there be a
better chance of favourable variations, arising from the large num-
ber of individuals of the same species there supported, but the con-
ditions of life are much more complex from the large number of
PRODUCTION OF NEW FORMS III
already existing species; and if some of these many species become
modified and improved, others will have to be improved in a cor-
responding degree, or they will be exterminated. Each new form,
also, as soon as it has been much improved, will be able to spread
over the open and continuous area, and will thus come into com-
petition with many other forms. Moreover, great areas, though now
continuous, will often, owing to former oscillations of level, have
existed in a broken condition; so that the good effects of isolation
will generally, to a certain extent, have concurred. Finally, I con-
clude that, although small isolated areas have been in some respects
highly favourable for the production of new species, yet that the
course of modification will generally have been more rapid on large
areas; and what is more important, that the new forms produced
on large areas, which already have been victorious over many com-
petitors, will be those that will spread most widely, and will give
rise to the greatest number of new varieties and species. They will
thus play a more important part in the changing history of the
organic world.
In accordance with this view, we can, perhaps, understand some
facts which will be again alluded to in our chapter on Geographical
Distribution; for instance, the fact of the productions of the smaller
continent of Australia now yielding before those of the larger Eu-
ropaeo-Asiatic area. Thus, also, it is that continental productions
have everywhere become so largely naturalised on islands. On a
small island, the race for fife will have been less severe, and there
will have been less modification and less extermination. Hence, we
can understand how it is that the flora of Madeira, according to
Oswald Heer, resembles to a certain extent the extinct tertiary flora
of Europe. All fresh-water basins, taken together, make a small
area compared with that of the sea or of the land. Consequently,
the competition between fresh-water productions will have been less
severe than elsewhere; new forms will have been then more slowly
produced, and old forms more slowly exterminated. And it is in
fresh-water basins that we find seven genera of Ganoid fishes,
remnants of a once preponderant order: and in fresh water we find
some of the most anomalous forms now known in the world as the
Ornithorhynchus and Lepidosiren, which, like fossils, connect to a
II2
ORIGIN OF SPECIES
certain extent orders at present widely sundered in the natural scale.
These anomalous forms may be called living fossils; they have en-
dured to the present day, from having inhabited a confined area,
and from having been exposed to less varied, and therefore less
severe, competition.
To sum up, as far as the extreme intricacy of the subject permits,
the circumstances favourable and unfavourable for the production
of new species through natural selection. I conclude that for ter-
restrial productions a large continental area, which has undergone
many oscillations of level, vrill have been the most favourable for
the production of many new forms of life, fitted to endure for a
long time and to spread widely. Whilst the area existed as a con-
tinent, the inhabitants will have been numerous in individuals and
kinds, and will have been subjected to severe competition. When
converted by subsidence into large separate islands, there will still
have existed many individuals of the same species on each island;
intercrossing on the confines of the range of each new species will
have been checked; after physical changes of any kind, immigration
will have been prevented, so that new places in the polity of each
island will have had to be filled up by the modification of the old
inhabitants; and time will have been allowed for the varieties in
each to become well modified and perfected. When, by renewed
elevation, the islands were reconverted into a continental area, there
will again have been very severe competition: the most favoured or
improved varieties will have been enabled to spread: there will have
been much extinction of the less improved forms, and the relative
proportional numbers of the various inhabitants of the reunited con-
tinent will again have been changed; and again there will have been
a fair field for natural selection to improve still further the inhabi-
tants, and thus to produce new species.
That natural selection generally acts with extreme slowness I fully
admit. It can act only when there are places in the natural polity of
a district which can be better occupied by the modification of some
of its existing inhabitants. The occurrence of such places will often
depend on physical changes, which generally take place very slowly,
and on the immigration of better adapted forms being prevented.
As some few of the old inhabitants become modified, the mutual
EXTINCTION CAUSED BY NATURAL SELECTION II3
relations of others will often be disturbed; and this will create new
places, ready to be filled up by better adapted forms; but all this
will take place very slowly. Although all the individuals of the
same species differ in some slight degree from each other, it would
often be long before differences of the right nature in various parts
of the organisation might occur. The result would often be greatly
retarded by free intercrossing. Many will exclaim that these several
causes are amply sufficient to neutralise the power of natural selec-
tion. I do not believe so. But I do believe that natural selection will
generally act very slowly, only at long intervals of time, and only
on a few of the inhabitants of the same region. I further believe
that these slow, intermittent results accord well with what geology
tells us of the rate and manner at which the inhabitants of the world
have changed.
Slow though the process of selection may be, if feeble man can
do much by artificial selection, I can see no limit to the amount of
change, to the beauty and complexity of the co-adaptations between
all organic beings, one with another and with their physical con-
ditions of life, which may have been affected in the long course of
time through nature’s power of selection, that is by the survival of
the fittest.
EXTINCTION CAUSED BY NATURAL SELECTION
This subject will be more fully discussed in our chapter on Geol-
ogy; but it must here be alluded to from being intimately connected
with natural selection. Natural selection acts solely through the
preservation of variations in some way advantageous, which con-
sequently endure. Owing to the high geometrical rate of increase
of all organic beings, each area is already fully stocked with in-
habitants; and it follows from this, that as the favoured forms in-
crease in number, so, generally, will the less favoured decrease and
become rare. Rarity, as geology tells us, is the precursor to extinction.
We can see that any form which is represented by few individuals
will run a good chance of utter extinction, during great fluctuations
in the nature of the seasons, or from a temporary increase in the
number of its enemies. But we may go further than this; for, as
new forms are produced, unless we admit that specific forms can go
1 14 ORIGIN OF SPECIES
on indefinitely increasing in number^ many old forms must become
extinct. That the number of specific forms has not indefinitely in-
creased, geology plainly tells us; and we shall presently attempt to
show why it is that the number of species throughout the world has
not become immeasurably great.
We have seen that the species which are most numerous in in-
dividuals have the best chance of producing favourable variations
within any .given period. We have evidence of this, in the facts
stated in the second chapter, showing that it is the common and
diffused or dominant species which offer the greatest number of
recorded varieties. Hence, rare species will be less quickly modified
or improved within any given period; they will consequently be
beaten in the race for life by the modified and improved descendants
of the commoner species.
From these several considerations I think it inevitably follows,
that as new species in the course of time are formed through natural
selection, others will become rarer and rarer, and finally extinct.
The forms which stand in closest competition with those undergoing
modification and improvement, will naturally suffer most. And we
have seen in the chapter on the Struggle for Existence, that it is the
most closely-allied forms,— varieties of the same species, and species
of the same genus or of related genera,— -which, from having nearly
the same structure, constitution, and habits, generally come into the
severest competition with each other; consequently, each new variety
or species, during the progress of its formation, will generally press
hardest on its nearest kindred, and tend to exterminate them. We
see the same process of extermination amongst our domesticated
productions, through the selection of improved forms by man. Many
curious instances could be given showing how quickly new breeds
of cattle, sheep, and other animals, and varieties of flowers, take the
place of older and inferior kinds. In Yorkshire, it is historically
known that the ancient black cattle were displaced by the long-
horns, and that these “were swept away by the shorthorns” (I
quote the words of an agricultural writer) “as if by some murderous
pestilence.”
DIVERGENCE Op CHARACTER
II5
DIVERGENCE OF CHARACTER
The principle, which I have designated by this term, is of high
importance, and explains, as I believe, several important facts. In
the first place, varieties, even strongly marked ones, though having
somewhat of the character of species — as is shown by the hopeless
doubts in many cases how to rank them — yet certainly differ far
less from each other than do good and distinct species. Neverthe-
less, according to my view, varieties are species in the process of
formation, or are, as I have called them, incipient species. How,
then, does the lesser difference between varieties become augmented
into the greater difference between species? That this does habit-
ually happen, we must infer from most of the innumerable species
throughout nature presenting well-marked differences; whereas
varieties, the supposed prototypes and parents of future well-marked
species, present slight and ill-defined differences. Mere chance, as
we may call it, might cause one variety to differ in some character
from its parents, and the offspring of this variety again to differ
from its parent in the very same character and in a greater degree;
but this alone would never account for so habitual and large a de-
gree of difference as that between the species of the same genus.
As has always been my practice, I have sought light on this head
from our domestic productions. We shall here find something
analogous. It will be admitted that the production of races so dif-
ferent as shorthorn and Hereford cattle, race and cart horses, the
several breeds of pigeons, etc., could never have been effected by
the mere chance accumulation of similar variations during many
successive generations. In practice, a fancier is, for instance, struck
by a pigeon having a slightly shorter beak; another fancier is struck
by a pigeon having a rather longer .beak; and on the acknowledged
principle that “fanciers do not and will not admire a medium
standard, but like extremes,’' they both go on (as has actually oc-
curred with the sub-breeds of the tumbler pigeon) choosing and
breeding from birds with longer and longer beaks, or with shorter
and shorter beaks. Again, we may suppose that at an early period
of history, the men of one nation or district required swifter horses,
whilst those of another required stronger and bulkier horses. The
Il6 ORIGIN OF SPECIES
early differences would be very slight; but, in the course of time,
from the continued selection of swifter horses in the one case, and
of stronger ones in the other, the differences would become greater,
and would be noted as forming two sub-breeds. Ultimately, after
the lapse of centuries, these sub-breeds would become converted
into two well-established and distinct breeds. As the differences
became greater, the inferior animals with intermediate characters,
being neither very swift nor very strong, would not have been used
for breeding, and will thus have tended to disappear. Here, then,
we see in man’s productions the action of what may be called the
principle of divergence, causing differences, at first barely appre-
ciable, steadily to increase, and the breeds to diverge in character,
both from each other and from their common parent.
But how, it may be asked, can any analogous principle apply in
nature? I believe it can and does apply most efficiently (though it
was a long time before I saw how), from the simple circumstance
that the more diversified the descendants from any one species be-
come in structure, constitution, and habits, by so much will they be
better enabled to seize on many and widely diversified places in the
polity of nature, and so be enabled to increase in numbers.
We can clearly discern this in the case of animals with simple
habits. Take the case of a carnivorous quadruped, of which the
number that can be supported in any country has long ago arrived
at its full average. If its natural power of increase be allowed to act,
it can succeed in increasing (the country not undergoing any change
in conditions) only by its varying descendants seizing on places at
present occupied by other animals; some of them, for instance, being
enabled to feed on new kinds of prey, either dead or alive; some
inhabiting new stations, climbing trees, frequenting water, and some
perhaps becoming less carnivorous. The more diversified in habits
and structure the descendants of our carnivorous animals become,
the more places they will be enabled to occupy. What applies to one
animal will apply throughout all time to all animals— that is, if they
vary— for otherwise natural selection can effect nothing. So it will
be with plants. It has been experimentally proved, that if a plot of
ground be sown with one species of grass, and a similar plot be sown
with several distinct genera of grasses, a greater number of plants
DIVERGENCE OF CHARACTER II7
and a greater weight of dry herbage can be raised in the latter than
in the former case. The same has been found to hold good when
one variety and several mixed varieties of wheat have been sown on
equal spaces of ground. Hence, if any one species of grass were to go
on varying, and the varieties were continually selected which dif-
fered from each other in the same manner, though in a very slight
degree, as do the distinct species and genera of grasses, a greater
number of individual plants of this species, including its modified
descendants, would succeed in living on the same piece of ground.
And we know that each species and each variety of grass is annually
sowing almost countless seeds; and is thus striving, as it may be
said, to the utmost to increase in number. Consequendy, in the
course of many thousand generations, the most distinct varieties of
any one species of grass would have the best chance of succeeding
and of increasing in numbers, and thus of supplanting the less dis-
tinct varieties; and varieties, when rendered very distinct from each
other, take the rank of species.
The truth of the principle that the greatest amount of life can be
supported by great diversification of structure, is seen under many
natural circumstances. In an extremely small area, especially if
freely open to immigration, and where the contest between individual
and individual must be very severe, we always find great diversity
in its inhabitants. For instance, I found that a piece of turf, three
feet by four in size, which had been exposed for many years to
exactly the same conditions, supported twenty species of plants, and
these belonged to eighteen genera and to eight orders, which shows
how much these plants differed from each other. So it is with the
plants and insects on small and uniform islets : also in small ponds
of fresh water. Farmers find that they can raise most food by a rota-
tion of plants belonging to the most different orders; nature follows
what may be called a simultaneous rotation. Most of the animals
and plants which live close round any small piece of ground, could
live on it (supposing its nature not to be in any way peculiar), and
may be said to be striving to the utmost to live there; but, it is seen,
that where they come into the closest competition, the advantages of
diversification of structure, with the accompanying differences of
habit and constitution, determine that the inhabitants, which thus
11 8 ORIGIN OF SPECIES
jostle each other most closely, shall, as a general rule, belong to what
we call different genera and orders.
The same principle is seen in the naturalisation of plants through
man's agency in foreign lands. It might have been expected that the
plants which would succeed in becoming naturalised in any land
would generally have been closely allied to the indigenes; for these
are commonly looked at as specially created and adapted for their
own country. It might also, perhaps, have been expected that natural-
ised plants would have belonged to a few groups more especially
adapted to certain stations in their new homes. But the case is very
different; and Alph. de Candolle has well remarked, in his great and
admirable work, that floras gain by naturalisation, proportionally
with the number of the native genera and species, far more in new
genera than in new species. To give a single instance: in the last
edition of Dr. Asa Gray's ‘Manual of the Flora of the Northern
United States,’ 260 naturalised plants are enumerated, and these
belong to 162 genera. We thus see that these naturalised plants are
of a highly diversified nature. They differ, moreover, to a large
extent, from the indigenes, for out of the 162 naturalised genera, no
less than 100 genera are not there indigenous, and thus a large pro-
portional addition is made to the genera now living in the United
States.
By considering the nature of the plants or animals which have in
any country struggled successfully with the indigenes, and have there
become naturalised, we may gain some crude idea in what manner
some of the natives would have to be modified, in order to gain an
advantage over their compatriots; and we may at least infer that
diversification of structure, amounting to new generic differences,
would be profitable to them.
The advantage of diversification of structure in the inhabitants
of the same region is, in fact, the same as that of the physiological
division of labor in the organs of the same individual body— a sub-
ject so well elucidated by Milne Edwards. No physiologist doubts
that a stomach adapted to digest vegetable matter alone, or flesh
alone, draws most nutriment from these substances. So in the gen-
eral economy of any land, the more widely and perfectly the animals
and plants are diversified for different habits of life, so will a greater
EFFECTS OF NATURAL SELECTION II9
number of individuals be capable of there supporting themselves.
A set of animals, with their organisation but little diversified, could
hardly compete with a set more perfectly diversified in structure. It
may be doubted, for instance, whether the Australian marsupials,
which are divided into groups differing but little from each other,
and feebly representing, as Mr. Waterhouse and others have re-
marked, our carnivorous, ruminant, and rodent mammals, could
successfully compete with these well-developed orders. In the Aus-
tralian mammals, we see the process of diversification in an early
and incomplete stage of development.
THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELECTION THROUGH
DIVERGENCE OF CHARACTER AND EXTINCTION, ON THE DESCENDANTS OF
A COMMON ANCESTOR
After the foregoing discussion, which has been much compressed,
we may assume that the modified descendants of any one species
will succeed so much the better as they become more diversified in
structure, and are thus enabled to encroach on places occupied by
other beings. Now let us see how this principle of benefit being
derived from divergence of character, combined with the principles
of natural selection and of extinction, tends to act.
The accompanying diagram will aid us in understanding this
rather perplexing subject. Let A to L represent the species of a
genus large in its own country; these species are supposed to resemble
each other in unequal degrees, as is so generally the case in nature,
and as is represented in the diagram by the letters standing at
unequal distances. I have said a large genus, because, as we saw in
the second chapter, on an average more species vary in large genera
than in small genera; and the varying species of the large genera
present a greater number of varieties. We have, also, seen that the
species, which are the commonest and the most widely diffused,
vary more than do the rare and restricted species. Let (A) be a
common, widely-diffused, and varying species, belonging to a genus
large in its own country. The branching and diverging dotted lines
of unequal lengths proceeding from (A), may represent its varying
"offspring. The variations are supposed to be extremely slight, but of
the most diversified nature; they are not supposed all to appear
120
UJbtlVjrliM WX*
a>* qn p>4 bH fH o'^ i’*
A B C D E
simultaneously, but often after long intervals of time; nor are they
all supposed to endure for equal periods. Only those variations
which are in some way profitable will be preserved or naturally
selected. And here the importance of the principle of benefit derived
from divergence of character comes in; for this will generally lead to
the most different or divergent variations (represented by the outer
EFFECTS OF NATURAL SELECTION
I2I
E F G H I K L
•. ! \ \ ! /
dotted lines) being preserved and accumulated by natural selection.
When a dotted line reaches one of the horizontal lines, and is there
marked by a small numbered letter, a sufficient amount of variation
is supposed to have been accumulated to form it into a fairly well-
marked variety, such as wpuld be thought worthy of record in a
systematic work.
122 ORIGIN OF SPECIES
The intervals between the horizontal lines in the diagram, may
represent each a thousand or more generations. After a thousand
generations, species (A) is supposed to have produced two fairly
well-marked varieties, namely d and These two varieties will
generally still be exposed to the same conditions which made their
parents variable, and the tendency to variability is in itself hereditary;
consequently they will likewise tend to vary, and commonly in
nearly the same manner as did their parents. Moreover, these two
varieties, being only slighdy modified forms, will tend to inherit
those advantages which made their parent (A) more numerous than
most of the other inhabitants of the same country; they will also
partake of those more general advantages which made the genus to
which the parent-species belonged, a large genus in its own country.
And all these circumstances are favorable to the production of new
varieties.
If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand
generations. And after this interval, variety is supposed in the
diagram to have produced variety which will, owing to the prin-
ciple of divergence, differ more from (A) than did variety
Variety is supposed to have produced two varieties, namely
and s^y differing from each other, and more considerably from their
common parent (A). We may continue the process by similar steps
for any length of time; some of the varieties, after each thousand
generations, producing only a single variety, but in a more and more
modified condition, some producing two or three varieties, and some
failing to produce any. Thus the varieties or modified descendants
of the common parent (A), will generally go on increasing in num-
ber and diverging in character. In the diagram the process is repre-
sented up to the ten-thousandth generation, and under a condensed
and simplified form up to the fourteen-thousandth generation.
But I must here remark that I do not suppose that the process ever
goes on so regularly as is represented in the diagram, though in itself
made somewhat irregular, nor that it goes on continuously; it is far
more probable that each form remains for long periods unaltered,
and then again undergoes modification. Nor do I suppose that the
most divergent varieties are invariably preserved; a medium form
EFFECTS OF NATURAL SELECTION I23
may often long endure, and may or may not produce more than one
modified descendant; for natural selection will always act according
to the nature of the places which are either unoccupied or not per-
fectly occupied by other beings; and this will depend on infinitely
complex relations. But as a general rule, the more diversified in
structure the descendants from any one species can be rendered, the
more places they will be enabled to seize on, and the more their
modified progeny will increase. In our diagram the line of succession
is broken at regular intervals by small numbered letters marking the
successive forms which have become sufficiently distinct to be
recorded as varieties. But these breaks are imaginary, and might
have been inserted anywhere, after intervals long enough to allow
the accumulation of a considerable amount of divergent varia-
tion.
As all the modified descendants from a common and widely-
diffused species, belonging to a large genus, will tend to partake of
the same advantages which made their parent successful in -life, they
will generally go on multiplying in number as well as diverging
in character; this is represented in the diagram by the several
divergent branches proceeding from (A). The modified off-
spring from the later and more highly improved branches in the
lines of descent, will, it is probable, often take the place of, and so
destroy, the earlier and less improved branches: this is represented
in the diagram by some of the lower branches not reaching to the
upper horizontal lines. In some cases, no doubt, the process of
modification will be confined to a single line of descent, and the
number of modified descendants will not be increased; although the
amount of divergent modification may have been augmented. This
case would be represented in the diagram, if all the lines proceeding
from (A) were removed, excepting that from i to In the same
way the English race horse and English pointer have apparendy
both gone on slowly diverging in character from their original stocks,
without either having given off any firesh branches or races.
After ten thousand generations, species (A) is supposed to have
produced three forms, ° and which, from having diverged
in character during the successive generations, will have come to
differ largely, but perhaps unequally, from each other and from their
ORIGIN OF SPECIES
124
common parent. If we suppose the amount of change between each
horizontal Hne in our diagram to be excessively small, these three
forms may still be only well-marked varieties; but we have only to
suppose the steps in the process of modification to be more numerous
or greater in amount, to convert these three forms into doubtful or
at least into well-defined species. Thus the diagram illustrates the
steps by which the small differences distinguishing varieties are in-
creased into the larger differences distinguishing species. By con-
tinuing the same process for a greater number of generations (as
shown in the diagram in a condensed and simplified manner), we
get eight species, marked by the letters between and all de-
scended from (A). Thus, as I believe, species are multiplied, and
genera are formed.
In a large genus it is probable that more than one species would
vary. In the diagram I have assumed that a second species (I) has
produced, by analogous steps, after ten thousand generations, either
two well-marked varieties and or two species, according to
the amount of change supposed to be represented between the
horizontal lines. After fourteen thousand generations, six new
species, marked by the letters to are supposed to have been
produced. In any genus, the species which are already very different
in character from each other, will generally tend to produce the
greatest number of modified descendants; for these will have the
best chance of seizing on new and widely different places in the
polity of nature: hence in the diagram I have chosen the extreme
species (A), and the nearly extreme species (I), as those which
have largely varied, and have given rise to new varieties and species.
The other nine species (marked by capital letters) of our original
genus, may for long but unequal periods continue to transmit
unaltered descendants; and this is shown in the diagram by the
dotted lines unequally prolonged upwards.
But during the process of modification, represented in the diagram,
another of our principles, namely that of extinction, will have played
an important part. As in each fully stocked country natural selection
necessarily acts by the selected form having some advantage in the
struggle for life over other forms, there will be a constant tendency
in the improved descendants of any one species to supplant and
EFFECTS OF NATURAL SELECTION I25
extermina-te in each stage o£ descent their predecessors and their
original progenitor. For it should be remembered that the compe-
tition will generally be most severe between those forms which are
most nearly related to each other in habits, constitution, and structure.
Hence all the intermediate forms between the earlier and later states,
that is between the less and more improved states of the same species,
as well as the original parent-species itself, will generally tend to
become extinct. So it probably will be with many whole collateral
lines of descent which will be conquered by later and improved lines.
If, however, the modified offspring of a species get into some distinct
country, or become quickly adapted to some quite new station, in
which offspring and progenitor do not come into competition, both
may continue to exist.
If, 'then, our diagram be assumed to represent a considerable
amount of modification, species (A) and all the earher varieties
will have become extinct, being replaced by eight new species
to nj }^) ; and species (I) will be replaced by six to 2:^^) new
species.
But we may go further than this. The original species of our
genus were supposed to resemble each other in unequal degrees, as
is so generally the case in nature; species (A) being more nearly
related to B, C, and D, than to the other species; and species (I) more
to G, H, K, L, than to the others. These two species (A) and (I)
were also supposed to be very common and widely diffused species,
so that they must originally have had some advantage over most of
the other species of the genus. Their modified descendants, fourteen
in number at the fourteen-thousandth generation, will probably have
inherited some of the same advantages; they have also been modified
and improved in a diversified manner at each stage of descent, so
as to have become adapted to many related places in the natural
economy of their country. It seems, therefore, extremely probable
that they will have taken the places of, and thus exterminated, not
only their parents (A) and (I), but likewise some of the original
species which were most nearly related to their parents. Hence very
few of the original species will have transmitted offspring to the
fourteen-thousandth generation. We may suppose that only one,
(F), of the two species (E) and (F) which were least closely related
126 ORIGIN OF SPECIES
to the other nine original species, has transmitted descendants to this
late stage o£ descent.
The new species in our diagram, descended from the original
eleven species, will now be fifteen in number. Owing to the divergent
tendency of natural selection, the extreme amount of difference in
character between species and will be much greater than that
between the most distinct of the original eleven species. The new
species, moreover, will be allied to each other in a widely different
manner. Of the eight descendants from (A) the three marked
will be nearly related from having recently branched off
from and f\ from having diverged at an earlier period from
will be in some degree distinct from the three first-named species;
and lasdy, and will be nearly related one to the other, but,
from having diverged at the first commencement of the process of
modification, will be widely different from the other five species,
and may constitute a sub-genus or a distinct genus.
The six descendants from (I) will form two sub-genera or genera.
But as the original species (I) differed largely from (A), standing
nearly at the extreme end of the original genus, the six descendants
from (I) will, owing to inheritance alone, differ considerably from
the eight descendants from (A); the two groups, moreover, are
supposed to have gone on diverging in different directions. The
intermediate species, also (and this is a very important consideration),
which connected the original species (A) and (I), have all become,
excepting (F), extinct, and have left no descendants. Hence the six
new species descended from (I), and the eight descendants from
(A), will have to be ranked as very distinct genera, or even as distinct
sub-families.
Thus it is, as I believe, that two or more genera are produced by
descent with modification, from two or more species of the same
genus. And the two or more parent-species are supposed to be
descended from some one species of an earlier genus. In our diagram,
this is indicated by the broken lines, beneath the capital letters, con-
verging in sub-branches downwards towards a single point; this
point represents a species, the supposed progenitor of our several
sub-genera and genera.
It is worth while to reflect for a moment on the character of the
EFFECTS OF NATUEIAL SELECTION IT]
new species which is supposed not to have diverged much in
character, but to have retained the form of (F), either unaltered or
altered only in a slight degree. In this case, its affinities to the other
fourteen new species will be of a curious and circuitous nature. Being
descended from a form which stood between the parent-species (A)
and (I), now supposed to be extinct and unknown, it will be in some
degree intermediate in character between the two groups descended
from these two species. But as these two groups have gone on diverg-
ing in character from the type of their parents, the new species (f^^)
will not be directly intermediate between them, but rather between
types of the two groups; and every naturalist will be able to call
such cases before his mind.
In the diagram, each horizontal line has hitherto been supposed
to represent a thousand generations, but each may represent a million
or more generations; it may also represent a section of the successive
strata of the earth’s crust including extinct remains. We shall, when
we come to our chapter on Geology, have to refer again to this
subject, and I think we shall then see that the diagram throws light
on the affinities of extinct beings, which, though generally belonging
to the same orders, families, or genera, with those now living, yet
are often, in some degree, intermediate in character between existing
groups; and we can understand this fact, for the extinct species lived
at various remote epochs when the branching lines of descent had
diverged less.
I see no reason to limit the process of modification, as now ex-
plained, to the formation of genera alone. If, in the diagram, we
suppose the amount of change represented by each successive group
of diverging dotted lines to be great, the forms marked to
those marked and f\ and those marked to will form three
very distinct genera. We shall also have two very distinct genera
descended from (I), differing widely from the descendants of (A).
Those two groups of genera will thus form two distinct families, or
orders, according to the amount of divergent modification supposed
to be represented in the diagram. And the two new families, or
orders, are descende'd from two species of the original genus, and
these are supposed to be descended from some still more ancient and
unknown form.
128 ORIGIN OF SPECIES
We have seen that in each country it is the species belonging to
the larger genera which oftenest present varieties or incipient species.
This, indeed, might have been expected; for, as natural selection acts
through one form having some advantage over other forms in the
struggle for existence, it will chiefly act on those which already have
some advantage; and the largeness of any group shows that its
species have inherited from a common ancestor some advantage in
common. Hence, the struggle for the production of new and modi-
fied descendants will mainly lie between the larger groups which
are all trying to increase in number. One large group will slowly
conquer another large group, reduce its numbers, and thus lessen
its chance of further variation and improvement. Within the same
large group, the later and more highly perfected sub-groups, from
branching out and seizing on many new places in the polity of
Nature, will constantly tend to supplant and destroy the earlier and
less improved sub-groups. Small and broken groups and sub-groups
will finally disappear. Looking to the future, we can predict that
the groups of organic beings which are now large and triumphant,
and which are least broken up, that is, which have as yet suffered
least extinction, will, for a long period, continue to increase. But
which groups will ultimately prevail, no man can predict; for we
know that many groups, formerly most extensively developed, have
now become extinct. Looking still more remotely to the future, we
may predict that, owing to the continued and steady increase of the
larger groups, a multitude of smaller groups will become utterly
extinct, and leave no modified descendants; and consequently, that,
of 'the species living at any one period extremely few will transmit
descendants to a remote futurity. I shall have to return to this subject
in the chapter on Classification, but I may add that as, according
to this view, extremely few of die more ancient species have trans-
mitted descendants to the present day, and, as all the descendants of
the same species form a class, we can understand how it is that there
exist so few classes in each main division of the animal and vegetable
kingdoms. Although few of the most ancient species have left modi-
fied descendants, yet, at remote geological periods, the earth may
have been almost as well peopled with species of many genera,
families, orders, and classes, as at the present time.
EFFECTS OF NATURAL SELECTION
129
ON THE DEGREE TO WHICH ORGANISATION TENDS TO ADVANCE
Natural Selection acts exclusively by the preservation and accumu-
lation of variations, which are beneficial under the organic and
inorganic conditions to which each creature is exposed at all periods
of life. The ultimate result is that each creature tends to become more
and more improved in relation to its conditions. This improvement
inevitably leads to the gradual advancement of the organisation of
the greater number of living beings throughout the world. But here
we enter on a very intricate subject, for naturalists have not defined
to each other’s satisfaction what is meant by an advance in organisa-
tion. Amongst the vertebrata the degree of intellect and an approach
in structure to man clearly come into play. It might be thought that
the amount of change which the various parts and organs pass
through in their development from the embryo to maturity would
suffice as a standard of comparison; but there are cases, as with
certain parasitic crustaceans, in which several parts of the structure
become less perfect, so that the mature animal cannot be called higher
than its larva. Von Baer’s standard seems the most widely appli-
cable and the best, namely, the amount of differentiation of the
parts of the same organic being, in the adult state, as I should be
inclined to add, and their specialisation for different functions; or,
as Milne Edwards would express it, the completeness of the division
of physiological labour. But we shall see how obscure this subject
is if we look, for instance, to fishes, amongst which some naturalists
rank those as highest which, like the sharks, approach nearest to
amphibians; whilst other naturalists range the common bony or
teleostean fishes as the highest, inasmuch as they are most stricdy
fish-like, and differ most from the other vertebrate classes. We see
still more plainly the obscurity of the subject by turning to plants,
amongst which the standard of intellect is of course quite excluded;
and here some botanists rank those plants as highest which have
every organ, as sepals, petals, stamens, and pistils, fully developed in
each flower; whereas other botanists, probably with more truth, look
at the plants which have their several organs much modified and
reduced in number, as the highest.
If we take as the standard of high organisation, the amount of
130 ORIGIN OF SPECIES
differentiation and specialisation of the several organs in each being
when adult (and this will include the advancement of the brain for
intellectual purposes), natural selection clearly leads towards this
standard: for all physiologists admit that the specialisation of organs,
inasmuch as in this state they perform their functions better, is an
advantage to each being j and hence the accumulation of variations
tending towards specialisation is within the scope of natural selection.
On the other hand, we can see, bearing in mind that all organic
beings are striving to increase at a high ratio and to seize on every
unoccupied or less well occupied place in the economy of nature,
that it is quite possible for natural selection gradually to fit a being
to a situation in which several organs would be superfluous or
useless: in such cases there would be retrogression in the scale of
organisation. Whether organisation on the whole has actually
advanced from the remotest geological periods to the present day
will be more conveniently discussed in our chapter on Geological
Succession.
But it may be objected that if all organic beings thus tend to rise
in the scale, how is it that throughout the world a multitude of the
lowest forms still exist; and how is it that in each great class some
forms are far more highly developed than others? Why have not
the more highly developed forms everywhere supplanted and ex-
terminated the lower? Lamarck, who beheved in an innate and
inevitable tendency towards perfection in all organic beings, seems
to have felt this difficulty so strongly, that he was led to suppose that
new and simple forms are continually being produced by spontaneous
generation. Science has not as yet proved the truth of this belief,
whatever the future may reveal. On our theory the continued exist-
ence of lowly organisms offers no difficulty; for natural selection,
or the survival of the fittest, does not necessarily include progressive
development—it only takes advantage of such variations as arise
and are beneficial to each creature under its complex relations of
life. And it may be asked what advantage, as far as we can see, would
it be to an infusorian animalcule — to an intestinal worm — or even to
an earthworm, to be highly organised. If it were no advantage, these
forms would be left, by natural selection, unimproved or but little
improved, and might remain for indefinite ages in their present lowly
EFFECTS OF NATURAL SELECTION I3I
condition. And geology tells us that some of the lowest forms, as
the infusoria and rhizopods, have remained for an enormous period
in nearly their present state. But to suppose that most of the many
now existing low forms have not in the least advanced since the first
dawn of life would be extremely rash; for every naturalist who has
dissected some of the beings now ranked as very low in the scale,
must have been struck with their really wondrous and beautiful
organisation.
Nearly the same remarks are applicable if we look to the different
grades of organisation within the same great group; for instance, in
the Vertebrata, to the co-existence of mammals and fish— amongst
Mammalia, to the co-existence of man and the ornithorhynchus —
amongst fishes, to the co-existence of the shark and the lancelet
(Amphioxus), which latter fish in the extreme simplicity of its
structure approaches the invertebrate classes. But mammals and fish
hardly come into competition with each other; the advancement of
the whole class of mammals, or of certain members in this class, to
the highest grade, would not lead to their taking the place of fishes.
Physiologists believe that the brain must be bathed by warm blood
to be highly active, and this requires aerial respiration; so that warm-
blooded mammals when inhabiting the water lie under a disadvan-
tage in having to come continually to the surface to breathe. With
fishes, members of the shark family would not tend to supplant the
lancelet; for the lancelet, as I hear from Fritz Muller, has as sole
companion and competitor on the barren sandy shore of South Brazil,
an anomalous annelid. The three lowest orders of mammals, namely,
marsupials, edentata, and rodents, co-exist in South America in the
same region with numerous monkeys, and probably interfere little
with each other. Although organisation, on the whole, may have
advanced and be still advancing throughout the world, yet the scale
will always present many degrees of perfection; for the high advance-
ment of certain whole classes, or of certain members of each class,
does not at all necessarily lead to the extinction of those groups with
which they do not enter into close competition. In some cases, as we
shall hereafter see, lowly organised forms appear to have been pre-
served to the present day, from inhabiting confined or peculiar sta-
tions, where they have been subjected to less severe competition, and
ORIGIN OF SPECIES
132
where their scanty numbers have retarded the chance of favorable
variations arising.
Finally, I believe that many lowly organised forms now exist
throughout the world, from various causes. In some cases variations
or individual differences of a favorable nature may never have arisen
for natural selection to act on and accumulate. In no case, probably,
has time sufficed for the utmost possible amount of development.
In some few cases there has been what we must call retrogression of
organisation. But the main cause lies in the fact that under very
simple conditions of life a high organisation would be of no service,
—possibly would be of actual disservice, as being of a more delicate
nature, and more liable to be put out of order and injured.
Looking to the first dawn of life, when all organic beings, as we
may believe, presented the simplest structure, how, it has been asked,
could the first steps in the advancement or differentiation of parts
have arisen ? Mr. Herbert Spencer would probably answer, that, as
soon as simple unicellular organism came by growth or division to
be compounded of several cells, or became attached to any supporting
surface, his law “that homologous units of any order became differ-
entiated in proportion as their relations to incident forces became
different” would come into action. But as we have no facts to guide
us, speculation on the subject is almost useless. It is, however, an
error to suppose that there would be no struggle for existence, and,
consequently, no natural selection, until many forms had been pro-
duced: variations in a single species inhabiting an isolated station
might be beneficial, and thus the whole mass of individuals might
be modified, or two distinct forms might arise. But, as I remarked
towards the close of the Introduction, no one ought to feel surprise
at much remaining as yet unexplained on the origin of species, if
we make due allowance for our profound ignorance on the mutual
relations of the inhabitants of the world at the present time, and still
more so during past ages.
CONVERGENCE OF CHARACTER
Mr. H. C. Watson thinks that I have overrated the importance
of divergence of character (in which, however, he apparently be-
lieves), and that convergence, as it may be called, has likewise played
CONVERGENCE OF CHARACTER
^33
a part. If two species, belonging to two distinct though allied genera,
had both produced a large number of new and divergent forms, it is
conceivable that these might approach each other so closely that they
would have all to be classed under the same genus; and thus the
descendants of two distinct genera would converge into one. But it
would in most cases be extremely rash , to attribute to convergence a
close and general similarity of structure in the modified descendants
of widely distinct forms. The shape of a crystal is determined solely
by the molecular forces, and it is not surprising that dissimilar sub-
stances should sometimes assume the same form; but with organic
beings we should bear in mind that the form of each depends on an
infinitude of complex relations, namely on the variations which have
arisen, these being due to causes far too intricate to be followed out, —
on the nature of the variations which have been preserved or selected,
and this depends on the surrounding physical conditions, and in a
still higher degree on the surrounding organisms with which each
being has come into competition, — and lastly, on inheritance (in itself
a fluctuating element) from innumerable progenitors, all of which
have had their forms determined through equally complex relations.
It is incredible that the descendants of two organisms, which had
originally differed in a marked manner, should ever afterwards con-
verge so closely as to lead to a near approach to identity throughout
their whole organisation. If this had occurred, we should meet with
the same form, independendy of genetic connection, recurring in
widely separated geological formations; and the balance of evidence
is opposed to any such an admission.
Mr. Watson has also objected that the continued action of natural
selection, together with divergence of character, would tend to make
an indefinite number of specific forms. As far as mere inorganic
conditions are concerned, it seems probable that a sufficient number
of species would soon become adapted to all considerable diversities
of heat, moisture, etc.; but I fully admit that the mutual relations of
organic beings are more important; and as the number of species in
any country goes on increasing, the organic conditions of life must
become more and more complex. Consequently there seems at first
sight no limit to the amount of profitable diversification of structure,
and therefore no limit to the number of species which might be
134 ORIGIN OF SPECIES
produced. We do not know that even the most prolific area is fully
stocked with specific forms: at the Cape of Good Hope and in Aus-
tralia, which support such an astonishing number of species, many
European plants have become naturalised. But geology shows us,
that from an early part of the tertiary period the number of species
of shells, and that from the middle part of this same period the
number of mammals, has not gready or at all increased. What then
checks an indefinite increase in the number of species? The amount
of life (I do not mean the number of specific forms) supported on
an area must have a limit, depending so largely as it does on physical
conditions; therefore, if an area be inhabited by very many species,
each or nearly each species will be represented by few individuals;
and such species will be liable to extermination from accidental
fluctuations in the nature of the seasons or in the number of their
enemies. The process of extermination in such cases would be rapid,
whereas the production of new species must always be slow. Imagine
the extreme case of as many species as individuals in England, and
the first severe winter or very dry summer would exterminate
thousands on thousands of species. Rare species, and each species
will become rare if the number of species in any country becomes
indefinitely increased, will, on the principle often explained, present
within a given period few favorable variations; consequently, the
process of giving birth to new specific forms would thus be retarded.
When any species becomes very rare, close interbreeding will help
to exterminate it; authors have thought that this comes into play in
accounting for the deterioration of the aurochs in Lithuania, of red
deer in Scotland, and of bears in Norway, etc. Lastly, and this I am
inclined to think is the most important element, a dominant species,
which has already beaten many competitors in its own home, will
tend to spread and supplant many others. Alph. de Candolle has
shown that those species which spread widely, tend generally to
spread very widely; consequently, they will tend to supplant and
exterminate several species in several areas, and thus check the in-
ordinate increase of specific forms throughout the world. Dr. Hooker
has recently shown that in the southeast corner of Australia, where,
apparently, there are many invaders from different quarters of the
globe, the endemic Australian species have been greatly reduced in
SUMMARY OF CHAPTER I35
number. How much weight to attribute to these several considera-
tions I will not pretend to say; but conjointly they must limit in each
country the tendency to an indefinite augmentation of specific forms.
SUMMARY OF CHAPTER
If under changing conditions of life organic beings present indi-
vidual differences in almost every part of their structure, and this
cannot be disputed; if there be, owing to their geometrical rate of
increase, a severe struggle for life at some age, season, or year, and
this certainly cannot be disputed; then, considering the infinite com-
plexity of the relations of all organic beings to each other and to their
conditions of life, causing an infinite diversity in structure, constitu-
tion, and habits, to be advantageous to them, it would be a most
extraordinary fact if no variations had ever occurred useful to each
being s own welfare, in the same manner as so many variations have
occurred useful to man. But if variations useful to any organic being
ever do occur, assuredly individuals thus characterised will have the
best chance of being preserved in the struggle for life; and from the
strong principle of inheritance, these will tend to produce offspring
similarly characterised. This principle of preservation, or the sur- *
vival of the fittest, I have called Natural Selection. It leads to the ;
improvement of each creature in relation to its organic and inorganic
conditions of life; and consequently, in most cases, to what must be
regarded as an advance in organisation. Nevertheless, low and simple
forms will long endure if well fitted for their simple conditions of
life.
Natural selection, on the principle of qualities being inherited at
corresponding ages, can modify the egg, seed, or young, as easily as
the adult. Amongst many animals, sexual selection will have given
its aid to ordinary selection, by assuring to the most vigorous and
best adapted males the greatest number of offspring. Sexual selection
will also give characters useful to the males alone, in their struggles
or rivalry with other males; and these characters will be transmitted
to one sex or to both sexes, according to the form of inheritance
which prevails.
Whether natural selection has really thus acted in adapting the
various forms of life to their several conditions and stations, must be
136 ORIGIN OF SPECIES
judged by the general tenor and balance of evidence given in the
following chapters. But we have already seen how it entails extinc-
tion; and how largely extinction has acted in the world’s history,
geology plainly declares. Natural selection, also, leads to divergence
of character; for the more organic beings diverge in structure, habits,
and constitution, by so much the more can a large number be sup-
ported on the area,— of which we see proof by looking to the inhabi-
tants of any small spot, and to the productions naturalised in foreign
lands. Therefore, during the modification of the descendants of any
one species, and during the incessant struggle of all species to increase
in numbers, the more diversified the descendants become, the better
will be their chance of success in the battle for life. Thus the small
differences distinguishing varieties of the same species, steadily tend
to increase, till they equal the greater differences between species of
the same genus, or even of distinct genera.
We have seen that it is the common, the widely diffused and
widely ranging species, belonging to the larger genera within each
class, which vary most; and these tend to transmit to their modified
offspring that superiority which now makes them dominant in their
own countries. Natural selection, as has just been remarked, leads to
divergence of character and to much extinction of the less improved
and intermediate forms of life. On these principles, the nature of
the affinities, and the generally well-defined distinctions between the
innumerable organic beings in each class throughout the world,
may be explained. It is a truly wonderful fact— the wonder of which
we are apt to overlook from familiarity— that all animals and all
plants throughout all time and space should be related to each other
in groups, subordinate to groups, in the manner which we every-
where behold— namely, varieties of the same specie most closely
related, species of the same genus less closely and unequally related,
forming sections and sub-genera, species of distinct genera much less
closely related, and genera related in different degrees, forming sub-
families, families, orders, subthe outer
and inner flowers in some compositous and umbelliferous plants.
Every one is familiar with the difference between the ray and central
florets of, for instance, the daisy, and this difference is often accom-
panied with the partial or complete abortion of the reproductive
organs. But in some of these plants, the seeds also differ in shape and
sculpture. These differences have sometimes been attributed to the
pressure of the involucra on the florets, or to their mutual pressure,
and the shape of the seeds in the ray-florets of some Compositae
countenances this idea; but with the Umbelliferae, it is by no means,
as Dr. Hooker informs me, the species with the densest heads which
most frequently differ in their inner and outer flowers. It might have
been thought that the development of the ray-petals by drawing
nourishment from the reproductive organs causes their abortion;
but this can hardly be the sole cause, for in some Compositae the
seeds of the outer and inner florets differ, without any difference in
the corolla. Possibly these several differences may be connected with
the different flow of nutriment towards the central and external
flowers: we know, at least, that with irregular flowers, those nearest
to the axis are most subject to peloria, that is to become abnormally
symmetrical. I may add, as an instance of this fact, and as a striking
case of correlation, that in many pelargoniums, the two upper petals
in the central flower of the truss often lose their patches of darker
colour; and when this occurs, the adherent nectary is quite aborted;
the central flower thus becoming peloric or regular. When the colour
is absent from only one of the two upper petals, the nectary is not
quite aborted but is much shortened.
With respect to the development of the corolla, Sprengel’s idea that
the ray-florets serve to attract insects, whose agency is highly ad-
vantageous or necessary for the fertilisation of these plants, is highly
probable; and if $0, natural selection may have come into play. But
ORIGIN OF SPECIES
150
with respect to the seeds, it seems impossible that their differences in
shape, which are not always correlated with any difference in the
corolla, can be in any way beneficial; yet in the Umbelliferae these
differences are of such apparent importance — ^the seeds being some-
times orthospermous in the exterior flowers and coelospermous in
the central flowers, — that the elder De Candolle founded his main
divisions in the order on such characters. Hence modifications of
structure, viewed by systematists as of high value, may be wholly due
to the laws of variation and correlation, without being, as far as we
can judge, of the slightest service to the species.
We may often falsely attribute to correlated variation structures
which are common to whole groups of species, and which in truth
are simply due to inheritance; for an ancient progenitor may have
acquired through natural selection some one modification in struc-
ture, and, after thousands of generations, some other and independent
modification; and these two modifications, having been transmitted
to a whole group of descendants with diverse habits, would naturally
be thought to be in some necessary manner correlated. Some other
correlations are apparendy due to the manner in which natural se-
lection can alone act. For instance, Alph. de Candolle has remarked
that winged seeds are never found in fruits which do not open; I
should explain this rule by the impossibility of seeds gradually be-
coming winged through natural selection, unless the capsules were
open: for in this case alone could -the seeds, which were a litde better
adapted to be wafted by the wind, gain an advantage over others less
well fitted for wide dispersal.
COMPENSATION AND ECONOMY OF GROWTH
The elder Geoffroy and Goethe propounded, at about the same
time, their law of compensation or balancement of growth; or, as
Goethe expressed it, “in order to spend on one side, nature is forced
to economise on the other side.” I think this holds true to a certain
extent with our domestic productions: if nourishment flows to one
part or organ in excess, it rarely flows, at least in excess, to another
part; thus it is diflScult to get a cow to give much milk and to fatten
readily. The same varieties of the cabbage do not yield abundant and
nutritious foliage and a copious supply of oil-bearing seeds. When
COMPENSATION AND ECONOMY OF GROWTH I5I
the seeds in our fruits become atrophied, the fruit itself gains largely
in size and quality. In our poultry, a large tuft of feathers on the
head is generally accompanied by a diminished comb and a large
beard by diminished wattles. With species in a state of nature it
can hardly be maintained that the law is of universal application; but
many good observers, more especially botanists, believe in its truth. I
will not, however, here give any instances, for I see hardly any way of
distinguishing between the effects, on the one hand, of a part being
largely developed through natural selection and another and adjoin-
ing part being reduced by this same process or by disuse, and, on the
other hand, the actual withdrawal of nutriment from one part owing
to the excess of growth in another and adjoining part.
I suspect, also, that some of the cases of compensation which have
been advanced, and likewise some other facts, may be merged under
a more general principle, namely, that natural selection is continually
trying to economise every part of the organisation. If under changed
conditions of life a structure, before useful, becomes less useful, its
diminution will be favoured, for it will profit the individual not to
have its nutriment wasted in building up a useless structure. I can
thus only understand a fact with which I was much struck when ex-
amining cirripedes, and of which many analogous instances could
be given: namely, that when a cirripede is parasitic within another
cirripede and is thus protected, it loses more or less completely its own
shell or carapace. This is the case with the male Ibla, and in a truly
extraordinary manner with the Proteolepas: for the carapace in all
other cirripedes consists of the three highly important anterior seg-
ments of the head enormously developed, and furnished with great
nerves and muscles; but in the parasitic and protected Proteolepas,
the whole anterior part of the head is reduced to the merest rudiment
attached to the bases of the prehensile antenna. Now the saving of a
large and complex structure, when rendered superfluous, would be a
decided advantage to each successive individual of the species; for in
the struggle for life to which every animal is exposed, each would
have a better chance of supporting itself, by less nutriment being
wasted.
Thus, as I believe, natural selection will tend in the long run to re-
duce any part of the organisation, as soon as it becomes, through
ORIGIN OF SPECIES
152
changed habits, superfluous, without by any means causing some
other part to he largely developed in a corresponding degree. And,
conversely, that natural selection may perfectly well succeed in largely
developing an organ without requiring as a necessary compensation
the reduction of some adjoining part.
MULTIPLE, RUDU^IENTARY, AND LOWLY ORGANISED STRUCTURES
ARE VARIABLE
It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both
with varieties and species, that when any part or organ is repeated
many times in the same individual (as the vertebrae in snakes, and
the stamens in polyandrous flowers) >the number is variable; whereas
the same part or organ, when it occurs in lesser numbers, is constant.
The same author as well as some botanists have further remarked
that multiple parts are extremely liable to vary in structure. As
“vegetative repetition,” to use Prof. Owen’s expression, is a sign of
low organisation, the foregoing statements accord with the common
opinion of naturalists, that beings which stand low in the scale of
nature are more variable than those which are higher. I presume
that lowness here means ithat the several parts of the organisation
have been but little specialised for particular functions; and as long
as the same part has to perform diversified work, we can perhaps see
why it should remain variable, that is, why natural selection should
not have preserved or rejected each little deviation of form so care-
fully as when the part has to serve for some one special purpose. In
the same way that a knife which has to cut all sorts of things may be
of almost any shape; whilst a tool for some particular purpose must
be of some particular shape. Natural selection, it should never be for-
gotten, can act solely through and for the advantage of each being.
Rudimentary parts, as it is generally admitted, are apt to be highly
variable. We shall have to recur to this subject; and I will here only
add that their variability seems to result from their uselessness, and
consequently from natural selection having had no power to check
deviations in their structure.
STRUCTURES VARIABLE
153
A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR
MANNER, IN COMPARISON WITH THE SAME PART IN ALLIED SPECIES,
TENDS TO BE HIGHLY VARIABLE
Several years ago I was much struck by a remark, to the above
effect, made by Mr. Waterhouse. Professor Owen, also, seems to
have come to a nearly similar conclusion. It is hopeless to attempt to
convince any one of the truth of the above proposition without giving
the long array of facts which I have collected, and which cannot pos-
sibly be here introduced. I can only state my conviction that it is a
rule of high generality. I am aware of several causes of error, but I
hope that I have made due allowances for them. It should be under-
stood that the rule by no means applies to any part, however un-
usually developed, unless it be unusually developed in one species
or in a few species in comparison with the same part in many closely
allied species. Thus, the wing of a bat is a most abnormal struc-
ture in the class of mammals, but the rule would not apply here,
because the whole group of bats possesses wings; it would apply only
if some one species had wings developed in a remarkable manner in
comparison with the other species of the same genus. The rule
applies very strongly in the case of secondary sexual characters, when
displayed in any unusual manner. The term, secondary sexual char-
acters, used by Hunter, relates to characters which are attached to one
sex, but are not directly connected with the act of reproduction. The
rule applies to males and females; but more rarely to the females, as
they seldom offer remarkable secondary sexual characters. The rule
being so plainly applicable in the case of secondary sexual characters,
may be due to the great variability of these characters, whether or not
displayed in any unusual manner— of which fact I think there can
be little doubt. But that our rule is not confined to secondary sexual
characters is clearly shown in the case of hermaphrodite cirripedes;
I particularly attended to Mr. Waterhouse s remark, whilst investi-
gating this Order, and I am fully convinced that the rule almost
always holds good. I shall, in a future work, give a list of all the
more remarkable cases; I will here give only one, as it illustrates the
rule in its largest application. The opercular valves of sessile cirri-
pedes (rock barnacles) are, in every sense of the word, very impor-
ORIGIN OF SPECIES
154
tant structures, and they differ extremely little even in distinct genera;
but in the several species of one genus, Pyrgoma, these valves pre-
sent a marvellous amount of diversification; the homologous valves
in the different species being sometimes wholly unlike in shape; and
the amount of variation in the individuals of the same species is
so great, that it is no exaggeration to state that the varieties of the
same species differ more from each other in the characters derived
from these important organs, than do the species belonging to other
distinct genera.
As with birds the individuals of the same species, inhabiting the
same country, vary extremely litde, I have particularly attended to
them; and the rule certainly seems to hold good in this class. I can-
not make out that it applies to plants, and this would have seriously
shaken my belief in its truth, had not the great variability in plants
made it particularly difficult to compare their relative degrees of
variability.
When we see any part or organ developed in a remarkable degree
or manner in a species, the fair presumption is that it is of high im-
portance to that species: nevertheless it is in this case eminently liable
to variation.' Why should this be so? On the view that each species
has been independently created, with all its parts as we now see them,
I can see no explanation. But on the view that groups of species are
descended from some other species, and have been modified through
natural selection, I think we can obtain some light. First let me make
some preliminary remarks. If, in our domestic animals, any part or
the whole animal be neglected, and no selection be applied, that part
(for instance, the comb in the Dorking fowl) or the whole breed
will cease to have a uniform character: and the breed may be said to
be degenerating. In rudimentary organs, and in those which have
been but little specialised for any particular purpose, and perhaps in
polymorphic groups, we see a nearly parallel case; for in such cases
natural selection either has not or cannot have come into full play,
and thus the organisation is left in a fluctuating condition. But what
here more particularly concerns us is, that those points in our domes-
tic animals, which at the present time are undergoing rapid change by
continued selection, are also eminently liable to variation. Look at
the individuals of the same breed of the pirrortrk onri QAP whst a
STRUCTURES VARIABLE 1 55
prodigious amount of difference there is in the beaks of tumblers, in
the beaks and watde of carriers, in the carriage and tail of fantails,
etc., these being the points now mainly attended to by English fan-
ciers. Even in the same sub-breed, as in that of the short-faced tum-
/bler, it is notoriously difficult to breed nearly perfect birds, many
« departing widely from the standard. There may truly be said to be
a constant struggle going on between, on the one hand, the tendency
to reversion to a less perfect state, as well as an innate tendency to
new variations, and, on the other hand, the power of steady selec-
tion to keep the breed true. In the long run selection gains the day,
and we do not expect to fail so completely as to breed a bird as coarse
as a common tumbler pigeon from a good short-faced strain.- But
as long as selection is rapidly going on, much variability in the
parts undergoing modification may always be expected.
Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other
species of the same genus, we may conclude that this part has under-
gone an extraordinary amount of modification since the period when
the several species branched off from the common progenitor of the
genus. This period will seldom be remote in any extreme degree, as
species rarely endure for more than one geological period. An extraor-
dinary amount of modification implies an unusually large and long-
continued amount of variability, which has continually been accumu-
lated by natural selection for the benefit of the species. But as the
variability of the extraordinarily developed part or organ has been
so great and long-continued within a period not excessively remote,
we might, as a general rule, still expect to find more variability in
such parts than in other parts of the organisation which have re-
mained for a much longer period nearly constant. And this, I am
convinced, is the case. That the struggle between natural selection
on the one hand, and the tendency to reversion and variability on
the other hand; will in the course of time cease; and that the most
abnormally developed organs may be made constant, I see no reason
to doubt. Hence, when an organ, however abnormal it may be, has
been transmitted in approximately the same condition to many modi-
fied descendants, as in the case of the wing of the bat, it must have
existed, according to our theory, for an immense period in nearly
ORIGIN OF SPECIES
156
the same state; and thus it has come not to be more variable than
any other structure. It is only in those cases in which die modifica-
tion has been comparatively recent and extraordinarily great that we
ought to find the generative variability, as it may be called, still
present in a high degree. For in 'this case the variability will seldom
as yet have been fixed by the continued selection of the individuals
varying in the required manner and degree, and by the continued
rejection of those tending to revert to a former and less modified
condition,
SPECIFIC CHARACTERS MORE VARIABLE THAN GENERIC CHARACTERS
The principle discussed under the last heading may be applied to
our present subject. It is notorious that specific characters are more
variable than generic. To explain by a simple example what is
meant: if in a large genus of plants some species had blue flowers
and some had red, the colour would be only a specific character, and
no one would be surprised at one of the blue species varying into red,
or conversely; but if all the species had blue flowers, the colour would
become a generic character, and its variation would be a more un-
usual circumstance. I have chosen this example because the explana-
tion which most naturalists would advance is not here applicable,
namely, that specific characters are more variable than generic, be-
cause they are taken from parts of less physiological importance than
those commonly used for classing genera. I believe this explanation
is partly, yet only indirectly, true; I shall, however, have to return to
this point in the chapter on Classification. It would be almost super-
fluous to adduce evidence in support of the statement, that ordinary
specific characters are more variable than generic; but with respect
to important characters, I have repeatedly noticed in works on
natural history, that when an author remarks with surprise that some
important organ or part, which is generally very constant throughout
a large group of species, differs considerably in closely allied species,
it is often variable in the individuals of the same species. And this
fact shows that a character, which is generally of generic value, when
it sinks in value and becomes only of specific value, often becomes
variable, though its physiological importance may remain the same.
Something of the same kind applies to monstrosities: at least Is.
SECONDARY SEXUAL CHARACTERS VARIABLE I57
Geoffrey St, Hilaire apparently entertains no doubt, that the more
an organ normally differs in the different species of the same group,
the more subject it is to anomalies in the individuals.
On the ordinary view of each species having been independendy
created, why should that part of the structure, which differs from
the same part in other independently created species of the same
genus, be more variable than those parts which are closely alike in
the several species? I do not see that any explanadon can be given.
But on the view that species are only strongly marked and fixed varie-
ties, we might expect often to find them still continuing to vary in
those parts of their structure which have varied within a moderately
recent period, and which have thus come to differ. Or to state the
case in another manner: the points in which all the species of a genus
resemble each other, and in which they differ from alhed genera, are
called generic characters; and these characters may be attributed to
inheritance from a common progenitor, for it can rarely have hap-
pened that natural selection will have modified several distinct spe-
cies, fitted to more or less widely different habits, in exactly the same
manner: and as these so-called generic characters have been inherited
from before the period when the several species first branched off from
their common progenitor, and subsequently have not varied or come
to differ in any degree, or only in a slight degree, it is not probable
that they should vary at the present day. On the other hand, the
points in which species differ from other species of the same genus
are called specific characters; and as these specific characters have
varied and come to differ since the period when the species branched
off from a common progenitor, it is probable that they should still
often be in some degree variable, — ^at least more variable than those
parts of the organisation which have for a very long period remained
constant.
SECONDARY SEXUAL CHARACTERS VARIABLE
I think it will be admitted by naturalists, without my entering on
details, that secondary sexual characters are highly variable. It will
also be admitted that species of the same group differ from each
other more widely in their secondary sexual characters, than in other
parts of their organisation: compare, for instance, the amount of
158 ORIGIN OF SPECIES
difierence between the males of gallinaceous birds, in which second-
ary sexual characters are strongly displayed, with the amount of
difference between the females. The cause of the original variabil-
ity of these characters is not manifest; but we can see why they
should not have been rendered as constant and uniform as others,
for they are accumulated by sexual selection, which is less rigid in
its action than ordinary selection, as it does not entail death, but only
gives fewer offspring to the less favoured males. Whatever the cause
may be of the variability of secondary sexual characters, as they are
highly variable, sexual selection will have had a wide scope for action,
and may thus have succeeded in giving to the species of the
same group a greater amount of difference in these than in other
respects.
It is a remarkable fact, that the secondary differences between the
two sexes of the same species are generally displayed in the very same
parts of the organisation in which the species of the same genus
differ from each other. Of this fact I will give in illustration the two
first instances which happen to stand on my list; and as the differ-
ences in these cases are of a very unusual nature, the relation can
hardly be accidental. The same number of joints in the tarsi is a
character common to very large groups of beetles, but in the Engidas,
as Westwood has remarked, the number varies greatly; and the
number likewise differs in the two sexes of the same species. Again
in the fossorial hymenoptera, the neuration of the wings is a char-
acter of the highest importance, because common to large groups;
but in certain genera the neuration differs in the different species,
and likewise in the two sexes of the same species. Sir J. Lubbock
has recently remarked, that several minute crustaceans offer excellent
illustrations of this law. “In Pontella, for instance, the sexual charac-
ters are afforded mainly by the anterior antennae and by the fifth pair
of legs: the specific differences also are principally given by these
organs.” This relation has a clear meaning on my view: I look at
all the species of the same genus as having as certainly descended
from a common progenitor, as have the two sexes of any one spe-
cies. Consequently, whatever part of the structure of the common
progenitor, or of its early descendants, became variable, variations of
this part would, it is highly probable, be taken advantage of by
DISTINCT SPECIES PRESENT VARIATIONS I59
natural and sexual selection, in order to fit the several species to their
several places in the economy of nature, and likewise to fit the two
sexes of the same species to each other, or to fit the males to struggle
with other males for the possession of the females.
Finally, then, I conclude that the greater variability of specific char-
acters, or those which distinguish species from species, than of
generic characters, or those which are possessed by all the species;
that the frequent extreme variability of any part which is developed
in a species in an extraordinary manner in comparison with the same
part in its congeners; and the slight degree of variability in a part,
however extraordinarily it may be developed, if it be common to a
whole group of species; that the great variability of secondary sexual
characters, and their great difference in closely allied species; that
secondary sexual and ordinary specific differences are generally dis-
played in the same parts of the organisation, — are all principles closely
connected together. All being mainly due to the species of the same
group being the descendants of a common progenitor, from whom
they have inherited much in common, to parts which have recently
and largely varied being more likely still to go on varying than parts
which have long been inherited and have not varied, to natural selec-
tion having more or less completely, according to the lapse of time,
overmastered the tendency to reversion and to further variability,—
to sexual selection being less rigid than ordinary selection,— and to
variations in the same parts having been accumulated by natural and
sexual selection, and having been thus adapted for secondary sexual,
and for ordinary purposes.
BISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS, SO THAT A VARIETY
OF ONE SPECIES OFTEN ASSUMES A CHARACTER PROPER TO AN ALLIED
SPECIES, OR REVERTS TO SOME OF THE CHARACTERS OF AN EARLY
PROGENITOR
These propositions will be most readily understood by looking to
our domestic races. The most distinct breeds of the pigeon, in coun-
tries widely apart, present sub-varieties with reversed feathers on
the head, and with feathers on the feet, characters not possessed by
the aboriginal rock pigeon; these then are analogous variations in
l6o ORIGIN OF SPECIES
two or more distinct races. The frequent presence of fourteen or even
•sixteen tail-feathers in the pouter may be considered as a variation
representing the normal structure of another race, the fan-tail. I
presume that no one will doubt that all such analogous variations are
due to the several races of the pigeon having inherited from a com-
mon parent the same constitution and tendency to variation, when
acted on by similar unknown influences. In the vegetable kingdom
we have a case of analogous variation, in the enlarged stems, or as
commonly called roots, of the Swedish turnip and rutabaga, plants
which several botanists rank as varieties produced by cultivation from
a common parent: if this be not so, the case will then be one of
analogous variation in two so-called distinct species; and to these a
third may be added, namely, the common turnip. According to the
ordinary view of each species having been independently created, we
should have to attribute this similarity in the enlarged stems of these
three plants, not to the vera causa of community of descent, and a con-
sequent tendency to vary in a like manner, but to three separated
yet closely related acts of creation. Many similar cases of analogous
variation have been observed by Naudin in the great gourd family,
and by various authors in our cereals. Similar cases occurring with
insects under natural conditions have lately been discussed with much
ability by Mr. Walsh, who has grouped them under his law of
Equable Variability.
With pigeons, however, we have another case, namely, the oc-
casional appearance in all the breeds, of slaty-blue birds with two
black bars on the wings, white loins, a bar at the end of the tail, with
the outer feathers externally edged near their basis with white. As
all these marks are characteristic of the parent rock pigeon, I presume
that no one will doubt that this is a case of reversion, and not of a
new yet analogous variation appearing in the several breeds. We
may, I think, confidently come to this conclusion, because, as we
have seen, these coloured marks are eminently liable to appear in the
crossed offspring of two distinct and differently coloured breeds;
and in this case there is nothing in the external conditions of life to
cause the reappearance of the slaty-blue, with the several marks, be-
yond the influence of the mere act of crossing, on the laws of
inheritance.
DISTINCT SPECIES PRESENT VARIATIONS l6l
No doubt it is a very surprising fact that characters should re-
appear after having been lost for many, probably for hundreds of
generations. But when a breed has been crossed only once by some
other breed, the offspring occasionally show for many generations
a tendency to revert in character to the foreign breed — some say, for
a dozen or even a score of generations. After twelve generations, the
proportion of blood, to use a common expression, from one ancestor,
is only one in 2,048; and yet, as we see, it is generally believed that a
tendency to reversion is retained by this remnant of foreign blood. In
a breed which has not been crossed but in which both parents have
lost some character which their progenitor possessed, the tendency,
whether strong or weak, to reproduce the lost character might, as
was formerly remarked, for all that we can see to the contrary, be
transmitted for almost any number of generations. When a charac-
ter which has been lost in a breed, reappears after a great number of
generations, the most probable hypothesis is, not that one individual
suddenly takes after an ancestor removed by some hundred genera-
tions, but that in each successive generation the character in question
has been lying latent, and at last, under unknown favourable con-
ditions, is developed. With the barb pigeon, for instance, which very
rarely produces a blue bird, it is probable that there is a latent
tendency in each generation to produce blue plumage. The abstract
improbability of such a tendency being transmitted through a vast
number of generations, is not greater than that of quite useless or
rudimentary organs being similarly transmitted. A mere tendency
to produce a rudiment is indeed sometimes thus inherited.
As all the species of the same genus are supposed to be descended
from a common progenitor, it might be expected that they would
occasionally vary in an analogous manner; so that the varieties of two
or more species would resemble each other, or that a variety of one
species would resemble in certain characters another and distinct
species, this other species being, according to our view, only a well-
marked and permanent variety. But characters exclusively due to
analogous variation would probably be of an unimportant nature,
for the preservation of all functionally important characters will have
been determined through natural selection, in accordance with the
different habits of the species. It might further be expected* that
ORIGIN OF SPECIES
162
the species of the same genus would occasionally exhibit reversions
to long lost characters. As, however, we do not know the common
ancestor of any natural group, we cannot distinguish between re-
visionary and analogous characters. If, for instance, we did not know
that the parent rock pigeon was not feather-footed or turn-crowned,
we could not have told, whether such characters in our domestic
breeds were reversions or only analogous variations; but we might
have inferred that the blue colour was a case of reversion from the
number of the markings, which are correlated with this tint, and
which would not probably have all appeared together from simple va-
riation. More especially we might have inferred this, from the blue
colour and the several marks so often appearing when differently
coloured breeds are crossed. Hence, although under nature it must
generally be left doubtful, what cases are reversions to formerly exist-
ing characters, and what are new but analogous variations, yet we
ought, on our theory, sometimes to find the varying offspring of a
species assuming characters which are already present in other mem-
bers of the same group. And this undoubtedly is the case.
The difficulty in distinguishing variable species is largely due to
the varieties mocking, as it were, other species of the same genus. A
considerable catalogue, also, could be given of forms intermediate
between two other forms, which themselves can only doubtfully be
ranked as species; and this shows, unless all these closely allied forms
be considered as independendy created species, that they have in
varying assumed some of the characters of the others. But the best
evidence of analogous variations is afforded by parts or organs which
are generally constant in character, but which occasionally vary so as
to resemble, in some degree, the same part or organ in an allied
species. I have collected a long list of such cases; but here, as before,
I lie under the great disadvantage of not being able to give them. I
can only repeat that such cases certainly occur, and seem to me very
remarkable.
I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several
species of the same genus, partly under domestication and partly
under nature. It is a case almost certainly of reversion. The ass some-
times has very distinct transverse bars on its legs, like those on the
DISTINCT SPECIES PRESENT VARIATIONS 163
legs of the zebra. It has been asserted that these are plainest in the
foal, and, from inquiries which I have made, I believe this to be
true. The stripe on the shoulder is sometimes double, and is very
variable in length and oudine. A white ass, but not an albino, has
been described without either spinal or shoulder stripe: and these
stripes are sometimes very obscure, or actually quite lost, in dark-
coloured asses. The koulan of Pallas is said to have been seen with
a double shoulder stripe. Mr. Blyth has seen a specimen of the
hemionus with a distinct shoulder stripe, though it properly has
none; and I have been informed by Colonel Poole that the foals of
this species are generally striped on the legs, and faintly on the
shoulder. The quagga, though so plainly barred like a zebra over
the body, is without bars on the legs; but Dr. Gray has figured one
specimen with very distinct zebra-like bars on the hocks.
With respect to the horse, I have collected cases in England of the
spinal stripe in horses of the most distinct breeds, and of all colours:
transverse bars on the legs are not rare in duns, mouse-duns, and in
one instance in a chestnut; a faint shoulder stripe may sometimes be
seen in duns, and I have seen a trace in a bay horse. My son made a
careful examination and sketch for me of a dun Belgian cart horse
with a double stripe on each shoulder and with leg stripes; I have
myself seen a dun Devonshire pony, and a small dun Welsh pony
has been carefully described to me, both with three parallel stripes on
each shoulder.
In the northwest part of India the Kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined
this breed for the Indian Government, a horse without stripes is not
considered as purely bred. The spine is always striped; the legs are
generally barred, and the shoulder stripe, which is sometimes double
and sometimes treble, is common; the side of the face, moreover, is
sometimes striped. The stripes are often plainest in the foal, and
sometimes quite disappear in old horses. Colonel Poole has seen
both gray and bay Kattywar horses striped when first foaled. I have
also reason to suspect, from information given me by Mr. W. W.
Edwards, that with the English race horse the spinal stripe is much
commoner in the foal than in the full-grown animal. I have myself
recently bred a foal from a bay mare (offspring of a Turkoman horse
164 ORIGIN OF SPECIES
and a Flemish mare) by a bay English race horse; this foal when a
week old was marked on its hinder quarters and on its forehead with
numerous, very narrow, dark, zebra-like bars, and its legs were feebly
striped. All the stripes soon disappeared completely. Without here
entering on further details, I may state that I have collected cases of
leg and shoulder stripes in horses of very different breeds in various
countries from Britain to Eastern China; and from Norway in the
north to the Malay Archipelago in the south. In all parts of the
world these stripes occur far oftenest in duns and mouse-duns; by the
term dun a large range of colour is included, from one between
brown and black to a close approach to cream colour.
I am aware that Colonel Hamilton Smith, who has written on this
subject, believes that the several breeds of the horse are descended
from several aboriginal species, one of which, the dun, was striped;
and that the above-described appearances are all due to ancient crosses
with the dun stock. But this view may be safely rejected, for it is
highly improbable that the heavy Belgian cart horse, Welsh ponies,
Norwegian cobs, the lanky Kattywar race, etc., inhabiting the most
distant parts of the world, should all have been crossed with one sup-
posed aboriginal stock.
Now let us turn to the effects of crossing the several species of the
horse-genus. Rollin asserts, that the common mule from the ass
and horse is particularly apt to have bars on its legs; according to
Ivir. Gosse, in certain parts of the United States, about nine out of
ten mules have striped legs. I once saw a mule with its legs so much
striped that any one might have thought that it was a hybrid zebra;
and Mr. W. C. Martin, in his excellent treatise on the horse, has
given a figure of a similar mule. In four coloured drawings, which
I have seen, of hybrids between the ass and zebra, 'the legs were much
more plainly barred than the rest of the body; and in one of them
there was a double shoulder stripe. In Lord Morton’s famous hybrid,
from a chestnut mare and male quagga, the hybrid, and even the pure
offspring subsequently produced from the same mare by a black
Arabian sire, were much more plainly barred across the legs than is
even the pure quagga. Lastly, and this is another most remarkable
rase, a hybrid has been figured by Dr. Gray (and he informs me
that he knows of a second case) from the ass and the hemionus; and
DISTINCT SPECIES PRESENT VARIATIONS 1 65
this hybrid, though the ass only occasionally has stripes on his legs
and the hemionus has none and has not even a shoulder stripe,
nevertheless had all four legs barred, and had three short shoulder
stripes, like those on the dun Devonshire and Welsh ponies, and even
had some zebra-like stripes on the sides of its face. With respect to
this last fact, I was so convinced that not even a stripe of colour ap-
pears from what is commonly called chance, that I was led solely
from the occurrence of the face stripes on this hybrid from the ass
and hemionus to ask Colonel Poole whether such face stripes ever
occurred in the eminently striped Kattywar breed of horses, and
was, as we have seen, answered in the affirmative.
What now are we to say to these several facts? We see several dis-
tinct species of the horse-genus becoming, by simple variation, striped
on the legs like a zebra, or striped on the shoulders like an ass. In the
horse we see this tendency strong whenever a dun tint appears— -a
tint which approaches to that of the general colouring of the other
species of the genus. The appearance of the stripes is not accom-
panied by any change of form or by any other new character. We see
this tendency to become striped most strongly displayed in hybrids
from between several of the most distinct species. Now observe the
case of the several breeds of pigeons: they are descended from a
pigeon (including two or three sub-species or geographical races)
of a bluish colour, with certain bars and other marks; and when any
breed assumes by simple variation a bluish tint, these bars and other
marks invariably reappear; but 'without any other change of form
or character. When the oldest and truest breeds of various colours
are crossed, we see a strong tendency for the blue tint and bars and
marks to reappear in the mongrels, I have stated that the most prob-
able hypothesis to account for the reappearance of very ancient char-
acters, is — that there is a tendency in the young of each successive
generation to produce the long-lost character, and that this tendency,
from unknown causes, sometimes prevails. And we have just seen
that in several species of the horse-genus the stripes are either plainer
or appear more commonly in the young than in the old. Call the
breeds of pigeons, some of which have bred true for centuries, spe-
cies; and how exacdy parallel is the case with that of the species of
the horse-genus! For myself, I venture confidently to look back thou-
ORIGIN OF SPECIES
l66
sands on thousands of generations, and I see an animal striped like
a zebra, but perhaps otherwise very differendy constructed, the com-
mon parents of our domestic horse (whether or not it be descended
from one or more wild stocks) of the ass, the hemionus, quagga, and
zebra.
He who believes that each equine species was independendy
created, will, I presume, assert that each species has been created with
a tendency to vary, both under nature and under domestication, in
this particular manner, so as often to become striped like the other
species of the genus; and that each has been created with a strong
tendency, when crossed with species inhabiting distant quarters of
the world, to produce hybrids resembling in their stripes, not their
own parents, but other species of the genus. To admit this view is,
as it seems to me, to reject a real for an unreal, or at least for an
unknown, cause. It makes the works of God a mere mockery and
deception; I would almost as soon believe with the old and ignorant
cosmogonists, that fossil shells had never lived, but had been created
in stone so as to mock the shells living on the seashore.
SUMMARY
Our ignorance of the laws of variation is profound. Not in one
case out of a hundred can we pretend to assign any reason why this
or that part has varied. But whenever we have the means of insti-
tuting a comparison, the same laws appear to have acted in produc-
ing the lesser differences between varieties of the same species, and
the greater differences between species of the same genus. Changed
conditions generally induce mere fluctuating variability, but some-
times they cause direct and definite effects; and these may become
strongly marked in the course of time, though we have not sufficient
evidence on this head. Habit in producing constitutional peculiarities
and use in strengthening and disuse in weakening and diminishing
organs, appear in many cases to have been potent in their
effects. Homologous parts tend to vary in the same manner, and
homologous parts tend to cohere. Modifications in hard parts and in
external parts sometimes affect softer and internal parts. When one
part is largely developed, perhaps it tends to draw nourishment from
the adjoining parts; and every part of the structure which can be
SUMMARY 167
saved without detriment will be saved. Changes of structure at an
early age may affect parts subsequendy developed; and many cases of
correlated variation, the nature of which we are unable to under-
stand, undoubtedly occur. Multiple parts are variable in number
and in structure, perhaps arising from such parts not having been
closely specialised for any particular function, so that their modi-
fications have not been closely checked by natural selection. It fol-
lows probably from this same cause, that organic beings low in the
scale are more variable than those standing higher in the scale, and
which have their whole organisation more specialised. Rudimentary
organs, from being useless, are not regulated by natural selection, and
hence are variable. Specific characters — that is, the characters which
have come to differ since the several species of the same genus
branched off from a common parent— are more variable than generic
characters, or those which have long been inherited, and have not dif-
fered within this same period. In these remarks we have referred to
special parts or organs being still variable, because they have recently
varied and thus come to differ; but we have also seen in the second
chapter that the same principle applies to the whole individual; for in
a district where many species of a genus are found— that is, where
there has been much former variation and differentiation, or where
the manufactory of new specific forms has been actively at work — in
that district and amongst these species, we now find, on an average,
most varieties. Secondary sexual characters are highly variable, and
such characters differ much in the species of the same group. Vari-
ability in the same parts of the organisation has generally been taken
advantage of in giving secondary sexual differences to the two sexes
of the same species, and specific differences to the several species of
the same genus. Any part or organ developed to an extraordinary
size or in an extraordinary manner, in comparison with the same
part or organ in the allied species, must have gone through an ex-
traordinary amount of modification since the genus arose; and thus
we can understand why it should often still be variable in a much
higher degree than other parts; for variation is a long-continued and
slow process, and natural selection will in such cases not as yet have
had time to overcome the tendency to further variability and to re-
version to a less modified state. But when a species with any ex-
ORIGIN OF SPECIES
l68
traordinarily developed organ has become the parent of many modi-
fied descendants— which on our view must be a very slow process,
requiring a long lapse of time— in this case, natural selection has suc-
ceeded in giving a fixed character to the organ, in however extraordi-
nary a manner it may have been developed. Species inheriting nearly
the same constitution from a common parent, and exposed to similar
influences, naturally tend to present analogous variations, or these
same species may occasionally revert to some of the characters of
their ancient progenitors. Although new and important modifica-
tions may not arise from reversion and analogous variation, such
modifications will add to the beautiful and harmonious diversity of
nature.
Whatever the cause may be of each slight diflference between the
offspring and their parents— and a cause for each must exist— we
have reason to believe that it is the steady accumulation of beneficial
differences which has given rise to all the more important modifica-
tions of structure in relation to the habits of each species.
CHAPTER VI
Difficulties of the Theory
Difficulties of the theory of descent with modification — ^Absence or rarity
of transitional varieties — ^Transitions in habits of life — ^Diversified
habits in the same species — Species with habits widely difierent from
those of their allies— -Organs of extreme perfection — ^Modes of transi-
tion — Cases of difficulty — ^Natura non facit saltum — Organs of small
importance — Organs not in all cases absolutely perfect — ^The law of
Unity of Type and of the Conditions of Existence embraced by the
theory of Natural Selection.
I ONG before the reader has arrived at this part of my work, a
crowd of difficulties will have occurred to him. Some of
them are so serious that to this day I can hardly reflect on
them without being in some degree staggered; but, to the best of
my judgment, the greater number are only apparent, and those that
are real are not, I think, fatal to theory.
These difficulties and objections may be classed under the follow-
ing heads : — First, why, if species have descended from other species
by fine gradations, do we not everywhere see innumerable transi-
tional forms? Why is not all nature in confusion, instead of the
species being, as we see them, well defined?
Secondly, is it possible that an animal having, for instance, the
structure and habits of a bat, could have been formed by the modi-
fication of some other animal with widely different habits and struc-
ture? Can we believe that natural selection could produce, on the
one hand, an organ of trifling importance, such as the tail of a giraffe,
which serves as a fly-flapper, and, on the other hand, an organ so
wonderful as the eye?
Thirdly, can instincts be acquired and modified through natural
selection? What shall we say to the instinct which leads the bee to
make cells, and which has practically anticipated the discoveries of
profound mathematicians ?
Fourthly, how can we account for species, when crossed, being
169
ORIGIN OF SPECIES
170
sterile and producing sterile offspring, whereas, when varieties are
crossed, their fertility is unimpaired?
The two first heads will here be discussed; some miscellaneous
objections in the following chapter; Instinct and Hybridism in the
two succeeding chapters.
ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES
As natural selection acts solely by the preservation of profitable
modifications, each new form will tend in a fully-stocked country to
take the place of, and finally to exterminate, its own less improved
parent-form and other less-favoured forms with which it comes into
competition. Thus extinction and natural selection go hand in hand.
Hence, if we look at each species as descended from some unknown
form, both the parent and all the transitional varieties will generally
have been exterminated by the very process of the formation and
perfection of the new form.
But, as by this theory innumerable transitional forms must have
existed, why do we not find them embedded in countless numbers
in the crust of the earth ? It will be more convenient to discuss this
question in the chapter on the Imperfection of the Geological Record;
and I will here only state that I believe the answer mainly lies in the
record being incomparably less perfect than is generally supposed.
The crust of the earth is a vast museum; but the natural collections
have been imperfecdy made, and only at long intervals of time.
But it may be urged that when several closely-allied species inhabit
the same territory, we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from
north to south over a continent, we generally meet at successive inter-
vals with closely allied or representative species, evidently filling
nearly the same place in the natural economy of the land. These
representative species often meet and interlock; and as the one
becomes rarer and rarer, the other becomes more and more frequent,
till the one replaces the other. But if we compare these species where
they intermingle, they are generally as absolutely distinct from each
other in every detail of structure as are specimens taken from the
metropolis inhabited by each. By my theory these allied species are
descended from a common parent; and during the process of modi-
TRANSITIONAL VARIETIES I7I
fication, each has become adapted to the conditions of life of its own
region, and has supplanted and exterminated its original parent-form
and all the transitional varieties between its past and present states.
Hence we ought not to expect at the present time to meet with
numerous transitional varieties in each region, though they must have
existed there, and may be embedded there in a fossil condition. But
in the intermediate region, having intermediate conditions of life,
why do we not now find closely-linking intermediate varieties ? This
difficulty for a long time quite confounded me. But I think it can
be in large part explained.
In the first place we should be extremely cautious in inferring,
becaus*e an area is now continuous, that it has been continuous during
a long period. Geology would lead us to believe that most continents
have been broken up into islands even during the later tertiary
periods; and in such islands distinct species might have been sep-
arately formed without the possibility of intermediate varieties exist-
ing in the intermediate zones. By changes in the form of the land
and of climate, marine areas now continuous must often have existed
within recent times in a far less continuous and uniform condition
than at present. But I will pass over this way of escaping from the
difficulty; for I believe that many perfectly defined species have been
formed on strictly continuous areas; though I do not doubt that the
formerly broken condition of areas now continuous, has played an
important part in the formation of new species, more especially with
freely-crossing and wandering animals.
In looking at species as they are now distributed over a wide area,
we generally find them tolerably numerous over a large territory,
then becoming somewhat abrupdy rarer and rarer on the confines,
and finally disappearing. Hence the neutral territory between two
representative species is generally narrow in comparison with the
territory proper to each. We see the same fact in ascending moun-
tains, and sometimes it is quite remarkable how abruptly, as
Alph. de Candolle has observed, a common alpine species disappears.
The same fact has been noticed by E. Forbes in sounding the depths
of the sea with the dredge. To those who look at climate and the
physical conditions of life as the all-important elements of distribu-
tion, these facts ought to cause surprise, as climate and height or
ORIGIN OF SPECIES
172
depth graduate away insensibly. But when we bear in mind that
almost every species, even in its metropolis, would increase im-
mensely in numbers, were it not for other competing species; that
nearly all either prey on or serve as prey for others; in short, that each
organic being is either directly or indirectly related in the most im-
portant manner to other organic beings,— we see that the range of
the inhabitants of any country by no means exclusively depends
on insensibly changing physical conditions, but in a large part on the
presence of other species, on which it lives, or by which it is destroyed,
or with which it comes into competition; and as these species are
already defined objects, not blending one into another by insensible
gradations, the range of any one species, depending as k does bn the
range of others, will tend to be sharply defined. Moreover, each
species on the confines of its range, where it exists in lessened num-
bers, will, during fluctuations in the number of its enemies or of its
prey, or in the nature of the seasons, be extremely liable to utter
extermination; and thus its geographical range will come to be still
more sharply defined.
As allied or representative species, when inhabiting a continuous
area, are generally distributed in such a manner that each has a wide
range, with a comparatively narrow neutral territory between them,
in which they become rather suddenly rarer and rarer; then, as
varieties do not essentially diflfer from species, the same rule will
probably apply to both; and if we take a varying species inhabiting
a very large area, we shall have to adapt two varieties to two large
areas, and a third variety to a narrow intermediate zone. The inter-
mediate variety, consequently, will exist in lesser numbers from in-
habiting a narrow and lesser area; and practically, as far as I can
make out, this rule holds good with varieties in a state of nature,
I have met with striking instances of the rule in the case of varie-
ties intermediate between well-marked varieties in the genus Balanus.
And it would appear from information given me by Mr. Watson,
Dr. Asa Gray, and Mr. Wollaston, that generally, when varieties
intermediate between two other forms occur, they are much rarer
numerically than the forms which they connect. Now, if we may
trust these facts and inferences, and conclude that varieties linking
two other varieties together generally have existed in lesser numbers
TRANSITIONAL VARIETIES 1 73
than the forms which they connect, then we can understand why
intermediate varieties should not endure for very long periods: —
why, as a general rule, they should be exterminated and disappear,
sooner than the forms which they originally linked together.
For any form existing in lesser numbers would, as already re-
marked, run a greater chance of being exterminated than one exist-
ing in large numbers; and in this particular case the intermediate
form would be eminently liable to the inroads of closely-allied forms
existing on both sides of it. But it is a far more important con-
sideration, that during the process of further modification, by which
two varieties are supposed to be converted and perfected into two
distinct species, the two which exist in larger numbers, from in-
habiting larger areas, will have a great advantage over the inter-
mediate variety, which exists in smaller numbers in a narrow and
intermediate zone. For forms existing in larger numbers will have
a better chance, within any given period, of presenting further
favourable variations for natural selection to seize on, than will the
rarer forms which exist in lesser numbers. Hence, the more common
forms, in the race for life, will tend to beat and supplant the less
common forms, for these will be more slowly modified and im-
proved. It is the same principle which, as I believe, accounts for the
common species in each country, as shown in the second chapter,
presenting on an average a greater number of well-marked varieties
than do the rarer species. I may illustrate what I mean by supposing
three varieties of sheep to be kept, one adapted to an extensive
mountainous region; a second to a comparatively narrow, hilly tract;
and a third to the wide plains at the base; and that the inhabitants
are all trying with equal steadiness and skill to improve their stocks
by selection; the chances in this case will be strongly in favour of
the great holders on the mountains or on the plains, improving their
breeds more quickly than the small holders on the intermediate
narrow, hilly tract; and consequently the improved mountain or
plain breed will soon take the place of the less improved hill breed;
and thus the two breeds, which originally existed in greater num-
bers, will come into close contact with each other, without the inter-
position of the supplanted, intermediate hill variety.
To sum up, I believe that species come to be tolerably well-defined
ORIGIN OF SPECIES
174
objects, and do not at any one period present an inextricable chaos
of varying and intermediate links: first, because new varieties are
very slowly formed, for variation is a slow process, and natural
selection can do nothing until favourable individual differences or
variations occur, and until a place in the natural polity of the country
can be better filled by some modification of some one or more of its
inhabitants. And such new places will depend on slow changes of
climate, or on the occasional immigration of new inhabitants, and,
probably, in a still more important degree, on some of the old in-
habitants becoming slowly modified, with the new forms thus pro-
duced and the old ones acting and reacting on each other. So that,
in any one region and at any one time, we ought to see only a few
species presenting slight modifications of structure in some degree
permanent; and this assuredly we do see.
Secondly, areas now continuous must often have existed within
the recent period as isolated portions, in which many forms, more
especially amongst the classes which unite for each birth and wander
much, may have separately been rendered sufficiently distinct to rank
as representative species. In this case, intermediate varieties between
the several representative species and their common parent, must
formerly have existed within each isolated portion of the land, but
these links during the process of natural selection will have been
supplanted and exterminated, so that they will no longer be found
in a living state.
Thirdly, when two or more varieties have been formed in differ-
ent portions of a stricdy continuous area, intermediate varieties will,
it is probable, at first have been formed in the intermediate zones,
but they will generally have had a short duration. For these inter-
mediate varieties will, from reasons already assigned (namely from
what we know of the actual distribution of closely allied or repre-
sentative species, and likewise of acknowledged varieties), exist in
the intermediate zones in lesser numbers than the varieties which
they tend to connect. From this cause alone the intermediate varieties
will be liable to accidental extermination; and during the process
of further modification through natural selection, they will almost
certainly be beaten and supplanted by the forms which they con-
nect; for these from existing in greater numbers will, in the aggre-
TRANSITIONS OF ORGANIC BEINGS 1 75
gate, present more varieties, and thus be further improved through
natural selection and gain further advantages.
Lastly, lopking not to any one time, but to all time, if my theory
be true, numberless intermediate varieties, linking closely together
all the species of the same group, must assuredly have existed; but
the very process of natural selection constantly tends, as has been
so often remarked, to exterminate the parent-forms and the inter-
mediate links. Consequendy evidence of their former existence
could be found only amongst fossil remains, which are preserved, as
we shall attempt to show in a future chapter, in an extremely im-
perfect and intermittent record.
ON THE ORIGIN AND TRANSITIONS OF ORGANIC BEINGS WITH
PECULIAR HABITS AND STRUCTURE
It has been asked by the opponents of such views as I hold, how,
for instance, could a land carnivorous animal have been converted
into one with aquatic habits; for how could the animal in its tran-
sitional state have subsisted? It would be easy to show that there
now exist carnivorous animals presenting close intermediate grades
from strictly terrestrial to aquatic habits; and as each exists by a
struggle for life, it is clear that each must be well adapted to its place
in nature. Look at the Mustek vison of North America, which has
webbed feet, and which resembles an otter in its fur, short legs,
and form of tail. During the summer this animal dives for and
preys on fish, but during the long winter it leaves the frozen waters,
and preys, like other pole-cats, on mice and land animals. If a
different case had been taken, and it had been asked how an in-
sectivorous quadruped could possibly have been converted into a
flying bat, the question would have been far more difScult to answer.
Yet I think such difficulties have litde weight.
Here, as on other occasions, I lie under a heavy disadvantage,
for, out of the many striking cases which I have collected, I can
give only one or two instances of transitional habits and structures
in allied species; and of diversified habits, either constant or occa-
sional, in the same species. And it seems to me that nothing less
than a long list of such cases is sufficient to lessen the difficulty in
any particular case like that of the bat.
ORIGIN OF SPECIES
176
Look at the family of squirrels; here we have the finest gradation
from animals with their tails only slightly flattened, and from others,
as Sir J. Richardson has remarked, with the posterior part of their
bodies rather wide and with the skin on their flanks rather full, to
the so-called flying squirrels; and flying squirrels have their limbs
and even the base of the tail united by a broad expanse of skin, which
serves as a parachute and allows them to glide through the air to
an astonishing distance from tree to tree. We cannot doubt that
each structure is of use to each kind of squirrel in its own country,
by enabling it to escape birds or beasts of prey, to collect food more
quickly, or, as there is reason to believe, to lessen the danger from
occasional falls. But it does not follow from this fact that the struc-
ture of each squirrel is the best that it is possible to conceive under
all possible conditions. Let the climate and vegetation change, let
other competing rodents or new beasts of prey immigrate, or old
ones become modified, and all analogy would lead us to believe that
some at least of the squirrels would decrease in numbers or become
exterminated, unless they also became modified and improved in
structure in a corresponding manner. Therefore, I can see no
difficulty, more especially under changing conditions of life, in
the continued preservation of individuals with fuller and fuller
flank-membranes, each modification being useful, each being propa-
gated, until, by the accumulated effects of this process of natural
selection, a perfect so-called flying squirrel was produced.
Now look at the Galeopithecus or so-called flying lemur, which
formerly was ranked amongst bats, but is now believed to belong to
the Insectivora. An extremely wide flank-membrane stretches from
the corners of the jaw to the tail, and includes the limbs with the
elongated fingers. This flank-membrane is furnished with an ex-
tensor muscle. Although no graduated links of structure, fitted for
gliding through the air, now connect the Galeopithecus with the
other Insectivora, yet there is no difficulty in supposing that such
links formerly existed, and that each was developed in the same
manner as with the less perfecdy gliding squirrels; each grade of
structure having been useful to its possessor. Nor can I see any
insuperable difficulty in further believing that the membrane con-
nected fingers and fore-arm of the Galeopithecus might have been
TRANSITIONS OF ORGANIC BEINGS I77
greatly lengthened by natural selection; and this, as far as the organs
o£ flight are concerned, would have converted the animal into a bat.
In certain bats in which the wing-membrane extends from the top
of the shoulder to the tail and includes the hind-legs, we perhaps
see traces of an apparatus originally fitted for gliding through the
air rather than for flight.
If about a dozen genera of birds were to become extinct, who
would have ventured to surmise that birds might have existed which
used their wings solely as flappers, like the logger-headed duck
(Micropterus of Eyton) ; as fins in the water and as front-legs on
the land, like the penguin; as sails, like the ostrich; and function-
ally for no purpose, like Apteryx? Yet the structure of each of
these birds is good for it, under the conditions of life to which it
is exposed, for each has to live by a struggle; but it is not necessarily
the best possible under all possible conditions. It must not be in-
ferred from these remarks that any of the grades of wing-structure
here alluded to, which perhaps may all be the result of disuse, indi-
cate the steps by which birds actually acquired their perfect power
of flight; but they serve to show what diversified means of transi-
tion are at least possible.
Seeing that a few members of such water-breathing classes as
the Crustacea and Mollusca are adapted to live on the land; and
seeing that we have flying birds and mammals, flying insects of the
most diversified types, and formerly had flying reptiles, it is con-
ceivable that flying-fish, wliich now glide far through the air, slighdy
rising and turning by the aid of their fluttering fins, might have been
modified into perfectly winged animals. If this had been effected,
who would have ever imagined that in an early transitional state
they had been the inhabitants of the open ocean, and had used their
incipient organs of flight exclusively, as far as we know, to escape
being devoured by other fish?
When we see any structure highly perfected for any particular
habit, as the wings of a bird for Alight, we should bear in mind that
animals displaying early transitional grades of the structure will
seldom have survived to the present day, for they will have been
supplanted by their successors, which were gradually rendered more
perfect through natural selection. Furthermore, we may conclude
ORIGIN OF SPECIES
178
that transitional states between structures fitted for very different
habits of life will rarely have been developed at an early period in
great numbers and under many subordinate forms. Thus, to return
to our imaginary illustration of the flying-fish, it does not seem
probable that fishes capable of true flight would have been developed
under many subordinate forms, for taking prey of many kinds in
many ways, on the land and in the water, until their organs of flight
had come to a high state of perfection, so as to have given them
a decided advantage over other animals in the battle for life. Hence
the chance of discovering species with transitional grades of struc-
ture in a fossil condition will always be less, from their having
existed in lesser numbers, than in the case of species with fully de-
veloped structures.
I will now give two or three instances both of diversified and
of changed habits in the individuals of the same species. In either
case it would be easy for natural selection to adapt the structure
of the animal to its changed habits, or exclusively to one of its
several habits. It is, however, difficult to decide, and immaterial
for us, whether habits generally change first and structure after-
wards; or whether slight modifications of structure lead to changed
habits; both probably often occurring almost simultaneously. Of
cases of changed habits it will suffice merely to allude to that of
the many British insects which now feed on exotic plants, or ex-
clusively on artificial substances. Of diversified habits innumerable
instances could be given: I have often watched a tyrant flycatcher
(Saurophagus sulphuratus) in South America, hovering over one
spot and then proceeding to another, like a kestrel, and at other
times standing stationary on the margin of water, and then dashing
into it like a kingfisher at a fish. In our own country the larger
titmouse (Parus major) may be seen climbing branches, almost
like a creeper; it sometimes, like a shrike, kills small birds by blows
on the head; and I have many times seen and heard it hammering
the seeds of the yew on a branch, and thus breaking them like a
nuthatch. In North America the black bear was seen by Hearne
swimming for hours with widely open mouth, thus catching, almost
like a whale, insects in the water.
As we sometimes see individuals following habits different from
TRANSITIONS OF ORGANIC BEINGS 179
those proper to their species and to the other species of the same
genus, we might expect that such individuals would occasionally
give rise to new species, having anomalous habits, and with their
structure either slightly or considerably modified from that of their
type. And such instances occur in nature. Can a more striking in-
stance of adaptation be given than that of a woodpecker for climb-
ing trees and seizing insects in the chinks of the bark? Yet in North
America there are woodpeckers which feed largely on fruit, and
others with elongated wings which chase insects on the wing. On
the plains of La Plata, where hardly a tree grows, there is a wood-
pecker (Colaptes campestris) which has two toes before and two
behind, a long pointed tongue, pointed tail-feathers, sufficiendy stiff
to support the bird in a vertical position on a post, but not so stiff
as in the typical woodpeckers, and a straight strong beak. The beak,
however, is not so straight or so strong as in the typical woodpeckers,
but it is strong enough to bore into wood. Hence this Colaptes in all
the essential parts of its structure is a woodpecker. Even in such
trifling characters as the colouring, the harsh tone of the voice, and
undulatory flight, its close blood-relationship to our common wood-
pecker is plainly declared; yet, as I can assert, not only from my
own observations, but from those of the accurate Azara, in certain
large districts it does not climb trees, and it makes its nest in holes
in banks! In certain other districts, however, this same woodpecker,
as Mr. Hudson states, frequents trees, and bores holes in the trunk
for its nest. I may mention as another illustration of the varied
habits of this genus, that a Mexican Colaptes has been described
by De Saussure as boring holes into hard wood in order to lay
up a store of acorns.
Petrels are the most aerial and oceanic of birds, but ii; the quiet
sounds of Tierra del Fuego, the Puffinuria berardi, in its general
habits, in its astonishing power of diving, in its manner of swim-
ming and of flying when made to take flight, would be mistaken
by any one for an auk or a grebe; nevertheless it is essentially a
petrel, but with many parts of its organisation profoundly modified
in relation to its new habits of life; whereas the woodpecker of La
Plata has had its structure only slightly modified. In the case of
the water-ouzel, the acutest observer, by examining its dead body,
ORIGIN OF SPECIES
i8o
would never have suspected its sub-aquatic habits; yet this bird,
which is allied to the thrush family, subsists by diving— using its
wings under water, and grasping stones with its feet. All the mem^
bers of the great order of Hymenopterous insects are terrestrial,
excepting the genus Proctotrupes, which Sir John Lubbock has
discovered to be aquatic in its habits; it often enters the water and
dives about by the use not of its legs but of its wings, and remains
as long as four hours beneath the surface; yet it exhibits no modi-
fication in structure in accordance with its abnormal habits.
He who believes that each being has been created as we now
see it, must occasionally have felt surprise when he has met with
an animal having habits and structure not in agreement. What
can be plainer than that the webbed feet of ducks and geese are
formed for swimming? Yet there are upland geese with webbed
feet which rarely go near >the water; and no one, except Audubon,
has seen the frigate-bird, which has all its four toes webbed, alight
on the surface of the ocean. On the other hand, grebes and coots are
eminently aquatic, although their toes are only bordered by mem-
brane. What seems plainer than that the long toes, not furnished
with membrane of the Grallatores, are formed for walking over
swamps and floating plants? — ^the water-hen and land-rail are mem-
bers of this order, yet the first is nearly as aquatic as the coot, and
the second is nearly as terrestrial as the quail or partridge. In such
cases, and many others could be given, habits have changed without
a corresponding change of structure. The webbed feet of the up-
land goose may be said to have become almost rudimentary in
function, though not in structure. In the frigate-bird, the deeply
scooped membrane between the toes shows that structure has begun
to change.
He who believes in separate and innumerable acts of creation
may say, that in these cases it has pleased the Creator to cause a being
of one type to take the place of one belonging to another type; but
this seems to me only re-stating the fact in dignified language. He
who believes in the struggle for existence and in the principle of
natural selection, will acknowledge 'that every organic being is
constantly endeavouring to increase in numbers; and that if any
one being varies ever so litde, either in habits or structure, and thus
ORGANS OF EXTREME PERFECTION l8l
gains an advantage over some other inhabitant of the same country,
it will seize on the place of that inhabitant, however different that
may be from its own place. Hence it will cause him no surprise that
there should be geese and frigate-birds with webbed feet, living on
the dry land and rarely alighting on the water, that there should be
long-toed corncrakes, living in meadows instead of in swamps;
that there should be woodpeckers where hardly a tree grows; that
there should be diving thrushes and diving Hymenoptera, and
petrels with the habits of auks.
ORGANS OF EXTREME PERFECTION AND COMPLICATION
To suppose that the eye with all its inimitable contrivances for
adjusting the focus to different distances, for admitting diiferent
amounts of light, and for the correction of spherical and chromatic
aberration, could have been formed by natural selection, seems, I
freely confess, absurd in the highest degree. When it was first said
that the sun stood still and the world turned round, the common
sense of mankind declared the doctrine false; but the old saying
of Vox populi, vox Dei, as every philosopher knows, cannot be
trusted in science. Reason tells me, that if numerous gradations from
a simple and imperfect eye to one complex and perfect can be shown
to exist, each grade being useful to its possessor, as is certainly the
case; if further, the eye ever varies and the variations be inherited,
as is likewise certainly the case; and if such variations should be
useful to any animal under changing conditions of life, then the
difficulty of believing that a perfect and complex eye could be
formed by natural selection, though insuperable by our imagination,
should not be considered as subversive of the theory. How a nerve
comes to be sensitive to light, hardly concerns us more than how
life itself originated; but I may remark that, as some of the lowest
organisms, in which nerves cannot be detected, are capable of per-
ceiving light, it does not seem impossible that certain sensitive
elements in their sarcode should become aggregated and developed
into nerves, endowed with this special sensibility.
In searching for the gradations through w'hich an organ in any
species has been perfected, we ought to look exclusively to its lineal
progenitors; but this is scarcely ever possible, and we are forced to
ORIGIN OF SPECIES
182
lcK)k to other species and genera of the same group, that is to the
collateral descendants from the same parent-form, in order to see
what gradations are possible, and for the chance of some gradations
having been transmitted in an unaltered or little altered condition.
But the state of the same organ in distinct classes may incidentally
throw light on the steps by which it has been perfected.
The simplest organ which can be called an eye consists of an optic
nerve, surrounded by pigment-cells and covered by translucent skin,
but without any lens or other refractive body. We may, however,
according to M. Jourdain, descend even a step lower and find aggre-
gates of pigment-cells, apparently serving as organs of vision, without
any nerves, and resting merely on sarcodic tissue. Eyes of the above
simple nature are not capable of distinct vision, and serve only to
distinguish light from darkness. In certain star-fishes, small de-
pressions in the layer of pigment which surrounds the nerve are
filled, as described by the author just quoted, with transparent
gelatinous matter, projecting with a convex surface, like the cornea
in the higher animals. He suggests that this serves not to form an
image, but only to concentrate the luminous rays and render their
perception more easy. In this concentration of the rays we gain the
first and by far the most important step towards the formation of
a true, picture-forming eye; for we have only to place the naked
extremity of the optic nerve, which in some of the lower animals
lies deeply buried in the body, and in some near the surface, at the
right distance from the concentrating apparatus, and an image will
be formed on it.
In the great class of the Articulata, we may start from an optic
nerve simply coated with pigment, the latter sometimes forming
a sort of pupil, but destitute of a lens or other optical contrivance.
With insects it is now known that the numerous facets on the cornea
of their great compound eyes form true lenses, and that the cones
include curiously modified nervous filaments. But these organs in
the Articulata are so much diversified that Muller formerly made
three main classes with seven subdivisions, besides a fourth main
class of aggregated simple eyes.
When we reflect on these facts, here given much too briefly, with
ORGANS OF EXTREME PERFECTION 1 83
respect to the wide, diversified, and graduated range o£ structure
in the eyes of the lower animals; and when we bear in mind how
small the number of all Hving forms must be in comparison with
those which have become extinct, the difficulty ceases to be very
great in believing that natural selection may have converted the
simple apparatus of an optic nerve, coated with pigment and
invested by transparent membrane, into an optical instrument as
perfect as is possessed by any member of the Articulate class.
He who will go thus far, ought not to hesitate to go one step
further, if he finds on finishing this volume that large bodies of
facts, otherwise inexplicable, can be explained by the theory of
modification through natural selection; he ought to admit that
a structure even as perfect as an eagle’s eye might thus be formed,
although in this case he does not know the transitional states. It
has been objected that in order to modify the eye and still preserve
it as a perfect instrument, many changes would have to be effected *
simultaneously, which, it is assumed, could not be done through >
natural selection; but as I have attempted to show in my work
on the variation of domestic animals, it is not necessary to suppose
that the modifications were all simultaneous, if they were extremely
slight and gradual. Different kinds of modification would, also,
serve for the same general purpose: as Mr. Wallace has remarked,
‘If a lens has too short or too long a focus, it may be amended either
by an alteration of curvature, or an alteration of density; if the
curvature be irregular, and the rays do not converge to a point,
then any increased regularity of curvature will be an improvement.
So the contraction of the iris and the muscular movements of the
eye are neither of them essential to vision, but only improvements
which might have been added and perfected at any stage of the
construction of the instrument.” Within the highest division of
the animal kingdom, namely, the Vertebrata, we can start from an
eye so simple, that it consists, as in the lancelet, of a litde sack of
transparent skin, furnished with a nerve and lined with pigment,
but destitute of any other apparatus. In fishes and reptiles, as Owen
has remarked, “the range of gradations of dioptric structures is
very great.” It is a significant fact that even in man, according to
ORIGIN OF SPECIES
184
tke high authority of Virchow, the beautiful crystalline lens is
formed in the embryo by an accumulation of epidermic cells, lying
in a sack-like fold of the skin; and the vitreous body is formed
from embryonic sub-cutaneous tissue* To arrive, however, at a just
conclusion regarding the formation of the eye, with all its mar-
vellous yet not absolutely perfect characters, it is indispensable that
the reason should conquer the imagination; but I have felt the diffi-
culty far too keenly to be surprised at others hesitating to extend the
principle of natural selection to so startling a length.
It is scarcely possible to avoid comparing the eye with a telescope.
We know that this instrument has been perfected by the long-
continued efforts of the highest human intellects; and we naturally
infer that the eye has been formed by a somewhat analogous process.
But may not this inference be presumptuous ? Have we any right
to assume that the Creator works by intellectual powers like those
of man? If we must compare the eye to an optical instrument,
we ought in imagination to take a -thick layer of transparent tissue,
with spaces filled with fluid, and with a nerve sensitive to light be-
neath, and then suppose every part of this layer to be continually
changing slowly in density, so as to separate into layers of different
densities and thicknesses, placed at different distances from each
other, and with the surfaces of each layer slowly changing in form.
Further we must suppose that there is a power, represented by
natural selection or the survival of the fittest, always intently watch-
ing each slight alteration in the transparent layers; and carefully
preserving each which, under varied circumstances, in any way
or in any degree, tends to produce a distincter image. We must
suppose each new state of the instrument to be multiplied by the
million; each to be preserved until a better one is produced, and
then the old ones to be all destroyed. In living bodies, variation will
cause the slight alterations, generation will multiply them almost
infinitely, and natural selection will pick out with unerring skill
each improvement. Let this process go on for millions of years;
and during each year on millions of individuals of many kinds;
and may we not believe that a living optical instrument might
thus be formed as superior to one of glass, as the works of the
Creator are to those of man?
MODES OF TRANSITION
185
MODES OF TRANSITION
If it could be demonstrated that any complex organ existed, which
could not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can
find out no such case. No doubt many organs exist of which we do
not know the transitional grades, more especially if we look to
much-isolated species, round which, according to the theory, there
has been much extinction. Or again, if we take an organ common
to all the members of a class, for in this latter case the organ must
have been originally formed at a remote period, since which all
the many members of the class have been developed; and in order
to discover the early transitional grades through which the organ
has passed, we should have to look to very ancient ancestral forms,
long since become extinct.
We should be extremely cautious in concluding that an organ
could not have been formed by transitional gradations of some
kind. Numerous cases could be given amongst the lower animals
of the same organ performing at the same time wholly distinct
functions; thus in the larva of the dragon-fly and in the fish Cobites
the alimentary canal respires, digests, and excretes. In the Hydra,
the animal may be turned inside out, and -the exterior surface will
then digest and the stomach respire. In such cases natural selection
might specialise, if any advantage were thus gained, the whole or
part of an organ, which had previously performed two functions,
for one function alone, and thus by insensible steps gready change
its nature. Many plants are known which regularly produce at the
same time differently constructed flowers; and if such plants were
to produce one kind alone, a great change would be effected with
comparative suddenness in -the character of the species. It is, how-
ever, probable that the two sorts of flowers borne by the same plant
were originally differentiated by finely graduated steps, which may
still be followed in some few cases.
Again, two distinct organs, or the same organ under two very
different forms, may simultaneously perform in the same individual
the same function, and this is an extremely important means of
transition: to give one instance, — ^there are fish with gills or branchiae
l86 ORIGIN OF SPECIES
that breathe the air dissolved in the water, at the same time that
they breathe free air in their swim bladders, this latter organ being
divided by highly vascular partitions and having a ductus pneumati-
cus for the supply of air. To give another instance from the vegetable
kingdom; plants climb by three distinct means, by spirally twining,
by clasping a support with their sensitive tendrils, and by the emis-
sion of aerial roodets; these three means are usually found in distinct
groups, but some few species exhibit two of the means, or even all
three, combined in the same individual. In all such cases one of the
two organs might readily be modified and perfected so as to perform
all the work, being aided during the progress of modification by the
other organ; and then this other organ might be modified for some
other and quite disdnot purpose, or be wholly obliterated.
The illustration of the swim bladder in fishes is a good one, be-
cause it shows us clearly the highly important fact that an organ
originally constructed for one purpose, namely, flotation, may be
converted into one for a widely different purpose, namely, respira-
tion. The swim bladder has, also, been worked in as an accessory to
the auditory organs of certain fishes. All physiologists admit that the
swim bladder is homologous, or “ideally similar’’ in position and
structure with the lungs of the higher vertebrate animals: hence
there is no reason to doubt that the swim bladder has actually
been converted into lungs, or an organ used exclusively for res-
piration.
According to this view it may be inferred that all vertebrate ani-
mals with true lungs are descended by ordinary generation from an
ancient and unknown prototype, which was furnished with a float-
ing apparatus or swim bladder. We can thus, as I infer from Owen’s
interesting description of these parts, understand the strange fact
that every particle of food and drink which we swallow has to pass
over the orifice of the trachea, with some risk of falling into the lungs,
notwithstanding the beautiful contrivance by which the glottis is
closed. In the higher Vertebrata the branchiae have wholly disap-
peared— but in the embryo the slits on the sides of the neck and
the loop-like course of the arteries still mark their former position.
But it is conceivable that the now utterly lost branchiae might have
been gradually worked in by natural selection for some distinct pur-
MODES OF TRANSITION 1 87
pose: for instance, Landois has shown that the wings of insects
are developed from the trachea; it is therefore highly probable that
in this great class organs which once served for respiration have
been actually converted into organs for flight.
In considering transitions of organs, it is so important to bear
in mind the probability of conversion from one function to another,
that I will give another instance. Pedunculated cirripedes have two
minute folds of skin, called by me the ovigerous frena, which serve,
through the means of a sticky secretion, to retain the eggs until they
are hatched within the sack. These cirripedes have no branchiae,
the whole surface of the body and of the sack, together with the
small frena, serving for respiration. The Balanidae or sessile cir-
ripedes, on the other hand, have no ovigerous frena, the eggs lying
loose at the bottom of the sack, within the well-enclosed shell; but
they have, in the same relative position with the frena, large, much-
folded membranes, which freely communicate with the circulatory
lacunae of the sack and body, and which have been considered by
all naturalists to act as branchiae. Now I think no one will dispute
that the ovigerous frena in the one family are strictly homologous
with the branchiae of the other family; indeed, they graduate into
each other. Therefore it need not be doubted that the two little
folds of skin, which originally served as ovigerous frena, but which,
likewise, very slightly aided in the act of respiration, have been
gradually converted by natural selection into branchiae, simply
through an increase in their size and the obliteration of their ad-
hesive glands. If all pedunculated cirripedes had become extinct,
and they have suflfered far more extinction than have sessile cirri-
pedes, who would ever have imagined that the branchiae in this
latter family had originally existed as organs for preventing the
ova from being washed out of the sack?
There is another possible mode of transition, namely, through the
acceleration or retardation of the period of reproduction. This has
lately been insisted on by Professor Cope and others in the United
States. It is now known that some animals are capable of reproduc-
tion at a very early age, before they have acquired their perfect
characters; and if this power became thoroughly well developed in
a species, it seems probable that the adult stage of development
ORIGIN OF SPECIES
l88
would sooner or later be lost; and in this case^ especially if the larva
differed much from the mature form, the character of the species
would be greatly changed and degraded. Again, not a few animals,
after arriving at maturity, go on changing in character during nearly
their whole lives. With mammals, for instance, the form of the
skull is often much altered with age, of which Dr. Murie has given
some striking instances with seals; every one knows how the
horns of stags become more and more branched, and the plumes
of some birds become more finely developed, as they grow older.
Professor Cope states that the teeth of certain lizards change much
in shape with advancing years; with crustaceans not only many
trivial, but some important parts assume a new character, as recorded
by Fritz Muller, after maturity. In all such cases,— and many could
be given,— if the age for reproduction were retarded, the character
of the species, at least in its adult state, would be modified; nor is
it improbable that the previous and earlier stages of development
would in some cases be hurried through and finally lost. Whether
species have often or ever been modified through this comparatively
sudden mode of transition, I can form no opinion; but if this has
occurred, it is probable that the differences between the young and
the mature, and between the mature and the old, were primordially
acquired by graduated steps.
SPECIAL DIFFICULTIES OF THE THEORY OF NATURAL SELECTION
Although we must be extremely cautious in concluding that any
organ could not have been produced by successive, small transitional
gradations, yet undoubtedly serious cases of difficulty occur.
One of the most serious is that of neuter insects, which are often
differently constructed from either the males or fertile females;
but this case will be treated of in the next chapter. The electric
organs of fishes offer another case of special difficulty; for it is im-
possible to conceive by what steps these wondrous organs have
been produced. But this is not surprising, for we do not even
know of what use they are. In the Gymnotus and Torpedo they no
doubt serve as powerful means of defence, and perhaps for securing
prey; yet in the ray, as observed by Matteucci, an analogous organ
in the tail manifests but litde electricity even when the animal is
DIFFICULTIES OF THE THEORY 189
gready irritated; so little, that it can hardly be of any use for the
above purposes. Moreover, in the ray, besides the organ just referred
to, there is, as Dr. R. McDonnell has shown, another organ near the
head, not known to be electrical, but which appears to be the real
homologue of the electric battery in the torpedo. It is generally
admitted that there exists between these organs and ordinary muscle
a close analogy, in intimate structure, in the distribution of the
nerves, and in the manner in which they are acted on by various
reagents. It should, also, be especially observed that muscular con-
traction is accompanied by an electrical discharge; and, as Dr. Rad-
cliffe insists, “in the electrical apparatus of the torpedo during rest,
there would seem to be a charge in every respect like that which is
met with in muscle and nerve during rest, and the discharge of the
torpedo, instead of being peculiar, may be only another form of the
discharge which attends upon the action of muscle and motor
nerve.” Beyond this we cannot at present go in the way of ex-
planation; but as we know so litde about the uses of these organs,
and as we know nothing about the habits and structure of the pro-
genitors of the existing electric fishes, it would be extremely bold to
maintain that no serviceable transitions are possible by which these
organs might have been gradually developed.
These organs appear at first to offer another and far more serious
difficulty; for they occur in about a dozen kinds of fish, of which
several are widely remote in their affinities. When the same organ
is found in several members of the same class, especially if in mem-
bers having very different habits of life, we may generally attribute
its presence to inheritance from a common ancestor; and its absence
in some of the members to loss through disuse or natural selection.
So that, if the electric organs had been inherited from some one
ancient progenitor, we might have expected that all electric fishes
would have been specially related to each other; but this is far
from the case. Nor does geology at all lead to the belief that most
fishes formerly possessed electric organs, which their modified de-
scendants have now lost. But when we look at the subject more
closely, we find in the several fishes provided with electric organs,
that those are situated in different parts of the body, — ^that they
differ in construction, as in the arrangement of the plates, and.
ORIGIN OF SPECIES
190
accordmg to Pacini, in the process or means by which the electricity
is excited— and lastly, in being supplied with nerves proceeding
from different sources, and this is perhaps the most important of
all the differences. Hence in the several fishes furnished with
electric organs, these cannot be considered as homologous, but only
as analogous in function. Consequently there is no reason to sup-
pose that they have been inherited from a common progenitor;
for had this been the case they would have closely resembled each
other in all respects. Thus the difBculty of an organ, apparently
' the same, arising in several remotely allied species, disappears, leav-
ing only the lesser yet still great difficulty; namely, by what gradu-
ated steps these organs have been developed in each separate group
of fishes.
The luminous organs which occur in a few insects, belonging to
widely different families, and which are situated in different parts
of the body, offer, under our present state of ignorance, a difficulty
almost exactly parallel with that of the electric organs. Other similar
cases could be given; for instance in plants, the very curious con-
trivance of a mass of pollen-grains, borne on a foot-stalk with an
adhesive gland, is apparently the same in Orchis and Asclepias,—
genera almost as remote as is possible amongst flowering plants;
but here again the parts are not homologous. In all cases of beings,
far removed from each other in the scale of organisation, which
are furnished with similar and peculiar organs, it will be found that
although the general appearance and function of the organs may
be the same, yet fundamental differences between them can always
be detected. For instance, the eyes of cephalopods or cuttle-fish and
of vertebrate animals appear wonderfully alike; and in such widely
sundered groups, no part of this resemblance can be due to in-
heritance from a common progenitor. Mr. Mivart has advanced
this case as one of special difficulty, but I am unable to see the force
of his argument. An organ for vision must be formed of trans-
parent tissue, and must include some sort of lens for throwing an
image at the back of a darkened chamber. Beyond this superficial
resemblance, there is hardly any real similarity between the eyes
of cuttle-fish and vertebrates, as may be seen by consulting Hensen’s
admirable memoir on these organs in the Cephalopoda. It is im-
possible for me here to enter on details, but I may specify a few
DIFFICULTIES OF THE THEORY I9I
of the points of difference. The crystalline lens in the higher cuttle-
fish consists of two parts, placed one behind the other Hke two
lenses, both having a very different structure and disposition to
what occurs in the vertebrata. The retina is wholly different, with
an actual inversion of the elemental parts, and with a large nervous
ganglion included within the membranes of the eye. The relations
of the muscles are as different as it is possible to conceive, and so in
other points. Hence it is not a little difficult to decide how far even
the same terms ought to be employed in describing the eyes of the
Cephalopoda and Vertebrata. It is, of course, open to any one to
deny that the eye in either case could have been developed through
the natural selection of successive slight variations; but if this be
admitted in the one case, it is clearly possible in the other; and
fundamental differences of structure in the visual organs of two
groups might have been anticipated, in accordance with this view
of their manner of formation. As two men have sometimes inde-
pendently hit on the same invention, so in the several foregoing
cases it appears that natural selection, working for the good of each
being, and taking advantage of all favourable variations, has pro-
duced similar organs, as far as function is concerned, in distinct
organic beings, which owe none of their structure in common to
inheritance from a common progenitor.
Fritz Miiller, in order to test the conclusions arrived at in this
volume, has followed out with much care a nearly similar line of
argument. Several families of crustaceans include a few species,
possessing an. air-breathing apparatus and fitted to live out of the
water. In two of these families, which were more especially examined
by Muller, and which are nearly related to each other, the species
agree most closely in all important characters; namely in their
sense organs, circulating system, in the position of the tufts of hair
within their complex stomachs, and lastly in the whole structure of
the water-breathing branchiae, even to the microscopical hooks by
which they are cleansed. Hence it might have been expected that in
the few species belonging to both families which live on the land,
the equally-important air-breathing apparatus would have, been the
same; for why should this one apparatus, given for the same purpose,
have been made to differ, whilst all the other important organs were
closely similar, or rather, identical.
ORIGIN OF SPECIES
192
Fritz Muller argues that this close similarity in so many points
o£ structure must, in accordance with the views advanced by me,
be accounted for by inheritance from a common progenitor. But as
that vast majority of the species in the above two families, as well as
most other crustaceans, are aquatic in their habits, it is improbable in
the highest degree that their common progenitor should have been
adapted for breathing air. Muller was thus led carefully to examine
the apparatus in the air-breathing species; and he found it to differ
in each in several important points, as in the position of the orifices,
in the manner in which they are opened and closed, and in some
accessory details. Now such differences are intelHgible, and might
even have been expected, on the supposition that species belonging
to distinct families had slowly become adapted to Hve more and
more out of water, and to breathe the air. For these species, from
belonging to distinct families, would have differed to a certain
extent, and in accordance with the principle that the nature of each
variation depends on two factors, viz., the nature of the organism
and that of the surrounding conditions, their variability assuredly
would not have been exactly the same. Consequendy natural selec-
tion would have had different materials or variations to work on,
in order to arrive at the same functional result; and the structures
thus acquired would almost necessarily have differed. On the
hypothesis of separate acts of creation the whole case remains unin-
telligible. This line of argument seems to have had great weight
in leading Fritz Miiller to accept the views maintained by me in this
volume.
Another distinguished zoologist, the late Professor ClaparMe,
has argued in the same manner, and has arrived at the same result.
He shows that there are parasitic mites (Acaridse), belonging to
distinct sub-families and families, which are furnished with hair-
claspers. These organs must have been independently developed,
as they could not have been inherited from a common progenitor;
and in the several groups they are formed by the modification of
the fore-legs, —of the hind-legs, —of the maxilk or lips,—and of
appendages on the under side of the hind part of the body.
In the foregoing cases, we see the same end gained and the same
function performed, in beings not at all or only remotely allied,
DIFFICULTIES OF THE THEORY I93
by organs in appearance, though not in development, closely similar.
On the other hand, it is a common rule throughout nature that
the same end should be gained, even sometimes in the case of closely-
related beings, by the most diversified means. How differently con-
structed is the feathered wing of a bird and the membrane-covered
wing of a bat; and still more so the four wings of a butterfly, the
two wings of a fly, and the two wings with the elytra of a beetle.
Bivalve shells are made to open and shut, but on what a number of
patterns is the hinge constructed, — ^from the long row of neatly
interlocking teeth in a Nucula to the simple ligament of a Mussel!
Seeds are disseminated by their minuteness, — ^by their capsule being
converted into a light balloon-like envelope, — ^by being embedded
in pulp or flesh, formed of the most diverse parts, and rendered
nutritious, as well as conspicuously coloured, so as to attract and
be devoured by birds, — ^by having hooks and grapnels of many kinds
and serrated awns, so as to adhere to the fur of quadrupeds, — and
by being furnished with wings and plumes, as different in shape as
they are elegant in structure, so as to be wafted by every breeze.
I will give one other instance; for this subject of the same end being
gained by the most diversified means well deserves attention. Some
authors maintain that organic beings have been formed in many
ways for the sake of mere variety, almost like toys in a shop, but
such a view of nature is incredible. With plants having separated
sexes, and with those in which, though hermaphrodites, the pollen
does not spontaneously fall on the stigma, some aid is necessary for
their fertilisation. With several kinds this is effected by the pollen-
grains, which are light and incoherent, being blown by the wind
through mere chance on to the stigma; and this is the simplest plan
which can well be conceived. An almost equally simple, though
very different, plan occurs in many plants in which a symmetrical
flower secretes a few drops of nectar, and is consequently visited by
‘‘insects; and these carry the pollen from the anthers to the stigma.
From this simple stage we may pass through an inexhaustible
number of contrivances, all for the same purpose and effected in
essentially the same manner, but entailing changes in every part of
the flower. The nectar may be stored in variously shaped receptacles,
with the stamens and pistils modified in many ways, sometimes
forming trap-like contrivances, and sometimes capable of neatly
ORIGIN OF SPECIES
194
adapted movements through irritability or elasticity. From such
structures we may advance till we come to such a case of extraor-
dinary adaptions as that lately described by Dr. Criiger in the Cory-
anthes. This orchid has part of its labellum or lower lip hollowed
out into a great bucket, into which drops of almost pure water con-
tinually fall from two secreting horns which stand above it; and
when the bucket is half full, the water overflows by a spout on one
side. The basal part of the labellum stands over the bucket, and is
itself hollowed out into a sort of chamber with two lateral en-
trances; within this chamber there are curious fleshy ridges. The
most ingenious man, if he had not witnessed what takes place, could
never have imagined what purpose all these parts serve. But Dr.
Criiger saw crowds of large humble-bees visiting the gigantic flowers
of this orchid, not in order to suck nectar, but to gnaw off the ridges
within the chamber above the bucket; in doing this they frequently
pushed each other into the bucket, and their wings being thus
wetted they could not fly away, but were compelled to crawl out
through the passage formed by the spout or overflow. Dr. Criiger
saw a “continual procession” of bees thus crawling out of their
involuntary bath. The passage is narrow, and is roofed over by the
column, so that a bee, in forcing its way out, first rubs its back against
the viscid stigma and then against the viscid glands of the pollen-
masses. The pollen-masses are thus glued to the back of the bee which
first happens to crawl out through the passage of a lately expanded
flower, and are thus carried away. Dr. Criiger sent me a flower
in spirits of wine, with a bee which he had killed before it had quite
crawled out, with a pollen-mass still fastened to its back. When the
bee, thus provided, flies to another flower, or to the same flower a
second time, and is pushed by its comrades into the bucket and then
crawls out by the passage, the pollen-mass necessarily comes first into
contact with the viscid stigma, and adheres to it, and the flower is
fertilised. Now at last we see the full use of every part of the flower,
of the water-secreting horns, of the bucket half full of water, which
prevents the bees from flying away, and forces them to crawl out
through the spout, and rub against the properly placed viscid pollen-
masses and the viscid stigma.
The construction of the flower in another closely allied orchid,
DIFFICULTIES OF THE THEORY I95
namely the Catasetum, is widely different, though serving the same
end; and is equally curious. Bees visit these flowers, like those o£
the Coryanthes, in order to gnaw the labellum; in doing this they
inevitably touch a long, tapering, sensitive projection, or, as I have
called it, the antenna. This antenna, when touched, transmits a
sensation or vibration to a certain membrane which is instantly rup-
tured; this sets free a spring by which the pollen-mass is shot forth,
like an arrow, in the right direction, and adheres by its viscid ex-
tremity to the back of the bee. The pollen-mass -of the male plant
(for the sexes are separate in this orchid) is thus carried to the
flower of the female plant, where it is brought into contact with the
stigma, which is viscid enough to break certain elastic threads, and
retain the pollen, thus effecting fertilisation.
How, it may be asked, in the foregoing and in innumerable
other instances, can we understand the graduated scale of complex-
ity and the multifarious means for gaining the same end. The
answer no doubt is, as already remarked, that when two forms vary,
which already differ from each other in some slight degree, the
variability will not be of the same exact nature, and consequendy
the results obtained through natural selection for the same general
purpose will not be the same. We should also bear in mind that
every highly developed organism has passed through many changes;
and that each modified structure tends to be inherited, so that each
modification will not readily be quite lost, but may be again and
again further altered. Hence the structure of each part of each
species, for whatever purpose it may serve, is the sum of many in-
herited changes, through which the species has passed during its
successive adaptations to changed habits and conditions of life.
Finally then, although in many cases it is most difficult even to
conjecture by what transitions organs have arrived at their present
state; yet, considering how small the proportion of living and
known forms is to the extinct and unknown, I have been astonished
how rarely an organ can be named, towards which no transitional
grade is known to lead. It certainly is true, that new organs appear-
ing as if created for some special purpose, rarely or never appear
in any being;— as indeed is shown by that old, but somewhat exag-
gerated, canon in natural history of “Natura non facit saltum.”
ORIGIN OF SPECIES
196
We meet with this admission in the writings of almost every ex-
perienced naturalist; or as Milne Edwards has well expressed it,
‘'Nature is prodigal in variety, but niggard in innovation.” Why, on
the theory of Creation, should there be so much variety and so little
real novelty? Why should all the parts and organs of many inde-
pendent beings, each supposed to have been separately created for
its proper place in nature, be so commonly linked together by gradu-
ated steps? Why should not Nature take a sudden leap from struc-
ture to structure? On the theory of natural selection, we can clearly
understand why she should not; for natural selection acts only by
taking advantage of slight successive variations; she can never
take a great and sudden leap, but must advance by short and sure,
though slow steps.
ORGANS OF LinXE APPARENT IMPORTANCE, AS AFFECTED
BY NATURAL SELECTION
As natural selection acts by life and death,— by the survival of
the fittest, and by the destruction of the less well-fitted individuals,
—I have sometimes felt great difficulty in understanding the origin
or formation of parts of little importance; almost as great, though
of a very different kind, as in the case of the most perfect and com-
plex organs.
In the first place, we are much too ignorant in regard to the whole
economy of any one organic being, to say what slight modifications
would be of importance or not. In a former chapter I have given
instances of very trifling characters, such as the down on fruit and
the colour of its flesh, the colour of the skin and hair of quadrupeds,
which, from being correlated with constitutional differences or
from determining the attacks of insects, might assuredly be acted
on by natural selection. The tail of the giraffe looks like an arti-
ficially constructed fly-flapper; and it seems at first incredible that
this could have been adapted for its present purpose by successive
slight modifications, each better and better fitted, for so trifling an
object as to drive away flies; yet we should pause before being too
positive even in this case, for we know that the distribution and ex-
istence of cattle and other animals in South America absolutely de-
pend on their power of resisting the attacks of insects: so that indi-
ORGANS AFFECTED 197
viduals which could by any means defend themselves from these
small enemies, would be able to range into new pastures and thus
gain a great advantage. It is not that the larger quadrupeds are
actually destroyed (except in some rare cases) by flies, but they are
incessantly harassed and their strength reduced, so that they are
more subject to disease, or not so well enabled in a coming dearth
to search for food, or to escape from beasts of prey.
Organs now of trifling importance have probably in some cases
been of high importance to an early progenitor, and, after having
been slowly perfected at a former period, have been transmitted to
existing species in nearly the same date, although now of very slight
use; but any actually injurious deviations in their structure would
of course have been checked by natural selection. Seeing how im-
portant an organ of locomotion the tail is in most aquatic animals,
its general presence and use for many purposes in so many land
animals, which in their lungs or modified swim bladders betray their
aquatic origin, may perhaps be thus accounted for. A well-developed
tail having been formed in an aquatic animal, it might subsequently
come to be worked in for all sorts of purposes,— as a fly-flapper, an
organ of prehension, or as an aid in turning, as in the case of the
dog, though the aid in this latter respect must be slight, for the hare,
with hardly any tail, can double still more quickly.
In the second place, we may easily err in attributing importance
to characters, and in believing that they have been developed through
natural selection. We must by no means overlook the effects of the
definite action of changed conditions of life, of so-called spontaneous
variations, which seem to depend in a quite subordinate degree on
the nature of the conditions, of the tendency to reversion to long-lost
characters, of the complex laws of growth, such as of correlation,
compensation, of the pressure of one part on another, etc., and finally
of sexual selection, by which characters of use to one sex are often
gained and then" transmitted more or less perfectly to the other sex,
though of no use to this sex. But structures thus indirectly gained,
although at first of no advantage to a species, may subsequently
have been taken advantage of by its modified descendants, under
new conditions of life and newly acquired habits.
If green woodpeckers alone had existed, and we did not know
198 ORIGIN OF SPECIES
that there were many black and pied kinds, I dare say that we should
have thought that Ae green colour was a beautiful adaptation to
conceal this tree-frequenting bird from its enemies; and conse-
quently that it was a character of importance, and had been ac-
quired through natural selection; as it is, the colour is probably in
chief part due to sexual* selection. A trailing palm in the Malay
Archipelago climbs the loftiest trees by the aid of exquisitely con-
structed hooks clustered around the ends of the branches, and this
contrivance, no doubt, is of the highest service to the plant; but as
we see nearly similar hooks on many trees which are not climbers,
and which, as there is reason to believe from the distribution of the
thorn-bearing species in Africa and South America, serve as a defence
against browsing quadrupeds, so the spikes on the palm may at
first have been developed for this object, and subsequently have been
improved and taken advantage of by the plant, as it underwent
further modification and became a climber. The naked skin on the
head of a vulture is generally considered as a direct adaptation for
wallowing in putridity; and so it may be, or it may possibly be due
to the direct action of putrid matter; but we should be very cautious
in drawing any such inference, when we see that the skin on the
head of the clean-feeding male turkey is likewise naked. The sutures
in the skulls of young mammals have been advanced as a beautiful
adaptation for aiding parturition, and no doubt they facilitate, or
may be indispensable for this act; but as sutures occur in the skulls
of young birds and reptiles, which have only to escape from a
broken egg, we may infer that this structure has arisen from the
laws of growth, and has been taken advantage of in the parturition
of the higher animals.
We are profoundly ignorant of the cause of each slight variation
or individual difference; and we are immediately made conscious
of this by reflecting on the differences between the breeds of our
domesticated animals in different countries, more especially in the
less civilised countries where there has been but little methodical
selection. Animals kept by savages in different countries often have
to struggle for their own subsistence, and are exposed to a certain
extent to natural selection, and individuals with slightly different
UTILITARIAN DOCTRINE 199
constitutions would succeed best under different climates. With
catde susceptibility to the attacks of flies is correlated with colour,
as is the liability to be poisoned by certain plants; so that even colour
would be thus subjected to the action of natural selection. Some
observers are convinced that a damp climate affects the growth of
the hair, and that with the hair the horns are correlated. Mountain
breeds always differ from lowland breeds; and a mountainous
country would probably affect the hind limbs from exercising them
more, and possibly even the form of the pelvis; and then by the law
of homologous variation, the front limbs and the head , would prob-
ably be affected. The shape, also, of the pelvis might affect by
pressure the shape of certain parts of the young in the womb. The
laborious breathing necessary in high regions tends, as we have
good reason to believe, to increase the size of the chest; and again
correlation would come into play. The effects of lessened exercise
together with abundant food on the whole organisation is probably
still more important; and this, as H. von Nathusius has lately shown
in his excellent treatise, is apparently one chief cause of the great
modification which the breed of swine have undergone. But we are
far too ignorant to speculate on the relative importance of the several
known and unknown causes of variation; and I have made these
remarks only to show that, if we are unable to account for the
characteristic differences of our several domestic breeds, which never-
theless are generally admitted to have arisen through ordinary
generation from one or a few parent-stocks, we ought not to lay
too much stress on our ignorance of the precise cause of ‘the slight
analogous differences between true species.
UTILITARIAN DOCTRINE, HOW FAR TRUE: BEAUTY, HOW
ACQUIRED
The foregoing remarks lead me to say a few words on the protest
lately made by some naturalists, against the utilitarian doctrine that
every detail of structure has been produced for the good of its
possessor. They believe that many structures have been created
for the sake of beauty, to delight man or the Creator (but this latter
point is beyond the scope of scientific discussion), or for the sake
200
ORIGIN OF SPECIES
o£ mere variety, a view already discussed. Such doctrines, if true,
would be absolutely fatal to my theory. I fully admit that many
structures are now of no direct use to their possessors, and may
never have been of any use to their progenitors; but this does not
prove that they were formed solely for beauty or variety. No doubt
the definite action of changed conditions, and the various causes of
modifications, lately specified, have all produced an effect, probably
a great effect, independently of any advantage thus gained. But a
still more important consideration is that .the chief part of the organ-
isation of every living creature is due to inheritance; and conse-
quendy, though each being assuredly is well fitted for its place in
nature, many structures have now no very close and direct relation
to present habits of life. Thus, we can hardly believe that the webbed
feet of the upland goose or of the frigate-bird are of special use to
these birds; we cannot believe that the similar bones in the arm of
the monkey, in the fore-leg of the horse, in the wing of the bat, and
in the flipper of the seal, are of special use to these animals. We may
safely attribute these structures to inheritance. But webbed feet
no doubt were as useful to the progenitor of the upland goose and of
the frigate-bird, as they now are to the most aquatic of Hving birds.
So we may believe that the progenitor of the seal did not possess
a flipper, but a foot with five toes fitted for walking or grasping; and
we may further venture to believe that the several bones in the limbs
of the monkey, horse, and bat, were originally developed, on the
principle of utility, probably through the reduction of more numer-
ous bones in the fin of some ancient fish-like progenitor of the whole
class. It is scarcely possible to decide how much allowance ought
to be made for such causes of change, as the definite action of
external conditions, so-called spontaneous variations, and the com-
plex laws of growth; but with these important exceptions, we
may conclude that the structure of every living creature either
now is, or was formerly, of some direct or indirect use to its pos-
sessor.
With respect to the belief that organic beings have been created
beautiful for the delight of man,— a belief which it has been pro-
nounced is subversive of my whole theory,— I may first remark that
the sense of beauty obviously depends on the nature of the mind,
UTILITARIAN DOCTRINE 201
irrespective of any real quality in the admired object; and that the
idea of what is beautiful, is not innate or unalterable. We see this,
for instance, in the men of different races admiring an entirely dif-
ferent standard of beauty in their women. If beautiful objects had
been created solely for man’s gratification, it ought to be shown
that before man appeared, there was less beauty on the face of the
earth than since he came on the stage. Were the beautiful volute
and cone shells of the Eocene epoch, and the gracefully sculptured
ammonites of the Secondary period, created that man might ages
afterwards admire them in his cabinet? Few objects are more beau-
tiful than the minute siliceous cases of the diatomacex: were these
created that they might be examined and admired under the higher
powers of the microscope? The beauty in this latter case, and in
many others, is apparently wholly due to symmetry of growth.
Flowers rank amongst the most beautiful productions of nature;
but they have been rendered conspicuous in contact with the green
leaves, and in consequence at the same time beautiful, so that they
may be easily observed by insects. I have come to this conclusion
from finding it an invariable rule that when a flower is fertilised
by the wind it never has a gaily-coloured corolla. Several plants
habitually produce ftwo kinds of flowers; one kind open and col-
oured so as to attract insects; the other closed, not coloured, destitute
of nectar, and never visited by insects. Hence we may conclude
that, if insects had not been developed on the face of the earth, our
plants would not have been decked with beautiful flowers, but
would have produced only such poor flowers as we see on our fir,
oak, nut and ash trees, on grasses, spinach, docks, and nettles, which
are all fertilised through the agency of the wind. A similar line of
argument holds good with fruits; that a ripe strawberry or cherry
is as pleasing to the eye as to the palate— that the gaily-coloured
fruit of the spindle-wood tree and the scarlet berries of the holly are
beautiful objects, — will be admitted by every one. But -this beauty
serves merely as a guide to birds and beasts, in order that the fruit
may be devoured and the matured seeds disseminated: I infer that
this is the case from having as yet found no exception to the rule
that seeds are always thus disseminated when embedded within a
fruit of any kind (that is within a fleshy or pulpy envelope), if it
202
ORIGIN OF SPECIES
be coloured of any brilliant tint, or rendered conspicuous by being
white or black.
On the other hand, I willingly admit that a great number of male
animals, as all our most gorgeous birds, some fishes, reptiles, and
mammals, and a host of magnificently coloured butterflies, have
been rendered beautiful for beauty’s sake; but this has been effected
through sexual selection, that is, by the more beautiful males having
been continually preferred by the females, and not for the delight of
man. So it is with the music of birds. We may infer from all this
that a nearly similar taste for beautiful colours and for musical sounds
runs through a large part of the animal kingdom. When the female
is as beautifully coloured as the male, which is not rarely the case
with birds and butterflies, the cause apparently lies in the colours
acquired through sexual selection having been transmitted to both
sexes, instead of to the males alone. How the sense of beauty in its
simplest form— that is, the reception of a peculiar kind of pleasure
from certain colours, forms, and sounds— was first developed in the
mind of man and of the lower animals, is a very obscure subject.
The same sort of difSculty is presented, if we enquire how it is
that certain flavours and odours give pleasure, and others displeasure.
Habit in all these cases appears to have come to a certain extent
into play; but there must be some fundamental cause in the consti-
tution of the nervous system in each species.
Natural selection cannot possibly produce any modification in a
species exclusively for the good of another species; though through-
out nature one species incessantly takes advantage of, and profits
by, the structures of others. But natural selection can and does often
produce structures for the direct injury of other animals, as we
see in the fang of the adder, and in the ovipositor of the ichneumon,
by which its eggs are deposited in the living bodies of other insects.
If it could be proved that any part of the structure of any one species
had been formed for the exclusive good of another species, it would
annihilate my theory, for such could not have been produced through
natural selection. Although many statements may be found in
works on natural histo^ to this effect, I cannot find even one which
seems to me of any weight. It is admitted that the rattlesnake has a
poison-fang for its own defence, and for the destruction of its prey;
UTILITARIAN DOCTRINE 203
but some authors suppose that at the same time it is. furnished with
a ratde for its own injury, namely, to warn its prey. I would almost
as soon believe that the cat curls the end of its tail when preparing
to spring, in order to warn the doomed mouse. It is a much more
probable view that the rattlesnake uses its rattle, the cobra expands
its frill, and the puff-adder swells whilst hissing so loudly and
harshly, in order to alarm the many birds and beasts which are
known to attack even the most venomous species. Snakes act on
the same principle which makes the hen ruffle her feathers and
expand her wings when a dog approaches her chickens. But I have
not space here to enlarge on the many ways by which animals en-
deavour to frighten away their enemies.
Natural selection will never produce in a being any structure
more injurious than beneficial to that being, for natural selection
acts solely by and for the good of each. No organ will be formed,
as Paley has remarked, for the purpose of causing pain or for
doing an injury to its possessor. If a fair balance be struck
between the good and evil caused by each part, each will be found
on the whole advantageous. After the lapse of time, under changing
conditions of life, if any part comes to be injurious, it will be modi-
fied; or if it be not so, the being will become extinct as myriads
have become extinct.
Natural selection tends only to make each organic being as per-
fect as, or slightly more perfect than, the other inhabitants of the
same country with which it comes into competition. And we see
that this is the standard of perfection attained under nature. The en-
demic productions of New Zealand, for instance, are perfect, one
compared with another;' but they are now rapidly yielding before
the advancing legions of plants and animals introduced from Europe.
Natural selection will not produce absolute perfection, nor do we
always meet, as far as we can judge, with this high standard under
nature. The correction for the aberration of light is said by Miiller
not to be perfect even in that most perfect organ, the human eye.
Helmholtz, whose judgment no one will dispute, after describing
in the strongest terms the wonderful power of the human eye, adds
these remarkable words: “That which we have discovered in the
way of inexactness and imperfection in the optical machine and in
ORIGIN OF SPECIES
204
the image on the retina, is as nothing in comparison with the incon-
gruities which we have just come across in the domain of the sen-
sations, One might say that nature has taken delight in accumulat-
ing contradictions in order to remove all foundations from the theory
of a pre-existing harmony between the external and internal worlds.”
If our reason leads us to admire with enthusiasm a multitude of in-
imitable contrivances in nature, this same reason tells us, though we
may easily err on both sides, that some other contrivances are less
perfect. Can we consider the sting of the bee as perfect, which,
when used against many kinds of enemies, cannot be withdrawn,
owing to the backward serratures, and thus inevitably causes the
death of the insect by tearing out its viscera?
If we look at the sting of the bee, as having existed in a remote
progenitor, as a boring and serrated instrument, like that in so
many members of the same great order, and that it has since been
modified, but not perfected for its present purpose, with the poison
originally adapted for some other object, such as to produce galls,
since intensified, we can perhaps understand how it is that the use
of >the sting should so often cause the insect’s own death: for if on
the whole the power of stinging be useful to the social community,
it will fulfil all the requirements of natural selection, though it may
cause the death of some few members. If we admire the truly
wonderful power of scent by which the males of many insects find
their females, can we admire the production for this single purpose
of thousands of drones, which are utterly useless to the community
for any other purpose, and which are ultimately slaughtered by
their industrious and sterile sisters? It may be difficult, but we ought
to admire the savage instinctive hatred of the queen-bee, which urges
her to destroy the young queens, her daughters, as soon as they are
born, or to perish herself in the combat; for undoubtedly this is for
the good of the community; and maternal love or maternal hatred,
though the latter fortunately is most rare, is all the same to the in-
exorable principles of natural selection. If we admire the several
ingenious contrivances by which orchids and many other plants are
fertilised through insect agency, can we consider as equally perfect
the elaboration of dense clouds of pollen by our fir-trees, so that a
few granules may be wafted by chance on to the ovules?
SUMMARY
205
summary: the law of unity of type and of the conditions of
EXISTENCE EMBRACED BY THE THEORY OF NATURAL SELECTION
We have in this chapter discussed some of the difficulties and ob-
jections which may be urged against the theory. Many of them are
serious; but I think that in the discussion light has been thrown on
several facts, which on the belief of independent acts of creation
are utterly obscure. We have seen that species at any one period are
not indefinitely variable, and are not linked together by a multitude
of intermediate gradations, partly because the process of natural
selection is always very slow, and at any one time acts only on a
few forms; and partly because the very process of natural selection
implies the continual supplanting and extinction of preceding and
intermediate gradations. Closely allied species, now living on a con-
tinuous area, must often have been formed when the area was not
continuous, and when the conditions of life did not insensibly grad-
uate away from one part to another. When two varieties are foriried
in two districts of a continuous area, an intermediate variety will
often be formed, fitted for an intermediate zone; but from reasons
assigned, the intermediate variety will usually exist in lesser num-
bers than the two forms which it connects; consequently the two
latter, during the course of further modification, from existing in
greater numbers, will have a great advantage over the less numerous
intermediate variety, and will thus generally succeed in supplanting
and exterminating it.
We have seen in this chapter how cautious we should be in con-
cluding that the most different habits of life could not graduate into
each other; that a bat, for instance, could not have been formed by
natural selection from an animal which at first only glided through
the air.
We have seen that a species under new conditions of life may
change its habits; or it may have diversified habits, with some very
unlike those of its nearest congeners. Hence we can understand,
bearing in mind that each organic being is trying to live wherever
it can live, how it has arisen that there are upland geese with webbed
feet, ground woodpeckers, diving thrushes, and petrels with the
habits of auks.
ORIGIN OF SPECIES
206
Although the belief that an organ so perfect as the eye could have
been formed by natural selection, is enough to stagger any one; yet
in the case of any organ, if we know of a long series of gradations in
complexity, each good for its possessor, then, under changing condi-
tions of life, there is no logical impossibility in the acquirement of
any conceivable degree of perfection through natural selection. In
the cases in which we know of no intermediate or transitional states,
we should be extremely cautious in concluding that none can have
existed, for the metamorphoses of many organs show what wonder-
ful changes in function are at least possible. For instance, a swim
bladder has apparently been converted into an air-breathing lung.
The same organ having performed simultaneously very different
functions, and then having been in part or in whole specialised for
one function; and two distinct organs having performed at the
same time the same function, the one having been perfected whilst
aided by the other, must often have largely facilitated transitions.
We have seen that in two beings widely remote from each other
in the natural scale, organs semng for the same purpose and in
external appearance closely similar may have been separately and
independently formed; but when such organs are closely examined,
essential differences in their structure can almost always be detected;
and this naturally follows from the principle of natural selection.
On the other hand, the common rule throughout nature is infinite
diversity of structure for gaining the same end; and this again
naturally follows from the same -great principle.
In many cases we are far too ignorant to be enabled to assert that
a part or organ is so unimportant for the welfare of a species, that
modifications in its structure could not have been slowly accumu-
lated by means of natural selection. In many other cases, modifica-
tions are probably the direct result of the laws of variation or of
growth, independently of any good having been thus gained. But
even such structures have often, as we may feel assured, been sub-
sequently taken advantage of, and still further modified, for the
good of species under new conditions of life. We may, also, believe
that a part formerly of high importance has frequently been retained
(as the tail of an aquatic animal by its terrestrial descendants),
SUMMARY
207
though it has become o£ such small importance that it could not,
in its present state, have been acquired by means of natural selec-
tion.
Natural selection can produce nothing in one species for the ex-
clusive good or injury of another; though it may well produce
parts, organs, and excretions highly useful or even indispensable,'
or again highly injurious to another species, but in all cases at the
same time useful to the possessor. In each well-stocked country
natural selection acts through the competition of the inhabitants,
and consequently leads to success in the battle for life, only in ac-
cordance with the standard of that particular country. Hence the
inhabitants of one country, generally the smaller one, often yield to
the inhabitants of another and generally the larger country. For in
the larger country there will have existed more individuals and more
diversified forms, and the competition will have been severer, and
thus the standard of perfection will have been rendered higher.
Natural selection will not necessarily lead to absolute perfection;
nor, as far as we can judge by our limited faculties, can absolute
perfection be everywhere predicated.
On the theory of natural selection we can clearly understand the
full meaning of that old canon in natural history, “Natura non facit
saltum.” This canon, if we look to the present inhabitants alone of
the world, is not strictly correct; but if we include all those of past
times, whether known or unknown, it must on this theory be strictly
true.
It is generally acknowledged that all organic beings have been
formed on two great laws— -Unity of Type, and the Conditions of
Existence. By unity of type is meant that fundamental agreement in
structure which we see in organic beings of the same class, and which
is quite independent of their habits of life. On my theory, unity of
type is explained by unity of descent. The expression of conditions
of existence, so often insisted on by the illustrious Cuvier, is fully
embraced by the principle of natural selection. For natural selection
acts by either now adapting the varying parts of each being to its
organic and inorganic conditions of life; or by having adapted them
during past periods of time: the adaptations being aided in many
ORIGIN OF SPECIES
208
cases by the increased use or disuse of parts, being affected by the
direct action of the external conditions of life, and subjected in all
cases to the several laws of growth and variation. Hence, in fact, the
law of the Conditions of Existence is the higher law; as it includes,
through the inheritance of former variations and adaptations, that
of Unity of Type.
CHAPTER VII
Miscellaneous Objections to the Theory of Natural Selection
Longevity — ^Modifications not necessarily simultaneous — Modifications
apparently of no direct service — ^Progressive development — Charac-
ters o£ small functional importance, the most constant — Supposed
incompetence of natural selection to account for the incipient stages
of useful structures — Causes which interfere with the acquisition
through natural selection of useful structures — Gradations of struc-
ture with changed functions — ^Widely different organs in members
of the same class, developed from one and the same source — Reasons
for disbelieving in great and abrupt modifications.
I WILL devote this chapter to the consideration of various mis-
cellaneous objections which have been advanced against my
views, as some of the previous discussions may thus be made
clearer; but it would be useless to discuss all of them, as many have
been made by writers who have not taken the trouble to understand
the subject. Thus a distinguished German naturalist has asserted
that the weakest part of my theory is, that I consider ail organic
beings as imperfect; what I have really said is, that all are not as
perfect as they might have been in relation to their conditions; and
this is shown to be the case by so many native forms in many
quarters of the world having yielded their places to intruding
foreigners. Nor can organic beings, even if they were at any one
time perfectly adapted to their conditions of life, have remained so,
when their conditions changed, unless they themselves likewise
changed; and no one will dispute that the physical conditions of
each country, as well as the numbers and kinds of its inhabitants
have undergone many mutations.
A critic has lately insisted, with some parade of mathematical
accuracy, that longevity is a great advantage to all species, so that
he who believes in natural selection “must arrange his genealogical
tree” in such a manner that all the descendants have longer lives than
their progenitors! Cannot our critic conceive that a biennial plant
or one of the lower animals might range into a cold climate and
209
210
ORIGIN OF SPECIES
perish there every winter; and yet, owing to advantages gained
through natural selection, survive from year to year, by means of its
seeds or ova? Mr. E. Ray Lankester has recently discussed this
subject, and he concludes, as far as its extreme complexity allows him
to form a judgment, that longevity is generally related to the stand-
ard of each species in the scale of organisation, as well as to the
amount of expenditure in reproduction and in general activity. And
these conditions have, it is probable, been largely determined through
natural selection.
It has been argued that, as none of the animals and plants of
Egypt, of which we know anything, have changed during the last
three or four thousand years, so probably have none in any part of
the world. But, as Mr. G. H. Lewes has remarked, this line of
argument proves too much, for the ancient domestic races figured on
the Egyptian monuments, or embalmed, are closely similar or even
identical with those now living; yet all naturalists admit that such
races have been produced through the modification of their original
types. The many animals which have remained unchanged since
the commencement of the glacial period, would have been an
incomparably stronger case, for these have been exposed to great
changes of climate and have migrated over great distances; whereas,
in Egypt, during the last several thousand years, the conditions of
life, as far as we know, have remained absolutely uniform. The fact
of little or no modification having been effected since the glacial
period would have been of some avail against those who believe in
an innate and necessary law of development, but is powerless against
the doctrine of natural selection or the survival of the fittest, which
implies that when variations or individual differences of a beneficial
nature happen to arise, these will be preserved; but this will be
effected only under certain favourable circumstances.
The celebrated pakontologist, Bronn, at the close of his German
translation of this work, asks, how, on the principle of natural
selection, can a variety live side by side with the parent species? If
both have become fitted for slighdy different habits of life or con-
ditions, they might live together; and if we lay on one side poly-
morphic species, in which the variability seems to be of a peculiar
nature, and all mere temporary variations, such as size, albinism,
THEORY OF NATURAL SELECTION
211
etc., the more permanent varieties are generally found, as far as I
can discover, inhabiting distinct stations, — such as high land or low
land, dry or moist districts. Moreover, in the case of animals which
wander much about and cross freely, their varieties seem to be
generally confined to distinct regions.
Bronn also insists that distinct species never differ from each other
in single characters, but in many parts; and he asks, how it always
comes that many parts of the organisation should have been modified
at the same time through variation and natural selection ? But there
is no necessity for supposing that all the parts of any being have
been simultaneously modified. The most striking modifications,
excellently adapted for some purpose, might, as was formerly
remarked, be acquired by successive variations, if slight, first in one
part and then in another; and as they would be transmitted all
together, they would appear to us as if they had been simultaneously
developed. The best answer, however, to the above objection is
afforded by those domestic races which have been modified, chiefly
through man’s power of selection, for some special purpose. Look at
the race and dray horse, or at the grey-hound and mastiff. Their
whole frames and even their mental characteristics have been modi-
fied; but if we could trace each step in the history of their trans-
formation, — and the latter steps can be traced, — ^we should not see
great and simultaneous changes, but first one part and then another
slightly modified and improved. Even when selection has been
applied by man to some one character alone, — of which our culti-
vated plants offer the best instances,— it will invariably be found that
although this one part, whether it be the flower, fruit, or leaves, has
been greatly changed, almost all the other parts have been slightly
modified. This may be attributed partly to the principle of correlated
growth, and partly to so-called spontaneous variation.
A much more serious objection has been urged by Bronn, and
recently by Broca, namely, that many characters appear to be of no
service whatever to their possessors, and therefore cannot have been
influenced through natural selection. Bronn adduces the length of
the ears and tails in the different species of hares and mice, — ^the com-
plex folds of enamel in the teeth of many animals, and a multitude
of analogous cases. With respect to plants, this subject has been
ORIGIN OF SPECIES
212
discussed by Nageli in an admirable essay. He admits that natural
selection has effected much, but he insists that the families of plants
differ chiefly from each other in morphological characters, which
appear to be quite unimportant for the welfare of the species. He
consequently believes in an innate tendency towards progressive and
more perfect development. He specifies the arrangement of the cells
in the tissues, and of the leaves on the axis, as cases in which natural
selection could not have acted. To these may be added the numerical
divisions in the parts of the flower, the position of the ovules, the
shape of the seed, when not of any use for dissemination, etc.
There is much force in the above objection. Nevertheless, we
ought, in the first place, to be extremely cautious in pretending to
decide what structures now are, or have formerly been, of use to
each species. In the second place, it should always be borne in mind
that when one part is modified, so vwll be other parts, through certain
dimly seen causes, such as an increased or diminished flow of nutri-
ment to a part, mutual pressure, an early developed part affecting one
subsequently developed, and so forth,— as well as through other
causes which lead to the many mysterious cases of correlation, which
we do not in the least understand. These agencies may be all grouped
together, for the sake of brevity, under the expression of the laws of
grovTth. In the third place, we have to allow for the direct and
definite action of changed conditions of life, and for so-called spon-
taneous variations, in which the nature of the conditions apparently
plays a quite subordinate part. Bud variations, such as the appearance
of a moss-rose on a common rose, or of a nectarine on a peach-tree,
offer good instances of spontaneous variations; but even in these
cases, if we bear in mind the power of a minute drop of poison in
producing complex galls, we ought not to feel too sure that the
above variations are not the effect of some local change in the nature
of the sap, due to some change in the conditions. There must be
some efficient cause for each slight individual difference, as well as
for more strongly marked variations which occasionally arise; and
if the unknown cause were to act persistently, it is almost certain
that all the individuals of the species would be similarly modified.
In the earlier editions of this work I under-rated, as it now seems
probable, the frequency and importance of modifications due to
THEORY OF NATURAL SELECTION 213
spontaneous variability. But it is impossible to attribute to this cause
the innumerable structures which are so well adapted to the habits
of life of each species. I can no more believe in this, than that the
well-adapted form of a race-horse or greyhound, which before the
principle of selection by man was well understood, excited so much
surprise in the minds of the older naturalists, can thus be ex-
plained.
It may be worth while to illustrate some of the foregoing remarks.
With respect to the assumed inutility of various parts and organs, it
is hardly necessary to observe that even in the higher and best-known
animals many structures exist, which are so highly developed that
no one doubts that they are of importance, yet their use has not
been, or has only recendy been, ascertained. As Bronn gives the
length of the ears and tail in the several species of mice as instances,
though trifling ones, of differences in structure which can be of no
special use, I may mention that, according to Dr. SchobI, the external
ears of the common mouse are supplied in an extraordinary manner
with nerves, so that they no doubt serve as tactile organs; hence the
length of the ears can hardly be quite unimportant. We shall, also,
presently see that the tail is a highly useful prehensile organ to
some of the species; and its use would be much influenced by its
length.
With respect to plants, to which on account of Nageli’s essay I
shall confine myself in the following remarks, it will be admitted
that the flowers of orchids present a multitude of curious structures,
which a few years ago would have been considered as mere morpho-
logical differences without any special function; but they are now
known to be of the highest importance for the fertilisation of the
species through the aid of insects, and have probably been gained
through natural selection. No one until lately would have imagined
that in dimorphic and trimorphic plants the different lengths of the
stamens and pistils, and their arrangement, could have been of any
service, but now we know this to be the case.
In certain whole groups of plants the ovules stand erect, and in
others they are suspended; and within the same ovarium of some
few plants, one ovule holds the former and a second ovule the latter
position. These positions seem at first purely morphological, or of
ORIGIN OF SPECIES
214
no physiological signification; but Dr. Hooker informs me that
within the same ovarium, the upper ovules alone in some cases, and
in other cases the lower ones alone are fertilised; and he suggests
that this probably depends on the direction in which the pollen-tubes
enter the ovarium. If so, the position of the ovules, even when one
is erect and the other suspended within the same ovarium, would
follow from the selection of any slight deviations in position which
favoured their fertilisation, and the production of seed.
Several plants belonging to distinct orders habitually produce
flowers of two kinds,— the one open of the ordinary structure, the
other closed and imperfect. These two kinds of flowers sometimes
differ wonderfully in structure, yet may be seen to graduate into
each other on the same plant. The ordinary and open flowers can
be intercrossed; and the benefits which certainly are derived from
this process are thus secured. The closed and imperfect flowers are,
however, manifestly of high importance, as they yield with the
utmost safety a large stock of seed, with the expenditure of wonder-
fully little pollen. The two kinds of flowers often differ much, as
just stated, in structure. The petals in the imperfect flowers almost
always consist of mere rudiments, and the pollen-grains are reduced
in diameter. In Ononis columnae five of the alternate stamens are
rudimentary; and in some species of Viola three stamens are in this
state, two retaining their proper function, but being of very small
size. In six out of thirty of the closed flowers in an Indian violet
(name unknown, for the plants have never produced with me per-
fect flowers), the sepals are reduced from the normal number of
five to three. In one section of the Malpighiaceae the closed flowers,
according to A. de Jussieu, are still further modified, for the five
stamens which stand opposite to the sepals are all aborted, a sixth
stamen standing opposite to a petal being alone developed; and this
stamen is not present in the ordinary flowers of these species; the
style is aborted; and the ovaria are reduced from three to two. Now
although natural selection may well have had the power to prevent
some of the flowers from expanding, and to reduce the amount of
pollen, when rendered by the closure of the flowers superfluous, yet
hardly any of the above special modifications can have been thus
determined, but must have followed from the laws of growth,
THEORY OF NATURAL SELECTION 215
including the functional inactivity of parts, during the progress of
the reduction of the pollen and the closure of the flowers.
It is so necessary to appreciate the important effects of the laws
of growth, that I will give some additional cases of another kind,
namely of differences in the same part or organ, due to differences
in relative position on the same plant. In the Spanish chestnut, and
in certain fir-trees, the angles of divergence of the leaves differ,
according to Schacht, in the nearly horizontal and in the upright
branches. In the common rue and some other plants, one flower,
usually the central or terminal one, opens first, and has five sepals
and petals, and five divisions to the ovarium; whilst all the other
flowers on the plant are tetramerous. In the British Adoxa the upper-
most flower generally has two calyx-lobes with the other organs
tetramerous, whilst the surrounding flowers generally have three
calyx-lobes with the other organs pentamerous. In many Com-
positse and Umbelliferse (and in some other plants) the circum-
ferential flowers have their corollas much more developed than those
of the centre; and this seems often connected with the abortion of
the reproductive organs. It is a more curious fact, previously referred
to, that the achenes or seeds of the circumference and centre some-
times differ greatly in form, colour, and other characters. In Cartha-
mus and some other Compositse the central achenes alone are fur-
nished with a pappus; and in Hyoseris the same head yields achenes
of three different forms. In certain Umbelliferae the exterior seeds,
according to Tausch, are orthospermous, and the central one coelo-
spermous, and this is a character which was considered by De
Candolle to be in other species of the highest systematic importance.
Prof. Braun mentions a Fumariaceous genus in which the flowers
in the lower part of the spike bear oval, ribbed, one-seeded nudets;
and in the upper part of the spike, lanceolate, two-valved, and two-
seeded siliques. In these several cases, with the exception of that of
the well developed ray-florets, which are of service in making the
flowers conspicuous to insects, natural selection cannot, as far as we
can judge, have come into play, or only in- a quite subordinate
manner. All these modifications follow from the relative position
and inter-action of the parts; and it can hardly be doubted that if
all the flowers and leaves on the same plant had been subjected to
2i6 origin of species
the same external and internal condition, as are the flowers and
leaves in certain positions, all would have been modified in the
same manner.
In numerous other cases we find modifications of structure, which
are considered by botanists to be generally of a highly important
nature, affecting only some of the flowers on the same plant, or
occurring on distinct plants, which grow close together under the
same conditions. As these variations seem of no special use to the
plants, they cannot have been influenced by natural selection. Of
their cause we are quite ignorant; we cannot even attribute them,
as in the last class of cases, to any proximate agency, such as relative
position. I will give only a few instances. It is so common to observe
on the same plant, flowers indifferently tetramerous, pentamerous,
etc., that I need not give examples; but as numerical variations are
comparatively rare when the parts are few, I may mention that,
according to De Candolle, the flowers of Papaver bracteatum offer
either two sepals with four petals (which is the common type with
poppies), or three sepals with six petals. The manner in which the
petals are folded in the bud is, in most groups, a very constant
morphological character; but Professor Asa Gray states that with
some species of Mimulus, the aestivation is almost as frequently that
of the Rhinanthideae as of the Antirrhinideae, to which latter tribe
the genus belongs. Aug. St. Hilaire gives the following cases: the
genus Zanthoxylon belongs to a division of the Rutaceae with a single
ovary, but in some species flowers may be found on the same plant,
and even in the same panicle, with either one or two ovaries. In
Helianthemum the capsule has been described as unilocular or 3-
locular; and in H. mutabile, “Une lame, plus ou moins large,
s’etend entre le pericarpe et le placenta.” In the flowers of Saponaria
officinalis, Dr. Masters has observed instances of both marginal and
free central placentation. Lasdy, St. Hilaire found towards the
southern extreme of the range of Gomphia oleaeformis two forms
which he did not at first doubt were distinct species, but he subse-
quently saw them growing on the same bush; and he then adds,
“Voila done dans un meme individu des loges et un style qui se
rattachent tantot a un axe verticale et tantot a un gynobase.”
We thus see that with plants many morphological changes may be
THEORY OF NATURAL SELECTION 21 7
attributed to the laws o£ growth and the inter-action of parts, inde-
pendently of natural selection. But with respect to Nageli’s doctrine
of an innate tendency towards perfection or progressive develop-
ment, can it be said in the case of these strongly pronounced varia-
tions, that the plants have been caught in the act of progressing
towards a higher state of development? On the contrary, I should
infer from the mere fact of the parts in question differing or varying
greatly on the same plant, that such modifications were of extremely
small importance to the plants themselves, of whatever importance
they may generally be to us for our classifications. The acquisition
of a useless part can hardly be said to raise an organism in the natural
scale; and in the case of the imperfect, closed flowers above described,
if any new principle has to be invoked, it must be one of retrogres-
sion rather than of progression; and so it must be with many
parasitic and degraded animals. We are ignorant of the exciting
cause of the above specified modifications; but if the unknown cause
were to act almost uniformly for a length of time, we may infer
that the result would be almost uniform; and in this case all
the individuals of the species would be modified in the same
manner.
From the fact of the above characters being unimportant for the
welfare of the species, any slight variations which occurred in them
would not have been accumulated and augmented through natural
selection. A structure which has been developed through long-
continued selection, when it ceases to be of service to a species,
generally becomes variable, as we see with rudimentary organs; for
it will no longer be regulated by this same power of selection. But
when, from the nature of the organism and of the conditions, modi-
fications have been induced which are unimportant for the welfare
of the species, they may be, and apparently often have been, trans-
mitted in nearly the same state to numerous, otherwise modified,
descendants. It cannot have been of much importance to the greater
number of mammals, birds, or reptiles, whether they were clothed
with hair, feathers, or scales; yet. hair has been .transmitted to almost
all mammals, feathers to all birds, and scales to all true reptiles. A
structure, whatever it may be, which is common to many allied
forms, is ranked by us as of high systematic importance, and con-
2i8 origin of species
sequently is often assumed to be of high vital importance to the
species. Thus, as I am inclined to believe, morphological differences,
which we consider as important — such as the arrangement of the
leaves, the divisions of the flower or of the ovarium, the position of
the ovules, etc. — ^first appeared in many cases as fluctuating varia-
tions, which sooner or later became constant through the nature of
the organism and of the surrounding conditions, as well as through
the intercrossing of distinct individuals, but not through natural
selection; for as these morphological characters do not affect the
welfare of the species, any slight deviations in them could not have
been governed or accumulated through this latter agency. It is a
strange result which we thus arrive at, namely that characters of
slight vital importance to the species, are the most important to the
systematist; but, as we shall hereafter see when we treat of the genetic
principle of classification, this is by no means so paradoxical as it
may at first appear.
Although we have no good evidence of the existence in organic
beings of an innate tendency towards progressive development, yet
this necessarily follows, as I have attempted to show in the fourth
chapter, through the continued action of natural selection. For the
best definition which has ever been given of a high standard of
organisation is the degree to which the parts have been specialised
or differentiated; and natural selection tends towards this end, inas-
much as the parts are thus enabled to perform their functions more
efficiently.
A distinguished zoologist, Mr. St. George Mivart, has recently
collected all the objections which have ever been advanced by myself
and others against the theory of natural selection, as propounded by
Mr. Wallace and myself, and has illustrated them with admirable
art and force. When thus marshalled, they make a formidable array;
and as it forms no part of Mr. Mivart’s plan to give the various facts
and considerations opposed to his conclusions, no slight effort of
reason and memory is left to the reader, who may wish to weigh the
evidence on both sides. When discussing special cases, Mr. Mivart
passes over the effects of the increased use and disuse of parts, which
I have always maintained to be highly important, and have treated
THEORY OF NATURAL SELECTION 21 ^
in my ‘Variation under Domestication’ at greater length than, as I
believe, any other writer. He likewise often assumes that I attribute
nothing to variation, independently of natural selection, whereas in
the work just referred to I have collected a greater number of well-
established cases than can be found in any other work known to me.
My judgment may not be trustworthy, but after reading with care
Mr. Mivart’s book, and comparing each section with what I have
said on the same head, I never before felt so strongly convinced of
the general truth of the conclusions here arrived at, subject, of course,
in so intricate a subject, to much partial error.
All Mr. Mivart’s objections will be, or have been, considered in
the present volume. The one new point which appears to have
struck many readers is, “that natural selection is incompetent to
account for the incipient stages of useful structures.” This subject
is intimately connected with that of the gradation of characters, often
accompanied by a change of function, — ^for instance, the conversion
of a swim bladder into lungs,— points which were discussed in the
last chapter under two headings. Nevertheless, I will here consider
in some detail several of the cases advanced by Mr. Mivart, selecting
those which are the most illustrative, as want of space prevents me
from considering all.
The giraffe, by its lofty stature, much elongated neck, fore legs,
head and tongue, has its whole frame beautifully adapted for brows-
ing on the higher branches of trees. It can thus obtain food beyond
the reach of the other Ungulata or hoofed animals inhabiting the
same country; and this must be a great advantage to it during
dearths. The Niata cattle in South America show us how small a
difference in structure may make, during such periods, a great dif-
ference in preserving an animal’s life. These cattle can browse as
well as others on grass, but from the projection of the lower jaw
they cannot, during the often recurrent droughts, browse on the
twigs of trees, reeds, etc., to which food the common cattle and
horses are then driven; so that at these times the Niatas perish, if not
fed by their owners. Before coming to Mr. Mivart’s objections, it
may be well to explain once again how natural selection will act in
all ordinary cases. Man has modified some of his animals, without
necessarily having attended to special points of structure, by simply
220
ORIGIN OF SPECIES
preserving and breeding from the fleetest individuals, as with the
race-horse and greyhound, or as with the game-cock, by breeding
from the victorious birds* So under nature with the nascent giraffe,
the individuals which were the highest browsers and were able dur-
ing dearths to reach even an inch or two above the others, will often
have been preserved; for they will have roamed over the whole
country in search of food. That the individuals of the same species
often differ slighdy in the relative lengths of all their parts may be
seen in many works of natural history, in which careful measure-
ments are given. These slight proportional differences, due to the
laws of growth and variation, are not of the slightest use or impor-
tance to most «pecies. But it will have been otherwise with the
nascent giraffe, considering its probable habits of life; for those indi-
viduals which had some one part or several parts of their bodies
rather more elongated than usual, would generally have survived.
These will have intercrossed and left offspring, either inheriting the
same bodily peculiarities, or with a tendency to vary again in the
same manner; whilst the individuals, less favoured in the same
respects, will have been the most liable to perish.
We here see that there is no need to separate single pairs, as man
does, when he methodically improves a breed; natural selection will
preserve and thus separate all the superior individuals, allowing them
freely to intercross, and will destroy all the inferior individuals. By
this process long-continued, which exactly corresponds with what
I have called unconscious selection by man, combined no doubt in
a most important manner with the inherited effects of the increased
use of parts, it seems to me almost certain that an ordinary hoofed
quadruped might be converted into a giraffe.
To this conclusion Mr. Mivart brings forward two objections.
One is that the increased size of the body would obviously require
an increased supply of food, and he considers it as “very problemati-
cal whether the disadvantages thence arising would not, in times of
scarcity, more than counterbalance the advantages.” But as the
giraffe does actually exist in large numbers in South Africa, and as
some of the largest antelopes in the world, taller than an ox, abound
there, why should we doubt that, as far as size is concerned, inter-
mediate gradations could formerly have existed there, subjected as
221
THEORY OF NATURAL SELECTION
now to severe dearths? Assuredly the being able to reach, at each
stage of increased size, to a supply of food, left untouched by the
other hoofed quadrupeds of the country, would have been of some
advantage to the nascent giraffe. Nor must we overlook the fact,
that increased bulk would act as a protection against almost all
beasts of prey excepting the lion; and against this animal, its tall
neck, — and the taller the better, — ^would, as Mr. Chauncey Wright
has remarked, serve as a watch-tower. It is from this cause, as Sir
S. Baker remarks, that no animal is more difficult to stalk than the
giraffe. This animal also uses its long neck as a means of offence or
defence, by violently swinging its head armed with stump-like horns.
The preservation of each species can rarely be determined by any
one advantage but by the union of all, great and small.
Mr. Mivart then asks (and this is his second objection), if natural
selection be so potent, and if high browsing be so great an advantage,
why has not any other hoofed quadruped acquired a long neck and
lofty stature, besides the giraffe, and, in lesser degree, the camel,
guanaco, and macrauchenia? Or, again, why has not any member
of the group acquired a long proboscis? With respect to South
Africa, which was formerly inhabited by numerous herds of the
giraffe, the answer is not difficult, and can best be given by an illus-
tration. In every meadow in England in which trees grow, we see
the lower branches trimmed or planed to an exact level by the brows-
ing of the horses or cattle; and what advantage would it be, for
instance, to sheep, if kept there, to acquire slightly longer necks? In
every district some one kind of animal will almost certainly be able
to browse higher than the others; and it is almost equally certain that
this one kind alone could have its neck elongated for this purpose,
through natural selection and the effects of increased use. In South
Africa the competition for browsing on the higher branches of the
acacias and other trees must be between giraffe and giraffe, and not
with the other ungulate animals.
Why, in other quarters of the world, various animals belonging
to this same order have not acquired either an elongated neck or a
proboscis, cannot be distinctly answered; but it is as unreasonable to
expect a distinct answer to such a question, as why some event in
the history of mankind did not occur in one country, whilst it did in
222
ORIGIN OF SPECIES
another. We are ignorant with respect to the conditions which
determine the numbers and range of each species; and we cannot
even conjecture what changes of structure would be favourable to
its increase in some new country. We can, however, see in a general
manner that various causes might have interfered with the develop-
ment of a long neck or proboscis. To reach the foliage of a con-
siderable height (without climbing, for which hoofed animals are
singularly ill-constructed) implies greatly increased bulk of body;
and we know that some areas support singularly few large quadru-
peds, for instance South America, though it is so luxuriant; whilst
South Africa abounds with them to art unparalleled degree. Why
this should be so, we do not know; nor why the later tertiary periods
should have been much more favourable for their existence than the
present time. Whatever the causes may have been, we can see that
certain districts and times would have been much more favourable
than others for the development of so large a quadruped as the
giraffe.
In order that an animal should acquire some structure specially
and largely developed, it is almost indispensable that several other
parts should be modified and coadapted. Although every part of
the body varies slightly, it does not follow that the necessary parts
should always vary in the right direction and to the right degree.
With the different species of our domesticated animals we know
that the parts vary in a different manner and degree; and that some
species are much more variable than others. Even if the fitting vari-
ations did arise, it does not follow that natural selection would be
able to acfon them, and produce a structure which apparently would
be beneficial to the species. For instance, if the number of indi-
viduals existing in a country is determined chiefly through destruc-
tion by beasts, of prey,-— by external or internal parasites, etc.,— as
seems often to be the case, then natural selection will be able to do
little, or will be greatly retarded, in modifying any particular struc-
ture for obtaining food. Lasdy, natural selection is a slow process,
and the same favourable conditions must long endure in order that
any marked, effect should thus be produced. Except by assigning
such general and vague reasons, we cannot explain why, in many
quarters of the world, hoofed quadrupeds have not acquired much
THEORY OF NATURAL SELECTION 223
elongated necks or other means for browsing on the higher branches
of trees.
Objections of the same nature as the foregoing have been advanced
by many writers. In each case various causes, besides the general
ones just indicated, have probably interfered with the acquisition
through natural selection of structures, which it is thought would be
beneficial to certain species. One writer asks, why has not the ostrich
acquired the power of flight.? But a moment’s reflection will show
what an enormous supply of food would be necessary to give to this
bird of the desert force to move its huge body through the air.
Oceanic islands are inhabited by bats and seals, but by no terrestrial
mammals; yet as some of these bats are peculiar species, they must
have long inhabited their present homes. Therefore Sir C. Lyell
asks, and assigns certain reasons in answer, why have not seals and
bats given birth on such islands to forms fitted to live on the land?
But seals would necessarily be first converted into terrestrial car-
nivorous animals of considerable size, and bats into terrestrial insec-
tivorous animals; for the former there would be no prey; for the
bats ground insects would serve as food, but these would already be
largely preyed on by the reptiles or birds, which first colonise and
abound on most oceanic islands. Gradations of structure, with each
stage beneficial to a changing species, will be favoured only under
certain peculiar conditions. A stricdy terrestrial animal, by occasion-
ally hunting for food in shallow water, then in streams or lakes,
might at last be converted into an animal so thoroughly aquatic as
to brave the open ocean. But seals would not find on oceanic islands
the conditions favourable to their gradual reconversion into a terres-
trial form. Bats, as formerly shown, probably acquired their wings
by at first gliding through the air from tree to tree, like the so-called
flying squirrels, for the sake of escaping from their enemies, or for
avoiding falls; but when the power of true flight had once been
acquired, it would never be reconverted back, at least for the above
purposes, into the less efficient power of gliding through the air.
Bats might, indeed, like many birds, have had their wings gready
reduced in size, or completely lost, through disuse; but in this case
it would be necessary that they should first have acquired the power
of running quickly on the ground, by the aid of their hind legs alone,
ORIGIN OF SPECIES
224
so as to compete with birds or other ground animals; and for such
a change a bat seems singularly ill-fitted. These conjectural remarks
have been made merely to show that a transition of structure, with
each step beneficial, is a highly complex affair; and that there is
nothing strange in a transition not having occurred in any particular
case.
Lastly, more than one writer has asked, why have some animals
had their mental powers more highly developed than others, as such
development would be advantageous to all? Why have not apes
acquired the intellectual powers of man? Various causes could be
assigned; but as they are conjectural, and their relative probability
cannot be weighed, it would be useless , to give them. A definite
answer to the latter question ought not to be expected, seeing that
no one can solve the simpler problem why, of two races of savages,
one has risen higher in the scale of civilisation than the other; and
this apparendy implies increased brain-power.
We will return to Mr. Mivards other objections. Insects often
resemble for the sake of protection various objects, such as green or
decayed leaves, dead twigs, bits of lichen, flowers, spines, excrement
of birds, and living insects; but to this latter point I shall hereafter
recur. The resemblance is often wonderfully close, and is not con-
fined to colour, but extends to form, and even to the manner in
which the insects hold themselves. The caterpillars which project
motionless like dead twigs from the bushes on which they feed,
offer an excellent instance of a resemblance of this kind. The cases
of the imitation of such objects as the excrement of birds, are rare
and exceptional On this head, Mr. Mivart remarks, “As, according
to Mr. Darwin’s theory, there is a constant tendency to indefinite
variation, and as the minute incipient variations will be in all direc-
tions, they must tend to neutralise each other, and at first to form
such unstable modifications that it is difflcult, if not impossible, to
see how such indefinite oscillations of infinitesimal beginnings can
ever build up a sufficiently appreciable resemblance to a leaf, bamboo,
or other object, for Natural Selection to seize upon and perpetuate,”
But in all the foregoing cases the insects in their original state no
doubt presented some rude and accidental resemblance to an object
commonly found in the stations frequented by them. Nor is this at
THEORY OF NATURAL SELECTION 225
all improbable, considering the almost infinite number of surround-
ing objects and the diversity in form and colour of the hosts of
insects which exist. As some rude resemblance is necessary for the
first start, we can understand how it is that the larger and higher
animals do not (with the exception, as far as I know, of one fish)
resemble for the sake of protection special objects, but only the
surface which commonly surrounds them, and this chiefly in colour.
Assuming that an insect originally happened to resemble in some
degree a dead twig or a decayed leaf, and that it varied slightly in
many ways, then all the variations which rendered the insect at all
more like any such object, and thus favoured its escape, would be
preserved, whilst other variations would be neglected and ultimately
lost; or, if they rendered the insect at all less like the imitated object,
they would be eliminated. There would indeed be force in Mr,
Mivart’s objection, if we were to attempt to account for the above
resemblances, independently of natural selection, through mere
fluctuating variability; but as the case stands there is none.
Nor can I see any force in Mr. Mivart’s difficulty with respect to
“the last touches of perfection in the mimicry;” as in the case given
by Mr. Wallace, of a walking stick insect (Ceroxylus laceratus),
which resembles “a stick grown over by a creeping moss or junger-
mannia.” So close was this resemblance, that a native Dyak main-
tained that the foliaceous excrescences were really moss. Insects are
preyed on by birds and other enemies, whose sight is probably
sharper than ours, and every grade in resemblance which aided an
insect to escape notice or detection, would tend towards its preserva-
tion; and the more perfect the resemblance so much the better for the
insect. Considering the nature of the differences between the species
in the group which includes the above Ceroxylus, there is nothing
improbable in this insect having varied in the irregularities on its
surface, and in these having become more or less green-coloured;
for in every group the characters which differ in the several species
are the most apt to vary, whilst the generic characters, or those
common to all the species, are the most constant.
The Greenland whale is one of the most wonderful animals in the
world, and the baleen, or whalebone, one of its greatest peculiarities.
ORIGIN OF SPECIES
226
The baleen consists of a row, on each side, of the upper jaw, of about
300 plates or laminae, which stand close together transversely to the
longer axis of the mouth. Within the main row there are some
subsidiary rows. The extremities and inner margins of all the plates
are frayed into stiff brisdes, which clothe the whole gigantic palate,
and serve to strain or sift the water, and thus to secure the minute
prey on which these great animals subsist. The middle and longest
lamina in the Greenland whale is ten, twelve, or even fifteen feet
in length; but in the different species of Cetaceans there are grada-
tions in length; the middle lamina being in one species, according
to Scoresby, four feet, in another three, in another eighteen inches,
and in the Bal^noptera rostrata only about nine inches in length.
The quality of the whalebone also differs in the different species.
With respect to the baleen, Mr. Mivart remarks that if it “had once
attained such a size and development as to be at all useful, then its
preservation and augmentation within serviceable limits would be
promoted by natural selection alone. But how to obtain the begin-
ning of such useful development?” In answer, it may be asked, why
should not the early progenitors of the whales with baleen have
possessed a mouth constructed something like the lamellated beak
of a duck? Ducks, like whales, subsist by sifting the mud and
water; and the family has sometimes been called Criblatores, or
sifters. I hope that I may not be misconstrued into saying that the
progenitors of whales did actually possess mouths lamellated like the
beak of a duck. I wish only to show that this is not incredible, and
that the immense plates of baleen in the Greenland whale might
have been developed from such lamellse by finely graduated steps,
each of service to its possessor.
The beak of a shoveller duck (Spatula clypeata) is a more beauti-
ful and complex structure than the mouth of a whale. The upper
mandible is furnished on each side (in the specimen examined by
me) with a row or comb formed of 188 thin, elastic lamelk,
obliquely bevelled so as to be pointed, and placed transversely to the
longer axis of the mouth. They arise from the palate, and are
attached by flexible membrane to the sides of the mandible. Those
standing towards the middle are the longest, being about one-third
of an inch in length, and they project .14 of an inch beneath the
THEORY OF NATURAL SELECTION 227
edge. At their bases there is a short subsidiary row of obliquely
transverse lamellae. In these several respects they resemble the plates
of baleen in the mouth of a whale. But towards the extremity of the
beak they differ much, as they project inwards, instead of straight
downwards. The entire head of the shoveller, though incomparably
less bulky, is about one-eighteenth of the length of the head of a
moderately large Balaenoptera rostrata, in which species the baleen is
only nine inches long; so that if we were to make the head of the
shoveller as long as that of the Balanoptera, the lamellae would be
six inches in length, — ^that is, two-thirds of the length of the baleen
in this species of whale. The lower mandible of the shoveller-duck
is furnished with lamellae of equal length with those above, but
finer; and in being thus furnished it differs conspicuously from the
lower jaw of a whale, which is destitute of baleen. On the other
hand, the extremities of these lower lamellae are frayed into fine
bristly points, so that they thus curiously resemble the plates of
baleen. In the genus Prion, a member of the distinct family of the
Petrels, the upper mandible alone is furnished with lamellae, which
are well developed and project beneath the margin; so that the beak
of this bird resembles in this respect the mouth of a whale.
From the highly developed structure of the shoveller’s beak we
may proceed (as I have learnt from information and specimens sent
to me by Mr. Salvin), without any great break, as far as fitness for
sifting is concerned, through the beak of the Merganetta armata, and
in some respects through that of the Aix sponsa, to the beak of the
common duck. In this latter species, the lamellae are much coarser
than in the shoveller, and are firmly attached to the sides of the
mandible; they are only about fifty in number on each side, and
do not project at all beneath the margin. They are square-topped,
and are edged with translucent hardish tissue, as if for crushing food.
The edges of the lower mandible are crossed by numerous fine ridges,
which project very little. Although the beak is thus very inferior as
a sifter to that of the shoveller, yet this bird, as every one knows,
constantly uses it for this purpose. There are other species, as I hear
from Mr. Salvin, in which the lamellae are considerably less developed
than in the common duck; but I do not know whether they use
their beaks for sifting the water.
ORIGIN OF SPECIES
228
Turning to another group of the same family. In the Egyptian
goose (Chenalopex) the beak closely resembles that of the common
duck; but the lamellae are not so numerous, nor so distinct from each
other, nor do they project so much inwards; yet this goose, as I am
informed by Mr. E. Bartlett, “uses its bill like a duck by throwing
the waters out at the corners.” Its chief food, however, is grass,
which it crops like the common goose. In this latter bird, the lamellae
of the upper mandible are much coarser than in the common duck,
almost confluent, about twenty-seven in number on each side, and
terminating upwards in teeth-Hke knobs. The palate is also covered
with hard rounded knobs. The edges of the lower mandible are
serrated with teeth much more prominent, coarser, and sharper than
in the duck. The common goose does not sift the water, but uses
its beak exclusively for tearing or cutting herbage, for which pur-
pose it is so well fitted, that it can crop grass closer than almost any
other animal. There are other species of geese, as I hear from Mr.
Bardett, in which the lamellse are less developed than in the common
goose.
We thus see that a member of the duck family, with a beak con-
structed like that of the common goose and adapted solely for
grazing, or even a member with a beak having less well-developed
lamellae, might be converted by small changes into a species like the
Egyptian goose, —this into one like the common duck,— and, lastly,
into one like the shoveller, provided with a beak almost exclusively
adapted for sifting the water; for this bird could hardly use any part
of its beak, except the hooked tip, for seizing or tearing solid food.
The beak of a goose, as I may add, might also be converted by small
changes into one provided with prominent, recurved teeth, like those
of the Merganser (a member of the same family), serving for the
widely diilerent purpose of securing live fish.
Returning to the whales. The Hyperoodon bidens is destitute of
true teeth in an efficient condition, but its palate is roughened,
according to LacepMe, with small, unequal, hard points of horn.
There is, therefore, nothing improbable in supposing that some
early cetacean form was provided with similar points of horn on the
palate, but rather more regularly placed, and which, like the knobs
on the beak of the goose, aided it in seizing or tearing its food. If so,
THEORY OF NATURAL SELECTION 229
it will hardly be denied that the points might have been converted
through variation and natural selection into lamellae as well-
developed as those of the Egyptian goose, in which case they would
have been used both for seizing objects and for sifting the water;
then into lamella like those of the domestic duck; and so onwards,
until they became as well constructed as those of the shoveller, in
which case they would have served exclusively as a sifting apparatus.
From this stage, in which the lamellae would be two-thirds of the
length of the plates of baleen in the Balaenoptera rostrata, gradations,
which may be observed in still-existing Cetaceans, lead us onwards
to the enormous plates of baleen in the Greenland whale. Nor is
there the least reason to doubt that each step in this scale might
have been as serviceable to certain ancient Cetaceans, with the func-
tions of the parts slowly changing during the progress of develop-
ment, as are the gradations in the beaks of the different existing
members of the duck family. We should bear in mind that each
species of duck is subjected to a severe struggle for existence, and
that the structure of every part of its frame must be well adapted
to its conditions of life.
The Pleuronectidse, or flatfish, are remarkable for their asymmetri-
cal bodies. They rest on one side,— -in the greater number of species
on the left, but in some on the right side; and occasionally reversed
adult specimens occur. The lower, or resting-surface, resembles at
first sight the ventral surface of an ordinary fish: it is of a white
color, less developed in many ways than the upper side, with the
lateral fins often of smaller size. But the eyes offer the most remark-
able peculiarity; for they are both placed on the upper side of the
head. During early youth, however, they stand opposite to each
other, and the whole body is then symmetrical, with both sides
equally coloured. Soon the eye proper to the lower side begins to
glide slowly round the head to the upper side; but does not pass
right through the skull, as was formerly thought to be the case. It
is obvious that unless the lower eye did thus travel round, it could
not be used by the fish whilst lying in its habitual position on one
side. The lower eye would, also, have been liable to be abraded by
the sandy bottom. That the Pleuronectid^ are admirably adapted
by their flattened and asymmetrical structure for their habits of life.
ORIGIN OF SPECIES
230
is manifest from several species, such as soles, flounders, etc., being
extremely common. The chief advantages thus gained seem to be
protection from their enemies, and facility for feeding on the ground.
The different members, however, of the family present, as Schiodte
remarks, “a long series of forms exhibiting a gradual transition from
Hippoglossus pinguis, which does not in any considerable degree
alter the shape in which it leaves the ovum, to the soles, which are
entirely thrown to one side.”
Mr. Mivart has taken up this case, and remarks that a sudden
spontaneous transformation in the position of the eyes is hardly con-
ceivable, in which I quite agree with him. He then adds: “If the
transit was gradual, then how such transit of one eye a minute
fraction of the journey towards the other side of the head could
benefit the individual is, indeed, far from clear. It seems, even, that
such an incipient transformation must rather have been injurious.”
But he might have found an answer to this objection in the excel-
lent observations published in 1867 by Malm. The Pleuronectidae,
whilst very young and still symmetrical, with their eyes standing on
opposite sides of the head, cannot long retain a vertical position,
owing to the excessive depth of their bodies, the small size of their
lateral fins, and to their being destitute of a swim bladder. Hence
soon growing tired, they fall to the bottom on one side. Whilst thus
at rest they often twist, as Malm observed, the lower eye upwards,
to see above them; and they do this so vigorously that the eye is
pressed hard against the upper part of the orbit. The forehead
between the eyes consequently becomes, as could be plainly seen,
temporarily contracted in breadth. On one occasion Malm saw a
young fish raise and depress the lower eye through an angular dis-
tance of about seventy degrees.
We should remember that the skull at this early age is car-
tilaginous and flexible, so that it readily yields to muscular action.
It is also known with the higher animals, even after early youth,
that the skull yields and is altered in shape, if the skin or muscles
be permanently contracted through disease or some accident. With
long-eared rabbits, if one ear lops forwards and downwards, its
weight drags forward all the bones of the skull on the same side,
of which I have given a figure. Malm states that the newly hatched
THEORY OF NATURAL SELECTION 23 1
young o£ perches, salmon, and several other symmetrical fishes^ have
the habit of occasionally resting on one side at the bottom; and he
has observed that they often then strain their lower eyes so as to look
upwards; and their skulls are thus rendered rather crooked. These
fishes, however, are soon able to hold themselves in a vertical posi-
tion, and no permanent effect is thus produced. With the Pleuro-
nectidx, on the other hand, the older they grow the more habitually
they rest on one side, owing to the increasing flatness of their bodies,
and a permanent effect is thus produced on the form of the head,
and on the position of the eyes. Judging from analogy, the tendency
to distortion would no doubt be increased through the principle of
inheritance. Schiodte believes, in opposition to some other natural-
ists, that the Pleuronectidse are not quite symmetrical even in the
embryo; and if this be so, we could understand how it is that certain
species, whilst young, habitually fall over and rest on the left side,
and other species on the right side. Malm adds, in confirmation of
the above view, that the adult Trachypterus arcticus, which is not a
member of the Pleuronectidae, rests on its left side at the bottom, and
swims diagonally through the water; and in this fish, the two sides
of the head are said to be somewhat dissimilar. Our great authority
on Fishes, Dr. Gunther, concludes his abstract of Malm’s paper, by
remarking that “the author gives a very simple explanation of the
abnormal condition of the. Pleuronectoids.”
We thus see that the first stages of the transit of the eye from one
side of the head to the other, which Mr. Mivart considers would be
injurious, may be attributed to the habit, no doubt beneficial to the
individual and to the species, of endeavouring to look upwards with
both eyes, whilst resting on one side at the bottom. We may also
attribute to the inherited effects of use the fact of the mouth in sev-
eral kinds of flat-fish being bent towards the lower surface, with the
jaw bones stronger and more effective on this, the eyeless side of the
head, than on the other, for the sake, as Dr. Traquair supposes, of
feeding with ease on the ground. Disuse, on the other hand, will
account for the less developed condition of the whole inferior half of
the body, including the lateral fins; though Yarrell thinks that the
reduced size of these fins is advantageous to the fi,sh, as “there is so
much less room for their action, than with the larger fins above.”
232 ORIGIN OF SPECIES
Perhaps the lesser number of teeth in the proportion of four to seven
in the upper halves of the two jaws of the plaice, to twenty-five to
thirty in bhe lower halves, may likewise be accounted for by disuse.
From the colourless state of the ventral surface of most fishes and of
many other aninials, we may reasonably suppose that the, absence of
colour in flat-fish on the side, whether it be the right or leftj which is
undermost, is due to the exclusion of light. But it cannot be supposed
that the peculiar speckled appearance of the upper side of the sole, so
like the sandy bed of the sea, or the power in some species, as recently
shown by Pouchet, of changing their colour in accordance with the
surrounding surface, or the presence of bony tubercles on the upper
side of the turbot, are due to the action of the light. Here natural
selection has probably come into play, as well as in adapting the gen-
eral shape of the body of these fishes, and many other peculiarities, to
their habits of life. We should keep in mind, as I have before insisted,
that the inherited effects of the increased use of parts, and perhaps of
their disuse, will be strengthened by natural selection. For all spon-
taneous variations in the right direction will thus be preserved; as
will those individuals^ which inherit in the highest degree the effects
of the increased and beneficial use of any part. How much to attrib-
ute in each particular case to the effects of use, and how much to
natural selection, it seems impossible to decide.
I may give another instance of a structure which apparendy owes
its origin exclusively to use or habit. The extremity of the tail in
some American monkeys has been converted into a wonderfully
perfect prehensile organ, and serves as a fifth hand. A reviewer who
agrees with Mr, Mivart in every detail, remarks on this structure:
“It is impossible to believe that in any number of ages the first slight
incipient tendency to grasp could preserve the lives of the individuals
possessing it, or favour their chance of having and of rearing off-
spring.” But there is no necessity for any such belief. Habit, and
this almost implies that some benefit great or small is thus derived,
would in all probability ^ufficeior the’ work. Brehm saw the young
of an African monkey’ (Cercdpithecus) clinging to the under sur-
face of their mother by their hands, and at the same time they
hooked their little tails round that of their mother. Professor Hens-
low kept in confinement some harvest mice (Mus messorius) which
THEORY OF NATURAL SELECTION 233
do not possess a structurally prehensile tail; but he frequently
observed that they curled their tails round the branches of a bush
placed in the cage, and thus aided themselves in cUmbing. I have
received an analogous account from Dr. Gunther, who has seen a
mouse thus suspend itself. If the harvest mouse had been more
strictly arboreal, it would perhaps have had its tail rendered struc-
turally prehensile, as is the case with some members of the same
order. Why Cercopithecus, considering its habits whilst young, has
not become thus provided, it would be difficult to say. It is, however,
possible that the long tail of this monkey may be of more service to
it as a balancing organ in making its prodigious leaps, than as a
prehensile organ.
The mammary glands are common to the whole class of mammals,
and are indispensable for their existence; they must, therefore, have
been developed at an extremely remote period, and we can know
nothing positively about their manner of development. Mr. Mivart
asks: “Is it conceivable that the young of any animal was ever saved
from destruction by accidentally sucking a drop of scarcely nutritious
fluid from an accidentally hypertrophied cutaneous gland of its
mother? And even if one was so, what chance was there of the
perpetuation of such a variation?” But the case is not here put fairly.
It is admitted by most evolutionists that mammals are descended
from a marsupial form; and if so, the mammary glands will have
been at first developed within the marsupial sack. In the case of the
fish (Hippicampus) the eggs are hatched, and the young are reared
for a time, within a sack of this nature; and an American naturalist,
Mr, Lockwood, believes from what he has seen of the development
of the young, that they are nourished by a secretion from the cuta-
neous glands of the sack. Now with the early progenitors of
mammals, almost before they deserved to be thus designated, is it
not at least possible that the young might have been similarly
nourished? And in this case, the individuals which secreted a fluid,
in some degree or manner the most nutritious, so as to partake of
the nature of milk, would in the long run have reared a larger num-
ber of well-nourished offspring, than would the individuals which
secreted a poorer fluid; and thus the cutaneous glands, which are the
234 ORIGIN OF SPECIES
homologues of the mammary glands, would have been improved or
rendered more effective. It accords with the widely extended prin-
ciple of specialisation, that the glands over a certain space of the
sack should have become more highly developed than the remainder;
and they would then have formed a breast, but at first without a
nipple, as we see in the Ornithorhyncus, at the base of the mam-
malian series. Through what agency the glands over a certain space
became more highly specialised than the others, I will not pretend to
decide, whether in part through compensation of growth, the effects
of use, or of natural selection.
The development of the mammary glands would have been of no
service, and could not have been effected through natural selection,
unless the young at the same time were able to partake of the secre-
tion. There is no greater difficulty in understanding how young
mammals have instinctively learnt to suck the breast, than in under-
standing how unhatched chickens have learnt to break the egg-shell
by tapping against it with their specially adapted beaks; or how a
few hours after leaving the shell they have learnt to pick up grains
of food. In such cases the most probable solution seems to be, that
the habit was at first acquired by practice at a more advanced age,
and afterwards transmitted to the offspring at an earlier age. But the
young kangaroo is said not to suck, only to cling to the nipple of its
mother, who has the power of injecting milk into the mouth of her
helpless, half-formed offspring. On this head Mr. Mivart remarks:
“Did no special provision exist, the young one must infallibly be
choked by the intrusion of the milk into the windpipe. But there is
a special provision. The larynx is so elongated that it rises up into
^ the posterior end of the nasal passage, and is thus enabled to give
free entrance to the air for the lungs, while the milk passes harm-
lessly on each side of this elongated larynx, and so safely attains the
gullet behind it.” Mr. Mivart then asks how did natural selection
remove in the adult kangaroo (and in most other mammals, on the
assumption that they are descended from a marsupial form), “this
at least perfecdy innocent and harmless structure?” It may be sug-
gested in answer that the voice, which is certainly of high impor-
tance to many animals, could hardly have been used with full force
as long as the larynx entered the nasal passage; and Professor Flower
THEORY OF NATURAL SELECTION 235
has suggested to me that this structure would have greatly interfered
with an animal swallowing solid food.
We will now turn for a short space to the lower divisions of the
animal kingdom. The Echinodermata (starfishes, sea urchins, etc.)
are furnished with remarkable organs, called pedicellariae, which
consist, when well developed, of a tridactyle forceps— that is, of one
formed of three serrated arms, neatly fitting together and placed on
the summit of a flexible stem, moved by muscles. These forceps can
seize firmly hold of any object; and Alexander Agassiz has seen an
Echinus or sea urchin rapidly passing particles of excrement from
forceps to forceps down certain lines of its body, in order that its
shell should not be fouled. But there is no doubt that besides remov-
ing dirt of all kinds, they subserve other functions; and one of these
apparently is defence.
With respect to these organs, Mr. Mivart, as on so many previous
occasions, asks: “What would be the utility of the first rudimentary
beginnings of such structures, and how could such incipient bud-
dings have ever preserved the life of a single Echinus?” He adds,
“Not even the sudden development of the snapping action could
have been beneficial without the freely moveable stalk, nor could the
latter have been efficient without the snapping jaws, yet no minute
merely indefinite variations could simultaneously evolve these com-
plex co-ordinations of structure; to deny this seems to do no less than
to affirm a starding paradox.” Paradoxical as this may appear to
Mr. Mivart, tridactyle forcepses, immovably fixed at the base, but
capable of a snapping acdon, certainly exist on some starfishes; and
this is intelligible if they serve, at least in part, as a means of defence.
Mr. Agassiz, to whose great kindness I am indebted for much
information on the subject, informs me that there are other star-
fishes, in which one of the three arms of the forceps is reduced to a
support for the other two; and again, other genera in which the
third arm is completely lost. In Echinoneus, the shell is described
by M. Perrier as bearing two kinds of pedicellariae, one resembling
those of Echinus, and the other those of Spatangus; and such cases
are always interesting as affording the means of apparendy sud-
den transitions, through the abortion of one of the two states of an
organ.
236 ORIGIN OF SPECIES
With respect to the steps by which these curious organs have been
evolved, Mr. Agassiz infers from his own researches and those of
Miiller, that both in starfishes and sea urchins the pedicellariae must
undoubtedly be looked at as modified spines. This may be inferred
from their manner of development in the individual, as well as from
a long and perfect series of gradations in different species and genera,
from simple granules to ordinary spines, to perfect tridactyle pedi-
cellarias. The gradation extends even to the manner in which
ordinary spines and the pedicellarise with their supporting calcareous
rods are articulated to the shell. In certain genera of starfishes, “the
very combinations needed to show that the pedicellari^ are only
modified branching spines” may be found. Thus we have fixed
spines, with three equi-distant, serrated, moveable branches, articu-
lated to near their bases; and higher up, on the same spine, three
other moveable branches. Now when the latter arise from the
summit of a spine they form in fact a rude tridactyle pedicellaria,
and such may be seen on the same spine together with the three
lower branches. In this case the identity in nature between the arms
of the pedicellarise and the moveable branches of a spine, is unmis-
takable. It is generally admitted that the ordinary spines serve as a
protection; and if so, there can be no reason to doubt that those
furnished with serrated and moveable branches likewise serve for
the same purpose; and they would thus serve still more effectively
as soon as by meeting together they acted as a prehensile or snapping
apparatus. Thus every gradation, from an ordinary fixed spine to a
fixed pedicellaria, would be of service.
In certain genera of starfishes, these organs, instead of being fixed
or borne on an immovable support, are placed on the summit of a
flexible and muscular, though short, stem; and in this case they
probably subserve some additional function besides defence. In the
-sea urchins the steps can be followed by which a fixed spine becomes
articulated to the shell, and is thus rendered moveable. I wish I had
space here to give a fuller abstract of Mr. Agassiz’s interesting ob-
servations on the development of the pedicellarise. All possible
gradations, as he adds, may likewise be found between the pedicel-
larise of the starfishes and the hooks of the ophiurans, another group
of the Echinodermata; and again between the pedicellarise of sea
THEORY OF NATURAL SELECTION 237
urchins and the anchors of the Holothuriae, also belonging to the
same great class.
Certain- compound - animals, or zoophytes, as they have been
termed, namely the Polyzoa, are provided with curious organs called
avicularia. These differ much in structure in the different species.
In their most perfect condition, they curiously resemble the head
and beak of a vulture in miniature, seated on a neck and capable of
movement, as is likewise the lower jaw or mandible. In one species
observed by me all the avicularia on the same branch often moved
simultaneously backwards and forwards, with the lower jaw widely
open, through an angle of about 90°, in the course of five seconds;
and their movement caused the whole polyzoary to tremble. When
the jaws are touched with a needle they seize it so firmly that the
branch can thus be shaken.
Mr. Mivart adduces this case, chiefly on account of the supposed
difficulty of organs, namely the avicularia of the Polyzoa and the
pedicellariae of the Echinodermata, which he considers as ‘'essentially
similar,” having been developed through natural selection in widely
distinct divisions of the animal kingdom. But, as far as structure is
concerned, I can see no similarity between tridactyle pedicellariae and
avicularia. The latter resemble somewhat more closely the chelae or
pincers of crustaceans; and Mr. Mivart might have adduced with
equal appropriateness this resemblance as a special difficulty; or even
their resemblance to the head and beak of a bird. The avicularia
are believed by Mr. Busk, Dr. Smitt, and Dr, Nitsche— naturalists
who have carefully studied this group — to be homologous with the
zooids and their cells which compose the zoophyte; the moveable
lip or lid of the cell corresponding with the lower and moveable
mandible of the avicularium. Mr. Busk, however, does not know of
any gradations now existing between a zooid and an avicularium.
It is therefore impossible to conjecture by what serviceable gradations
the one could have been converted into the other: but it by no means
follows from this that such gradations have not existed.
As the chelae of crustaceans resemble in some degree the avicularia
of Polyzoa, both serving as pincers, it may be worth while to show
that with the former a long series of serviceable- gradations still
238 ORIGIN OF SPECIES
exists* In the first and simplest stage, the terminal segment o£ a
limb shut down either on the square summit of the broad penulti-
mate segment or against one whole side; and is thus enabled to catch
hold of an object; but the limb still serves as an organ of locomotion.
We next find one corner of the broad penultimate segment slightly
prominent, sometimes furnished with irregular teeth; and against
these the terminal segment shuts down. By an increase in the size
of this projection, with its shape, .as well as that of the terminal seg-
ment, slightly modified and improved, the pincers are rendered more
and more perfect until we have at last an instrument as efficient
as the chek of a lobster; and all these gradations can be actually
traced.
Besides the avicularia, the Polyzoa possess curious organs called
vibracula. These generally consist of long bristles, capable of move-
ment and easily excited. In one species examined by me the vibracula
were slighdy curved and serrated along the outer margin; and all of
them on the same polyzoary often moved simultaneously; so that,
acting like long oars, they swept a branch rapidly across the object
glass of my microscope. When a branch was placed on its face, the
vibracula became entangled, and they made violent efforts to free
themselves. They are supposed to serve as a defence, and may be
seen, as Mr. Busk remarks, “to sweep slowly and carefully over the
surface of the polyzoary, removing what might be noxious to the
delicate inhabitants of the cells when their tentacula are protruded.”
The avicularia, like the vibracula, probably serve for defence, but
they also catch and kill small living animals, which it is believed are
afterwards swept by the currents within reach of the tentacula of the
zooids. Some species are provided with avicularia and vibracula;
some with avicularia alone, and a few with vibracula alone.
It is not easy to imagine two objects more widely different in
appearance than a bristle or vibraculum, and an avicularium like
the head of a bird; yet they are almost certainly homologous and
have been developed from the same common source, namely a zooid
with its cell. Hence we can understand how it is that these organs
graduate in some cases, as I am informed by Mr. Busk, into each
other. Thus with the avicularia of several species of Lepralia, the
moveable mandible is so much produced and is so like a bristle,
THEORY OF NATURAL SELECTION 239
that the presence of the upper or fixed beak alone serves to determine
its avicularian nature. The vibracula may have been directly de-
veloped from the lips of the cells, without having passed through
the avicularian stage; but it seems more probable that they have
passed through this stage, as during the early stages of the trans-
formation, the other parts of the cell with the included zooid could
hardly have disappeared at once. In many cases the vibracula have
a grooved support at the base, which seems to represent the fixed
beak; though this support in some species is quite absent. This
view of the development of the vibracula, if trustworthy, is interest-
ing; for supposing that all the species provided with avicularia had
become extinct, no one with the most vivid imagination would ever
have thought that the vibracula had originally existed as part of an
organ, resembling a bird’s head or an irregular box or hood. It is
interesting to see two such widely different organs developed from
a common origin; and as the moveable lip of the cell serves as a
protection to the zooid, there is no difficulty in believing that all the
gradations, by which the lip became converted first into the lower
mandible of an avicularium and then into an elongated bristle, like-
wise served as a protection in different ways and under different
circumstances.
In the vegetable kingdom Mr. Mivart only alludes to two cases,
namely the structure of the flowers of orchids, and the movements
of climbing plants. With respect to the former, he says, “The ex-
planation of their origin is deemed thoroughly unsatisfactory— ut-
terly insuificient to explain the incipient, infinitesimal beginnings
of structures which are of utility only when they are considerably
developed.” As I have fully treated this subject in another work, I
will here give only a few details on one alone of the most striking
peculiarities of the flowers of orchids, namely their pollinia. A pol-
linium when highly developed consists of a mass of pollen-grains,
affixed to an elastic foot-stalk or caudicle, and this to a little mass of
extremely viscid matter. The pollinia are by this means transported
by insects from one flower to the stigma of another. In some orchids
there is no caudicle to the pollen-masses, and the grains are merely
tied together by fine threads; but as these are not confined to orchids.
240 ORIGIN OF SPECIES
they need not here be considered; yet I may mention that at the
base o£ the orchidaceous series, in Cypripedium, we can see how the
threads were probably first developed In other orchids the threads
cohere at one end of the pollen-masses; and this forms the first or
nascent trace of a caudicle. That this is the origin of the caudicle,
even when of considerable length and highly developed, we have
good evidence in the aborted pollen-grains which can sometimes be
detected embedded within the central and solid parts.
With respect to the second chief peculiarity, namely, the little
mass of viscid matter attached to the end of the caudicle, a long
series of gradations can be specified, each of plain service to the
plant. In most flowers belonging to other orders the stigma secretes
a litde viscid matter. Now in certain orchids similar viscid matter
is secreted, but in much larger quantities, by one alone of the three
stigmas; and this stigma, perhaps in consequence of the copious
secretion, is rendered sterile. When an insect visits a flower of this
kind, it rubs off some of the viscid matter and thus at the same time
drags away some of the pollen-grains. From this simple condition,
which differs but little from that of a multitude of common flowers,
there are endless gradations,— to species in which the pollen-mass
terminates in a very short, free caudicle,— to others in which the
caudicle becomes firmly attached to the viscid matter, with the
sterile stigma itself much modified. In this latter case we have a
pollinium in its most highly developed and perfect condition. He
who will carefully examine the flowers of orchids for himself will
not deny the existence of the above series of gradations— from a
mass of pollen-grains merely tied together by threads, with the
stigma differing but litde from that of an ordinary flower, to a highly
complex pollinium, admirably adapted for transportal by insects;
nor will he deny that ail the gradations in the several species are
admirably adapted in relation to the general structure of each flower
for its fertilisation by different insects. In this, and in almost every
other case, the enquiry may be pushed further backwards; and it
may be asked how did the stigma of an ordinary flower become
viscid, but as we do not know the full history of any one group of
beings, it is as useless to ask, as it is hopeless to attempt answering,
such questions.
THEORY OF NATURAL SELECTION 24I
We will now turn to climbing plants. These can be arranged in
a long series, from those which simply twine round a support, to
those which I have called leaf-climbers, and to those provided with
tendrils. In these two latter classes the stems have generally, but
not always, lost the power of twining, though they retain the power
of revolving, which the tendrils likewise possess. The gradations
from leaf-climbers to tendril-bearers are wonderfully close, and cer-
tain plants may be indifferently placed in either class. But in ascend-
ing the series from simple twiners to leaf-climbers, an important
quality is added, namely sensitiveness to a touch, by which means
the foot-stalks of the leaves or flowers, or these modified and con-
verted into tendrils, are excited to bend round and clasp the touching
object. He who will read my memoir on these plants will, I think,
admit that all the many gradations in function and structure be-
tween simple twiners and tendril-bearers are in each case beneficial
in a high degree to the species. For instance, it is clearly a great
advantage to a twining plant to become a leaficHmber; and it is
probable that every twiner which possessed leaves with long foot-
stalks would have been developed into a leaf-climber, if the foot-
stalks had possessed in any slight degree the requisite sensitiveness
to a touch.
As twining is the simplest means of ascending a support, and
forms the basis of our series, it may naturally be asked how did
plants acquire this power in an incipient degree, afterwards to be
improved and increased through natural selection. The power of
twining depends, firstly, on the stems whilst young being extremely
flexible (but this is a character common to many plants which are
not climbers); and, secondly, on their continually bending to all
points of the compass, one after the other in succession, in the same
order. By this movement the stems are inclined to all sides, and
are made to move round and round. As soon as the lower part of
a stem strikes against any object and is stopped, the upper part still
goes on bending and revolving, and thus n^essarily twines round
and up the support. The revolving movement ceases after the early
growth of each shoot. As in many widely separated families of
plants, single species and single genera possess the power of re-
volving, and have thus become twiners, they must have independ-
242 ORIGIN OF SPECIES
ently acquired it, and cannot have inherited it from a common
progenitor. Hence I was led to predict that some slight tendency
to a movement of this kind would be found to be far from uncom-
mon with plants which did not climb; and that this had afforded
the basis for natural selection to work on and improve. When I
made this prediction, I knew of only one imperfect case, namely of
the young flower-peduncles of a Maurandia which revolved slightly
and irregularly, like the stems of twining plants, but without mak-
ing any use of this habit. Soon afterwards Fritz Miiller discovered
that the young stems of an Alisma and of a Linum,— plants which
do not climb and are widely separated in the natural system,— re-
volved plainly, though irregularly; and he states that he has reason
to suspect that this occurs with some other plants. These slight
movements appear to be of no service to the plants in question; any-
how, they are not of the least use in the way of climbing, which is
the point that concerns us. Nevertheless we can see that if the
stems of these plants had been flexible, and if under the conditions
to which they are exposed it had profited them to ascend to a height,
then the habit of slighdy and irregularly revolving might have been
increased and utilised through natural selection, until they had be-
come converted into well-developed twining species.
With respect to the sensitiveness of the foot-stalks of the leaves
and flowers, and of tendrils, nearly the same remarks are applicable
as in the case of the revolving movements of twining plants. As a
vast number of species, belonging to widely distinct groups, are
endowed with this kind of sensitiveness, it ought to be found in a
nascent condition in many plants which have not become climbers.
This is the case: I observed that the young flower-peduncles of the
above Maurandia curved themselves a little towards the side which
was touched. Morren found in several species of Oxalis that the
leaves and their foot-stalks moved, especially after exposure to a hot
sun, when they were gently and repeatedly touched, or when the
plant was shaken. I repeated these observations on some other
species of Oxalis with the same result; in some of them the move-
ment was distinct, but was best seen in the young leaves; in others
it was extremely slight. It is a more important fact that according
to the high authority of Hofmeister, the young shoots and leaves
THEORY OF NATURAL SELECTION 245
of aU plants move after being shaken; and with climbing plants it
is, as we know, only during the early stages of growth that the foot-
stalks and tendrils are sensitive.
It is scarcely possible that the above slight movements, due to a
touch or shake, in the young and growing organs of plants, can be
of any functional importance to them. But plants possess, in
obedience to various stimuli, powers of movement, which are of
manifest importance to them; for instance, towards and more rarely
from the light,— in opposition to, and more rarely in the direction
of, the attraction of gravity. When the nerves and muscles of an
animal are excited by galvanism or by the absorption of strychnine,
the consequent movements may be called an incidental result, for
the nerves and muscles have not been rendered specially sensitive
to these stimuli. So with plants it appears that, from having the
power of movement in obedience to certain stimuli, they are ex-
cited in an incidental manner by a touch, or by being shaken. Hence
there is no great difficulty in admitting that in the case of leaf-
climbers and tendril-bearers, it is this tendency which has been
taken advantage of and increased through natural selection. It is,
however, probable, from reasons which I have assigned in my
memoir, that this will have occurred only with plants which had
already acquired the power of revolving, and had thus become
twiners.
I have already endeavoured to explain how plants became twiners,
namely, by the increase of a tendency to slight and irregular re-
volving movements, which were at first of no use to them; this
movement, as well as that due to a touch or shake, being the in-
cidental result of the power of moving, gained for other and
beneficial purposes. Whether, during the gradual development of
climbing plants, natural selection has been aided by the inherited
effects of use, I will not pretend to decide; but we know that cer-
tain periodical movements, for instance the so-called sleep of plants,
are governed by habit.
I have now considered enough, perhaps more than enough, of
the cases, selected with care by a skilful naturalist, to prove that
natural selection is incompetent to account for the incipient stages
ORIGIN OF SPECIES
244
of useful structures; and I have shown, as I hope, that there is no
great difficulty on this head. A good opportunity has thus been
afforded for enlarging a little on gradations of structure, often as-
sociated with changed functions,— an important subject, which was
not treated at sufficient length in the former editions of this work.
I will now briefly recapitulate the foregoing cases.
With the giraffe, the continued preservation of the individuals of
some extinct high-reaching ruminant, which had the longest necks,
legs, etc., and could browse a litde above the average height, and
the continued destruction of those which could not browse so high,
would have sufficed for the production of this remarkable quad-
ruped; but the prolonged use of all the parts together with inherit-
ance will have aided in an important manner in their co-ordination.
With the many insects which imitate various objects, there is no
improbability in the belief that an accidental resemblance to some
common object was in each case the foundation for the work of
natural selection, since perfected through the occasional preservation
of slight variations which made the resemblance at all closer; and
this will have been carried on as long as the insect continued to vary,
and as long as a more and more perfect resemblance led to its escape
from sharp-sighted enemies. In certain species of whales there is a
tendency to the formation of irregular little points of horn on the
palate; and it seems to be quite within the scope of natural selection
to preserve all favourable variations, until the points were converted
first into lamellated knobs or teeth, like those on the beak of a goose,
—then into short lamelk, like those of the domestic ducks— and
then into lamella, as perfect as those of the shoveller-duck,— and
finally into the gigantic plates of baleen, as in the mouth of the
Greenland whale. In the family of the ducks, the lamelk are first
used as teeth, then pardy as teeth and partly as a sifting apparatus,
and at last almost exclusively for this latter purpose.
With such structures as the above lamelk of horn or whale-bone,
habit or use can have done little or nothing, as far as we can judge,
towards their development. On the other hand, the transportal of
the lower eye of a flat-fish to the upper side of the head, and the
formation of a prehensile tail, may be attributed almost wholly to
continued use, together with inheritance. With respect to the mam-
THEORY OF NATURAL SELECTION 245
mse of the higher animals, the most probable conjecture is that
primordially the cutaneous glands over the whole surface of a
marsupial sack secreted a nutritious fluid; and that these glands
were improved in function through natural selection, and concen-
trated into a confined area, in which case they would have formed
a mamma. There is no more difiiculty in understanding how the
branched spines of some ancient Echinoderm, which served as a
defence, became developed through natural selection into tridactyle
pedicellariae, than in understanding the development of the pincers
of crustaceans, through slight, serviceable modifications in the ulti-
mate and penultimate segments of a limb, which was at first used
solely for locomotion. In the avicularia and vibracula of the Polyzoa
we have organs widely different in appearance developed from the
same source; and with the vibracula we can understand how the
successive gradations might have been of service. With the pollinia
of orchids, the threads which originally served to tie together the
pollen-grains, can be traced cohering into caudicles; and the steps
can likewise be followed by which viscid matter, such as that secreted
by the stigmas of ordinary flowers, and still subserving nearly but
not quite the same purpose, became attached to the free ends of the
caudicles; — ^all these gradations being of manifest benefit to the
plants in question. With respect to climbing plants, I need not re-
peat what has been so lately said.
It has often been asked, if natural selection be so potent, why has
not this or that structure been gained by certain species, to which it
would apparently have been advantageous? But it is unreasonable
to expect a precise answer to such questions, considering our igno-
rance of the past history of each species, and of the conditions which
at the. present-day. determine its numbers and range. In most cases
only general reasons, but in some few cases special reasons, can be
assigned. Thus to adapt a species to new habits of life, many co-
ordinated modifications are almost indispensable, and it may often
have happened that the requisite parts did not vary in the right
manner or to the right degree. Many species must have been pre-
vented from increasing in numbers through destructive agencies,
which stood in no relation to certain structures, which we imagine
would have been gained through natural selection from appearing
246 ORIGIN OF SPECIES
to us advantageous to the species- In this case, as the struggle for
life did not depend on such structures, they could not have been
acquired through natural selection. In many cases complex and
long-enduring conditions, often of a peculiar nature, are necessary
for the development of a structure; and the requisite conditions may
seldom have concurred. The beHef that any given structure, which
we think, often erroneously, would have been beneficial to a species,
would have been gained under all circumstances through natural
selection, is opposed to what we can understand of its manner of
action. Mr. Mivart does not deny that natural selection has effected
something; but he considers it as “demonstrably insufficient” to ac-
count for the phenomena which I explain by its agency. His chief
arguments have now been considered, and the others will hereafter
be considered. They seem to me to partake little of the character
of demonstration, and to have litde weight in comparison with
those in favour of the power of natural selection, aided by the other
agencies often specified. I am bound to add, that some of the facts
and arguments here used by me, have been advanced for the same
purpose in an able article lately published in the 'Medico-Chirurgical
Review.’
At the present day almost all naturalists admit evolution under
some form. Mr. Mivart believes that species change through “an
internal force or tendency,” about which it is not pretended that
anything is known. That species have a capacity for change will be
admitted by all evolutionists; but there is no need, as it seems to me,
to invoke any internal force beyond the tendency to ordinary va-
riability, which through the aid of selection by man has given rise
to many well-adapted domestic races, and which through the aid
of natural selection would equally well give rise by graduated steps
to natural races or species. The final result will generally have been,
as already explained, an advance, but in some few cases a retrogres-
sion, in organisation.
Mr. Mivart is further inclined to believe, and some naturalists
agree with him, that new species manifest themselves “with sudden-
ness and by modifications appearing at once.” For instance, he sup-
poses that the differences between the extinct three-toed Hipparion
and the horse arose suddenly. He thinks it difficult to believe that
THEORY OF NATURAL SELECTION 247
the wing of a bird ‘‘was developed in any other way than by a com-
paratively sudden modification of a marked and important kind;”
and apparently he would extend the same view to the wings of bats
and pterodactyles. This conclusion, which implies great breaks or
discontinuity in the series, appears to me improbable in the highest
degree.
Every one who believes in slow and gradual evolution, will of
course admit that specific changes may have been as abrupt and as
great as any single variation which we meet with under nature, or
even under domestication. But as species are more variable when
domesticated or cultivated than under their natural conditions, it is
not probable that such great and abrupt variations have often
occurred under nature, as are known occasionally to arise under
domestication. Of these latter variations several may be attributed
to reversion; and the> characters which thus reappear were, it is
probable, in many cases at first gained in a gradual manner. A still
greater number must be called monstrosities, such as six-fingered
men, porcupine men, Ancon sheep, Niata cattle, &c.; and as they
are widely different in character from natural species, they throw
very litde light on our subject. Excluding such cases of abrupt
variations, the few which remain would at best constitute, if found
in a state of nature, doubtful species, closely related to their parental
types.
My reasons for doubting whether natural species have changed
as abrupdy as have occasionally domestic races, and for endrely dis-
believing that they have changed in the wonderful manner indicated
by Mr. Mivart, are as follows. According to our experience, abrupt
and strongly marked variations occur in our domesdcated produc-
tions, singly and at rather long intervals of time. If such occurred
under nature, they would be liable, as formerly explained, to be
lost by accidental causes of destruction and by subsequent inter-
crossing; and so it is known to be under domestication, unless
abrupt variations of this kind are specially preserved and separated
by the care of man. Hence, in order that a new species should sud-
denly appear in the manner supposed by Mr. Mivart, it is almost
necessary to believe, in opposition to all analogy, that several won-
derfully changed individuals appeared simultaneously within the
248 ORIGIN OF SPECIES
same district. This diflEcuky, as in the case of unconscious selection
by man, is avoided on the theory of gradual evolution, through the
preservation of a large number of individuals, which varied more
or less in any favourable direction, and of the destruction of a large
number which varied in an opposite manner.
That many species have been evolved in an extremely gradual
manner, there can hardly be a doubt. The species and even the
genera of many large natural families are so closely allied together,
that it is difficult to distinguish not a few of them. On every con-
tinent in proceeding from north to south, from lowland to upland,
etc., we meet with a host of closely related or representative species;
as we likewise do on certain distinct continents, which we have
reason to believe were formerly connected. But in making these
and the following remarks, I am compelled to allude to subjects
hereafter to be discussed. Look at the many outlying islands round
a continent, and see how many of their inhabitants can be raised
only to the rank of doubtful species. So it is if we look to past
times, and compare the species which have just passed away with
those still living within the same areas; or if we compare the fossil
species embedded in the sub-stages of the same geological formation.
It is indeed manifest that multitudes of species are related in the
closest manner to other species that still exist, or have lately existed;
and it will hardly be maintained that such species have been de-
veloped in an abrupt or sudden manner. Nor should it be forgotten,
when we look to the special parts of allied species, instead of to
distinct species, that numerous and wonderfully fine gradations can
be traced, connecting together widely different structures.
Many large groups of facts are intelligible only on the principle
that species have been evolved by very small steps. For instance, the
fact that the species included in the larger genera are more closely
related to each other, and present a greater number of varieties than
do the species in the smaller genera. The former are also grouped
in litde clusters, like varieties round species; and they present other
analogies with varieties, as was shown in our second chapter. On
this same principle we can understand how it is that specific charac-
ters are more variable than generic characters; and how the parts
which are developed in an extraordinary de^ee or manner are more
THEORY OF NATURAL SELECTION 249
variable than other parts of the same species. Many analogous facts,
all pointing in the same direction, could be added.
Although very many species have almost certainly been produced
by steps not greater than those separating fine- varieties; yet it may
be maintained that some have been developed in a different and
abrupt manner. Such an admission, however, ought not to be made
without strong evidence being assigned. The vague and in some
respects false analogies, as they have been shown to be by Mr.
Chauncey Wright, which have been advanced in favour of this
view, such as the sudden crystallisation of inorganic substances, or
the falling of a facetted spheroid from one facet to another, hardly
deserve consideration. One class of facts, however, namely, the
sudden appearance of new and distinct forms of life in our geo-
logical formations, supports at first sight the belief in abrupt de-
velopment. But the value of this evidence depends entirely on the
perfection of the geological record, in relation to periods remote in
the history of the world. If the record is as fragmentary as many
geologists strenuously assert, there is nothing strange in new forms
appearing as if suddenly developed.
Unless we admit transformations as prodigious as those advocated
by Mr. Mivart, such as the sudden development of the wings of
birds or bats, or the sudden conversion of a Hipparion into a horse,
hardly any light is thrown by the belief in abrupt modifications on
the deficiency of connecting links in our geological formations. But
against the belief in such abrupt changes, embryology enters a
strong protest. It is notorious that the wings of birds and bats, and
the legs of horses or other quadrupeds, are undistinguishable at an
early embryonic period, and that they become differentiated by in-
sensibly fine steps. Embryological resemblances of all kinds can be
accounted for, as we shall hereafter see, by the progenitors of our
existing species having varied after early youth, and having trans-
mitted their newly acquired characters to their offspring, at a cor-
responding age. The embryo is thus left almost unaffected, and
serves as a record of the past condition of the species. Hence it is
that existing species during the early stages of their development
so often resemble ancient and extinct forms belonging to the same
class. On this view of the meaning of embryological resemblances,
250 ORIGIN OF SPECIES
and indeed on any view, it is incredible that an animal should have
undergone such momentous and abrupt transformations, as those
above indicated; and yet should not bear even a trace in its embryonic
condition of any sudden modification, every detail in its structure
being developed by insensibly fine steps.
He who believes that some ancient form was transformed sud-
denly through an internal force or tendency into, for instance, one
furnished with wings, will be almost compelled to assume, in opposi-
tion to ail analogy, that many individuals varied simultaneously.
It cannot be denied that such abrupt and great changes of structure
are widely different from those which most species apparendy have
undergone. He will further be compelled to believe that many
structures beautifully adapted to all the other parts of the same
creature and to the surrounding conditions, have been suddenly
produced; and of such complex and wonderful co-adaptations, he
will not be able to assign a shadow of an explanation. He will be
forced to admit that these great and sudden transformations have
left no trace of their action on the embryo. To admit all this is, as
it seems to me, to enter into the realms of miracle, and to leave
those of Science.
CHAPTER VIII
Instinct
Instincts comparable with habits, but different in their origin — ^Instincts
graduated — Aphides and ants — ^Instincts variable — ^Domestic in-
stincts, their origin — ^Natural instincts of the cuckoo, molothrus,
ostrich, and parasitic bees — ^Slave-making ants — Hive-bee, its cell-
making instinct — Changes of instinct and structure not necessarily
simultaneous — Difficulties of the theory of the Natural Selection of
instincts — ^Neuter or sterile insects — -Summary.
M any instincts are so wonderful that their development
will probably appear to the reader a difficulty sufficient to
overthrow my whole theory. I may here premise, that I
have nothing to do with the origin of the mental powers, any more
than I have with that of life itself. We are concerned only with the
diversities of instinct and of the other mental faculties in animals of
the same class.
I will not attempt any definition of instinct. It would be easy to
show that several distinct mental actions are commonly embraced by
this term; but every one understands what is meant, when it is said
that instinct impels the cuckoo to migrate and to lay her eggs in other
birds’ nests. An action, which we ourselves require experience to en-
able us to perform, when performed by an animal, more especially by
a very young one, without experience, and when performed by many
individuals in the same way, without their knowing for what pur-
pose it is performed, is usually said to be instinctive. But I could
show that none of these characters are universal. A little dose of
judgment or reason, as Pierre Huber expresses it, often comes into
play, even with animals low in the scale of nature.
Frederick Cuvier and several of the older metaphysicians have com-
pared instinct with habit. This comparison gives, I thinks an accu-
rate notion of the frame of mind under which an instinctive action
is performed, but not necessarily of its origin. How unconsciously
many habitual actions are performed, indeed not rarely in direct
ORIGIN OF SPECIES
252
opposition to our conscious will! yet they may be modified by the
will or reason. Habits easily become associated with other habits,
with certain periods of time, and states of the body. When once
acquired, they often remain constant throughout life. Several other
points of resemblance between instincts and habits could be pointed
out. As in repeating a well-known song, so in instincts, one action
follows another by a sort of rhythm; if a person be interrupted in a
song, or in repeating anything by rote, he is generally forced to go
back to recover the habitual train of thought; so P. Huber found it
was with a caterpillar, which makes a very complicated hammock;
for if he took a caterpillar which had completed its hammock up
to, say, the sixth stage of construction, and put it into a hammock
completed up only to the third stage, the caterpillar simply re-per-
formed the fourth, fifth, and sixth stages of construction. If, how-
ever, a caterpillar were taken out of a hammock made up, for in-
stance, to the third stage, and were put into one finished up to the
sixth stage, so that much of its work was already done for it, far from
deriving any benefit from this, it was much embarrassed, and in
order to complete its hammock, seemed forced to start from the
third stage, where it had left off, and thus tried to complete the
already finished work.
If we suppose any habitual action to become inherited — and it can
be shown that this does sometimes happen — then the resemblance
between what originally was a habit and an instinct becomes so close
as not to be distinguished. If Mozart, instead of playing the piano-
forte at three years old with wonderfully litde practice, had played a
tune with no practice at all, he might truly be said to have done so
instinctively. But it would be a serious error to suppose that the
greater number of instincts have been acquired by habit in one gen-
eration, and then transmitted by inheritance to succeeding genera-
tions. It can be clearly shown that the most wonderful instincts with
which we are acquainted, namely, those of the hive-bee and of many
ants, could not possibly have been acquired by habit.
It will be universally admitted that instincts are as important as
corporeal structures for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to a
INSTINCT
253
species; and if it can be shown that instincts do vary ever so little,
then I can see no difficulty in natural selection preserving and con-
tinually accumulating variations of instinct to any extent that was
profitable. It is thus, as I believe, that all the most complex and won-
derful instincts have originated. As modifications of corporeal struc-
ture arise from, and are increased by, use or habit, and are diminished
or lost by disuse, so I do not doubt it has been with instincts. But I
believe that the effects of habit are in many cases of subordinate im-
portance to the effects of the natural selection of what may be called
-spontaneous variations of instincts;— that is of variations produced
by the same unknown causes which produce slight deviations of
bodily structure.
No complex instinct can possibly be produced through natural
selection, except by the slow and gradual accumulation of numerous
slight, yet profitable, variations. Hence, as in the cases of corporeal
structures, we ought to find in nature, not the actual transitional
gradations by which each complex instinct has been acquired — ^for
these could be found only in the lineal ancestors of each species— but
we ought to find in the collateral lines of descent some evidence of
such gradations; or we ought at least be able to show that gradations
of some kind are possible; and this we certainly can do. I have been
surprised to find, making allowance for the instincts of animals hav-
ing been but litde observed except in Europe and North America,
and for no instinct being known amongst extinct species, how very
generally gradations, leading to the most complex instincts, can be dis-
covered. Changes of instinct may sometimes be facilitated by the
same species having different instincts at different periods of life, or
at different seasons of the year, or when placed under different cir-
cumstances, etc.; in which case either the one or the other instinct
might be preserved by natural selection. And such instances of
diversity of instinct in the same species can be shown to occur in
nature.
Again, as in the case of corporeal structure, and conformably to
my theory, the instinct of each species is good for itself, but has
never, as far as we can judge, been produced for the exclusive good
of others. One of the strongest instances of an animal apparently
performing an action for the sole good of another, with which I am
254 ORIGIN OF SPECIES
acquainted, is that of aphides voluntarily yielding, as was first ob-
served by Huber, their sweet excretion to ants: that they do so vol-
untarily, the following facts show: I removed all the ants from a
group of about a dozen aphides on a dock-plant, and prevented their
attendance during several hours. After this interval, I felt sure that
the aphides would want to excrete. I watched them for some time
through a lens, but not one excreted; I then tickled and stroked them
with a hair in the same manner, as well as I could, as the ants do
with their antennae; but not one excreted. Afterwards I allowed an
ant to visit them, and it immediately seemed, by its eager way of
running about, to be well aware what a rich flock it had discovered;
it then began to play with its antennae on the abdomen first of one
aphis and then of another; and each, as soon as it felt the antennae,
immediately lifted up its abdomen and excreted a limpid drop of
sweet juice, which was eagerly devoured by the ant. Even the
quite young aphides behaved in this manner, showing that the action
was instinctive, and not the result of experience. It is certain, from
the observations of Huber, that the aphides show no dislike to the
ants: if the latter be not present they are at last compelled to. eject
their excretion. But as the excretion is extremely viscid, it is no doubt
a convenience to the aphides to have it removed; therefore probably
they do not excrete solely for the good of the ants. Although there is
no evidence that any animal performs an action for the exclusive
good of another species, yet each tries to take advantage of the
instincts of others, as each takes advantage of the weaker bodily
structure of other species. So again certain instincts cannot be con-
sidered as absolutely perfect; but as details on this and other such
points are not indispensable, they may be here passed over.
As some degree of variation in instincts under a state of na-
ture, and the inheritance of such variations, are indispensable for
the action of natural selection, as many instances as possible ought to
be given; but want of space prevents me. I can only assert that in-
stincts certainly do vary— for instance, the migratory instinct, both in
extent and direction, and in its* total loss. So it is with the nests of
birds, which vary pardy in dependence on the situations chosen, and
on the nature and temperature of the country inhabited, but often
from causes wholly unknown to us: Audubon has given several re-
CHANGES OF HABIT OR INSTINCT 255
markable cases of differences in the nests of the same species in the
northern and southern United States. Why, it has been asked, if
instinct be variable, has it not granted to the bee ‘‘the ability to use
some other material when wax was deficient”? But what other
natural material could bees use? They will work, as I have seen,
with wax hardened with vermilion or softened with lard. Andrew
Knight observed that his bees, instead of laboriously collecting prop-
olis, used a cement of wax and turpentine, with which he had cov-
ered decorticated trees. It has lately been shown that bees, instead
of searching for pollen, will gladly use a very different substance,
namely oatmeal. Fear of any particular enemy is certainly an instinc-
tive quality, as may be seen in nestling birds, though it is strengthened
by experience, and by the sight of fear of the same enemy in other
animals. The fear of man is slowly acquired, as I have elsewhere
shown, by the various animals which inhabit desert islands; and we
see an instance of this even in England, in the greater wildness of
all our large birds in comparison with our small birds; for the large
birds have been most persecuted by man. We may safely attribute the
greater wildness of our large birds to this cause; for in uninhabited
islands large birds are not more fearful than small; and the magpie,
so wary in England, is tame in Norway, as is the hooded crow in
Egypt.
That the mental qualities of animals of the same kind, born in a
state of nature, vary much, could be shown by many facts. Several
cases could also be adduced of occasional and strange habits in wild
animals, which, if advantageous to the species, might have given rise,
through natural selection, to new instincts. But I am well aware that
these general statements, without the facts in detail, will produce but
a feeble effect on the reader’s mind. I can only repeat my assurance,
that I do not speak without good evidence.
INHERITED CHANGES OF HABIT OR INSTINCT IN
DOMESTICATED ANIMALS
The possibility, or even probability, of inherited variations of in-
stinct in a state of nature will be strengthened by briefly considering
a few cases under domestication. We shall thus be enabled to see
the part which habit and the selection of so-called spontaneous varia-
ORIGIN OF SPECIES
256
tions have played in modifying the mental qualities of our domestic
animals. It is notorious how much domestic animals vary in their
mental qualities. With cats, for instance, one naturally takes to
catching rats, and another mice, and these tendencies are known to
be inherited. One cat, according to Mr. St. John, always brought
home game-birds, another hares or rabbits, and another hunted on
marshy ground and almost nighdy caught woodcocks or snipes. A
number of curious and authentic instances could be given of various
shades of disposition and of taste, and likewise of the oddest tricks,
associated with certain frames of mind or periods of time, being in-
herited. But let us look to the familiar case of the breeds of the dogs:
it cannot be doubted that young pointers (I have myself seen a strik-
ing instance) will sometimes point and even back other dogs the very
first time that they are taken out; retrieving is certainly in some de-
gree inherited by retrievers; and a tendency to run round, instead of
at, a flock of sheep, by shepherd dogs. I cannot see that these actions,
performed without experience by the young, and in nearly the same
manner by each individual, performed with eager delight by each
breed, and without the end being known—for the young pointer can
no more know that he points to aid his master, than the white butter-
fly knows why she lays her eggs on the leaf of the cabbage— I cannot
see that these actions differ essentially from true instincts. If we were
to behold one kind of wolf, when young and without any training,
as soon as it scented its prey, stand motionless like a statue, and then
slowly crawl forward with a peculiar gait; and another kind of wolf
rushing round, instead of at, a herd of deer, and driving them to a
distant point, we should assuredly call these actions instinctive.
Domestic instincts, as they may be called, are certainly far less fixed
than natural instincts; but they have been acted on by far less rigorous
selection, and have been transmitted for an incomparably shorter
period, under less fixed conditions of life.
How strongly these domestic instincts, habits, and dispositions are
inherited, and how curiously they become mingled, is well shown
when difierent breeds of dogs are crossed. Thus it is known that
a cross with a bull-dog has affected for many generations the courage
and obstinacy of greyhounds; and a cross with a greyhound has
given to a whole family of shepherd-dogs a tendency to hunt hares.
CHAxNGES OF HABIT OR INSTINCT
257
These domestic instincts, when thus tested by crossing, resemble
natural instincts, which in a like manner become curiously blended
together, and for a long period exhibit traces of the instincts of either
parent: for example, Le Roy describes a dog, whose great-grandfather
was a wolf, and this dog showed a trace of its wild parentage only in
one way, by not coming in a straight line to his master when called.
Domestic instincts are sometimes spoken of as actions which have
become inherited solely from long-continued and compulsory habit;
but this is not true. No one would ever have thought of teaching, or
probably could have taught, the tumbler-pigeon to tumble, — an action
which, as I have witnessed, is performed by young birds that have
never seen a pigeon tumble. We may believe that some one pigeon
showed a slight tendency to this strange habit, and that the long-con-
tinued selection of the best individuals in successive generations made
tumblers what they now are; and near Glasgow there are house-tum-
blers, as I hear from Mr. Brent, which cannot fly eighteen inches high
without going head over heels. It may be doubted whether any one
would have thought of training a dog to point, had not some one dog
naturally shown a tendency in this line; and this is known occasion-
ally to happen, as I once saw, in a pure terrier: the act of pointing
is probably, as many have thought, only the exaggerated pause of an
animal preparing to spring on its prey. When the first tendency to
point was once displayed, methodical selection and the inherited
effects of compulsory training in each successive generation would
soon complete the work; and unconscious selection is still in progress,
as each man tries to procure, without intending to improve the breed,
dogs which stand and hunt best. On the other hand, habit alone in
some cases has sufficed; hardly any animal is more difficult to tame
than the young of the wild rabbit; scarcely any animal is tamer
than the young of the tame rabbit; but I can hardly suppose that
domestic rabbits have often been selected for tameness alone; so that
we must attribute at least the greater part of the inherited change
from extreme wildness to extreme tameness, to habit and long-
continued close confinement.
Natural instincts are lost under domestication: a remarkable in-
stance of this is seen in those breeds of fowls which very rarely or
never become “broody,’’ that is, never wish to sit on their eggs. Famil-
258 ORIGIN OF SPECIES
iarity alone prevents our seeing how largely and how permanendy the
minds o£ our domestic animals have been modified. It is scarcely pos-
sible to doubt that the love of man has become instinctive in the dog.
All wolves, foxes, jackals, and species of the cat genus, when kept
tame, are most eager to attack poultry, sheep, and pigs; and this
tendency has been found incurable in dogs which have been brought
home as puppies from countries such as Tierra del Fuego and Aus-
tralia, where the savages do not keep these domestic animals. How
rarely, on the other hand, do our civilised dogs, even when quite
young, require to be taught not to attack poultry, sheep, and pigs!
No doubt they occasionally do make an attack, and are then beaten;
and if not cured, they are destroyed; so that habit and some degree of
selection have probably concurred in civilising by inheritance our
dogs. On the other hand, young chickens have lost, wholly by habit,
that fear of the dog and cat which no doubt was originally instinc-
tive in them; for I am informed by Captain Hutton that the young
chickens of the parent-stock, the Callus bankiva, when reared in
India under a hen, are at first excessively wild. So it is with young
pheasants reared in England under a hen. It is not that chickens have
lost all fear, but fear only of dogs and cats, for if the hen gives the
danger-chuckle, they will run (more especially young turkeys) from
under her, and conceal themselves in -the surrounding grass or
thickets; and this is evidently done for the instinctive purpose of
allowing, as we see in wild ground-birds, their mother to fly away.
But this instinct retained by our chickens has become useless under
domestication, for the mother-hen has almost lost by disuse the
power of flight.
Hence, we may conclude that under domestication instincts have
been acquired, and natural instincts have been lost, partly by habit,
and partly by man selecting and accumulating, during successive gen-
erations, peculiar mental habits and actions, which at first appeared
from what we must in our ignorance call an accident. In some cases
compulsory habit alone has sufficed to produce inherited mental
changes; in other cases, compulsory habit has done nothing, and all
has been the result of selection, pursued both methodically and un-
consciously: but in most cases habit and selection have probably
concurred.
INSTINCTS OF THE CUCKOO
259
SPECUL INSTINCTS
We shall, perhaps, best understand how instincts in a state of na-
ture have become modified by selection, by considering a few cases.
I will select only three, — namely, the instinct which leads the cuckoo
to lay her eggs in other birds’ nests; the slave-making instinct of
certain ants; and the cell-making power of the hive-bee. These two
latter instincts have generally and jusdy been ranked by naturalists
as the most wonderful of all known instincts.
Instincts of the Cuc\oo—lx. is supposed by some naturahsts that the
more immediate cause of the instinct of the cuckoo is, that she lays
her eggs, not daily, but at intervals of two or three days; so that, if
she w'ere to make her own nest and sit on her own eggs, those first
laid would have to be left for some time unincubated, or there would
be eggs and young birds of different ages in the same nest. If this
were the case, the process of laying and hatching might be inconven-
iently long, more especially as she migrates at a very early period;
and the first hatched young would probably have to be fed by the
male alone. But the American cuckoo is in this predicament; for
she makes her own nest, and has eggs and young successively hatched,
all at the same time. It has been both asserted and denied that the
American cuckoo occasionally lays her eggs in other birds’ nests; but
I have lately heard from Dr. Merrell, of Iowa, that he once found in
Illinois a young cuckoo together with a young jay in the nest of a
Blue jay (Garrulus cristatus) ; and as both were nearly full feathered,
there could be no mistake in their identification. I could also give
several instances of various birds which have been known occasion-
ally to lay their eggs in other birds’ nests. Now let us suppose that
the ancient progenitor of our European cuckoo had the habits of the
American cuckoo, and that she occasionally laid an egg in another
bird’s nest. If the old bird profited by this occasional habit through
being enabled to migrate earlier or through any other cause; or if the
young were made more vigorous by advantage being taken of the
mistaken instinct of another species than when reared by their own
mother, encumbered as she could hardly fail to be by having eggs and
young of different ages at the same time; then the old birds or the
fostered young would gain an advantage. And analogy would lead
ORIGIN OF SPECIES
260
US to believe, that the young thus reared would be apt to follow by in-
heritance the occasional and aberrant habit of their mother, and in
their turn would be apt to lay their eggs in other birds’ nests, and thus
be more successful in rearing their young. By a continued process of
this nature, I believe that the strange instinct of our cuckoo has been
generated. It has, also, recently been ascertained on sufficient evi-
dence, by Adolf Muller, that the cuckoo occasionally lays her eggs on
the bare ground, sits on them, and feeds her young. This rare event
is probably a case of reversion to the long-lost, aboriginal instinct of
nidification.
It has been objected that I have not noticed other related instincts
and adaptations of structure in the cuckoo, which are spoken of as
necessarily co-ordinated. But in all cases, speculation on an instinct
known to us only in a single species, is useless, for we have hitherto
had no facts to guide us. Until recently the instincts of the Euro-
pean and of the non-parasitic American cuckoo alone were known;
now, owing to Mr. Ramsay’s observations, we have learnt something
about three Australian species, which lay their eggs in other birds’
nests. The chief points to be referred to are three: first, that the
common cuckoo, with rare exceptions, lays only one egg in a nest,
so that the large and voracious young bird receives ample food.
Secondly, that the eggs are remarkably small, not exceeding those
of the skylark, —a bird about one-fourth as large as the cuckoo. That
the small size of the egg is a real case of adaptation we may infer
from the fact of the non-parasitic American cuckoo laying full-sized
eggs. Thirdly, that the young cuckoo, soon after birth, has the in-
stinct, the strength, and a properly shaped back for ejecting its foster-
brothers, which then perish from cold and hunger. This has been
boldly called a beneficial arrangement, in order that the young
cuckoo may get sufficient food, and that its foster-brothers may perish
before they had acquired much feeling!
Turning now to the Australian species; though these birds gener-
ally lay only one egg in a nest, it is not rare to find two and even
three eggs in the same nest. In the Bronze cuckoo the eggs vary
greatly in size, from eight to ten lines in length. Now if it had been
of an advantage to this species to have laid eggs even smaller than
those now laid, so as to have deceived certain foster-parents, or, as is
INSTINCTS OF THE CUCKOO 261
more probable, to have been hatched within a shorter period (for it
is asserted that there is a relation between the size of eggs and the
period of their incubation), then there is no difficulty in beUeving
that a race or species might have been formed which would have
laid smaller and smaller eggs; for these would have been more safely
hatched and reared. Mr. Ramsay remarks that two of the Australian
cuckoos, when they lay their eggs in an open nest, manifest a decided
preference for nests containing eggs similar in colour to their own.
The European species apparendy manifests some tendency towards
a similar instinct, but not rarely departs from it, as is shown by her
laying her dull and pale-coloured eggs in the nest of the Hedge-
warbler with bright greenish-blue eggs. Had our cuckoo invariably
displayed the above instinct, it would assuredly have been added to
those which it is assumed must all have been acquired together.
The eggs of the Australian Bronze cuckoo vary, according to Mr.
Ramsay, to an extraordinary degree in colour; so that in this respect,
as well as in size, natural selection might have secured and fixed any
advantageous variation.
In the case of the European cuckoo, the offspring of the foster-
parents are commonly ejected from the nest within three days after
the cuckoo is hatched; and as the latter at this age is in a most
helpless condition, Mr. Gould was formerly inclined to believe that
the act of ejection was performed by the foster-parents themselves.
But he has now received a trustworthy account of a young cuckoo
which was actually seen, whilst still blind and not able even to hold
up its own head, in the act of ejecting its foster-brothers. One of
these was replaced in the nest by the observer, and was again thrown
out. With respect to the means by which this strange and odious
instinct was acquired, if it were of great importance for the young
cuckoo, as is probably the case, to receive as much food as possible
soon after birth, I can see no special difficulty in its having gradually
acquired, during successive generations, the blind desire, the strength,
and structure necessary for the work of ejection; for those young
cuckoos which had such habits and structure best developed would
be the most securely reared. The first step towards the acquisition
of the proper instinct might have been mere unintentional restless-
ness on the part of the young bird, when somewhat advanced in
ORIGIN OF SPECIES
262
age and strength; the habit having been afterwards improved, and
transmitted to an earlier age. I can see no more difficulty in this,
than in the unhatched young of other birds acquiring the instinct
to break through their own shells; — or than in young snakes acquir-
ing in their upper jaws, as Owen has remarked, a transitory sharp
tooth for cutting through the tough egg-shell. For if each part is
liable to individual variations at aU ages, and the variations tend to
be inherited at a corresponding or earlier age, — ^propositions which
cannot be disputed, — then the instincts and structure of the young
could be slowly modified as surely as those of the adult; and both
cases must stand or fall together with the whole theory of natural
selection.
Some species of Molothrus, a widely distinct genus of American
birds, allied to our starlings, have parasitic habits like those of the
cuckoo; and the species present an interesting gradation in the per-
fection of their instincts. The sexes of Molothrus badius are stated
by an excellent observer, Mr. Hudson, sometimes to live promiscu-
ously together in flocks, and sometimes to pair. They either build
a nest of their own, or seize on one belonging to some other bird,
occasionally throwing out the nestlings of the stranger. They either
lay their eggs in the nest thus appropriated, or oddly enough build
one for themselves on the top of it. They usually sit on their own
eggs and rear their own young; but Mr. Hudson says it is probable
that they are occasionally parasitic, for he has seen the young of this
species following old birds of a distinct kind and clamouring to be
fed by them. The parasitic habits of another species of Molothrus,
the M. bonariensis, are much more highly developed than those
of the last, but are still far from perfect. This bird, as far as it is
known, invariably lays its eggs in the nests of strangers; but it is
remarkable that several together sometimes commence to build an
irregular untidy nest of their own, placed in singularly ill-adapted
situations, as on the leaves of a large thistle. They never, however, as
far as Mr. Hudson has ascertained, complete a nest for themselves.
They often lay so many eggs — ^from fifteen to twenty — ^in the same
foster-nest, that few or none can possibly be hatched. They have,
moreover, the extraordinary habit of pecking holes in the eggs,
INSTINCTS OF THE MOLOTHRUS 263
whether of their own species or of their foster-parents, which they
find in the appropriated nests. They drop also many eggs on the bare
ground, which are thus wasted. A third species, the M. pecoris of
North America, has acquired instincts as perfect as those of the
cuckoo, for it never lays more than one egg in a foster-nest, so that
the young bird is securely reared. Mr. Hudson is a strong dis-
believer in evolution, but he appears to have been so much struck
by the imperfect instincts of the Molothrus bonariensis that he quotes
my words, and asks, ‘‘Must we consider these habits, not as espe-
cially endowed or created instincts, but as small consequences of one
general law, namely, transition?”
Various birds, as has already been remarked, occasionally lay their
eggs in the nests of other birds. This habit is not very uncommon
with the Gallinace^, and throws some light on the singular instinct
of the ostrich. In this family several hen birds unite and lay first
a few eggs in one nest and then in another; and these are hatched by
the males. This instinct may probably be accounted for by the fact
of the hens laying a large number of eggs, but, as with the cuckoo,
at intervals of two or three days. The instinct, however, of the
American ostrich, as in the case of the Molothrus bonariensis, has
not as yet been perfected; for a surprising number of eggs lie strewed
over the plains, so that in one day’s hunting I picked up no less than
twenty lost and wasted eggs.
Many bees are parasitic, and regularly lay their eggs in the nests
of other kinds of bees. This case is more remarkable than that of the
cuckoo; for these bees have not only had their instincts but their
structure modified in accordance with their parasitic habits; for they
do not possess the pollen-collecting apparatus which would have
been indispensable if they had stored up food for their own young.
Some species of Sphegidae (wasp-like insects) are likewise parasitic;
and M. Fabre has lately shown good reason for believing that,
although the Tachytes nigra generally makes its own burrow and
stores it with paralysed prey for its own larvae, yet that, when this
insect finds a burrow already made and stored by another sphex, it
takes advantage of the prize, and becomes for the occasion parasitic.
In this case, as with that of the Molothrus or cuckoo, I can see no
ORIGIN OF SPECIES
264
difficulty in natural selection making an occasional habit permanent,
if of advantage to the species, and if the insect whose nest and stored
food are feloniously appropriated, be not thus exterminated.
Slave-ma{ing instinct,— This remarkable instinct was first dis-
covered in the Formica (Polyerges) rufescens by Pierre Huber, a
better observer even than his celebrated father. This ant is absolutely
dependent on its slaves; without their aid, the species would certainly
become extinct in a single year. The males and fertile females do
no work of any kind, and the workers or sterile females, though most
energetic and courageous in capturing slaves, do no other work.
They are incapable of making their own nests, or of feeding their
own larvcT. When the old nest is found inconvenient, and they have
to migrate, it is the slaves which determine the migration, and
actually carry their masters in their jaws.
