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The Five-Foot Shelf of Books 


The Origin of Species 

By Charles Darwin 

W//A Introductions and ^otes 
Wo/ume II 

P. F. Collier & Son Corporation 


Copyright, 1909 
By p. F. Collier & Son 


"But with regard to the material world, 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 Revealed 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 24th, l8§g. 
Sixth Edition, January, i8'/2. 





Of the Progress of Opinion on the Origin of Species .... 9 


Variation under Domestication 23 

Variation under Nature 54 

Struggle for Existence 71 

Natural Selection; or the Survival of the Fittest 87 

Laws of Variation 138 

Difficulties op the Theory 169 

Miscellaneous Objections to the Theory of Natural Selection 209 

Instinct 251 

Hybridism 285 

4 coNTE^^^s 

On the Imperfection of the Geological Record 319 

On the Geological Succession of Organic Beings 349 

Geographical Distribution 378 

Geographical Distribution — continued 409 


Mutual Affinities of Organic Beings: Morphology — Embry- 
ology — Rudimentary Organs 431 

Recapitulation and Conclusion 478 

Glossary 507 

Index 517 


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 
stria 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 constandy, 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, and three years later he withdrew from London to the litde 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 paf)er 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 
Linnzan Society. In November of the following year "The Origin of 


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, BufFon, 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 sf)ecies 
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 
sjjecies on Galapagos ArchijDelago. 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 


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 upwn 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 imjxirtance 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 imfx)rtant pwints." 




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 
sf>ecies 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 Buffon. But as his opinions fluctuated gready at different 
{jeriods, 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 jusdy-celebrated naturalist first published his views 
in 1801; he much enlarged them in 1809 in his 'Philosophie Zoologique,' 

' Aristotle, in his 'Physicz 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. 


and subsequently, in 1815, in the Introduction to his 'Hist. Nat. des 
Animaux sans Vertebres.' In tliese 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- 

Geoffroy Saint-Hilaire, as is stated in his 'Life,' written by his son, 
suspected, as early as 1795, that what we call sf>ecies 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 "tnonde ambiant," 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 r^server entierement a I'avenir, suppose meme que I'avenir doive avoir 
prise sur lui." 

' I have taken the date of the first publication of Lamarck from Isidore Geoffroy 
Saint-Hilaire's ('Hist. Nat. G^n^ralc,' torn. iL p. 405, 1859) excellent history of 
opinion on this subject. In this work a full account is foven of BufTon's conclusions 
on the same subject. It b 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, 
cattle 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 yeari 


In 1 813, 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 distinctly 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 
consequently 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 'Amaryllidacea:' (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 sf>ecies. 

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 


sjjecies, 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 1 83 1 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. In the 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 


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 apparently 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 


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 Brst appeared in their 
respective homes, "he knew not how," or by some process "he knew not 

This address was delivered after the papers by Mr. Wallace and myself 
on the Origin of Species, presently 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 palxontologists 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 app>ears (Ibid., vol. iii. p. 798) is inac- 
curate and without evidence. I also gave some extraas from a corre- 
spondence between Professor Owen and the Editor of the 'Lxmdon 
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 
cither partially or wholly again fallen into error. It is consolatory to me 
that others find Professor Owen's controversial writings as difficult 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 Geoflroy Saint-Hilaire, in his lectures delivered in 1850 
(of which a r^sum^ appeared in the 'Revue et Mag. de Zoolog.,' Jan. 
1851), briefly gives his reason for believing that specific characters "sont 
fix^s, pour chaque esp^, tant qu'elle se pierp^tue au milieu des memes 


circonstances: ils se tnodifaent, si les circonstances ambiantes viennent k 
changer." "En resume, l' observation des animaux sauvages demontre 
d^ja la variabilite limitee des especes. Les experiences sur les animaux 
sauvages devenus domestiques, et sur les animaux domestiques redevenus 
sauvages, la demontrent plus clairctnent encore. Ces memes experiences 
prouvent, de plus, que les differences produites peuvent etre de vaUur 
generiqtte." 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 AfHnity,' the dif&cult 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 

In 1852 M. Naudin, a distinguished botanist, expressly stated, in an 
admirable paper on the Origin of S{x;cies ('Revue Horticole,' p. 102; 
since pardy 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^; fatalitc pour les 
uns; pour les autres, volontd providentielle, dont Taction incessante sur 
les etres vivants determine, a toutes les epoques de I'existence du monde, 
la forme, le volume, et la dur^e de chacun d'eux en raison de sa destin^ 
dans I'ordre de choses dont il fait partie. C'est cette puissance qui 
harmonise chaque membra a I'ensemble, en I'appropriam a la fonction 


qu'il doit remplir dans I'organisme generale de la nature, fonction qui 
est pour lui sa raison d'etre." ' 

In 1853 a celebrated geologist, Count Keyserling ('Bulletin de la Soc. 
Geolog.,' 2nd ser., torn. 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- 

A well-known French botanist, M. Lecoq, writes in 1854 ('Etudes sur 
Geograph. Bot.,' torn. i. p. 250), "On voit que nos recherches sur la 
fixit^ ou la variation de I'espece, nous conduisent directement aux id^s 
^mises, par deux hommes justement celebres, Geoffroy 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 ']o\itnal of the Linnean Society' conuins 
papers, read July ist, 1858, by Mr. Wallace and myself, in which, as 

' From references in Bronn's 'Untcrsuchungen iibcr die EntwickcIunRs-Gesctze,' 
it appears that the celebrated botanist and palzontologist, 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'Esp^e," it scemi 
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. 


stated in the introductory remarks to this volume, the theory of Natural 
Selection is promulgated by Mr. Wallace with admirable force and 

Von Baer, towards whom all zoologists feel so profound a respect, 
expressed about the year 1859 (see 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 sjjecies 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 oppjosed to the general analogy of nature. If, on the 
other hand, we view 'Persistent Types' in relation to that hypothesis 
which supposes the sjiecies 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. 


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 f)erhaps be made out on this question by patiendy 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 exactly 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, 



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 apparendy 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 

In considering the origin of species, it is quite conceivable that a 
naturalist, reflecting on the mutual affinities 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 jusdy 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 structure, 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 prepwsterous 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 vdition 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 


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 
difficulties 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 f)erfected into a highly developed being or into an elab- 
orately constructed organ; secondly, the subject of Instinct, or the mental 
f)owers 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 


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 recently entertained, 
and which I formerly entertained — namely, 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. 



Variation under Domestication 

Causes of variability — Effects 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. 


WHEN 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. 



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- 
cestor. 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 


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, causing 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 


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 af?ected 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 wi^h 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. 



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. Impwrtant 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 jjeculiarities 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<oloured 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 


"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 with 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 unimpKjrtant 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 apjjears, 
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 


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 often 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 Ufe 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 f)eculiarities 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- 
resfxjnding 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 


great difficulty in proving its truth : we may safely conclude that very 
many of the most strongly marked domestic varieties could not f)Os- 
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 pwultry of various breeds, and esculent 
vegetables, for an unUmited number of generations, would be opposed 
to all experience. 


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 the 


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 diffel 
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 difference 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. 


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 
litde 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 animals, 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 


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 
Canidar 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 oppxssition 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 apfjcars 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- 


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 resembUng 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. 


Believing that it 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 


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 outhne almost like that 
(if 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 trumpxjter 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 vertebra! vary in number; as does the number of the ribs, 
together with their relative breadth and the presence of processes. 


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 scutelL-c 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, 
[X)uter, 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- 


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<ivilised 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 Columbidx 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 


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 perfecdy 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 rt 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 


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 supjxjse 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 sp)ecies presenting certain very abnormal char- 
acters, as compared with all other Columbidac, 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. 


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 pxjints 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 ^tgyptian dynasty, about 3000 b.c^ as was 
{X)inted 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. 


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 sjjecies 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 
sp>ecies. 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? 


Let US now briefly consider the steps by which domestic races have 
been produced, either from one or from several allied species. Some 


effect may be attributed to the direct and definite action of the 
external conditions of hfe, 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 seedling. 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 purpKJse; 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 Uttle 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 pwwer 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 jx)wer 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 


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, sf)eaking 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 indispiensable 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 direcdon, 
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 suflScient 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 


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. 


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 encyclopsedia. 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. 


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 


expectation of permanently altering the breed. Nevertheless we 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 Enghsh 
pointer has been greatly changed within the last century, and in this 
case the change has, it is believed, been chiefly efleaed 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 


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 

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 f)ear, 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 


variety, sowing its seeds, and, when a slightly better variety chanced 
to appear, selecting it, and so onwards. But the gardeners of 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 savnges, 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 


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 pwuter 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 apj^jar to explain what has sometimes been noticed, 
namely, that we know hardly anything about the origin or history 


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. 


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- 


ful 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 efTectually 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 sHghtly 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 respea 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 


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 
Geoffroy 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. 


To sum up on the origin of 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. 


Variation Under Nature 

Variability — Individual differences — Doubtful species — Wide ranging, 
much diffused, and common sf>ecies, vary most — Species of the 
larger genera in each country vary more frequendy than the species 
of the smaller genera — Many of the sjjecies of the larger genera 
resemble varieties in being very closely, but unequally, related to each 
other, and in having restricted ranges. 

"ORE 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 sjjeaks 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 

It may be doubted whether sudden and considerable deviations of 
structure such as we occasionally see in our domestic productions, 



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 


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 imporunce 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 faas, that parts which must be called 
important, whether viewed under a physiological or dassificatory 


point of view, sometimes vary in the individuals of the same species. 
I am convinced that the most experienced naturaUst 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 

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 


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 Miiller 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 antenns 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 apf)ears 
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<apsule 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 


common fact that the female produces offspring of two sexes which 
sometimes differ from each other in a wonderful manner. 


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 beUeve 
that many of these doubtful and closely aUied 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 conunonly 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 

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. 


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 Eurof)e, 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-sp)ecies, 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 sf)ecies 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 universally ranked by naturalists as true 
species. Nevertheless, no certain criterion can possibly be given by 


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 
archip)elago, 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. WoUaston'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 naturahsts 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- 


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 likewise 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 arc 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 


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 CandoUe, 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 difTer 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 s^iecies 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 exactly the same relation to each other, as do the four 
or five provisionally admitted species which closely surround the 


typical Quercus robur. Finally, De Candolie 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 strictly to fulfil the definition above given of a 
true species. It should be added that De Candolie 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 paLtontology, 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 difBculty; 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. 


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. 



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 propwrtional 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 


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. 


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 sp>ecies 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 


to the size o£ the genera to which the species belong. Again, plants 
low in the scale o£ organisation are generally much more widely 
difTused 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 looking 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 exp>ect that 
the circumstances would generally be still favourable to variation. 
On the other hand, if we look at each sp)ecies 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, 


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 greatly in size; and that large genera 
have often come to their maxima, decline, and disappeared. All that 
we want to show is, that, where many sf)ecies of a genus have been 
formed, on an average many are still forming; and this certainly 
holds good. 


There are other relations between the sj^ecies 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 


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, little 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 im[X)rtant 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 

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 


Mr. Watson as doubtful species, but which are almost universally 
ranked by British botanists as good and true species. 


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 aprt to be closely, but unequally, allied together, forming 
little clusters round other species. Species very closely allied to other 
species apparently 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 
species 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. 


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 sjiecies of the same genus — The 
relation of organism to organism the most important of all relations. 

EFORE 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 entitled to hold, if the existence of any well-marked vari- 
eties 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 
Again, it may be asked, how is it that varieties, which I have called 



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 slight 
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 Httle 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, evidently 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 on distribution, rarity, abundance, extinction, and vari- 


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. 


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. 


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 Hfe, and during some season 


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 literally not be standing-room for his pro- 
geny. Linna!us 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, 
after 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 
following seasons. Still more striking is the evidence from our 
domestic animals of many kinds which have run wild in several 
parts of the world; if the statements of the rate of increase of slow- 
breeding cattle and horses in South America, and latterly in Australia, 
had not been well authenticated, they would have been incredible. 


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 
thisde, 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 diiTusion 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 
destruaion 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 
of eggs, and another, like the hippobosca, a single one; but this dif- 


ference does not determine how many individuals of 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 the number of the species 
will almost instantaneously increase to any amount. 


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 


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 


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 
suffer 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 comjietitors 
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<apped 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 
gardens 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 in a small tract, epidemics — at least, this 
seems generally to occur with our game animals — often ensue; and 
here we have a limiting check independent of the struggle for life. 
But even some of these so-called epidemics appear to be due to para- 


side 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. 
1 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. 


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 


planted part of 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 the 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 cattle 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; 


and Azara and Rengger have shown that this is caused by the greater 
number in Paraguay of 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 af?ect 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 plants 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. 


Humblebees alone visit red clover, as other bees cannot reach the 
nectar. It has been suggested that moths may fertiUse the clovers; 
but I doubt whether they could do so in the case of the red clover, 
from their weight not being sufficient to depress the wing petals. 
Hence we may infer as highly probable that, if the whole genus of 
humblebees 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 feline 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 prop)ortional 
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 


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. 




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 compnitition for 
food or residence, or from which it 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 
beetle, 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 beetle, the structure of its legs, so well 
adapted for diving, allows it to compete with other aquatic insects. 


to huiM for its own prey, and to escape serving as prey to other 

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 Ufe 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, 


as it is difScult 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 Ufe 
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. 


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. 

HOW 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 not 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 



what infinitely varied diversities of structure might be of use to 
each being under changing conditions of Hfe. Can it, then, be 
thought improbable, seeing that variations useful to man have un- 
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 
pxjssibly 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 


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 Uttle 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 change, 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 


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 hve, 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 p)ossession 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 


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 

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 life 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 hyf)othesis 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 


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 f)eaches far 
more than those with other coloured flesh. If, with all the aids of 
art, these slight differences make a great difTerence 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- 
impxjrtant, 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. 


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 animal's 
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 


the beak of 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 Uttle 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 effi- 
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. 


Inasmuch as peculiarities often appear under domestication in one 
sex and become hereditarily attached to that sex, so no doubt it will 


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 


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 partly 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 life, 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. 



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 frequently 
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 


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 
oflspring, 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. Supf)osing 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 slightly 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 


without the aid of 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 Leguminosz, 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 consequently would have the best chance 
of flourishing and surviving. The plants which produced flowers 
with the largest glands or nectaries, excreting most nectar, would 


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 full-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 


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 dioeciously 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 little 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 
incarnatum) 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 


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 


selection banish the belief of the continued creation of new organic 
beings, or of any great and sudden modification in their structure. 


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 reproduaion 
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 1 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 00 
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 
jjerpetuity 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 


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 fertiUty is greatly diminished if these visits be prevented. 
Now, it is scarcely f>ossible 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 suflicient, 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 (xjssible to raise pure seedlings, so largely do they naturally 
cross. In numerous other cases, far from self-fertilisation being 
favoured, there are sp)ecial contrivances which effectually prevent the 
stigma receiving jxillen 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 


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 
Sprengcl, 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 


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 

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 2^aland, 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 fertihsing 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 


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. 


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 beUeve, 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 ? 


In the case of 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 little 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 


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<ontinued 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 and 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 


improved at a slow rate; and this may sometimes be of much im- 
portance. If, however, an isolated area be very small, either from 
being 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- 

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 Ufe 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 


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- 
p)etitors, 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 imjx)rtant 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- 
ropxo-Asiatic area. Thus, also, it is that continental productions 
have everywhere become so largely naturalised on islands. On a 
small island, the race for life 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 


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. 1 conclude that for ter- 
restrial productions a large continental area, which has undergone 
many oscillations of level, will 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 jxiHty 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 


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. 


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 


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 sjjecies 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 



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 1 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 


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-estabhshed 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 


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. Consequently, 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 
exacdy 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 


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 

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 


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. 


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 


a'* 1'* />'* 

b'-t f<* 


i'* m"* 

a'" f'o 

m"> E" 

>W \t^ 


>w 4/^* 

k^ I' '-Is^Jm* 

\U' .}:jjr ''••}/>»' 

a'^!/ %' 

Hk* \!-4* 

■••A' Mivw* 

A' '•■':■■■ 

(-* '''--Wfn-f 


j' Hv'w^ i 

'■,!// 1 i 


'/: i : 



\ \ 




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 


/«'* F "^ n'4 r'-f iv"f y^ y'4 z'4 









1 \ \ i / \ ^ 


; 1 \ i / \ 



E" F"' w'" z'° 

\ tu9\i/ --.i/'i' 

j u' T^^''U-' '^ij/i^ 

i u'\/ ti>'--.^i/ \!^i' 

1 ! UO^ \^ 

' 1 \./ .•/ , 

i i ■■•••-. ../" 

\ j t^ -^jA' 

\ ! i \ / . / 

\ i 1 Mi' i 

i \ \ \ ! ,' 1 
: ! : \ \ 1 ; .' I J 

• i 1 •«•■ i 1 

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 would be thought worthy of record in a 
systematic work. 


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 a' and mK 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 tAvo 
varieties, being only slightly 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 

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 a' is supposed in the 
diagram to have produced variety a', which will, owing to the prin- 
ciple of divergence, differ more from (A) than did variety a'. 
Variety w' is supposed to have produced two varieties, namely m* 
and /, 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 simpHfied 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 


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 sufEciendy 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- 

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 a' to a'". In the same 
way the English race horse and English pointer have apparently 
both gone on slowly diverging in character from their original stocks, 
without either having given off any fresh branches or races. 

After ten thousand generations, sp)ecies (A) is supposed to have 
produced three forms, a'", /"* and m^", which, from having diverged 
in character during the successive generations, will have come to 
differ largely, birt perhaps unequally, from each other and from their 


common parent. If we suppose the amount of change between each 
horizontal Une 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 sp)ecies. Thus the diagram illustrates the 
steps by which the small dilTerences 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 a" and m", 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 z'°) 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 n" to 2", 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 


exterminate in each stage of 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 earlier varieties 
will have become extinct, being replaced by eight new species (a" 
to /»"); and species (I) will be replaced by six («'* to 2") new 

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 sf)ecies 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 


to the other nine original species, has transmitted descendants to this 
late stage of 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 a" and z" 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 a'*, 
q^*, p'*, will be nearly related from having recently branched off 
from a'"; i", and /", from having diverged at an earlier period from 
a', will be in some degree distinct from the three first-named species; 
and lastly, o", P* and m" 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 

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 


new species f", 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 a** to />", 
those marked ^" and /", and those marked o" to w", 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 descended from two species of the original genus, and 
these are supposed to be descended from some still more ancient and 
unknown form. 


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 
sp)ecies 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 poUty 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 sf)ecies 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 the 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. 



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 Uving 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 


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; 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 Bt 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 

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 believed 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 Htde 
improved, and might remain for indefinite ages in their present lowly 


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 

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 o£ 
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 Miiller, 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 


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. 


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 


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, independently 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 


produced. We do not know that even the most prolific area is fully 
stocked with specific forms: at the Caf)e 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 greatly or at all increased. What then 
checks an indefinite increase in the number of sp>ecies ? The amount 
of life (I do not mean the number of specific forms) supported on 
an area must have a Hmit, 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 sp)ecific 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 


number. How much weight to attribute to these several considera- 
tions I will not pretend to say; but conjointly they must limit in each 
coimtry the tendency to an indefinite augmentation of specific forms. 


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, owang 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 

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 


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 selertion, 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 sp)ecies, 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 sjjecies, 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, sp)ecies of distinct genera much less 
closely related, and genera related in different degrees, forming sub- 
families, families, orders, sub-classes, and classes. The several sub- 
ordinate groups in any class cannot be ranked in a single file, but 
seem clustered round points, and these round other points, and so on 
in almost endless cycles. If species had been independently created, 
no explanation would have been possible of this kind of classification; 


but it is explained through inheritance and the complex action of 
natural selection, entailing extinction and divergence of character, 
as we have seen illustrated in the diagram. 

The affinities of all the beings of the same class have sometimes 
been represented by a great tree. 1 believe this simile largely speaks 
the truth. The green and budding twigs may represent existing 
species; and those produced during former years may represent the 
long succession of extinct sp)ecies. At each period of growth all the 
growing twigs have tried to branch out on all sides, and to overtop 
and kill the surrounding twigs and branches, in the same manner as 
species and groups of species have at all times overmastered other 
species in the great battle for life. The limbs divided into great 
branches, and these into lesser and lesser branches, were themselves 
once, when the tree was young, budding twigs; and this connection 
of the former and present buds, by ramifying branches, may well 
represent the classification of all extinct and living species in groups 
subordinate to groups. Of the many twigs which flourished when 
the tree was a mere bush, only two or three, now grown into great 
branches, yet survive and bear the other branches; so with the species 
which lived during long-past geological periods, very few have left 
living and modified descendants. From the first growth of the tree, 
many a limb and branch has decayed and dropped off; and these 
fallen branches of various sizes may represent those whole orders, 
families, and genera which have now no living representatives, and 
which are known to us only in a fossil state. As we here and there 
see a thin straggling branch springing from a fork low down in a 
tree, and which by some chance has been favoured and is still alive 
on its summit, so we occasionally see an animal like the Ornitho- 
rhynchus or Lepidosiren, which in some small degree connects by 
its affinities two large branches of life, and which has apparently been 
saved from fatal comf)etition by having inhabited a protected station. 
As buds give rise by growth to fresh buds, and these, if vigorous, 
branch out and overtop on all sides many a feebler branch, so by 
generation I believe it has been with the great Tree of Life, which 
fills with its dead and broken branches the crust of the earth, and 
covers the surface with its ever-branching and beautiful ramifications. 

Laws of Variation 

EHects of changed conditions — Use and disuse, combined with natural 
selection; organs of flight and of vision — Acclimatisation — Corre- 
lated variation — Compensation and economy of growth — False corre- 
lations — Multiple, rudimentary, and lowly organised structures 
variable — Parts developed in an unusual manner are highly variable; 
specific characters more variable than generic: secondary sexual 
characters variable — Species of the same genus vary in an analogous 
manner — Reversions to long-lost characters — Summary. 

I HAVE hitherto sometimes spoken as if the variations — so com- 
mon and multiform with organic beings under domestication, 
and in a lesser degree with those under nature — were due to 
chance. This, of course, is a wholly incorrect expression, but it 
serves to acknowledge plainly our ignorance of the cause of each 
particular variation. Some authors believe it to be as much the func- 
tion of the reproductive system to produce individual differences, or 
slight deviations of structure, as to make the child like its parents. 
But the fact of variations and monstrosities occurring much more 
frequently under domestication than under nature, and the greater 
variability of species having wide ranges than of those with restricted 
ranges, lead to the conclusion that variability is generally related to 
the conditions of life to which each sjjecies has been exposed during 
several successive generations. In the first chapter I attempted to 
show that changed conditions act in two ways, directly on the whole 
organisation or on certain parts alone, and indirectly through the 
reproductive system. In all cases there are two factors, the nature 
of the organism, which is much the most important of the two, and 
the nature of the conditions. The direct action of changed conditions 
leads to definite or indefinite results. In the latter case the organisa- 
tion seems to become plastic, and we have much fluctuating varia- 
bility. In the former case the nature of the organism is such that it 
yields readily, when subjected to certain conditions, and all, or nearly 
all the individuals become modified in the same way. 



It is very difficult to decide how far changed conditions, such as 
of climate, food, etc., have acted in a definite manner. There is 
reason to believe that in the course of time the effects have been 
greater than can be proved by clear evidence. But we may safely 
conclude that the innumerable complex coadaptations of structure, 
which we see throughout nature between various organic beings, 
cannot be attributed simply to such action. In the following cases 
the conditions seem to have produced some slight definite effect: 
E. Forbes asserts that shells at their southern limit, and when living 
in shallow water, are more brightly coloured than those of the same 
species from further north or from a greater depth; but this certainly 
does not always hold good. Mr. Gould believes that birds of the same 
species are more brightly coloured under a clear atmosphere, than 
when living near the coast or on islands; and Wollaston is con- 
vinced that residence near the sea affects the colours of insects. 
Moquin-Tandon gives a list of plants which, when growing near the 
seashore, have their leaves in some degree fleshy, though not else- 
where fleshy. These slightly varying organisms are interesting in as 
far as they present characters analogous to those possessed by the 
species which are confined to similar conditions. 

When a variation is of the slightest use to any being, we cannot 
tell how much to attribute to the accumulative action of natural 
selection, and how much to the definite action of the conditions of 
life. Thus, it is well known to furriers that animals of the same 
species have thicker and better fur the further north they live; but 
who can tell how much of this difference may be due to the warmest 
clad individuals having been favoured and preserved during many 
generations, and how much to the action of the severe climate.' For 
it would appear that climate has some direct action on the hair of our 
domestic quadrupeds. 

Instances could be given of similar varieties being produced from 
the same species under external conditions of life as different as can 
well be conceived; and, on the other hand, of dissimilar varieties be- 
ing produced under apparently the same external conditions. Again, 
innumerable instances are known to every naturalist, of species keep- 
ing true, or not varying at all, although hving under the most oppo- 
site climates. Such considerations as these incline me to lay less 


weight on the direa action of the surrounding conditions, than on a 
tendency to vary, due to causes of which we are quite ignorant. 

In one sense the conditions of Ufe may be said, not only to cause 
variability, either directly or indirectly, but likewise to include natural 
selection, for the conditions determine whether this or that variety 
shall survive. But when man is the selecting agent, we clearly see 
that the two elements of change are distinct; variability is in some 
manner excited, but it is the will of man which accumulates the 
variations in certain directions; and it is this latter agency which 
answers to the survival of the fittest under nature. 


From the facts alluded to in the first chapter, I think there can 
be no doubt that use in our domestic animals has strengthened and 
enlarged certain parts, and disuse diminished them; and that such 
modifications are inherited. Under free nature, we have no standard 
of comparison, by which to judge of the effects of long-continued 
use or disuse, for we know not the parent-forms; but many animals 
jwssess structures which can be best explained by the effects of disuse. 
As Professor Owen has remarked, there is no greater anomaly in 
nature than a bird that cannot fly; yet there are several in this state. 
The logger-headed duck of South America can only flap along the 
surface of the water, and has its wings in nearly the same condition 
as the domestic Aylesbury duck: it is a remarkable fact that the 
young birds, according to Mr. Cunningham, can fly, while the adults 
have lost this power. As the larger ground-feeding birds seldom take 
flight except to escape danger, it is probable that the nearly wingless 
condition of several birds, now inhabiting or which lately inhabited 
several oceanic islands, tenanted by no beasts of prey, has been 
caused by disuse. The ostrich indeed inhabits continents, and is 
exposed to danger from which it cannot escape by flight, biH it can 
defend itself by kicking its enemies, as efficiently as many quadru- 
peds. We may believe that the progenitor of the ostrich genus had 
habits like those of the bustard, and that, as the size and weight of its 
body were increased during successive generations, its legs were used 
more, and its wings less, until they became incapable of flight. 


Kirby has remarked (and I have observed the same fact) that 
the anterior tarsi, or feet, of many male dung-feeding beetles are 
often broken off; he examined seventeen specimens in his own col- 
lection, and not one had even a relic left. In the Onites apelles the 
tarsi are so habitually lost, that the insect has been described as not 
having them. In some other genera they are present, but in a rudi- 
mentary condition. In the Ateuchus or sacred beetle of the Egyp- 
tians, they are totally deficient. The evidence that accidental mutila- 
tions can be inherited is at present not decisive; but the remarkable 
cases observed by Brown-Sequard in guinea pigs, of the inherited 
effects of operations, should make us cautious in denying this 
tendency. Hence it will perhaps be safest to look at the entire ab- 
sence of the anterior tarsi in Ateuchus, and their rudimentary con- 
dition in some other genera, not as cases of inherited mutilations, 
but as due to the effects of long-continued disuse; for as many dung- 
feeding beetles are generally found with their tarsi lost, this must 
happen early in life; therefore the tarsi cannot be of much im- 
portance or be much used by these insects. 

In some cases we might easily put down to disuse modifications 
of structure which are wholly, or mainly, due to natural selection. 
Mr. Wollaston has discovered the remarkable fact that 200 beetles, 
out of the 550 species (but more are now known) inhabiting Madeira, 
are so far deficient in wings that they cannot fly; and that, of the 
twenty-nine endemic genera, no less than twenty-three have all their 
species in this condition! Several facts, — namely, that beetles in many 
parts of the world are frequently blown to sea and perish; that the 
beetles in Madeira, as observed by Mr. Wollaston, lie much con- 
cealed, until the wind lulls and the sun shines; that the proportion of 
wingless beetles is larger on the expwsed Desertas than in Madeira 
itself; and especially the extraordinary fact, so strongly insisted on 
by Mr. Wollaston, that certain large groups of beetles, elsewhere 
excessively numerous, which absolutely require the use of their 
wings, are here almost entirely absent; — these several considerations 
make me believe that the wingless condition of so many Madeira 
beetles is mainly due to the action of natural selection, combined 
probably with disuse. For during many successive generations each 
individual beetle which flew least, either from its wings having been 


ever so little less perfectly developed or from indolent habit, will 
have had the best chance of surviving from not being blown out to 
sea; and, on the other hand, those beetles which most readily took 
to flight would oftenest have been blown to sea, and thus destroyed. 

The insects in Madeira which are not ground-feeders, and which, 
as certain flower-feeding coleoptera and lepidoptera, must habitually 
use their wings to gain their subsistence, have, as Mr. Wollaston 
suspects, their wings not at all reduced, but even enlarged. This is 
quite compatible with the action of natural selection. For when a 
new insect first arrived on the island, the tendency of natural selec- 
tion to enlarge or to reduce the wings, would def)end on whether a 
greater number of individuals were saved by successfully batding 
with the winds, or by giving up the attempt and rarely or never 
flying. As with mariners shipwrecked near a coast, it would have 
been better for the good swimmers if they had been able to swim still 
further, whereas it would have been better for the bad swimmers if 
they had not been able to swim at all and had stuck to the wreck. 

The eyes of moles and of some burrowing rodents are rudimentary 
in size, and in some cases are quite covered by skin and fur. This 
state of the eyes is probably due to gradual reduction from disuse, 
but aided perhaps by natural selection. In South America, a burrow- 
ing rodent, the tucutucu, or Ctenomys, is even more subterranean in 
its habits than the mole; and 1 was assured by a Spaniard, who had 
often caught them, that they were frequently blind. One which I 
kept aUve was certainly in this condition, the cause, as appeared on 
dissection, having been inflammation of the nictitating membrane. 
As frequent inflammations of the eyes must be injurious to any 
animal, and as eyes are certainly not necessary to animals having 
subterranean habits, a reduction in their size, with the adhesion of 
the eyelids and growth of fur over them, might in such case be an 
advantage; and if so, natural selection would aid the effects of disuse. 

It is well known that several animals, belonging to the most 
different classes, which inhabit the caves of Carniola and of Ken- 
tucky, are blind. In some of the crabs the footstalk for the eye 
remains, though the eye is gone; — the stand for the telescope is there, 
though the telescope with its glasses has been lost. As it is difficult 
to imagine that eyes, though useless, could be in any way injurious 


to animals living in darkness, their loss may be attributed to disuse. 
In one of the blind animals, namely, the cave rat (Neotoma), two of 
which were captured by Professor Silliman at above half a mile 
distance from the mouth of the cave, and therefore not in the pro- 
foundest depths, the eyes were lustrous and of large size; and these 
animals, as I am informed by Professor Silliman, after having been 
exposed for about a month to a graduated light, acquired a dim 
perception of objects. 

It is difficult to imagine conditions of Ufe more similar than 
deep limestone caverns under a nearly similar climate; so that, in 
accordance with the old view of the blind animals having been 
separately created for the American and European caverns, very close 
similarity in their organisation and affinities might have been ex- 
pected. This is certainly not the case if we look at the two whole 
faunas; and with respect to the insects alone, Schiodte has remarked, 
"We are accordingly prevented from considering the entire phe- 
nomenon in any other light than something purely local, and the 
similarity which is exhibited in a few forms between the Mammoth 
cave (in Kentucky) and the caves in Carniola, otherwise than as a 
very plain expression of that analogy which subsists generally be- 
tween the fauna of Europe and of North America." On my view 
we must suppose that American animals, having in most cases ordi- 
nary powers of vision, slowly migrated by successive generations from 
the outer world into the deeper and deeper recesses of the Kentucky 
caves, as did European animals into the caves of Europe. We have 
some evidence of this gradation of habit; for, as Schiodte remarks, 
"We accordingly look upon the subterranean faunas as small rami- 
fications which have penetrated into the earth from the geographi- 
cally limited faunas of the adjacent tracts, and which, as they ex- 
tended themselves into darkness, have been accommodated to sur- 
rounding circumstances. Animals not far remote from ordinary 
forms, prepare the transition from light to darkness. Next follow 
those that are constructed for twilight; and, last of all, those destined 
for total darkness, and whose formation is quite peculiar." These 
remarks of Schiodte's, it should be understood, apply not to the 
same, but to distinct species. By the time that an animal had reached, 
after numberless generations, the deepest recesses., disuse will on 


this view have more or less perfectly obliterated its eyes, and natural 
selection will often have effeaed other changes, such as an increase 
in the length of the antennae or palpi, as a compensation for blindness. 
Notwithstanding such modifications, we might expect still to see in 
the cave animals of America, affinities to the other inhabitants of that 
continent, and in those of Europe to the inhabitants of the European 
continent. And this is the case with some of the American cave 
animals, as I hear from Professor Dana; and some of the European 
cave insects are very closely allied to those of the surrounding country. 
It would be difficult to give any rational explanation of the affinities 
of the blind cave animals to the other inhabitants of the two con- 
tinents on the ordinary view of their independent creation. That 
several of the inhabitants of the caves of the Old and New Worlds 
should be closely related, we might expect from the well-known 
relationship of most of their other productions. As a blind species 
of Bathyscia is found in abundance on shady rocks far from caves, 
the loss of vision in the cave species of this one genus has probably 
had no relation to its dark habitation; for it is natural that an insect 
already deprived of vision should readily become adapted to dark 
caverns. Another blind genus (Anophthalmus) offers this remark- 
able peculiarity, that the species, as Mr. Murray observes, have not 
as yet been found anywhere except in caves, yet those which inhabit 
the several caves of Eurof)e and America are distinct; but it is possible 
that the progenitors of these several species, whilst they were fur- 
nished with eyes, may formerly have ranged over both continents, 
and then have become extinct, excepting in their present secluded 
abodes. Far from feeling surprise that some of the cave animals 
should be very anomalous, as Agassiz has remarked in regard to the 
blind fish, the Amblyopsis, and as is the case with the blind Proteus 
with reference to the reptiles of Europe, I am only surprised that 
more wrecks of ancient life have not been preserved, owing to the 
less severe competition to which the scanty inhabitants of these dark 
abodes will have been exposed. 


Habit is hereditary with plants, as in the period of flowering, in 
the time of sleep, in the amount of rain requisite for seeds to germi- 


nate, etc^ and this leads me to say a few words on acclimatisation. 
As it is extremely common for distinct species belonging to the same 
genus to inhabit hot and cold countries, if it be true that all the 
species of the same genus are descended from a single parent-form, 
acclimatisation must be readily effected during a long course of 
descent. It is notorious that each species is adapted to the chmate of 
its own home: species from an arctic or even from a temperate region 
cannot endure a tropical climate, or conversely. So again, many 
succulent plants cannot endure a damp climate. But the degree of 
adaptation of species to the climates under which they live is often 
overrated. We may infer this from our frequent inability to predict 
whether or not an imported plant will endure our climate, and 
from the number of plants and animals brought from different 
countries which are here perfectly healthy. We have reason to believe 
that species in a state of nature are closely limited in their ranges 
by the competition of other organic beings quite as much as, or 
more than, by adaptation to particular climates. But whether or not 
this adaptation is in most cases very close, we have evidence with 
some few plants, of their becoming, to a certain extent, naturally 
habituated to different temf)eratures; that is, they become accli- 
matised; thus the pines and rhododendrons, raised from seed col- 
lected by Dr. Hooker from the same species growing at different 
heights on the Himalaya, were found to possess in this country 
different constitutional powers of resisting cold. Mr. Thwaites in- 
forms me that he has observed similar facts in Ceylon; analogous 
observations have been made by Mr. H. C. Watson on European 
species of plants brought from the Azores to England; and I could 
give other cases. In regard to animals, several authentic instances 
could be adduced of species having largely extended, within historical 
times, their range from warmer to cooler latitudes, and conversely; 
but we do not positively know that these animals were stricdy 
adapted to their native climate, though in all ordinary cases we 
assume such to be the case; nor do we know that they have subse- 
quently become specially acclimatised to their new homes, so as to 
be better fitted for them than they were at first. 

As we may infer that our domestic animals were originally chosen 
by uncivilised man because they were useful and because they bred 


readily under confinement, and not because they were subsequently 
found capable of far-extended transportation, the common and ex- 
traordinary capacity in our domestic animals of not only withstand- 
ing the most different climates, but of being perfectly fertile (a far 
severer test) under them, may be used as an argument that a large 
proportion of other animals now in a state of nature could easily be 
brought to bear widely different climates. We must not, however, 
push the foregoing argument too far, on account of the probable 
origin of some of our domestic animals from several wild stocks; the 
blood, for instance, of a tropical and arctic wolf may perhaps be 
mingled in our domestic breeds. The rat and mouse cannot be con- 
sidered as domestic animals, but they have been transported by man 
to many parts of the world, and now have a far wider range than 
any other rodent; for they live under the cold climate of Faroe in 
the north and of the Falklands in the south, and on many an island 
in the torrid zones. Hence adaptation to any special climate may be 
looked at as a quality readily grafted on an innate wide flexibility 
of constitution, common to most animals. On this view, the capacity 
of enduring the most different climates by man himself and by his 
domestic animals, and the fact of the extinct elephant and rhinoceros 
having formerly endured a glacial climate, whereas the living species 
are now all tropical or sub-tropical in their habits, ought not to be 
looked at as anomalies, but as examples of a very common flexi- 
bility of constitution, brought, under peculiar circumstances, into 

How much of the acclimatisation of species to any peculiar cli- 
mate is due to mere habit, and how much to the natural selection of 
varieties having different innate constitutions, and how much to 
both means combined, is an obscure question. That habit or cus- 
tom has some influence, I must believe, both from analogy and from 
the incessant advice given in agricultural works, even in the ancient 
encyclopaedias of China, to be very cautious in transporting animals 
from one district to another. And as it is not likely that man should 
have succeeded in selecting so many breeds and sub-breeds with con- 
stitutions specially fitted for their own districts, the result must, I 
think, be due to habit. On the other hand, natural selection would 
inevitably tend to preserve those individuals which were born with 


constitutions best adapted to any country which they inhabited. In 
treatises on many kinds of cultivated plants, certain varieties are said 
to withstand certain climates better than others; this is strikingly 
shown in works on fruit trees pubUshed in the United States, in 
which certain varieties are habitually recommended for the northern 
and others for the southern States; and as most of these varieties are 
of recent origin, they cannot owe their constitutional differences to 
habit. The case of the Jerusalem artichoke, which is never propagated 
in England by seed, and of which consequently new varieties have 
not been produced, has even been advanced, as proving that ac- 
climatisation cannot be effected, for it is now as tender as ever it was! 
The case, also, of the kidney bean has been often cited for a similar 
purpose, and with much greater weight; but imtil someone will sow, 
during a score of generations, his kidney beans so early that a very 
large proportion are destroyed by frost, and then collect seed from the 
few survivors, with care to prevent accidental crosses, and then again 
get seed from these seedlings, with the same precautions, the experi- 
ment cannot be said to have been tried. Nor let it be supposed that 
differences in the constitution of seedling kidney beans never appear, 
for an account has been published how much more hardy some seed- 
hngs are than others; and of this fact I have myself observed striking 

On the whole, we may conclude that habit, or use and disuse, have, 
in some cases, played a considerable part in the modification of the 
constitution and structure; but that the effects have often been largely 
combined with, and sometimes overmastered by, the natural selection 
of innate variations. 


I mean by this expression that the whole organisation is so tied to- 
gether, during its growth and development, that when slight varia- 
tions in any one part occur, and are accumulated through natural se- 
lection, other parts become modified. This is a very important 
subject, most imperfectly understood, and no doubt wholly different 
classes of facts may be here easily confounded together. We shall 
presently see that simple inheritance often gives the false appearance 
of correlation. One of the most obvious real cases is, that variations 


of Structure arising in the young or larvx naturally tend to affect the 
structure of the mature animal. The several parts of the body which 
are homologous, and which, at an early embryonic jseriod, are identi- 
cal in structure, and which are necessarily exposed to similar con- 
ditions, seem eminently liable to vary in a like manner: we see this 
in the right and left sides of the body varying in the same manner; in 
the front and hind legs, and even in the jaws and limbs, varying 
together, for the lower jaw is believed by some anatomists to be 
homologous with the limbs. These tendencies, I do not doubt, may 
be mastered more or less completely by natural selection; thus a 
family of stags once existed with an antler only on one side; and if 
this had been of any great use to the breed, it might probably have 
been rendered permanent by selection. 

Homologous parts, as has been remarked by some authors, tend to 
cohere; this is often seen in monstrous plants: and nothing is more 
common than the union of homologous parts in normal structures, 
as in the union of the jjetals into a tube. Hard parts seem to affect 
the form of adjoining soft parts; it is believed by some authors that 
with birds the diversity in the shape of the pelvis causes the remark- 
able diversity in the shape of their kidneys. Others believe that the 
shape of the pelvis in the human mother influences by pressure the 
shape of the head of the child. In snakes, according to Schlegel, the 
form of the body and the manner of swallowing determine the posi- 
tion and form of several of the most important viscera. 

The nature of the bond is frequently quite obscure. M. Is. Geoffroy 
St. Hilaire has forcibly remarked, that certain malcon formations 
frequently, and that others rarely, co-exist, without our being able 
to assign any reason. What can be more singular than the relation 
in cats between complete whiteness and blue eyes with deafness, or 
between the tortoise-shell colour and the female sex; or in pigeons be- 
tween their feathered feet and skin betwixt the outer toes, or between 
the presence of more or less down on the young pigeon when first 
hatched, with the future colour of its plumage; or, again, the rela- 
tion between the hair and teeth in the naked Turkish dog, though 
here no doubt homology comes into play? With respect to this latter 
case of correlation, I think it can hardly be accidental, that the two 
orders of mammals which are most abnormal in their dermal cover- 


ings, viz^ Cetacea (whales) and Edentata (armadilloes, scaly ant- 
eaters, etc.), are likewise on the whole the most abnormal in their 
teeth; but there are so many exceptions to this rule, as Mr. Mivart 
has remarked, that it has little value. 

I know of no case better adapted to show the importance of the 
laws of correlation and variation, independently of utility and there- 
fore of natural selection, than that of the difTerence between the 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 Composita: 
countenances this idea; but with the Umbelliferaj, it is by no means, 
as Dr. Hooker informs me, the sfjecies 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 Composite 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 pietals, 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 so, natural selection may have come into play. But 


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 Umbellifera: 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 apparently 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 httle better 
adapted to be wafted by the wind, gain an advantage over others less 
well fitted for wide dispersal. 


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 sp)end 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 difficult 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 


the seeds in our fruits become atrophied, the fruit itself gains largely 
in size and quality. In our pwultry, 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 
cirrif)ede 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 antennae. 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 

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 


changed habits, superfluous, without by any means causing some 
other part to be 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. 


It seems to be a rule, as remarked by Is. Geoflroy 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 f)olyandrous 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 that 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. 



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 1 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- 


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 sjjecies 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 little, 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 

When we see any part or organ develof)ed 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 sfjecies, 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 pigeon, and see what a 


prodigious amount of difference there is in the beaks of tumblers, in 
the beaks and wattle 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 


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 the 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 


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 sf)ecies 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 fKjint in the chapter on Classification. It would be almost super- 
fluous to adduce evidence in suppwrt 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. 


Geoffroy 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 o£ each sp)ecies having been independently 
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 explanation 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 allied 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 sf)ecies 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 


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 sf>ecies 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 


difference 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 

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 spwcies 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 Engidac, 
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 


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 variabihty 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 Hkely 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. 


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 


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 fantail. 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 sf)ecies; 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 


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 


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<rowned, 
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 

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 


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 outline. 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 strijjed 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 resp)ect 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 strif)e 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 strif)es is not 
considered as purely bred. The spine is always striped; the legs are 
generally barred, and the shoulder strifje, which is sometimes double 
and sometimes treble, is common; the side of the face, moreover, is 
sometimes striped. The strijjes 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 


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 p)onies, 
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 
Mr. Gosse, in certain parts of the United States, about nine out of 
ten mules have stripjed 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 
case, 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 


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 strip)ed 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, sjje- 
cies; and how exactly parallel is the case with that of the species of 
the horse-genus! For myself, I venture confidently to look back thou- 


sands on thousands of generations, and I see an animal striped like 
a zebra, but perhaps otherwise very differently 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 

He who believes that each equine species was independently 
created, will, I presume, assert that each sp)ecies 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. 


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 


saved without detriment will be saved. Changes of structure at an 
early age may affect parts subsequently 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 dif?er 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 sp)ecies 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 sf)ecies, and sp)ecific differences to the several sjjecies 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 sp)ecies, 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- 


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 develof)ed. 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 

Whatever the cause may be of each slight difference 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. 

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 different 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 Tyf)e and of the Conditions of Existence embraced by the 
theory of Natural Selection. 

10NG before the reader has arrived at this part of my work, a 
crowd of difficulties will have occurred to him. Some o£ 
-^ 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 sf)ecies have descended from other sp>ecies 
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 



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. 


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 parei>t 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 imperfectly 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 sp>ecies, 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- 


fication, each has become adapted to the conditions of hfe 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, 
because 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 sjiecies, 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 abruptly 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 


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 comf)etition; and as these sf)ecies are 
already defined objects, not blending one into another by insensible 
gradations, the range of any one sf)ecies, depending as it does on 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 differ 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. WoUaston, 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 


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 


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 sf)ecies 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 strictly 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- 


gate, present more varieties, and thus be further improved through 
natural selection and gain further advantages. 

Lastly, looking 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. Consequently 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. 


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 Mustela 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 difficult to answer. 
Yet I think such difficulties have little 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 sjjecies. 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. 


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<alled 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<alled 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 perfectly gliding squirrels; each grade of 
structure having been useful to its pxDssessor. 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 


greatly lengthened by natural selection; and this, as far as the organs 
of 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, which now glide far through the air, slightly 
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 flight, 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 


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, Hke 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 
hke 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 


those proper to their species and to the other species of the same 
genus, we might exjject 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 woodfjecker 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, sufficiently stifl 
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 in 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, 


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 correspHDnding 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 
scoofjed 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 


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. 


To suppose that the eye with all its inimitable contrivances for 
adjusting the focus to different distances, for admitting different 
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 which 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 


look 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<ells 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 Miiller 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 


respect to the wide, diversified, and graduated range of structure 
in the eyes of the lower animals; and when we bear in mind how 
small the number of all living 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 pxjssessed 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 
shght 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 little 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 


the 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 in<o 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 supjwse 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? 



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 extinrtion. 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 greatly 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 branchiz 


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 distinct 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- 

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 1 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 branchii 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- 


pose: for instance, Landois has shown that the wings of insects 
are developed from the trachex; 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 branchix, 
the whole surface of the body and of the sack, together with the 
small frena, serving for respiration. The Balanidz 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 branchix. Now I think no one will dispute 
that the ovigerous frena in the one family are strictly homologous 
with the branchii 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 branchix, 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 suffered far more extinction than have sessile cirri- 
pedes, who would ever have imagined that the branchix 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 pxjwer became thoroughly well developed in 
a species, it seems probable that the adult stage of development 


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 shap)e with advancing years; with crustaceans not only many 
trivial, but some important parts assume a new character, as recorded 
by Fritz Miiller, 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. 


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 little electricity even when the animal is 


greatly 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 little 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 develof>ed. 

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 exjiected 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, 


according to Pacini, in the process or means by which the elearicity 
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 difficulty 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 sp)ecial 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 


of the points of difference. The crystalline lens in the higher cuttle- 
fish consists of two parts, placed one behind the other like 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 Miiller, 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-impKjrtant 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. 


Fritz Miiller argues that this close similarity in so many points 
of 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 sf^ecies 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. Miiller was thus led carefully to examine 
the apparatus in the air-breathing sf)ecies; 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 intelligible, and might 
even have been expected, on the supposition that species belonging 
to distinct families had slowly become adapted to live 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. Consequently 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 argunxent seems to have had great weight 
in leading Fritz Miiller to accept the views maintained by me in this 

Another distinguished zoologist, the late Professor Claparede, 
has argued in the same manner, and has arrived at the same result. 
He shows that there are parasitic mites (Acaridjc), 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 maxillae 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, 


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, biK 
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 
nimiber of contrivances, all for the same purpose and effected in 
essentially the same manner, birt 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 neady 


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 Qjry- 
anthcs. 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 spKJut 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 ofl 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 prollen-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 sp)out, and rub against the properly placed viscid f)ollen- 
masses and the viscid stigma. 
The construction of the flower in another closely allied orchid, 


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 pwllen, 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 Ufe. 

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 sf)ecial 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." 


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. 


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 purpxjse 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- 


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<alled 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 


that there were many black and pied kinds, I dare say that we should 
have thought that the 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 


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 shaf)e, 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 sjjeculate on the relative imf)ortance 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. 


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 


of 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 sjjecified, 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- 
quently, 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 living 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 utiUty, 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, scxalled spontaneous variations, and the com- 
plex laws of growth; but with these important exceptions, we 
may conclude that the structure of every Uving creature either 
now is, or was formerly, of some direct or indirect use to its pos- 

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. 


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 diatomaceae: 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 fertiUsed 
by the wind it never has a gaily<oloured corolla. Several plants 
habitually produce two 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<oloured 
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 


be coloured of any brilliant tint, or rendered conspicuous by being 
white or black. 

On the other hand, I wiUingly 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 difficulty 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 ovif)ositor of the ichneumon, 
by which its eggs are deposited in the Uving 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 history 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; 


but some authors suppose that at the same time it is furnished with 
a rattle 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 Eurojje. 
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 


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: the law oh 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 difficuhies 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 indep)endent 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 formed 
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. 


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 jjerfection 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 serving 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 sfjecies 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), 


though it has become of such small importance that it could not, 
in its present state, have been acquired by means of natural selec- 

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 stricdy 

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 


cases by the increased use or disuse of parts, being affected by the 
direct action of the external conditions of Hfe, 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. 


Miscellaneous Objections to the Theory of Natural Selection 

Longevity — Modifications not necessarily simultaneous — Modifications 
apparently of no direct service — Progressive developnient — Charac- 
ters of 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 all 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 



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 palarontologist, 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 slightly 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, 


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<alled 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 


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 
shaf>e 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 will 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 
growth. In the third place, we have to allow for the direct and 
definite action of changed conditions of life, and for so<alled 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 pxaison 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 


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- 

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 recently 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. Schobl, 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 

With respect to plants, to which on account of Niigeli'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 
sf)ecies 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 


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 columnse 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 Malpighiacex 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 sjjecies; 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. 


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 impwrtant 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 f)entamerous. In many Com- 
posita: and Umbelliferx (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 Compositas the central achenes alone are fur- 
nished with a pappus; and in Hyoseris the same head yields achenes 
of three different forms. In certain Umbelliferac 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 nutlets; 
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 


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 estivation is almost as frequently that 
of the Rhinanthideac 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 Rutacex 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 mains 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. Lastly, St. Hilaire found towards the 
southern extreme of the range of Gomphia olexformis 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 


attributed to the laws of growth and the inter-action of parts, inde- 
pendently of natural selection. But with respect to Niigeli'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 

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- 


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 appjear. 

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 speciaHsed 
or differentiated; and natural selection tends towards this end, inas- 
much as the parts are thus enabled to perform their functions more 

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 sp)ecial 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 


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 girafTe, 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 


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 slightly 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 species. 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 


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 
exfject 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 


another. We are ignorant with resp)ect 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 suppwrt singularly few large quadru- 
peds, for instance South America, though it is so luxuriant; whilst 
South Africa abounds with them to an 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 

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 act on 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. Lastly, 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 


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 p)eculiar conditions. A strictly 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 greatly 
reduced in size, or completely lost, through disuse; but in this case 
it would be necessary that they shoiJd first have acquired the power 
of running quickly on the ground, by the aid of their hind legs alone, 


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 

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 apparently implies increased brain-power. 

We will return to Mr. Mivart's 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 difficult, 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 ujxin 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 


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. 


The baleen consists of a row, on each side, of the upper jaw, of about 
300 plates or lamina:, 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 bristles, 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 Balacnoptera 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 lamella: by finely graduated steps, 
each of service to its possessor. 

The beak of a shoveller duck (Spatula dypeata) 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 lamella:, 
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 


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 Balacnoptera 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 sp>ecies of whale. The lower mandible of the shoveller-duck 
is furnished with lamella: 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 lamella: are frayed into fine 
bristly points, so that they thus curiously resemble the plates o£ 
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. 


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 oird, the lamellx 
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-like 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. 
Bartlett, in which the lamellae are less developed than in the common 

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-developjed 
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 different 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 Lacejjede, 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, 


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 lamellae 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 dif?erent 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 Pleuronectidaf, 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 Pleuronectidae are admirably adapted 
by their flattened and asymmetrical structure for their habits of life, 


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 Pleuronectidac, 
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 f)ermanently 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 


young of 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- 
nectidac, 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 Pleuronectidx 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 Pleuronectidx, 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. Giinther, 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 fish, as "there is so 
much less room for their action, than with the larger fins above." 


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 the lower halves, may likewise be accounted for by disuse. 
From the colourless state of the ventral surface of most fishes and of 
many other animals, we may reasonably suppose that the absence of 
colour in flat-fish on the side, whether it be the right or left, 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 sfjecies, as recently 
shown by Pouchet, of changing their colour in accordance with the 
siu-rounding 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 apparently 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 suffice for the work. Brehm saw the young 
of an African monkey (Cercopithecus) 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 


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 climbing. I have 
received an analogous account from Dr. Giinther, 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. Lxxkwood, 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 


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, 1 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 sjjecial 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 perfectly 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 


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 pedicellarii, 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 utiUty 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 startling paradox." Paradoxical as this may appear to 
Mr. Mivart, tridactyle forcepses, immovably fixed at the base, but 
capable of a snapping action, 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 pedicellariac, one resembling 
those of Echinus, and the other those of Spatangus; and such cases 
are always interesting as affording the means of apparently sud- 
den transitions, through the abortion of one of the two states of an 


With respect to the steps by which these curious organs have been 
evolved, Mr. Agassiz infers from his own researches and those of 
MiJlier, that both in starfishes and sea urchins the {jedicellarix 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- 
cellariac. The gradation extends even to the manner in which 
ordinary spines and the pedicellaria: with their supporting calcareous 
rods are articulated to the shell. In certain genera of starfishes, "the 
very combinations needed to show that the pedicellariae 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 pedicellaria 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 purpwse; 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 pedicellaria:. All possible 
gradations, as he adds, may likewise be found between the pedicel- 
lariac of the starfishes and the hooks of the ophiurans, another group 
of the Echinodermata; and again between the pedicellarix of sea 


urchins and the anchors of the Holothurix, 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 difler 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 
pedicellarijE 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 pedicellariac and 
avicularia. The latter resemble somewhat more closely the chelx 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 cheli 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 


exists. In the first and simplest stage, the terminal segment of 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 shaf)e, 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 chela: of a lobster; and all these gradations can be actually 

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 slightly 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 sf)ecies 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, 


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 hps 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 Up 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 

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 insufficient 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, 


they need not here be considered; yet I may mention that at the 
base of 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 little 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 |X)llen-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 
poUinium 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 little from that of an ordinary flower, to a highly 
complex jx)llinium, admirably adapted for transportal by insects; 
nor will he deny that all 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. 


We will now turn to climbing plants. These can be arranged in 
a long series, from those which simply twine round a sup()ort, to 
those which I have called lea£<limbers, 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<limbers 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 leaf -climber; 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 necessarily 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- 


ently acquired it, and cannot have inherited it from a common 
progenitor. Hence 1 was led to predict that some shght tendency 
to a movement of this kind would be found to be far from uncom- 
mon with plants which did not chmb; and that this had alTorded 
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 expwsed it had profited them to ascend to a height, 
then the habit of sHghtly and irregularly revolving might have been 
increased and utihsed 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 


of all 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 

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<alled 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 


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 little 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 lamellae, like those of the domestic ducks — and 
then into lamellae, 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 lamellse are first 
used as teeth, then partly as teeth and partly as a sifting apparatus, 
and at last almost exclusively for this latter purpose. 

With such structures as the above lamella; 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- 


mx 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 difficulty in understanding how the 
branched spines of some ancient Echinoderm, which served as a 
defence, became developed through natural selection into tridactyle 
pedicellarii, 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 indisp)ensable, 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 


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 p)eculiar nature, are necessary 
for the development of a structure; and the requisite conditions may 
seldom have concurred. The belief 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 little 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 

At the present day almost all naturalists admit evolution under 
some form. Mr. Mivart believes that sp)ecies 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 


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, wfcich implies great breaks or 
discontinuity in the series, appears to me improbable in the highest 

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 little 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 

My reasons for doubting whether natural species have changed 
as abrupdy as have occasionally domestic races, and for entirely 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 domesticated produc- 
tions, singly and at rather long intervals of time. If such occurred 
under nature, they would be hable, 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 


same district. This difficulty, 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 Utde 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 degree or manner are more 


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 beUef 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. 


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 all 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. 



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. 

MANY 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 think, 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 



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 little 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 


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 little 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 

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 


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; 1 then tickled and stroked them 
with a hair in the same mani\er, as well as I could, as the ants do 
with their antennx; but not one excreted. Afterwards 1 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 antenna; 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 sjjecies, 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 partly 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- 


markable cases of diflerences 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 abiUty 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 quahty, 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 1 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 

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. 


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- 


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 nightly 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 Exed 
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 different 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. 


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 J can hardly suppose that 
domestic rabbits have often been selected for lameness alone; so that 
we must attribute at least the greater part of the inherited change 
from extreme wildness to extreme lameness, 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- 


iarity alone prevents our seeing how largely and how permanently the 
minds of 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 Gallus 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 



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 o£ 
certain ants; and the cell-making power of the hive-bee. These two 
latter instincts have generally and justly been ranked by naturalists 
as the most wonderful of all known instincts. 

Instincts of the Ct(c/(oo. — It 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, i£ 
she were 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 s^iecies 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 


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 Miiller, 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 

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 


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 believing 
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 apparently 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 impxDrtance 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 


age and strength; the habit having been afterwards improved, and 
transmitted to an earher 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 all ages, and the variations tend to 
be inherited at a corresfxinding 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 

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. 


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 o£ 
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 app)ears 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 Gallinaceac, and throws some Ught 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 f)ollen-collecting apparatus which would have 
been indispensable if they had stored up food for their own young. 
Some sf)ecies of Sphegidse (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 


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

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


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

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


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

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

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


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

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

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


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


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

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


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

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

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


distance of radius X V2, or radius X 141421 (or at some lesser 
distance), from the centres of the six surrounding spheres in the 
same layer; and at the same distance from the centres of the adjoin- 
ing spheres in the other and parallel layer; then, if planes of inter- 
section between the several spheres in both layers be formed, there 
will result a double layer of hexagonal prisms united together by 
pyramidal bases formed of three rhombs; and the rhombs and the 
sides of the hexagonal prisms will have every angle identically the 
same with the best measurements which have been made of the cells 
of the hive-bee. But I hear from Professor Wyman, who has made 
numerous careful measurements, that the accuracy of the workman- 
ship of the bee has been greatly exaggerated; so much so, that what- 
ever the typical form of the cell may be, it is rarely, if ever, realised. 

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

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


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

I then put into the hive, instead of a thick, rectangular piece of wax, 
a thin and narrow, knife-edged ridge, coloured with vermilion. 
The bees instantly began on both sides to excavate litde basins near 
to each other, in the same way as before; but the ridge of wax was 
so thin, that the bottoms of the basins, if they had been excavated to 
the same depth as in the former experiment, would have broken into 
each other from the opposite sides. The bees, however, did not suffer 
this to happen, and they stopped their excavations in due time; so 
that the basins, as soon as they had been a little deef)ened, came to 
have flat bases; and these flat bases, formed by thin little plates of 
the vermilion wax left ungnawed, were situated, as far as the eye 
could judge, exactly along the planes of imaginary intersection be- 
tween the basins on the opposite sides of the ridge of wax. In some 
parts, only small portions, in other parts, large portions of a rhombic 
plate were thus left between the opposed basins, but the work, from 
the unnatural state of things, had not been neady performed. The 
bees must have worked at very nearly the same rate in circularly 
gnawing away and deepening the basins on both sides of the ridge 
of vermilion wax, in order to have thus succeeded in leaving flat 
plates between the basins, by stopping work at the planes of inter- 

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


of wax, perceiving when they have gnawed the wax away to the 
proper thinness, and then stopping their work. In ordinary combs 
it has appeared to me that the bees do not always succeed in working 
at exactly the same rate from the opposite sides; for I have noticed 
half<ompleted rhombs at the base of a just commenced cell, which 
were slightly concave on one side, where I suppose that the bees 
had excavated too quickly, and convex on the opposed side where 
the bees had worked less quickly. In one well marked instance, I 
put the comb back into the hive, and allowed the bees to go on 
working for a short time, and again examined the cell, and I found 
that the rhombic plate had been completed, and had become perfectly 
fiat: it was absolutely impossible, from the extreme thinness of the 
little plate, that they could have effected this by gnawing away the 
convex side; and I suspect that the bees in such cases stand on oppo- 
site sides and push and bend the ductile and warm wax (which as I 
have tried is easily done) into its proper intermediate plane, and thus 
flatten it. 

From the experiment of the ridge of vermilion wax we can see 
that, if the bees were to build for themselves a thin wall of wax, they 
could make their cells of the proper shape, by standing at the proper 
distance from each other, by excavating at the same rate, and by 
endeavouring to make equal spherical hollows, but never allowing 
the spheres to break into each other. Now bees, as may be clearly 
seen by examining the edge of a growing comb, do make a rough, 
circumferential wall or rim all round the comb; and they gnaw this 
away from the opposite sides, always working circularly as they 
deepen each cell. They do not make the whole three-sided pyramidal 
base of any one cell at the same time, but only that one rhombic 
plate which stands on the extreme growing margin, or the two 
plates, as the case may be; and they never complete the upper edges 
of the rhombic plates, until the hexagonal walls are commenced. 
Some of these statements differ from those made by the justly cele- 
brated elder Huber, but I am convinced of their accuracy; and if 
I had space, I could show that they are conformable with my theory. 

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


wax; but I will not here enter on details. We see how important a 
part excavation plays in the construction of the cells; but it would 
be a great error to suppose that the bees cannot build up a rough 
wall of wax in the proper position — that is, along the plane of inter- 
section between two adjoining spheres. I have several sjjecimens 
showing clearly that they can do this. Even in the rude circum- 
ferential rim or wall of wax round a growing comb, flexures may 
sometimes be observed, corresponding in position to the planes of 
the rhombic basal plates of future cells. But the rough wall of wax 
has in every case to be finished ofl, by being largely gnawed away 
on both sides. The manner in which the bees build is curious; they 
always make the first rough wall from ten to twenty times thicker 
than the excessively thin finished wall of the cell, which will ulti- 
mately be left. We shall understand how they work, by supposing 
masons first to pile up a broad ridge of cement, and then to begin 
cutting it away equally on both sides near the ground, till a smooth, 
very thin wall is left in the middle; the masons always piling up the 
cut-away cement, and adding fresh cement on the summit of the 
ridge. We shall thus have a thin wall steadily growing upward but 
always crowned by a gigantic coping. From all the cells, both those 
just commenced and those completed, being thus crowned by a 
strong coping of wax, the bees can cluster and crawl over the comb 
without injuring the delicate hexagonal walls. These walls, as Profes- 
sor Miller has kindly ascertained for me, vary greatly in thickness; 
being, on an average of twelve measurements made near the border 
of the comb, ^-J^ of an inch in thickness; whereas the basal rhom- 
boidal plates are thicker, nearly in the proportion of three to two, 
having a mean thickness, from twenty-one measurements, of -rir 
of an inch. By the above singular manner of building, strength is 
continually given to the comb, with the utmost ultimate economy 
of wax. 

It seems at first to add to the difficulty of understanding how the 
cells are made, that a multitude of bees all work together; one bee 
after working a short time at one cell going to another, so that, as 
Huber has stated, a score of individuals work even at the commence- 
ment of the first cell. I was able practically to show this fact, by 
covering the edges of the hexagonal walls of a single cell, or the 


extreme margin of the circumferential rim of a growing comb, with 
an extremely thin layer of melted vermilion wax; and I invariably 
found that the colour was most delicately diffused by the bees — as 
delicately as a painter could have done it with his brush — by atoms 
of the coloured wax having been taken from the spot on which it 
had been placed, and worked into the growing edges of the cells all 
round. The work of construction seems to be a sort of balance 
struck between many bees, all instinctively standing at the same 
relative distance from each other, all trying to sweep equal spheres, 
and then building up, or leaving ungnawed, the planes of intersec- 
tion between these spheres. It was really curious to note in cases of 
difficulty, as when two pieces of comb met at an angle, how 
often the bees would pull down and rebuild in different ways the 
same cell, sometimes recurring to a shape which they had at first 

When bees have a place on which they can stand in their proper 
positions for working, — for instance, on a slip of wood, placed directly 
under the middle of a comb growing downwards, so that the comb 
has to be built over one face of the sHp — in this case the bees can lay 
the foundations of one wall of a new hexagon, in its strictly proper 
place, projecting beyond the other completed cells. It suffices that 
the bees should be enabled to stand at their proper relative distances 
from each other and from the walls of the last completed cells, and 
then, by striking imaginary spheres, they can build up a wall inter- 
mediate between two adjoining spheres; but, as far as I have seen, 
they never gnaw away and finish of? the angles of a cell till a large 
part both of that cell and of the adjoining cells has been built. 
This capacity in bees of laying down under certain circumstances 
a rough wall in its proper place between two just-commenced cells, 
is important, as it bears on a fact, which seems at first subversive of 
the foregoing theory; namely, that the cells on the extreme margin 
of wasp-combs are sometimes strictly hexagonal; but I have not 
space here to enter on this subject. Nor does there seem to me any 
great difficulty in a single insect (as in the case of a queen-wasp) 
making hexagonal cells, if she were to work alternately on the inside 
and outside of two or three cells commenced at the same ume, always 
standing at the proper relative distance from the parts of the cells 


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

As natural selection acts only by the accumulation of slight modi- 
fications of structure or instinct, each profitable to the individual 
under its conditions of life, it may reasonably be asked, how a long 
and graduated succession of modified architectural instincts, all 
tending towards the present perfect plan of construction, could have 
profited the progenitors of the hive-bee? I think the answer is not 
difficult: cells constructed like those of the bee or the wasp gain in 
strength, and save much in labour and space, and in the materials 
of which they are constructed. With respect to the formation of 
wax, it is known that bees are often hard pressed to get sufficient 
nectar, and I am informed by Mr. Tegetmeier that it has been experi- 
mentally proved that from twelve to fifteen f)ounds of dry sugar are 
consumed by a hive of bees for the secretion of a pound of wax; so 
that a prodigious quantity of fluid nectar must be collected and 
consumed by the bees in a hive for the secretion of the wax necessary 
for the construction of their combs. Moreover, many bees have to 
remain idle for many days during the process of secretion. A large 
store of honey is indispensable to support a large stock of bees during 
the winter; and the security of the hive is known mainly to depend 
on a large number of bees being supported. Hence the saving of wax 
by largely saving honey and the time consumed in collecting the 
honey must be an important element of success to any family 
of bees. Of course the success of the sp>ecies may be dependent on 
the number of its enemies, or parasites, or on quite distinct causes, 
and so be altogether independent of the quantity of honey which the 
bees can collect. But let us suppose that this latter circumstance 
determined, as it probably often has determined, whether a bee allied 
to our humble-bees could exist in large numbers in any country; 
and let us further suppose that the community lived through the 
winter, and consequently required a store of honey: there can in 
this case be no doubt that it would be an advantage to our imaginary 
humble-bee, if a slight modification in her instincts led her to make 
her waxen cells near together, so as to intersect a Httle; for a wall in 
common even to two adjoining cells would save some little labour 
and wax. Hence it would continually be more and more advan- 


tageous to our humble-bees, if they were to make their cells more 
and more regular, nearer together, and aggregated into a mass, like 
the cells of the Melipona; for in this case a large part of the bounding 
surface of each cell would serve to bound the adjoining cells, and 
much labour and wax would be saved. Again, from the same cause, 
it would be advantageous to the Melipona, if she were to make her 
cells closer together, and more regular in every way than at present; 
for then, as we have seen, the spherical surfaces would wholly dis- 
apf)ear and be replaced by plane surfaces; and the Melijxjna would 
make a comb as perfect as that of the hive-bee. Beyond this stage of 
perfection in architecture, natural selection could not lead; for the 
comb of the hive-bee, as far as we can see, is absolutely perfect in 
economising labour and wax. 

Thus, as I believe, the most wonderful of all known instincts, that 
of the hive-bee, can be explained by natural selection having taken 
advantage of numerous, successive, slight modifications of simpler 
instincts; natural selection having, by slow degrees, more and more 
perfectly led the bees to sweep equal spheres at a given distance from 
each other in a double layer, and to build up and excavate the wax 
along the planes of intersection; the bees, of course, no more know- 
ing that they swept their spheres at one particular distance from each 
other, than they know what are the several angles of the hexagonal 
prisms and of the basal rhombic plates; the motive power of the 
process of natural selection having been the construction of cells of 
due strength and of the proper size and shape for the larvae, this being 
effected with the greatest possible economy of labour and wax; that 
individual swarm which thus made the best cells with least labour, 
and least waste of honey in the secretion of wax, having succeeded 
best, and having transmitted their newly acquired economical in- 
stincts to new swarms, which in their turn will have had the best 
chance of succeeding in the struggle for existence. 

instincts: neuter and sterile INSECTS 

It has been objected to the foregoing view of the origin of instincts 
that "the variations of structure and of instinct must have been 
simultaneous and accurately adjusted to each other as a modification 


in the one without an immediate corresponding change in the other 
would have been fatal." The force of this objection rests entirely 
on the assumption that the changes in the instincts and structure 
are abrupt. To take as an illustration the case of the larger titmouse 
(Parus major) alluded to in a previous chapter; this bird often holds 
the seeds of the yew between its feet on a branch, and hammers with 
its beak till it gets at the kernel. Now what special difficulty would 
there be in natural selection preserving all the slight individual vari- 
ations in the shape of the beak, which were better and better adapted 
to break open the seeds, until a beak was formed, as well constructed 
for this purpose as that of the nuthatch, at the same time that habit, 
or compulsion, or spontaneous variations of taste, led the bird to 
become more and more of a seed-eater? In this case the beak is 
supposed to be slowly modified by natural selection, subsequently 
to, but in accordance with, slowly changing habits or taste; but let 
the feet of the titmouse vary and grow larger from correlation with 
the beak, or from any other unknown cause, and it is not improbable 
that such larger feet would lead the bird to climb more and more 
until it acquired the remarkable climbing instinct and power of the 
nuthatch. In this case a gradual change of structure is supposed to 
lead to changed instinctive habits. To take one more case: few in- 
stincts are more remarkable than that which leads the swift of the 
Eastern Islands to make its nest wholly of inspissated saliva. Some 
birds build their nests of mud, believed to be moistened with saliva; 
and one of the swifts of North America makes its nest (as I have 
seen) of sticks agglutinated with saliva, and even with flakes of this 
substance. Is it then very improbable that the natural selection of 
individual swifts, which secreted more and more saliva, should at 
last produce a sf)ecies with instincts leading it to neglect other mate- 
rials, and to make its nest exclusively of inspissated saliva? And so 
in other cases. It must, however, be admitted that in many instances 
we cannot conjecture whether it was instinct or structure which 
first varied. 

No doubt many instincts of very difficult explanation could be 
opposed to the theory of natural selection — cases, in which we cannot 
see how an instinct could have originated; cases, in which no inter- 
mediate gradations are known to exist; cases of instincts of such 


trifling importance, that they could hardly have been acted on by 
natural selection; cases of instincts almost identically the same in 
animals so remote in the scale of nature, that we cannot account for 
their similarity by inheritance from a common progenitor, and conse- 
quently must believe that they were independendy acquired through 
natural selection. I will not here enter on these several cases, but 
will confine myself to one special difficulty, which at first appeared 
to me insuperable, and actually fatal to the whole theory. I allude 
to the neuters or sterile females in insect-communities; for these 
neuters often differ widely in instinct and in structiu-e from both 
the males and fertile females, and yet, from being sterile, they cannot 
propagate their kind. 

The subject well deserves to be discussed at great length, but I will 
here take only a single case, that of working or sterile ants. How the 
workers have been rendered sterile is a difficulty; but not much 
greater than that of any other striking modification of structure; 
for it can be shown that some insects and other articulate animals 
in a state of nature occasionally become sterile; and if such inserts 
had been social, and it had been profitable to the community that a 
number should have been annually born capable of work, but in- 
capable of procreation, I can see no esf>ecial difficulty in this having 
been effected through natural selection. But I must pass over this 
preliminary difficulty. The great difficulty Ues in the working ants 
differing widely from both the males and the fertile females in 
structure, as in the shape of the thorax, and in being destitute of 
wings and sometimes of eyes, and in instinct. As far as instinct alone 
is concerned, the wonderful difference in this respect between the 
workers and the perfect females, would have been better exemplified 
by the hive-bee. If a working ant or other neuter insect had been an 
ordinary animal, I should have unhesitatingly assumed that all its 
characters had been slowly acquired through natural selection; 
namely, by individuals having been born with slight profitable modi- 
fications, which were inherited by the offspring; and that these again 
varied and again were selected, and so onwards. But with the work- 
ing ant we have an insect differing greatly from its parents, yet 
absolutely sterile, so that it could never have transmitted successively 
acquired modifications of structure or instinct to its progeny. It may 


well be asked how is it possible to reconcile this case with the theory 
of natural selection? 

First, let it be remembered that we have innumerable instances, 
both in our domestic productions and in those in a state of nature, of 
all sorts of dilTerences of inherited structure which are correlated with 
certain ages, and with either sex. We have differences correlated 
not only with one sex, but with that short period when the reproduc- 
tive system is active, as in the nuptial plumage of many birds, and 
in the hooked jaws of the male salmon. We have even slight dif- 
ferences in the horns of different breeds of cattle in relation to an 
artificially imperfect state of the male sex; for oxen of certain breeds 
have longer horns than the oxen of other breeds, relatively to the 
length of the horns in both the bulls and cows of these same breeds. 
Hence I can see no great difficulty in any character becoming cor- 
related with the sterile condition of certain members of insect- 
communities: the difficulty lies in understanding how such cor- 
related modifications of structure could have been slowly accumulated 
by natural selection. 

This difficulty, though appearing insuperable, is lessened, or, as I 
believe, disappears, when it is remembered that selection may be 
applied to the family, as well as to the individual, and may thus 
gain the desired end. Breeders of cattle wish the flesh and fat to 
be well marbled together: an animal thus characterised has been 
slaughtered, but the breeder has gone with confidence to the same 
stock and has succeeded. Such faith may be placed in the power of 
selection, that a breed of catde, always yielding oxen with extraor- 
dinarily long horns, could, it is probable, be formed by carefully 
watching which individual bulls and cows, when matched, produce 
oxen with the longest horns; and yet no one ox would ever have 
propagated its kind. Here is a better and real illustration : according 
to M. Verlot, some varieties of the double annual stock, from having 
been long and carefully selected to the right degree, always produce 
a large proportion of seedlings bearing double and quite sterile 
flowers; but they likewise yield some single and fertile plants. These 
latter, by which alone the variety can be propagated, may be com- 
pared with the fertile male and female ants, and the double sterile 
plants with the neuters of the same community. As with the varieties 


of the stock, so with social insects, selection has been applied to the 
family, and not to the individual, for the sake of gaining a service- 
able end. Hence, we may conclude that slight modifications of struc- 
ture or of instinct, correlated with the sterile condition of certain 
members of the community, have proved advantageous: consequently 
the fertile males and females have flourished, and transmitted to 
their fertile offspring a tendency to produce sterile members with 
the same modifications. This process must have been repeated many 
times, until that prodigious amount of difference between the fertile 
and sterile females of the same species has been produced which we 
see in many social insects. 

But we have not as yet touched on the acme of the difficulty; 
namely, the fact that the neuters of several ants differ, not only from 
the fertile females and males, but from each other, sometimes to an 
almost incredible degree, and are thus divided into two or even three 
castes. The castes, moreover, do not commonly graduate into each 
other, but are perfectly well defined; being as distinct from each 
other as arc any two species of the same genus, or rather as any two 
genera of the same family. Thus, in Eciton, there are working and 
soldier neuters, with jaws and instincts extraordinarily different: in 
Cryptocerus, the workers of one caste alone carry a wonderful sort 
of shield on their heads, the use of which is quite unknown: in the 
Mexican Myrmecocystus, the workers of one caste never leave the 
nest; they are fed by the workers of another caste, and they have an 
enormously developed abdomen which secretes a sort of honey, sup)- 
plying the place of that excreted by the aphides, or the domestic cattle 
as they may be called, which our European ants guard and imprison. 

It will indeed be thought that I have an overweening confidence 
in the principle of Natural Selection, when I do not admit that such 
wonderful and well-established facts at once annihilate the theory. 
In the simpler case of neuter insects all of one caste, which, as I 
believe, have been rendered different from the fertile males and 
females through natural selection, we may conclude from the analogy 
of ordinary variations, that the successive, slight, profitable modifica- 
tions did not first arise in all the neuters in the same nest, but in 
some few alone; and that by the survival of the communities with 
females which produced most neuters having the advantageous modi- 


fication, all the neuters ultimately came to be thus characterized. 
According to this view we ought occasionally to find in the same 
nest neuter insects, presenting gradations of structure; and this we 
do find, even not rarely considering how few neuter insects out of 
Europe have been carefully examined. Mr. F. Smith has shown that 
the neuters of several British ants difTer surprisingly from each other 
in size and sometimes in colour; and that the extreme forms can 
be linked together by individuals taken out of the same nest: I have 
myself compared perfect gradations of this kind. It sometimes hap- 
pens that the larger or the smaller sized workers are the most numer- 
ous; or that both large and small are numerous, whilst those of an 
intermediate size are scanty in numbers. Formica flava has larger 
and smaller workers, with some few of intermediate size; and, in 
this species, as Mr. F. Smith has observed, the larger workers have 
simple eyes (ocelli), which though small can be plainly distinguished, 
whereas the smaller workers have their ocelli rudimentary. Having 
carefully dissected several specimens of these workers, I can affirm 
that the eyes are far more rudimentary in the smaller workers than 
can be accounted for merely by their proportionally lesser size; and 
I fully believe, though I dare not assert so positively, that the workers 
of intermediate size have their ocelli in an exactly intermediate con- 
dition. So that here we have two bodies of sterile workers in the 
same nest, differing not only in size, but in their organs of vision, 
yet connected by some few members in an intermediate condition. 
I may digress by adding, that if the smaller workers had been the 
most useful to the community, and those males and females had 
been continually selected, which produced more and more of the 
smaller workers, until all the workers were in this condition; we 
should then have had a species of ant with neuters in nearly the 
same condition as those of Myrmica. For the workers of Myrmica 
have not even rudiments of ocelli, though the male and female ants 
of this genus have well-developed ocelli. 

I may give one other case: so confidently did I expect occasionally 
to find gradations of important structures between the different 
castes of neuters in the same species, that I gladly availed myself 
of Mr. F. Smith's offer of numerous sp)ecimens from the same nest 
of the driver ant (Anomma) of West Africa. The reader will per- 


haps best appreciate the amount of diflerence in these workers by 
my giving not the actual measurements, but a strictly accurate illus- 
tration: the difference was the same as if we were to see a set of 
workmen building a house, of whom many were five feet four inches 
high, and many sixteen feet high; but we must in addition suppose 
that the larger workmen had heads four instead of three times as 
big as those of the smaller men, and jaws nearly five times as big. 
The jaws, moreover, of the working ants of the several sizes differed 
wonderfully in shape, and in the form and number of the teeth. But 
the important fact for us is, that, though the workers can be grouped 
into castes of different sizes, yet they graduate insensibly into each 
other, as does the widely-different structure of their jaws. I speak 
confidently on this latter point, as Sir J. Lubbock made drawings 
for me, with the camera lucida, of the jaws which I dissected from 
the workers of the several sizes. Mr. Bates, in his interesting 'Natural- 
ist on the Amazons,' has described analogous cases. 

With these facts before me, I believe that natural selection, by 
acting on the fertile ants or parents, could form a species which should 
regularly produce neuters, all of large size with one form of jaw, or 
all of small size with widely different jaws; or lastly, and this is the 
greatest difficulty, one set of workers of one size and structure, and 
simultaneously another set of workers of a different size and struc- 
ture; — a graduated series having first been formed, as in the case of 
the driver ant, and then the extreme forms having been produced in 
greater and greater numbers, through the survival of the parents 
which generated them, until none with an intermediate structure 
were produced. 

An analogous explanation has been given by Mr. Wallace, of the 
equally complex case, of certain Malayan butterflies regularly appear- 
ing under two or even three distinct female forms; and by Fritz 
Miiller, of certain Brazilian crustaceans likewise appearing under 
two widely distinct male forms. But this subject need not here be 

I have now explained how, I believe, the wonderful fact of two 
distinctly defined castes of sterile workers existing in the same nest, 
both widely different from each other and from their parents, has 
originated. We can see how useful their production may have been 


to a scx:ial community of ants, on the same principle that the division 
of labour is useful to civilised man. Ants, however, work by in- 
herited instincts and by inherited organs or tools, whilst man works 
by acquired knowledge and manufactured instruments. But I must 
confess, that, with all my faith in natural selection, I should never 
have anticipated that this principle could have been efficient in so 
high a degree, had not the case of these neuter insects led me to this 
conclusion. I have, therefore, discussed this case, at some little but 
wholly insufficient length, in order to show the power of natural 
selection, and likewise because this is by far the most serious special 
difficulty which my theory has encountered. The case, also, is very 
interesting, as it proves that with animals, as with plants, any amount 
of modification may be effected by the accumulation of numerous, 
shght, spontaneous variations, which are in any way profitable, with- 
out exercise or habit having been brought into play. For pecuUar 
habits confined to the workers or sterile females, however long they 
might be followed, could not possibly af?ect the males and fertile 
females, which alone leave descendants. I am surprised that no one 
has hitherto advanced this demonstrative case of neuter insects, 
against the well-known doctrine of inherited habit, as advanced by 


I have endeavoured in this chapter briefly to show that the mental 
qualities of our domestic animals vary, and that the variations are 
inherited. Still more briefly I have attempted to show that instincts 
vary slightly in a state of nature. No one will dispute that instincts 
are of the highest importance to each animal. Therefore there is no 
real difficulty, under changing conditions of life, in natural selection 
accumulating to any extent slight modifications of instinct which 
are in any way useful. In many cases habit or use and disuse have 
probably come into play. I do not pretend that the facts given in this 
chapter strengthen in any great degree my theory; but none of the 
cases of difficulty, to the best of my judgment, annihilate it. On the 
other hand, the fact that instincts are not always absolutely perfect 
and are liable to mistakes; — that no instinct can be shown to have 
been produced for the good of other animals, though animals take 


advantage of the instincts of others; — that the canon in natural 
history, of "Natura non facit saltum," is applicable to instincts as 
well as to corporeal structure, and is plainly explicable on the fore- 
going views, but is otherwise inexplicable, — all tend to corroborate 
the theory of natural selection. 

This theory is also strengthened by some few other facts in regard 
to instincts; as by that common case of closely allied, but distinct, 
species, when inhabiting distant parts of the world and living under 
considerably different conditions of life, yet often retaining nearly 
the same instincts. For instance, we can understand, on the principle 
of inheritance, how it is that the thrush of tropical South America 
lines its nest with mud, in the same peculiar manner as does our 
British thrush; how it is that the Hornbills of Africa and India have 
the same extraordinary instinct of plastering up and imprisoning the 
females in a hole in a tree, with only a small hole left in the plaster 
through which the males feed them and their young when hatched; 
how it is that the male wrens (Troglodytes) of North America build 
"cock-nests," to roost in, like the males of our Kitty-wrens, — a habit 
wholly unlike that of any other known bird. Finally, it may not be 
a logical deduction, but to my imagination it is far more satisfactory 
to look at such instincts as the young cuckoo ejecting its foster- 
brothers, — ants making slaves, — the larvae of ichneumonidac feeding 
within the live bodies of caterpillars, — not as spxjcially endowed or 
created instincts, but as small consequences of one general law 
leading to the advancement of all organic beings, — namely, multiply, 
vary, let the strongest live and the weakest die. 



Distinction between the sterility of first crosses and of hybrids — Sterility 
various in degree, not universal, aflected by close interbreeding, 
removed by domestication — Laws governing the sterility of hybrids 
— Sterility not a special endowment, but incidental on other difler- 
ences, not accumulated by natural selection — Causes of the sterility 
of first crosses and of hybrids — Parallelism between the effects of 
changed conditions of life and of crossing — Dimorphism and Tri- 
morphism — Fertility of varieties when crossed, and of their mongrel 
offspring not universal — Hybrids and mongrels compared independ- 
ently of their fertility — Summary. 

THE view commonly entertained by naturalists is that species, 
when intercrossed, have been specially endowed with ste- 
rility, in order to prevent their confusion. This view certainly 
seems at first highly probable, for species living together could hardly 
have been kept distinct had they been capable of freely crossing. 
The subject is in many ways impxjrtant for us, more especially as the 
sterility of species when first crossed, and that of their hybrid off- 
spring, cannot have been acquired, as I shall show, by the preserva- 
tion of successive profitable degrees of sterility. It is an incidental 
result of differences in the reproductive systems of the parent-species. 

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

Pure species have of course their organs of reproduction in a 
perfect condition, yet when intercrossed they produce either few or 
no offspring. Hybrids, on the other hand, have their reproductive 
organs functionally impotent, as may be clearly seen in the state of 
the male element in both plants and animals; though the formative 
organs themselves are perfect in structure, as far as the microscope 
reveals. In the first case the two sexual elements which go to form 
the embryo are perfect; in the second case they are either not at all 



developed, or are imperfectly developed. This distinction is impor- 
tant, when the cause of the sterility, which is common to the two 
cases, has to be considered. The distinction probably has been slurred 
over, owing to the sterility in both cases being looked on as a special 
endowment, beyond the province of our reasoning powers. 

The fertility of varieties, that is of the forms known or believed to 
be descended from common parents, when crossed, and likewise the 
fertility of their mongrel offspring, is, with reference to my theory, 
of equal importance with the sterility of sfjecies; for it seems to make 
a broad and clear distinction between varieties and species. 


First, for the sterility of species when crossed and of their hybrid 
offspring. It is impossible to study the several memoirs and works 
of those two conscientious and admirable observers, Kolreuter and 
Gartner, who almost devoted their lives to this subject, without being 
deeply impressed with the high generality of some degree of sterility. 
Kolreuter makes the rule universal; but then he cuts the knot, for in 
ten cases in which he found two forms, considered by most authors 
as distinct species, quite fertile together, he unhesitatingly ranks them 
as varieties. Gartner, also, makes the rule equally universal; and he 
disputes the entire fertility of Kolreuter's ten cases. But in these and 
in many other cases, Gartner is obliged carefully to count the seeds, 
in order to show that there is any degree of sterility. He always com- 
pares the maximum number of seeds produced by two species when 
first crossed, and the maximum produced by their hybrid offspring, 
with the average number produced by their pure parent-species in a 
state of nature. But causes of serious error here intervene: a plant, 
to be hybridised, must be castrated, and, what is often more impor- 
tant, must be secluded in order to prevent pollen being brought to it 
by insects from other plants. Nearly all the plants experimented on 
by Gartner were potted, and were kept in a chamber in his house. 
That these processes are often injurious to the fertility of a plant 
cannot be doubted; for Gartner gives in his table about a score of 
cases of plants which he castrated, and artificially fertilised with their 
own pollen, and (excluding all cases such as the Leguminosac, in 
which there is an acknowledged difficulty in the manipulation) half 


o£ these twenty plants had their fertility in some degree impaired. 
Moreover, as Gartner repeatedly crossed some forms, such as the 
common red and blue pimpernels (Anagallis arvensis and cceulea), 
which the best botanists rank as varieties, and found them absolutely 
sterile, we may doubt whether many species are really so sterile, 
when intercrossed, as he believed. 

It is certain, on the one hand, that the sterility of various species 
when crossed is so different in degree and graduates away so insen- 
sibly, and, on the other hand, that the fertility of pure species is so 
easily affected by various circumstances, that for all practical pur- 
poses it is most difficult to say where perfect fertiUty ends and ste- 
rility begins. 1 think no better evidence of this can be required than 
that the two most experienced observers who have ever lived, namely 
Kolreuter and Gartner, arrived at diametrically opposite conclusions 
in regard to some of the very same forms. It is also most instructive 
to compare — but I have not space here to enter into details — the 
evidence advanced by our best botanists on the question whether 
certain doubtful forms should be ranked as species or varieties, with 
the evidence from fertility adduced by different hybridisers, or by 
the same observer from experiments made during different years. It 
can thus be shown that neither sterility nor fertility affords any cer- 
tain distinction between species and varieties. The evidence from 
this source graduates away, and is doubtful in the same degree as 
is the evidence derived from other constitutional and structural 

In regard to the sterility of hybrids in successive generations; 
though Gartner was enabled to rear some hybrids, carefully guarding 
them from a cross with either pure parent, for six or seven, and in 
one case for ten generations, yet he asserts positively that their fer- 
tility never increases, but generally decreases greatly and suddenly. 
With respect to this decrease, it may first be noticed that when any 
deviation in structure or constitution is common to both parents, 
this is often transmitted in an augmented degree to the offspring; 
and both sexual elements in hybrid plants are already affected in 
some degree. But I believe that their fertility has been diminished 
in nearly all these cases by an independent cause, namely, by too 
close interbreeding. I have made so many experiments and collected 


so many facts, showing on the one hand that an occasional cross with 
a distinct individual or variety increases the vigour and fertility of 
the offspring, and on the other hand that very close interbreeding 
lessens their vigour and fertility, that I cannot doubt the correctness 
of this conclusion. Hybrids are seldom raised by experimentalists 
in great numbers; and as the parent-species, or other allied hybrids, 
generally grow in the same garden, the visits of insects must be 
carefully prevented during the flowering season; hence hybrids, if 
left to themselves, will generally be fertilised during each generation 
by pollen from the same flower; and this would probably be injuri- 
ous to their fertility, already lessened by their hybrid origin. I am 
strengthened in this conviction by a remarkable statement repeatedly 
made by Gartner, namely, that if even the less fertile hybrids be 
artificially fertilised with hybrid pollen of the same kind, their fer- 
tility, notwithstanding the frequent ill effects from manipulation, 
sometimes decidedly increases, and goes on increasing. Now, in the 
process of artificial fertilisation, pollen is as often taken by chance 
(as I know from my own experience) from the anthers of another 
flower, as from the anthers of the flower itself which is to be fer- 
tilised; so that a cross between two flowers, though probably often 
on the same plant, would be thus effected. Moreover, whenever 
complicated experiments are in progress, so careful an observer as 
Gartner would have castrated his hybrids, and this would have 
ensured in each generation a cross with pollen from a distinct flower, 
either from the same plant or from another plant of the same hybrid 
nature. And thus, the strange fact of an increase of fertility in the 
successive generations of artificially fertilised hybrids, in contrast 
with those spontaneously self-fertilised, may, as I believe, be accounted 
for by too close interbreeding having been avoided. 

Now let us turn to the results arrived at by a third most experi- 
enced hybridiser, namely, the Hon. and Rev. W. Herbert. He is as 
emphatic in his conclusion that some hybrids are perfectly fertile — 
as fertile as the pure parent-species — as are Kolreuter and Gartner 
that some degree of sterility between distinct species is a universal 
law of nature. He experimented on some of the very same species 
as did Gartner. The difference in their results may, I think, be in 
part accounted for by Herbert's great horticultural skill, and by his 


having hot-houses at his command. Of his many important state- 
ments I will here give only a single one as an example, namely, that 
"every ovule in a pod of Crinum capense fertilised by C. revolutum 
produced a plant, which I never saw to occur in a case of its natural 
fecundation." So that here we have perfect or even more than 
commonly perfect fertility, in a first cross between two distinct 

This case of the Crinum leads me to refer to a singular fact, 
namely, that individual plants of certain species of Lobelia, Ver- 
bascum and Passiflora, can easily be fertilised by pollen from a dis- 
tinct species, but not by pollen from the same plant, though this 
pollen can be proved to be perfectly sound by fertilising other plants 
or species. In the genus Hippeastrum, in Corydalis, as shown by 
Professor Hildebrand, in various orchids as shown by Mr. Scott and 
Fritz Miiller, all the individuals are in this peculiar condition. So 
that with some species, certain abnormal individuals, and in other 
species all the individuals, can actually be hybridised much more 
readily than they can be fertilised by pollen from the same individual 
plant! To give one instance, a bulb of Hippeastrum aulicum pro- 
duced four flowers; three were fertilised by Herbert with their own 
pollen, and the fourth was subsequently fertilised by the pollen of a 
compound hybrid descended from three distinct species: the result 
was that "the ovaries of the three first flowers soon ceased to grow, 
and after a few days p)erished entirely, whereas the pod impregnated 
by the pollen of the hybrid made vigorous growth and rapid progress 
to maturity, and bore good seed, which vegetated fredy." Mr. 
Herbert tried similar experiments during many years, and always 
with the same result. These cases serve to show on what slight and 
mysterious causes the lesser or greater fertility of a species sometimes 

The practical experiments of horticulturists, though not made 
with scientific precision, deserve some notice. It is notorious in how 
complicated a manner the species of Pelargonium, Fuchsia, Calceo- 
laria, Petunia, Rhododendron, etc., have been crossed, yet many of 
these hybrids seed freely. For instance, Herbert asserts that a hybrid 
from Calceolaria integrifolia and plantaginea, species most widely 
dissimilar in general habit, "reproduces itself as perfecdy as if it had 


been a natural species from the mountains of Chili." I have taken 
some pains to ascertain the degree of fertiHty of some of the complex 
crosses of Rhododendrons, and I am assured that many of them 
are perfectly fertile. Mr. C. Noble, for instance, informs me that he 
raises stocks for grafting from a hybrid between Rhod. ponticum 
and catawbiense, and that this hybrid "seeds as freely as it is possible 
to imagine." Had hybrids, when fairly treated, always gone on 
decreasing in fertility in each successive generation, as Gartner be- 
lieved to be the case, the fact would have been notorious to nursery- 
men. Horticulturists raise large beds of the same hybrid, and such 
alone are fairly treated, for by insect-agency the several individuals 
are allowed to cross freely with each other, and the injurious influ- 
ence of close interbreeding is thus prevented. Any one may readily 
convince himself of the efficiency of insect-agency by examining the 
flowers of the more sterile kinds of hybrid Rhododendrons, which 
produce no pollen, for he will find on their stigmas plenty of pollen 
brought from other flowers. 

In regard to animals, much fewer experiments have been carefully 
tried than with plants. If our systematic arrangements can be trusted, 
that is, if the genera of animals are as distinct from each other as are 
the genera of plants, then we may infer that animals more widely 
distinct in the scale of nature can be crossed more easily than in the 
case of plants; but the hybrids themselves are, I think, more sterile. 
It should, however, be borne in mind that, owing to few animals 
breeding freely under confinement, few experiments have been fairly 
tried: for instance, the canary bird has been crossed with nine distinct 
species of finches, but, as not one of these breeds freely in confine- 
ment, we have no right to expect that the first crosses between them 
and the canary, or that their hybrids, should be perfectly fertile. 
Again, with respect to the fertility in successive generations of the 
more fertile hybrid animals, I hardly know of an instance in which 
two families of the same hybrid have been raised at the same time 
from different parents, so as to avoid the ill effects of close inter- 
breeding. On the contrary, brothers and sisters have usually been 
crossed in each successive generation, in opposition to the constantly 
repeated admonition of every breeder. And in this case, it is not at 


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

Although I know of hardly any thoroughly well-authenticated 
cases of perfectly fertile hybrid animals, I have reason to believe that 
the hybrids from Cervulus vaginalis and Reevesii, and from Phasi- 
anus colchicus with P. torquatus, are perfectly fertile. M. Quatre- 
fages states that the hybrids from two moths (Bombyx cynthia and 
arrindia) were proved in Paris to be fertile inter se for eight genera- 
tions. It has lately been asserted that two such distinct species as the 
hare and rabbit, when they can be got to breed together, produce 
offspring which are highly fertile when crossed with one of the 
parent-species. The hybrids from the common and Chinese geese 
(A. cygnoides), species which are so different that they are generally 
ranked in distinct genera, have often bred in this country with either 
pure parent, and in one single instance they have bred inter se. This 
was effected by Mr. Eyton, who raised two hybrids from the same 
parents, but from different hatches; and from these two birds he 
raised no less than eight hybrids (grandchildren of the pure geese) 
from one nest. In India, however, these cross-bred geese must be far 
more fertile; for 1 am assured by two eminendy capable judges, 
namely Mr. Blyth and Captain Hutton, that whole flocks of these 
crossed geese are kept in various parts of the country; and as they 
are kept for profit, where neither pure parent-species exists, they 
must certainly be highly or perfectly fertile. 

With our domesticated animals, the various races when crossed 
together are quite fertile; yet in many cases they are descended from 
two or more wild sp)ecies. From this fact we must conclude either 
that the aboriginal parent-species at first produced perfectly fertile 
hybrids, or that the hybrids subsequently reared under domestica- 
tion became quite fertile. This latter alternative, which was first 
propounded by Pallas, seems by far the most probable, and can, 
indeed, hardly be doubted. It is, for instance, almost certain that our 
dogs are descended from several wild stocks; yet, with perhaps the 
exception of certain indigenous domestic dogs of South America, 
all are quite fertile together; but analogy makes me greatly doubt, 
whether the several aboriginal species would at first have freely bred 


together and have produced quite fertile hybrids. So again I have 
lately acquired decisive evidence that the crossed offspring from the 
Indian humped and common cattle are inter se perfectly fertile; and 
from the observations by Riitimeyer on their important osteological 
differences, as well as from those by Mr. Blyth on their differences 
in habits, voice, constitution, etc., these two forms must be regarded 
as good and distinct species. The same remarks may be extended 
to the two chief races of the pig. We must, therefore, either give up 
the belief of the universal sterility of species when crossed; or we 
must look at this sterility in animals, not as an indelible character- 
istic, but as one capable of being removed by domestication. 

Finally, considering all the ascertained facts on the intercrossing of 
plants and animals, it may be concluded that some degree of sterility, 
both in first crosses and in hybrids, is an extremely general result; 
but that it cannot, under our present state of knowledge, be consid- 
ered as absolutely universal. 


We will now consider a little more in detail the laws governing 
the sterility of first crosses and of hybrids. Our chief object will be 
to see whether or not these laws indicate that species have been 
specially endowed with this quality, in order to prevent their crossing 
and blending together in utter confusion. The following conclusions 
are drawn up chiefly from Gartner's admirable work on the hybridi- 
sation of plants. I have taken much pains to ascertain how far they 
apply to animals, and, considering how scanty our knowledge is in 
regard to hybrid animals, I have been surprised to find how generally 
the same rules apply to both kingdoms. 

It has been already remarked, that the degree of fertility, both of 
first crosses and of hybrids, graduates from zero to perfect fertility. 
It is surprising in how many curious ways this gradation can be 
shown; but only the barest outline of the facts can here be given. 
When pollen from a plant of one family is placed on the stigma of 
a plant of a distinct family, it exerts no more influence than so much 
inorganic dust. From this absolute zero of fertility, the pollen of 
different species applied to the stigma of some one species of the 
same genus, yields a perfect gradation in the number of seeds pro- 


duced, up to nearly complete or even quite complete fertility; and, 
as we have seen, in certain abnormal cases, even to an excess of 
fertility, beyond that which the plant's own pollen produces. So in 
hybrids themselves, there are some which never have produced, and 
probably never would produce, even with the pollen of the pure 
parents, a single fertile seed: but in some of these cases a first trace 
of fertility may be detected, by the pollen of one of the pure parent- 
species causing the flower of the hybrid to wither earlier than it 
otherwise would have done; and the early withering of the flower 
is well known to be a sign of incipient fertilisation. From this 
extreme degree of sterility we have self-fertilised hybrids producing 
a greater and greater number of seeds up to perfect fertility. 

The hybrids raised from two species which are very difficult to 
cross, and which rarely produce any offspring, are generally very 
sterile; but the parallelism between the difficulty of making a first 
cross, and the sterility of the hybrids thus produced — two classes of 
facts which are generally confounded together — is by no means 
strict. There are many cases, in which two pure species, as in the 
genus Verbascum, can be united with unusual facility, and produce 
numerous hybrid-offspring, yet these hybrids are remarkably sterile. 
On the other hand, there are species which can be crossed very rarely, 
or with extreme difficulty, but the hybrids, when at last produced, 
are very fertile. Even within the limits of the same genus, for 
instance in Dianthus, these two opposite cases occur. 

The fertility, both of first crosses and of hybrids, is more easily 
affected by unfavorable conditions, than is that of pure species. But 
the fertility of first crosses is likewise innately variable; for it is not 
always the same in degree when the same two species are crossed 
under the same circumstances; it depends in part upon the constitu- 
tion of the individuals which happen to have been chosen for the 
experiment. So it is with hybrids, for their degree of fertility is often 
found to differ greatly in the several individuals raised from seed out 
of the same capsule and exposed to the same conditions. 

By the term systematic affinity is meant, the general resemblance 
between species in structure and constitution. Now the fertility 
of first crosses, and of the hybrids produced from them, is largely 
governed by their systematic affinity. This is clearly shown by 


hybrids never having been raised between species ranked by system- 
atists in distinct families; and on the other hand, by very closely 
allied species generally uniting with facility. But the correspondence 
between systematic affinity and the facility of crossing is by no means 
strict. A multitude of cases could be given of very closely allied 
sfjecies which will not unite, or only with extreme difficulty; and on 
the other hand of very distinct species which unite with the utmost 
faciUty. In the same family there may be a genus, as Dianthus, in 
which very many species can most readily be crossed; and another 
genus, as Silene, in which the most persevering efforts have failed 
to produce between extremely close species a single hybrid. Even 
within the Umits of the same genus, we meet with this same differ- 
ence; for instance, the many species of Nicotiana have been more 
largely crossed than the species of almost any other genus; but 
Gartner found that N. acuminata, which is not a particularly distinct 
sjjecies, obstinately failed to fertiUse, or to be fertilised by no less 
than eight other species of Nicotiana. Many analogous facts could 
be given. 

No one has been able to point out what kind or what amount of 
difference, in any recognisable character, is sufficient to prevent two 
species crossing. It can be shown that plants most widely different 
in habit and general appearance, and having strongly marked differ- 
ences in every part of the flower, even in the pollen, in the fruit, and 
in the cotyledons, can be crossed. Annual and perennial plants, 
deciduous and evergreen trees, plants inhabiting different stations 
and fitted for extremely different climates, can often be crossed 
with ease. 

By a reciprocal cross between two species, I mean the case, for 
instance, of a female ass being first crossed by a stallion, and then a 
mare by a male ass; these two sp)ecies may then be said to have been 
reciprocally crossed. There is often the widest possible difference 
in the facility of making reciprocal crosses. Such cases are highly 
important, for they prove that the capacity in any two species to 
cross is often completely independent of their systematic affinity, 
that is of any difference in their structure or constitution, excepting 
in their reproductive systems. The diversity of the result in recipro- 
cal crosses between the same two species was long ago observed by 


Kolreuter. To give an instance: Mirabilis jalapa can easily be fer- 
tilised by the pollen of M. longiflora, and the hybrids thus produced 
are sufficiendy fertile; but Kolreuter tried more than two hundred 
times, during eight following years, to fertilise reciprocally M. longi- 
flora with the pollen of M. jalapa, and utterly failed. Several other 
equally striking cases could be given. Thuret has observed the same 
fact with certain sea-weeds or Fuci. Gartner, moreover, found that 
this difference of facility in making reciprocal crosses is extremely 
common in a lesser degree. He has observed it even between closely 
related forms (as Matthiola annua and glabra) which many bot- 
anists rank only as varieties. It is also a remarkable fact, that hybrids 
raised from reciprocal crosses, though of course compounded of the 
very same two sjjecies, the one sp>ecies having first been used as the 
father and then as the mother, though they rarely differ in external 
characters, yet generally differ in fertility in a small, and occasionally 
in a high degree. 

Several other singular rules could be given from Gartner: for 
instance, some species have a remarkable power of crossing with 
other species; other species of the same genus have a remarkable 
power of impressing their likeness on their hybrid offspring; but 
these two powers do not at all necessarily go together. There are 
certain hybrids which, instead of having, as is usual, an intermediate 
character between their two parents, always closely resemble one of 
them; and such hybrids, though externally so like one of their pure 
parent-species, are with rare exceptions extremely sterile. So again 
amongst hybrids which are usually intermediate in structure between 
their parents, exceptional and abnormal individuals sometimes are 
born, which closely resemble one of their pure parents; and these 
hybrids are almost always utterly sterile, even when the other hybrids 
raised from seed from the same capsule have a considerable degree 
of fertility. These facts show how completely the fertility of a hybrid 
may be independent of its external resemblance to either pure parent. 

Considering the several rules now given, which govern the fer- 
tility of first crosses and of hybrids, we see that when forms, which 
must be considered as good and distinct species, are united, their 
fertility graduates from zero to perfect fertility, or even to fertility 
under certain conditions in excess; that their fertility, besides being 


eminently susceptible to favourable and unfavourable conditions, is 
innately variable; that it is by no means always the same in degree in 
the first cross and in the hybrids produced from this cross; that the 
fertility of hybrids is not related to the degree in which they 
resemble in external appearance either parent; and lastly, that the 
facility of making a first cross between any two species is not always 
governed by their systematic affinity or degree of resemblance to 
each other. This latter statement is clearly proved by the difference 
in the result of reciprocal crosses between the same two species, for, 
according as the one species or the other is used as the father or the 
mother, there is generally some difference, and occasionally the 
widest possible difference, in the facility of effecting an union. The 
hybrids, moreover, produced from reciprocal crosses often differ in 

Now do these complex and singular rules indicate that species 
have been endowed with sterility simply to prevent their becoming 
confounded in nature? I think not. For why should the sterility 
be so extremely different in degree, when various species are crossed, 
all of which we must suppwse it would be equally important to keep 
from blending together? Why should the degree of sterility be 
innately variable in the individuals of the same species? Why should 
some SfHxies cross with facility, and yet produce very sterile hybrids; 
and other species cross with extreme difficulty, and yet produce fairly 
fertile hybrids? Why should there often be so great a difference in 
the result of a reciprocal cross between the same two species ? Why, 
it may even be asked, has the production of hybrids been permitted ? 
To grant to species the special power of producing hybrids, and then 
to stop their further propagation by different degrees of sterility, 
not strictly related to the facility of the first union between their 
parents, seenis a strange arrangement. 

The foregoing rules and facts, on the other hand, appear to me 
clearly to indicate that the sterility both of first crosses and of hybrids 
is simply incidental or dependent on unknown differences in their 
reproductive systems; the differences being of so peculiar and limited 
a nature, that, in reciprocal crosses between the same two species, the 
male sexual element of the one will often freely act on the female 
sexual element of the other, but not in a reversed direction. It will 


be advisable to explain a little more fully by an example what I mean 
by sterility being incidental on other differences, and not a sjjecially 
endowed quality. As the capacity of one plant to be grafted or 
budded on another is unimportant for their welfare in a state o£ 
nature, I presume that no one will suppose that this capacity is a 
specially endowed quality, but will admit that it is incidental on 
differences in the laws of growth of the two plants. We can some- 
times see the reason why one tree will not take on another, from 
differences in their rate of growth, in the hardness of their wood, in 
the period of the flow or nature of their sap, etc.; but in a multitude 
of cases we can assign no reason whatever. Great diversity in the 
size of two plants, one being woody and the other herbaceous, one 
being evergreen and the other deciduous, an adaptation to widely 
different climates, do not always prevent the two grafting together. 
As in hybridisation, so with grafting, the capacity is limited by sys- 
tematic affinity, for no one has been able to graft together trees 
belonging to quite distinct families; and, on the other hand, closely 
allied species, and varieties of the same species, can usually, but not 
invariably, be grafted with ease. But this capacity, as in hybridisation, 
is by no means absolutely governed by systematic affinity. Although 
many distinct genera within the same family have been grafted 
together, in other cases sfjecies of the same genus will not take on 
each other. The pear can be grafted far more readily on the quince, 
which is ranked as a distinct genus, than on the apple, which is a 
member of the same genus. Even different varieties of the pear take 
with different degrees of facility on the quince; so do different vari- 
eties of the apricot and peach on certain varieties of the plum. 

As Gartner found that there was sometimes an innate difference 
in different individuals of the same two species in crossing; so 
Sageret believes this to be the case with different individuals of the 
same two species in being grafted together. As in reciprocal crosses, 
the facility of effecting an union is often very far from equal, so it 
sometimes is in grafting; the common gooseberry, for instance, can- 
not be grafted on the currant, whereas the currant will take, though 
with difficulty, on the gooseberry. 

We have seen that the sterility of hybrids, which have their repro- 
ductive organs in an imperfect condition, is a different case from 


the difficulty of uniting two pure species, which have their repro- 
ductive organs perfect; yet these two distinct classes of cases run to a 
large extent parallel. Something analogous occurs in grafting; for 
Thouin found that three species of Robinia, which seeded freely on 
their own roots, and which could be grafted with no great difficulty 
on a fourth species, when thus grafted were rendered barren. On the 
other hand, certain species of Sorbus, when grafted on other species 
yielded twice as much fruit as when on their own roots. We are 
reminded by this latter fact of the extraordinary cases of Hip- 
peastrum, Passiflora, etc., which seed much more freely when fer- 
tilised with the pollen of a distinct species, than when fertilised with 
pollen from the same plant. 

We thus see, that, although there is a clear and great difference 
between the mere adhesion of grafted stocks, and the union of the 
male and female elements in the act of reproduction, yet that there 
is a rude degree of parallelism in the results of grafting and of 
crossing distinct species. And as we must look at the curious and 
complex laws governing the facility with which trees can be grafted 
on each other as incidental on unknown differences in their vegeta- 
tive systems, so I believe that the still more complex laws governing 
the facility of first crosses are incidental on unknown differences in 
their reproductive systems. These differences in both cases, follow 
to a certain extent, as might have been expected, systematic affinity, 
by which term every kind of resemblance and dissimilarity between 
organic beings is attempted to be expressed. The facts by no means 
seem to indicate that the greater or lesser difficulty of either grafting 
or crossing various sp)ecies has been a sp)ecial endowment; although 
in the case of crossing, the difficulty is as important for the endurance 
and stability of specific forms, as in the case of grafting it is unim- 
portant for their welfare. 


At one time it appeared to me probable, as it has to others, that the 
sterility of first crosses and of hybrids might have been slowly 
acquired through the natural selection of slighdy lessened degrees 
of fertiUty, which, like any other variation, spontaneously appeared 
in certain individuals of one variety when crossed with those of 


another variety. For it would clearly be advantageous to two vari- 
eties or incipient species, if they could be kept from blending, on 
the same principle that, when man is selecting at the same time two 
varieties, it is necessary that he should keep them separate. In the 
first place, it may be remarked that species inhabiting distinct 
regions are often sterile when crossed; now it could clearly have been 
of no advantage to such separated species to have been rendered 
mutually sterile, and consequently this could not have been effected 
through natural selection; but it may perhaps be argued, that, if a 
species was rendered sterile with some one compatriot, sterility with 
other species would follow as a necessary contingency. In the second 
place, it is almost as much opposed to the theory of natural selection 
as to that of special creation, that in reciprocal crosses the male 
element of one form should have been rendered utterly impotent 
on a second form, whilst at the same time the male element of this 
second form is enabled freely to fertilise the first form; for this 
peculiar state of the reproductive system could hardly have been 
advantageous to either species. 

In considering the probability of natural selection having come 
into action, in rendering species mutually sterile, the greatest dif- 
ficulty will be found to lie in the existence of many graduated steps 
from slightly lessened fertility to absolute sterility. It may be admit- 
ted that it would profit an incipient species, if it were rendered 
in some slight degree sterile when crossed with its parent form or 
with some other variety; for thus fewer bastardised and deteriorated 
offspring would be produced to commingle their blood with the new 
sf)ecies in process of formation. But he who will take the trouble to 
reflect on the steps by which this first degree of sterility could be 
increased through natural selection to that high degree which is 
common with so many species, and which is universal with species 
which have been differentiated to a generic or family rank, will find 
the subject extraordinarily complex. After mature reflection it seems 
to me that this could not have been effected through natural selec- 
tion. Take the case of any two sjjecies which, when crossed, produced 
few and sterile offspring; now, what is there which could favour 
the survival of those individuals which happened to be endowed in 
a slightly higher degree with mutual infertility, and which thus 


approached by one small step towards absolute sterility? Yet an 
advance of this kind, if the theory of natural selection be brought 
to bear, must have incessantly occurred with many species, for a 
multitude are mutually quite barren. With sterile neuter insects we 
have reason to believe that modifications in their structure and 
fertility have been slowly accumulated by natural selection, from an 
advantage having been thus indirectly given to the community to 
which they belonged over other communities of the same species; 
but an individual animal not belonging to a social community, if 
rendered slightly sterile when crossed with some other variety, would 
not thus itself gain any advantage or indirectly give any advantage 
to the other individuals of the same variety, thus leading to their 

But it would be superfluous to discuss this question in detail; for 
with plants we have conclusive evidence that the sterility of crossed 
species must be due to some principle, quite independent of natural 
selection. Both Gartner and Kolreuter have proved that in genera 
including numerous species, a series can be formed from sjjecies 
which when crossed yield fewer and fewer seeds, to species which 
never produce a single seed, but yet are affected by the pollen of 
certain other species, for the germen swells. It is here manifestly 
impossible to select the more sterile individuals, which have already 
ceased to yield seeds; so that this acme of sterility, when the germen 
alone is affected, cannot have been gained through selection; and 
from the laws governing the various grades of sterility being so 
uniform throughout the animal and vegetable kingdoms, we may 
infer that the cause, whatever it may be, is the same or nearly the 
same in all cases. 

We will now look a little closer at the probable nature of the 
differences between species which induce sterility in first crosses and 
in hybrids. In the case of first crosses, the greater or less difficulty 
in effecting an union and in obtaining offspring apparently depends 
on several distinct causes. There must sometimes be a physical 
impossibility in the male element reaching the ovule, as would be the 
case with a plant having a pistil too long for the pollen-tubes to 
reach the ovarium. It has also been observed that when the pollen 


of one sjjecies is placed on the stigma of a distantly allied species, 
though the pollen-tubes protrude, they do not penetrate the stigmatic 
surface. Again, the male element may reach the female element but 
be incapable of causing an embryo to be developed, as seems to 
have been the case with some of Thuret's experiments on Fuci. No 
explanation can be given of these facts, any more than why certain 
trees cannot be grafted on others. Lasdy an embryo may be devel- 
oped, and then perish at an early period. This latter alternative has 
not been suiTiciently attended to; but I believe, from observations 
communicated to me by Mr. Hewitt, who has had great experience 
in hybridising pheasants and fowls, that the early death of the 
embryo is a very frequent cause of sterility in first crosses. Mr. Salter 
has recently given the results of an examination of about 500 eggs 
produced from various crosses between three species of Gallus and 
their hybrids; the majority of these eggs had been fertilised; and in 
the majority of the fertilised eggs, the embryos had either been par- 
tially developed and had then perished, or had become nearly 
mature, but the young chickens had been unable to break through 
the shells. Of the chickens which were born, more than four-fifths 
died within the first few days, or at latest weeks, "without any 
obvious cause, apparently from mere inability to live"; so that from 
the 500 eggs only twelve chickens were reared. With plants, hy- 
bridised embryos probably often perish in a Uke manner; at least it 
is known that hybrids raised from very distinct species are sometimes 
weak and dwarfed, and perish at an early age; of which fact Max 
Wichura has recently given some striking cases with hybrid willows. 
It may be here worth noticing that in some cases of parthenogenesis, 
the embryos within the eggs of silk moths which had not been 
fertilised, pass through their early stages of development and then 
perish like the embryos produced by a cross between distinct species. 
Until becoming acquainted with these facts, I was unwilling to 
believe in the frequent early death of hybrid embryos; for hybrids, 
when once born, are generally healthy and long-lived, as we see in 
the case of the common mule. Hybrids, however, are differendy 
circumstanced before and after birth; when born and living in a 
country where their two parents live, they are generally placed under 
suitable conditions of life. But a hybrid partakes of only half of the 


nature and constitution of its mother; it may therefore before birth, 
as long as it is nourished within its mother's womb, or within the 
egg or seed produced by the mother, be exposed to conditions in 
some degree unsuitable, and consequently be liable to perish at an 
early jjeriod; more especially as all very young beings are eminently 
sensitive to injurious or unnatural conditions of life. But after all, 
the cause more probably lies in some imperfection in the original 
act of impregnation, causing the embryo to be imperfectly devel- 
oped, rather than in the conditions to which it is subsequently ex- 

In regard to the sterility of hybrids, in which the sexual elements 
are imperfectly developed, the case is somewhat different. I have 
more than once alluded to a large body of facts showing that, when 
animals and plants are removed from their natural conditions, they 
are extremely liable to have their reproductive systems seriously 
affected. This, in fact, is the great bar to the domestication of 
animals. Between the sterility thus superinduced and that of hybrids, 
there are many points of similarity. In both cases the steriUty is 
independent of general health, and is often accompanied by excess 
of size or great luxuriance. In both cases the sterility occurs in vari- 
ous degrees; in both, the male element is the most liable to be 
affected; but sometimes the female more than the male. In both, 
the tendency goes to a certain extent with systematic affinity, for 
whole groups of animals and plants are rendered imf)otent by the 
same unnatural conditions; and whole groups of species tend to 
produce sterile hybrids. On the other hand, one Sf)ecies in a group 
will sometimes resist great changes of conditions with unimpaired 
fertility; and certain species in a group will produce unusually fer- 
tile hybrids. No one can tell till he tries, whether any particular 
animal will breed under confinement, or any exotic plant seed freely 
under culture; nor can he tell till he tries, whether any two species 
of a genus will produce more or less sterile hybrids. Lastly, when 
organic beings are placed during several generations under condi- 
tions not natural to them, they are extremely liable to vary, which 
seems to be partly due to their reproductive systems having been 
specially affected, though in a lesser degree than when sterility 
ensues. So it is with hybrids, for their offspring in successive genera- 


tions are eminently liable to vary, as every experimentalist has 

Thus we see that when organic beings are placed under new and 
unnatural conditions, and when hybrids are produced by the un- 
natural crossing of two species, the reproductive system, independ- 
ently of the general state of health, is affected in a very similar 
manner. In the one case, the conditions of life have been disturbed, 
though often in so slight a degree as to be inappreciable by us; in 
the other case, or that of hybrids, the external conditions have 
remained the same, but the organisation has been disturbed by two 
distinct structures and constitutions, including of course the repro- 
ductive systems, having been blended into one. For it is scarcely 
possible that two organisations should be compxjunded into one, 
without some disturbance occurring in the development, or peri- 
odical action, or mutual relations of the different parts and organs 
one to another or to the conditions of life. When hybrids are able to 
breed inter se, they transmit to their offspring from generation to 
generation the same compounded organisation, and hence we need 
not be surprised that their sterility, though in some degree variable, 
does not diminish; it is even apt to increase, this being generally the 
result, as before explained, of too close interbreeding. The above 
view of the sterility of hybrids being caused by two constitutions 
being compounded into one has been strongly maintained by Max 

It must, however, be owned that we cannot understand, on the 
above or any other view, several facts with respect to the sterility of 
hybrids; for instance, the unequal fertility of hybrids produced from 
reciprocal crosses; or the increased sterility in those hybrids which 
occasionally and exceptionally resemble closely either pure parent. 
Nor do I pretend that the foregoing remarks go to the root of the 
matter; no explanation is offered why an organism, when placed 
under natural conditions, is rendered sterile. All that I have at- 
tempted to show is, that in two cases, in some respeas allied, sterility 
is the common result, — in the one case from the conditions of life 
having been disturbed, in the other case from the organisation having 
been disturbed by two organisations being compounded into one. 

A similar parallelism holds good with an allied yet very different 


class of facts. It is an old and almost universal belief founded on a 
considerable body of evidence, which I have elsewhere given, that 
slight changes in the conditions of life are beneficial to all living 
things. We see this acted on by farmers and gardeners in their 
frequent exchanges of seed, tubers, etc., from one soil or climate to 
another, and back again. During the convalescence of animals, great 
benefit is derived from almost any change in their habits of life. 
Again, both with plants and animals, there is the clearest evidence 
that a cross between individuals of the same species, which differ to 
a certain extent, gives vigour and fertility to the offspring; and that 
close interbreeding continued during several generations between 
the nearest relations, if these be kept under the same conditions of 
life, almost always leads to decreased size, weakness, or sterility. 

Hence it seems that, on the one hand, slight changes in the condi- 
tions of life benefit all organic beings, and on the other hand, that 
slight crosses, that is, crosses between the males and females of the 
same species, which have been subjected to slightly different condi- 
tions, or which have slightly varied, give vigour and fertility to the 
offspring. But, as we have seen, organic beings long habituated to 
certain uniform conditions under a state of nature, when subjected, 
as under confinement, to a considerable change in their conditions, 
very frequently are rendered more or less sterile; and we know that 
a cross between two forms, that have become widely or specifically 
different, produce hybrids which are almost always in some degree 
sterile. I am fully f)ersuaded that this double parallelism is by no 
means an accident or an illusion. He who is able to explain why 
the elephant and a multitude of other animals are incapable of 
breeding when kept under only partial confinement in their native 
country, will be able to explain the primary cause of hybrids being 
so generally sterile. He will at the same time be able to explain how 
it is that the races of some of our domesticated animals, which have 
often been subjected to new and not uniform conditions, are quite 
fertile together, although they are descended from distinct species, 
which would probably have been sterile if aboriginally crossed. The 
above two parallel series of facts seem to be connected together by 
some common but unknown bond, which is essentially related to 
the principle of life; this principle, according to Mr. Herbert Spencer, 


being that life depends on, or consists in, the incessant action and 
reaction of various forces, which, as throughout nature, are always 
tending towards an equiHbrium; and when this tendency is sHghtly 
disturbed by any change, the vital forces gain in jwwer. 


This subject may be here briefly discussed, and will be found to 
throw some light on hybridism. Several plants belonging to distinct 
orders present two forms, which exist in about equal numbers and 
which differ in no respect except in their reproductive organs; one 
form having a long pistil with short stamens, the other a short pistil 
with long stamens; the two having differently sized pollen-grains. 
With trimorphic plants there are three forms likewise differing in the 
lengths of their pistils and stamens, in the size and colour of the 
pollen-grains, and in some other respects; and as in each of the three 
forms there are two sets of stamens, the three forms pnDssess altogether 
six sets of stamens and three kinds of pistils. These organs are so 
proportioned in length to each other, that half the stamens in two 
of the forms stand on a level with the stigma of the third form. 
Now I have shown, and the result has been confirmed by other 
observers, that, in order to obtain full fertility with these plants, it is 
necessary that the stigma of the one form should be fertilised by 
pollen taken from the stamens of corresponding height in another 
form- So that with dimorphic species two unions, which may be 
called legitimate, are fully fertile; and two, which may be called 
illegitimate, are more or less infertile. With trimorphic species six 
unions are legitimate, or fully fertile, — and twelve are illegitimate, 
or more or less infertile. 

The infertility which may be observed in various dimorphic and 
trimorphic plants, when they are illegitimately fertilised, that is, by 
pollen taken from stamens not corresponding in height with the 
pistil, differs much in degree, up to absolute and utter sterility; just 
in the same manner as occurs in crossing distinct species. As the 
degree of sterility in the latter case depends in an eminent degree on 
the conditions of life being more or less favourable, so I have found 
it with illegitimate unions. It is well known that if pollen of a dis- 
tinct species be placed on the stigma of a flower, and its own pollen 


be afterwards, even after a considerable interval of time, placed on 
the same stigma, its action is so strongly prepotent that it generally 
annihilates the effect of the foreign pollen; so it is with the pollen 
of the several forms of the same species, for legitimate pollen is 
strongly prepotent over illegitimate pollen, when both are placed on 
the same stigma. I ascertained this by fertilising several flowers, 
first illegitimately, and twenty-four hours afterwards legitimately, 
with pollen taken from a peculiarly coloured variety, and all the seed- 
lings were similarly coloured; this shows that the legitimate pollen, 
though applied twenty-four hours subsequently, had wholly de- 
stroyed or prevented the action of the previously applied illegitimate 
pollen. Again, as in making reciprocal crosses between the same two 
species, there is occasionally a great difference in the result, so the 
same thing occurs with trimorphic plants; for instance, the mid- 
styled form of Lythrum salicaria was illegitimately fertilised with 
the greatest ease by pollen from the longer stamens of the short- 
styled form, and yielded many seeds; but the latter form did not 
yield a single seed when fertilised by the longer stamens of the 
mid-styled form. 

In all these respects, and in others which might be added, the 
forms of the same undoubted species when illegitimately united be- 
have in exactly the same manner as do two distinct species when 
crossed. This led me carefully to observe during four years many 
seedlings, raised from several illegitimate unions. The chief result 
is that these illegitimate plants, as they may be called, are not fully 
fertile. It is possible to raise from dimorphic species, both long- 
styled and short-styled illegitimate plants, and from trimorphic 
plants all three illegitimate forms. These can then be properly united 
in a legitimate manner. When this is done, there is no apparent 
reason why they should not yield as many seeds as did their parents 
when legitimately fertilised. But such is not the case. They are all 
infertile, in various degrees; some being so utterly and incurably 
sterile that they did not yield during four seasons a single seed or 
even seed<apsule. The sterility of these illegitimate plants, when 
united with each other in a legitimate manner, may be strictly com- 
pared with that of hybrids when crossed inter se. If, on the other 
hand, a hybrid is crossed with either pure parent-species, the sterility 


is usually much lessened; and so it is when an illegitimate plant is 
fertilised by a legitimate plant. In the same manner as the sterility 
of hybrids does not always run parallel with the difficulty of making 
the first cross between the two parent-species, so the sterility of certain 
illegitimate plants was unusually great, whilst the sterility of the 
union from which they were derived was by no means great. With 
hybrids raised from the same seed-capsule the degree of sterility is 
innately variable, so it is in a marked manner with illegitimate 
plants. Lasdy, many hybrids are profuse and persistent flowerers, 
whilst other and more sterile hybrids produce few flowers, and are 
weak, miserable dwarfs; exactly similar cases occur with the illegiti- 
mate offspring of various dimorphic and trimorphic plants. 

Altogether there is the closest identity in character and behaviour 
between illegitimate plants and hybrids. It is hardly an exaggeration 
to maintain that illegitimate plants are hybrids, produced within the 
limits of the same species by the improper union of certain forms, 
whilst ordinary hybrids are produced from an improper union be- 
tween so-called distinct species. We have also already seen that there 
is the closest similarity in all respects between first illegitimate unions 
and first crosses between distinct species. This will perhaps be made 
more fully apparent by an illustration; we may suppose that a 
botanist found two well-marked varieties (and such occur) of the 
long-styled form of the trimorphic Lythrum salicaria, and that he 
determined to try by crossing whether they were specifically distinct. 
He would find that they yielded only about one-fifth of the projjer 
number of seed, and that they behaved in all the other above speci- 
fied respects as if they had been two distinct species. But to make 
the case sure, he would raise plants from his supposed hybridized 
seed, and he would find that the seedlings were miserably dwarfed 
and utterly sterile, and that they behaved in all other respects like 
ordinary hybrids. He might then maintain that he had actually 
proved, in accordance with the common view, that his two varieties 
were as good and as distinct species as any in the world; but he would 
be completely mistaken. 

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


criterion o£ specific distinction; secondly, because we may conclude 
that there is some unknown bond which connects the infertility of 
illegitimate unions with that of their illegitimate offspring, and we 
are led to extend the same view to first crosses and hybrids; thirdly, 
because we find, and this seems to me of especial importance, that 
two or three forms of the same sp)ecies may exist and may differ in 
no respect whatever, either in structure or in constitution, relatively 
to external conditions, and yet be sterile when united in certain ways. 
For we must remember that it is the union of the sexual elements of 
individuals of the same form, for instance, of two long-styled forms, 
which results in sterility; whilst it is the union of the sexual elements 
proper to two distinct forms which is fertile. Hence the case appears 
at first sight exactly the reverse of what occurs, in the ordinary unions 
of the individuals of the same species and with crosses between dis- 
tinct species. It is, however, doubtful whether this is really so; but I 
will not enlarge on this obscure subject. 

We may, however, infer as probable from the consideration of 
dimorphic and trimorphic plants, that the sterility of distinct species 
when crossed and of their hybrid progeny, depends exclusively on 
the nature of their sexual elements, and not on any difference in 
their structure or general constitution. We are also led to this same 
conclusion by considering reciprocal crosses, in which the male of 
one species cannot be united, or can be united with great difficulty, 
with the female of a second species, whilst the converse cross can be 
effected with [jerfeot facility. That excellent observer, Gartner, like- 
wise concluded that species when crossed are sterile owing to differ- 
ences confined to their reproductive systems. 


It may be urged, as an overwhelming argument, that there must 
be some essential distinction between species and varieties, inasmuch 
as the latter, however much they may differ from each other in 
external appearance, cross with perfect facility, and yield {perfectly 
fertile offspring. With some exceptions, presently to be given, I fully 
admit that this is the rule. But the subject is surrounded by difficul- 
ties, for, looking to varieties produced under nature, if two forms 


hitherto reputed to be varieties be found in any degree sterile 
together, they are at once ranked by most naturaUsts as species. For 
instance, the blue and red pimpernel, which are considered by most 
botanists as varieties, are said by Gartner to be quite sterile when 
crossed, and he consequently ranks them as undoubted species. If 
we thus argue in a circle, the fertility of all varieties produced under 
nature will assuredly have to be granted. 

If we turn to varieties, produced, or supposed to have been pro- 
duced, under domestication, we are still involved in some doubt. 
For when it is stated, for instance, that certain South American 
indigenous domestic dogs do not readily unite with European dogs, 
the explanation which will occur to every one, and probably the true 
one, is that they are descended from aboriginally distinct sp)ecies. 
Nevertheless the f)erfect fertility of so many domestic races, differing 
widely from each other in appearance, for instance those of the 
pigeon, or of the cabbage, is a remarkable fact; more especially when 
we reflect how many species there are, which, though resembling 
each other most closely, are utterly sterile when intercrossed. Several 
considerations, however, render the fertility of domestic varieties 
less remarkable. In the first place, it may be observed that the amount 
of external difference between two species is no sure guide to their 
degree of mutual sterility, so that similar differences in the case of 
varieties would be no sure guide. It is certain that with species the 
cause lies exclusively in differences in their sexual constitution. Now 
the varying conditions to which domesticated animals and cultivated 
plants have been subjected, have had so little tendency towards modi- 
fying the reproductive system in a manner leading to mutual 
sterility, that we have good grounds for admitting the directly oppo- 
site doctrine of Pallas, namely, that such conditions generally elim- 
inate this tendency; so that the domesticated descendants of species, 
which in their natural state probably would have been in some degree 
sterile when crossed, become perfectly fertile together. With plants, 
so far is cultivation from giving a tendency towards sterility between 
distinct species, that in several well-authenticated cases already 
alluded to, certain plants have been affected in an opposite manner, 
for they have become self-impotent whilst still retaining the capacity 
of fertilising, and being fertilised by, other species. If the Pallasian 


doctrine of the elimination of sterility through long-continued domes- 
tication be admitted, and it can hardly be rejected, it becomes in 
the highest degree improbable that similar conditions long con- 
tinued should likewise induce this tendency; though in certain cases, 
with sp)ecies having a p)eculiar constitution, sterility might occasion- 
ally be thus caused. Thus, as I believe, we can understand why with 
domesticated animals varieties have not been produced which are 
mutually sterile; and why with plants only a few such cases, imme- 
diately to be given, have been observed. 

The real difficulty in our present subject is not, as it appears to 
me, why domestic varieties have not become mutually infertile 
when crossed, but why this has so generally occurred with natural 
varieties, as soon as they have been permanently modified in a 
sufficient degree to take rank as species. We are far from precisely 
knowing the cause; nor is this surprising, seeing how profoundly 
ignorant we are in regard to the normal and abnormal action of the 
reproductive system. But we can see that sp)ecies, owing to their 
struggle for existence with numerous competitors, will have been ex- 
posed during long periods of time to more uniform conditions, than 
have domestic varieties; and this may well make a wide difference in 
the result. For we know how commonly wild animals and plants, 
when taken from their natural conditions and subjected to captivity, 
are rendered sterile; and the reproductive functions of organic 
beings which have always lived under natural conditions would 
probably in like manner be eminently sensitive to the influence of 
an unnatural cross. Domesticated productions, on the other hand, 
which, as shown by the mere fact of their domestication, were not 
originally highly sensitive to changes in their conditions of life, and 
which can now generally resist with undiminished fertility repeated 
changes of conditions, might be expected to produce varieties, which 
would be little liable to have their reproductive powers injuriously 
affected by the act of crossing with other varieties which had orig- 
inated in a like manner. 

I have as yet spoken as if the varieties of the same species were 
invariably fertile when intercrossed. But it is impossible to resist 
the evidence of the existence of a certain amount of sterility in the 
few following cases, which I will briefly abstract. The evidence is 


at least as good as that from which we beHeve in the sterility of a 
multitude of species. The evidence is, also, derived from hostile 
witnesses, who in all other cases consider fertility and sterility as safe 
criterions of specific distinction. Gartner kept during several years 
a dwarf kind of maize with yellow seeds, and a tall variety with 
red seeds growing near each other in his garden; and although these 
plants have separated sexes, they never naturally crossed. He then 
fertilised thirteen flowers of the one kind with pollen of the other; 
but only a single head produced any seed, and this one head pro- 
duced only five grains. Manipulation in this case could not have 
been injurious, as the plants have separated sexes. No one, I believe, 
has suspected that these varieties of maize are distinct species; and it 
is important to notice that the hybrid plants thus raised were them- 
selves perfectly fertile; so that even Gartner did not venture to con- 
sider the two varieties as specifically distinct. 

Girou de Buzareingues crossed three varieties of gourd, which 
like the maize has separated sexes, and he asserts that their mutual 
fertilisation is by so much the less easy as their differences are 
greater. How far these experiments may be trusted, I know not; 
but the forms experimented on are ranked by Sageret, who mainly 
founds his classification by the test of infertility, as varieties, and 
Naudin has come to the same conclusion. 

The following case is far more remarkable, and seems at first 
incredible; but it is the result of an astonishing number of experi- 
ments made during many years on nine species of Verbascum, by 
so good an observer and so hostile a witness as Gartner: namely 
that the yellow and white varieties when crossed produce less 
seed than the similarly coloured varieties of the same species. 
Moreover, he asserts that when yellow and white varieties of one 
species are crossed with yellow and white varieties of a distinct 
species, more seed is produced by the crosses between the similarly 
coloured flowers, than between those which are differently coloured. 
Mr. Scott also has experimented on the species and varieties of 
Verbascum; and although unable to confirm Gartner's results on 
the crossing of the distinct species, he finds that the dissimilarly 
coloured varieties of the same sf)ecies yield fewer seeds, in the pro- 
portion of eighty-six to lOO, than the similarly coloured varieties. 


Yet these varieties differ in no respect except in the colour of their 
flowers; and one variety can sometimes be raised from the seed of 

Kolreuter, whose accuracy has been confirmed by every subsequent 
observer, has proved the remarkable fact, that one particular variety 
of the common cobacco was more fertile than the other varieties, 
when crossed with a widely distinct species. He experimented on 
five forms which are commonly reputed to be varieties, and which 
he tested by the severest trial, namely, by reciprocal crosses, and he 
found their mongrel offspring perfectly fertile. But one of these 
five varieties, when used either as the father or mother, and crossed 
with the Nicotiana glutinosa, always yielded hybrids not so sterile 
as those which were produced from the four other varieties when 
crossed with N. glutinosa. Hence the reproductive system of this 
one variety must have been in some manner and in some degree 

From these facts it can no longer be maintained that varieties 
when crossed are invariably quite fertile. From the great difficulty 
of ascertaining the infertility of varieties in a state of nature, for a 
supposed variety, if proved to be infertile in any degree, would 
almost universally be ranked as a species; — from man attending only 
to external characters in his domestic varieties, and from such varie- 
ties not having been exposed for very long periods to uniform con- 
ditions of life; — from these several considerations we may conclude 
that fertility does not constitute a fundamental distinction between 
varieties and sf>ecies when crossed. The general sterility of crossed 
species may safely be looked at, not as a special acquirement or en- 
dowment, but as incidental on changes of an unknown nature in 
their sexual elements. 


Independently of the question of fertility, the offspring of species 
and of varieties when crossed may be compared in several other 
respects. Gartner, whose strong wish it was to draw a distinct line 
between species and varieties, could find very few, and, as it seems 
to me, quite unimportant differences between the so<alled hybrid 


offspring of species, and the so<allecl mongrel offspring of varieties. 
And, on the other hand, they agree most closely in many important 

I shall here discuss this subject with extreme brevity. The most 
important distinction is, that in the first generation mongrels are 
more variable than hybrids; but Gartner admits that hybrids from 
species which have long been cultivated are often variable in the 
first generation; and I have myself seen striking instances of this 
fact. Gartner further admits that hybrids between very closely allied 
sf)ecies are more variable than those from very distinct sp)ecies; and 
this shows that the difference in the degree of variability graduates 
away. When mongrels and the more fertile hybrids are propagated 
for several generations, an extreme amount of variability in the off- 
spring in both cases is notorious; but some few instances of both 
hybrids and mongrels long retaining a uniform character could be 
given. The variability, however, in the successive generations of 
mongrels is, perhaps, greater than in hybrids. 

This greater variability in mongrels than in hybrids does not 
seem at all surprising. For the parents of mongrels are varieties, 
and mostly domestic varieties (very few experiments having been 
tried on natural varieties), and this implies that there has been recent 
variability, which would often continue and would augment that 
arising from the act of crossing. The slight variability of hybrids in 
the first generation, in contrast with that in the succeeding genera- 
tions, is a curious fact and deserves attention. For it bears on the 
view which I have taken of one of the causes of ordinary variability; 
namely, that the reproductive system from being eminently sensitive 
to changed conditions of life, fails under these circumstances to per- 
form its proper function of producing offspring closely similar in 
all respects to the parent-form. Now, hybrids in the first generation 
are descended from species (excluding those long cultivated) which 
have not had their reproductive systems in any way affected, and 
they are not variable; but hybrids themselves have their reproduc- 
tive systems seriously affected, and their descendants are highly 

But to return to our comparison of mongrels and hybrids: Gartner 
states that mongrels are more liable than hybrids to revert to either 


parent-form; but this, if it be true, is certainly only a difference in 
degree. Moreover, Gartner expressly states that hybrids from long 
cultivated plants are more subject to reversion than hybrids from 
species in their natural state; and this probably explains the singular 
difference in the results arrived at by different observers: thus Max 
Wichura doubts whether hybrids ever revert to their parent-forms, 
and he experimented on uncultivated species of willows; whilst 
Naudin, on the other hand, insists in the strongest terms on the 
almost universal tendency to reversion in hybrids, and he experi- 
mented chiefly on cultivated plants. Gartner further states that 
when any two sjiecies, although most closely allied to each other, 
are crossed with a third species, the hybrids are widely different from 
each other; whereas if two very distinct varieties of one species are 
crossed with another sp)ecies, the hybrids do not differ much. But 
this conclusion, as far as I can make out, is founded on a single ex- 
periment; and seems directly opposed to the results of several ex- 
periments made by Kolreuter. 

Such alone are the unimportant differences which Gartner is 
able to point out between hybrid and mongrel plants. On the other 
hand, the degrees and kinds of resemblance in mongrels and in 
hybrids to their respective parents, more especially in hybrids pro- 
duced from nearly related species, follow, according to Gartner, 
the same laws. When two species are crossed, one has sometimes a 
prepotent power of impressing its likeness on the hybrid. So I believe 
it to be with varieties of plants; and with animals one variety cer- 
tainly often has this prepotent power over another variety. Hybrid 
plants produced from a reciprocal cross, generally resemble each 
other closely; and so it is with mongrel plants from a reciprocal 
cross. Both hybrids and mongrels can be reduced to either pure 
parent-form, by repeated crosses in successive generations with either 

These several remarks are apparendy applicable to animals; but 
the subject is here much complicated, partly owing to the existence 
of secondary sexual charaaers; but more especially owing to prepo- 
tency in transmitting likeness running more strongly in one sex than 
in the other, both when one species is crossed with another, and 


when one variety is crossed with another variety. For instance, I 
think those authors are right who maintain that the ass has a pre- 
potent power over the horse, so that both the mule and the hinny 
resemble more closely the ass than the horse; but that the prepotency 
runs more strongly in the male than in the female ass, so that the 
mule, which is the offspring of the male ass and mare, is more like 
an ass, than is the hinny, which is the offspring of the female ass 
and stallion. 

Much stress has been laid by some authors on the supposed fart, 
that it is only with mongrels that the offspring are not intermediate 
in character, but closely resemble one of their parents; but this does 
sometimes occur with hybrids, yet I grant much less frequently 
than with mongrels. Looking to the cases which I have collected of 
cross-bred animals closely resembling one parent, the resemblances 
seem chiefly confined to characters almost monstrous in their nature, 
and which have suddenly appeared — such as albinism, melanism, 
deficiency of tail or horns, or additional fingers and toes; and do not 
relate to characters which have been slowly acquired through selec- 
tion. A tendency to sudden reversions to the perfect character of 
either parent would, also, be much more likely to occur with mon- 
grels, which are descended from varieties often suddenly produced 
and semi-monstrous in character, than with hybrids, which are de- 
scended from species slowly and naturally produced. On the whole, 
I entirely agree with Dr. Prosper Lucas, who, after arranging an 
enormous body of facts with respect to animals, comes to the con- 
clusion that the laws of resemblance of the child to its parents are 
the same, whether the two parents differ little or much from each 
other, namely, in the union of individuals of the same variety, or of 
different varieties, or of distinct sjiecies. 

Independently of the question of fertiHty and sterility, in all other 
respects there seems to be a general and close similarity in the off- 
spring of crossed species, and of crossed varieties. If we look at 
species as having been specially created, and at varieties as having 
been produced by secondary laws, this similarity would be an 
astonishing fact. But it harmonises perfectly with the view that there 
is no essential distinction between species and varieties. 



First crosses between forms, sufficiently distinct to be ranked as 
sf)ecies, and their hybrids, are very generally, but not universally, 
sterile. The sterility is of all degrees, and is often so slight that the 
most careful experimentalists have arrived at diametrically opposite 
conclusions in ranking forms by this test. The sterility is innately 
variable in individuals of the same species, and is eminently sus- 
ceptible to the action of favourable and unfavourable conditions. 
The degree of sterility does not strictly follow systematic affinity, 
but is governed by several curious and complex laws. It is generally 
different, and sometimes widely different in reciprocal crosses be- 
tween the same two species. It is not always equal in degree in a 
first cross and in the hybrids produced from this cross. 

In the same manner as in grafting trees, the capacity in one species 
or variety to take on another, is incidental on differences, generally 
of an unknown nature, in their vegetative systems, so in crossing, the 
greater or less facility of one sfjecies to unite with another is inci- 
dental on unknown differences in their reproductive systems. There 
is no more reason to think that species have been specially endowed 
with various degrees of sterility to prevent their crossing and blending 
in nature, than to think that trees have been specially endowed with 
various and somewhat analogous degrees of difficulty in being 
grafted together in order to prevent their inarching in our forests. 

The sterility of first crosses and of their hybrid progeny has not 
been acquired through natural selection. In the case of first crosses 
it seems to depend on several circumstances; in some instances in 
chief part on the early death of the embryo. In the case of hybrids, 
it apparently depends on their whole organisation having been dis- 
turbed by being compounded from two distinct forms; the sterility 
being closely allied to that which so frequently affects pure sjiecies, 
when exposed to new and unnatural conditions of life. He who will 
explain these latter cases will be able to explain the sterility of hy- 
brids. This view is strongly supported by a parallelism of another 
kind: namely, that, firstly, slight changes in the conditions of life 
add to the vigour and fertility of all organic beings; and secondly, 
that the crossing of forms, which have been exposed to slightly 


difTerent conditions of life or which have varied, favours the size, 
vigour, and fertihty of their offspring. The facts given on the sterility 
of the illegitimate unions of dimorphic and trimorphic plants and 
of their illegitimate progeny, perhaps render it probable that some 
unknown bond in all cases connects the degree of fertility of first 
unions with that of their offspring. The consideration of these facts 
on dimorphism, as well as of the results of reciprocal crosses, clearly 
leads to the conclusion that the primary cause of the sterility of 
crossed species is confined to differences in their sexual elements. 
But why, in the case of distinct species, the sexual elements should 
so generally have become more or less modified, leading to their 
mutual infertility, we do not know; but it seems to stand in some 
close relation to species having been exposed for long periods of time 
to nearly uniform conditions of life. 

It is not surprising that the difficulty in crossing any two species, 
and the sterility of their hybrid offspring, should in most cases cor- 
respond, even if due to distinct causes: for both depend on the 
amount of difference between the species which are crossed. Nor 
is it surprising that the facility of effecting a first cross, and the 
fertility of the hybrids thus produced, and the capacity of being 
grafted together — though this latter capacity evidently depends on 
widely different circumstances — should all run, to a certain extent, 
parallel with the systematic affinity of the forms subjected to experi- 
ment; for systematic affinity includes resemblances of all kinds. 

First crosses between forms known to be varieties, or sufficiently 
alike to be considered as varieties, and their mongrel offspring, are 
very generally, but not, as is so often stated, invariably fertile. Nor 
is this almost universal and perfect fertility surprising, when it is 
remembered how liable we are to argue in a circle with respect to 
varieties in a state of nature; and when we remember that the 
greater number of varieties have been produced under domestication 
by the selection of mere external differences, and that they have not 
been long exposed to uniform conditions of life. It should also be 
esf)ecially kept in mind, that long<ontinued domestication tends to 
eliminate sterility, and is therefore little likely to induce this same 
quality. Independently of the question of fertility, in all other 
respects there is the closest general resemblance between hybrids and 


mongrels, — in their variability, in their power of absorbing each 
other by repeated crosses, and in their inheritance of characters from 
both parent-forms. Finally, then, although we are as ignorant of 
the precise cause of the sterility of first crosses and of hybrids as we 
are why animals and plants removed from their natural conditions 
become sterile, yet the facts given in this chapter do not seem to me 
opposed to the belief that species aboriginally existed as varieties. 


On the Imperfection of the Geological Record 

On the absence of intermediate varieties at the present day — On the 
nature of extinct intermediate varieties; on their number — On the 
lapse of time, as inferred from the rate of denudation and of deposi- 
tion — On the lapse of time as estimated by years — On the poorness 
of our palaeontological collections — On the intermittence of geologi- 
cal formations — On the denudation of granitic areas — On the absence 
of intermediate varieties in any one formation — On the sudden 
appearance of groups of sjjecies— On their sudden appearance in the 
lowest known fossiliferous strata — Antiquity of the habitable earth. 

IN the sixth chapter I enumerated the chief objections which 
might be justly urged against the views maintained in this 
volume. Most of them have now been discussed. One, namely 
the distinaness of specific forms, and their not being blended to- 
gether by innumerable transitional links, is a very obvious difficulty. 
I assigned reasons why such links do not commonly occur at the 
present day under the circumstances apparently most favourable for 
their presence, namely on an extensive and continuous area with 
graduated physical conditions. 1 endeavoured to show, that the life 
of each species depends in a more important manner on the presence 
of other already defined organic forms, than on climate, and, there- 
fore, that the really governing conditions of life do not graduate 
away quite insensibly like heat or moisture. I endeavoured, also, to 
show that intermediate varieties, from existing in lesser numbers 
than the forms which they connect, will generally be beaten out and 
exterminated during the course of further modification and im- 
provement. The main cause, however, of innumerable intermediate 
links not now occurring everywhere throughout nature, depends on 
the very process of natural selection, through which new varieties 
continually take the places of and supplant their parent-forms. But 
just in proportion as this process of extermination has acted on an 
enormous scale, so must the number of intermediate varieties, which 
have formerly existed, be truly enormous. Why then is not every 



geological formation and every stratum full of such intermediate 
links? Geology assuredly does not reveal any such finely-graduated 
organic chain; and this, perhaps, is the most obvious and serious 
objection which can be urged against the theory. The explanation 
lies, as I believe, in the extreme imperfection of the geological 

In the first place, it should always be borne in mind what sort of 
intermediate forms must, on the theory, have formerly existed. I 
have found it difficult, when looking at any two species, to avoid 
picturing to myself forms directly intermediate between them. But 
this is a wholly false view; we should always look for forms inter- 
mediate between each species and a common but unknown progeni- 
tor; and the progenitor will generally have differed in some respects 
from all its modified descendants. To give a simple illustration: 
the fantail and pouter pigeons are both descended from the rock- 
pigeon; if we possessed all the intermediate varieties which have 
ever existed, we should have an extremely close series between both 
and the rock-pigeon; but we should have no varieties directly in- 
termediate between the fantail and pouter; none, for instance, com- 
bining a tail somewhat expanded with a crop somewhat enlarged, 
the characteristic features of these two breeds. These two breeds, 
moreover, have become so much modified, that, if we had no his- 
torical or indirect evidence regarding their origin, it would not have 
been possible to have determined, from a mere comparison of their 
structure with that of the rock-pigeon, C. livia, whether they had 
descended from this species or from some other allied form, such 
as C. CEnas. 

So, with natural species, if we look to forms very distinct, for 
instance to the horse and tapir, we have no reason to suppose that 
links directly intermediate between them ever existed, but between 
each and an unknown common parent. The common parent will 
have had in its whole organisation much general resemblance to 
the tapir and to the horse; but in some points of structure may have 
differed considerably from both, even perhaps more than they differ 
from each other. Hence, in all such cases, we should be unable to 
recognise the parent-form of any two or more species, even if we 
closely compared the structure of the parent with that of its modified 


descendants, unless at the same time we had a nearly perfect chain 
of the intermediate links. 

It is just possible by the theory, that one of two living forms might 
have descended from the other; for instance, a horse from a tapir; 
and in this case direct intermediate links will have existed between 
them. But such a case would imply that one form had remained 
for a very long period unaltered, whilst its descendants had under- 
gone a vast amount of change; and the principle of competition 
between organism and organism, between child and parent, will 
render this a very rare event; for in all cases the new and improved 
forms of life tend to supplant the old and unimproved forms. 

By the theory of natural selection all living species have been con- 
nected with the parent-species of each genus, by differences not 
greater than we see between the natural and domestic varieties of 
the same species at the present day; and these parent-species, now 
generally extinct, have in their turn been similarly connected with 
more ancient forms; and so on backwards, always converging to 
the common ancestor of each great class. So that the number of 
intermediate and transitional links, between all living and extinct 
species, must have been inconceivably great. But assuredly, if this 
theory be true, such have lived upon the earth. 


Independently of our not finding fossil remains of such infinitely 
numerous connecting links, it may be objected that time cannot 
have sufficed for so great an amount of organic change, all changes 
having been effected slowly. It is hardly possible for me to recall to 
the reader who is not a practical geologist, the facts leading the 
mind feebly to comprehend the lapse of time. He who can read 
Sir Charles Lyell's grand work on the Principles of Geology, which 
the future historian will recognise as having produced a revolution 
in natural science, and yet does not admit how vast have been the 
past periods of time, may at once close this volume. Not that it 
suffices to study the Principles of Geology, or to read special treatises 
by difTerent observers on separate formations, and to mark how 
each author attempts to give an inadequate idea of the duration of 


each formation, or even of each stratum. We can best gain some 
idea of past time by knowing the agencies at work, and learning 
how deeply the surface of the land has been denuded, and how much 
sediment has been deposited. As Lyell has well remarked, the extent 
and thickness of our sedimentary formations are the result and the 
measure of the denudation which the earth's crust has elsewhere 
undergone. Therefore a man should examine for himself the great 
piles of superimposed strata, and watch the rivulets bringing down 
mud, and the waves wearing away the sea<liffs, in order to com- 
prehend something about the duration of past time, the monuments 
of which we see all around us. 

It is good to wander along the coast, when formed of moderately 
hard rocks, and mark the process of degradation. The tides in most 
cases reach the cliffs only for a short time twice a day, and the waves 
eat into them only when they are charged with sand or pebbles; for 
there is good evidence that pure water effects nothing in wearing 
away rock. At last the base of the cliff is undermined, huge frag- 
ments fall down, and these, remaining fixed, have to be worn away 
atom by atom, until after being reduced in size they can be rolled 
about by the waves, and then they are more quickly ground into 
pebbles, sand, or mud. But how often do we see along the bases of 
retreating cliffs rounded boulders, all thickly clothed by marine pro- 
ductions, showing how little they are abraded and how seldom they 
are rolled about! Moreover, if we follow for a few miles any line of 
rocky cliff, which is undergoing degradation, we find that it is only 
here and there, along a short length or round a promontory, that 
the cliffs are at the present time suffering. The appearance of the 
surface and the vegetation show that elsewhere years have elapsed 
since the waters washed their base. 

We have, however, recently learnt from the observations of Ram- 
say, in the van of many excellent observers — of Jukes, Geikie, Croll, 
and others, that subaerial degradation is a much more important 
agency than coast-action, or the power of the waves. The whole 
surface of the land is exposed to the chemical action of the air and 
of the rain-water with its dissolved carbonic acid, and in colder 
countries to frost; the disintegrated matter is carried down even 
gentle slopes during heavy rain, and to a greater extent than might 


be supposed, especially in arid districts, by the wind; it is then 
transported by the streams and rivers, which when rapid deepen 
their channels, and triturate the fragments. On a rainy day, even in 
a gently undulating country, we see the effects of subaerial degrada- 
tion in the muddy rills which flow down every slope. Messrs. 
Ramsay and Whitaker have shown, and the observation is a most 
striking one, that the great lines of escarpment in the Wealden dis- 
trict and those ranging across England, which formerly were looked 
at as ancient sea-coasts, cannot have been thus formed, for each line 
is composed of one and the same formation, whilst our sea<liffs 
are everywhere formed by the intersection of various formations. 
This being the case, we are compelled to admit that the escarpments 
owe their origin in chief part to the rocks of which they are com- 
posed having resisted subaerial denudation better than the surround- 
ing surface; this surface consequently has been gradually lowered, 
with the lines of harder rock left projecting. Nothing impresses 
the mind with the vast duration of time, according to our ideas of 
time, more forcibly than the conviction thus gained that subaerial 
agencies which apparently have so little power, and which seem to 
work so slowly, have produced great results. 

When thus impressed with the slow rate at which the land is 
worn away through subaerial and littoral action, it is good, in order 
to appreciate the past duration of time, to consider on the one hand, 
the masses of rock which have been removed over many extensive 
areas, and on the other hand the thickness of our sedimentary 
formations. I remember having been much struck when viewing 
volcanic islands, which have been worn by the waves and pared all 
round into perpendicular cliffs of one or two thousand feet in 
height; for the gentle slope of the lava-streams, due to their formerly 
Uquid state, showed at a glance how far the hard, rocky beds had 
once extended into the open ocean. The same story is told still more 
plainly by faults, — those great cracks along which the strata have 
been upheaved on one side, or thrown down on the other, to the 
height or depth of thousands of feet; for since the crust cracked, and 
it makes no great difference whether the upheaval was sudden, or, 
as most geologists now believe, was slow and effected by many 
starts, the surface of the land has been so completely planed down 


that no trace of these vast dislocations is externally visible. The 
Craven fault, for instance, extends for upwards of thirty miles, and 
along this line the vertical displacement of the strata varies from 
600 to 3000 feet. Professor Ramsay has published an account of a 
downthrow in Anglesea of 2,300 feet; and he informs me that he 
fully believes that there is one in Merionethshire of 12,000 feet; yet 
in these cases there is nothing on the surface of the land to show 
such prodigious movements; the pile of rocks on either side of the 
crack having been smoothly swept away. 

On the other hand, in all parts of the world the piles of sedi- 
mentary strata are of wonderful thickness. In the Cordillera I 
estimated one mass of conglomerate at ten thousand feet; and al- 
though conglomerates have probably been accumulated at a quicker 
rate than finer sediments, yet from being formed of worn and 
rounded fjebbles, each of which bears the stamp of time, they are 
good to show how slowly the mass must have been heap)ed together. 
Professor Ramsay has given me the maximum thickness, from actual 
measurement in most cases, of the successive formations in different 
parts of Great Britain; and this is the result: — 


Palaeozoic strata (not including igneous beds) 57.154 

Secondary strata •3ii9o 

Tertiary strata 2,240 

— making altogether 72,584 feet; that is, very nearly thirteen and 
three-quarters British miles. Some of the formations, which are 
represented in England by thin beds, are thousands of feet in thick- 
ness on the Continent. Moreover, between each successive formation, 
we have, in the opinion of most geologists, blank periods of enormous 
length. So that the lofty pile of sedimentary rocks in Britain gives 
but an inadequate idea of the time which has elapsed during their 
accumulation. The consideration of these various facts impresses 
the mind almost in the same manner as does the vain endeavour to 
grapple with the idea of eternity. 

Nevertheless this impression is partly false. Mr. Croll, in an in- 
teresting paper, remarks that we do not err "in forming too great a 
conception of the length of geological periods," but in estimating 
them by years. When geologists look at large and complicated 


phenomena, and then at the figures representing several million 
years, the two produce a totally different effect on the mind, and the 
figures are at once pronounced too small. In regard to subaerial 
denudation, Mr. CroU shows, by calculating the known amount of 
sediment annually brought down by certain rivers, relatively to 
their areas of drainage, that 1,000 feet of solid rock, as it became 
gradually disintegrated, would thus be removed from the mean 
level of the whole area in the course of six million years. 

This seems an astonishing result, and some considerations lead 
to the suspicion that it may be too large, but even if halved or 
quartered it is still very surprising. Few of us, however, know 
what a million really means: Mr. Croll gives the following illustra- 
tion: Take a narrow strip of pajx;r, eighty-three feet four inches in 
length, and stretch it along the wall of a large hall; then mark off 
at one end the tenth of an inch. This tenth of an inch will represent 
one hundred years, and the entire strip a million years. But let it 
be borne in mind, in relation to the subject of this work, what a 
hundred years implies, represented as it is by a measure utterly in- 
significant in a hall of the above dimensions. Several eminent 
breeders, during a single lifetime, have so largely modified some of 
the higher animals, which propagate their kind much more slowly 
than most of the lower animals, that they have formed what well 
deserves to be called a new sub-breed. Few men have attended 
with due care to any one strain for more than half a century, so 
that a hundred years represents the work of two breeders in succes- 
sion. It is not to be supposed that species in a state of nature ever 
change so quickly as domestic animals under the guidance of 
methodical selection. The comparison would be in every way fairer 
with the effects which follow from unconscious selection, that is 
the preservation of the most useful or beautiful animals, with no 
intention of modifying the breed; but by this process of unconscious 
selection, various breeds have been sensibly changed in the course 
of two or three centuries. 

Species, however, probably change much more slowly, and within 
the same country only a few change at the same time. This slow- 
ness follows from all the inhabitants of the same country being 
already so well adapted to each other, that new places in the polity 


of nature do not occur until after long intervals, due to the occur- 
rence of physical changes of some kind, or through the immigration 
of new forms. Moreover variations or individual differences of the 
right nature, by which some of the inhabitants might be better 
fitted to their new places under the altered circumstances, would 
not always occur at once. Unfortunately we have no means of 
determining, according to the standard of years, how long a period 
it takes to modify a species; but to the subject of time we must 


Now let us turn to our richest geological museums, and what a 
paltry display we behold! That our collections are imperfect, is 
admitted by every one. The remark of that admirable palaeontologist, 
Edward Forbes, should never be forgotten, namely, that very many 
fossil species are known and named from single and often broken 
specimens, or from a few specimens collected on some one spot. 
Only a small portion of the surface of the earth has been geologically 
explored, and no part with sufficient care, as the important dis- 
coveries made every year in Europe prove. No organism wholly 
soft can be preserved. Shells and bones decay and disappear when 
left on the bottom of the sea, where sediment is not accumulating. 
We probably take a quite erroneous view, when we assume that 
sediment is being deposited over nearly the whole bed of the sea, at 
a rate sufficiently quick to embed and preserve fossil remains. 
Throughout an enormously large proportion of the ocean, the bright 
blue tint of the water bespeaks its purity. The many cases on record 
of a formation conformably covered, after an immense interval of 
time, by another and later formation, without the underlying bed 
having suffered in the interval any wear and tear, seem explicable 
only on the view of the bottom of the sea not rarely lying for ages 
in an unaltered condition. The remains which do become embedded, 
if in sand or gravel, will, when the beds are upraised, generally be 
dissolved by the percolation of rain-water charged with carbonic 
acid. Some of the many kinds of animals which live on the beach 
between high and low water mark seem to be rarely preserved. For 
instance, the several species of the Chthamalinx (a sub-family of 


sessile cirripedes) coat the rocks all over the world in infinite num- 
bers; they are all strictly littoral, with the exception of a single 
Mediterranean species, which inhabits deep water, and this has been 
found fossil in Sicily, whereas not one other species has hitherto 
been found in any tertiary formation; yet it is known that the genus 
Chthamalus existed during the Chalk period. Lastly, many great 
deposits requiring a vast length of time for their accumulation, are 
entirely destitute of organic remains, without our being able to as- 
sign any reason: one of the most striking instances is that of the 
Flysch formation, which consists of shale and sandstone, several 
thousand, occasionally even six thousand feet in thickness, and ex- 
tending for at least 300 miles from Vienna to Switzerland; and 
although this great mass has been most carefully searched, no fos- 
sils, except a few vegetable remains, have been found. 

With respect to the terrestrial productions which lived during the 
Secondary and Palarozoic periods, it is superfluous to state that our 
evidence is fragmentary in an extreme degree. For instance, until 
recently not a land-shell was known belonging to either of these 
vast periods, with the exception of one species discovered by Sir C. 
Lyell and Dr. Dawson in the carboniferous strata of North America; 
but now land-shells have been found in the lias. In regard to mam- 
miferous remains, a glance at the historical table published in Lyell's 
Manual will bring home the truth, how accidental and rare is their 
preservation, far better than pages of detail. Nor is their rarity 
surprising, when we remember how large a proportion of the bones 
of tertiary mammals have been discovered either in caves or in 
lacustrine deposits; and that not a cave or true lacustrine bed is 
known belonging to the age of our secondary or palaeozoic forma- 

But the imperfection in the geological record largely results from 
another and more important cause than any of the foregoing; 
namely, from the several formations being separated from each 
other by wide intervals of time. This doctrine has been emphatically 
admitted by many geologists and palarontologists, who, like E. 
Forbes, entirely disbelieve in the change of species. When we see 
the formations tabulated in written works, or when we follow them 
in nature, it is difficult to avoid believing that they are closely con- 


secutive. But we know, for instance, from Sir R. Murchison's great 
work on Russia, what wide gaps there are in that country between 
the superimposed formations; so it is in North America, and in 
many other parts of the world. The most skilful geologist, if his 
attention had been confined exclusively to these large territories, 
would never have suspected that, during the periods which were 
blank and barren in his own country, great piles of sediment, 
charged with new and peculiar forms of life, had elsewhere been 
accumulated. And if, in each separate territory, hardly any idea 
can be formed of the length of time which has elapsed between the 
consecutive formations, we may infer that this could nowhere be 
ascertained. The frequent and great changes in the mineralogical 
composition of consecutive formations, generally implying great 
changes in the geography of the surrounding lands, whence the 
sediment was derived, accord with the belief of vast intervals of 
time having elapsed between each formation. 

We can, I think, see why the geological formations of each region 
are almost invariably intermittent; that is, have not followed each 
other in close sequence. Scarcely any fact struck me more when 
examining many hundred miles of the South American coasts, 
which have been upraised several hundred feet within the recent 
fjeriod, than the absence of any recent deposits sufficiently extensive 
to last for even a short geological period. Along the whole west 
coast, which is inhabited by a {jeculiar marine fauna, tertiary beds 
are so poorly developed that no record of several successive and 
peculiar marine faunas will probably be preserved to a distant age. 
A little reflection will explain why, along the rising coast of the 
western side of South America, no extensive formations with recent 
or tertiary remains can anywhere be found, though the supply of 
sediment must for ages have been great, from the enormous degrada- 
tion of the coast-rocks and from muddy streams entering the sea. 
The explanation, no doubt, is, that the littoral and sub-littoral de- 
posits are continually worn away, as soon as they are brought up 
by the slow and gradual rising of the land within the grinding 
action of the coast-waves. 

We may, I think, conclude that sediment must be accumulated in 
extremely thick, solid, or extensive masses, in order to withstand 


the incessant action of the waves, when first upraised and during 
successive oscillations of level, as well as the subsequent subaerial 
degradation. Such thick and extensive accumulations of sediment 
may be formed in two ways; either in profound depths of the sea, 
in which case the bottom will not be inhabited by so many and 
such varied forms of life, as the more shallow seas; and the mass 
when upraised will give an imperfect record of the organisms which 
existed in the neighbourhood during the period of its accumulation. 
Or, sediment may be deposited to any thickness and extent over a 
shallow bottom, if it continue slowly to subside. In this latter case, 
as long as the rate of subsidence and the supply of sediment nearly 
balance each other, the sea will remain shallow and favourable for 
many and varied forms, and thus a rich fossiliferous formation, 
thick enough, when upraised, to resist a large amount of denudation, 
may be formed. 

I am convinced that nearly all our ancient formations, which are 
throughout the greater part of their thickness rich in fossils, have 
thus been formed during subsidence. Since publishing my views on 
this subject in 1845, I have watched the progress of Geology, and 
have been surprised to note how author after author, in treating of 
this or that great formation, has come to the conclusion that it was 
accumulated during subsidence. I may add, that the only ancient 
tertiary formation on the west coast of South America, which has 
been bulky enough to resist such degradation as it has as yet suf- 
fered, but which will hardly last to a distant geological age, was 
deposited during a downward oscillation of level, and thus gained 
considerable thickness. 

All geological facts tell us plainly that each area has undergone 
numerous slow oscillations of level, and apparently these oscillations 
have affected wide spaces. Consequently, formations rich in fossils 
and sufficiently thick and extensive to resist subsequent degradation, 
will have been formed over wide spaces during periods of subsidence, 
but only where the supply of sediment was sufficient to keep the 
sea shallow and to embed and preserve the remains before they had 
time to decay. On the other hand, as long as the bed of the sea 
remains stationary, thicl{^ deposits cannot have been accumulated in 
the shallow parts, which are the most favourable to life. Still less 


can this have happened during the alternate periods of elevation; 
or, to speak more accurately, the beds which were then accumulated 
will generally have been destroyed by being upraised and brought 
within the limits of the coast-action. 

These remarks apply chiefly to littoral and sub-littoral deposits. 
In the case of an extensive and shallow sea, such as that within a 
large part of the Malay Archipelago, where the depth varies from 
thirty or forty to sixty fathoms, a widely extended formation might 
be formed during a period of elevation, and yet not suffer excessively 
from denudation during its slow upheaval; but the thickness of the 
formation could not be great, for owing to the elevatory movement 
it would be less than the depth in which it was formed; nor would 
the deposit be much consolidated, nor be capped by overlying forma- 
tions, so that it would run a good chance of being worn away by 
atmospheric degradation and by the action of the sea during sub- 
sequent oscillations of level. It has, however, been suggested 
by Mr. Hopkins, that if one part of the area, after rising and be- 
fore being denuded, subsided, the deposit formed during the 
rising movement, though not thick, might afterwards become pro- 
tected by fresh accumulations, and thus be preserved for a long 

Mr. Hopkins also expresses his belief that sedimentary beds of 
considerable horizontal extent have rarely been completely destroyed. 
But all geologists, excepting the few who believe that our present 
metamorphic schists and plutonic rocks once formed the primordial 
nucleus of the globe, will admit that these latter rocks have been 
stript of their covering to an enormous extent. For it is scarcely 
possible that such rocks could have been solidified and crystallized 
whilst uncovered; but if the metamorphic action occurred at pro- 
found depths of the ocean, the former protecting mantle of rock 
may not have been very thick. Admitting then that gneiss, mica- 
schist, granite, diorite, etc., were once necessarily covered up, how 
can we account for the naked and extensive areas of such rocks in 
many parts of the world, except on the belief that they have sub- 
sequently been completely denuded of all overlying strata? That 
such extensive areas do exist cannot be doubted; the granitic region 
of Parime is described by Humboldt as being at least nineteen times 


as large as Switzerland. South of the Amazon, Boue colours an 
area composed of rocks of this nature as equal to that of Spain, 
France, Italy, part of Germany, and the British Islands, all con- 
joined. This region has not been carefully explored, but from the 
concurrent testimony of travellers, the granitic area is very large; 
thus. Von Eschwege gives a detailed section of these rocks, stretch- 
ing from Rio de Janeiro for 260 geographical miles inland in a 
straight line; and I travelled for 150 miles in another direction, and 
saw nothing but granitic rocks. Numerous specimens, collected 
along the whole coast from near Rio Janeiro to the mouth of the 
Plata, a distance of 1,100 geographical miles, were examined by me, 
and they all belonged to this class. Inland, along the whole northern 
bank of the Plata, I saw, besides modern tertiary beds, only one 
small patch of slightly metamorphosed rock, which alone could 
have formed a part of the original capping of the granitic series. 
Turning to a well-known region, namely, to the United States and 
Canada, as shown in Professor H. D. Rogers's beautiful map, I have 
estimated the areas by cutting out and weighing the paper, and I 
find that the metamorphic (excluding "the semi-metamorphic") 
and granitic rocks exceed, in the proportion of 19 to 12.5, the whole 
of the newer Paleozoic formations. In many regions the meta- 
morphic and granitic rocks would be found much more widely 
extended than they appear to be, if all the sedimentary beds were 
removed which rest unconformably on them, and which could not 
have formed part of the original mantle under which they were 
crystallized. Hence it is probable that in some parts of the world 
whole formations have been completely denuded, with not a wreck 
left behind. 

One remark is here worth a passing notice. During p>eriods of 
elevation the area of the land and of the adjoining shoal parts of 
the sea will be increased, and new stations will often be formed: — 
all circumstances favourable, as previously explained, for the forma- 
tion of new varieties and species; but during such periods there 
will generally be a blank in the geological record. On the other 
hand, during subsidence, the inhabited area and number of in- 
habitants will decrease (excepting on the shores of a continent when 
first broken up into an archipelago), and consequently, during sub- 


sidence, though there will be much extinction, few new varieties or 
species will be formed; and it is during these very periods of sub- 
sidence that the deposits which are richest in fossils have been 


From these several considerations, it cannot be doubted that the 
geological record, viewed as a whole, is extremely imperfect; but if 
we confine our attention to any one formation, it becomes much 
more difficult to understand why we do not therein find closely 
graduated varieties between the allied species which lived at its 
commencement and at its close. Several cases are on record of the 
same species presenting varieties in the upper and lower parts of 
the same formation; thus, Trautschold gives a number of instances 
with Ammonites; and Hilgendorf has described a most curious case 
of ten graduated forms of Planorbis multiformis in the successive 
beds of a fresh-water formation in Switzerland. Although each 
formation has indisputably required a vast number of years for its 
deposition, several reasons can be given why each should not com- 
monly include a graduated series of links between the sf)ecies which 
lived at its commencement and close; but I cannot assign due pro- 
portional weight to the following considerations. 

Although each formation may mark a very long lapse of years, 
each probably is short compared with the period requisite to change 
one species into another. I am aware that two paleontologists, whose 
opinions are worthy of much deference, namely Bronn and Wood- 
ward, have concluded that the average duration of each formation is 
twice or thrice as long as the average duration of specific forms. 
But insuperable difficulties, as it seems to me, prevent us from com- 
ing to any just conclusion on this head. When we see a species first 
apf>earing in the middle of any formation, it would be rash in the 
extreme to infer that it had not elsewhere previously existed. So 
again when we find a species disappearing before the last layers have 
been deposited, it would be equally rash to suppose that it then 
became extinct. We forget how small the area of Europe is com- 
pared with the rest of the world; nor have the several stages of the 


same formation throughout Europe been correlated with perfect 

We may safely infer that with marine animals of all kinds there 
has been a large amount of migration due to climatal and other 
changes; and when we see a species first appearing in any formation, 
the probability is that it only then first immigrated into that area. 
It is well known, for instance, that several species appear somewhat 
earlier in the palaeozoic beds of North America than in those of 
Eurof)e; time having apparently been required for their migration 
from the American to the European seas. In examining the latest 
deposits in various quarters of the world, it has everywhere been 
noted, that some few still existing species are common in the deposit, 
but have become extinct in the immediately surrounding sea; or, 
conversely, that some are now abundant in the neighbouring sea, but 
are rare or absent in this particular deposit. It is an excellent lesson 
to reflect on the ascertained amount of migration of the inhabitants 
of Europe during the glacial epoch, which forms only a part of one 
whole geological period; and likewise to reflect on the changes of 
level, on the extreme change of climate, and on the great lapse of 
time, all included within the same glacial period. Yet it may be 
doubted whether, in any quarter of the world, sedimentary deposits, 
including fossil remains, have gone on accumulating within the 
same area during the whole of this period. It is not, for instance, 
probable that sediment was deposited during the whole of the 
glacial fjeriod near the mouth of the Mississippi, within that limit of 
depth at which marine animals can best flourish: for we know that 
great geographical changes occurred in other parts of America during 
this space of time. When such beds as were deposited in shallow 
water near the mouth of the Mississippi during some part of the 
glacial period shall have been upraised, organic remains will prob- 
ably first appear and disapf)ear at different levels, owing to the 
migrations of species and to geographical changes. And in the dis- 
tant future, a geologist, examining those beds, would be tempted to 
conclude that the average duration of life of the embedded fossils 
had been less than that of the glacial period, instead of having been 
really far greater, that is, extending from before the glacial epoch to 
the present day. 


In order to get a perfect gradation between two forms in the upper 
and lower parts of the same formation, the deposit must have gone 
on continuously accumulating during a long period, sufficient for 
the slow process of modification; hence the deposit must be a very 
thick one; and the species undergoing change must have lived in the 
same district throughout the whole time. But we have seen that a 
thick formation, fossiliferous throughout its entire thickness, can 
accumulate only during a jjeriod of subsidence; and to keep the 
depth approximately the same, which is necessary that the same 
marine species may live on the same space, the supply of sediment 
must nearly counterbalance the amount of subsidence. But this 
same movement of subsidence will tend to submerge the area whence 
the sediment is derived, and thus diminish the supply, whilst the 
downward movement continues. In fact, this nearly exact balancing 
between the supply of sediment and the amount of subsidence is 
probably a rare contingency; for it has been observed by more than 
one paleontologist, that very thick deposits are usually barren of 
organic remains, except near their upper or lower limits. 

It would seem that each separate formation, like the whole pile 
of formations in any country, has generally been intermittent in its 
accumulation. When we see, as is so often the case, a formation 
composed of beds of widely different mineralogical composition, we 
may reasonably suspect that the process of deposition has been more 
or less interrupted. Nor will the closest inspection of a formation 
give us any idea of the length of time which its deposition may have 
consumed. Many instances could be given of beds only a few feet 
in thickness, representing formations, which are elsewhere thousands 
of feet in thickness, and which must have required an enormous 
period for their accumulation; yet no one ignorant of this fact would 
have even susfjected the vast lapse of time represented by the thinner 
formation. Many cases could be given of the lower beds of a forma- 
tion having been upraised, denuded, submerged, and then re-covered 
by the upper beds of the same formation, — facts, showing what wide, 
yet easily overlooked, intervals have occurred in its accumulation. 
In other cases we have the plainest evidence in great fossilised trees, 
still standing upright as they grew, of many long intervals of time 
and changes of level during the process of deposition, which would 


not have been suspected, had not the trees been preserved: thus Sir 
C. Lyell and Dr. Dawson found carboniferous beds 1,400 feet thick 
in Nova Scotia, with ancient root-bearing strata, one above the other 
at no less than sixty-eight different levels. Hence, when the same 
species occurs at the bottom, middle, and top of a formation, the 
probability is that it has not lived on the same spot during the whole 
period of deposition, but has disappeared and reappeared, perhaps 
many times, during the same geological period. Consequently if it 
were to undergo a considerable amount of modification during the 
deposition of any one geological formation, a section would not in- 
clude all the fine intermediate gradations which must, on our theory, 
have existed, but abrupt, though perhaps slight, changes of form. 

It is all-important to remember that naturaUsts have no golden rule 
by which to distinguish species and varieties; they grant some litde 
variability to each species, but when they meet with a somewhat 
greater amount of difference between any two forms, they rank both 
as species, unless they are enabled to connect them together by the 
closest intermediate gradations; and this, from the reasons just as- 
signed, we can seldom hope to effect in any one geological section. 
Supposing B and C to be two species, and a third, A, to be found in 
an older and underlying bed; even if A were strictly intermediate 
between B and C, it would simply be ranked as a third and distinct 
species, unless at the same time it could be closely connected by inter- 
mediate varieties with either one or both forms. Nor should it be 
forgotten, as before explained, that A might be the actual progenitor 
of B and C, and yet would not necessarily be strictly intermediate be- 
tween them in all respects. So that we might obtain the parent-sp)ecies 
and its several modified descendants from the lower and upper beds 
of the same formation, and unless we obtained numerous transitional 
gradations, we should not recognise their blood-relationship, and 
should consequently rank them as distinct species. 

It is notorious on what excessively slight differences many 
palaeontologists have founded their species; and they do this the 
more readily if the specimens come from different substages of the 
same formation. Some experienced conchologists are now sinking 
many of the very fine species of D'Orbigny and others into the rank 
of varieties; and on this view we do find the kind of evidence of 


change which on the theory we ought to find. Look again at the 
later tertiary deposits, which include many shells believed by the 
majority of naturalists to be identical with existing species; but some 
excellent naturahsts, as Agassiz and Pictet, maintain that all these 
tertiary species are specifically distinct, though the distinction is ad- 
mitted to be very slight; so that here, unless we believe that these 
eminent naturalists have been misled by their imaginations, and 
that these late tertiary species really present no difference whatever 
from their living representatives, or unless we admit, in opposition 
to the judgment of most naturalists, that these tertiary species are all 
truly distinct from the recent, we have evidence of the frequent oc- 
currence of slight modifications of the kind required. If we look to 
rather wider intervals of time, namely, to distinct but consecutive 
stages of the same great formation, we find that the embedded fossils, 
though universally ranked as specifically different, yet are far more 
closely related to each other than are the species found in more widely 
separated formations; so that here again we have undoubted evidence 
of change in the direction required by the theory; but to this latter 
subject I shall return in the following chapter. 

With animals and plants that propagate rapidly and do not wan- 
der much, there is reason to suspect, as we have formerly seen, that 
their varieties are generally at first local; and that such local varieties 
do not spread widely and supplant their parent-forms until they 
have been modified and perfected in some considerable degree. Ac- 
cording to this view, the chance of discovering in a formation in any 
one country all the early stages of transition between any two forms, 
is small, for the successive changes are supposed to have been local 
or confined to some one spot. Most marine animals have a wide 
range; and we have seen that with plants it is those which have the 
widest range, that oftenest present varieties; so that, with shells and 
other marine animals, it is probable that those which had the widest 
range, far exceeding the limits of the known geological formations in 
Europe, have oftenest given rise, first to local varieties and ultimately 
to new species; and this again would greatly lessen the chance of our 
being able to trace the stages of transition in any one geological 

It is a more important consideration, leading to the same result, as 


lately insisted on by Dr. Falconer, namely, that the period during 
which each species underwent modification, though long as measured 
by years, was probably short in comparison with that during which 
it remained without undergoing any change. 

It should not be forgotten, that at the present day, with perfect 
specimens for examination, two forms can seldom be connected by 
intermediate varieties, and thus proved to be the same species, until 
many specimens are collected from many places; and with fossil spe- 
cies this can rarely be done. We shall, perhaps, best perceive the im- 
probability of our being enabled to connect species by numerous, 
fine, intermediate, fossil links, by asking ourselves whether, for in- 
stance, geologists at some future period will be able to prove that our 
different breeds of cattle, sheep, horses, and dogs are descended from 
a single stock or from several aboriginal stocks; or, again, whether 
certain sea-shells inhabiting the shores of North America, which are 
ranked by some conchologists as distinct species from their European 
representatives, and by other conchologists as only varieties, are really 
varieties, or are, as it is called, specifically distinct. This could be 
effected by the future geologist only by his discovering in a fossil 
state numerous intermediate gradations; and such success is improb- 
able in the highest degree. 

It has been asserted over and over again, by writers who believe 
in the immutability of species, that geology yields no linking forms. 
This assertion, as we shall see in the next chapter, is certainly errone- 
ous. As Sir J. Lubbock has remarked, "Every species is a link between 
other allied forms." If we take a genus having a score of species, 
recent and extinct, and destroy four-fifths of them, no one doubts that 
the remainder will stand much more distinct from each other. If the 
extreme forms in the genus happen to have been thus destroyed, the 
genus itself will stand more distinct from other allied genera. What 
geological research has not revealed, is the former existence of in- 
finitely numerous gradations, as fine as existing varieties, connecting 
together nearly all existing and extinct sjjecies. But this ought not 
to be expected; yet this has been repeatedly advanced as a most seri- 
ous objection against my views. 

It may be worth while to sum up the foregoing remarks on the 
causes of the imperfection of the geological record under an imagi- 


nary illustration. The Malay Archipelago is about the size of Europe 
from the North Cape to the Mediterranean, and from Britain to Rus- 
sia; and therefore equals all the geological formations which have 
been examined with any accuracy, excepting those of the United 
States of America. I fully agree with Mr. Godwin-Austen, that the 
present condition of the Malay Archipelago, with its numerous large 
islands separated by wide and shallow seas, probably represents the 
former state of Europe, whilst most of our formations were accumu- 
lating. The Malay Archipelago is one of the richest regions in organic 
beings; yet if all the species were to be collected which have ever 
lived there, how imperfectly would they represent the natural 
history of the world! 

But we have every reason to believe that the terrestrial productions 
of the archip)elago would be preserved in an extremely imperfect 
manner in the formations which we suppose to be there accumulat- 
ing. Not many of the strictly littoral animals, or of those which lived 
on naked submarine rocks, would be embedded; and those embedded 
in gravel or sand would not endure to a distant epoch. Wherever 
sediment did not accumulate on the bed of the sea, or where it did 
not accumulate at a sufficient rate to protect organic bodies from 
decay, no remains could be preserved. 

Formations rich in fossils of many kinds, and of thickness sufficient 
to last to an age as distant in futurity as the secondary formations Ue 
in the past, would generally be formed in the archipelago only dur- 
ing periods of subsidence. These periods of subsidence would be 
separated from each other by immense intervals of time, during 
which the area would be either stationary or rising; whilst rising, the 
fossiliferous formations on the steeper shores would be destroyed, 
almost as soon as accumulated, by the incessant coast-action, as we 
now see on the shores of South America. Even throughout the ex- 
tensive and shallow seas within the archipelago, sedimentary beds 
could hardly be accumulated of great thickness during the periods of 
elevation, or become capped and protected by subsequent deposits, so 
as to have a good chance of enduring to a very distant future. During 
the periods of subsidence, there would probably be much extinction 
of life; during the periods of elevation, there would be much varia- 
tion, but the geological record would then be less perfect. 


It may be doubted whether the duration of any one great period 
of subsidence over the whole or part of the archipelago, together with 
a contemporaneous accumulation of sediment, would exceed the 
average duration of the same specific forms; and these contingencies 
are indispensable for the preservation of all the transitional gradations 
between any two or more sf)ecies. If such gradations were not all 
fully preserved, transitional varieties would merely appear as so many 
new, though closely allied species. It is also probable that each great 
period of subsidence would be interrupted by oscillations of level, and 
that slight climatal changes would intervene during such lengthy 
periods; and in these cases the inhabitants of the archipelago would 
migrate, and no closely consecutive record of their modifications 
could be preserved in any one formation. 

Very many of the marine inhabitants of the archipelago now range 
thousands of miles beyond its confines; and analogy plainly leads to 
the belief that it would be chiefly these far-ranging species, though 
only some of them, which would oftenest produce new varieties; 
and the varieties would at first be local or confined to one place, but 
if possessed of any decided advantage, or when further modified and 
improved, they would slowly spread and supplant their parent-forms. 
When such varieties returned to their ancient homes, as they would 
differ from their former state in a nearly uniform, though perhaps 
extremely slight degree, and as they would be found embedded in 
slightly different sub-stages of the same formation, they would, ac- 
cording to the principles followed by many palaeontologists, be ranked 
as new and distinct species. 

If then there be some degree of truth in these remarks, we have no 
right to expect to find, in our geological formations, an infinite num- 
ber of those fine transitional forms which, on our theory, have con- 
nected all the past and present species of the same group into one 
long and branching chain of life. We ought only to look for a few 
links, and such assuredly we do find — some more distandy, some 
more closely, related to each other; and these links, let them be ever 
so close, if found in different stages of the same formation, would, by 
many paleontologists, be ranked as distinct species. But I do not 
pretend that I should ever have suspected how f>oor was the record 
in the best preserved geological sections, had not the absence of 


innumerable transitional links between the species which lived at 
the commencement and close of each formation, pressed so hardly 
on my theory. 


The abrupt manner in which whole groups of sp)ecies suddenly 
appear in certain formations, has been urged by several palxontolo- 
gists — for instance, by Agassiz, Pictet, and Sedgwick — as a fatal ob- 
jection to the belief in the transmutation of species. If numerous 
species, belonging to the same genera or families, have really started 
into life at once, the fact would be fatal to the theory of evolution 
through natural selection. For the development by this means of a 
group of forms, all of which are descended from some one progenitor, 
must have been an extremely slow process; and the progenitors must 
have lived long before their modified descendants. But we continu- 
ally overrate the perfection of the geological record, and falsely infer, 
because certain genera or families have not been found beneath a 
certain stage, that they did not exist before that stage. In all cases 
positive palacontological evidence may be implicitly trusted; nega- 
tive evidence is worthless, as experience has so often shown. We con- 
tinually forget how large the world is, compared with the area over 
which our geological formations have been carefully examined; we 
forget that groups of species may elsewhere have long existed, and 
have slowly multiplied, before they invaded the ancient archipela- 
goes of Europe and the United States. We do not make due allow- 
ance for the intervals of time which have elapsed between our con- 
secutive formations, — longer perhaps in many cases than the time 
required for the accumulation of each formation. These intervals will 
have given time for the multiplication of species from some one par- 
ent-form: and in the succeeding formation, such groups or species 
will appear as if suddenly created. 

I may here recall a remark formerly made, namely, that it might re- 
quire a long succession of ages to adapt an organism to some new 
and peculiar line of life, for instance, to fly through the air; and con- 
sequently that the transitional forms would often long remain con- 
fined to some one region; but that, when this adaptation had once 


been effected, and a few species had thus acquired a great advantage 
over other organisms, a comparatively short time would be necessary 
to produce many divergent forms, which would spread rapidly and 
widely, throughout the world. Professor Pictet, in his excellent re- 
view of this work, in commenting on early transitional forms, and 
taking birds as an illustration, cannot see how the successive modifi- 
cations of the anterior limbs of a supposed prototype could possibly 
have been of any advantage. But look at the penguins of the South- 
ern Ocean; have not these birds their front limbs in this precise in- 
termediate state of "neither true arms nor true wings".? Yet these 
birds hold their place victoriously in the battle for life; for they exist 
in infinite numbers and of many kinds. I do not suppose that we 
here see the real transitional grades through which the wings of 
birds have passed; but what special difficulty is there in believing that 
it might profit the modified descendants of the penguin, first to be- 
come enabled to flap along the surface of the sea like the logger- 
headed duck, and ultimately to rise from its surface and glide through 
the air.' 

I will now give a few examples to illustrate the foregoing remarks, 
and to show how liable we are to error in supposing that whole 
groups of species have suddenly been produced. Even in so short an 
interval as that between the first and second editions of Pictet's great 
work on Pala;ontology, published in 1844-46 and 1853-57, '^^ <^°"' 
elusions on the first appearance and disappearance of several groups 
of animals have been considerably modified; and a third edition 
would require still further changes. I may recall the well-known fact 
that in geological treatises, published not many years ago, mammals 
were always spoken of as having abruptly come in at the commence- 
ment of the tertiary series. And now one of the richest known accu- 
mulations of fossil mammals belongs to the middle of the secondary 
series; and true mammals have been discovered in the new red sand- 
stone at nearly the commencement of this great series. Cuvier used 
to urge that no monkey occurred in any tertiary stratum; but now 
extinct sf)ecies have been discovered in India, South America, and in 
Europe, as far back as the miocene stage. Had it not been for the 
rare accident of the preservation of footsteps in the new red sandstone 
of the United States, who would have ventured to suppose that no less 


than at least thirty different bird-like animals, some of gigantic size, 
existed during that period? Not a fragment of bone has been dis- 
covered in these beds. Not long ago, palxontologists maintained that 
the whole class of birds came suddenly into existence during the 
eocene period; but now we know, on the authority of Professor 
Owen, that a bird certainly lived during the deposition of the upper 
greensand; and still more recently, that strange bird, the Arche- 
opteryx, with a long lizard-like tail, bearing a pair of feathers on 
each joint, and with its wings furnished with two free claws, has 
been discovered in the oolitic slates of Solenhofen. Hardly any recent 
discovery shows more forcibly than this, how little we as yet know 
of the former inhabitants of the world. 

I may give another instance, which, from having passed under 
my own eyes, has much struck me. In a memoir on Fossil Sessile 
Cirripedes, I stated that, from the large number of existing and ex- 
tinct tertiary species; from the extraordinary abundance of the indi- 
viduals of many species all over the world, from the arctic regions to 
the equator, inhabiting various zones of depths from the upper tidal 
limits to fifty fathoms; from the perfect manner in which specimens 
are preserved in the oldest tertiary beds; from the ease with which 
even a fragment of a valve can be recognized; from all these cir- 
cumstances, I inferred that, had sessile cirripedes existed during the 
secondary periods, they would certainly have been preserved and 
discovered; and as not one species had then been discovered in beds 
of this age, I concluded that this great group had been suddenly de- 
veloped at the commencement of the tertiary series. This was a sore 
trouble to me, adding as I then thought one more instance of the 
abrupt appearance of a great group of species. But my work had 
hardly been published, when a skilful palarontologist, M. Bosquet, 
sent me a drawing of a perfect specimen of an unmistakeable sessile 
cirripede, which he had himself extracted from the chalk of Belgium. 
And, as if to make the case as striking as possible, this cirripede was 
a Chthamalus, a very common, large, and ubiquitous genus, of which 
not one species has as yet been found even in any tertiary stratum. 
Still more recently, a Pyrgoma, a member of a distinct sub-family of 
sessile cirripedes, has been discovered by Mr. Woodward in the upp)er 
chalk; so that we now have abundant evidence of the existence of this 
group of animals during the secondary period. 


The case most frequently insisted on by palaeontologists of the ap- 
parently sudden appearance of a whole group of sp)ecies, is that of 
the teleostean fishes, low down, according to Agassiz, in the Chalk 
period- This group includes the large majority of existing species. 
But certain Jurassic and Triassic forms are now commonly admitted 
to be teleostean; and even some palaeozoic forms have thus been 
classed by one high authority. If the teleosteans had really appeared 
suddenly in the northern hemisphere at the commencement of the 
chalk formation, the fact would have been highly remarkable; but 
it would not have formed an insup>erable difficulty, unless it could 
Ukewise have been shown that at the same period the species were 
suddenly and simultaneously develofjed in other quarters of the 
world. It is almost superfluous to remark that hardly any fossil-fish 
are known from south of the equator; and by running through 
Pictet's Paleontology it will be seen that very few sjjecies are known 
from several formations in Europe. Some few families of fish now 
have a confined range; the teleostean fishes might formerly 
have had a similarly confined range, and after having been largely 
developed in some one sea, have spread widely. Nor have we any 
right to suppose that the seas of the world have always been so freely 
open from south to north as they are at present. Even at this day, 
if the Malay Archipelago were converted into land, the tropical parts 
of the Indian Ocean would form a large and {perfectly enclosed 
basin, in which any great group of marine animals might be midti- 
plied; and here they would remain confined, until some of the species 
became adapted to a cooler climate, and were enabled to double the 
Southern capes of Africa or Australia, and thus reach other and 
distant seas. 

From these considerations, from our ignorance of the geology of 
other countries beyond the confines of Europe and the United States, 
and from the revolution in our palxontological knowledge effected 
by the discoveries of the last dozen years, it seems to me to be about as 
rash to dogmatize on the succession of organic forms throughout the 
world, as it would be for a naturalist to land for five minutes on a 
barren point in Australia, and then to discuss the number and range 
of its productions. 



There is another and allied difficulty, which is much more serious. 
I allude to the manner in which species belonging to several of the 
main divisions of the animal kingdom suddenly appear in the lowest 
known fossiliferous rocks. Most of the arguments which have con- 
vinced me that all the existing species of the same group are de- 
scended from a single progenitor, apply with equal force to the earli- 
est known sf)ecies. For instance, it cannot be doubted that all the 
Cambrian and Silurian trilobites are descended from some one 
crustacean, which must have lived long before the Cambrian age, 
and which probably differed greatly from any known animal. Some 
of the most ancient animals, as the Nautilus, Lingula, etc., do not 
differ much from living sf)ecies; and it cannot on our theory be suf>- 
posed, that these old species were the progenitors of all the species 
belonging to the same groups which have subsequently appeared, for 
they are not in any degree intermediate in character. 

Consequently, if the theory be true, it is indisputable that before 
the lowest Cambrian stratum was deposited long periods elapsed, 
as long as, or probably far longer than, the whole interval from the 
Cambrian age to the present day; and that during these vast periods 
the world swarmed with living creatures. Here we encounter a for- 
midable objection; for it seems doubtful whether the earth, in a fit 
state for the habitation of living creatures, has lasted long enough. 
Sir W. Thompson concludes that the consolidation of the crust can 
hardly have occurred less than twenty or more than 400 million 
years ago, but probably not less than ninety-eight or more than 200 
million years. These very wide limits show how doubtful the data 
are; and other elements may have hereafter to be introduced into 
the problem. Mr. Croll estimates that about sixty million years have 
elapsed since the Cambrian period, but this, judging from the small 
amount of organic change since the commencement of the Glacial 
epoch, appears a very short time for the many and great mutations of 
life, which have certainly occurred since the Cambrian formation; 
and the previous 140 million years can hardly be considered as suffi- 
cient for the development of the varied forms of life which already 


existed during the Cambrian period. It is, however, probable, as Sir 
WiHiam Thompson insists, that the world at a very early period 
was subjected to more rapid and violent changes in its physical 
conditions than those now occurring; and such changes would have 
tended to induce changes at a corresponding rate in the organisms 
which then existed. 

To the question why we do not find rich fossiliferous deposits 
belonging to these assumed earliest periods prior to the Cambrian 
system, I can give no satisfactory answer. Several eminent geolo- 
gists, with Sir R. Murchison at their head, were until recently con- 
vinced that we beheld in the organic remains of the lowest Silurian 
stratum the first dawn of life. Other highly competent judges, as 
Lyell and E. Forbes, have disputed this conclusion. We should not 
forget that only a small portion of the world is known with accuracy. 
Not very long ago M. Barrande added another and lower stage, 
abounding with new and peculiar sjjecies, beneath the then known 
Silurian system; and now, still lower down in the Lower Cambrian 
formation, Mr. Hicks has found in South Wales beds rich in tri- 
lobites, and containing various molluscs and annelids. The presence 
of phosphatic nodules and bituminous matter, even in some of the 
lowest azoic rocks, probably indicates life at these periods; and the 
existence of the Eozoon in the Laurentian formation of Canada is 
generally admitted. There are three great series of strata beneath the 
Silurian system in Canada, in the lowest of which the Eozoon is 
found. Sir W. Logan states that their "united thickness may possi- 
bly far surpass that of all the succeeding rocks, from the base of the 
palaeozoic series to the present time. We are thus carried back to a 
period so remote that the appearance of the so-called primordial 
fauna (of Barrande) may by some be considered as a comparatively 
modern event." The Eozoon belongs to the most lowly organised of 
all classes of animals, but is highly organised for its class; it existed in 
countless numbers, and, as Dr. Dawson has remarked, certainly 
preyed on other minute organic beings, which must have lived in 
great numbers. Thus the words, which I wrote in 1859, about the ex- 
istence of living beings long before the Cambrian period, and which 
are almost the same with those since used by Sir W. Logan, have 
proved true. Nevertheless, the difficulty of assigning any good reason 


for the absence of vast piles of strata rich in fossils beneath the Cam- 
brian system is very great. It does not seem probable that the most 
ancient beds have been quite worn away by denudation, or that their 
fossils have been wholly obliterated by metamorphic action, for if 
this had been the case we should have found only small remnants of 
the formations next succeeding them in age, and these would always 
have existed in a partially metamorphosed condition. But the descrip- 
tions which we possess of the Silurian deposits over immense ter- 
ritories in Russia and in North America, do not support the view, that 
the older a formation is, the more invariably it has suffered extreme 
denudation and metamorphism. 

The case at present must remain inexplicable; and may be truly 
urged as a valid argument against the views here entertained. To 
show that it may hereafter receive some explanation, I will give the 
following hypothesis. From the nature of the organic remains which 
do not appear to have inhabited profound depths, in the several for- 
mations of Europe and of the United States; and from the amount of 
sediment, miles in thickness, of which the formations are composed, 
we may infer that from first to last large islands or tracts of land, 
whence the sediment was derived, occurred in the neighbourhood of 
the now existing continents of Europe and North America. The 
same view has since been maintained by Agassiz and others. But we 
do not know what was the state of things in the intervals between the 
several successive formations; whether Europe and the United States 
during these intervals existed as dry land, or as a submarine surface 
near land, on which sediment was not deposited, or as the bed on an 
open and unfathomable sea. 

Looking to the existing oceans, which are thrice as extensive as the 
land, we see them studded with many islands; but hardly one truly 
oceanic island (with the exception of New Zealand, if this can be 
called a truly oceanic island) is as yet known to afford even a rem- 
nant of any palaeozoic and secondary formation. Hence, we may per- 
haps infer that during the palxozoic and secondary periods, neither 
continents nor continental islands existed where our oceans now 
extend; for had they existed, pala;ozoic and secondary formations 
would in all probability have been accumulated from sediment de- 
rived from their wear and tear; and these would have been at least 


partially upheaved by the oscillations of level, which must have inter- 
vened during these enormously long periods. If then we may infer 
anything from these facts, we may infer that, where our oceans now 
extend, oceans have extended from the remotest period of which we 
have any record; and on the other hand, that where continents now 
exist, large tracts of land have existed, subjected no doubt to great 
oscillations of level, since the Cambrian period. The colored map ap- 
pended to my volume on Coral Reefs, led me to conclude that the 
great oceans are still mainly areas of subsidence, the great archipela- 
goes still areas of oscillations of level, and the continents areas of ele- 
vation. But we have no reason to assume that things have thus re- 
mained from the beginning of the world. Our continents seem to 
have been formed by a preponderance, during many oscillations of 
level, of the force of elevation ; but may not the areas of preponderant 
movement have changed in the lapse of ages ? At a period long ante- 
cedent to the Cambrian epoch, continents may have existed where 
oceans are now spread out; and clear and open oceans may have ex- 
isted where our continents now stand. Nor should we be justified in 
assuming that if, for instance, the bed of the Pacific Ocean were now 
converted into a continent we should there find sedimentary forma- 
tions in a recognisable condition older than the Cambrian strata, sup- 
posing such to have been formerly deposited; for it might well hap- 
pen that strata which had subsided some miles nearer to the centre of 
the earth, and which had been pressed on by an enormous weight of 
superincumbent water, might have undergone far more meta- 
morphic action than strata which have always remained nearer to the 
surface. The immense areas in some parts of the world, for instance 
in South America, of naked metamorphic rocks, which must have 
been heated under great pressure, have always seemed to me to re- 
quire some special explanation; and we may perhaps believe that we 
see in these large areas, the many formations long anterior to the 
Cambrian epoch in a completely metamorphosed and denuded con- 

The several difficulties here discussed, namely — that, though we 
find in our geological formations many links between the species 
which now exist and which formerly existed, we do not find infinitely 
numerous fine transitional forms closely joining them all together; — 


the sudden manner in which several groups of species first appear in 
our European formations; — the almost entire absence, as at present 
known, of formations rich in fossils beneath the Cambrian strata, — 
are all undoubtedly of the most serious nature. We see this in the fact 
that the most eminent palaeontologists, namely, Cuvier, Agassiz, Bar- 
rande, Pictet, Falconer, E. Forbes, etc., and all our greatest geologists, 
as Lyell, Murchison, Sedgwick, etc., have unanimously, often vehe- 
mently, maintained the immutability of species. But Sir Charles Lyell 
now gives the support of his high authority to the other side; and 
most geologists and palxontologists are much shaken in their for- 
mer belief. Those who believe that the geological record is in any 
degree perfect, will undoubtedly at once reject the theory. For my 
part, following out Lyell's metaphor, I look at the geological record 
as a history of the world imperfectly kept, and written in a changing 
dialect; of this history we jx)ssess the last volume alone, relating 
only to two or three countries. Of this volume, only here and there 
a short chapter has been preserved; and of each page, only here and 
there a few lines. Each word of the slowly-changing language, more 
or less different in the successive chapters, may represent the forms of 
life, which are entombed in our consecutive formations, and which 
falsely apf>ear to have been abruptly introduced. On this view, the 
difficulties above discussed are greatly diminished, or even disappear. 


On the Geological Succession of Organic Beings 

On the slow and successive apf)earance of new species — On their different 
rates of change — Species once lost do not reappear — Groups of 
species follow the same general rules in their appearance and dis- 
appearance as do single species — On extinction—On simultaneous 
changes in the forms of life throughout the world — On the affinities 
of extinct species to each other and to living species — On the state 
of development of ancient forms — On the succession of the same 
types within the same areas — Summary of preceding and present 

IET us now see whether the several facts and laws relating to 
the geological succession of organic beings accord best with 
_^ the common view of the immutability of species, or with 
that of their slow and gradual modification, through variation and 
natural selection. 

New species have appeared very slowly, one after another, both on 
the land and in the waters. Lyell has shown that it is hardly possible 
to resist the evidence on this head in the case of the several tertiary 
stages; and every year tends to fill up the blanks between the stages, 
and to make the proportion between the lost and existing forms more 
gradual. In some of the most recent beds, though undoubtedly of 
high antiquity if measured by years, only one or two species are 
extinct, and only one or two are new, having appeared there for the 
first time, either locally, or, as far as we know, on the face of the 
earth. The secondary formations are more broken; but, as Bronn has 
remarked, neither the appearance nor disappearance of the many 
species embedded in each formation has been simultaneous. 

Species belonging to different genera and classes have not changed 
at the same rate, or in the same degree. In the older tertiary beds 
a few living shells may still be found in the midst of a multitude 
of extinct forms. Falconer has given a striking instance of a similar 
fact, for an existing crocodile is associated with many lost mammals 



and reptiles in the sub-Himalayan deposits. The Silurian Lingula 
differs but little from the living species of this genus; whereas most 
of the other Silurian molluscs and all the crustaceans have changed 
gready. The productions of the land seem to have changed at a 
quicker rate than those of the sea, of which a striking instance has 
been observed in Switzerland. There is some reason to beUeve that 
organisms high in the scale, change more quickly than those that 
are low: though there are exceptions to this rule. The amount of 
organic change, as Pictet has remarked, is not the same in each 
successive so<alled formation. Yet if we compare any but the most 
closely related formations, all the species will be found to have 
undergone some change. When a species has once disappeared from 
the face of the earth, we have no reason to believe that the same 
identical form ever reappears. The strongest apparent exception to 
this latter rule is that of the so-called "colonies" of M. Barrande, 
which intrude for a period in the midst of an older formation, and 
then allow the preexisting fauna to reappear; but Lyell's explanation, 
namely, that it is a case of temporary migration from a distinct 
geographical province, seems satisfactory. 

These several facts accord well with our theory, which includes no 
fixed law of development, causing all the inhabitants of an area to 
change abruptly, or simultaneously, or to an equal degree. The 
process of modification must be slow, and will generally affect only a 
few species at the same time; for the variability of each species is 
independent of that of all others. Whether such variations or indi- 
vidual differences as may arise will be accumulated through natural 
selection in a greater or less degree, thus causing a greater or less 
amount of (permanent modification, will dep)end on many complex 
contingencies — on the variations being of a beneficial nature, on the 
freedom of intercrossing, on the slowly changing physical conditions 
of the country, on the immigration of new colonists, and on the 
nature of the other inhabitants with which the varying species come 
into competition. Hence it is by no means surprising that one species 
should retain the same identical form much longer than others; or, 
if changing, should change in a less degree. We find similar relations 
between the existing inhabitants of distinct countries; for instance, 
the land shells and coleopterous insects of Madeira have come to 


differ considerably from their nearest allies on the continent of 
Europe, whereas the marine shells and birds have remained un- 
altered. We can perhaps understand the apparently quicker rate 
of change in terrestrial and in more highly organised productions 
compared with marine and lower productions, by the more complex 
relations of the higher beings to their organic and inorganic condi- 
tions of life, as explained in a former chapter. When many of the 
inhabitants of any area have become modified and improved, we can 
understand, on the principle of competition, and from the all- 
important relations of organism to organism in the struggle for life, 
that any form which did not become in some degree modified and 
improved, would be liable to extermination. Hence we see why all 
the species in the same region do at last, if we look to long enough 
intervals of time, become modified, for otherwise they would become 

In members of the same class the average amount of change during 
long and equal periods of time, may, perhaps, be nearly the same; 
but as the accumulation of enduring formation, rich in fossils, 
depends on great masses of sediment being deposited on subsiding 
areas, our formations have been almost necessarily accumulated at 
wide and irregularly intermittent intervals of time; consequently 
the amount of organic change exhibited by the fossils embedded in 
consecutive formations is not equal. Each formation, on this view, 
does not mark a new and complete act of creation, but only an 
occasional scene, taken almost at hazard in an ever slowly changing 

We can clearly understand why a species when once lost should 
never reappear, even if the very same conditions of life, organic and 
inorganic, should recur. For though the offspring of one species 
might be adapted (and no doubt this has occurred in innumerable 
instances) to fill the place of another species in the economy of 
nature, and thus supplant it; yet the two forms — the old and the new 
— would not be identically the same; for both would almost certainly 
inherit different characters from their distinct progenitors; and 
organisms already differing would vary in a different manner. For 
instance, it is possible, if all our fantail pigeons were destroyed, that 
fanciers might make a new breed hardly distinguishable from the 


present breed; but if the parent rock pigeon were likewise destroyed, 
and under nature we have every reason to believe that parent-forms 
are generally supplanted and exterminated by their improved off- 
spring, it is incredible that a fantail, identical with the existing 
breed, could be raised from any other species of pigeon, or even from 
any other well-established race of the domestic pigeon, for the succes- 
sive variations would almost certainly be in some degree different, 
and the newly-formed variety would probably inherit from its pro- 
genitor some characteristic differences. 

Groups of species, that is, genera and families, follow the same 
general rules in their appearance and disappearance as do single 
sp)ecies, changing more or less quickly, and in a greater or lesser 
degree. A group, when it has once disappeared, never reappears; 
that is, its existence, as long as it lasts, is continuous. I am aware 
that there are some apparent exceptions to this rule, but the excep- 
tions are surprisingly few, so few that E. Forbes, Pictet, and Wood- 
ward (though all strongly opposed to such views as I maintain) 
admit its truth; and the rule strictly accords with the theory. For all 
the species of the same group, however long it may have lasted, are 
the modified descendants one from the other, and all from a com- 
mon progenitor. In the genus Lingula, for instance, the species which 
have successively appeared at all ages must have been connected by 
an unbroken series of generations, from the lowest Silurian stratum 
to the present day. 

We have seen in the last chapter that whole groups of species some- 
times falsely appear to have been abruptly developed; and I have 
attempted to give an explanation of this fact, which if true would be 
fatal to my views. But such cases are certainly exceptional; the 
general rule being a gradual increase in number, until the group 
reaches its maximum, and then, sooner or later, a gradual decrease. 
If the number of the species included within a genus, or the number 
of the genera within a family, be represented by a vertical line of 
varying thickness, ascending through the successive geological 
formations, in which the species are found, the line will sometimes 
falsely appear to begin at its lower end, not in a sharp point, but 
abruptly; it then gradually thickens upwards, often keeping of 
equal thickness for a space, and ultimately thins out in the upper 


beds, marking the decrease and final extinction of the species. This 
gradual increase in number of the species of a group is strictly con- 
formable with the theory, for the species of the same genus, and the 
genera of the same family, can increase only slowly and progressively; 
the process of modification and the production of a number of allied 
forms necessarily being a slow and gradual process, — one species first 
giving rise to two or three varieties, these being slowly converted 
into species, which in their turn produce by equally slow steps other 
varieties and species, and so on, like the branching of a great tree 
from a single stem, till the group becomes large. 


We have as yet only spoken incidentally of the disappearance of 
species and of groups of species. On the theory of natural selection, 
the extinction of old forms and the production of new and improved 
forms are intimately connected together. The old notion of all the 
inhabitants of the earth having been swept away by catastrophes at 
successive periods is very generally given up, even by those geologists, 
as Elie de Beaumont, Murchison, Barrande, etc., whose general views 
would naturally lead them to this conclusion. On the contrary, we 
have every reason to believe, from the study of the tertiary forma- 
tions, that species and groups of species gradually disappear, one 
after another, first from one spot, then from another, and finally 
from the world. In some few cases, however, as by the breaking of 
an isthmus and the consequent irruption of a multitude of new 
inhabitants into an adjoining sea, or by the final subsidence of an 
island, the process of extinction may have been rapid. Both single 
species and whole groups of species last for very unequal periods; 
some groups, as we have seen, have endured from the earliest known 
dawn of life to the present day; some have disappeared before the 
close of the palxozoic period. No fixed law seems to determine the 
length of time during which any single species or any single genus 
endures. There is reason to believe that the extinction of a whole 
group of species is generally a slower process than their production : 
if their appearance and disappearance be represented, as before, by 
a vertical line of varying thickness the line is found to tap)er more 
gradually at its upper end, which marks the progress of extermina- 


tion, than at its lower end, which marks the first appearance and the 
early increase in number of the species. In some cases, however, the 
extermination of whole groups, as of ammonites, towards the close 
of the secondary period, has been wonderfully sudden. 

The extinction of species has been involved in the most gratuitous 
mystery. Some authors have even supposed that, as the individual 
has a definite length of life, so have species a definite duration. No 
one can have marvelled more than I have done at the extinction of 
species. When I found in La Plata the tooth of a horse embedded 
with the remains of Mastodon, Megatherium, Toxodon, and other 
extinct monsters, which all co-existed with still living shells at a very 
late geological f)eriod, I was filled with astonishment; for, seeing that 
the horse, since its introduction by the Spaniards into South America, 
has run wild over the whole country and has increased in numbers 
at an unparalleled rate, 1 asked myself what could so recently have 
exterminated the former horse under conditions of hfe apparently 
so favourable. But my astonishment was groundless. Professor Owen 
soon perceived that the tooth, though so like that of the existing 
horse, belonged to an extinct species. Had this horse been still living, 
but in some degree rare, no naturalist would have felt the least 
surprise at its rarity; for rarity is the attribute of a vast number of 
species of all classes, in all countries. If we ask ourselves why this or 
that species is rare, we answer that something is unfavourable in its 
conditions of life; but what that something is we can hardly ever tell. 
On the supposition of the fossil horse still existing as a rare species, 
we might have felt certain, from the analogy of all other mammals, 
even of the slow-breeding elephant, and from the history of the 
naturalisation of the domestic horse in South America, that under 
more favourable conditions it would in a very few years have stocked 
the whole continent. But we could not have told what the unfavour- 
able conditions were which checked its increase, whether some one 
or several contingencies, and at what period of the horse's life, and 
in what degree they severally acted. If the conditions had gone on, 
however slowly, becoming less and less favourable, we assuredly 
should not have perceived the fact, yet the fossil horse would cer- 
tainly have become rarer and rarer, and finally extinct; — its place 
being seized on by some more successful competitor. 


It is most difficult always to remember that the increase of every 
creature is constantly being checked by unperceived hostile agencies; 
and that these same unperceived agencies are amply sufficient to 
cause rarity, and finally extinction. So little is this subject understood, 
that I have heard surprise repeatedly expressed at such great mon- 
sters as the Mastodon and the more ancient Dinosaurians having 
become extinct; as if mere bodily strength gave victory in the batde 
of life. Mere size, on the contrary, would in some cases determine, 
as has been remarked by Owen, quicker extermination from the 
greater amount of requisite food. Before man inhabited India or 
Africa, some cause must have checked the continued increase of the 
existing elephant. A highly capable judge, Dr. Falconer, believes 
that it is chiefly insects which, from incessantly harassing and weak- 
ening the elephant in India, check its increase; and this was Bruce's 
conclusion with respect to the African elephant in Abyssinia. It is 
certain that insects and blood-sucking bats determine the existence 
of the larger naturalized quadrupeds in several parts of South 

We see in many cases in the more recent tertiary formations, that 
rarity precedes extinction; and we know that this has been the 
progress of events with those animals which have been exterminated, 
either locally or wholly, through man's agency. I may repeat what 
I published in 1845, namely, that to admit that species generally 
become rare before they become extinct — to feel no surprise at the 
rarity of a species, and yet to marvel greatly when the species ceases 
to exist, is much the same as to admit that sickness in the individual 
is the forerunner of death — to feel no surprise at sickness, but, when 
the sick man dies, to wonder and to suspect that he died by some 
deed of violence. 

The theory of natural selection is grounded on the belief that each 
new variety and ultimately each new sjjecies, is produced and main- 
tained by having some advantage over those with which it comes 
into competition; and the consequent extinction of the less favoured 
forms almost inevitably follows. It is the same with our domestic 
productions; when a new and slightly improved variety has been 
raised, it at first supplants the less improved varieties in the same 
neighbourhood; when much improved it is transported far and near. 


like our short-horn cattle, and takes the place of other breeds in other 
countries. Thus the appearance of new forms and the disappearance 
of old forms, both those naturally and those artificially produced, are 
bound together. In flourishing groups, the number of new specific 
forms which have been produced within a given time has at some 
periods probably been greater than the number of the old specific 
forms which have been exterminated; but we know that species have 
not gone on indefinitely increasing, at least during the later geologi- 
cal epochs, so that, looking to later times, we may believe that the 
production of new forms has caused the extinction of about the 
same number of old forms. 

The competition will generally be most severe, as formerly 
explained and illustrated by examples, between the forms which 
are most like each other in all respects. Hence the improved and 
modified descendants of a species will generally cause the extermi- 
nation of the parent species; and if many new forms have been 
developed from any one species, the nearest allies of that species, />., 
the species of the same genus, will be the most liable to extermina- 
tion. Thus, as I believe, a number of new species descended from 
one species, that is a new genus, comes to supplant an old genus, 
belonging to the same family. But it must often have happened that 
a new species belonging to some one group has seized on the place 
occupied by a species belonging to a distinct group, and thus have 
caused its extermination. If many allied forms be developed from 
the successful intruder, many will have to yield their places; and it 
will generally be the allied forms, which will suffer from some 
inherited inferiority in common. But whether it be sf)ecies belong- 
ing to the same or to a distinct class, which have yielded their places 
to other modified and improved species, a few of the sufferers may 
often be preserved for a long time, from being fitted to some peculiar 
line of life, or from inhabiting some distant and isolated station, 
where they will have escaped severe competition. For instance, some 
sjjecies of Trigonia, a great genus of shells in the secondary forma- 
tions, survive in the Australian seas; and a few members of the great 
and almost extinct group of Ganoid fishes still inhabit our fresh 
waters. Therefore the utter extinction of a group is generally, as we 
have seen, a slower process than its production. 


With respect to the apparently sudden extermination of whole 
families or orders, as of trilobites at the close of the palaeozoic period 
and of ammonites at the close of the secondary period, we must 
remember what has been already said on the probable wide intervals 
of time between our consecutive formations; and in these intervals 
there may have been much slow extermination. Moreover, when, by 
sudden immigration or by unusually rapid development, many 
species of a new group have taken possession of an area, many of 
the older species will have been exterminated in a correspondingly 
rapid manner; and the forms which thus yield their places will 
commonly be allied, for they will partake of the same inferiority in 

Thus, as it seems to me, the manner in which single species and 
whole groups of species become extinct accord well with the theory 
of natural selection. We need not marvel at extinction; if we must 
marvel, let it be at our own presumption in imagining for a moment 
that we understand the many complex contingencies on which the 
existence of each species depends. If we forget for an instant that 
each species tends to increase inordinately, and that some check is 
always in action, yet seldom perceived by us, the whole economy of 
nature will be utterly obscured. Whenever we can precisely say why 
this species is more abundant in individuals than that; why this 
species and not another can be naturalised in a given country; then, 
and not until then, we may justly feel surprise why we cannot 
account for the extinction of any particular species or group of 


Scarcely any palxontological discovery is more striking than the 
fact that the forms of life change almost simultaneously throughout 
the world. Thus our European Chalk formation can be recognised 
in many distinct regions, under the most different climates, where 
not a fragment of the mineral chalk itself can be found; namely in 
North America, in equatorial South America, in Tierra del Fuego, 
at the Cape of Good Hope, and in the peninsula of India. For at 
these distant points, the organic remains in certain beds present an 


unmistakeable resemblance to those of the Chalk. It is not that the 
same species are met with; for in some cases not one species is 
identically the same, but they belong to the same families, genera, 
and sections of genera, and sometimes are similarly characterised in 
such trifling points as mere superficial sculpture. Moreover, other 
forms, which are not found in the Chalk of Europe, but which occur 
in the formations either above or below, occur in the same order at 
these distant points of the world. In the several successive palxozoic 
formations of Russia, Western Europe, and North America, a similar 
parallelism in the forms of life has been observed by several authors; 
so it is, according to Lyell, with the European and North American 
tertiary deposits. Even if the few fossil species which are common to 
the Old and New Worlds were kept wholly out of view, the general 
parallelism in the successive forms of life, in the paleozoic and 
tertiary stages, would still be manifest, and the several forma- 
tions could be easily correlated. 

These observations, however, relate to the marine inhabitants of 
the world: we have not sufficient data to judge whether the pro- 
ductions of the land and of fresh water at distant points change in 
the same parallel manner. We may doubt whether they have thus 
changed: if the Megatherium, Mylodon, Macrauchenia, and Toxo- 
don had been brought to Europe from La Plata, without any infor- 
mation in regard to their geological position, no one would have 
suspected that they had co-existed with sea-shells all still living; but 
as these anomalous monsters co-existed with the Mastodon and 
Horse, it might at least have been inferred that they had lived during 
one of the later tertiary stages. 

When the marine forms of life are spoken of as having changed 
simultaneously throughout the world, it must not be supposed that 
this expression relates to the same year, or to the same country, or 
even that it has a very strict geological sense; for if all the marine 
animals now living in Europe, and all those that lived in Europe 
during the pleistocene period (a very remote period as measured by 
years, including the whole glacial epoch) were compared with those 
now existing in South America or in Australia, the most skilful 
naturalist would hardly be able to say whether the present or the 
pleistocene inhabitants of Europe resembled most closely those of 


the southern hemisphere. So, again, several highly competent 
observers maintain that the existing productions of the United 
States are more closely related to those which lived in Europe during 
certain late tertiary stages, than to the present inhabitants of Europe; 
and if this be so, it is evident that fossiliferous beds now deposited 
on the shores of North America would hereafter be liable to be 
classed with somewhat older European beds. Nevertheless, looking 
to a remotely future epoch, there can be little doubt that all the 
more modern marine formations, namely, the upper pliocene, the 
pleistocene and strictly modern beds of Europe, North and South 
America, and Australia, from containing fossil remains in some 
degree allied, and from not including those forms which are found 
only in the older underlying deposits, would be correctly ranked as 
simultaneous in a geological sense. 

The fact of the forms of life changing simultaneously, in the above 
large sense, at distant parts of the world, has greatly struck those 
admirable observers, MM. de Verneuil and d'Archiac. After refer- 
ring to the parallelism of the palarozoic forms of life in various parts 
of Europe, they add, "If, struck by this strange sequence, we turn our 
attention to North America, and there discover a series of analogous 
phenomena, it will appear certain that all these modifications of 
species, their extinction, and the introduction of new ones, cannot 
be owing to mere changes in marine currents or other causes more 
or less local and temporary, but depend on general laws which 
govern the whole animal kingdom." M. Barrande has made forcible 
remarks to precisely the same effect. It is, indeed, quite futile to 
look to changes of currents, climate, or other physical conditions, as 
the cause of these great mutations in the forms of life throughout the 
world, under the most different climates. We must, as Barrande has 
remarked, look to some sp)ecial law. We shall see this more clearly 
when we treat of the present distribution of organic beings, and find 
how slight is the relation between the physical conditions of various 
countries and the nature of their inhabitants. 

This great fact of the parallel succession of the forms of life 
throughout the world, is explicable on the theory of natural selection. 
New species are formed by having some advantage over older forms; 
and the forms, which are already dominant, or have some advantage 


over the other forms in their own country, give birth to the greatest 
number of new varieties or incipient species. We have distinct evi- 
dence on this head, in the plants which are dominant, that is, which 
are commonest and most widely diffused, producing the greatest 
number of new varieties. It is also natural that the dominant, vary- 
ing, and far-spreading species, which have already invaded to a 
certain extent the territories of other species, should be those which 
would have the best chance of spreading still further, and of giving 
rise in new countries to other new varieties and species. The process 
of diffusion would often be very slow, depending on climatal and 
geographical changes, on strange accidents, and on the gradual 
acclimatisation of new species to the various climates through which 
they might have to pass, but in the course of time the dominant 
forms would generally succeed in spreading and would ultimately 
prevail. The diffusion would, it is probable, be slower with the 
terrestrial inhabitants of the distinct continents than with the marine 
inhabitants of the continuous sea. We might therefore exfject to 
find, as we do find, a less strict degree of parallelism in the succession 
of the productions of the land than with those of the sea. 

Thus, as it seems to me, the parallel, and, taken in a large sense, 
simultaneous, succession of the same forms of life throughout the 
world, accords well with the principle of new spjecies having been 
formed by dominant species spreading widely and varying; the new 
sf)ecies thus produced being themselves dominant, owing to their 
having had some advantage over their already dominant parents, as 
well as over other species, and again spreading, varying, and pro- 
ducing new forms. The old forms which are beaten and which 
yield their places to the new and victorious forms, will generally be 
allied in groups, from inheriting some inferiority in common; and 
therefore, as new and improved groups spread throughout the world, 
old groups disappear from the world; and the succession of forms 
everywhere tends to correspond both in their first appearance and 
final disappearance. 

There is one other remark connected with this subject worth 
making. I have given my reasons for believing that most of our 
great formations, rich in fossils, were deposited during periods of 
subsidence; and that blank intervals of vast duration, as far as fossils 


are concerned, occurred during the periods when the bed of the sea 
was either stationary or rising, and hkewise when sediment was not 
thrown down quickly enough to embed and preserve organic 
remains. During these long and blank intervals I suppose that the 
inhabitants of each region underwent a considerable amount of 
modification and extinction, and that there was much migration 
from other parts of the world. As we have reason to believe that 
large areas are affected by the same movement, it is probable that 
strictly contemporaneous formations have often been accumulated 
over very wide spaces in the same quarter of the world; but we are 
very far from having any right to conclude that this has invariably 
been the case, and that large areas have invariably been affected by 
the same movements. When two formations have been deposited in 
two regions during nearly, but not exactly, the same period, we 
should find in both, from the causes explained in the foregoing 
paragraphs, the same general succession in the forms of life; but 
the species would not exactly correspond; for there will have been 
a little more time in the one region than in the other for modifica- 
tion, extinction, and immigration. 

I suspect that cases of this nature occur in Europe. Mr. Prestwich, 
in his admirable Memoirs on the eocene deposits of England and 
France, is able to draw a close general parallelism between the suc- 
cessive stages in the two countries; but when he compares certain 
stages in England with those in France, although he finds in both 
a curious accordance in the numbers of the species belonging to the 
same genera, yet the species themselves differ in a manner very diffi- 
cult to account for considering the proximity of the two areas, — 
unless, indeed, it be assumed that an isthmus separated two seas 
inhabited by distinct, but contemporaneous, faunas. Lyell has made 
similar observations on some of the later tertiary formations. Bar- 
rande, also, shows that there is a striking general parallelism in the 
successive Silurian deposits of Bohemia and Scandinavia; neverthe- 
less he finds a surprising amount of difference in the species. If the 
several formations in these regions have not been deposited during 
the same exact periods, — a formation in one region often corre- 
sponding with a blank interval in the other, — and if in both regions 
the species have gone on slowly changing during the accumulation 


of the several formations and during the long intervals of time 
between them; in this case the several formations in the two regions 
could be arranged in the same order, in accordance with the general 
succession of the forms of life, and the order would falsely appear 
to be strictly parallel; nevertheless the species would not be all the 
same in the apparently corresponding stages in the two regions. 


Let US now look to the mutual affinities of extinct and living 
species. All fall into a few grand classes; and this fact is at once 
explained on the principle of descent. The more ancient any form 
is, the more, as a general rule, it differs from hving forms. But, as 
Buckland long ago remarked, extinct species can all be classed either 
in still existing groups, or between them. That the extinct forms of 
life help to fill up the intervals between existing genera, families, 
and orders, is certainly true; but as this statement has often been 
ignored or even denied, it may be well to make some remarks on this 
subject, and to give some instances. If we confine our attention 
either to the Hving or to the extinct species of the same class, the 
series is far less perfect than if we combine both into one general 
system. In the writings of Professor Owen we continually meet 
with the expression of generalised forms, as applied to extinct ani- 
mals; and in the writings of Agassiz, of prophetic or synthetic types; 
and these terms imply that such forms are in fact intermediate or 
connecting links. Another distinguished paleontologist, M. Gaudry, 
has shown in the most striking manner that many of the fossil 
mammals discovered by him in Attica serve to break down the inter- 
vals between existing genera. Cuvier ranked the Ruminants and 
Pachyderms as two of the most distinct orders of mammals: but 
SO many fossil links have been disentombed that Owen has had to 
alter the whole classification, and has placed certain pachyderms in 
the same sub-order with ruminants; for example, he dissolves by 
gradations the apparently wide interval between the pig and the 
camel. The Ungulata or hoofed quadrupeds are now divided into 
the even-toed or odd-toed divisions; but the Macrauchenia of South 
America connects to a certain extent these two grand divisions. No 


one will deny that the Hippahon is intermediate between the 
existing horse and certain older ungulate forms. What a wonderful 
connecting link in the chain of mammals is the Typotherium from 
South America, as the name given to it by Professor Gervais 
expresses, and which cannot be placed in any existing order. The 
Sirenia form a very distinct group of mammals, and one of the 
most remarkable peculiarities in the existing dugong and lamentin is 
the entire absence of hind limbs without even a rudiment being left; 
but the extinct Halitherium had, according to Professor Flower, an 
ossified thigh-bone "articulated to a well-defined acetabulum in the 
pelvis," and it thus makes some approach to ordinary hoofed quad- 
rupeds, to which the Sirenia are in other respects aUied. The ceta- 
ceans or whales are widely different from all other mammals, but 
the tertiary 2^uglodon and Squalodon, which have been placed by 
some naturalists in an order by themselves, are considered by Profes- 
sor Huxley to be undoubtedly cetaceans, "and to constitute con- 
necting links with the aquatic carnivora." 

Even the wide interval between birds and reptiles has been shown 
by the naturalist just quoted to be partially bridged over in the 
most unexpected manner, on the one hand, by the ostrich and extinct 
Archeopteryx, and on the other hand, by the Compsognathus, one 
of the dinosaurians — that group which includes the most gigantic of 
all terrestrial reptiles. Turning to the Invertebrata, Barrande asserts 
(a higher authority could not be named) that he is every day 
taught that, although paleozoic animals can certainly be classed 
under existing groups, yet that at this ancient period the groups were 
not so distinctly separated from each other as they now are. 

Some writers have objected to any extinct species, or group of 
species, being considered as intermediate between any two living 
species, or groups of species. If by this term it is meant that an 
extinct form is directly intermediate in all its characters between two 
living forms or groups, the objection is probably valid. But in a 
natural classification many fossil species certainly stand between 
living species, and some extinct genera between living genera, even 
between genera belonging to distinct families. The most common 
case, especially with respect to very distinct groups, such as fish and 
reptiles, seems to be, that, supposing them to be distinguished at the 


present day by a score of characters, the ancient members are sep- 
arated by a somewhat lesser number of characters; so that the two 
groups formerly made a somewhat nearer approach to each other 
than they now do. 

It is a common belief that the more ancient a form is, by so much 
the more it tends to connect by some of its characters groups now 
widely separated from each other. This remark no doubt must be 
restricted to those groups which have undergone much change in the 
course of geological ages; and it would be difficult to prove the 
truth of the proposition, for every now and then a Uving animal, 
as the Lepidosiren, is discovered having affinities directed towards 
very distinct groups. Yet if we compare the older reptiles and batra- 
chians, the older fish, the older cephalopods, and the eocene mam- 
mals, with the more recent members of the same classes, we must 
admit that there is truth in the remark. 

Let us see how far these several facts and inferences accord with 
the theory of descent with modification. As the subject is somewhat 
complex, I must request the reader to turn to the diagram in the 
fourth chapter. We may suppose that the numbered letters in italics 
represent genera, and the dotted lines diverging from them the 
species in each genus. The diagram is much too simple, too few 
genera and too few species being given, but this is unimportant for 
us. The horizontal lines may represent successive geological forma- 
tions, and all the forms beneath the uppermost line may be con- 
sidered as extinct. The three existing genera a", ly'*, p'*, will form 
a small family; ^'* and /" a closely allied family or sub-family; and 
0", ir", m", a third family. These three families, together with the 
many extinct genera on the several lines of descent diverging from 
the parent-form (A) will form an order, for all will have inherited 
something in common from their ancient progenitor. On the prin- 
ciple of the continued tendency to divergence of character, which 
was formerly illustrated by this diagram, the more recent any form 
is, the more it will generally differ from its ancient progenitor. 
Hence we can understand the rule that the most ancient fossils differ 
most from existing forms. We must not, however, assume that 
divergence of character is a necessary contingency; it depends solely 
on the descendants from a species being thus enabled to seize on 


many and different places in the economy of nature. Therefore it is 
quite possible, as we have seen in the case of some Silurian forms, 
that a species might go on being slightly modified in relation to its 
slightly altered conditions of life, and yet retain throughout a vast 
period the same general characteristics. This is represented in the 
diagram by the letter f'*. 

All the many forms, extinct and recent, descended from (A), 
make, as before remarked, one order; and this order, from the con- 
tinued effects of extinction and divergence of character, has become 
divided into several sub-families and families, some of which are 
supposed to have perished at different periods, and some to have 
endured to the present day. 

By looking at the diagram we can see that if many of the extinct 
forms supposed to be imbedded in the successive formations, were 
discovered at several points low down in the series, the three existing 
families on the uppermost line would be rendered less distinct from 
each other. If, for instance, the genera a', a", d", f, m^, m', w', were 
disinterred, these three families would be so closely linked together 
that they probably would have to be united into one great family, in 
nearly the same manner as has occurred with ruminants and certain 
pachyderms. Yet he who objected to consider as intermediate the 
extinct genera, which thus link together the living genera of three 
families, would be partly justified, for they are intermediate, not 
directly, but only by a long and circuitous course through many 
widely different forms. If many extinct forms were to be discovered 
above one of the horizontal lines or geological formations — for in- 
stance, above No. VI. — but none from beneath this line, then only 
two of the families (those on the left hand, a'*, etc., and ^'*, etc.) 
would have to be united into one; and there would remain two 
families, which would be less distinct from each other than they 
were before the discovery of the fossils. So again if the three families 
formed of eight genera (a'* to w'*), on the uppermost line, be sup- 
posed to differ from each other by half-a-dozen important char- 
acters, then the families which existed at the period marked VI. 
would certainly have differed from each other by a less number of 
characters; for they would at this early stage of descent have diverged 
in a less degree from their common progenitor. Thus it comes that 


ancient and extinct genera are often in a greater or less degree inter- 
mediate in character between their modified descendants, or between 
their collateral relations. 

Under nature the process will be far more complicated than is 
represented in the diagram; for the groups will have been more 
numerous; they will have endured for extremely unequal lengths 
of time, and will have been modified in various degrees. As we 
possess only the last volume of the geological record, and that in a 
very broken condition, we have no right to expect, except in rare 
cases, to fill up the wide intervals in the natural system, and thus to 
unite distinct families or orders. All that we have a right to expect 
is, that those groups which have, within known geological periods, 
undergone much modification, should in the older formations make 
some slight approach to each other; so that the older members should 
differ less from each other in some of their characters than do the 
existing members of the same groups; and this by the concurrent 
evidence of our best palaeontologists is frequently the case. 

Thus, on the theory of descent with modification, the main facts 
with respect to the mutual affinities of the extinct forms of life to 
each other and to living forms, are explained in a satisfactory man- 
ner. And they are wholly inexplicable on any other view. 

On this same theory, it is evident that the fauna during any one 
great period in the earth's history will be intermediate in general 
character between that which preceded and that which succeeded it. 
Thus the sp)ecies which lived at the sixth great stage of descent in 
the diagram are the modified offspring of those which lived at the 
fifth stage, and are the parents of those which became still more 
modified at the seventh stage; hence they could hardly fail to be 
nearly intermediate in character between the forms of life above and 
below. We must, however, allow for the entire extinction of some 
preceding forms, and in any one region for the immigration of new 
forms from other regions, and for a large amount of modification 
during the long and blank interval between the successive forma- 
tions. Subject to these allowances, the fauna of each geological 
period undoubtedly is intermediate in character, between the pre- 
ceding and succeeding faunas. I need give only one instance, namely, 
the manner in which the fossils of the Devonian system, when this 


system was first discovered, were at once recognised by palaeontolo- 
gists as intermediate in character between those of the overlying 
carboniferous, and underlying Silurian systems. But each fauna is 
not necessarily exactly intermediate, as unequal intervals of time 
have elapsed between consecutive formations. 

It is no real objection to the truth of the statement that the fauna 
of each f)eriod as a whole is nearly intermediate in character between 
the preceding and succeeding faunas, that certain genera offer excep- 
tions to the rule. For instance, the species of mastodons and ele- 
phants, when arranged by Dr. Falconer in two series, — in the first 
place according to their mutual affinities, and in the second place 
according to their periods of existence, — do not accord in arrange- 
ment. The sp)ecies extreme in character are not the oldest or the 
most recent; nor are those which are intermediate in character, inter- 
mediate in age. But supposing for an instant, in this and other such 
cases, that the record of the first appearance and disappearance of 
the species was complete, which is far from the case, we have no 
reason to believe that forms successively produced necessarily endure 
for corresponding lengths of time. A very ancient form may occa- 
sionally have lasted much longer than a form elsewhere subsequently 
produced, especially in the case of terrestrial productions inhabiting 
separated districts. To compare small things with great; if the 
principal living and extinct races of the domestic pigeon were 
arranged in serial affinity, this arrangement would not closely accord 
with the order in time of their production, and even less with the 
order of their disappearance; for the parent rock pigeon still lives; 
and many varieties between the rock pigeon and the carrier have 
become extinct; and carriers which are extreme in the important 
character of length of back originated earlier than short-beaked 
tumblers, which are at the opposite end of the scries in this respect. 

Closely connected with the statement, that the organic remains 
from an intermediate formation are in some degree intermediate in 
character, is the fact, insisted on by all palaeontologists, that fossils 
from two consecutive formations are far more closely related to each 
other, than are the fossils from two remote formations. Pictet gives 
as a well-known instance, the general resemblance of the organic 
remains from the several stages of the Chalk formation, though the 


species are distinct in each stage. This fact alone, from its generality, 
seems to have shaken Professor Pictet in his belief in the immuta- 
bility of species. He who is acquainted with the distribution of 
existing species over the globe, will not attempt to account for the 
close resemblance of distinct species in closely consecutive forma- 
tions, by the physical conditions of the ancient areas having remained 
nearly the same. Let it be remembered that the forms of life, at 
least those inhabiting the sea, have changed almost simultaneously 
throughout the world, and therefore under the most different cli- 
mates and conditions. Consider the prodigious vicissitudes of climate 
during the pleistocene period, which includes the whole glacial 
epoch, and note how little the specific forms of the inhabitants of the 
sea have been affected. 

On the theory of descent, the full meaning of the fossil remains 
from closely consecutive formations being closely related, though 
ranked as distinct species, is obvious. As the accumulation of each 
formation has often been interrupted, and as long blank intervals 
have intervened between successive formations, we ought not to 
expect to find, as I attempted to show in the last chapter, in any one 
or in any two formations, all the intermediate varieties between the 
species which appeared at the commencement and close of these 
periods: but we ought to find after intervals, very long as measured 
by years, but only moderately long as measured geologically, closely 
allied forms, or, as they have been called by some authors, repre- 
sentative species; and these assuredly we do find. We find, in short, 
such evidence of the slow and scarcely sensible mutations of specific 
forms, as we have the right to expect. 


We have seen in the fourth chapter that the degree of differentia- 
tion and specialisation of the parts in organic beings, when arrived 
at maturity, is the best standard, as yet suggested, of their degree of 
perfection or highness. We have also seen that, as the specialisation 
of parts is an advantage to each being, so natural selection will tend 
to render the organisation of each being more specialised and per- 
fect, and in this sense higher; not but that it may leave many crea- 


tures with simple and unimproved structures fitted for simple 
conditions of life, and in some cases will even degrade or simplify 
the organisation, yet leaving such degraded beings better fitted for 
their new walks of life. In another and more general manner, new 
species become superior to their predecessors; for they have to beat in 
the struggle for life all the older forms, with which they come into 
close competition. We may therefore conclude that if under a nearly 
similar climate the eocene inhabitants of the world could be put into 
competition with the existing inhabitants, the former would be 
beaten and exterminated by the latter, as would the secondary by 
the eocene, and the palaeozoic by the secondary forms. So that by 
this fundamental test of victory in the battle for life, as well as by 
the standard of the specialisation of organs, modern forms ought, 
on the theory of natural selection, to stand higher than ancient forms. 
Is this the case? A large majority of palaeontologists would answer 
in the affirmative; and it seems that this answer must be admitted 
as true, though difficult of proof. 

It is no valid objection to this conclusion, that certain Brachiopods 
have been but slightly modified from an extremely remote geological 
epoch; and that certain land and fresh-water shells have remained 
nearly the same, from the time when, as far as is known, they first 
appeared. It is not an insuperable difficulty that Foraminifera have 
not, as insisted on by Dr. Carpenter, progressed in organisation since 
even the Laurentian ejXKh; for some organisms would have to 
remain fitted for simple conditions of life, and what could be better 
fitted for this end than these lowly organised Protozoa? Such objec- 
tions as the above would be fatal to my view, if it included advance 
in organisation as a necessary contingent. They would Ukewise be 
fatal, if the above Foraminifera, for instance, could be proved to have 
first come into existence during the Laurentian epoch, or the above 
Brachiof)ods during the Cambrian formation; for in this case, there 
would not have been time sufficient for the development of these 
organisms up to the standard which they had then reached. When 
advanced up to any given point, there is no necessity, on the theory 
of natural selection, for their further continued progress; though 
they will, during each successive age, have to be slightly modified, 
so as to hold their places in relation to slight changes in their condi- 


tions. The foregoing objections hinge on the question whether we 
really know how old the world is, and at what period the various 
forms of life first appeared; and this may well be disputed. 

The problem whether organisation on the whole has advanced is 
in many ways excessively intricate. The geological record, at all 
times imperfect, does not extend far enough back, to show with 
unmistakeable clearness that within the known history of the world 
organisation has largely advanced. Even at the present day, looking 
to members of the same class, naturalists are not unanimous which 
forms ought to be ranked as highest : thus, some look at the selaceans 
or sharks, from their approach in some important points of structure 
to reptiles, as the highest fish; others look at the teleosteans as the 
highest. The ganoids stand intermediate between the selaceans and 
teleosteans; the latter at the present day are largely preponderant in 
number; but formerly selaceans and ganoids alone existed; and in 
this case, according to the standard of highness chosen, so will it be 
said that fishes have advanced or retrograded in organisation. To 
attempt to compare members of distinct types in the scale of high- 
ness seems hopeless; who will decide whether a cuttle-fish be higher 
thaa a bee — that insect which the great Von Baer believed to be "in 
fact more highly organised than a fish, although upon another type"? 
In the complex struggle for life it is quite credible that crustaceans, 
not very high in their own class, might beat cephalopods, the highest 
molluscs; and such crustaceans, though not highly developed, 
would stand very high in the scale of invertebrate animals, if judged 
by the most decisive of all trials — the law of battle. Beside these 
inherent difficulties in deciding which forms are the most advanced 
in organisation, we ought not solely to compare the highest mem- 
bers of a class at any two periods — though undoubtedly this is one 
and perhaps the most important element in striking a balance — but 
we ought to compare all the members, high and low, at two periods. 
At an ancient epoch the highest and lowest molluscoidal animals, 
namely, cephalopods and brachiopods, swarmed in numbers; at the 
present time both groups are greatly reduced, whilst others, inter- 
mediate in organisation, have largely increased; consequently some 
naturalists maintain that molluscs were formerly more highly 
developed than at present; but a stronger case can be made out on 


the opposite side, by considering the vast reduction of the brachio- 
pods, and the fact that our existing cephalopods, though few in 
number, are more highly organised than their ancient representa- 
tives. We ought also to compare the relative proportional numbers 
at any two periods of the high and low classes throughout the world: 
if, for instance, at the present day fifty thousand kinds of vertebrate 
animals exist, and if we knew that at some former period only ten 
thousand kinds existed, we ought to look at this increase in number 
in the highest class, which implies a great displacement of lower 
forms, as a decided advance in the organisation of the world. We 
thus see how hopelessly difficult it is to compare with perfect fairness 
under such extremely complex relations, the standard of organisation 
of the imperfectly known faunas of successive periods. 

We shall appreciate this difficulty more clearly, by looking to 
certain existing faunas and floras. From the extraordinary manner 
in which Eurojiean productions have recently spread over New 
Zealand, and have seized on places which must have been previously 
occupied by the indigenes, we must believe, that if all the animals 
and plants of Great Britain were set free in New Zealand, a multi- 
tude of British forms would in the course of time become thoroughly 
naturalised there, and would exterminate many of the natives. On 
the other hand, from the fact that hardly a single inhabitant of the 
southern hemisphere has become wild in any part of Eurojje, we 
may well doubt whether, if all the productions of New Zealand were 
set free in Great Britain, any considerable number would be enabled 
to seize on places now occupied by our native plants and animals. 
Under this point of view, the productions of Great Britain stand 
much higher in the scale than those of New Zealand. Yet the most 
skilful naturalist, from an examination of the species of the two 
countries, could not have foreseen this result. 

Agassiz and several other highly competent judges insist that 
ancient animals resemble to a certain extent the embryos of recent 
animals belonging to the same classes; and that the geological suc- 
cession of extinct forms is nearly parallel with the embryological 
development of existing forms. This view accords admirably well 
with our theory. In a future chapter I shall attempt to show that the 
adult differs from its embryo, owing to variations having super- 


vened at a not early age, and having been inherited at a correspond- 
ing age. This process, whilst it leaves the embryo almost unaltered, 
continually adds, in the course of successive generations, more and 
more difference to the adult. Thus the embryo comes to be left as a 
sort of picture, preserved by nature, of the former and less modified 
condition of the species. This view may be true, and yet may never 
be capable of proof. Seeing, for instance, that the oldest known 
mammals, reptiles, and fishes strictly belong to their proper classes, 
though some of these old forms are in a slight degree less distinct 
from each other than are the typical members of the same groups at 
the present day, it would be vain to look for animals having the 
common embryological character of the Vertebrata, until beds rich 
in fossils are discovered far beneath the lowest Cambrian strata — a 
discovery of which the chance is small. 


Mr. Clift many years ago showed that the fossil mammals from 
the Australian caves were closely allied to the living marsupials of 
that continent. In South America, a similar relationship is mani- 
fest, even to an uneducated eye, in the gigantic pieces of armour, 
like those of the armadillo, found in several parts of La Plata; and 
Professor Owen has shown in the most striking manner that most 
of the fossil mammals, buried there in such numbers, are related to 
South American types. This relationship is even more clearly seen 
in the wonderful collection of fossil bones made by MM. Lund and 
Clausen in the caves of Brazil. I was so much impressed with these 
facts that I strongly insisted, in 1H39 and 1845, on this "law of the 
succession of types," — on "this wonderful relationship in the same 
continent between the dead and the living." Professor Owen has 
subsequently extended the same generalisation to the mammals of 
the Old World. We see the same law in this author's restorations of 
the extinct and gigantic birds of New Zealand. We see it also in the 
birds of the caves of Brazil. Mr. Woodward has shown that the 
same law holds good with sea shells, but, from the wide distribution 
of most molluscs, it is not well displayed by them. Other cases could 
be added, as the relation between the extinct and living land shells 


of Madeira; and between the extinct and living brackish water 
shells of the Aralo-Caspian Sea. 

Now what does this remarkable law of the succession of the same 
types within the same areas mean ? He would be a bold man who, 
after comparing the present climate of Australia and of parts of 
South America, under the same latitude, would attempt to account, 
on the one hand through dissimilar physical conditions, for the dis- 
similarity of the inhabitants of these two continents; and, on the 
other hand through similarity of conditions, for the uniformity of 
the same types in each continent during the later tertiary periods. 
Nor can it be pretended that it is an immutable law that marsupials 
should have been chiefly or solely produced in Australia; or that 
Edentata and other American types should have been solely produced 
in South America. For we know that Europe in ancient times was 
peopled by numerous marsupials; and I have shown in the publica- 
tions above alluded to, that in America the law of distribution of 
terrestrial mammals was formerly different from what it now is. 
North America formerly partook strongly of the present character 
of the southern half of the continent; and the southern half was 
formerly more closely allied, than it is at present, to the northern 
half. In a similar manner we know, from Falconer and Cautley's 
discoveries, that Northern India was formerly more closely related in 
its mammals to Africa than it is at the present time. Analogous 
facts could be given in relation to the distribution of marine 

On the theory of descent with modification, the great law of the 
long enduring, but not immutable, succession of the same types 
within the same areas, is at once explained; for the inhabitants of 
each quarter of the world will obviously tend to leave in that quarter, 
during the next succeeding period of time, closely allied though in 
some degree modified descendants. If the inhabitants of one conti- 
nent formerly differed greatly from those of another continent, so 
will their modified descendants still differ in nearly the same manner 
and degree. But after very long intervals of time, and after great 
geographical changes, permitting much intermigration, the feebler 
will yield to the more dominant forms, and there will be nothing 
immutable in the distribution of organic beings. 


It may be asked in ridicule, whether I suppose that the mega- 
therium and other allied huge monsters, which formerly lived in 
South America, have left behind them the sloth, armadillo, and 
anteater, as their degenerate descendants. This cannot for an instant 
be admitted. These huge animals have become wholly extinct, and 
have left no progeny. But in the caves of Brazil, there are many 
extinct species which are closely allied in size and in all other char- 
acters to the species still living in South America; and some of these 
fossils may have been the actual progenitors of the living species. 

It must not be forgotten that, on our theory, all the species of the 
same genus are the descendants of some one species; so that, if six 
genera, each having eight species, be found in one geological forma- 
tion, and in a succeeding formation there be six other allied or repre- 
sentative genera each with the same number of species, then we may 
conclude that generally only one species of each of the older genera 
has left modified descendants, which constitute the new genera con- 
taining the several species; the other seven species of each old genus 
having died out and left no progeny. Or, and this will be a far 
commoner case, two or three species in two or three alone of the six 
older genera will be the parents of the new genera : the other species 
and the other old genera having become utterly extinct. In failing 
orders, with the genera and species decreasing in numbers as is the 
case with the Edentata of South America, still fewer genera and 
species will leave modified blood-descendants. 


I have attempted to show that the geological record is extremely 
imperfect; that only a small portion of the globe has been geo- 
logically explored with care; that only certain classes of organic 
beings have been largely preserved in a fossil state; that the number 
both of specimens and of species, preserved in our museums, is abso- 
lutely as nothing compared with the number of generations which 
must have passed away even during a single formation; that, owing 
to subsidence being almost necessary for the accumulation of deposits 
rich in fossil species of many kinds, and thick enough to outlast 
future degradation, great intervals of time must have elapsed between 
most of our successive formations; that there has probably been more 


extinction during the periods of subsidence, and more variation 
during the periods of elevation, and during the latter the record will 
have been least perfectly kept; that each single formation has not 
been continuously deposited; that the duration of each formation is 
probably short compared with the average duration of specific forms; 
that migration has played an important part in the first appearance 
of new forms in any one area and formation; that widely ranging 
species are those which have varied most frequently, and have often- 
est given rise to new species; that varieties have at first been local; 
and lastly, although each species must have passed through numer- 
ous transitional stages, it is probable that the periods, during which 
each underwent modification, though many and long as measured 
by years, have been short in comparison with the periods during 
which each remained in an unchanged condition. These causes, 
taken conjointly, will to a large extent explain why — though we do 
find many links — we do not find interminable varieties, connecting 
together all extinct and existing forms by the finest graduated steps. 
It should also be constantly borne in mind that any linking variety 
between two forms, which might be found, would be ranked, unless 
the whole chain could be perfectly restored, as a new and distinct 
species; for it is not pretended that we have any sure criterion by 
which species and varieties can be discriminated. 

He who rejects this view of the imperfection of the geological 
record, will rightly reject the whole theory. For he may ask in vain 
where are the numberless transitional links which must formerly 
have connected the closely allied or representative species, found in 
the successive stages of the same great formation? He may dis- 
believe in the immense intervals of time which must have elapsed 
between our consecutive formations; he may overlook how impor- 
tant a part migration has played, when the formations of any one 
great region, as those of Europe, are considered; he may urge the 
apparent, but often falsely apparent, sudden coming in of whole 
groups of sjjecies. He may ask where are the remains of those 
infinitely numerous organisms which must have existed long before 
the Cambrian system was deposited? We now know that at least 
one animal did then exist; but I can answer this last question only 
by supposing that where our oceans now extend they have extended 


for an enormous period, and where our oscillating continents now 
stand they have stood since the commencement of the Cambrian 
system; but that, long before that epoch, the world presented a 
widely different aspect; and that the older continents, formed of 
formations older than any known to us, exist now only as remnants 
in a metamorphosed condition, or Ue still buried under the ocean. 

Passing from these difficulties, the other great leading facts in 
paljcontology agree admirably with the theory of descent with modi- 
fication through variation and natural selection. We can thus under- 
stand how it is that new species come in slowly and successively; how 
species of different classes do not necessarily change together, or at 
the same rate, or in the same degree; yet in the long run that all 
undergo modification to some extent. The extinction of old forms is 
the almost inevitable consequence of the production of new forms. 
We can understand why, when a species has once disappeared, it 
never reappears. Groups of species increase in numbers slowly, and 
endure for unequal periods of time; for the process of modification is 
necessarily slow, and depends on many complex contingencies. The 
dominant species belonging to large and dominant groups tend to 
leave many modified descendants, which form new sub-groups and 
groups. As these are formed, the species of the less vigorous groups, 
from their inferiority inherited from a common progenitor, tend to 
become extinct together, and to leave no modified offspring on the 
face of the earth. But the utter extinction of a whole group of 
species has sometimes been a slow process, from the survival of a 
few descendants, lingering in protected and isolated situations. 
When a group has once wholly disappeared, it does not reappear; 
for the link of generation has been broken. 

We can understand how it is that dominant forms which spread 
widely and yield the greatest number of varieties tend to people the 
world with allied, but modified, descendants; and these will gener- 
ally succeed in displacing the groups which are their inferiors in the 
struggle for existence. Hence, after long intervals of time, the pro- 
ductions of the world appear to have changed simultaneously. 

We can understand how it is that all the forms of life, ancient and 
recent, make together a few grand classes. We can understand, 
from the continued tendency to divergence of character, why, the 


more ancient a form is, the more it generally differs from those now 
living; why ancient and extinct forms often tend to fill up gaps 
between existing forms, sometimes blending two groups, previously 
classed as distinct, into one; but more commonly bringing them only 
a little closer together. The more ancient a form is, the more often 
it stands in some degree intermediate between groups now distinct; 
for the more ancient a form is, the more nearly it will be related to, 
and consequently resemble, the common progenitor of groups, since 
become widely divergent. Extinct forms are seldom directly inter- 
mediate between existing forms; but are intermediate only by a long 
and circuitous course through other extinct and different forms. We 
can clearly see why the organic remains of closely consecutive forma- 
tions are closely allied; for they are closely linked together by genera- 
tion. We can clearly see why the remains of an intermediate 
formation are intermediate in character. 

The inhabitants of the world at each successive period in its history 
have beaten their predecessors in the race for life, and are, in so far, 
higher in the scale, and their structure has generally become more 
specialised; and this may account for the common belief held by so 
many palaeontologists, that organisation on the whole has progressed. 
Extinct and ancient animals resemble to a certain extent the embryos 
of the more recent animals belonging to the same classes, and this 
wonderful fact receives a simple explanation according to our views. 
The succession of the same types of structure within the same areas 
during the later geological periods ceases to be mysterious, and is 
intelligible on the principle of inheritance. 

If, then, the geological record be as imperfect as many believe, 
and it may at least be asserted that the record cannot be proved to 
be much more perfect, the main objections to the theory of natural 
selection are greatly diminished or disappear. On the other hand, 
all the chief laws of palaeontology plainly proclaim, as it seems to 
me, that species have been produced by ordinary generation: old 
forms having been supplanted by new and improved forms of life, 
the products of Variation and the Survival of the Fittest. 


Geographical Distribution 

Present distribution cannot be accounted for by differences in physical 
conditions — Importance of barriers — Affinity of the productions of 
the same continent — Centres of creation — Means of dispersal, by 
changes of chmate and of the level of the land, and by occasional 
means — Dispersal during the Glacial period — Alternate Glacial 
periods in the North and South. 

IN considering the distribution of organic beings over the face 
of the globe, the first great fact which strikes us is, that neither 
the similarity nor the dissimilarity of the inhabitants of various 
regions can be wholly accounted for by climatal and other physical 
conditions. Of late, almost every author who has studied the subject 
has come to this conclusion. The case of America alone would 
almost suffice to prove its truth; for if we exclude the arctic and 
northern temperate parts, all authors agree that one of the most 
fundamental divisions in geographical distribution is that between 
the New and Old Worlds; yet if we travel over the vast American 
continent, from the central parts of the United States to its extreme 
southern point, we meet with the most diversified conditions; humid 
districts, arid deserts, lofty mountains, grassy plains, forests, marshes, 
lakes, and great rivers, under almost every temperature. There is 
hardly a climate or condition in the Old World which cannot be 
paralleled in the New — at least as closely as the same species generally 
require. No doubt small areas can be pointed out in the Old World 
hotter than any in the New World; but these are not inhabited by a 
fauna different from that of the surrounding districts; for it is rare 
to find a group of organisms confined to a small area, of which the 
conditions are peculiar in only a slight degree. Notwithstanding 
this general parallelism in the conditions of the Old and New 
Worlds, how widely different are their living productions! 

In the southern hemisphere, if we compare large tracts of land in 
Australia, South Africa, and western South America, between latl- 



tudes 25° and 35^, we shall find parts extremely similar in all their 
conditions, yet it would not be possible to point out three faunas 
and floras more utterly dissimilar. Or, again, we may compare the 
productions of South America south of latitude 35^ with those north 
of 25 , which consequently are separated by a space of ten degrees of 
latitude, and are exposed to considerably different conditions; yet 
they are incomparably more closely related to each other than they 
are to the productions of Australia or Africa under nearly the same 
climate. Analogous facts could be given with respect to the inhabi- 
tants of the sea. 

A second great fact which strikes us in our general review is, that 
barriers of any kind, or obstacles to free migration, are related in a 
close and important manner to the differences between the produc- 
tions of various regions. We see this in the great differences in nearly 
all the terrestrial produaions of the New and Old Worlds, excepting 
in the northern parts, where the land almost joins, and where, under 
a slightly different climate, there might have been free migration 
for the northern temperate forms, as there now is for the strirtly 
arctic productions. We see the same fact in the great difference be- 
tween the inhabitants of Australia, Africa, and South America under 
the same latitude; for these countries are almost as much isolated 
from each other as is possible. On each continent, also, we see the 
same fact; for on the opposite sides of lofty and continuous moun- 
tain-ranges, of great deserts and even of large rivers, we find different 
productions; though as mountain-chains, deserts, etc., are not as 
impassable, or likely to have endured so long, as the oceans separat- 
ing continents, the differences are very inferior in degree to those 
characteristic of distinct continents. 

Turning to the sea, we find the same law. The marine inhabitants 
of the eastern and western shores of South America are very distinct, 
with extremely few shells, Crustacea, or echinodermata in common; 
but Dr. Giinther has recently shown that about thirty per cent, of the 
fishes are the same on the opposite sides of the isthmus of Panama; 
and this fact has led naturalists to believe that the isthmus was for- 
merly open. Westward of the shores of America, a wide space of 
open ocean extends, with not an island as a halting-place for emi- 
grants; here we have a barrier of another kind, and as soon as this 


is passed we meet in the eastern islands of the Pacific with another 
and totally distinct fauna. So that three marine faunas range far 
northward and southward in parallel lines not far from each other, 
under corresponding climates; but from being separated from each 
other by impassable barriers, either of land or open sea, they are 
almost wholly distinct. On the other hand, proceeding still farther 
westward from the eastern islands of the tropical parts of the Pacific, 
we encounter no impassable barriers, and we have innumerable 
islands as halting-places, or continuous coasts, until, after travelling 
over a hemisphere, we come to the shores of Africa; and over this 
vast space we meet with no well-defined and distinct marine faunas. 
Although so few marine animals are common to the above-named 
three approximate faunas of Eastern and Western America and the 
eastern Pacific islands, yet many fishes range from the Pacific into 
the Indian Ocean, and many shells are common to the eastern islands 
of the Pacific and the eastern shores of Africa on almost exactly 
opposite meridians of longitude. 

A third great fact, partly included in the foregoing statement, is 
the affinity of the productions of the same continent or of the same 
sea, though the species themselves are distinct at different points and 
stations. It is a law of the widest generality, and every continent 
offers innumerable instances. Nevertheless, the naturalist, in travel- 
ling, for instance, from north to south, never fails to be struck by 
the manner in which successive groups of beings, specifically distinct, 
though nearly related, replace each other. He hears from closely 
allied, yet distinct kinds of birds, notes nearly similar, and sees their 
nests similarly constructed, but not quite alike, with eggs coloured 
in nearly the same manner. The plains near the Straits of Magellan 
are inhabited by one species of Rhea (American ostrich), and north- 
ward the plains of La Plata by another species of the same genus; 
and not by a true ostrich or emu, like those inhabiting Africa and 
Australia under the same latitude. On these same plains of La Plata 
we see the agouti and bizcacha, animals having nearly the same 
habits as our hares and rabbits, and belonging to the same order of 
rodents, but they plainly display an American type of structure. 
We ascend the lofty peaks of the Cordillera, and we find an alpine 


species of bizcacha; we look to the waters, and we do not find the 
beaver or musk-rat, but the coypu and capybara, rodents of the South 
American type. Innumerable other instances could be given. If 
we look to the islands off the American shore, however much they 
may differ in geological structure, the inhabitants are essentially 
American, though they may be all peculiar species. We may look 
back to past ages, as shown in the last chapter, and we find Ameri- 
can types then prevailing on the American continent and in the 
American seas. We see in these facts some deep organic bond, 
throughout space and time, over the same areas of land and water, 
independently of physical conditions. The naturalist must be dull 
who is not led to inquire what this bond is. 

The bond is simply inheritance, that cause which alone, as far as 
we positively know, produces organisms quite like each other, or, as 
we see in the case of varieties, nearly alike. The dissimilarity of the 
inhabitants of different regions may be attributed to modification 
through variation and natural selection, and probably in a sub- 
ordinate degree to the definite influence of different physical con- 
ditions. The degrees of dissimilarity will depend on the migration 
of the more dominant forms of life from one region into another 
having been more or less effectually prevented, at periods more or 
less remote; — on the nature and number of the former immigrants; 
— and on the action of the inhabitants on each other in leading to the 
preservation of different modifications; the relation of organism to 
organism in the struggle for life being, as I have already often re- 
marked, the most important of all relations. Thus the high im- 
portance of barriers comes into play by checking migration; as does 
time for the slow process of modification through natural selection. 
Widely ranging species, abounding in individuals, which have 
already triumphed over many competitors in their own widely ex- 
tended homes, will have the best chance of seizing on new places, 
when they spread out into new countries. In their new homes they 
will be exposed to new conditions, and will frequently undergo 
further modification and improvement; and thus they will become 
still further victorious, and will produce groups of modified descend- 
ants. On this principle of inheritance with modification we can 


understand how it is that sections of genera, whole genera, and even 
families, are confined to the same areas, as is so commonly and 
notoriously the case. 

There is no evidence, as was remarked in the last chapter, of the 
existence of any law of necessary development. As the variability 
of each species is an independent property, and will be taken ad- 
vantage of by natural selection, only so far as it profits each indi- 
vidual in its complex struggle for hfe, so the amount of modifica- 
tion in different species will be no uniform quantity. If a number 
of species, after having long competed with each other in their old 
home, were to migrate in a body into a new and afterwards isolated 
country, they would be little liable to modification; for neither 
migration nor isolation in themselves effect any thing. These prin- 
ciples come into play only by bringing organisms into new relations 
with each other and in a lesser degree with the surrounding physical 
conditions. As we have seen in the last chapter that some forms 
have retained nearly the same character from an enormously remote 
geological (period, so certain species have migrated over vast spaces, 
and have not become greatly or at all modified. 

According to these views, it is obvious that the several species of 
the same genus, though inhabiting the most distant quarters of the 
world, must originally have proceeded from the same source, as they 
are descended from the same progenitor. In the case of those species 
which have undergone during whole geological periods little modifi- 
cation, there is not much difficulty in believing that they have mi- 
grated from the same region; for during the vast geographical and 
climatal changes which have supwrvened since ancient times, almost 
any amount of migration is possible. But in many other cases, in 
which we have reason to believe that the species of a genus have 
been produced within comparatively recent times, there is great diffi- 
culty on this head. It is also obvious that the individuals of the same 
species, though now inhabiting distant and isolated regions, must 
have proceeded from one spot, where their parents were first pro- 
duced: for, as has been explained, it is incredible that individuals 
identically the same should have been produced from parents spe- 
cifically distinct. 



We are thus brought to the question which has been largely dis- 
cussed by naturalists, namely, whether species have been created at 
one or more points of the earth's surface. Undoubtedly there are 
many cases of extreme difficulty in understanding how the same 
species could possibly have migrated from some one point to the 
several distant and isolated points, where now found. Nevertheless 
the simplicity of the view that each species was first produced within 
a single region captivates the mind. He who rejects it, rejects the vera 
causa of ordinary generation with subsequent migration, and calls 
in the agency of a miracle. It is universally admitted, that in most 
cases the area inhabited by a species is continuous; and that when 
a plant or animal inhabits two pxiints so distant from each other, 
or with an interval of such a nature, that the space could not have 
been easily passed over by migration, the fact is given as something 
remarkable and exceptional. The incapacity of migrating across a 
wide sea is more clear in the case of terrestrial mammals than per- 
haps with any other organic beings; and, accordingly, we find no 
inexplicable instances of the same mammals inhabiting distant 
points of the world. No geologist feels any difficulty in Great Britain 
possessing the same quadrupeds with the rest of Europe, for they 
were no doubt once united. But if the same species can be pro- 
duced at two separate points, why do we not find a single mammal 
common to Europe and Australia or South America? The condi- 
tions of life are nearly the same, so that a multitude of European 
animals and plants have become naturalised in America and Aus- 
traUa; and some of the aboriginal plants are identically the same at 
these distant points of the northern and southern hemispheres. The 
answer, as I believe, is, that mammals have not been able to mi- 
grate, whereas some plants, from their varied means of dispersal, 
have migrated across the wide and broken interspaces. The great 
and striking influence of barriers of all kinds, is intelligible only on 
the view that the great majority of species have been produced on 
one side, and have not been able to migrate to the opposite side. 
Some few families, many sub-families, very many genera, and a still 
greater number of sections of genera, are confined to a single region; 


and it has been observed by several naturalists that the most natural 
genera, or those genera in which the species are most closely related 
to each other, are generally confined to the same country, or if they 
have a wide range that their range is continuous. What a strange 
anomaly it would be, if a directly opposite rule were to prevail, 
when we go down one step lower in the series, namely, to the indi- 
viduals of the same species, and these had not been, at least at first, 
confined to some one region! 

Hence it seems to me, as it has to many other naturalists, that the 
view of each species having been produced in one area alone, and 
having subsequently migrated from that area as far as its powers 
of migration and subsistence under past and present conditions f)er- 
mitted, is the most probable. Undoubtedly many cases occur, in 
which we cannot explain how the same species could have passed 
from one point to the other. But the geographical and climatal 
changes which have certainly occurred within recent geological 
times, must have rendered discontinuous the formerly continuous 
range of many species. So that we are reduced to consider whether 
the exceptions to continuity of range are so numerous and of so 
grave a nature, that we ought to give up the belief, rendered prob- 
able by general considerations, that each species has been produced 
within one area, and has migrated thence as far as it could. It would 
be hopelessly tedious to discuss all the exceptional cases of the same 
species, now living at distant and separated points, nor do I for a 
moment pretend that any explanation could be offered of many in- 
stances. But, after some preliminary remarks, I will discuss a few of 
the most striking classes of facts; namely, the existence of the same 
species on the summits of distant mountain ranges, and at distant 
points in the arctic and antarctic regions; and secondly (in the 
following chapter), the wide distribution of fresh-water produc- 
tions; and thirdly, the occurrence of the same terrestrial species on 
islands and on the nearest mainland, though separated by hundreds 
of miles of open sea. If the existence of the same species at distant 
and isolated points of the earth's surface, can in many instances be 
explained on the view of each species having migrated from a single 
birthplace; then, considering our ignorance with respect to former 
climatal and geographical changes and to the various occasional 


means of transport, the belief that a single birthplace is the law, 
seems to me incomparably the safest. 

In discussing this subject, we shall be enabled at the same time 
to consider a point equally important for us, namely, whether the 
several species of a genus which must on our theory all be descended 
from a common progenitor, can have migrated, undergoing modi- 
fication during their migration, from some one area. If, when most 
of the species inhabiting one region are different from those of 
another region, though closely allied to them, it can be shown that 
migration from the one region to the other has probably occurred at 
some former period, our general view will be much strengthened; 
for the explanation is obvious on the principle of descent with modi- 
fication. A volcanic island, for instance, upheaved and formed at 
the distance of a few hundreds of miles from a continent, would 
probably receive from it in the course of time a few colonists, and 
their descendants, though modified, would still be related by in- 
heritance to the inhabitants of that continent. Cases of this nature 
are common, and are, as we shall hereafter see, inexplicable on the 
theory of independent creation. This view of the relation of the 
species of one region to those of another, does not differ much from 
that advanced by Mr. Wallace, who concludes that "every species 
has come into existence coincident both in space and time with a 
preexisting closely allied species." And it is now well known that 
he attributes this coincidence to descent with modification. 

The question of single or multiple centres of creation differs from 
another though allied question, — namely, whether all the indi- 
viduals of the same species are descended from a single pair, or 
single hermaphrodite, or whether, as some authors suppose, from 
many individuals simultaneously created. With organic beings which 
never intercross, if such exist, each species must be descended from 
a succession of modified varieties, that have supplanted each other, 
but have never blended with other individuals or varieties of the 
same species; so that, at each successive stage of modification, all 
the individuals of the same form will be descended from a single 
parent. But in the great majority of cases, namely, with all organ- 
isms which habitually unite for each birth, or which occasionally 
iiKercross, the individuals of the same species inhabiting the same 


area will be kept nearly uniform by intercrossing; so that many indi- 
viduals will go on simultaneously changing, and the whole amount 
of modification at each stage will not be due to descent from a single 
parent. To illustrate what I mean: our English race-horses differ 
from the horses of every other breed; but they do not owe their 
difference and superiority to descent from any single pair, but to 
continued care in the selecting and training of many individuals 
during each generation. 

Before discussing the three classes of facts, which I have selected 
as presenting the greatest amount of difficulty on the theory of 
"single centres of creation," I must say a few words on the means 
of dispersal. 


Sir C. Lyell and other authors have ably treated this subject. I 
can give here only the briefest abstract of the more important facts. 
Change of climate must have had a powerful influence on migration. 
A region now impassable to certain organisms from the nature of 
its climate, might have been a high road for migration, when the 
climate was different. I shall, however, presently have to discuss 
this branch of the subject in some detail. Changes of level in the land 
must also have been highly influential: a narrow isthmus now 
separates two marine faunas; submerge it, or let it formerly have been 
submerged, and the two faunas will now blend together, or may 
formerly have blended. Where the sea now extends, land may at a 
former period have connected islands or possibly even continents 
together, and thus have allowed terrestrial productions to pass from 
one to the other. No geologist disputes that great mutations of level 
have occurred within the period of existing organisms. Edward 
Forbes insisted that all the islands in the Atlantic must have been 
recently connected with Europe or Africa, and Europe likewise with 
America. Other authors have thus hypothetically bridged over every 
ocean, and united almost every island with some mainland. If indeed 
the arguments used by Forbes are to be trusted, it must be admitted 
that scarcely a single island exists which has not recently been united 
to some continent. This view cuts the Gordian knot of the dispersal 
of the same species to the most distant points, and removes many a 


difficulty; but to the best of my judgment we are not authorised in 
admitting such enormous geographical changes within the period of 
existing species. It seems to me that we have abundant evidence of 
great oscillations in the level of the land or sea; but not of such vast 
changes in the position and extension of our continents, as to have 
united them within the recent period to each other and to the several 
intervening oceanic islands. I freely admit the former existence of 
many islands, now buried beneath the sea, which may have served 
as halting-places for plants and for many animals during their migra- 
tion. In the coral-producing oceans such sunken islands are now 
marked by rings of coral or atolls standing over them. Whenever it 
is fully admitted, as it will some day be, that each species has pro- 
ceeded from a single birthplace, and when in the course of time we 
know something definite about the means of distribution, we shall 
be enabled to speculate with security on the former extension of the 
land. But 1 do not believe that it will ever be proved that within the 
recent period most of our continents which now stand quite separate, 
have been continuously, or almost continuously united with each 
other, and with the many existing oceanic islands. Several facts in 
distribution — such as the great difference in the marine faunas on 
the opposite sides of almost every continent, — the close relation of 
the tertiary inhabitants of several lands and even seas to their present 
inhabitants, — the degree of affinity between the mammals inhabiting 
islands with those of the nearest continent, being in part determined 
(as we shall hereafter see) by the depth of the intervening ocean, — 
these and other such facts are opposed to the admission of such pro- 
digious geographical revolutions within the recent period, as are 
necessary on the view advanced by Forbes and admitted by his 
followers. The nature and relative proportions of the inhabitants of 
oceanic islands are likewise opposed to the beUef of their former 
continuity with continents. Nor does the almost universally volcanic 
composition of such islands favour the admission that they are the 
wrecks of sunken continents; — if they had originally existed as con- 
tinental mountain ranges, some at least of the islands would have 
been formed, like other mountain summits, of granite, metamorphic 
schists, old fossiliferous and other rocks, instead of consisting of 
mere piles of volcanic matter. 


I must now say a few words on what are called accidental means, 
but which more properly should be called occasional means of dis- 
tribution. I shall here confine myself to plants. In botanical works, 
this or that plant is often stated to be ill adapted for wide dissemi- 
nation; but the greater or less facilities for transport across the sea 
may be said to be almost wholly unknown. Until I tried, with Mr. 
Berkeley's aid, a few experiments, it was not even known how far 
seeds could resist the injurious action of sea water. To my surprise 
I found that out of eighty-seven kinds, sixty-four germinated after 
an immersion of twenty-eight days, and a few survived an immer- 
sion of 137 days. It deserves notice that certain orders were far more 
injured than others: nine Leguminosae were tried, and, with one 
exception, they resisted the salt water badly; seven species of the 
allied orders, Hydrophyllacese and Polemoniacex, were all killed by 
a month's immersion. For convenience' sake I chiefly tried small 
seeds without the capsule or fruit; and as all of these sank in a few 
days they could not have been floated across wide spaces of the sea, 
whether or not they were injured by the salt water. Afterwards I 
tried some larger fruits, capsules, etc., and some of these floated for 
a long time. It is well known what a difference there is in the buoy- 
ancy of green and seasoned timber; and it occurred to me that floods 
would often wash into the sea dried plants or branches with seed- 
capsules or fruit attached to them. Hence I was led to dry the stems 
and branches of ninety-four plants with ripe fruit, and to place them 
on sea water. The majority sank quickly, but some which, whilst 
green, floated for a very short time, when dried floated much longer; 
for instance, ripe hazelnuts sank immediately, but when dried they 
floated for ninety days, and afterwards when planted germinated; 
an asparagus-plant with ripe berries floated for twenty-three days, 
when dried it floated for eighty-five days, and the seeds afterwards 
germinated; the ripe seeds of Helosciadium sank in two days, when 
dried they floated for above ninety days, and afterwards germinated. 
Altogether, out of the ninety-four dried plants, eighteen floated for 
above twenty-eight days; and some of the eighteen floated for a very 
much longer period. So that as f4 kinds of seeds germinated after 
an immersion of twenty-eight days; and as H distinct species with 
ripe fruit (but not all the same species as in the foregoing experi- 


ment) floated, after being dried, for above twenty-eight days, we 
may conclude, as far as anything can be inferred from these scanty 
facts, that the seeds of ^Vir kinds of plants of any country might be 
floated by sea currents during twenty-eight days, and would retain 
their power of germination. In Johnston's Physical Atlas, the average 
rate of the several Atlantic currents is thirty-three miles per diem 
(some currents running at the rate of sixty miles per diem) ; on this 
average, the seeds of iVir plants belonging to one country might be 
floated across 924 miles of sea to another country, and when stranded, 
if blown by an inland gale to a favourable spot, would germinate. 

Subsequently to my experiments, M. Martens tried similar ones, 
but in a much better manner, for he placed the seeds in a box in the 
actual sea, so that they were alternately wet and exposed to the air 
like really floating plants. He tried ninety-eight seeds, mostly dif- 
ferent from mine; but he chose many large fruits and likewise seeds 
from plants which live near the sea; and this would have favoured 
both the average length of their flotation and their resistance to the 
injurious action of the salt water. On the other hand, he did not 
previously dry the plants or branches with the fruit; and this, as we 
have seen, would have caused some of them to have floated much 
longer. The result was that if of his seeds of different kinds floated 
for forty-two days, and were then capable of germination. But I do 
not doubt that plants exposed to the waves would float for a less 
time than those protected from violent movement as in our experi- 
ments. Therefore it would pwrhaps be safer to assume that the seeds 
of about iV^ plants of a flora, after having been dried, could be 
floated across a space of sea 900 miles in width, and would then 
germinate. The fact of the larger fruits often floating longer than 
the small, is interesting; as plants with large seeds or fruit which, as 
Alph. de Candolle has shown, generally have restricted ranges, could 
hardly be transported by any other means. 

Seeds may be occasionally transported in another manner. Drift 
timber is thrown up on most islands, even on those in the midst of 
the widest oceans; and the natives of the coral islands in the Pacific 
procure stones for their tools, solely from the roots of drifted trees, 
these stones being a valuable royal tax. I find that when irregularly 
shaped stones are embedded in the roots of trees, small parcels of 


earth are frequently enclosed in their interstices and behind them, — 
so perfectly that not a particle could be washed away during the 
longest transport: out of one small portion of earth thus completely 
enclosed by the roots of an oak about 50 years old, three dicoty- 
ledonous plants germinated; I am certain of the accuracy of this 
observation. Again, I can show that the carcases of birds, when 
floating on the sea, sometimes escape being immediately devoured: 
and many kinds of seeds in the crops of floating birds long retain 
their vitality: peas and vetches, for instance, are killed by even a 
few days' immersion in sea water; but some taken out of the crop 
of a pigeon, which had floated on artificial sea water for thirty days, 
to my surprise nearly all germinated. 

Living birds can hardly fail to be highly effective agents in the 
transportation of seeds. 1 could give many facts showing how 
frequently birds of many kinds are blown by gales to vast distances 
across the ocean. We may safely assume that under such circum- 
stances their rate of flight would often be thirty-five miles an hour; 
and some authors have given a far higher estimate. I have never 
seen an instance of nutritious seeds passing through the intestines 
of a bird; but hard seeds of fruit pass uninjured through even the 
digestive organs of a turkey. In the course of two months, I picked 
up in my garden twelve kinds of seeds, out of the excrement of small 
birds, and these seemed perfect, and some of them, which were tried, 
germinated. But the following fact is more important: the crops 
of birds do not secrete gastric juice, and do not, as I know by trial, 
injure in the least the germination of seeds; now, after a bird has 
found and devoured a large supply of food, it is positively asserted 
that all the grains do not pass into the gizzard for twelve or even 
eighteen hours. A bird in this interval might easily be blown to the 
distance of five hundred miles, and hawks are known to look out for 
tired birds, and the contents of their torn crops might thus readily 
get scattered. Some hawks and owls bolt their prey whole, and, after 
an interval of from twelve to twenty hours, disgorge pellets, which, 
as I know from experiments made in the Zoological Gardens, include 
seeds capable of germination. Some seeds of the oat, wheat, millet, 
canary, hemp, clover, and beet germinated after having been from 
twelve to twenty-one hours in the stomachs of different birds of 


prey; and two seeds of beet grew after having been thus retained for 
two days and fourteen hours. Fresh-water fish, I find, eat seeds of 
many land and water plants; fish are frequently devoured by birds, 
and thus the seeds might be transported from place to place. I forced 
many kinds of seeds into the stomachs of dead fish, and then gave 
their bodies to fishing-eagles, storks, and pelicans; these birds, after 
an interval of many hours, either rejected the seeds in pellets or 
passed them in their excrement; and several of these seeds retained 
the power of germination. Certain seeds, however, were always 
killed by this process. 

Locusts are sometimes blown to great distances from the land; I 
myself caught one 370 miles from the coast of Africa, and have 
heard of others caught at greater distances. The Rev. R. T. Lowe 
informed Sir C. Lyell that in November 1844 swarms of locusts 
visited the island of Madeira. They were in countless numbers, as 
thick as the flakes of snow in the heaviest snowstorm, and extended 
upwards as far as could be seen with a telescope. During two or three 
days they slowly careered round and round in an immense ellipse, 
at least five or six miles in diameter, and at night alighted on the 
taller trees, which were completely coated with them. They then 
disappeared over the sea, as suddenly as they had appeared, and have 
not since visited the island. Now, in parts of Natal it is believed 
by some farmers, though on insufficient evidence, that injurious 
seeds are introduced into their grassland in the dung left by the 
great flights of locusts which often visit that country. In consequence 
of this belief Mr. Weale sent me in a letter a small packet of the dried 
pellets, out of which I extracted under the microscope several seeds, 
and raised from them seven grass plants, belonging to two species, 
of two genera. Hence a swarm of locusts, such as that which visited 
Madeira, might readily be the means of introducing several kinds of 
plants into an island lying far from the mainland. 

Although the beaks and feet of birds are generally clean, earth 
sometimes adheres to them: in one case I removed sixty-one grains, 
and in another case twenty-two grains of dry argillaceous earth from 
the foot of a partridge, and in the earth there was a pebble as large 
as the seed of a vetch. Here is a better case: the leg of a woodcock 
was sent to me by a friend, with a little cake of dry earth attached to 


the shank, weighing only nine grains; and this contained a seed of 
the toad-rush (Juncus bufonius) which germinated and flowered. 
Mr. Swaysland, of Brighton, who during the last forty years has paid 
close attention to our migratory birds, informs me that he has often 
shot wagtails (Motacillae), wheatears, and whincats (Saxicolx), on 
their first arrival on our shores, before they had alighted; and he has 
several times noticed litde cakes of earth attached to their feet. Many 
facts could be given showing how generally soil is charged with 
seeds. For instance. Prof. Newton sent me the leg of a red-legged 
partridge (Caccabis rufa) which had been wounded and could not 
fly, with a ball of hard earth adhering to it, and weighing six and a 
half ounces. The earth had been kept for three years, but when 
broken, watered and placed under a bell-glass, no less than eighty- 
two plants sprung from it: these consisted of twelve monocoty- 
ledons, including the common oat, and at least one kind of grass, 
and of seventy dicotyledons, which consisted, judging from the 
young leaves, of at least three distinct species. With such facts before 
us, can we doubt that the many birds which are annually blown by 
gales across great spaces of ocean, and which annually migrate — for 
instance, the millions of quails across the Mediterranean — must 
occasionally transport a few seeds embedded in dirt adhering to 
their feet or beaks? But I shall have to recur to this subject. 

As icebergs are known to be sometimes loaded with earth and 
stones, and have even carried brushwood, bones, and the nest of a 
land-bird, it can hardly be doubted that they must occasionally, as 
suggested by Lyell, have transported seeds from one part to another 
of the arctic and antarctic regions; and during the Glacial period 
from one part of the now temperate regions to another. In the 
Azores, from the large number of plants common to Europe, in 
comparison with the species on the other islands of the Atlantic, 
which stand nearer to the mainland, and (as remarked by Mr. H. C. 
Watson) from their somewhat northern character in comparison 
with the latitude, I suspected that these islands had been partly 
stocked by ice-borne seeds, during the Glacial epoch. At my request 
Sir C. Lyell wrote to M. Hartung to inquire whether he had observed 
erratic boulders on these islands, and he answered that he had found 
large fragments of granite and other rocks, which do not occur in 


the archipelago. Hence we may safely infer that icebergs formerly 
landed their rocky burthens on the shores of these mid-ocean islands, 
and it is at least possible that they may have brought thither some 
few seeds of northern plants. 

Considering that these several means of transport, and that other 
means, which without doubt remain to be discovered, have been in 
action year after year for tens of thousands of years, it would, I 
think, be a marvellous fact if many plants had not thus become 
widely transported. These means of transport are sometimes called 
accidental, but this is not strictly correct: the currents of the sea are 
not accidental, nor is the direction of prevalent gales of wind. It 
should be observed that scarcely any means of transport would carry 
seeds for very great distances: for seeds do not retain their vitality 
when exposed for a great length of time to the action of sea water; 
nor could they be long carried in the crops or intestines of birds. 
These means, however, would suffice for occasional transport across 
tracts of sea some hundred miles in breadth, or from island to island, 
or from a continent to a neighbouring island, but not from one 
distant continent to another. The floras of distant continents would 
not by such means become mingled; but would remain as distinct 
as they now are. The currents, from their course, would never bring 
seeds from North America to Britain, though they might and do 
bring seeds from the West Indies to our western shores, where, if 
not killed by their long immersion in salt water, they could not 
endure our climate. Almost every year, one or two land birds are 
blown across the whole Atlantic Ocean, from North America to 
the western shores of Ireland and England; but seeds could be 
transported by these rare wanderers only by one means, namely, by 
dirt adhering to their feet or beaks, which is in itself a rare accident. 
Even in this case, how small would be the chance of a seed falling 
on favourable soil, and coming to maturity! But it would be a great 
error to argue that because a well-stocked island, like Great Britain, 
has not, as far as is known (and it would be very difficult to prove 
this), received within the last few centuries, through occasional 
means of transport, immigrants from Europe or any other continent, 
that a poorly-stocked island, though standing more remote from the 
mainland, would not receive colonists by similar means. Out of a 


hundred kinds of seeds or animals transported to an island, even if 
far less well-stocked than Britain, perhaps not more than one would 
be so well fitted to its new home, as to become naturalised. But this 
is no valid argument against what would be effected by occasional 
means of transport, during the long lapse of geological time, whilst 
the island was being upheaved, and before it had become fully 
stocked with inhabitants. On almost bare land, with few or no 
destructive insects or birds living there, nearly every seed which 
chanced to arrive, if fitted for the climate, would germinate and 


The identity of many plants and animals, on mountain-summits, 
separated from each other by hundreds of miles of lowlands, where 
Alpine sjjecies could not fxjssibly exist, is one of the most striking 
cases known of the same species living at distant points, without the 
apparent possibility of their having migrated from one point to the 
other. It is indeed a remarkable fact to see so many plants of the 
same species living on the snowy regions of the Alps or Pyrenees, 
and in the extreme northern parts of Europe; but it is far more 
remarkable, that the plants on the White Mountains, in the United 
States of America, are all the same with those of Labrador, and 
nearly all the same, as we hear from Asa Gray, with those on the 
loftiest mountains of Europe. Even as long ago as 1747, such facts 
led Gmelin to conclude that the same species must have been in- 
dependently created at many distinct points; and we might have 
remained in this same belief, had not Agassiz and others called 
vivid attention to the Glacial period, which, as we shall immediately 
see, affords a simple explanation of these facts. We have evidence 
of almost every conceivable kind, organic and inorganic, that, within 
a very recent geological period, central Europe and North America 
suffered under an arctic climate. The ruins of a house burnt by fire 
do not tell their tale more plainly than do the mountains of Scotland 
and Wales, with their scored flanks, polished surfaces, and perched 
boulders, of the icy streams with which their valleys were lately 
filled. So greatly has the climate of Europe changed, that in north- 
ern Italy, gigantic moraines, left by old glaciers, are now clothed 


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

The former influence of the glacial climate on the distribution 
of the inhabitants of Europe, as explained by Edward Forbes, is 
substantially as follows. But we shall follow the changes more 
readily, by supposing a new glacial period slowly to come on, and 
then pass away, as formerly occurred. As the cold came on, and 
as each more southern zone became fitted for the inhabitants of 
the north, these would take the places of the former inhabitants of 
the temperate regions. The latter, at the same time, would travel 
further and further southward, unless they were stopped by barriers, 
in which case they would perish. The mountains would become 
covered with snow and ice, and their former Alpine inhabitants 
would descend to the plains. By the time that the cold had reached 
its maximum, we should have an arctic fauna and flora, covering 
the central parts of Europe, as far south as the Alps and Pyrenees, 
and even stretching into Spain. The now temperate regions of the 
United States would likewise be covered by arctic plants and animals 
and these would be nearly the same with those of Europe; for the 
present circumpolar inhabitants, which we suppose to have every- 
where travelled southward, are remarkably uniform round the 

As the warmth returned, the arctic forms would retreat north- 
ward, closely followed up in their retreat by the productions of the 
more temperate regions. And as the snow melted from the bases 
of the mountains, the arctic forms would seize on the cleared and 
thawed ground, always ascending, as the warmth increased and the 
snow still further disappeared, higher and higher, whilst their 
brethren were pursuing their northern journey. Hence, when the 
warmth had fully returned, the same species, which had lately lived 
together on the European and North American lowlands, would 
again be found in the arctic regions of the Old and New Worlds, 
and on many isolated mountain summits far distant from each other. 

Thus we can understand the identity of many plants at points so 
immensely remote as the mountains of the United States and those 
of Europe. We can thus also understand the fact that the Alpine 


plants of each mountain range are more especially related to the 
arctic forms living due north or nearly due north of them: for the 
first migration when the cold came on, and the re-migration on the 
returning warmth, would generally have been due south and north. 
The Alpine plants, for example, of Scotland, as remarked by Mr. 
H. C. Watson, and those of the Pyrenees, as remarked by Ramond, 
are more especially allied to the plants of northern Scandinavia; 
those of the United States, to Labrador; those of the mountains of 
Siberia to the arctic regions of that country. These views, grounded 
as they are on the perfectly well-ascertained occurrence of a former 
Glacial period, seem to me to explain in so satisfactory a manner 
the present distribution of the Alpine and arctic productions of 
Europe and America, that when in other regions we find the same 
species on distant mountain summits, we may almost conclude, 
without other evidence, that a colder climate formerly permitted 
their migration across the intervening lowlands, now become too 
warm for their existence. 

As the arctic forms moved first southward and afterwards back- 
wards to the north, in unison with the changing climate, they will 
not have been exposed during their long migrations to any great 
diversity of temperature; and as they all migrated in a body together, 
their mutual relations will not have been much disturbed. Hence, 
in accordance with the principles inculcated in this volume, these 
forms will not have been liable to much modification. But with the 
Alpine productions, left isolated from the moment of the returning 
warmth, first at the bases and ultimately on the summits of the 
mountains, the case will have been somewhat different; for it is 
not likely that all the same arctic species will have been left on 
mountain ranges far distant from each other, and have survived 
there ever since; they will also in all probability, have become 
mingled with ancient Alpine species, which must have existed on 
the mountains before the commencement of the Glacial epoch, and 
which during the coldest period will have been temporarily driven 
down to the plains; they will, also, have been subsequently exposed 
to somewhat different climatal influences. Their mutual relations 
will thus have been in some degree disturbed; consequently they 
will have been liable to modification; and they have been modified; 


for if we compare the present Alpine plants and animals of the 
several great European mountain ranges one with another, though 
many of the species remain identically the same, some exist as 
varieties, some as doubtful forms or sub-species, and some as dis- 
tinct yet closely allied species representing each other on the several 

In the foregoing illustration I have assumed that at the com- 
mencement of our imaginary Glacial period, the arctic productions 
were as uniform round the polar regions as they are at the present 
day. But it is also necessary to assume that many sub-arctic and 
some few temf)erate forms were the same round the world, for 
some of the species which now exist on the lower mountain-slof)es 
and on the plains of North America and Europe are the same; and 
it may be asked how I account for this degree of uniformity in the 
sub-arctic and temf)erate forms round the world, at the commence- 
ment of the real Glacial period. At the present day, the sub-arctic 
and northern temperate productions of the Old and New Worlds 
are separated from each other by the whole Atlantic Ocean and by 
the northern part of the Pacific. During the Glacial period, when 
the inhabitants of the Old and New Worlds lived farther southwards 
than they do at present, they must have been still more completely 
separated from each other by wider spaces of ocean; so that it may 
well be asked how the same species could then or previously have 
entered the two continents. The explanation, I believe, lies in the 
nature of the climate before the commencement of the Glacial 
period. At this, the newer Pliocene period, the majority of the 
inhabitants of the world were specifically the same as now, and we 
have good reason to believe that the climate was warmer than at 
the present day. Hence we may suppose that the organisms which 
now live under latitude 60°, lived during the Pliocene period, farther 
north under the Polar Circle, in latitude 66°-67°; and that the 
present arctic productions then lived on the broken land still nearer 
to the pole. Now, if we look at a terrestrial globe, we see under the 
Polar Circle that there is almost continuous land from western 
Europe, through Siberia, to eastern America. And this continuity 
of the circumpolar land, with the consequent freedom under a more 
favourable climate for intermigration, will account for the supposed 


uniformity of the sub-arctic and temperate productions of the Old 
and New Worlds, at a period anterior to the Glacial epoch. 

Believing, from reasons before alluded to, that our continents 
have long remained in nearly the same relative position, though 
subjected to great oscillations of level, I am strongly incUned to 
extend the above view, and to infer that during some still earlier 
and still warmer period, such as the older Pliocene period, a large 
number of the same plants and animals inhabited the almost con- 
tinuous circumpolar land; and that these plants and animals, both 
in the Old and New Worlds, began slowly to migrate southwards 
as the climate became less warm, long before the commencement 
of the Glacial period. We now see, as I believe, their descendants, 
mostly in a modified condition, in the central parts of Europe and 
the United States. On this view we can understand the relationship 
with very little identity, between the productions of North America 
and Europe, — a relationship which is highly remarkable, considering 
the distance of the two areas, and their separation by the whole 
Atlantic Ocean. We can further understand the singular fact re- 
marked on by several observers that the productions of Europe and 
America during the later tertiary stages were more closely related 
to each other than they are at the present time; for during these 
warmer periods the northern parts of the Old and New Worlds 
will have been almost continuously united by land, serving as a 
bridge, since rendered impassable by cold, for the intermigration of 
their inhabitants. 

During the slowly decreasing warmth of the Pliocene period, as 
soon as the species in common, which inhabited the New and Old 
Worlds, migrated south of the Polar Circle, they will have been 
completely cut off from each other. This separation, as far as the 
more temperate productions are concerned, must have taken place 
long ages ago. As the plants and animals migrated southward, they 
will have become mingled in the one great region with the native 
American productions, and would have had to compete with them; 
and in the other great region, with those of the Old World. Con- 
sequently we have here everything favourable for much modification, 
— for far more modification than with the Alpine productions, left 
isobted, within a much more recent period, on the several mountain- 


ranges and on the arctic lands of Europe and North America. Hence, 
it has come, that when we compare the now Hving productions of 
the temperate regions of the New and Old Worlds, we find very 
few identical species (though Asa Gray has lately shown that more 
plants are identical than was formerly supposed), but we find in 
every great class many forms, which some naturalists rank as geo- 
graphical races, and others as distinct species; and a host of closely 
allied or representative forms which are ranked by all naturalists as 
specifically distinct. 

As on the land, so in the waters of the sea, a slow southern migra- 
tion of a marine fauna, which, during the Pliocene or even a some- 
what earlier p)eriod, was nearly uniform along the continuous shores 
of the Polar Circle, will account, on the theory of modification, for 
many closely allied forms now living in marine areas completely 
sundered. Thus, I think, we can understand the presence of some 
closely allied, still existing and extinct tertiary forms, on the eastern 
and western shores of temperate North America; and the still more 
striking fact of many closely allied crustaceans (as described in 
Dana's admirable work), some fish and other marine animals, 
inhabiting the Mediterranean and the seas of Japan, — these two 
areas being now completely separated by the breadth of a whole 
continent and by wide spaces of ocean. 

These cases of close relationship in species either now or formerly 
inhabiting the seas on the eastern and western shores of North 
America, the Mediterranean and Japan, and the temperate lands of 
North America and Europe, are inexplicable on the theory of crea- 
tion. We cannot maintain that such species have been created alike, 
in correspondence with the nearly similar physical conditions of the 
areas; for if we compare, for instance, certain parts of South America 
with parts of South Africa or Australia, we see countries closely 
similar in all their physical conditions, with their inhabitants utterly 


But we must return to our more immediate subject. I am con- 
vinced that Forbes' view may be largely extended. In Europe we 
meet with the plainest evidence of the Glacial period, from the 


western shores of Britain to the Oural range, and southward to the 
Pyrenees. We may infer from the frozen mammals and nature of 
the mountain vegetation, that Siberia was similarly affected. In the 
Lebanon, according to Dr. Hooker, perj^etual snow formerly covered 
the central axis, and fed glaciers which rolled 4,000 feet down the 
valleys. The same observer has recently found great moraines at a 
low level on the Atlas range in North Africa. Along the Himalaya, 
at points 900 miles apart, glaciers have left the marks of their former 
low descent; and in Sikkim, Dr. Hooker saw maize growing on 
ancient and gigantic moraines. Southward of the Asiatic continent, 
on the opposite side of the equator, we know, from the excellent 
researches of Dr. J. Haast and Dr. Hector, that in New Zealand 
immense glaciers formerly descended to a low level; and the same 
plants found by Dr. Hooker on widely separated mountains in this 
island tell the same story of a former cold period. From facts com- 
municated to me by the Rev. W. B. Clarke, it appears also that there 
are traces of former glacial action on the mountains of the south- 
eastern corner of Australia. 

Looking to America: in the northern half, ice-borne fragments 
of rock have been observed on the eastern side of the continent, as 
far south as latitude 36°-37'', and on the shores of the Pacific, 
where the climate is now so different, as far south as lat. 46°, 
Erratic boulders have, also, been noticed on the Rocky Mountains. 
In the Cordillera of South America, nearly under the equator, 
glaciers once extended far below their present level. In Central 
Chile I examined a vast mound of detritus with great boulders, 
crossing the Portillo valley, which, there can hardly be a doubt, 
once formed a huge moraine; and Mr. D. Forbes informs me that 
he found in various parts of the Cordillera, from latitude 13° to 30° 
S., at about the height of 12,000 feet, deeply-furrowed rocks, re- 
sembling those with which he was familiar in Norway, and likewise 
great masses of detritus, including grooved pebbles. Along this 
whole space of the Cordillera true glaciers do not now exist even 
at much more considerable heights. Farther south on both sides of 
the continent, from lat. 41 ° to the southernmost extremity, we have 
the clearest evidence of former glacial action, in numerous immense 
boulders transported far from their parent source. 


From these several facts, namely from the glacial action having 
extended all round the northern and southern hemispheres — from 
the period having been in a geological sense recent in both hemi- 
spheres — from its having lasted in both during a great length of 
time, as may be inferred from the amount of work effected — and 
lastly from glaciers having recently descended to a low level along 
the whole line of the Cordillera, it at one time appeared to me that 
we could not avoid the conclusion that the temperature of the whole 
world had been simultaneously lowered during the Glacial period. 
But now, Mr. CroU, in a series of admirable memoirs, has attempted 
to show that a glacial condition of climate is the result of various 
physical causes, brought into operation by an increase in the eccen- 
tricity of the earth's orbit. All these causes tend towards the same 
end; but the most powerful appears to be the indirect influence of 
the eccentricity of the orbit upon oceanic currents. According to 
Mr. Croll, cold periods regularly recur every ten or fifteen thousand 
years; and these at long intervals are extremely severe, owing to 
certain contingencies, of which the most important, as Sir C. Lyell 
has shown, is the relative position of the land and water. Mr. Croll 
believes that the last great Glacial period occurred about 240,000 
years ago, and endured with slight alterations of climate for about 
160,000 years. With respect to more ancient Glacial periods, several 
geologists are convinced from direct evidence that such occurred 
during the Miocene and Eocene formations, not to mention still 
more ancient formations. But the most important result for us, 
arrived at by Mr. Croll, is that whenever the northern hemisphere 
passes through a cold period the tempwrature of the southern hemi- 
sphere is actually raised, with the winters rendered much milder, 
chiefly through changes in the direction of the ocean currents. So 

diversely it will be with the northern hemisphere, whilst the 
xjuthern passes through a Glacial period. This conclusion throws 
so much light on geographical distribution that I am strongly in- 
clined to trust in it; but I will first give the facts, which demand an 

In South America, Dr. Hooker has shown that besides many 
closely allied species, between forty and fifty of the flowering plants 
of Tierra del Fuego, forming no inconsiderable part of its scanty 


flora, are common to North America and Europe, enormously 
remote as these areas in opposite hemispheres are from each other. 
On the lofty mountains of equatorial America a host of peculiar 
species belonging to European genera occur. On the Organ moun- 
tains of Brazil, some few temperate European, some Antarctic, and 
some Andean genera were found by Gardner, which do not exist 
in the low intervening hot countries. On the Silla of Caraccas, the 
illustrious Humboldt long ago found species belonging to genera 
characteristic of the Cordillera. 

In Africa, several forms characteristic of Europe and some few 
representatives of the flora of the Cape of Good Hope occur on the 
mountains of Abyssinia. At the Cape of Good Hope a very few 
European species, believed not to have been introduced by man, 
and on the mountains several representative European forms are 
found, which have not been discovered in the intertropical parts of 
Africa. Dr. Hooker has also lately shown that several of the plants 
living on the upper parts of the lofty island of Fernando Po and on 
the neighbouring Cameroon mountains, in the Gulf of Guinea, are 
closely related to those on the mountains of Abyssinia, and likewise 
to those of temperate Europe. It now also appears, as I hear from 
Dr. Hooker, that some of these same temperate plants have been 
discovered by the Rev. R. T. Lowe on the mountains of the Cape 
Verde Islands. This extension of the same temperate forms, almost 
under the equator, across the whole continent of Africa and to the 
mountains of the Cape Verde archipelago, is one of the most astonish- 
ing facts ever recorded in the distribution of plants. 

On the Himalaya, and on the isolated mountain-ranges of the 
peninsula of India, on the heights of Ceylon, and on the volcanic 
cones of Java, many plants occur, either identically the same or 
representing each other, and at the same time representing plants 
of Europe not found in the intervening hot lowlands. A list of the 
genera of plants collected on the loftier peaks of Java, raises a pic- 
ture of a collection made on a hillock in Europe! Still more striking 
is the fact that p)eculiar Australian forms are represented by certain 
plants growing on the summits of the mountains of Borneo. Some 
of these Australian forms, as I hear from Dr. Hooker, extend along 
the heights of the peninsula of Malacca, and are thinly scattered on 


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

On the southern mountains of Australia, Dr. F. Miiller has dis- 
covered several European species; other species, not introduced by 
man, occur on the lowlands; and a long list can be given, as I am 
informed by Dr. Hooker, of European genera, found in Australia, 
but not in the intermediate torrid regions. In the admirable 'Intro- 
duction to the Flora of New Zealand,' by Dr. Hooker, analogous 
and striking facts are given in regard to the plants of that large 
island. Hence we see that certain plants growing on the more lofty 
mountains of the tropics in all parts of the world, and on the tem- 
perate plains of the north and south, are either the same sf>ecies or 
varieties of the same species. It should, however, be observed that 
these plants are not strictly arctic forms; for, as Mr. H. C. Watson 
has remarked, "in receding from polar towards equatorial latitudes, 
the Alpine or mountain floras really become less and less arctic." 
Besides these identical and closely allied forms, many species in- 
habiting the same widely sundered areas, belong to genera not now 
found in the intermediate tropical lowlands. 

These brief remarks apply to plants alone; but some few analogous 
facts could be given in regard to terrestrial animals. In marine pro- 
ductions, similar cases likewise occur; as an example, I may quote 
a statement by the highest authority, Professor Dana, that "it is 
certainly a wonderful fact that New Zealand should have a closer 
resemblance in its Crustacea to Great Britain, its antipode, than to 
any other part of the world." Sir J. Richardson, also, speaks of the 
reappearance on the shores of New Zealand, Tasmania, etc., of 
northern forms of fish. Dr. Hooker informs me that twenty-five 
species of Algx are common to New Zealand and to Europe, but 
have not been found in the intermediate tropical seas. 

From the foregoing facts, namely, the presence of temperate forms 
on the highlands across the whole of equatorial Africa, and along 
the peninsula of India, to Ceylon and the Malay Archipelago, and 
in a less well-marked manner across the wide expanse of tropical 
South America, it appears almost certain that at some former period, 
no doubt during the most severe part of a Glacial period, the low- 
lands of these great continents were everywhere tenanted under the 


equator by a considerable number of temperate forms. At this 
period the equatorial climate at the level of the sea was probably 
about the same with that now experienced at the height of from 
five to six thousand feet under the same latitude, or perhaps even 
rather cooler. During this, the coldest period, the lowlands under 
the equator must have been clothed with a mingled tropical and 
temf)erate vegetation, like that described by Hooker as growing 
luxuriantly at the height of from four to five thousand feet on the 
lower slopes of the Himalayas, but with perhaps a still greater pre- 
ponderance of temperate forms. So again in the mountainous 
islands of Fernando Po, in the Gulf of Guinea, Mr. Mann found 
temperate European forms beginning to appear at the height of 
about five thousand feet. On the mountains of Panama, at the 
height of only two thousand feet, Dr. Seemann found the vegetation 
like that of Mexico, "with forms of the torrid zone harmoniously 
blended with those of the temperate." 

Now let us see whether Mr. Croll's conclusion that when the 
northern hemisphere suffered from the extreme cold of the great 
Glacial period, the southern hemisphere was actually warmer, throws 
any clear light on the present apparently inexplicable distribution 
of various organisms in the temperate parts of both hemispheres, 
and on the mountains of the tropics. The Glacial period, as measured 
by years, must have been very long; and when we remember over 
what vast spaces some naturalised plants and animals have spread 
within a few centuries, this period will have been ample for any 
amount of migration. As the cold became more and more intense, 
we know that arctic forms invaded the temperate regions; and, 
from the facts just given, there can hardly be a doubt that some of 
the more vigorous, dominant and widest-spreading temperate forms 
invaded the equatorial lowlands. The inhabitants of these hot low- 
lands would at the same time have migrated to the tropical and 
sub-tropical regions of the south, for the southern hemisphere was 
at this period warmer. On the decline of the Glacial f)eriod, as both 
hemispheres gradually recovered their former temf)eratures, the 
northern temperate forms living on the lowlands under the equator, 
would have been driven to their former homes or have been de- 
stroyed, being replaced by the equatorial forms returning from the 


south. Some, however, of the northern temperate forms would 
almost certainly have ascended any adjoining high land, where, if 
suiliciently lofty, they would have long survived like the arctic forms 
on the mountains of Europe. They might have survived, even if 
the climate was not perfectly fitted for them, for the change of 
temperature must have been very slow, and plants undoubtedly 
possess a certain capacity for acclimatisation, as shown by their 
transmitting to their offspring difJerent constitutional powers ol 
resisting heat and cold. 

In the regular course of events the southern hemisphere would 
in its turn be subjected to a severe Glacial period, with the northern 
hemisphere rendered warmer; and then the southern temperate 
forms would invade the equatorial lowlands. The northern forms 
which had before been left on the mountains would now descend 
and mingle with the southern forms. These latter, when the warmth 
returned, would return to their former homes, leaving some few 
,sj)ecies on the mountains, and carrying southward with them some 
of the northern temf)crate forms which had descended from their 
mountain fastnesses. Thus, we should have some few species 
identically the same in the northern and southern temperate zones 
and on the mountains of the intermediate tropical regions. But the 
species left during a long time on these mountains, or in opposite 
hemispheres, would have to compete with many new forms and 
would be exposed to somewhat different physical conditions; hence 
they would be eminently liable to modification, and would generally 
now exist as varieties or as representative species; and this is the 
case. We must, also, bear in mind the occurrence in both hemi- 
spheres of former Glacial periods; for these will account, in accord- 
ance with the same principles, for the many quite distinct species 
inhabiting the same widely separated areas, and belonging to genera 
not now found in the intermediate torrid zones. 

It is a remarkable fact strongly insisted on by Hooker in regard 
to America, and by Alph. de CandoUe in regard to Australia, that 
many more identical or slightly modified species have migrated 
from the north to the south, than in a reversed direction. We see, 
however, a few southern forms on the mountains of Borneo and 
Abyssinia. I suspect that this preponderant migration from the 


north to the south is due to the greater extent of land in the north, 
and to the northern forms having existed in their own homes in 
greater numbers, and having consequently been advanced through 
natural selection and competition to a higher stage of perfection, or 
dominating power, than the southern forms. And thus, when the 
two sets became commingled in the equatorial regions, during the 
alternations of the Glacial periods, the northern forms were the more 
pHJwerful and were able to hold their places on the mountains, and 
afterwards to migrate southward with the southern forms; but not 
so the southern in regard to the northern forms. In the same man- 
ner at the present day, we see that very many European productions 
cover the ground in La Plata, New Zealand, and to a lesser degree 
in Australia, and have beaten the natives; whereas extremely few 
southern forms have become naturaUsed in any part of the northern 
hemisphere, though hides, wool, and other objects likely to carry 
seeds have been largely imported into Europe during the last two 
or three centuries from La Plata and during the last forty or fifty 
years from Australia. The Neilgherrie mountains in India, however, 
ofler a partial exception; for here, as I hear from Dr. Hooker, 
Australian forms are rapidly sowing themselves and becoming 
naturalised. Before the last great Glacial period, no doubt the inter- 
tropical mountains were stocked with endemic Alpine forms; but 
these have almost everywhere yielded to the more dominant forms 
generated in the larger areas and more efficient workshops of the 
north. In many islands the native productions are nearly equalled, 
or even outnumbered, by those which have become naturalised; and 
this is the first stage towards their extinction. Mountains are islands 
on the land, and their inhabitants have yielded to those produced 
within the larger areas of the north, just in the same way as the 
inhabitants of real islands have everywhere yielded and are still 
yielding to continental forms naturalised through man's agency. 

The same principles apply to the distribution of terrestrial animals 
and of marine productions, in the northern and southern temperate 
zones, and on the intertropical mountains. When, during the height 
of the Glacial period, the ocean currents were widely different to 
what they now are, some of the inhabitants of the temperate seas 
might have reached the equator; of these a few would perhaps at 


once be abie to migrate southward, by keeping to the cooler currents, 
whilst others might remain and survive in the colder depths until 
the southern hemisphere was in its turn subjected to a glacial climate 
and permitted their further progress; in nearly the same manner as, 
according to Forbes, isolated spaces inhabited by arctic productions 
exist to the present day in the deeper parts of the northern temperate 

I am far from supposing that all the difficulties in regard to the 
distribution and affinities of the identical and allied species, which 
now live so widely separated in the north and south, and sometimes 
on the intermediate mountain-ranges, are removed on the views 
above given. The exact lines of migration cannot be indicated. We 
cannot say why certain species and not others have migrated; why 
certain species have been modified and have given rise to new forms, 
whilst others have remained unaltered. We cannot hope to explain 
such facts, until we can say why one species and not another becomes 
naturalised by man's agency in a foreign land; why one species 
ranges twice or thrice as far, and is twice or thrice as common, as 
another species within their own homes. 

Various special difficulties also remain to be solved; for instance, 
the occurrence, as shown by Dr. Hooker, of the same plants at points 
so enormously remote as Kerguelen Land, New Zealand, and 
Fuegia; but icebergs, as suggested by Lyell, may have been con- 
cerned in their dispersal. The existence at these and other distant 
points of the southern hemisphere, of species, which, though distinct, 
belong to genera exclusively confined to the south, is a more remark- 
able case. Some of these species are so distinct, that we cannot sup- 
pose that there has been time since the commencement of the last 
Glacial period for their migration and subsequent modification to 
the necessary degree. The facts seem to indicate that distinct species 
belonging to the same genera have migrated in radiating lines from 
a common genera; and I am inclined to look in the southern, as in 
the northern hemisphere, to a former and warmer period, before the 
commencement of the last Glacial period, when the antarctic lands, 
now covered with ice, supported a highly peculiar and isolated flora. 
It may be suspected that before this flora was exterminated during 
the last Glacial epoch, a few forms had been already widely dispersed 


to various points of the southern hemisphere by occasional means of 
transport, and by the aid as halting-places, of now sunken islands. 
Thus the southern shores of America, Australia, and New Zealand 
may have become slightly tinted by the same pecuUar forms of life. 
Sir C. Lyell in a striking passage has speculated, in language 
almost identical with mine, on the effects of great alterations of 
climate throughout the world on geographical distribution. And we 
have now seen that Mr. Croll's conclusion that successive Glacial 
periods in the one hemisphere coincide with warmer periods in the 
opposite hemisphere, together with the admission of the slow modi- 
fication of species, explains a multitude of facts in the distribution 
of the same and of the allied forms of life in all parts of the globe. 
The living waters have flowed during one period from the north 
and during another from the south, and in both cases have reached 
the equator; but the stream of life has flowed with greater force 
from the north than in the opposite direction, and has consequently 
more freely inundated the south. As the tide leaves its drift in 
horizontal lines, rising higher on the shores where the tide rises 
highest, so have the living waters left their living drift on our moun- 
tain summits, in a line gently rising from the arctic lowlands to 
a great altitude under the equator. The various beings thus left 
stranded may be compared with savage races of man, driven up and 
surviving in the mountain fastnesses of almost every land, which 
serves as a record, full of interest to us, of the former inhabitants of 
the surrounding lowlands. 


Geographical Distribution — continued 

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


AS lakes and river systems are separated from each other by bar- 
/ \ riers of land, it might have been thought that fresh-water 
jL .m. productions would not have ranged widely within the same 
country, and as the sea is apparently a still more formidable barrier, 
that they would never have extended to distant countries. But the 
case is exactly the reverse. Not only have many fresh-water species, 
belonging to different classes, an enormous range, but allied species 
prevail in a remarkable manner throughout the world. When first 
collecting in the fresh waters of Brazil, I well remember feeUng much 
surprise at the similarity of the fresh-water insects, shells, etc., and 
at the dissimilarity of the surrounding terrestrial beings, compared 
with those of Britain. 

But the wide ranging power of fresh-water productions can, I 
think, in most cases be explained by their having become fitted, in 
a manner highly useful to them, for short and frequent migrations 
from pond to [X)nd, or from stream to stream, within their own coun- 
tries; and liability to wide dispersal would follow from this capacity 
as an almost necessary consequence. We can here consider only a few 
cases; of these, some of the most difficult to explain are presented by 
fish. It was formerly believed that the same fresh-water species never 
existed on two continents distant from each other. But Dr. Giin- 
ther has lately shown that the Galaxias attenuatus inhabits Tasmania, 
New Zealand, the Falkland Islands, and the mainland of South 
America. This is a wonderful case, and probably indicates dispersal 



from an antarctic centre during a former warm period. This case, 
however, is rendered in some degree less surprising by the species of 
this genus having the power of crossing by some unknown means 
considerable spaces of open ocean : thus there is one species common 
to New Zealand and to the Auckland Islands, though separated by a 
distance of about 230 miles. On the same continent fresh-water fish 
often range widely, and as if capriciously; for in two adjoining river- 
systems some of the species may be the same, and some wholly 

It is probable that they are occasionally transported by what- may 
be called accidental means. Thus fishes still alive are not very rarely 
dropped at distant points by whirlwinds; and it is known that the ova 
retain their vitality for a considerable time after removal from the 
water. Their dispersal may, however, be mainly attributed to changes 
in the level of the land within the recent period, causing rivers to flow 
into each other. Instances, also, could be given of this having oc- 
curred during floods, without any change of level. The wide differ- 
ence of the fish on the opposite sides of most mountain ranges, which 
are continuous, and which consequently must from an early period 
have completely prevented the inosculation of the river system on 
the two sides, leads to the same conclusion. Some fresh-water fish 
belong to very ancient forms, and in such cases there will have been 
ample time for great geographical changes, and consequendy time 
and means for much migration. Moreover Dr. Gunther has recently 
been led by several considerations to infer that with fishes the same 
forms have a long endurance. Salt-water fish can with care be slowly 
accustomed to live in fresh water; and, according to Valenciennes, 
there is hardly a single group of which all the members are confined 
to fresh water, so that a marine species belonging to a fresh-water 
group might travel far along the shores of the sea, and could, it is 
probable, become adapted without much difficulty to the fresh waters 
of a distant land. 

Some species of fresh-water shells have very wide ranges, and allied 
species which, on our theory, are descended from a common parent, 
and must have prcxeeded from a single source, prevail throughout 
the world. Their distribution at first perplexed me much, as 
their ova are not likely to be transported by birds; and the ova, 


as well as the adults, are immediately killed by sea water. I 
could not even understand how some naturalised sjjecies have spread 
rapidly throughout the same country. But two facts, which I have 
observed — and many others no doubt will be discovered — throw some 
light on this subject. When ducks suddenly emerge from a pond 
covered with duckweed, 1 have twice seen these little plants adhering 
to their backs; and it has happened to me, in removing a little duck- 
weed from one aquarium to another, that I have unintentionally 
stocked the one with fresh-water shells from the other. But another 
agency is perhaps more effectual : I suspended the feet of a duck in 
an aquarium, where many ova of fresh-water shells were hatching; 
and I found that numbers of the extremely minute and just-hatched 
shells crawled on the feet, and clung to them so firmly that when 
taken out of the water they could not be jarred off, though at a some- 
what more advanced age they would voluntarily drop off. These 
just-hatched molluscs, though aquatic in their nature, survived on 
the duck's feet, in damp air, from twelve to twenty hours; and in this 
length of time a duck or heron might fly at least six or seven hun- 
dred miles, and if blown across the sea to an oceanic island, or to any 
other distant point, would be sure to alight on a pxxsl or rivulet. Sir 
Charles Lyell informs me that a Dytiscus has been caught with an 
Ancylus (a fresh-water shell like a limpet) firmly adhering to it; 
and a water-beetle of the same family, a Colymbetes, once flew on 
board the 'Beagle', when forty-five miles distant from the nearest 
land: how much farther it might have been blown by a favouring 
gale no one can tell. 

With respect to plants, it has long been known what enormous 
ranges many fresh-water, and even marsh species, have, both over 
continents and to the most remote oceanic islands. This is strikingly 
illustrated, according to Alph. de CandoUe, in those large groups of 
terrestrial plants, which have very few aquatic members; for the 
latter seem immediately to acquire, as if in consequence, a wide range. 
I think favourable means of dispersal explain this fact. I have be- 
fore mentioned that earth occasionally adheres in some quantity to 
the feet and beaks of birds. Wading birds, which frequent the muddy 
edges of ponds, if suddenly flushed, would be the most likely to have 
muddy feet. Birds of this order wander more than those of any 


Other; and they are occasionally found on the most remote and barren 
islands of the open ocean; they would not be Ukely to alight on the 
surface of the sea, so that any dirt on their feet would not be washed 
off; and when gaining the land, they would be sure to fly to their 
natural fresh-water haunts. I do not believe that botanists are aware 
how charged the mud of ponds is with seeds; I have tried several lit- 
tle experiments, but will here give only the most striking case: I took 
in February three table-spoonfuls of mud from three different points, 
beneath water, on the edge of a little pond: this mud when dried 
weighed only six and three-fourths ounces; I kept it covered up in my 
study for six months, pulling up and counting each plant as it grew; 
the plants were of many kinds, and were altogether 537 in number; 
and yet the viscid mud was ail contained in a breakfast cup! Consid- 
ering these facts, I think it would be an inexplicable circumstance if 
water-birds did not transport the seeds of fresh-water plants to un- 
stocked ponds and streams, situated at very distant points. The same 
agency may have come into play with the eggs of some of the smaller 
fresh-water animals. 

Other and unknown agencies probably have also played a part. I 
have stated that fresh-water fish eat some kinds of seeds, though they 
reject many other kinds after having swallowed them; even small fish 
swallow seeds of moderate size, as of the yellow water-lily and 
Potamogeton. Herons and other birds, century after century, have 
gone on daily devouring fish; they then take flight and go to other 
waters, or are blown across the sea; and we have seen that seeds retain 
their power of germination, when rejected many hours afterwards 
in {jellets or in the excrement. When I saw the great size of the 
seeds of that fine water-lily, the Nelumbium, and remembered Alph. 
de CandoIIe's remarks on the distribution of this plant, I thought that 
the means of its dispersal must remain inexplicable; but Audu- 
bon states that he found the seeds of the great southern water-lily 
(probably, according to Dr. Hooker, the Nelumbium luteum) in a 
heron's stomach. Now this bird must often have flown with its 
stomach thus well stocked to distant ponds, and then getting a hearty 
meal of fish, analogy makes me believe that it would have rejected 
the seeds in a pellet in a fit state for germination. 

In considering these several means of distribution, it should be re- 


membered that when a pond or stream is first formed, for instance, 
on a rising islet, it will be unoccupied; and a single seed or egg will 
have a good chance of succeeding. Although there will always be a 
struggle for life between the inhabitants of the same pond, however 
few in kind, yet as the number even in a well-stocked pond is small 
in comparison with the number of species inhabiting an equal area of 
land, the competition between them will probably be less severe than 
between terrestrial species; consequently an intruder from the waters 
of a foreign country would have a better chance of seizing on a new 
place, than in the case of terrestrial colonists. We should also remem- 
ber that many fresh-water productions are low in the scale of nature, 
and we have reason to believe that such beings become modified 
more slowly than the high; and this will give time for the migration 
of aquatic species. We should not forget the probability of many 
fresh-water forms having formerly ranged continuously over im- 
mense areas, and then having become extinct at intermediate points. 
But the wide distribution of fresh-water plants and of the lower ani- 
mals, whether retaining the same identical form or in some degree 
modified, apparently depends in main part on the wide dispersal of 
their seeds and eggs by animals, more especially by fresh-water birds, 
which have great powers of flight, and naturally travel from one piece 
of water to another. 


We now come to the last of the three classes of facts, which I have 
selected as presenting the greatest amount of difficulty with respect 
to distribution, on the view that not only all the individuals of the 
same species have migrated from some one area, but that allied spe- 
cies, although now inhabiting the most distant points, have proceeded 
from a single area, — the birthplace of their early progenitors. I have 
already given my reason for disbelieving in continental extensions 
within the (period of existing species, on so enormous a scale that all 
the many islands of the several oceans were thus stocked with their 
present terrestrial inhabitants. This view removes many difficulties, 
but it does not accord with all the facts in regard to the productions 
of islands. In the following remarks I shall not confine myself to the 
mere question of dispersal, but shall consider some other cases bearing 


on the truth o£ the two theories of independent creation and of 

descent with modification. 

The species of ^11 kinds which inhabit oceanic islands are few in 
number compared with those on equal continental areas: Alph. de 
Candolle admits this for plants, and WoUaston for insects. New 
Zealand, for instance, with its lofty mountains and diversified sta- 
tions, extending over 780 miles of latitude, together with the outlying 
islands of Auckland, Campbell, and Chatham, contain altogether 
only 960 kinds of flowering plants; if we compare this moderate num- 
ber with the species which swarm over equal areas in southwestern 
Australia or at the Cape of Good Hope, we must admit that some 
cause, independently of different physical conditions, has given rise 
to so great a difference in number. Even the uniform county of 
Cambridge has 847 plants, and the litde island of Anglesea 764, but 
a few ferns and a few introduced plants are included in these num- 
bers, and the comparison in some other respects is not quite fair. We 
have evidence that the barren island of Ascension aboriginally pos- 
sessed less than half-a-dozen flowering plants; yet many species have 
now become naturalised on it, as they have in New Zealand and on 
every other oceanic island which can be named. In St. Helena there 
is reason to believe that the naturalised plants and animals have nearly 
or quite exterminated many native productions. He who admits the 
doctrine of the creation of each separate species, will have to admit 
that a sufficient number of the best adapted plants and animals were 
not created for oceanic islands; for man has unintentionally stocked 
them far more fully and perfectly than did nature. 

Although in oceanic islands the species are few in number, the pro- 
portion of endemic kinds (/.r. those found nowhere else in the world) 
is often extremely large. If we compare, for instance, the number of 
endemic land shells in Madeira, or of endemic birds in the Galapagos 
archipelago, with the number found on any continent, and then com- 
pare the area of the island with that of the continent, we shall see 
that this is true. This fact might have been theoretically exfjected, 
for, as already explained, sfiecies occasionally arriving after long in- 
tervals of time in the new and isolated district, and having to compete 
with new associates, would be eminently liable to modification, and 
would often produce groups of modified descendants. But it by no 


means follows that, because in an island nearly all the species of one 
class are peculiar, those of another class, or of another section of the 
same class, are peculiar; and this difference seems to depend partly 
on the species which are not modified having immigrated in a body, 
so that their mutual relations have not been much disturbed; and 
pardy on the frequent arrival of unmodified immigrants from the 
mother country, with which the insular forms have intercrossed. It 
should be borne in mind that the offspring of such crosses would cer- 
tainly gain in vigour; so that even an occasional cross would produce 
more effect than might have been anticipated. 1 will give a few illus- 
trations of the foregoing remarks : in the Galapagos Islands there are 
twenty-six land birds; of these twenty-one (or perhaps twenty-three) 
are peculiar, whereas of the eleven marine birds only two are pecu- 
liar; and it is obvious that marine birds could arrive at these islands 
much more easily and frequendy than land birds. Bermuda, on the 
other hand, which lies at about the same distance from North Amer- 
ica as the Galapagos Islands do from South America, and which has 
a very pecuHar soil, does not possess a single endemic land bird; and 
we know from Mr. J. M. Jones' admirable account of Bermuda, that 
very many North American birds occasionally or even frequently 
visit this island. Almost every year, as I am informed by Mr. E. V. 
Harcourt, many European and African birds are blown to Madeira; 
this island is inhabited by ninety-nine kinds, of which one alone is 
peculiar, though very closely related to a European form; and three 
or four other species are confined to this island and to the Canaries. 
So that the Islands of Bermuda and Madeira have been stocked from 
the neighbouring continents with birds, which for long ages have 
there struggled together, and have become mutually co-adapted. 
Hence when settled in their new homes, each kind will have been 
kept by the others to its proper place and habits, and will conse- 
quently have been but little liable to modification. Any tendency to 
modification will also have been checked by intercrossing with the 
unmodified immigrants, often arriving from the mother country. 
Madeira again is inhabited by a wonderful number of peculiar land 
shells, whereas not one species of sea shell is peculiar to its shores; 
now, though we do not know how sea shells are dispersed, yet we 
can see that their eggs or larvx, perhaps attached to seaweed or float- 


ing timber, or to the feet of wading-birds, might be transported across 
three or four hundred miles of open sea far more easily than land 
shells. The different orders of insects inhabiting Madeira present 
nearly parallel cases. 

Oceanic islands are sometimes deficient in animals of certain whole 
classes, and their places are occupied by other classes; thus in the 
Galapagos Islands reptiles, and in New Zealand gigantic wingless 
birds, take, or recently took, the place of mammals. Although New 
Zealand is here spoken of as an oceanic island, it is in some degree 
doubtful whether it should be so ranked; it is of large size, and is not 
separated from Australia by a profoundly deep sea; from its geologi- 
cal character and the direction of its mountain ranges, the Rev. W. B. 
Clarke has lately maintained that this island, as well as New Cale- 
donia, should be considered as appurtenances of Australia. Turning 
to plants. Dr. Hooker has shown that in the Galapagos Islands the 
prop)ortional numbers of the different orders are very different from 
what they are elsewhere. All such differences in number, and the ab- 
sence of certain whole groups of animals and plants, are generally 
accounted for by supposed differences in the physical conditions of the 
islands; but this explanation is not a little doubtful. Facility of immi- 
gration seems to have been fully as important as the nature of the 

Many remarkable little facts could be given with respect to the 
inhabitants of oceanic islands. For instance, in certain islands 
not tenanted by a single mammal, some of the endemic plants 
have beautifully hooked seeds; yet few relations are more mani- 
fest than that hooks serve for the transportal of seeds in the 
wool or fur of quadrupeds. But a hooked seed might be carried 
to an island by other means; and the plant then becoming modi- 
fied would form an endemic species, still retaining its hooks, 
which would form a useless appendage like the shrivelled wings 
under the soldered wingcovers of many insular beetles. Again, 
islands often possess trees or bushes belonging to orders which 
elsewhere include only herbaceous sp)ecies; now trees, as Alph. 
de Candolle has shown, generally have, whatever the cause may 
be, confined ranges. Hence trees would be little likely to reach 
distant oceanic islands; and an herbaceous plant, which had no chance 


of successfully competing with the many fully developed trees grow- 
ing on a continent, might, when established on an island, gain an 
advantage over other herbaceous plants by growing taller and taller 
and overtopping them. In this case, natural selection would tend to 
add to the stature of the plant, to whatever order it belonged, and 
thus first convert it into a bush and then into a tree. 


With respect to the absence of whole orders or animals on oceanic 
islands, Bory St. Vincent long ago remarked that batrachians (frogs, 
toads, newts) are never found on any of the many islands with which 
the great oceans are studded. I have taken pains to verify this asser- 
tion, and have found it true, with the exception of New Zealand, 
New Caledonia, the Andaman Islands, and perhaps the Solomon 
Islands and the Seychelles. But I have already remarked that it is 
doubtful whether New Zealand and New Caledonia ought to be 
classed as oceanic islands; and this is still more doubtful with respect 
to the Andaman and Solomon groups and the Seychelles. This gen- 
eral absence of frogs, toads, and newts on so many true oceanic 
islands cannot be accounted for by their physical conditions: indeed 
it seems that islands are peculiarly fitted for these animals; for frogs 
have been introduced into Madeira, the Azores, and Mauritius, and 
have multiplied so as to become a nuisance. But as these animals and 
their spawn are immediately killed (with the exception, as far as 
known, of one Indian species) by sea water, there would be great 
difficulty in