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Title: Darwin, and After Darwin (Vol. 1 and 3, of 3) - An Exposition of the Darwinian Theory and a Discussion of - Post-Darwinian Questions
Author: Romanes, George John, 1848-1894
Language: English
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*** Start of this LibraryBlog Digital Book "Darwin, and After Darwin (Vol. 1 and 3, of 3) - An Exposition of the Darwinian Theory and a Discussion of - Post-Darwinian Questions" ***
[Illustration: Frontispiece-Charles Darwin]
DARWIN, AND AFTER DARWIN
_AN EXPOSITION OF THE DARWINIAN THEORY
AND A DISCUSSION OF
POST-DARWINIAN QUESTIONS_
BY
GEORGE JOHN ROMANES, M.A., LL.D., F.R.S.
_Honorary Fellow of Gonville and Caius College, Cambridge_
I
THE DARWINIAN THEORY
FOURTH EDITION
Chicago
THE OPEN COURT PUBLISHING COMPANY
1910
The Illustrations of this book (with the exception of the Frontispiece
and the colored plate facing page 332) are copyrighted under the title
"Darwinism Illustrated."
THE OPEN COURT PUBLISHING CO.
PRESS OF THE
BLAKELY-OSWALD PRINTING CO.
CHICAGO
[Illustration: Letter to Mr. Hegeler
Transcription:
Ch. Ch. Oxford:
March 15th 1892.
My dear Sir,
As we have now agreed that
the Open Court Publishing Company is to
undertake the American edition of my
work entitled "Darwin and after
Darwin," I have much pleasure
in transferring to you the copyright
thereof, with all that this
includes.
Thanking you very much for
the kindness and liberality which
have marked your conduct of these
negotiations,
I remain,
Yours very faithfully,
George J. Romanes
To
Edward C. Hegeler Esq.
La Salle, Ill. U. S.]
PREFACE
Several years ago Lord Rosebery founded, in the University of Edinburgh,
a lectureship on "The Philosophy of Natural History," and I was invited
by the Senatus to deliver the lectures. This invitation I accepted, and
subsequently constituted the material of my lectures the foundation of
another course, which was given in the Royal Institution, under the
title "Before and after Darwin." Here the course extended over three
years--namely from 1888 to 1890. The lectures for 1888 were devoted to
the history of biology from the earliest recorded times till the
publication of the "Origin of Species" in 1859; the lectures for 1889
dealt with the theory of organic evolution up to the date of Mr.
Darwin's death, in 1882; while those of the third year discussed the
further developments of this theory from that date till the close of the
course in 1890.
It is from these two courses--which resembled each other in comprising
between thirty and forty lectures, but differed largely in other
respects--that the present treatise has grown. Seeing, however, that it
has grown much beyond the bulk of the original lectures, I have thought
it desirable to publish the whole in the form of three separate works.
Of these the first--or that which deals with the purely historical side
of biological science--may be allowed to stand over for an indefinite
time. The second is the one which is now brought out and which, as its
sub-title signifies, is devoted to the general theory of organic
evolution as this was left by the stupendous labours of Darwin. As soon
as the translations shall have been completed, the third portion will
follow (probably in the Autumn season), under the sub-title,
"Post-Darwinian Questions."
As the present volume is thus intended to be merely a systematic
exposition of what may be termed the Darwinism of Darwin, and as on this
account it is likely to prove of more service to general readers than to
professed naturalists, I have been everywhere careful to avoid assuming
even the most elementary knowledge of natural science on the part of
those to whom the exposition is addressed. The case, however, will be
different as regards the next volume, where I shall have to deal with
the important questions touching Heredity, Utility, Isolation, &c.,
which have been raised since the death of Mr. Darwin, and which are now
being debated with such salutary vehemence by the best naturalists of
our time.
My obligations to the Senatus of the University of Edinburgh, and to the
Board of Management of the Royal Institution, have already been
virtually expressed; but I should like to take this opportunity of also
expressing my obligations to the students who attended the lectures in
the University of Edinburgh. For alike in respect of their large
numbers, their keen intelligence, and their generous sympathy, the
members of that voluntary class yielded a degree of stimulating
encouragement, without which the labour of preparing the original
lectures could not have been attended with the interest and the
satisfaction that I found in it. My thanks are also due to Mr. R. E.
Holding for the painstaking manner in which he has assisted me in
executing most of the original drawings with which this volume is
illustrated; and likewise to Messrs. Macmillan and Co. for kindly
allowing me to reprint--without special acknowledgment in every
case--certain passages from an essay which they published for me many
years ago, under the title "Scientific Evidences of Organic Evolution."
Lastly, I must mention that I am indebted to the same firm for
permission to reproduce an excellent portrait of Mr. Darwin, which
constitutes the frontispiece.
G. J. R.
CHRIST CHURCH, OXFORD,
_April 19th, 1892._
CONTENTS
PAGE
CHAPTER I.
INTRODUCTORY 1
CHAPTER II.
CLASSIFICATION 23
CHAPTER III.
MORPHOLOGY 50
CHAPTER IV.
EMBRYOLOGY 98
CHAPTER V.
PALÆONTOLOGY 156
CHAPTER VI.
GEOGRAPHICAL DISTRIBUTION 204
CHAPTER VII.
THE THEORY OF NATURAL SELECTION 251
CHAPTER VIII.
EVIDENCES OF THE THEORY OF NATURAL SELECTION 285
CHAPTER IX.
CRITICISMS OF THE THEORY OF NATURAL SELECTION 333
CHAPTER X.
THE THEORY OF SEXUAL SELECTION, AND CONCLUDING REMARKS 379
* * * * *
APPENDIX TO CHAPTER V. 421
NOTE A TO PAGE 257 443
NOTE B TO PAGE 295 445
NOTE C TO PAGE 394 448
INDEX 451
LIST OF ILLUSTRATIONS
FIG. PAGE
1. Successive forms of Paludina, from the Tertiary deposits of
Slavonia 19
2. Skeleton of Seal 52
3. Skeleton of Greenland Whale 53
4. Paddle of Whale compared with Hand of Man 54
5. Wing of Reptile, Mammal, and Bird 56
6. Skeleton of _Dinornis gravis_ 61
7. Hermit crabs compared with the cocoa-nut crab 64
8. Rudimentary or vestigial hind-limbs of Python 67
9. _Apteryx Australis_ 69
10. Illustrations of the nictitating membrane in various animals
named 75
11. Rudimentary, or vestigial and useless, muscles of the human
ear 76
12. Portrait of a young male gorilla 78
13. Portrait of a young male child 79
14. An infant, three weeks old, supporting its own weight 81
15. Sacrum of Gorilla compared with that of Man, showing the
rudimentary tail-bones of each 82
16. Diagrammatic outline of the human embryo when about seven
weeks old 83
17. Front and back view of adult human sacrum 84
18. _Appendix vermiformis_ in Orang and in Man 85
19. The same, showing variation in the Orang 85
20. Human ear 86
21. Foetus of an Orang 87
22. Vestigial characters of human ears 88
23. Hair-tracts on the arms and hands of Man, as compared with
those on the arms and hands of Chimpanzee 90
24. Molar teeth of lower jaw in Gorilla, Orang, and Man 93
25. Perforation of the humerus (supra-condyloid foramen) in three
species of Quadrumana where it normally occurs, and in Man,
where it does not normally occur 95
26. Antlers of stag, showing successive addition of branches in
successive years 100
27. Fission of a Protozoön 107
28. _Hydra viridis_, partly in section 111
29. Successive stages in the division of the ovum, or egg-cell,
of a worm 113
30. Ovarian ovum of a Mammal 121
31. Amoeboid movements of young egg-cells 122
32. Human ovum, mature and greatly magnified 123
33. Stages in the formation of the polar bodies in the ovum of a
star-fish 125
34. Fertilization of the ovum of an echinoderm 126
35. Fertilization of the ovum of a star-fish 127
36. Karyokinesis of a typical tissue-cell (epithelium of
Salamander) 129
37. Study of successive changes taking place in the nucleus of an
epithelium-cell, preparatory to division of the cell 131
38. Formation and conjugation of the pronuclei in _Ascaris
megalocephala_ 132, 133
39. Segmentation of ovum 135
40. The contents of an ovum in an advanced stage of segmentation,
drawn in perspective 135
41. Formation of the gastrula of _Amphioxus_ 137
42. Gastrulation 138
43. Gastrula of a Chalk Sponge 139
44. _Prophysema primordiale_, an extant gastræa-form 140
45. Ideal primitive vertebrate, seen from the left side 143
46. The same in transverse section through the ovaries 144
47. _Amphioxus lanceolatus_ 145
48. _Balanoglossus_ 148
49. A large Sea-lamprey (_Petromyzon marinus_) 148
50. Adult Shark (_Carcharias melanopterus_) 149
51. Diagram of heart and gill-arches of a fish 150
52. One gill-arch, with branchial fringe attached 150
53. Diagram of heart and gill-arches in a lizard 150
54. Ideal diagram of primitive gill-or aortic-arches 151
55. The same, modified for a bird 151
56. The same, modified for a mammal 151
57. A series of embryos at three comparable and progressive
stages of development, representing each of the classes of
vertebrated animals below the Mammalia 152
58. Another series of embryos, also at three comparable and
progressive stages of development, representing four different
divisions of the class Mammalia 153
59. Diagram of geological succession of the classes of the Animal
Kingdom 165
60. Skull of _Oreodon Culbertsoni_ 167
61,62. Horns of _Cervus dicrocerus_ 168
63. Horns of _C. matheronis_ 168
64. Horns of _C. pardinensis_ 168
65. Horns of _C. issiodorensis_ 168
66. Horns of _C. Sedgwickii_ 168
67. Successive stages in the development of an existing Deer's
Antlers 169
68. Homocercal tail 169
69. Heterocercal tail 170
70. Vertebrated but symmetrical fin (diphycercal) 170
71. Tail of _Archæopteryx_ 171
72. Tail of modern Bird 171
73. _Archæopteryx macura_, restored 172
74. Skeleton of Polar Bear 174
75. Skeleton of Lion 175
76. Anterior limb of Man, Dog, Hog, Sheep, and Horse 176
77. Posterior limb of Man, Monkey, Dog, Sheep, and Horse 177
78. Posterior limb of _Baptanodon discus_, and anterior limb of
_Chelydra serpentina_ 179
79. Paddle of a Whale 180
80. Fossil skeleton of _Phenacodus primævus_ 184
81. Bones of the foot of four different forms of the perissodactyl
type 186
82. Bones of the foot of four different forms of the artiodactyl
type 187
83. Feet and teeth In fossil pedigree of the Horse 189
84. _Palæotherium_. (Lower Tertiary of Paris Basin) 190
85. _Hipparion_. (New World Pliocene) 192
86. Comparative series of Brains 194
87. Ideal section through all the above stages 195
88. Skulls of Canadian Stag, _Cervalces Americanus_, and Elk 198
89. Transmutations of _Planorbis_ 200
90. Transformation of _Strombus_ 202
91. Pigeons. Drawn from life 298
92. Pigeons (_continued_). Drawn from life 299
93. Fowls. Drawn from life 300
94. Fowls (_continued_). Drawn from life 301
95. Pair of Japanese Fowls, long-tailed breed 302
96. Canaries. Drawn from life 303
97. Sebastopol, or Frizzled Goose 304
98. The Dingo, or wild dog of Australia 304
99. Dogs. Drawn from life 305
100. Dogs (_continued_). Drawn from life 306
101. The Hairless Dog of Japan 307
102. The skull of a Bull-dog compared with that of a Deer-hound 307
103. Rabbits. Drawn from life 308
104. Horses. Drawn from life 309
105. Sheep. Drawn from life 310
106. Cattle. Drawn from life 311
107. Wild Boar contrasted with a modern Domesticated Pig 312
108. Seasonal changes of colour in Ptarmigan (_Lagopus mutus_) 317
109. _Oedicneus crepitans_, showing the instinctive attitude of
concealment 320
110. Imitative forms and colours in insects 322
111. The larva of Puss Moth (_Cerura vinula_) 325
112. The larva of Puss Moth in disturbed attitude 326
113. Three cases of mimicry 328
114. Two further cases of mimicry; flies resembling a wasp in the
one and a bee in the other 329
115. A case of mimicry where a non-venomous species of snake
resembles a venomous one 330
116. A case of mimicry where a homopterous resembles a leaf-cutting
ant 332
117. Feather-footed pigeon 359
118. _Raia radiata_ 368
119. Electric organ of the Skate 369
120. Electric cells of _Raia radiata_ 370
121. The Garden Bower-bird (_Amblyornis inornata_) 382
122. Courtship of Spiders 388
123. Courtship of Spiders (_continued_) 389
124. The Bell-bird (_Chasmorhynchus niveus_) 396
125. _C. tricarunculatus_ 397
SECTION I
_EVOLUTION_
CHAPTER I.
INTRODUCTORY.
Among the many and unprecedented changes that have been wrought by Mr.
Darwin's work on the _Origin of Species_, there is one which, although
second in importance to no other, has not received the attention which
it deserves. I allude to the profound modification which that work has
produced on the ideas of naturalists with regard to method.
Having had occasion of late years somewhat closely to follow the history
of biological science, I have everywhere observed that progress is not
so much marked by the march of discovery _per se_, as by the altered
views of method which the march has involved. If we except what
Aristotle called "the first start" in himself, I think one may fairly
say that from the rejuvenescence of biology in the sixteenth century to
the stage of growth which it has now reached in the nineteenth, there is
a direct proportion to be found between the value of work done and the
degree in which the worker has thereby advanced the true conception of
scientific working. Of course, up to a certain point, it is notorious
that the revolt against the purely "subjective methods" in the sixteenth
century revived the spirit of _inductive_ research as this had been left
by the Greeks; but even with regard to this revolt there are two things
which I should like to observe.
In the first place, it seems to me, an altogether disproportionate value
has been assigned to Bacon's share in the movement. At most, I think, he
deserves to be regarded but as a literary exponent of the _Zeitgeist_ of
his century. Himself a philosopher, as distinguished from a man of
science, whatever influence his preaching may have had upon the general
public, it seems little short of absurd to suppose that it could have
produced any considerable effect upon men who were engaged in the
practical work of research. And those who read the _Novum Organon_ with
a first-hand knowledge of what is required for such research can
scarcely fail to agree with his great contemporary Harvey, that he wrote
upon science like a Lord Chancellor.
The second thing I should like to observe is, that as the revolt against
the purely subjective methods grew in extent and influence it passed to
the opposite extreme, which eventually became only less deleterious to
the interests of science than was the bondage of authority, and
addiction to _a priori_ methods, from which the revolt had set her free.
For, without here waiting to trace the history of this matter in detail,
I think it ought now to be manifest to everyone who studies it, that up
to the commencement of the present century the progress of science in
general, and of natural history in particular, was seriously retarded by
what may be termed the Bugbear of Speculation. Fully awakened to the
dangers of web-spinning from the ever-fertile resources of their own
inner consciousness, naturalists became more and more abandoned to the
idea that their science ought to consist in a mere observation of facts,
or tabulation of phenomena, without attempt at theorizing upon their
philosophical import. If the facts and phenomena presented any such
import, that was an affair for men of letters to deal with; but, as men
of science, it was _their_ duty to avoid the seductive temptations of
the world, the flesh, and the devil, in the form of speculation,
deduction, and generalization.
I do not allege that this ideal of natural history was either absolute
or universal; but there can be no question that it was both orthodox and
general. Even Linnæus was express in his limitations of true scientific
work in natural history to the collecting and arranging of species of
plants and animals. In accordance with this view, the _status_ of a
botanist or a zoologist was estimated by the number of specific names,
natural habitats, &c., which he could retain in his memory, rather than
by any evidences which he might give of intellectual powers in the way
of constructive thought. At the most these powers might legitimately
exercise themselves only in the direction of taxonomic work; and if a
Hales, a Haller, or a Hunter obtained any brilliant results in the way
of observation and experiment, their merit was taken to consist in the
discovery of facts _per se_: not in any endeavours they might make in
the way of combining their facts under general principles. Even as late
in the day as Cuvier this ideal was upheld as the strictly legitimate
one for a naturalist to follow; and although Cuvier himself was far from
being always loyal to it, he leaves no doubt regarding the estimate in
which he held the still greater deviations of his colleagues, St.
Hilaire and Lamarck.
Now, these traditional notions touching the severance between the facts
of natural history and the philosophy of it, continued more or less to
dominate the minds of naturalists until the publication of the _Origin
of Species_, in 1859. Then it was that an epoch was marked in this
respect, as in so many other respects where natural history is
concerned. For, looking to the enormous results which followed from a
deliberate disregard of such traditional canons by Darwin, it has long
since become impossible for naturalists, even of the strictest sect, not
to perceive that their previous bondage to the law of a mere ritual has
been for ever superseded by what verily deserves to be regarded as a new
dispensation. Yet it cannot be said, or even so much as suspected, that
Darwin's method in any way resembled that of pre-scientific days, the
revolt against which led to the straight-laced--and for a long time most
salutary--conceptions of method that we have just been noticing. Where,
then, is the difference? To me it seems that the difference is as
follows; and, if so, that not the least of our many obligations to
Darwin as the great organizer of biological science arises from his
having clearly displayed the true principle which ought to govern
biological research.
To begin with, he nowhere loses sight of the primary distinction between
fact and theory; so that, thus far, he loyally follows the spirit of
revolt against subjective methods. But, while always holding this
distinction clearly in view, his idea of the scientific use of facts is
plainly that of furnishing legitimate material for the construction of
theories. Natural history is not to him an affair of the herbarium or
the cabinet. The collectors and the species-framers are, as it were, his
diggers of clay and makers of bricks: even the skilled observers and the
trained experimentalists are his mechanics. Valuable as the work of all
these men is in itself, its principal value, as he has finally
demonstrated, is that which it acquires in rendering possible the work
of the architect. Therefore, although he has toiled in all the trades
with his own hands, and in each has accomplished some of the best work
that has ever been done, the great difference between him and most of
his predecessors consists in this,--that while to them the discovery or
accumulation of facts was an end, to him it is the means. In their eyes
it was enough that the facts should be discovered and recorded. In his
eyes the value of facts is due to their power of guiding the mind to a
further discovery of principles. And the extraordinary success which
attended his work in this respect of _generalization_ immediately
brought natural history into line with the other inductive sciences,
behind which, in this most important of all respects, she has so
seriously fallen. For it was the _Origin of Species_ which first clearly
revealed to naturalists as a class, that it was the duty of their
science to take as its motto, what is really the motto of natural
science in general,
Felix qui potuit rerum cognoscere causas.
Not facts, then, or phenomena, but causes or principles, are the
ultimate objects of scientific quest. It remains to ask, How ought this
quest to be prosecuted?
Well, in the second place, Darwin has shown that next only to the
importance of clearly distinguishing between facts and theories on the
one hand, and of clearly recognising the relation between them on the
other, is the importance of not being scared by the Bugbear of
Speculation. The spirit of speculation is the same as the spirit of
science, namely, as we have just seen, a desire to know the causes of
things. The _hypotheses non fingo_ of Newton, if taken to mean what it
is often understood as meaning, would express precisely the opposite
spirit from that in which all scientific research must necessarily take
its origin. For if it be causes or principles, as distinguished from
facts or phenomena, that constitute the final aim of scientific
research, obviously the advancement of such research can be attained
only by the framing of hypotheses. And to frame hypotheses is to
speculate.
Therefore, the difference between science and speculation is not a
difference of spirit; nor, thus far, is it a difference of method. The
only difference between them is in the subsequent process of verifying
hypotheses. For while speculation, in its purest form, is satisfied to
test her explanations only by the degree in which they accord with our
subjective ideas of probability--or with the "Illative Sense" of
Cardinal Newman,--science is not satisfied to rest in any explanation as
final until it shall have been fully verified by an appeal to objective
proof. This distinction is now so well and so generally appreciated that
I need not dwell upon it. Nor need I wait to go into any details with
regard to the so-called canons of verification. My only object is to
make perfectly clear, first, that in order to have any question to put
to the test of objective verification, science must already have so far
employed the method of speculation as to have framed a question to be
tested; and, secondly, that the point where science parts company with
speculation is the point where this testing process begins.
Now, if these things are so, there can be no doubt that Darwin was
following the truest method of inductive research in allowing any amount
of latitude to his speculative thought in the direction of scientific
theorizing. For it follows from the above distinctions that the danger
of speculation does not reside in the width of its range, or even in the
impetuosity of its vehemence. Indeed, the wider its reach, and the
greater its energy, the better will it be for the interests of science.
The only danger of speculation consists in its momentum being apt to
carry away the mind from the more laborious work of adequate
verification; and therefore a true scientific judgment consists in
giving a free rein to speculation on the one hand, while holding ready
the break of verification with the other. Now, it is just because Darwin
did both these things with so admirable a judgment, that he gave the
world of natural history so good a lesson as to the most effectual way
of driving the chariot of science.
This lesson we have now all more or less learnt to profit by. Yet no
other naturalist has proved himself so proficient in holding the balance
true. For the most part, indeed, they have now all ceased to confound
the process of speculation _per se_ with the danger of inadequate
verification; and therefore the old ideal of natural history as
concerned merely with collecting species, classifying affinities, and,
in general, tabulating facts, has been well-nigh universally
superseded. But this great gain has been attended by some measure of
loss. For while not a few naturalists have since erred on the side of
insufficiently distinguishing between fully verified principles of
evolution and merely speculative deductions therefrom, a still larger
number have formed for themselves a Darwinian creed, and regard any
further theorizing on the subject of evolution as _ipso facto_
unorthodox.
Having occupied the best years of my life in closely studying the
literature of Darwinism, I shall endeavour throughout the following
pages to avoid both these extremes. No one in this generation is able to
imitate Darwin, either as an observer or a generalizer. But this does
not hinder that we should all so far endeavour to follow his _method_,
as always to draw a clear distinction, not merely between observation
and deduction, but also between degrees of verification. At all events,
my own aim will everywhere be to avoid dogmatism on the one hand, and
undue timidity as regards general reasoning on the other. For everything
that is said justification will be given; and, as far as prolonged
deliberation has enabled me to do so, the exact value of such
justification will be rendered by a statement of at least the main
grounds on which it rests. The somewhat extensive range of the present
treatise, however, will not admit of my rendering more than a small
percentage of the facts which in each case go to corroborate the
conclusion. But although a great deal must thus be necessarily lost on
the one side, I am disposed to think that more will be gained on the
other, by presenting, in a terser form than would otherwise be
possible, the whole theory of organic evolution as I believe that it
will eventually stand. My endeavour, therefore, will be to exhibit the
general structure of this theory in what I take to be its strictly
logical form, rather than to encumber any of its parts by a lengthy
citation of facts. Following this method, I shall in each case give only
what I consider the main facts for and against the positions which have
to be argued; and in most cases I shall arrange the facts in two
divisions, namely, first those of largest generality, and next a few of
the most special character that can be found.
As explained in the Preface, the present instalment of the treatise is
concerned with the theory of evolution, from the appearance of the
_Origin of Species_ in 1859, to the death of its author in 1882; while
the second part will be devoted to the sundry post-Darwinian questions
which have arisen in the subsequent decade. To the possible criticism
that a disproportionate amount of space will thus be allotted to a
consideration of these post-Darwinian questions, I may furnish in
advance the following reply.
In the first place, besides the works of Darwin himself, there are a
number of others which have already and very admirably expounded the
evidences, both of organic evolution as a fact, and of natural selection
as a cause. Therefore, in the present treatise it seemed needless to go
beyond the ground which was covered by my original lectures, namely, a
condensed and connected, while at the same time a critical statement of
the main evidences, and the main objections, which have thus far been
published with reference to the distinctively Darwinian theory. Indeed
while re-casting this portion of my lectures for the present
publication, I have felt that criticism might be more justly urged from
the side of impatience at a reiteration of facts and arguments already
so well known. But while endeavouring, as much as possible, to avoid
overlapping the previous expositions, I have not carried this attempt to
the extent of damaging my own, by omitting any of the more important
heads of evidence; and I have sought to invest the latter with some
measure of novelty by making good what appears to me a deficiency which
has hitherto obtained in the matter of pictorial illustration. In
particular, there will be found a tolerably extensive series of
woodcuts, serving to represent the more important products of artificial
selection. These, like all the other original illustrations, have been
drawn either direct from nature or from a comparative study of the best
authorities. Nevertheless, I desire it to be understood that the first
part of this treatise is intended to retain its original character, as a
merely educational exposition of Darwinian teaching--an exposition,
therefore, which, in its present form, may be regarded as a compendium,
or hand-book, adapted to the requirements of a general reader, or
biological student as distinguished from those of a professed
naturalist.
The case, however, is different with the second instalment, which will
be published at no very distant date. Here I have not followed with
nearly so much closeness the material of my original lectures. On the
contrary, I have had in view a special class of readers; and, although I
have tried not altogether to sacrifice the more general class, I shall
desire it to be understood that I am there appealing to naturalists who
are specialists in Darwinism. One must say advisedly, naturalists who
are specialists in Darwinism, because, while the literature of Darwinism
has become a department of science in itself, there are nowadays many
naturalists who, without having paid any close attention to the subject,
deem themselves entitled to hold authoritative opinions with regard to
it. These men may have done admirable work in other departments of
natural history, and yet their opinions on such matters as we shall
hereafter have to consider may be destitute of value. As there is no
necessary relation between erudition in one department of science and
soundness of judgment in another, the mere fact that a man is
distinguished as a botanist or zoologist does not in itself qualify him
as a critic where specially Darwinian questions are concerned. Thus it
happens now, as it happened thirty years ago, that highly distinguished
botanists and zoologists prove themselves incapable as judges of general
reasoning. It was Darwin's complaint that for many years nearly all his
scientific critics either could not, or would not, understand what he
had written--and this even as regarded the fundamental principles of his
theory, which with the utmost clearness he had over and over again
repeated. Now the only difference between such naturalists and their
successors of the present day is, that the latter have grown up in a
Darwinian environment, and so, as already remarked, have more or less
thoughtlessly adopted some form of Darwinian creed. But this scientific
creed is not a whit less dogmatic and intolerant than was the more
theological one which it has supplanted; and while it usually
incorporates the main elements of Darwin's teaching, it still more
usually comprises gross perversions of their consequences. All this I
shall have occasion more fully to show in subsequent parts of the
present work; and allusion is made to the matter here merely for the
sake of observing that in future I shall not pay attention to
unsupported expressions of opinion from any quarter: I shall consider
only such as are accompanied with some statement of the grounds upon
which the opinion is held. And, even as thus limited, I do not think it
will be found that the following exposition devotes any disproportional
amount of attention to the contemporary movements of Darwinian thought,
seeing, as we shall see, how active scientific speculation has been in
the field of Darwinism since the death of Mr. Darwin.
* * * * *
Leaving, then, these post-Darwinian questions to be dealt with
subsequently, I shall now begin a systematic _résumé_ of the evidences
in favour of the Darwinian theory, as this was left to the world by
Darwin himself.
There is a great distinction to be drawn between the fact of evolution
and the manner of it, or between the evidence of evolution as having
taken place somehow, and the evidence of the causes which have been
concerned in the process. This most important distinction is frequently
disregarded by popular writers on Darwinism; and, therefore, in order to
mark it as strongly as possible, I will effect a complete separation
between the evidence which we have of evolution as a fact, and the
evidence which we have as to its method. In other words, not until I
shall have fully considered the evidence of organic evolution as a
process which somehow or another _has_ taken place, will I proceed to
consider _how_ it has taken place, or the causes which Darwin and others
have suggested as having probably been concerned in this process.
Confining, then, our attention in the first instance to a proof of
evolution considered as a fact, without any reference at all to its
method, let us begin by considering the antecedent standing of the
matter.
* * * * *
First of all we must clearly recognise that there are only two
hypotheses in the field whereby it is possible so much as to suggest an
explanation of the origin of species. Either all the species of plants
and animals must have been supernaturally created, or else they must
have been naturally evolved. There is no third hypothesis possible; for
no one can rationally suggest that species have been eternal.
Next, be it observed, that the theory of a continuous transmutation of
species is not logically bound to furnish a full explanation of _all_
the natural causes which it may suppose to have been at work. The
radical distinction between the two theories consists in the one
assuming an immediate action of some supernatural or inscrutable cause,
while the other assumes the immediate action of natural--and therefore
of possibly discoverable--causes. But in order to sustain this latter
assumption, the theory of descent is under no logical necessity to
furnish a full proof of _all_ the natural causes which may have been
concerned in working out the observed results. We do not know the
natural causes of many diseases; but yet no one nowadays thinks of
reverting to any hypothesis of a supernatural cause, in order to explain
the occurrence of any disease the natural causation of which is obscure.
The science of medicine being in so many cases able to explain the
occurrence of disease by its hypothesis of natural causes, medical men
now feel that they are entitled to assume, on the basis of a wide
analogy, and therefore on the basis of a strong antecedent presumption,
that all diseases are due to natural causes, whether or not in
particular cases such causes happen to have been discovered. And from
this position it follows that medical men are not logically bound to
entertain any supernatural theory of an obscure disease, merely because
as yet they have failed to find a natural theory. And so it is with
biologists and their theory of descent. Even if it be fully proved to
them that the causes which they have hitherto discovered, or suggested,
are inadequate to account for all the facts of organic nature, this
would in no wise logically compel them to vacate their theory of
evolution, in favour of the theory of creation. All that it would so
compel them to do would be to search with yet greater diligence for the
natural causes still undiscovered, but in the existence of which they
are, by their independent evidence in favour of the theory, bound to
believe.
In short, the issue is not between the theory of a supernatural cause
and the theory of any one particular natural cause, or set of
causes--such as natural selection, use, disuse, and so forth. The issue
thus far--or where only the _fact_ of evolution is concerned--is between
the theory of a supernatural cause as operating immediately in
numberless acts of special creation, and the theory of natural causes as
a whole, whether these happen, or do not happen, to have been hitherto
discovered.
This much by way of preliminaries being understood, we have next to
notice that whichever of the two rival theories we choose to entertain,
we are not here concerned with any question touching the origin of life.
We are concerned only with the origin of particular forms of life--that
is to say, with the origin of species. The theory of descent starts from
life as a _datum_ already granted. How life itself came to be, the
theory of descent, as such, is not concerned to show. Therefore, in the
present discussion, I will take the existence of life as a fact which
does not fall within the range of our present discussion. No doubt the
question as to the origin of life is in itself a deeply interesting
question, and although in the opinion of most biologists it is a
question which we may well hope will some day fall within the range of
science to answer, at present, it must be confessed, science is not in a
position to furnish so much as any suggestion upon the subject; and
therefore our wisdom as men of science is frankly to acknowledge that
such is the case.
* * * * *
We are now in a position to observe that the theory of organic evolution
is strongly recommended to our acceptance on merely antecedent grounds,
by the fact that it is in full accordance with what is known as the
principle of continuity. By the principle of continuity is meant the
uniformity of nature, in virtue of which the many and varied processes
going on in nature are due to the same kind of method, i. e. the method
of natural causation. This conception of the uniformity of nature is
one that has only been arrived at step by step through a long and
arduous course of human experience in the explanation of natural
phenomena. The explanations of such phenomena which are first given are
always of the supernatural kind; it is not until investigation has
revealed the natural causes which are concerned that the hypotheses of
superstition give way to those of science. Thus it follows that the
hypotheses of superstition which are the latest in yielding to the
explanations of science, are those which refer to the more recondite
cases of natural causation; for here it is that methodical investigation
is longest in discovering the natural causes. Thus it is only by degrees
that fetishism is superseded by what now appears a common-sense
interpretation of physical phenomena; that exorcism gives place to
medicine; alchemy to chemistry; astrology to astronomy; and so forth.
Everywhere the miraculous is progressively banished from the field of
explanation by the advance of scientific discovery; and the places where
it is left longest in occupation are those where the natural causes are
most intricate or obscure, and thus present the greatest difficulty to
the advancing explanations of science. Now, in our own day there are but
very few of these strongholds of the miraculous left. Nearly the whole
field of explanation is occupied by naturalism, so that no one ever
thinks of resorting to supernaturalism except in the comparatively few
cases where science has not yet been able to explore the most obscure
regions of causation. One of these cases is the origin of life; and,
until quite recently, another of these cases was the origin of species.
But now that a very reasonable explanation of the origin of species has
been offered by science, it is but in accordance with all previous
historical analogies that many minds should prove themselves unable all
at once to adjust themselves to the new ideas, and thus still linger
about the more venerable ideas of supernaturalism. But we are now in
possession of so many of these historical analogies, that all minds with
any instincts of science in their composition have grown to distrust, on
merely antecedent grounds, any explanation which embodies a miraculous
element. Such minds have grown to regard all these explanations as mere
expressions of our own ignorance of natural causation; or, in other
words, they have come to regard it as an _a priori_ truth that nature is
everywhere uniform in respect of method or causation; that the reign of
law universal; the principle of continuity ubiquitous.
Now, it must be obvious to any mind which has adopted this attitude of
thought, that the scientific theory of natural descent is recommended by
an overwhelming weight of antecedent presumption, as against the
dogmatic theory of supernatural design.
To begin with, we must remember that the fact of evolution--or, which is
the same thing, the fact of continuity in natural causation--has now
been unquestionably proved in so many other and analogous departments of
nature, that to suppose any interruption of this method as between
species and species becomes, on grounds of such analogy alone, well-nigh
incredible. For example, it is now a matter of demonstrated fact that
throughout the range of _inorganic_ nature the principles of evolution
have obtained. It is no longer possible for any one to believe with our
forefathers that the earth's surface has always existed as it now
exists. For the science of geology has proved to demonstration that seas
and lands are perpetually undergoing gradual changes of relative
positions--continents and oceans supplanting each other in the course of
ages, mountain-chains being slowly uplifted, again as slowly denuded,
and so forth. Moreover, and as a closer analogy, within the limits of
animate nature we know it is the universal law that every individual
life undergoes a process of gradual development; and that breeds, races,
or strains, may be brought into existence by the intentional use of
natural processes--the results bearing an unmistakeable resemblance to
what we know as natural species. Again, even in the case of natural
species themselves, there are two considerations which present enormous
force from an antecedent point of view. The first is that organic forms
are only then recognised as species when intermediate forms are absent.
If the intermediate forms are actually living, or admit of being found
in the fossil state, naturalists forthwith regard the whole series as
varieties, and name all the members of it as belonging to the same
species. Consequently it becomes obvious that naturalists, in their work
of naming species, may only have been marking out the cases where
intermediate or connecting forms have been lost to observation. For
example, here we have a diagram representing a very unusually complete
series of fossil shells, which within the last few years has been
unearthed from the Tertiary lake basins of Slavonia. Before the series
was completed, some six or eight of the then disconnected forms were
described as distinct species; but as soon as the connecting forms were
found--showing a progressive modification from the older to the newer
beds,--the whole were included as varieties of one species.
[Illustration: FIG. 1.--Successive forms of Paludina, from the
Tertiary deposits of Slavonia (after Neumayr).]
Of course, other cases of the same kind might be adduced, and therefore,
as just remarked, in their work of naming species naturalists may only
have been marking out the cases where intermediate forms have been lost
to observation. And this possibility becomes little less than a
certainty when we note the next consideration which I have to adduce,
namely, that in all their systematic divisions of plants and animals in
groups higher than species--such as genera, families, orders, and the
rest--naturalists have at all times recognised the fact that the one
shades off into the other by such imperceptible gradations, that it is
impossible to regard such divisions as other than conventional. It is
important to remember that this fact was fully recognised before the
days of Darwin. In those days the scientifically orthodox doctrine was,
that although species were to be regarded as fixed units, bearing the
stamp of a special creation, all the higher taxonomic divisions were
to be considered as what may be termed the artificial creation of
naturalists themselves. In other words, it was believed, and in many
cases known, that if we could go far enough back in the history of the
earth, we should everywhere find a tendency to mutual approximation
between allied _groups of species_; so that, for instance, birds and
reptiles would be found to be drawing nearer and nearer together, until
eventually they would seem to become fused in a single type; that the
existing distinctions between herbivorous and carnivorous mammals
would be found to do likewise; and so on with all the larger
group-distinctions, at any rate within the limits of the same
sub-kingdoms. But although naturalists recognised this even in the
pre-Darwinian days, they stoutly believed that a great exception was to
be made in the case of species. These, the lowest or initial members of
their taxonomic series, they supposed to be permanent--the miraculously
created units of organic nature. Now, all that I have at present to
remark is, that this pre-Darwinian exception which was made in favour of
species to the otherwise recognised principle of gradual change, was an
exception which can at no time have been recommended by any antecedent
considerations. At all times it stood out of analogy with the principle
of continuity; and, as we shall fully find in subsequent chapters, it is
now directly contradicted by all the facts of biological science.
There remains one other fact of high generality to which prominent
attention should be drawn from the present, or merely antecedent, point
of view. On the theory of special creation no reason can be assigned why
distinct specific types should present any correlation, either in time
or in space, with their nearest allies; for there is evidently no
conceivable reason why any given species, A, should have been specially
created on the same area and at about the same time as its nearest
representative, B,--still less, of course, that such should be a general
rule throughout all the thousands and millions of species which have
ever inhabited the earth. But, equally of course, on the theory of a
natural evolution this is so necessary a consequence, that if no
correlation of such a two-fold kind were observable, the theory would be
negatived. Thus the question whether there be any indication of such a
two-fold correlation may be regarded as a test-question as between the
two theories; for although the vast majority of extinct species have
been lost to science, there are a countless number of existing species
which furnish ample material for answering the question. And the answer
is so unequivocal that Mr. Wallace, who is one of our greatest
authorities on geographical distribution, has laid it down as a general
law, applicable to all the departments of organic nature, that, so far
as observation can extend, "every species has come into existence
coincident both in space and time with a pre-existing and closely allied
species." As it appears to me that the significance of these words
cannot be increased by any comment upon them, I will here bring this
introductory chapter to a close.
CHAPTER II.
CLASSIFICATION.
The first line of direct evidence in favour of organic evolution which I
shall open is that which may be termed the argument from Classification.
It is a matter of observable fact that different forms of plants and
animals present among themselves more or less pronounced resemblances.
From the earliest times, therefore, it has been the aim of philosophical
naturalists to classify plants and animals in accordance with these
resemblances. Of course the earliest attempts at such classification
were extremely crude. The oldest of these attempts with which we are
acquainted--namely, that which is presented in the books of Genesis and
Leviticus--arranges the whole vegetable kingdom in three simple
divisions of Grass, Herbs, and Trees; while the animal kingdom is
arranged with almost equal simplicity with reference, first to habitats
in water, earth, or air, and next as to modes of progression. These, of
course, were what may be termed common-sense classifications, having
reference merely to external appearances and habits of life. But when
Aristotle laboriously investigated the comparative anatomy of animals,
he could not fail to perceive that their entire structures had to be
taken into account in order to classify them scientifically; and, also,
that for this purpose the internal parts were of quite as much
importance as the external. Indeed, he perceived that they were of
greatly more importance in this respect, inasmuch as they presented so
many more points for comparison; and, in the result, he furnished an
astonishingly comprehensive, as well as an astonishingly accurate
classification of the larger groups of the animal kingdom. On the other
hand, classification of the vegetable kingdom continued pretty much as
it had been left by the book of Genesis--all plants being divided into
three groups, Herbs, Shrubs, and Trees. Nor was this primitive state of
matters improved upon till the sixteenth century, when Gesner
(1516-1565), and still more Cæsalpino (1519-1603), laid the foundations
of systematic botany.
But the more that naturalists prosecuted their studies on the anatomy of
plants and animals, the more enormously complex did they find the
problem of classification become. Therefore they began by forming what
are called artificial systems, in contradistinction to natural systems.
An artificial system of classification is a system based on the more or
less arbitrary selection of some one part, or set of parts; while a
natural classification is one that is based upon a complete knowledge of
all the structures of all the organisms which are classified.
Thus, the object of classification has been that of arranging organisms
in accordance with their natural affinities, by comparing organism with
organism, for the purpose of ascertaining which of the constituent
organs are of the most invariable occurrence, and therefore of the most
typical signification. A porpoise, for instance, has a large number of
teeth, and in this feature resembles most fish, while it differs from
all mammals. But it also gives suck to its young. Now, looking to these
two features alone, should we say that a porpoise ought to be classed as
a fish or as a mammal? Assuredly as a mammal; because the number of
teeth is a very variable feature both in fish and mammals, whereas the
giving of suck is an invariable feature among mammals, and occurs
nowhere else in the animal kingdom. This, of course, is chosen as a very
simple illustration. Were all cases as obvious, there would be but
little distinction between natural and artificial systems of
classification. But it is because the lines of natural affinity are, as
it were, so interwoven throughout the organic world, and because there
is, in consequence, so much difficulty in following them, that
artificial systems have to be made in the first instance as feelers
towards eventual discovery of the natural system. In other words, while
forming their artificial systems of classification, it has always been
the aim of naturalists--whether consciously or unconsciously--to admit
as the bases of their systems those characters which, in the then state
of their knowledge, seemed most calculated to play an important part in
the eventual construction of the natural system. If we were dealing with
the history of classification, it would here be interesting to note how
the course of it has been marked by gradual change in the principles
which naturalists adopted as guides to the selection of characters on
which to found their attempts at a natural classification. Some of these
changes, indeed, I shall have to mention later on; but at present what
has to be specially noted is, that through all these changes of theory
or principle, and through all the ever-advancing construction of their
taxonomic science, naturalists themselves were unable to give any
intelligible reason for the faith that was in them--or the faith that
over and above the artificial classifications which were made for the
mere purpose of cataloguing the living library of organic nature, there
was deeply hidden in nature itself a truly natural classification, for
the eventual discovery of which artificial systems might prove to be of
more or less assistance.
Linnæus, for example, expressly says--"You ask me for the characters of
the natural orders; I confess that I cannot give them." Yet he maintains
that, although he cannot define the characters, he knows, by a sort of
naturalist's instinct, what in a general way will subsequently be found
to be the organs of most importance in the eventual grouping of plants
under a natural system. "I will not give my reasons for the distribution
of the natural orders which I have published," he said: "you, or some
other person, after twenty or after fifty years, will discover them, and
see that I was right."
Thus we perceive that in forming their provisional or artificial
classifications, naturalists have been guided by an instinctive belief
in some general principle of natural affinity, the character of which
they have not been able to define; and that the structures which they
selected as the bases of their classifications when these were
consciously artificial, were selected because it seemed that they were
the structures most likely to prove of use in subsequent attempts at
working out the natural system.
This general principle of natural affinity, of which all naturalists
have seen more or less well-marked evidence in organic nature, and after
which they have all been feeling, has sometimes been regarded as
natural, but more often as supernatural. Those who regarded it as
supernatural took it to consist in a divine ideal of creation according
to types, so that the structural affinities of organisms were to them
expressions of an archetypal plan, which might be revealed in its
entirety when all organisms on the face of the earth should have been
examined. Those, on the other hand, who regarded the general principle
of affinity as depending on some natural causes, for the most part
concluded that these must have been utilitarian causes; or, in other
words, that the fundamental affinities of structure must have depended
upon fundamental requirements of function. According to this view, the
natural classification would eventually be found to stand upon a basis
of physiology. Therefore all the systems of classification up to the
earlier part of the present century went upon the apparent axiom, that
characters which are of most importance to the organisms presenting them
must be characters most indicative of natural affinities. But the truth
of the matter was eventually found to be otherwise. For it was
eventually found that there is absolutely no correlation between these
two things; that, therefore, it is a mere chance whether or not organs
which are of importance to organisms are likewise of importance as
guides to classification; and, in point of fact, that the general
tendency in this matter is towards an inverse instead of a direct
proportion. More often than not, the greater the value of a structure
for the purpose of indicating natural affinities, the less is its value
to the creatures presenting it.
Enough has now been said to show three things. First, that long before
the theory of descent was entertained by naturalists, naturalists
perceived the fact of natural affinities, and did their best to
construct a natural system of classification for the purpose of
expressing such affinities. Second, that naturalists had a kind of
instinctive belief in some one principle running through the whole
organic world, which thus served to bind together organisms in groups
subordinate to groups--that is, into species, genera, orders, families,
classes, sub-kingdoms, and kingdoms. Third, that they were not able to
give any very intelligible reason for this faith that was in them;
sometimes supposing the principle in question to be that of a
supernatural plan of organization, sometimes regarding it as dependent
on conditions of physiology, and sometimes not attempting to account for
it at all.
Of course it is obvious that the theory of descent furnishes the
explanation which is required. For it is now evident to evolutionists,
that although these older naturalists did not know what they were doing
when they were tracing these lines of natural affinity, and thus helping
to construct a natural classification--I say it is now evident to
evolutionists that these naturalists were simply tracing the lines of
genetic relationship. The great principle pervading organic nature,
which was seen so mysteriously to bind the whole creation together as in
a nexus of organic affinity, is now easily understood as nothing more or
less than the principle of Heredity. Let us, therefore, look a little
more closely at the character of this network, in order to see how far
it lends itself to this new interpretation.
The first thing that we have to observe about the nexus is, that it is a
nexus--not a single line, or even a series of parallel lines. In other
words, some time before the theory of descent was seriously entertained,
naturalists for the most part had fully recognised that it was
impossible to arrange either plants or animals, with respect to their
mutual affinities, in a ladder-like series (as was supposed to be the
type of classification by the earlier systematists), or even in map-like
groups (as was supposed to be the type by Linnæus). And similarly, also,
with respect to grades of organization. In the case of the larger
groups, indeed, it is usually possible to say that the members of this
group as a whole are more highly organized than the members of that
group as a whole; so that, for instance, we have no hesitation in
regarding the Vertebrata as more highly organized than the Invertebrata,
Birds than Reptiles, and so on. But when we proceed to smaller
subdivisions, such as genera and species, it is usually impossible to
say that the one type is more highly organized than another type. A
horse, for instance, cannot be said to be more highly organized than a
zebra or an ass; although the entire horse-genus is clearly a more
highly organized type than any genus of animal which is not a mammal.
In view of these facts, therefore, the system of classification which
was eventually arrived at before the days of Darwin, was the system
which naturalists likened to a tree; and this is the system which all
naturalists now agreed upon as the true one. According to this system, a
short trunk may be taken to represent the lowest organisms which cannot
properly be termed either plants or animals. This short trunk soon
separates into two large trunks, one of which represents the vegetable
and the other the animal kingdom. Each of these trunks then gives off
large branches signifying classes, and these give off smaller, but more
numerous branches, signifying families, which ramify again into orders,
genera, and finally into the leaves, which may be taken to represent
species. Now, in such a representative tree of life, the height of any
branch from the ground may be taken to indicate the grade of
organization which the leaves, or species, present; so that, if we
picture to ourselves such a tree, we may understand that while there is
a general advance of organization from below upwards, there are many
deviations in this respect. Sometimes leaves growing on the same branch
are growing at a different level--especially, of course, if the branch
be a large one, corresponding to a class or sub-kingdom. And sometimes
leaves growing on different branches are growing at the same level: that
is to say, although they represent species belonging to widely divergent
families, orders, or even classes, it cannot be said that the one
species is more highly organized than the other.
Now, this tree-like arrangement of species in nature is an arrangement
for which Darwin is not responsible. For, as we have seen, the detecting
of it has been due to the progressive work of naturalists for centuries
past; and even when it was detected, at about the commencement of the
present century, naturalists were confessedly unable to explain the
reason of it, or what was the underlying principle that they were
engaged in tracing when they proceeded ever more and more accurately to
define these ramifications of natural affinity. But now, as just
remarked, we can clearly perceive that this underlying principle was
none other than Heredity as expressed in family likeness,--likeness,
therefore, growing progressively more unlike with remoteness of
ancestral relationship. For thus only can we obtain any explanation of
the sundry puzzles and apparent paradoxes, which a working out of their
natural classifications revealed to botanists and zoologists during the
first half of the present century. It will now be my endeavour to show
how these puzzles and paradoxes are all explained by the theory that
natural affinities are merely the expression of genetic affinities.
First of all, and from the most general point of view, it is obvious
that the tree-like system of classification, which Darwin found already
and empirically worked out by the labours of his predecessors, is as
suggestive as anything could well be of the fact of genetic
relationship. For this is the form that every tabulation of family
pedigree must assume; and therefore the mere fact that a scientific
tabulation of natural affinities was eventually found to take the form
of a tree, is in itself highly suggestive of the inference that such a
tabulation represents a _family_ tree. If all species were separately
created, there can be no assignable reason why the ideas of earlier
naturalists touching the form which a natural classification would
eventually assume should not have represented the truth--why, for
example, it should not have assumed the form of a ladder (as was
anticipated in the seventeenth century), or of a map (as was anticipated
in the eighteenth), or, again, of a number of wholly unrelated lines,
circles, &c. (as certain speculative writers of the present century have
imagined). But, on the other hand, if all species were separately and
independently created, it becomes virtually incredible that we should
everywhere observe this progressive arborescence of characters common to
larger groups into more and more numerous, and more and more delicate,
ramifications of characters distinctive only of smaller and smaller
groups. A man would be deemed insane if he were to attribute the origin
of every branch and every twig of a real tree to a separate act of
special creation; and although we have not been able to witness the
growth of what we may term in a new sense the Tree of Life, the
structural relations which are now apparent between its innumerable
ramifications bear quite as strong a testimony to the fact of their
having been due to an organic growth, as is the testimony furnished by
the branches of an actual tree.
Or, to take another illustration. Classification of organic forms, as
Darwin, Lyell, and Häckel have pointed out, strongly resembles the
classification of languages. In the case of languages, as in the case of
species, we have genetic affinities strongly marked; so that it is
possible to some extent to construct a Language-tree, the branches of
which shall indicate, in a diagrammatic form, the progressive divergence
of a large group of languages from a common stock. For instance, Latin
may be regarded as a fossil language, which has given rise to a group of
living languages--Italian, Spanish, French, and, to a large extent,
English. Now what would be thought of a philologist who should maintain
that English, French, Spanish, and Italian were all specially created
languages--or languages separately constructed by the Deity, and by as
many separate acts of inspiration communicated to the nations which now
speak them--and that their resemblance to the fossil form, Latin, must
be attributed to special design? Yet the evidence of the natural
transmutation of species is in one respect much stronger than that of
the natural transmutation of languages--in respect, namely, of there
being a vastly greater number of cases all bearing testimony to the fact
of genetic relationship.
But, quitting now this most general point of view--or the suggestive
fact that what we have before us is a _tree_--let us next approach this
tree for the purpose of examining its structure more in detail. When we
do this, the fact of next greatest generality which we find is as
follows.
In cases where a very old form of life has continued to exist
unmodified, so that by investigation of its anatomy we are brought back
to a more primitive type of structure than that of the newer forms
growing higher up _upon the same branch_, two things are observable. In
the first place, the old form is less differentiated than the newer
ones; and, in the next place, it is seen much more closely to resemble
types of structure belonging to some of the other and larger branches of
the tree. The organization of the older form is not only _simpler_; but
it is, as naturalists say, more _generalized_. It comprises within
itself characters belonging to its own branch, and also characters
belonging to neighbouring branches, or to the trunk from which allied
branches spring. Hence it becomes a general rule of classification, that
it is by the lowest, or by the oldest, forms of any two natural groups
that the affinities between the two groups admit of being best detected.
And it is obvious that this is just what ought to be the case on the
theory of descent with divergent modification; while, upon the
alternative theory of special creation, no reason can be assigned why
the lowest or the oldest types should thus combine the characters which
afterwards become severally distinctive of higher or newer types.
Again, I have already alluded to the remarkable fact that there is no
correlation between the value of structures to the organisms which
present them, and their value to the naturalist for the purpose of
tracing natural affinity; and I have remarked that up to the close of
the last century it was regarded as an axiom of taxonomic science, that
structures which are of most importance to the animals or plants
possessing them must likewise prove of most importance in any natural
system of classification. On this account, all attempts to discover the
natural classification went upon the supposition that such a direct
proportion must obtain--with the result that organs of most
physiological importance were chosen as the bases of systematic work.
And when, in the earlier part of the present century, De Candolle found
that instead of a direct there was usually an inverse proportion between
the functional and the taxonomic value of a structure, he was unable to
suggest any reason for this apparently paradoxical fact. For, upon the
theory of special creation, no reason can be assigned why organs of
least importance to organisms should prove of most importance as marks
of natural affinity. But on the theory of descent with progressive
modification the apparent paradox is at once explained. For it is
evident that organs of functional importance are, other things equal,
the organs which are most likely to undergo different modifications in
different lines of family descent, and therefore in time to have their
genetic relationships in these different lines obscured. On the other
hand, organs or structures which are of no functional importance are
never called upon to change in response to any change of habit, or to
any change in the conditions of life. They may, therefore, continue to
be inherited through many different lines of family descent, and thus
afford evidence of genetic relationship where such evidence fails to be
given by any of the structures of vital importance, which in the course
of many generations have been required to change in many ways according
to the varied experiences of different branches of the same family.
Here, then, we have an empirically discovered rule in the science of
classification, the _raison d'être_ of which we are at once able to
appreciate upon the theory of evolution, whereas no possible explanation
of why it should ever have become a rule could be furnished upon the
theory of special creation.
Here, again, is another empirically determined rule. The larger the
_number_, as distinguished from the _importance_, of structures which
are found common to different groups, the greater becomes their value as
guides to the determination of natural affinity. Or, as Darwin puts it,
"the value of an aggregate of characters, even when none are important,
alone explains the aphorism enunciated by Linnæus, namely, that the
characters do not give the genus, but the genus gives the characters;
for this seems founded on the appreciation of many trifling points of
resemblance, too slight to be defined[1]."
[1] _Origin of Species_, p. 367.
Now it is evident, without comment, of how much value aggregates of
characters ought to be in classification, if the ultimate meaning of
classification be that of tracing lines of pedigree; whereas, if this
ultimate meaning were that of tracing divine ideals manifested in
special creation, we can see no reason why single characters are not
such sure tokens of a natural arrangement as are aggregates of
characters, even though the latter be in every other respect
unimportant. For, on the special creation theory, we cannot explain why
an assemblage, say of four or five trifling characters, should have been
chosen to mark some unity of plan, rather than some one character of
functional importance, which would have served at least equally well any
such hypothetical purpose. On the other hand, as Darwin remarks, "we
care not how trifling a character may be--let it be the mere inflection
of the angle of the jaw, the manner in which an insect's wing is folded,
whether the skin be covered with hair or feathers--if it prevail
throughout many and different species, especially those having very
different habits of life, it assumes high value; for we can account for
its presence in so many forms, with such different habits, only by
inheritance from a common parent. We may err in this respect in regard
to single points of structure, but when several characters, let them be
ever so trifling, concur throughout a large group of beings having
different habits, we may feel almost sure, on the theory of descent,
that these characters have been inherited from a common ancestor; and
we know that such aggregated characters have especial value in
classification[2]."
[2] _Origin of Species_, p. 372.
It is true that even a single character, if found common to a large
number of forms, while uniformly absent from others, is also regarded by
naturalists as of importance for purposes of classification, although
they recognise it as of a value subordinate to that of aggregates of
characters. But this also is what we should expect on the theory of
descent. If even any one structure be found to run through a number of
animals presenting different habits of life, the readiest explanation of
the fact is to be found in the theory of descent; but this does not
hinder that if several such characters always occur together, the
inference of genetic relationship is correspondingly confirmed. And the
fact that before this inference was ever drawn, naturalists recognised
the value of single characters in proportion to their constancy, and the
yet higher value of aggregates of characters in proportion to their
number--this fact shows that in their work of classification naturalists
empirically observed the effects of a cause which we have now
discovered, to wit, hereditary transmission of characters through
ever-widening groups of changing species.
There is another argument which appears to tell strongly in favour of
the theory of descent. We have just seen that non-adaptive structures,
not being required to change in response to change of habits or
conditions of life, are allowed to persist unchanged through many
generations, and thus furnish exceptionally good guides in the science
of classification--or, according to our theory, in the work of tracing
lines of pedigree. But now, the converse of this statement holds equally
true. For it often happens that adaptive structures are required to
change in different lines of descent in analogous ways, in order to meet
analogous needs; and, when such is the case, the structures concerned
have to assume more or less close resemblances to one another, even
though they have severally descended from quite different ancestors. The
paddles of a whale, for instance, most strikingly resemble the fins of a
fish as to their outward form and movements; yet, on the theory of
descent, they must be held to have had a widely different parentage.
Now, in all such cases where there is thus what is called an analogous
(or adaptive) resemblance, as distinguished from what is called an
homologous (or anatomical) resemblance--in all such cases it is
observable that the similarities do not extend further into the
structure of the parts than it is necessary that they should extend, in
order that the structures should both perform the same functions. The
whole anatomy of the paddles of a whale is quite unlike that of the fins
of a fish--being, in fact, that of the fore-limb of a mammal. The
change, therefore, which the fore-limb has here undergone to suit it to
the aquatic habits of this mammal, is no greater than was required for
that purpose: the change has not extended to any one feature of
_anatomical_ significance. This, of course, is what we should expect on
the theory of descent with modification of ancestral characters; but on
the theory of special creation it is not intelligible why there should
always be so marked a distinction between resemblances as analogical or
adaptive, and resemblances as homological or of meaning in reference to
a natural classification. To take another and more detailed instance,
the Tasmanian wolf is an animal separated from true wolves in a natural
system of classification. Yet its jaws and teeth bear a strong general
resemblance to those of all the dog tribe, although there are
differences of anatomical detail. In particular, while the dogs all have
on each side of the upper jaw four pre-molars and two molars, the
Tasmanian wolf has three pre-molars and four molars. Now there is no
reason, so far as their common function of dealing with flesh is
concerned, why the teeth of the Tasmanian wolf should not have resembled
homologically as well as analogically the teeth of a true wolf; and
therefore we cannot assign any intelligible reason why, if all the
species of the dog genus were separately created with one pattern of
teeth, the unallied Tasmanian wolf should have been furnished with what
is practically the same pattern from a functional point of view, while
differing from a structural point of view. But, of course, on the theory
of descent with modification, we can well understand why similarities of
habit should have led to similarities of structural appearance of an
adaptive kind in different lines of descent, without there being any
trace of such real or anatomical similarities as could possibly point to
genetic relationship.
Lastly, to adduce the only remaining argument from classification which
I regard as of any considerable weight, naturalists have found it
necessary, while constructing their natural classifications, to set
great store on what Mr. Darwin calls "chains of affinities." Thus, for
instance, "nothing can be easier than to define a number of characters
common to all birds; but with crustaceans any such definition has
hitherto been found impossible. There are crustaceans at the opposite
ends of the series, which have hardly a character in common; yet the
species at both ends, from being plainly allied to others, and these to
others, and so onwards, can be recognised as unequivocally belonging to
this, and to no other class of the articulata[3]." Now it is evident
that this progressive modification of specific types--where it cannot be
said that the continuity of resemblance is anywhere broken, and yet
terminates in modification so great that but for the connecting links no
one could divine a natural relationship between the extreme members of
the series,--it is evident that such chains of affinity speak most
strongly in favour of a transmutation of the species concerned, while it
is impossible to suggest any explanation of the fact in terms of the
rival theory. For if all the links of such a chain were separately
forged by as many acts of special creation, we can see no reason why B
should resemble A, C resemble B, and so on, but with ever slight though
accumulating differences, until there is no resemblance at all between A
and Z.
[3] _Origin of Species_, pp. 368-9.
* * * * *
I hope enough has now been said to show that all the general principles
and particular facts appertaining to the natural classification of
plants and animals, are precisely what they ought to be according to the
theory of genetic descent; while no one of them is such as might
be--and, indeed, used to be--expected upon the theory of special
creation. Therefore, the only possible way in which all this uniform
body of direct evidence can be met by a supporter of the latter theory,
is by falling back upon the argument from ignorance. We do not know, it
may be said, what hidden reasons there may have been for following all
these general principles in the separate creation of specific types.
Now, it is evident that this is a form of argument which admits of being
brought against all the actual--and even all the possible--lines of
evidence in favour of evolution. Therefore I deem it desirable thus
early in our proceedings to place this argument from ignorance on its
proper logical footing.
If there were any independent evidence in favour of special creation as
a _fact_, then indeed the argument from ignorance might be fairly used
against any sceptical cavils regarding the _method_. In this way, for
example, Bishop Butler made a legitimate use of the argument from
ignorance when he urged that it is no reasonable objection against a
revelation, _otherwise accredited_, to show that it has been rendered in
a form, or after a method, which we should not have antecedently
expected. But he could not have legitimately employed this argument,
except on the supposition that he had some independent evidence in
favour of the revelation; for, in the absence of any such independent
evidence, appeal to the argument from ignorance would have become a mere
begging of the question, by simply assuming that a revelation had been
made. And thus it is in the present case. A man, of course, may quite
legitimately say, _Assuming that the theory of special creation is
true_, it is not for us to anticipate the form or method of the
process. But where the question is as to whether or not the theory _is_
true, it becomes a mere begging of this question to take refuge in the
argument from ignorance, or to represent in effect that there is no
question to be discussed. And if, when the form or method is
investigated, it be found everywhere charged with evidence in favour of
the theory of descent, the case becomes the same as that of a supposed
revelation, which has been discredited by finding that all available
evidence points to a natural growth. In short, the argument from
ignorance is in any case available only as a negative foil against
destructive criticism: in no case has it any positive value, or value of
a constructive kind. Therefore, if a theory on any subject is destitute
of positive evidence, while some alternative theory is in possession of
such evidence, the argument from ignorance can be of no logical use to
the former, even though it maybe of such use to the latter. For it is
only the possession of positive evidence which can furnish a logical
justification of the argument from ignorance: in the absence of such
evidence, even the negative value of the argument disappears, and it
then implies nothing more than the gratuitous assumption of a theory.
* * * * *
I will now sum up the various considerations which have occupied us
during the present chapter.
First of all we must take note that the classification of plants and
animals in groups subordinate to groups is not merely arbitrary, or
undertaken only for a matter of convenience and nomenclature--such, for
instance, as the classification of stars in constellations. On the
contrary, the classification of a naturalist differs from that of an
astronomer, in that the objects which he has to classify present
structural resemblances and structural differences in numberless
degrees; and it is the object of his classification to present a tabular
statement of these facts. Now, long before the theory of evolution was
entertained, naturalists became fully aware that these facts of
structural resemblances running through groups subordinate to groups
were really facts of nature, and not merely poetic imaginations of the
mind. No one could dissect a number of fishes without perceiving that
they were all constructed on one anatomical pattern, which differed
considerably from the equally uniform pattern on which all mammals were
constructed, even although some mammals bore an extraordinary
resemblance to fish in external form and habits of life. And similarly
with all the smaller divisions of the animal and vegetable kingdoms.
Everywhere investigation revealed the bonds of close structural
resemblances between species of the same genus, resemblance less close
between genera of the same family, resemblance still less close between
families of the same order, resemblance yet more remote between orders
of the same class, and resemblance only in fundamental features between
classes of the same sub-kingdom, beyond which limit all anatomical
resemblance was found to disappear--the different sub-kingdoms being
formed on wholly different patterns. Furthermore, in tracing all these
grades of structural relationship, naturalists were slowly led to
recognise that the form which a natural classification must eventually
assume would be that of a tree, wherein the constituent branches would
display a progressive advance of organization from below upwards.
Now we have seen that although this tree-like arrangement of natural
groups was as suggestive as anything could well be of all the forms o£
life being bound together by the ties of genetic relationship, such was
not the inference which was drawn from it. Dominated by the theory of
special creation, naturalists either regarded the resemblance of type
subordinate to type as expressive of divine ideals manifested in such
creation, or else contented themselves with investigating the facts
without venturing to speculate upon their philosophical import. But even
those naturalists who abstained from committing themselves to any theory
of archetypal plans, did not doubt that facts so innumerable and so
universal must have been due to some one co-ordinating principle--that,
even though they were not able to suggest what it was, there must have
been some hidden bond of connexion running through the whole of organic
nature. Now, as we have seen, it is manifest to evolutionists that this
hidden bond can be nothing else than heredity; and, therefore, that
these earlier naturalists, although they did not know what they were
doing, were really tracing the lines of genetic descent as revealed by
degrees of structural resemblance,--that the arborescent grouping of
organic forms which their labours led them to begin, and in large
measure to execute, was in fact a family tree of life.
Here, then, is the substance of the argument from classification. The
mere fact that all organic nature thus incontestably lends itself to a
natural arrangement of group subordinate to group, when due regard is
paid to degrees of anatomical resemblance--this mere fact of itself
tells so weightily in favour of descent with progressive modification in
different lines, that even if it stood alone it would be entitled to
rank as one of our strongest pieces of evidence. But, as we have seen,
it does not stand alone. When we look beyond this large and general fact
of all the innumerable forms of life being thus united in a tree-like
system by an unquestionable relationship of some kind, to those smaller
details in the science of classification which have been found most
useful as guides for this kind of research, then we find that all these
details, or empirically discovered rules, are exactly what we should
have expected them to be, supposing the real meaning of classification
to have been that of tracing lines of pedigree.
In particular, we have seen that the most archaic types are both simpler
in their organization and more generalized in their characters than are
the more recent types--a fact of which no explanation can be given on
the theory of special creation. But, upon the theory of natural
evolution, we can without difficulty understand why the earlier forms
should have been the simpler forms, and also why they should have been
the most generalized. For it is out of the older forms that the newer
must have grown; and, as they multiplied, they must have become more and
more differentiated.
Again, we have seen that there is no correlation between the importance
of any structure from a classificatory point of view, and the importance
of that structure to the organism which presents it. On the contrary,
it is a general rule that "the less any part of the organization is
concerned with special habits, the more important it becomes for
classification." Now, from the point of view of special creation it is
unintelligible why unity of ideal should be most manifested by least
important structures, whereas from the point of view of evolution it is
to be expected that these life-serving structures should have been most
liable to divergent modification in divergent lines of descent, or in
adaptation to different conditions of life, while the trivial or less
important characters should have been allowed to remain unmodified. Thus
we can now understand why all primitive classifications were wrong in
principle when they went upon the assumption that divine ideals were
best exhibited by resemblances between life-serving (and therefore
adaptive) structures, with the result that whales were classed with
fishes, birds with bats, and so on. Nevertheless, these primitive
naturalists were quite logical; for, from the premises furnished by the
theory of special creation, it is much more reasonable to expect that
unity of ideal should be shown in plainly adaptive characters than in
trivial and more or less hidden anatomical characters. Moreover, long
after biological science had ceased consciously to follow any
theological theory, the apparent axiom continued to be entertained, that
structures of most importance to organisms must also be structures of
most importance to systematists. And when at last, in the present
century, this was found not to be the case, no reason could be suggested
why it was not the case. But now we are able fully to explain this
apparent anomaly.
Once more, we have seen that aggregates of characters presenting
resemblances to one another have always been found to be of special
importance as guides to classification. This, of course, is what we
should have expected, if the real meaning of classification be that of
tracing lines of pedigree; but on the theory of special creation no
reason can be assigned why single characters are not such sure tokens of
a natural arrangement as are aggregates of characters, however trivial
the latter may be. For it is obvious that unity of ideal might have been
even better displayed by everywhere maintaining the pattern of some one
important structure, than by doing so in the case of several unimportant
structures. Take an analogous instance from human contrivances. Unity of
ideal in the case of gun-making would be shown by the same principles of
mechanism running through all the different sizes and shapes of
gun-locks, rather than by the ornamental patterns engraved upon the
outside. Yet it must be supposed that in the mechanisms assumed to have
been constructed by special creation, it was the trivial details rather
than the fundamental principles of these mechanisms which were chosen by
the Divinity to display his ideals.
And this leads us to the next consideration--namely, that when in two
different lines of descent animals happen to adopt similar habits of
life, the modifications which they undergo in order to fit them for
these habits often induces striking resemblances of structure between
the two animals, as in the case of whales and fish. But in all such
instances it is invariably found that the resemblance is only
superficial and apparent: not anatomical or real. In other words, the
resemblance does not extend further than it is necessary that it
should, if both sets of organs are to be adapted to perform the same
functions. Now this, again, is just what one would expect to find as the
universal rule on the theory of descent, with modification of ancestral
characters. But, on the opposite theory of special creation, I know not
how it is to be explained that among so many instances of close
superficial resemblance between creatures belonging to different
branches of the tree of life, there are no instances of any real or
anatomical resemblance. So far as their structures are adapted to
perform a common function, there is in all such cases what may be termed
a deceptive appearance of some unity of ideal; but, when carefully
examined, it is always found that two apparently identical structures
occurring on different branches of the classificatory tree are in fact
fundamentally different in respect of their structural plan.
Lastly, we have seen that one of the guiding principles of
classification has been empirically found to consist in setting a high
value on "chains of affinities." That is to say, naturalists not
unfrequently meet with a long series of progressive modifications of
type, which, although it cannot be said that the continuity is anywhere
broken, at last leads to so much divergence of character that, but for
the intermediate links, the members at each end of the chain could not
be suspected of being in any way related. Well, such cases of chains of
affinity obviously tell most strongly in favour of descent with
continuous modification; while it is impossible to suggest why, if all
the links were separately forged by as many acts of special creation,
there should have been this gradual transmutation of characters carried
to the point where the original creative ideal has been so completely
transformed that, but for the accident of the chain being still
complete, no one of nature's interpreters could possibly have discovered
the connexion. For, as we have seen, this is not a case in which any
appeal can be logically made to the argument from ignorance of divine
method, unless some independent evidence could be adduced in favour of
special creation. And that no such independent evidence exists, it will
be the object of future chapters to show.
CHAPTER III.
MORPHOLOGY.
The theory of evolution supposes that hereditary characters admit of
being slowly modified wherever their modification will render an
organism better suited to a change in its conditions of life. Let us,
then, observe the evidence which we have of such adaptive modifications
of structure, in cases where the need of such modification is apparent.
We may begin by again taking the case of the whales and porpoises. The
theory of evolution infers, from the whole structure of these animals,
that their progenitors must have been terrestrial quadrupeds of some
kind, which gradually became more and more aquatic in their habits. Now
the change in the conditions of their life thus brought about would have
rendered desirable great modifications of structure. These changes would
have begun by affecting the least typical--that is, the least strongly
inherited--structures, such as the skin, claws, and teeth. But, as time
went on, the adaptation would have extended to more typical structures,
until the shape of the body would have become affected by the bones and
muscles required for terrestrial locomotion becoming better adapted for
aquatic locomotion, and the whole outline of the animal more fish-like
in shape. This is the stage which we actually observe in the seals,
where the hind legs, although retaining all their typical bones, have
become shortened up almost to rudiments, and directed backwards, so as
to be of no use for walking, while serving to complete the fish-like
taper of the body. (Fig. 2.) But in the whales the modification has gone
further than this so that the hind legs have ceased to be apparent
externally, and are only represented internally--and even this only in
some species--by remnants so rudimentary that it is difficult to make
out with certainty the homologies of the bones; moreover, the head and
the whole body have become completely fish-like in shape. (Fig. 3.) But
profound as are these alterations, they affect only those parts of the
organism which it was for the benefit of the organism to have altered,
so that it might be adapted to an aquatic mode of existence. Thus the
arm, which is used as a fin, still retains the bones of the shoulder,
fore-arm, wrist, and fingers, although they are all enclosed in a
fin-shaped sack, so as to render them useless for any purpose other than
swimming (Fig. 4.) Similarly, the head, although it so closely resembles
the head of a fish in shape, still retains the bones of the mammalian
skull in their proper anatomical relations to one another; but modified
in form so as to offer the least possible resistance to the water. In
short, it may be said that all the modifications have been effected with
the least possible divergence from the typical mammalian type, which is
compatible with securing so perfect an adaptation to a purely aquatic
mode of life.
[Illustration: FIG. 2.--Skeleton of Seal, 1/8 nat. size. Drawn from
nature (_R. Coll. Surg. Mus._).]
[Illustration: FIG. 3.--Skeleton of Greenland Whale, 1/100 nat.
size. The rudimentary bones of the pelvis are shown on a larger
scale in the upper drawing. (From Prof. Flower.)]
[Illustration: FIG. 4.--Paddle of Whale compared with Hand of Man.
Drawn from nature (_R. Coll. Surg. Mus._).]
Now I have chosen the case of the whale and porpoise group, because they
offer so extreme an example of profound modification of structure in
adaptation to changed conditions of life. But the same thing may be seen
in hundreds and hundreds of other cases. For instance, to confine our
attention to the arm, not only is the limb modified in the whale for
swimming, but in another mammal--the bat--it is modified for flying, by
having the fingers enormously elongated and overspread with a membranous
web.
In birds, again, the arm is modified for flight in a wholly different
way--the fingers here being very short and all run together, while the
chief expanse of the wing is composed of the shoulder and fore-arm. In
frogs and lizards, again, we find hands more like our own; but in an
extinct species of flying reptile the modification was extreme, the wing
having been formed by a prodigious elongation of the fifth finger, and a
membrane spread over it and the rest of the hand. (Fig. 5.) Lastly, in
serpents the hand and arm have disappeared altogether.
[Illustration: FIG. 5.--Wing of Reptile, Mammal, and Bird. Drawn
from nature (_Brit. Mus._).]
Thus, even if we confine our attention to a single organ, how wonderful
are the modifications which it is seen to undergo, although never losing
its typical character. Everywhere we find the distinction between
homology and analogy which was explained in the last chapter--the
distinction, that is, between correspondence of structure and
correspondence of function. On the one hand, we meet with structures
which are perfectly homologous and yet in no way analogous: the
structural elements remain, but are profoundly modified so as to perform
wholly different functions. On the other hand, we meet with structures
which are perfectly analogous, and yet in no way homologous: totally
different structures are modified to perform the same functions. How,
then, are we to explain these things? By design manifested in special
creation, or by descent with adaptive modification? If it is said by
design manifested in special creation, we must suppose that the Deity
formed an archetypal plan of certain structures, and that he determined
to adhere to this plan through all the modifications which those
structures exhibit. But, if so, why is it that some structures are
selected as typical and not others? Why should the vertebral skeleton,
for instance, be tortured into every conceivable variety of modification
in order to subserve as great a variety of functions; while another
structure, such as the eye, is made in different sub-kingdoms on
fundamentally different plans, notwithstanding that it has throughout to
perform the same function? Will any one have the hardihood to assert
that in the case of the skeleton the Deity has endeavoured to show his
_ingenuity_, by the manifold functions to which he has made the same
structure subservient; while in the case of the eye he has endeavoured
to show his _resources_, by the manifold structures which he has adapted
to serve the same function? If so, it becomes a most unfortunate
circumstance that, throughout both the vegetable and animal kingdoms,
all cases which can be pointed to as showing ingenious adaptation of the
same typical structure to the performance of widely different
functions--or cases of homology without analogy,--are cases which come
within the limits of the same natural group of plants and animals, and
therefore admit of being equally well explained by descent from a common
ancestry; while all cases of widely different structures performing the
same function--or cases of analogy without homology,--are to be found in
different groups of plants or animals, and are therefore suggestive of
independent variations arising in the different lines of hereditary
descent.
To take a specific illustration. The octopus, or devil-fish, belongs to
a widely different class of animals from a true fish; and yet its eye,
in general appearance, looks wonderfully like the eye of a true fish.
Now, Mr. Mivart pointed to this fact as a great difficulty in the way of
the theory of evolution by natural selection, because it must clearly be
a most improbable thing that so complicated a structure as the eye of a
fish should happen to be arrived at through each of two totally
different lines of descent. And this difficulty would, indeed, be a
formidable one to the theory of evolution, if the similarity were not
only analogical but homological. Unfortunately for the objection,
however, Darwin clearly showed in his reply that in no one anatomical or
homologous feature do the two structures resemble one another; so that,
in point of fact, the two organs do not resemble one another in any
particular further than it is necessary that they should, if both are to
be analogous, or to serve the same function as organs of sight. But now,
suppose that this had not been the case, and that the two structures,
besides presenting the necessary superficial or analogical resemblance,
had also presented an anatomical or homologous resemblance, with what
force might it have then been urged,--Your hypothesis of hereditary
descent with progressive modification being here excluded by the fact
that the animals compared belong to two widely different branches of the
tree of life, how are we to explain the identity of type manifested by
these two complicated organs of vision? The only hypothesis open to us
is intelligent adherence to an ideal plan or mechanism. But as this
cannot now be urged in any comparable case throughout the whole organic
world, we may on the other hand present it as a most significant fact,
that while within the limits of the same large branch of the tree of
life we constantly find the same typical structures modified so as to
perform very different functions, we never find any of these particular
types of structure in other large branches of the tree. That is to say,
we never find typical structures appearing except in cases where their
presence may be explained by the hypothesis of hereditary descent; while
in thousands of such cases we find these structures undergoing every
conceivable variety of adaptive modification.
Consequently, special creationists must fall back upon another position
and say,--Well, but it may have pleased the Deity to form a certain
number of ideal types, and never to have allowed the structures
occurring in one type to appear in any of the others. We
answer,--Undoubtedly such may have been the case; but, if so, it is a
most unfortunate thing for your theory, because the fact implies that
the Deity has planned his types in such a way as to suggest the
counter-theory of descent. For instance, it would seem most capricious
on the part of the Deity to have made the eyes of an innumerable number
of fish on exactly the same ideal type, and then to have made the eye of
the octopus so exactly like these other eyes in superficial appearance
as to deceive so accomplished a naturalist as Mr. Mivart, and yet to
have taken scrupulous care that in no one ideal particular should the
one type resemble the other. However, adopting for the sake of argument
this great assumption, let us suppose that God did lay down these
arbitrary rules for his own guidance in creation, and then let us see to
what the assumption leads. If the Deity formed a certain number of ideal
types, and determined that on no account should he allow any part of one
type to appear in any part of another, surely we should expect that
within the limits of the same type the same typical structures should
always be present. Thus, remember what efforts, so to speak, have been
made to maintain the uniformity of type in the case of the fore-limb as
previously explained, and should we not expect that in other and similar
cases a similar method should have been followed? Yet we repeatedly find
that this is not the case. Even in the whale, as we have seen, the
hind-limbs are either altogether absent or dwindled almost to nothing;
and it is impossible to see in what respect the hind-limbs are of any
less ideal value than the fore-limbs--which are carefully preserved in
all vertebrated animals except the snakes, and the extinct _Dinornis_,
where again we meet in this particular with a sudden and sublime
indifference to the maintenance of a typical structure. (Fig. 6.)[4] Now
I say that if the theory of ideal types is true, we have in these facts
evidence of a most unreasonable inconsistency. But the theory of descent
with continued adaptive modification fully explains all the known cases;
for in every case the degree of divergence from the typical structure
which an organism presents corresponds, in a general way, with the
length of time during which the divergence has been going on. Thus we
scarcely ever meet with any great departure from the typical form with
respect to one of the organs, without some of the other organs being so
far modified as of themselves to indicate, on the supposition of
descent with modification, that the animal or plant must have been
subject to the modifying influences for an enormously long series of
generations. And this combined testimony of a number of organs in the
same organism is what the theory of descent would lead us to expect,
while the rival theory of design can offer no explanation of the fact,
that when one organ shows a conspicuous departure from the supposed
ideal type, some of the other organs in the same organism should tend to
keep it company by doing likewise.
[4] It is, however, probable that all species of the genus retained
a tiny rudiment of wings in greatly dwindled scapulo-coracoid bones.
And Mr. H. O. Forbes has detected, in a recently exhumed specimen of
the latter, an indication of the glenoid cavity, for the
articulation of an extremely aborted humerus. (See _Nature_, Jan.
14th, 1892.)
[Illustration: FIG. 6.--Skeleton of _Dinornis gravis_, 1/16 nat.
size. Drawn from nature (_Brit. Mus._). As separate cuts on a larger
scale are shown, 1st, the sternum, as this appears in mounted
skeletons, and, 2nd, the same in profile, with its (hypothetical)
scapulo-coracoid attached.]
As an illustration both of this and of other points which have been
mentioned, I may draw attention to what seems to me a particularly
suggestive case. So-called soldier-or hermit-crabs, are crabs which have
adopted the habit of appropriating the empty shells of mollusks. In
association with this peculiar habit, the structure of these animals
differs very greatly from that of all other crabs. In particular, the
hinder part of the body, which occupies the mollusk-shell, and which
therefore has ceased to require any hard covering of its own, has been
suffered to lose its calcareous integument, and presents a soft fleshy
character, quite unlike that of the more exposed parts of the animal.
Moreover, this soft fleshy part of the creature is specially adapted to
the particular requirements of the creature by having its lateral
appendages--i. e. appendages which in other crustacea perform the
function of legs--modified so as to act as claspers to the inside of the
mollusk-shell; while the tail-end of the part in question is twisted
into the form of a spiral, which fits into the spiral of the
mollusk-shell. Now, in Keeling Island there is a large kind of crab
called _Birgus latro_, which lives upon land and there feeds upon
cocoa-nuts. The whole structure of this crab, it seems to me,
unmistakeably resembles the structure of a hermit-crab (see drawings on
the next page, Fig. 7). Yet this crab neither lives in the shell of a
mollusk, nor is the hinder part of its body in the soft and fleshy
condition just described: on the contrary, it is covered with a hard
integument like all the other parts of the animal. Consequently, I think
we may infer that the ancestors of _Birgus_ were hermit-crabs living in
mollusk-shells; but that their descendants gradually relinquished this
habit as they gradually became more and more terrestrial, while,
concurrently with these changes in habit, the originally soft posterior
parts acquired a hard protective covering to take the place of that
which was formerly supplied by the mollusk-shell. So that, if so, we now
have, within the limits of a single organism, evidence of a whole series
of morphological changes in the past history of its species. First,
there must have been the great change from an ordinary crab to a
hermit-crab in all the respects previously pointed out. Next, there must
have been the change back again from a hermit-crab to an ordinary crab,
so far as living without the necessity of a mollusk-shell is concerned.
From an evolutionary point of view, therefore, we appear to have in the
existing structure of _Birgus_ a morphological record of all these
changes, and one which gives us a reasonable explanation of why the
animal presents the extraordinary appearance which it does. But, on the
theory of special creation, it is inexplicable why this land-crab should
have been formed on the pattern of a hermit-crab, when it never has need
to enter the shell of a mollusk. In other words, its peculiar
structure is not specially in keeping with its present habits, although
so curiously allied to the similar structure of certain other crabs of
totally different habits, in relation to which the peculiarities are of
plain and obvious significance.
[Illustration: FIG. 7.--Hermit-crabs compared with the cocoa-nut
crab. On the left of the illustration one hermit-crab is represented
as occupying a mollusk-shell, and another (larger specimen) as it
appears when withdrawn from such a shell. On the right of the
illustration the cocoa-nut crab is represented in its natural
habitat on land. When full-grown, however, it is much larger than
our hermit-crabs. The latter are drawn from life, natural size, the
former from a specimen in the British Museum, 1/6 natural size.]
* * * * *
I will devote the remainder of this chapter to considering another
branch of the argument from morphology, to which the case of _Birgus_
serves as a suitable introduction: I mean the argument from rudimentary
structures.
Throughout both the animal and vegetable kingdoms we constantly meet
with dwarfed and useless representatives of organs, which in other and
allied kinds of animals and plants are of large size and functional
utility. Thus, for instance, the unborn whale has rudimentary teeth,
which are never destined to cut the gums; and throughout its life this
animal retains, in a similarly rudimentary condition, a number of organs
which never could have been of use to any kind of creature save a
terrestrial quadruped. The whole anatomy of its internal ear, for
example, has reference to hearing in air--or, as Hunter long ago
remarked, "is constructed upon the same principle as in the quadruped";
yet, as Owen says, "the outer opening and passage leading therefrom to
the tympanum can rarely be affected by sonorous vibrations of the
atmosphere, and indeed they are reduced, or have degenerated, to a
degree which makes it difficult to conceive how such vibrations can be
propagated to the ear-drum during the brief moments in which the opening
may be raised above the water."
[Illustration: FIG. 8.--Rudimentary or vestigial hind-limbs of
Python, as exhibited in the skeleton and on the external surface of
the animal. Drawn from nature, 1/4 nat. size (_Zoological
Gardens_).]
Now, rudimentary organs of this kind are of such frequent occurrence,
that almost every species presents one or more of them--usually, indeed,
a considerable number. How, then, are they to be accounted for? of
course the theory of descent with adaptive modification has a simple
answer to supply--namely, that when, from changed conditions of life, an
organ which was previously useful becomes useless, it will be suffered
to dwindle away in successive generations, under the influence of
certain natural causes which we shall have to consider in future
chapters. On the other hand, the theory of special creation can only
maintain that these rudiments are formed for the sake of adhering to an
ideal type. Now, here again the former theory appears to be triumphant
over the latter; for, without waiting to dispute the wisdom of making
dwarfed and useless structures merely for the whimsical motive assigned,
surely if such a method were adopted in so many cases, we should expect
that in consistency it would be adopted in all cases. This reasonable
expectation, however, is far from being realized. We have already seen
that in numberless cases, such as that of the fore-limbs of serpents, no
vestige of a rudiment is present. But the vacillating policy in the
matter of rudiments does not end here; for it is shown in a still more
aggravated form where within the limits of the same natural group of
organisms a rudiment is sometimes present and sometimes absent. For
instance, although in nearly all the numerous species of snakes there
are no vestiges of limbs, in the python we find very tiny rudiments of
the hind-limbs. (Fig. 8.) Now, is it a worthy conception of deity that,
while neglecting to maintain his unity of ideal in the case of nearly
all the numerous species of snakes, he should have added a tiny rudiment
in the case of the python--and even in that case should have maintained
his ideal very inefficiently, inasmuch as only two limbs, instead of
four, are represented? how much more reasonable is the naturalistic
interpretation; for here the very irregularity of their appearance in
different species, which constitutes rudimentary structures one of the
crowning difficulties to the theory of special design, furnishes the
best possible evidence in favour of hereditary descent; seeing that this
irregularity then becomes what may be termed the anticipated expression
of progressive dwindling due to inutility. Thus, for example, to return
to the case of wings, we have already seen that in an extinct genus of
bird, _dinornis_, these organs were reduced to such an extent as to
leave it still doubtful whether so much as the tiny rudiment
hypothetically supplied to fig. 6 (p. 61) was present in all the
species. And here is another well-known case of another genus of still
existing bird, which, as was the case with _dinornis_, occurs only in
new zealand. (Fig. 9.) Upon this island there are no four-footed
enemies--either existing or extinct--to escape from which the wings of
birds would be of any service. Consequently we can understand why on
this island we should meet with such a remarkable dwindling away of
wings.
[Illustration: FIG. 9.--_Apteryx Australis._ Drawn from life in the
Zoological Gardens, 1/8 nat. size. The external wing is drawn to a
scale in the upper part of the cut. The surroundings are supplied
from the most recent descriptions.]
Similarly, the logger-headed duck of South America can only flap along
the surface of the water, having its wings considerably reduced though
less so than the _Apteryx_ of New Zealand. But here the interesting fact
is that the young birds are able to fly perfectly well. Now, in
accordance with a general law to be considered in a future chapter, the
life-history of an individual organism is a kind of condensed
recapitulation of the life-history of its species. Consequently, we can
understand why the little chickens of the logger-headed duck are able to
fly like all other ducks, while their parents are only able to flap
along the surface of the water.
Facts analogous to this reduction of wings in birds which have no
further use for them, are to be met with also in insects under similar
circumstances. Thus, there are on the island of Madeira somewhere
between 500 and 600 species of beetles, which are in large part
peculiar to that island, though related to other--and therefore
presumably parent--species on the neighbouring continent. Now, no less
than 200 species--or nearly half the whole number--are so far deficient
in wings that they cannot fly. And, if we disregard the species which
are not peculiar to the island--that is to say, all the species which
likewise occur on the neighbouring continent, and therefore, as
evolutionists conclude, have but _recently_ migrated to the island,--we
find this very remarkable proportion. There are altogether 29 peculiar
genera, and out of these no less than 23 have _all_ their species in
this condition.
Similar facts have been recently observed by the Rev. A. E. Eaton with
respect to insects inhabiting Kerguelen Island. All the species which he
found on the island--viz. a moth, several flies, and numerous
beetles--he found to be incapable of flight; and therefore, as Wallace
observes, "as these insects could hardly have reached the islands in a
wingless state, even if there were any other known land inhabited by
them, which there is not, we must assume that, like the Madeiran
insects, they were originally winged, and lost their power of flight
because its possession was injurious to them"--Kerguelen Island being
"one of the stormiest places on the globe," and therefore a place where
insects could rarely afford to fly without incurring the danger of being
blown out to sea.
Here is another and perhaps an even more suggestive class of facts.
It is now many years ago since the editors of _Silliman's Journal_
requested the late Professor Agassiz to give them his opinion on the
following question. In a certain dark subterranean cave, called the
Mammoth cave, there are found some peculiar species of blind fishes. Now
the editors of _Silliman's Journal_ wished to know whether Prof. Agassiz
would hold that these fish had been specially created in these caves,
and purposely devoided of eyes which could never be of any use to them;
or whether he would allow that these fish had probably descended from
other species, but, having got into the dark cave, gradually lost their
eyes through disuse. Prof. Agassiz, who was a believer in special
creation, allowed that this ought to constitute a crucial test as
between the two theories of special design and hereditary descent. "If
physical circumstances," he said, "ever modified organized beings, it
should be easily ascertained here." And eventually he gave it as his
opinion, that these fish "were created under the circumstances in which
they now live, within the limits over which they now range, and with the
structural peculiarities which now characterise them."
Since then a great deal of attention has been paid to the fauna of this
Mammoth cave, and also to the faunas of other dark caverns, not only in
the New, but also in the Old World. In the result, the following general
facts have been fully established.
(1) Not only fish, but many representatives of other classes, have been
found in dark caves.
(2) Wherever the caves are totally dark, all the animals are blind.
(3) If the animals live near enough to the entrance to receive some
degree of light, they may have large and lustrous eyes.
(4) In all cases the species of blind animals are closely allied to
species inhabiting the district where the caves occur; so that the blind
species inhabiting American caves are closely allied to American
species, while those inhabiting European caves are closely allied to
European species.
(5) In nearly all cases structural remnants of eyes admit of being
detected, in various degrees of obsolescence. In the case of some of the
crustaceans of the Mammoth cave the foot-stalks of the eyes are present,
although the eyes themselves are entirely absent.
Now, it is evident that all these general facts are in full agreement
with the theory of evolution, while they offer serious difficulties to
the theory of special creation. As Darwin remarks, it is hard to imagine
conditions of life more similar than those furnished by deep limestone
caverns under nearly the same climate in the two continents of America
and Europe; so that, in accordance with the theory of special creation,
very close similarity in the organizations of the two sets of faunas
might have been expected. But, instead of this, the affinities of these
two sets of faunas are with those of their respective continents--as of
course they ought to be on the theory of evolution. Again, what would
have been the sense of creating useless foot-stalks for the imaginary
support of absent eyes, not to mention all the other various grades of
degeneration in other cases? So that, upon the whole, if we agree with
the late Prof. Agassiz in regarding these cave animals as furnishing a
crucial test between the rival theories of creation and evolution, we
must further conclude that the whole body of evidence which they now
furnish is weighing on the side of evolution.
So much, then, for a few special instances of what Darwin called
rudimentary structures, but what may be more descriptively
designated--in accordance with the theory of descent--obsolescent or
vestigial structures. It is, however, of great importance to add that
these structures are of such general occurrence throughout both the
vegetable and animal kingdoms, that, as Darwin has observed, it is
almost impossible to point to a single species which does not present
one or more of them. In other words, it is almost impossible to find a
single species which does not in this way bear some record of its own
descent from other species; and the more closely the structure of any
species is examined anatomically, the more numerous are such records
found to be. Thus, for example, of all organisms that of man has been
most minutely investigated by anatomists; and therefore I think it will
be instructive to conclude this chapter by giving a list of the more
noteworthy vestigial structures which are known to occur in the human
body. I will take only those which are found in adult man, reserving for
the next chapter those which occur in a transitory manner during earlier
periods of his life. But, even as thus restricted, the number of
obsolescent structures which we all present in our own persons is so
remarkable, that their combined testimony to our descent from a
quadrumanous ancestry appears to me in itself conclusive. I mean, that
even if these structures stood alone, or apart from any more general
evidences of our family relationships, they would be sufficient to prove
our parentage. Nevertheless, it is desirable to remark that of course
these special evidences which I am about to detail do not stand alone.
Not only is there the general analogy furnished by the general proof of
evolution elsewhere, but there is likewise the more special
correspondence between the whole of our anatomy and that of our nearest
zoological allies. Now the force of this latter consideration is so
enormous, that no one who has not studied human anatomy can be in a
position to appreciate it. For without special study it is impossible to
form any adequate idea of the intricacy of structure which is presented
by the human form. Yet it is found that this enormously intricate
organization is repeated in all its details in the bodies of the higher
apes. There is no bone, muscle, nerve, or vessel of any importance in
the one which is not answered to by the other. Hence there are hundreds
of thousands of instances of the most detailed correspondence, without
there being any instances to the contrary, if we pay due regard to
vestigial characters. The entire corporeal structure of man is an exact
anatomical copy of that which we find in the ape.
My object, then, here is to limit attention to those features of our
corporeal structure which, having become useless on account of our
change in attitude and habits, are in process of becoming obsolete, and
therefore occur as mere vestigial records of a former state of things.
For example, throughout the vertebrated series, from fish to mammals,
there occurs in the inner corner of the eye a semi-transparent eye-lid,
which is called the nictitating membrane. The object of this structure
is to sweep rapidly, every now and then, over the external surface of
the eye, apparently in order to keep the surface clean. But although the
membrane occurs in all classes of the sub-kingdom, it is more prevalent
in some than in others--e.g. in birds than in mammals. Even, however,
where it does not occur of a size and mobility to be of any use, it is
usually represented, in animals above fishes, by a functionless
rudiment, as here depicted in the case of man. (Fig. 10.)
[Illustration: FIG. 10.--Illustrations of the nictitating membrane
in the various animals named drawn from nature. The letter N
indicates the membrane in each case. In man it is called the _plica
semilunaris_, and is represented in the two lower drawings under
this name. In the case of the shark (_Galeus_) the muscular
mechanism is shown as dissected.]
Now the organization of man presents so many vestigial structures thus
referring to various stages of his long ancestral history, that it would
be tedious so much as to enumerate them. Therefore I will yet further
limit the list of vestigial structures to be given as examples, by not
only restricting these to cases which occur in our own organization; but
of them I shall mention only such as refer us to the very last stage of
our ancestral history--viz. structures which have become obsolescent
since the time when our distinctively human branch of the family tree
diverged from that of our immediate forefathers, the Quadrumana.
(1) _Muscles of the external ear._--These, which are of large size and
functional use in quadrupeds, we retain in a dwindled and useless
condition (Fig. 11). This is likewise the case in anthropoid apes; but
in not a few other Quadrumana (e.g. baboons, macacus, magots, &c.)
degeneration has not proceeded so far, and the ears are voluntarily
moveable.
[Illustration: FIG. 11.--Rudimentary, or vestigial and useless,
muscles of the human ear. (From _Gray's Anatomy_.)]
(2) _Panniculus carnosis._--A large number of the mammalia are able to
move their skin by means of sub-cutaneous muscle--as we see, for
instance, in a horse, when thus protecting himself against the sucking
of flies. We, in common with the Quadrumana, possess an active remnant
of such a muscle in the skin of the forehead, whereby we draw up the
eyebrows; but we are no longer able to use other considerable remnants
of it, in the scalp and elsewhere,--or, more correctly, it is rarely
that we meet with persons who can. But most of the Quadrumana (including
the anthropoids) are still able to do so. There are also many other
vestigial muscles, which occur only in a small percentage of human
beings, but which, when they do occur, present unmistakeable homologies
with normal muscles in some of the Quadrumana and still lower
animals[5].
[5] See especially Mr. John Wood's papers, _Proc. R. S._, xiii to
xvi, and xviii; also _Journ. Anat._, i and iii. In this connexion
Darwin refers to M. Richard, _Annls. d. Sc. Nat. Zoolg._, tom.
xviii, p. 13, 1852.
(3) _Feet._--It is observable that in the infant the feet have a strong
deflection inwards, so that the soles in considerable measure face one
another. This peculiarity, which is even more marked in the embryo than
in the infant (see p. 153), and which becomes gradually less and less
conspicuous even before the child begins to walk, appears to me a highly
suggestive peculiarity. For it plainly refers to the condition of
things in the Quadrumana, seeing that in all these animals the feet are
similarly curved inwards, to facilitate the grasping of branches. And
even when walking on the ground apes and monkeys employ to a great
extent the outside edges of their feet, as does also a child when
learning to walk. The feet of a young child are also extraordinarily
mobile in all directions, as are those of apes. In order to show these
points, I here introduce comparative drawings of a young ape and the
portrait of a young male child. These drawings, moreover, serve at the
same time to illustrate two other vestigial characters, which have
often been previously noticed with regard to the infant's foot. I allude
to the incurved form of the legs, and the lateral extension of the great
toe, whereby it approaches the thumb-like character of this organ in the
Quadrumana. As in the case of the incurved position of the legs and
feet, so in this case of the lateral extensibility of the great toe, the
peculiarity is even more marked in embryonic than in infant life. For,
as Prof. Wyman has remarked with regard to the foetus when about an inch
in length, "The great toe is shorter than the others; and, instead of
being parallel to them, is projected at an angle from the side of the
foot, thus corresponding with the permanent condition of this part in
the Quadrumana[6]." So that this organ, which, according to Owen, "is
perhaps the most characteristic peculiarity in the human structure,"
when traced back to the early stages of its development, is found to
present a notably less degree of peculiarity.
[6] _Proc. Nat. Hist. Soc._, Boston, 1863.
[Illustration: FIG. 12.--Portrait of a young male gorilla (after
Hartmann).]
[Illustration: FIG. 13.--Portrait of a young male child.
Photographed from life, when the mobile feet were for a short time
at rest in a position of extreme inflection.]
(4) _Hands._--Dr. Louis Robinson has recently observed that the grasping
power of the whole human hand is so surprisingly great at birth, and
during the first few weeks of infancy, as to be far in excess of present
requirements on the part of a young child. Hence he concludes that it
refers us to our quadrumanous ancestry--the young of anthropoid apes
being endowed with similar powers of grasping, in order to hold on to
the hair of the mother when she is using her arms for the purposes of
locomotion. This inference appears to me justifiable, inasmuch as no
other explanation can be given of the comparatively inordinate muscular
force of an infant's grip. For experiments showed that very young babies
are able to support their own weight, by holding on to a horizontal bar,
for a period varying from one half to more than two minutes[7]. With his
kind permission I here reproduce one of Dr. Robinson's instantaneous,
and hitherto unpublished, photographs of a very young infant. This
photograph was taken after the above paragraph (3) was written, and I
introduce it here because it serves to show incidentally--and perhaps
even better than the preceding figure--the points there mentioned with
regard to the feet and great toes. Again, as Dr. Robinson observes, the
attitude, and the disproportionately large development of the arms as
compared with the legs, give all the photographs a striking resemblance
to a picture of the chimpanzee "Sally" at the Zoological Gardens. For
"invariably the thighs are bent nearly at right angles to the body, and
in no case did the lower limbs hang down and take the attitude of the
erect position." He adds, "In many cases no sign of distress is evinced,
and no cry uttered, until the grasp begins to give way."
[7] _Nineteenth Century_, November, 1891.
[Illustration: FIG. 14.--An infant, three weeks old, supporting its
own weight for over two minutes. The attitude of the lower limbs,
feet, and toes, is strikingly simian. Reproduced from an
instantaneous photograph, kindly given for the purpose by Dr. L.
Robinson.]
(5) _Tail._--The absence of a tail in man is popularly supposed to
constitute a difficulty against the doctrine of his quadrumanous
descent. As a matter of fact, however, the absence of an external tail
in man is precisely what this doctrine would expect, seeing that the
nearest allies of man in the quadrumanous series are likewise destitute
of an external tail. Far, then, from this deficiency in man constituting
any difficulty to be accounted for, if the case were not so--i. e. if
man _did_ possess an external tail,--the difficulty would be to
understand how he had managed to retain an organ which had been
renounced by his most recent ancestors. Nevertheless, as the anthropoid
apes continue to present the rudimentary vestiges of a tail in a few
caudal vertebræ below the integuments, we might well expect to find a
similar state of matters in the case of man. And this is just what we do
find, as a glance at these two comparative illustrations will show.
(Fig. 15.) Moreover, during embryonic life, both of the anthropoid apes
and of man, the tail much more closely resembles that of the lower kinds
of quadrumanous animals from which these higher representatives of the
group have descended. For at a certain stage of embryonic life the tail,
both of apes and of human beings, is actually longer than the legs (see
Fig. 16). And at this stage of development, also, the tail admits of
being moved by muscles which later on dwindle away. Occasionally,
however, these muscles persist, and are then described by anatomists as
abnormalities. The following illustrations serve to show the muscles in
question, when thus found in adult man.
[Illustration: FIG. 15.--Sacrum of Gorilla compared with that of
Man, showing the rudimentary tail-bones of each. Drawn from nature
(_R. Coll. Surg. Mus._).]
[Illustration: FIG. 16.--Diagrammatic outline of the human embryo
when about seven weeks old, showing the relations of the limbs and
tail to the trunk (after Allen Thomson), _r_, the radial, and _u_,
the ulnar, border of the hand and fore-arm; _t_, the tibial, and
_f_, the fibular, border of the foot and lower leg; _au_, ear; _s_,
spinal cord; _v_, umbilical cord; _b_, branchial gill-slits; _c_,
tail.]
[Illustration: FIG. 17.--Front and back view of adult human sacrum,
showing abnormal persistence of vestigial tail-muscles. (The first
drawing is copied from Prof. Watson's paper in _Journl. Anat. and
Physiol._, vol. 79: the second is compiled from different
specimens.)]
(6) _Vermiform Appendix of the Cæcum._--This is of large size and
functional use in the process of digestion among many herbivorous
animals; while in man it is not only too small to serve any such
purpose, but is even a source of danger to life--many persons dying
every year from inflammation set up by the lodgement in this blind tube
of fruit-stones, &c.
In the orang it is longer than in man (Fig. 18), as it is also in the
human foetus proportionally compared with the adult. (Fig. 19.) In
some of the lower herbivorous animals it is longer than the entire body.
Like vestigial structures in general, however, this one is highly
variable. Thus the above cut (Fig. 19) serves to show that it may
sometimes be almost as short in the orang as it normally is in man--both
the human subjects of this illustration having been normal.
[Illustration: FIG. 18.--_Appendix vermiformis_ in Orang and in Man.
Drawn from dried inflated specimens in the Cambridge Museum by Mr.
J. J. Lister. _Il_, ilium; _Co_, colon; _C_, cæcum; W, a window cut
in the wall of the cæcum; X X X, the appendix.]
[Illustration: FIG. 19.--The same, showing variation in the Orang.
Drawn from a specimen in the Museum of the Royal College of
Surgeons.]
(7) _Ear._--Mr. Darwin writes:--
The celebrated sculptor, Mr. Woolner, informs me of one little
peculiarity in the external ear, which he has often observed both
in men and women.... The peculiarity consists in a little blunt
point, projecting from the inwardly folded margin, or helix. When
present, it is developed at birth, and, according to Prof. Ludwig
Meyer, more frequently in man than in woman. Mr. Woolner made an
exact model of one such case, and sent me the accompanying
drawing.... The helix obviously consists of the extreme margin of
the ear folded inwards; and the folding appears to be in some
manner connected with the whole external ear being permanently
pressed backwards. In many monkeys, which do not stand high in the
order, as baboons and some species of macacus, the upper portion of
the ear is slightly pointed, and the margin is not at all folded
inwards; but if the margin were to be thus folded, a slight point
would necessarily project towards the centre.... The following
wood-cut is an accurate copy of a photograph of the foetus of an
orang (kindly sent me by Dr. Nitsche), in which it may be seen how
different the pointed outline of the ear is at this period from its
adult condition, when it bears a close general resemblance to that
of man [including even the occasional appearance of the projecting
point shown in the preceding woodcut]. It is evident that the
folding over of the tip of such an ear, unless it changed greatly
during its further development, would give rise to a point
projecting inwards[8].
[8] _Descent of Man_, 2nd ed., pp. 15-16.
[Illustration: FIG. 20.--Human ear, modelled and drawn by Mr.
Woolner. _a_, the projecting point.]
[Illustration: FIG. 21.--Foetus of an Orang. Exact copy of a
photograph, showing the form of the ear at this early stage.]
The following woodcut serves still further to show vestigial
resemblances between the human ear and that of apes. The last two
figures illustrate the general resemblance between the normal ear of
foetal man and the ear of an adult orang-outang. The other two figures
on the lower line are intended to exhibit occasional modifications of
the adult human ear, which approximate simian characters somewhat more
closely than does the normal type. It will be observed that in their
comparatively small lobes these ears resemble those of all the apes; and
that while the outer margin of one is not unlike that of the Barbary
ape, the outer margin of the other follows those of the chimpanzee and
orang. Of course it would be easy to select individual human ears which
present either of these characters in a more pronounced degree; but
these ears have been chosen as models because they present both
characters in conjunction. The upper row of figures likewise shows the
close similarity of hair-tracts, and the direction of growth on the part
of the hair itself, in cases where the human ear happens to be of an
abnormally hirsute character. But this particular instance (which I do
not think has been previously noticed) introduces us to the subject of
hair, and hair-growth, in general.
[Illustration: FIG. 22.--Vestigial characters of human ears. Drawn
from nature.]
(8) _Hair._--Adult man presents rudimentary hair over most parts of the
body. Wallace has sought to draw a refined distinction between this
vestigial coating and the useful coating of quadrumanous animals, in the
absence of the former from the human back. But even this refined
distinction does not hold. On the one hand, the comparatively hairless
chimpanzee which died last year in the Zoological Gardens (_T. calvus_)
was remarkably denuded over the back; and, on the other hand, men who
present a considerable development of hair over the rest of their bodies
present it also on their backs and shoulders. Again, in all men the
rudimentary hair on the upper and lower arm is directed towards the
elbow--a peculiarity which occurs nowhere else in the animal kingdom,
with the exception of the anthropoid apes and a few American monkeys,
where it presumably has to do with arboreal habits. For, when sitting in
trees, the orang, as observed by Mr. Wallace, places its hands above its
head with its elbows pointing downwards: the disposition of hair on
the arms and fore-arms then has the effect of thatch in turning the
rain. Again, I find that in all species of apes, monkeys, and baboons
which I have examined (and they have been numerous), the hair on the
backs of the hands and feet is continued as far as the first row of
phalanges; but becomes scanty, or disappears altogether, on the second
row; while it is invariably absent on the terminal row. I also find that
the same peculiarity occurs in man. We all have rudimentary hair on the
first row of phalanges, both of hands and feet: when present at all, it
is more scanty on the second row; and in no case have I been able to
find any on the terminal row. In all cases these peculiarities are
congenital, and the total absence or partial presence of hair on the
second phalanges is constant in different species of Quadrumana. For
instance, it is entirely absent in all the chimpanzees, which I have
examined, while scantily present in all the orangs. As in man, it occurs
in a patch midway between the joints.
[Illustration: FIG. 23.--Hair-tracts on the arms and hands of Man,
as compared with those on the arms and hands of Chimpanzee. Drawn
from life.]
Besides showing these two features with regard to the disposition of
hair on the human arm and hand, the above woodcut illustrates a third.
By looking closely at the arm of the very hairy man from whom the
drawing was taken, it could be seen that there was a strong tendency
towards a whorled arrangement of the hairs on the backs of the wrists.
This is likewise, as a general rule, a marked feature in the arrangement
of hair on the same places in the gorilla, orang, and chimpanzee. In the
specimen of the latter, however, from which the drawing was taken, this
characteristic was not well marked. The downward direction of the hair
on the backs of the hands is exactly the same in man as it is in all
the anthropoid apes. Again, with regard to hair, Darwin notices that
occasionally there appears in man a few hairs in the eyebrows much
longer than the others; and that they seem to be representative of
similarly long and scattered hairs which occur in the chimpanzee,
macacus, and baboons.
Lastly, it may be here more conveniently observed than in the next
chapter on Embryology, that at about the sixth month the human foetus
is often thickly coated with somewhat long dark hair over the entire
body, except the soles of the feet and palms of the hands, which are
likewise bare in all quadrumanous animals. This covering, which is
called the lanugo, and sometimes extends even to the whole forehead,
ears, and face, is shed before birth. So that it appears to be useless
for any purpose other than that of emphatically declaring man a child of
the monkey.
(9) _Teeth._--Darwin writes:--
It appears as if the posterior molar or wisdom-teeth were tending
to become rudimentary in the more civilized races of man. These
teeth are rather smaller than the other molars, as is likewise the
case with the corresponding teeth in the chimpanzee and orang; and
they have only two separate fangs.... They are also much more
liable to vary, both in structure and in the period of their
development, than the other teeth. In the Melanian races, on the
other hand, the wisdom-teeth are usually furnished with three
separate fangs, and are usually sound [i. e. not specially liable
to decay]; they also differ from the other molars in size, less
than in the Caucasian races.
Now, in addition to these there are other respects in which the
dwindling condition of wisdom-teeth is manifested--particularly with
regard to the pattern of their crowns. Indeed, in this respect it would
seem that even in the anthropoid apes there is the beginning of a
tendency to degeneration of the molar teeth from behind forwards. For if
we compare the three molars in the lower jaw of the gorilla, orang, and
chimpanzee, we find that the gorilla has five well-marked cusps on all
three of them; but that in the orang the cusps are not so pronounced,
while in the chimpanzee there are only four of them on the third molar.
Now in man it is only the first of these three teeth which normally
presents five cusps, both the others presenting only four. So that,
comparing all these genera together, it appears that the number of
cusps is being reduced from behind forwards; the chimpanzee having lost
one of them from the third molar, while man has not only lost this, but
also one from the second molar,--and, it may be added, likewise
partially (or even totally) from the first molar, as a frequent
variation among civilized races. But, on the other hand, variations are
often met with in the opposite direction, where the second or the third
molar of man presents five cusps--in the one case following the
chimpanzee, in the other the gorilla. These latter variations,
therefore, may fairly be regarded as reversionary. For these facts I am
indebted to the kindness of Mr. C. S. Tomes.
[Illustration: FIG. 24.--Molar teeth of lower jaw in Gorilla, Orang,
and Man. Drawn from nature, nat. size (_R. Mus. Coll. Surg._).]
(10) _Perforations of the humerus._--The peculiarities which we have to
notice under this heading are two in number. First, the supra condyloid
foramen is a normal feature in some of the lower Quadrumana (Fig. 25),
where it gives passage to the great nerve of the fore-arm, and often
also to the great artery. In man, however, it is not a normal feature.
Yet it occurs in a small percentage of cases--viz., according to Sir W.
Turner, in about one per cent., and therefore is regarded by Darwin as a
vestigial character. Secondly, there is inter-condyloid foramen, which
is also situated near the lower end of the humerus, but more in the
middle of the bone. This occurs, but not constantly, in apes, and also
in the human species. From the fact that it does so much more frequently
in the bones of ancient--and also of some savage--races of mankind (viz.
in 20 to 30 per cent. of cases), Darwin is disposed to regard it also as
a vestigial feature. On the other hand, Prof. Flower tells me that in
his opinion it is but an expression of impoverished nutrition during
the growth of the bone.
[Illustration: FIG. 25.--Perforation of the humerus (supra-condyloid
foramen) in three species of Quadrumana where it normally occurs,
and in Man, where it does not normally occur. Drawn from nature (_R.
Coll. Surg. Mus._).]
(11) _Flattening of tibia._--In some very ancient human skeletons, there
has also been found a lateral flattening of the tibia, which rarely
occurs in any existing human beings, but which appears to have been
usual among the earliest races of mankind hitherto discovered. According
to Broca, the measurements of these fossil human tibiæ resemble those of
apes. Moreover, the bone is bent and strongly convex forwards, while its
angles are so rounded as to present the nearly oval section seen in
apes. It is in association with these ape-like human tibiæ that
perforated humeri of man are found in greatest abundance.
On the other hand, however, there is reason to doubt whether this form
of tibia in man is really a survival from his quadrumanous ancestry.
For, as Boyd-Dawkins and Hartmann have pointed out, the degree of
flattening presented by some of these ancient human bones is _greater_
than that which occurs in any existing species of anthropoid ape. Of
course the possibility remains that the unknown species of ape from
which man descended may have had its tibia more flattened than is now
observable in any of the existing species. Nevertheless, as some doubt
attaches to this particular case, I do not press it--and, indeed, only
mention it at all in order that the doubt may be expressed.
Similarly, I will conclude by remarking that several other instances of
the survival of vestigial structures in man have been alleged, which are
of a still more doubtful character. Of such, for example, are the
supposed absence of the genial tubercle in the case of a very ancient
jaw-bone of man, and the disposition of valves in human veins. From the
former it was argued that the possessor of this very ancient jaw-bone
was probably speechless, inasmuch as the tubercle in existing man gives
attachment to muscles of the tongue. From the latter it has been argued
that all the valves in the veins of the human body have reference, in
their disposition, to the incidence of blood-pressure when the attitude
of the body is horizontal, or quadrupedal. Now, the former case has
already broken down, and I find that the latter does not hold. But we
can well afford to lose such doubtful and spurious cases, in view of all
the foregoing unquestionable and genuine cases of vestigial structures
which are to be met with even within the limits of our own
organization--and even when these limits are still further limited by
selecting only those instances which refer to the very latest chapter of
our long ancestral history.
CHAPTER IV.
EMBRYOLOGY.
We will next consider what of late years has become the most important
of the lines of evidence, not only in favour of the general fact of
evolution, but also of its history: I mean the evidence which has been
yielded by the newest of the sciences, the science of Embryology. But
here, as in the analogous case of adult morphology, in order to do
justice to the mass of evidence which has now been accumulated, a whole
volume would be necessary. As in that previous case, therefore, I must
restrict myself to giving an outline sketch of the main facts.
First I will display what in the language of Paley we may call "the
state of the argument."
It is an observable fact that there is often a close correspondence
between developmental changes as revealed by any chronological series of
fossils which may happen to have been preserved, and developmental
changes which may be observed during the life-history of now existing
individuals belonging to the same group of animals. For instance, the
successive development of prongs in the horns of deer-like animals,
which is so clearly shown in the geological history of this tribe, is
closely reproduced in the life-history of existing deer. Or, in other
words, the antlers of an existing deer furnish in their development a
kind of _résumé_, or recapitulation, of the successive phases whereby
the primitive horn was gradually superseded by horns presenting a
greater and greater number of prongs in successive species of extinct
deer (Fig. 26). Now it must be obvious that such a recapitulation in the
life-history of an existing animal of developmental changes successively
distinctive of sundry allied, though now extinct species, speaks
strongly in favour of evolution. For as it is of the essence of this
theory that new forms arise from older forms by way of _hereditary_
descent, we should antecedently expect, if the theory is true, that the
phases of development presented by the individual organism would follow,
in their main outlines, those phases of development through which their
long line of ancestors had passed. The only alternative view is that as
species of deer, for instance, were separately created, additional
prongs were successively added to their antlers; and yet that, in order
to be so added to successive species every individual deer belonging to
later species was required to repeat in his own lifetime the process of
successive additions which had previously taken place in a remote series
of extinct species. Now I do not deny that this view is a possible view;
but I do deny that it is a probable one. According to the evolutionary
interpretation of such facts, we can see a very good _reason_ why the
life-history of the individual is thus a condensed _résumé_ of the
life-history of its ancestral species. But according to the opposite
view no reason can be assigned why such should be the case. In a
previous chapter--the chapter on Classification--we have seen that if
each species were created separately, no reason can be assigned why they
should all have been turned out upon structural patterns so strongly
suggestive of hereditary descent with gradual modifications, or slow
divergence--the result being group subordinated to group, with the most
generalized (or least developed) forms at the bottom, and the highest
products of organization at the top. And now we see--or shall
immediately see--that this consideration admits of being greatly
fortified by a study of the developmental history of every individual
organism. If it would be an unaccountable fact that every separately
created species should have been created with close structural
resemblances to a certain limited number of other species, less close
resemblances to certain further species, and so backwards; assuredly it
would be a still more unaccountable fact that every individual of every
species should exhibit in its own person a history of developmental
change, every term of which corresponds with the structural
peculiarities of its now extinct predecessors--and this in the exact
historical order of their succession in geological time. The more that
we think about this antithesis between the naturalistic and the
non-naturalistic interpretations, the greater must we feel the contrast
in respect of rationality to become; and, therefore, I need not spend
time by saying anything further upon the antecedent standing of the two
theories in this respect. The evidence, then, which I am about to adduce
from the study of development in the life-histories of individual
organisms, will be regarded by me as so much unquestionable evidence in
favour of similar processes of development in the life-histories of
their respective species--in so far, I mean, as the two sets of changes
admit of being proved parallel.
[Illustration: FIG. 26.--Antlers of Stag, showing successive
addition of branches in successive years. Drawn from nature (_Brit.
Mus._).]
In the only illustration hitherto adduced--viz. that of deers'
horns--the series of changes from a one-pronged horn to a fully
developed arborescent antler, is a series which takes place during the
adult life of the animal; for it is only when the breeding age has been
attained that horns are required to appear. But seeing that every animal
passes through most of the phases of its development, not only before
the breeding age has been attained, but even before the time of its own
birth, clearly the largest field for the study of individual development
is furnished by embryology. For instance, there is a salamander which
differs from most other salamanders in being exclusively terrestrial in
its habits. Now, the young of this salamander before their birth are
found to be furnished with gills, which, however, they are never
destined to use. Yet these gills are so perfectly formed, that if the
young salamanders be removed from the body of their mother shortly
before birth, and be then immediately placed in water, the little
animals show themselves quite capable of aquatic respiration, and will
merrily swim about in a medium which would quickly drown their own
parent. Here, then, we have both morphological and physiological
evidence pointing to the possession of gills by the ancestors of the
land salamander.
It would be easy to devote the whole of the present chapter to an
enumeration of special instances of the kinds thus chosen for purposes
of illustration; but as it is desirable to take a deeper, and therefore
a more general view of the whole subject, I will begin at the
foundation, and gradually work up from the earliest stages of
development to the latest. Before starting, however, I ask the reader to
bear in mind one consideration, which must reasonably prevent our
anticipating that in _every case_ the life-history of an individual
organism should present a _full_ recapitulation of the life-history of
its ancestral line of species. Supposing the theory of evolution to be
true, it must follow that in many cases it would have been more or less
disadvantageous to a developing type that it should have been obliged to
reproduce in its individual representatives all the phases of
development previously undergone by its ancestry--even within the limits
of the same family. We can easily understand, for example, that the
waste of material required for building up the useless gills of the
embryonic salamanders is a waste which, sooner or later, is likely to be
done away with; so that the fact of its occurring at all is in itself
enough to show that the change from aquatic to terrestrial habits on the
part of this species must have been one of comparatively recent
occurrence. Now, in as far as it is detrimental to a developing type
that it should pass through any particular ancestral phases of
development, we may be sure that natural selection--or whatever other
adjustive causes we may suppose to have been at work in the adaptation
of organisms to their surroundings--will constantly seek to get rid of
this necessity, with the result, when successful, of dropping out the
detrimental phases. Thus the foreshortening of developmental history
which takes place in the individual lifetime may be expected often to
take place, not only in the way of condensation, but also in the way of
excision. Many pages of ancestral history may be recapitulated in the
paragraphs of embryonic development, while others may not be so much as
mentioned. And that this is the true explanation of what embryologists
term "direct" development--or of a more or less sudden leap from one
phase to another, without any appearance of intermediate phases--is
proved by the fact that in some cases both direct and indirect
development occur within the same group of organisms, some genera or
families having dropped out the intermediate phases which other genera
or families retain.
* * * * *
The argument from embryology must be taken to begin with the first
beginning of individual life in the ovum. And, in order to understand
the bearings of the argument in this its first stage, we must consider
the phenomena of reproduction in the simplest form which these phenomena
are known to present.
The whole of the animal kingdom is divided into two great groups, which
are called the Protozoa and the Metazoa. Similarly, the whole of the
vegetable kingdom is divided into the Protophyta and the Metaphyta. The
characteristic feature of all the Protozoa and Protophyta is that the
organism consists of a single physiological cell, while the
characteristic of all the Metazoa and Metaphyta is that the organism
consists of a plurality of physiological cells, variously modified to
subserve different functions in the economy of the animal or plant, as
the case may be. For the sake of brevity, I shall hereafter deal only
with the case of animals (Protozoa and Metazoa); but it may throughout
be understood that everything which is said applies also to the case of
plants (Protophyta and Metaphyta).
A Protozoön (like a Protophyton) is a solitary cell, or a "unicellular
organism," while a Metazoön (like a Metaphyton) is a society of cells,
or a "multicellular organism." Now, it is only in the multicellular
organisms that there is any observable distinction of sex. In all the
unicellular organisms the phenomena of reproduction appear to be more or
less identical with those of growth. Nevertheless, as these phenomena
are here in some cases suggestively peculiar, I will consider them more
in detail.
A Protozoön is a single corpuscle of protoplasm which in different
species of Protozoa varies in size from more than one inch to less than
1/1000 of an inch in diameter. In some species there is an enveloping
cortical substance; in other species no such substance can be detected.
Again, in most species there is a nucleus, while in other species no
such differentiation of structure has hitherto been observed.
Nevertheless, from the fact that the nucleus occurs in the majority of
Protozoa, coupled with the fact that the demonstration of this body is
often a matter of extreme difficulty, not only in some of the Protozoa
where it has been but recently detected, but also in the case of certain
physiological cells elsewhere,--from these facts it is not unreasonable
to suppose that all the Protozoa possess a nucleus, whether or not it
admits of being rendered visible by histological methods thus far at our
disposal. If this is the case, we should be justified in saying, as I
have said, that a Protozoön is an isolated physiological cell, and, like
cells in general, multiplies by means of what Spencer and Häckel have
aptly called a process of discontinuous growth. That is to say, when a
cell reaches maturity, further growth takes place in the direction of a
severance of its substance--the separated portion thus starting anew as
a distinct physiological unit. But, notwithstanding the complex changes
which have been more recently observed to take place in the nucleus of
some Protozoa prior to their division, the process of multiplication by
division may still be regarded as a process of growth, which differs
from the previous growth of the individual cell in being attended by a
severance of continuity. If we take a suspended drop of gum, and
gradually add to its size by allowing more and more gum to flow into it,
a point will eventually be reached at which the force of gravity will
overcome that of cohesion, and a portion of the drop will fall away from
the remainder. Here we have a rough physical simile, although of course
no true analogy. In virtue of a continuous assimilation of nutriment,
the protoplasm of a cell increases in mass, until it reaches the size at
which the forces of disruption overcome those of cohesion--or, in other
words, the point at which increase of size is no longer compatible with
continuity of substance. Nevertheless, it must not be supposed that the
process is thus merely a physical one. The phenomena which occur even in
the simplest--or so-called "direct"--cell-division, are of themselves
enough to prove that the process is vital, or physiological; and this in
a high degree of specialization. But so, likewise, are all processes of
growth in organic structures; and therefore the simile of the drop of
gum is not to be regarded as a true analogy: it serves only to indicate
the fact that when cell-growth proceeds beyond a certain point
cell-division ensues. The size to which cells may grow before they thus
divide is very variable in different kinds of cells; for while some may
normally attain a length of ten or twelve inches, others divide before
they measure 1/1000 of an inch. This, however, is a matter of detail,
and does not affect the general physiological principles on which we are
at present engaged.
Now, as we have seen, a Protozoön is a single cell; for even although in
some of the higher forms of protozoal life a colony of cells may be
bound together in organic connexion, each of these cells is in itself an
"individual," capable of self-nourishment, reproduction, and, generally,
of independent existence. Consequently, when the growth of a Protozoön
ends in a division of its substance, the two parts wander away from each
other as separate organisms. (Fig. 27.)
[Illustration: FIG. 27.--Fission of a Protozoön. In the left-hand
drawing the process is represented as having advanced sufficiently
far to have caused a division and segregation both of the nucleus
and the vesicle. In the right-hand drawing the process is
represented as complete. _n_, N, severed nucleus; _vc_, severed
vesicle; _ps_, pseudopodia; _f_, ingested food.]
The next point we have to observe is, that in all cases where a cell or
a Protozoön multiplies by way of fissiparous division, the process
begins in the nucleus. If the nucleus divides into two parts, the whole
cell will eventually divide into two parts, each of which retains a
portion of the original nucleus, as represented in the above figure. If
the nucleus divides into three, four, or even, as happens in the
development of some embryonic tissues, into as many as six parts, the
cell will subdivide into a corresponding number, each retaining a
portion of the nucleus. Therefore, in all cases of fissiparous division,
the seat or origin of the process is the nucleus.
Thus far, then, the phenomena of multiplication are identical in all the
lowest or unicellular organisms, and in the constituent cells of all the
higher or multicellular. And this is the first point which I desire to
make apparent. For where the object is to prove a continuity between the
phenomena of growth and reproduction, it is of primary importance to
show--1st, that there is such a continuity in the case of all the
unicellular organisms, and, 2nd, that there are all the above points of
resemblance between the multiplication of cells in the unicellular and
in the multicellular organisms.
It remains to consider the points of difference, and, if possible, to
show that these do not go to disprove the doctrine of continuity which
the points of resemblance so forcibly indicate.
The first point of difference obviously is, that in the case of all the
multicellular organisms the two or more "daughter-cells," which are
produced by division of the "mother-cell," do not wander away from one
another; but, as a rule, they continue to be held in more or less close
apposition by means of other cells and binding membranes,--with the
result of giving rise to those various "tissues," which in turn go to
constitute the material of "organs." I cannot suppose, however, that any
advocate of discontinuity will care to take his stand at this point.
But, if any one were so foolish as to do so, it would be easy to
dislodge him by describing the state of matters in some of the Protozoa
where a number of unicellular "individuals" are organically united so as
to form a "colony." These cases serve to bridge this distinction between
Protozoa and Metazoa, of which therefore we may now take leave.
In the second place, there is the no less obvious distinction that the
result of cell-division in the Metazoa is not merely to multiply cells
all of the same kind: on the contrary, the process here gives rise to as
many different kinds of cells as there are different kinds of tissue
composing the adult organism. But no one, I should think, is likely to
oppose the doctrine of continuity on the ground of this distinction. For
the distinction is clearly one which must necessarily arise, if the
doctrine of continuity between unicellular and multicellular organisms
be true. In other words, it is a distinction which the theory of
evolution itself must necessarily pre-suppose, and therefore it is no
objection to the theory that its pre-supposition is realized. Moreover,
as we shall see better presently, there is no difficulty in
understanding why this distinction should have arisen, so soon as it
became necessary (or desirable) that individual cells, when composing a
"colony," should conform to the economic principle of the division of
labour--a principle, indeed, which is already foreshadowed in the
constituent parts of a single cell, since the nucleus has one set of
functions and its surrounding protoplasm another.
But now, in the third place, we arrive at a more important distinction,
and one which lies at the root of the others still remaining to be
considered. I refer to sexual propagation. For it is a peculiarity of
the multicellular organisms that, although many of them may likewise
propagate themselves by other means (Fig. 28), they all propagate
themselves by means of sexual congress. Now, in its essence, sexual
congress consists in the fusion of two specialized cells (or, as now
seems almost certain, of the nuclei thereof), so that it is out of such
a combination that the new individual arises by means of successive
cell-divisions, which, beginning in the fertilized ovum, eventually
build up all the tissues and organs of the body.
[Illustration: FIG. 28.--_Hydra viridis_, partly in section. M,
mouth; O, ovary, or bud containing female reproductive cells; T,
testis, or bud containing male reproductive cells. In addition to
these buds containing germinal elements alone, there is another
which illustrates the process of "gemmation"--i. e. the direct
out-growth of a fully formed offspring.]
This process clearly indicates very high specialization on the part of
germ-cells. For we see by it that although these cells when young
resemble all other cells in being capable of self-multiplication by
binary division (thus reproducing cells exactly like themselves), when
older they lose this power; but, at the same time, they acquire an
entirely new and very remarkable power of giving rise to a vast
succession of many different kinds of cells, all of which are mutually
correlated as to their several functions, so as to constitute a
hierarchy of cells--or, to speak literally, a multicellular
_co-organization_. Here it is that we touch the really important
distinction between the Protozoa and the Metazoa; for although I have
said that some of the higher Protozoa foreshadow this state of matters
in forming cell-colonies, it must now be noted that the cells composing
such colonies are all of the same kind; and, therefore, that the
principle of producing different kinds of cells which, by mutual
co-adaptation of functions, shall be capable of constructing a
multicellular Metazoön,--this great principle of _co-organization_ is
but dimly nascent in the cell-colonies of Protozoa. And its marvellous
development in the Metazoa appears ultimately to depend upon the highly
specialized character of germ-cells. Even in cases where multicellular
organisms are capable of reproducing their kind without the need of any
preceding process of fertilization (parthenogenesis), and even in the
still more numerous cases where complete organisms are budded forth from
any part of their parent organism (gemmation, Fig. 28), there is now
very good reason to conclude that these powers of a-sexual reproduction
on the part of multicellular organisms are all ultimately due to the
specialized character of their germ-cells. For in all these cases the
tissues of the parent, from which the budding takes place, were
ultimately derived from germ-cells--no matter how many generations of
budded organisms may have intervened. And that propagation by budding,
&c, in multicellular organisms is thus ultimately due to their
propagation by sexual methods, seems to be further shown by certain
facts which will have to be discussed at some length in my next volume.
Here, therefore, I will mention only one of them--and this because it
furnishes what appears to be another important distinction between the
Protozoa and the Metazoa.
In nearly all cases where a Protozoön multiplies itself by fission, the
process begins by a simple division of the nucleus. But when a Metazoön
is developed from a germ-cell, although the process likewise begins by a
division of the nucleus, this division is not a simple or direct one; on
the contrary, it is inaugurated by a series of processes going on within
the nucleus, which are so enormously complex, and withal so beautifully
ordered, that to my mind they constitute the most wonderful--if not also
the most suggestive--which have ever been revealed by microscopical
research. It is needless to say that I refer to the phenomena of
karyokinesis. A few pages further on they will be described more fully.
For our present purposes it is sufficient to give merely a pictorial
illustration of their successive phases; for a glance at such a
representation serves to reveal the only point to which attention has
now to be drawn--namely, the immense complexity of the processes in
question, and therefore the contrast which they furnish to the simple
(or "direct") division of the nucleus preparatory to cell-division in
the unicellular organisms. Here, then (Fig. 29), we see the complex
processes of karyokinesis in the first two stages of egg-cell division.
But similar processes continue to repeat themselves in subsequent
stages; and this, there is now good reason to believe, throughout _all_
the stages of cell-division, whereby the original egg-cell eventually
constructs an entire organism. In other words, all the cells composing
all the tissues of a multicellular organism, at all stages of its
development, are probably originated by these complex processes, which
differ so much from the simple process of direct division in the
unicellular organisms[9]. In this important respect, therefore, it does
at first sight appear that we have a distinction between the Protozoa
and the Metazoa of so pronounced a character, as fairly to raise the
question whether cell-division is fundamentally identical in unicellular
and in multicellular organisms.
[9] I say "probably," because analogy points in this direction. As a
matter of fact, in many cases of tissue-formation karyokinesis has
not hitherto been detected. But even if in such cases it does not
occur--i. e. if failure to detect its occurrence be not due merely
to still remaining imperfections of our histological methods,--the
large number of cases in which it has been seen to occur in the
formation of sundry tissues are of themselves sufficient to indicate
some important difference between cells derived from ova (metazoal),
and cells which have not been so derived (protozoal). Which is the
point now under discussion.
[Illustration: FIG. 29.--Successive stages in the division of the
ovum, or egg-cell, of a worm. (After Strasburger.) _a_ to _d_ show
the changes taking place in the nucleus and surrounding
cell-contents, which result in the first segmentation of the ovum at
_e_; _f_ and _g_ show a repetition of these changes in each of the
two resulting cells, leading to the second segmentation stage at
_h_.]
Lastly, the only other distinction of a physiologically significant kind
between a single cell when it occurs as a Protozoön and when it does so
as the unfertilized ovum of a Metazoön is, that in the latter case the
nucleus discharges from its own substance two minute protoplasmic masses
("polar bodies"), which are then eliminated from the cell altogether.
This process, which will be more fully described later on, appears to be
of invariable occurrence in the case of all egg-cells, while nothing
resembling it has ever been observed in any of the Protozoa.
We must now consider these several points of difference _seriatim_.
First, with regard to sexual propagation, we have already seen that this
is by no means the only method of propagation among the multicellular
organisms; and it now remains to add that, on the other hand, there is,
to say the least, a suggestive foreshadowing of sexual propagation among
the unicellular organisms. For although simple binary fission is here
the more usual mode of multiplication, very frequently two (rarely three
or more) Protozoa of the same species come together, fuse into a single
mass, and thus become very literally "one flesh." This process of
"conjugation" is usually (though by no means invariably) followed by a
period of quiescent "encystation"; after which the contents of the cyst
escape in the form of a number of minute particles, or "spores," and
these severally develope into the parent type. Obviously this process of
conjugation, when it is thus a preliminary to multiplication, appears to
be in its essence the same as fertilization. And if it be objected that
encystation and spore-formation in the Protozoa are not always preceded
by conjugation, the answer would be that neither is oviparous
propagation in the Metazoa invariably preceded by fertilization.
Nevertheless, that there are great distinctions between true sexual
propagation and this foreshadowing of it in conjugation I do not deny.
The question, however, is whether they be so great as to justify any
argument against an historical continuity between them. What, then, are
these remaining distinctions? Briefly, as we have seen, they are the
extrusion from egg-cells of polar bodies, and the occurrence, both in
egg-cells and their products (tissue-cells), of the process of
karyokinesis. But, as regards the polar bodies, it is surely not
difficult to suppose that, whatever their significance may be, it is
probably in some way or another connected with the high specialization
of the functions which an egg-cell has to discharge. Nor is there any
difficulty in further supposing that, whatever purpose is served by
getting rid of polar bodies, the process whereby they are got rid of was
originally one of utilitarian development--i. e. a process which at its
commencement did not betoken any difference of kind, or breach of
continuity, between egg-cells and cells of simpler constitution.
Lastly, with respect to karyokinesis, although it is true that the
microscope has in comparatively recent years displayed this apparently
important distinction between unicellular and multicellular organisms,
two considerations have here to be supplied. The first is, that in some
of the Protozoa processes very much resembling those of karyokinesis
have already been observed taking place in the nucleus preparatory to
its division. And although such processes do not present quite the same
appearances as are to be met with in egg-cells, neither do the
karyokinetic processes in tissue-cells, which in their sundry kinds
exhibit great variations in this respect. Moreover, even if such were
not the case, the bare fact that nuclear division is not invariably of
the simple or direct character in the case of all Protozoa, is
sufficient to show that the distinction now before us--like the one last
dealt with--is by no means absolute. As in the case of sexual
propagation, so in that of karyokinesis, processes which are common to
all the Metazoa are not wholly without their foreshadowings in the
Protozoa. And seeing how greatly exalted is the office of egg-cells--and
even of tissue-cells--as compared with that of their supposed ancestry
in protozoal cells, it seems to me scarcely to be wondered at if their
specializations of function should be associated with corresponding
peculiarities of structure--a general fact which would in no way
militate against the doctrine of evolution. Could we know the whole
truth, we should probably find that in order to endow the most primitive
of egg-cells with its powers of marshalling its products into a living
army of cell-battalions, such an egg-cell must have been passed through
a course of developmental specialization of so elaborate a kind, that
even the complex processes of karyokinesis are but a very inadequate
expression thereof.
Probably I have now said enough to show that, remarkable and altogether
exceptional as the properties of germ-cells of the multicellular
organisms unquestionably show themselves to be, yet when these
properties are traced back to their simplest beginnings in the
unicellular organisms, they may fairly be regarded as fundamentally
identical with the properties of living cells in general. Thus viewed,
no line of real demarcation can be drawn between growth and
reproduction, even of the sexual kind. The one process is, so to speak,
physiologically continuous with the other; and hence, so far as the
pre-embryonic stage of life-history is concerned, the facts cannot
fairly be regarded as out of keeping with the theory of evolution.
I will now pass on to consider the embryogeny of the Metazoa, beginning
at its earliest stage in the fertilization of the ovum. And here it is
that the constructive argument in favour of evolution which is derived
from embryology may be said properly to commence. For it is surely in
itself a most suggestive fact that all the Metazoa begin their life in
the same way, or under the same form and conditions. _Omne vivum ex
ovo._ This is a formula which has now been found to apply throughout the
whole range of the multicellular organisms. And seeing, as we have just
seen, that the ovum is everywhere a single cell, the formula amounts to
saying that, physiologically speaking, every Metazoön begins its life as
a Protozoön, and every Metaphyton as a Protophyton[10].
[10] Even when propagated by budding, a multicellular organism has
been ultimately derived from a germ-cell.
Now, if the theory of evolution is true, what should we expect to happen
when these germ-cells are fertilized, and so enter upon their severally
distinct processes of development? Assuredly we should expect to find
that the higher organisms pass through the same phases of development as
the lower organisms, up to the time when their higher characters begin
to become apparent. If in the life-history of species these higher
characters were gained by gradual improvement upon lower characters, and
if the development of the higher individual is now a general
recapitulation of that of its ancestral species, in studying this
recapitulation we should expect to find the higher organism successively
unfolding its higher characters from the lower ones through which its
ancestral species had previously passed. And this is just what we do
find. Take, for example, the case of the highest organism, Man. Like
that of all other organisms, unicellular or multicellular, his
development starts from the nucleus of a single cell. Again, like that
of all the Metazoa and Metaphyta, his development starts from the
specially elaborated nucleus of an egg-cell, or a nucleus which has been
formed by the fusion of a male with a female element[11]. When his
animality becomes established, he exhibits the fundamental anatomical
qualities which characterize such lowly animals as polyps and
jelly-fish. And even when he is marked off as a Vertebrate, it cannot be
said whether he is to be a fish, a reptile, a bird, or a beast. Later on
it becomes evident that he is to be a Mammal; but not till later still
can it be said to which order of mammals he belongs.
[11] It has already been stated that both parthenogenesis and
gemmation are ultimately derived from sexual reproduction. It may
now be added, on the other hand, that the earlier stages of
parthenogenesis have been observed to occur sporadically in all
sub-kingdoms of the Metaxoa, including the Vertebrata, and even the
highest class, Mammalia. These earlier stages consist in
_spontaneous_ segmentations of the ovum; so that even if a virgin
has ever conceived and borne a son, and even if such a fact in the
human species has been unique, still it would not betoken any breach
of physiological continuity. Indeed, according to Weismann's not
improbable hypothesis touching the physiological meaning of polar
bodies, such a fact need betoken nothing more than a slight
disturbance of the complex machinery of ovulation, on account of
which the ovum failed to eliminate from its substance an almost
inconceivably minute portion of its nucleus.
Here, however, we must guard against an error which is frequently met
with in popular expositions of this subject. It is not true that the
embryonic phases in the development of a higher form always resemble so
many adult stages of lower forms. This may or may not be the case; but
what always is the case is, that the embryonic phases of the higher
form resemble the corresponding phases of the lower forms. Thus, for
example, it would be wrong to suppose that at any stage of his
development a man resembles a jelly-fish. What he does resemble at an
early stage of his development is the essential or groundplan of the
jelly-fish, which that animal presents in _its_ embryonic condition, or
before it begins to assume its more specialized characters fitting it
for its own particular sphere of life. The similarities, therefore,
which it is the function of comparative embryology to reveal are the
similarities of type or morphological plan: not similarities of specific
detail. Specific details may have been added to this, that, and the
other species for their own special requirements, after they had
severally branched off from the common ancestral stem; and so could not
be expected to recur in the life-history of an independent specific
branch. The comparison therefore must be a comparison of embryo with
embryo; not of embryos with adult forms.
* * * * *
In order to give a general idea of the results thus far yielded by a
study of comparative embryology in the present connexion, I will devote
the rest of this chapter to giving an outline sketch of the most
important and best established of these results.
Histologically the ovum, or egg-cell, is nearly identical in all
animals, whether vertebrate or invertebrate. Considered as a cell it is
of large size, but actually it is not more than 1/100, and may be less
than 1/200 of an inch in diameter. In man, as in most mammals, it is
about 1/120. It is a more or less spherical body, presenting a thin
transparent envelope, called the _zona pellucida_, which
contains--first, the protoplasmic cell-substance or "yolk," within which
lies, second, the nucleus or germinal vesicle, within which again lies,
third, the nucleolus or germinal spot. This description is true of the
egg-cells of all animals, if we add that in the case of the lowest
animals--such as sponges, &c.--there is no enveloping membrane: the
egg-cell is here a naked cell, and its constituent protoplasm, being
thus unconfined, is free to perform protoplasmic movements, which it
does after the manner, and with all the activity, of an amoeba. But
even with respect to this matter of an enveloping membrane, there is no
essential difference between an ovum of the lowest and an ovum of the
highest animals. For in their early stages of development within the
ovary the ova of the highest animals are likewise in the condition of
naked cells, exhibiting amoebiform movements; the enveloping membrane
of an ovum being the product of a later development. Moreover this
membrane, when present, is usually provided with one or more minute
apertures, through which the spermatozoön passes when fertilizing the
ovum. It is remarkable that the spermatozoa know, so to speak, of the
existence of these gate-ways,--their snake-like movements being directed
towards them, presumably by a stimulus due to some emanation
therefrom[12]. In the mammalian ovum, however, these apertures are
exceedingly minute, and distributed all round the circumference of the
pellucid envelope, as represented in this illustration (Fig. 32).
[12] The spermatozooids of certain plants can be strongly attracted
towards a pipette which is filled with malic acid--crowding around
and into it with avidity.
[Illustration: FIG. 30.--Ovarian ovum of a Mammal, (_a_) magnified
and viewed under pressure, (_b_) burst by increased pressure, with
yolk and nucleus escaping: (_c_) the nucleus more freed from
yolk-substance. (From _Quain's Anatomy_, after Allen Thomson.)]
[Illustration: FIG. 31.--Amoeboid movements of young egg-cells, _a_,
Amoeboid ovum of _Hydra_ (from Balfour, after Kleitnenberg); _b_,
early ovum of _Toxopneustes variegatus_, with pseudopodia-like
processes (from Balfour, after Selenka); _c_, ovum of _Toxopneustes
lividus_, more nearly ripe (from Balfour, Hertwig). A1 to A4, the
primitive egg-cell of a Chalk-Sponge (_Leuculmis echinus_), in four
successive conditions of motion. B1 to B8, ditto of a Hermit-Crab
(_Chondracanthus cornutus_), in eight successive stages (after E.
von Beneden). C1 to C5, ditto of a Cat, in five successive stages
(after Pflüger). D, ditto of Trout; E, of a Hen; F, of Man. The
first series is taken from the _Encycl. Brit._; the second from
Häckel's _Evolution of Man_.]
[Illustration: FIG. 32.--Human ovum, mature and greatly magnified.
(After Häckel.)]
In thus saying that the ova of all animals are, so far as microscopes
can reveal, _substantially_ similar, I am of course speaking of the
egg-cell proper, and not of what is popularly known as the egg. The egg
of a bird, for example, is the egg-cell, _plus_ an enormous aggregation
of nutritive material, an egg-shell, and sundry other structures suited
to the subsequent development of the egg-cell when separated from the
parent's body. But all these accessories are, from our present point of
view, accidental or adventitious. What we have now to understand by the
ovum, the egg, or the egg-cell, is the microscopical germ which I have
just described. So far then as this germ is concerned, we find that all
multicellular organisms begin their existence in the same kind of
structure, and that this structure is anatomically indistinguishable
from that of the permanent form presented by the lowest, or unicellular
organisms. But although anatomically indistinguishable, physiologically
they present the sundry peculiarities already mentioned.
Now I have endeavoured to show that none of these peculiarities are such
as to exclude--or even so much as to invalidate--the supposition of
developmental continuity between the lowest egg-cells and the highest
protozoal cells. It remains to show in this place, and on the other
hand, that there is no breach of continuity between the lowest and the
highest egg-cells; but, on the contrary, that the remarkable uniformity
of the complex processes whereby their peculiar characters are exhibited
to the histologist, is such as of itself to sustain the doctrine of
continuity in a singularly forcible manner. On this account, therefore,
and also because the facts will again have to be considered in another
connexion when we come to deal with Weismann's theory of heredity, I
will here briefly describe the processes in question.
We have already seen that the young egg-cell multiplies itself by simple
binary division, after the manner of unicellular organisms in
general--thereby indicating, as also by its amoebiform movements, its
fundamental identity with such organisms in kind. But, as we have
likewise seen, when the ovum ceases to resemble these organisms, by
taking on its higher degree of functional capacity, it is no longer able
to multiply itself in this manner. On the contrary, its cell-divisions
are now of an endogenous character, and result in the formation of many
different kinds of cells, in the order required for constructing the
multicellular organism to which the whole series of processes eventually
give rise. We have now to consider these processes _seriatim_.
[Illustration: FIG. 33.--Stages in the formation of the polar bodies
in the ovum of a star-fish. (After Hertwig.) _g.v._, germinal
vesicle transformed into a spindle-shaped system of fibres; _p.'_,
the first polar body becoming extruded; _p._, _p._, both polar
bodies fully extruded; _f. pn._, female pronucleus, or residue of
the germinal vesicle.]
First of all the nucleus discharges its polar bodies, as previously
mentioned, and in the manner here depicted on the previous page. (Fig.
33.) It will be observed that the nucleus of the ovum, or the germinal
vesicle as it is called, gets rid first of one and afterwards of the
other polar body by an "indirect," or karyokinetic, process of division.
(Fig. 33.) Extrusion of these bodies from the ovum (or it may be only
from the nucleus) having been accomplished, what remains of the nucleus
retires from the circumference of the ovum, and is called the female
pronucleus. (Fig. 33. _f. pn._) The ovum is now ready for fertilization.
A similar emission of nuclear substance is said by some good observers
to take place also from the male germ-cell, or spermatozoön, at or about
the close of _its_ development. The theories to which these facts have
given rise will be considered in future chapters on Heredity.
Turning now to the mechanism of fertilization, the diagrams (Figs. 34,
35) represent what happens in the case of star-fish.
[Illustration: FIG. 34.--Fertilization of the ovum of an echinoderm.
(From _Quain's Anatomy_, after Selenka.) S, spermatozoön; _m. pr._,
male pronucleus; _f. pr._, female pronucleus. 1 to 4 correspond to D
to G in the next figure.]
[Illustration: FIG. 35.--Fertilization of the ovum of a star-fish.
(From the _Encycl. Brit._ after Fol.) A, spermatozoa in the
mucilaginous coat of the ovum; a prominence is rising from the
surface of the ovum towards a spermatozoön; B, they have almost met;
C, they have met; D, the spermatozoön enters the ovum through a
distinct opening; H, the entire ovum, showing extruded polar bodies
on its upper surface, and the moving together of the male and female
pronuclei; E, F, G, meeting and coalescence of the pronuclei.]
The sperm-cell, or spermatozoön, is seen in the act of penetrating the
ovum. In the first figure it has already pierced the mucilaginous coat
of the ovum, the limit of which is represented by a line through which
the tail of the spermatozoön is passing: the head of the spermatozoön is
just entering the ovum proper. It may be noted that, in the case of many
animals, the general protoplasm of the ovum becomes aware, so to speak,
of the approach of a spermatozoön, and sends up a process to meet it.
(Fig. 35, A, B, C.) Several--or even many--spermatozoa may thus enter
the coat of the ovum; but normally only one proceeds further, or right
into the substance of the ovum, for the purpose of effecting
fertilization. This spermatozoön, as soon as it enters the periphery of
the yolk, or cell-substance proper, sets up a series of remarkable
phenomena. First, its own head rapidly increases in size, and takes on
the appearance of a cell-nucleus: this is called the male pronucleus. At
the same time its tail begins to disappear, and the enlarged head
proceeds to make its way directly towards the nucleus of the ovum which,
as before stated, is now called the female pronucleus. The latter in its
turn moves towards the former, and when the two meet they fuse into one
mass, forming a new nucleus. Before the two actually meet, the
spermatozoön has lost its tail altogether; and it is noteworthy that
during its passage through the protoplasmic cell-contents of the ovum,
it appears to exercise upon this protoplasm an attractive influence; for
the granules of the latter in its vicinity dispose themselves around it
in radiating lines. All these various phenomena are depicted in the
above wood-cuts. (Figs. 34, 35.)
Fertilization having been thus effected by fusion of the male and female
pronuclei into a single (or new) nucleus, this latter body proceeds to
exhibit complicated processes of karyokinesis, which, as before shown,
are preliminary to nuclear division in the case of egg-cells. Indeed the
karyokinetic process may begin in both the pronuclei before their
junction is effected; and, even when their junction is effected, it does
not appear that complete fusion of the so-called chromatin elements of
the two pronuclei takes place. For the purpose of explaining what this
means, and still more for the purpose of giving a general idea of the
karyokinetic processes as a whole, I will quote the following
description of them, because, for terseness combined with lucidity, it
is unsurpassable.
[Illustration: FIG. 36.--Karyokinesis of a typical tissue-cell
(epithelium of Salamander). (After Flemming and Klein.) The series
from A to I represents the successive stages in the movement of the
chromatin fibres during division, excepting G, which represents the
"nucleus-spindle" of an egg-cell. A, resting nucleus; D,
wreath-form; E, single star, the loops of the wreath being broken;
F, separation of the star into two groups of U-shaped fibres; H,
diaster or double star; I, completion of the cell-division and
formation of two resting nuclei. In G the chromatin fibres are
marked _a_, and correspond to the "equatorial plate"; _b_,
achromatin fibres forming the nucleus-spindle; _c_, granules of the
cell-protoplasm forming a "polar star." Such a polar star is seen at
each end of the nucleus-spindle, and is not to be confused with the
diaster H, the two ends of which are composed of _chromatin_.]
Researches, chiefly due to Flemming, have shown that the nucleus in
very many tissues of higher plants and animals consists of a
capsule containing a plasma of "achromatin," not deeply stained by
re-agents, ramifying in which is a reticulum of "chromatin"
consisting of fibres which readily take a deep stain. (Fig. 36, A).
Further it is demonstrated that, when the cell is about to divide
into two, definite and very remarkable movements take place in the
nucleus, resulting in the disappearance of the capsule and in the
arrangement of its fibres first in the form of a wreath (D), and
subsequently (by the breaking of the loops formed by the fibres) in
the form of a star (E). A further movement within the nucleus leads
to an arrangement of the broken loops in two groups (F), the
position of the open ends of the broken loops being reversed as
compared with what previously obtained. Now the two groups diverge,
and in many cases a striated appearance of the achromatin substance
between the two groups of chromatin loops is observable (H). In
some cases (especially egg-cells) this striated arrangement of the
achromatin is then termed a "nucleus-spindle," and the group of
chromatin loops (G, _a_) is known as "the equatorial plate." At
each end of the nucleus-spindle in these cases there is often seen
a star consisting of granules belonging to the general protoplasm
of the cell (G, _c_). These are known as "polar stars." After the
separation of the two sets of loops (H) the protoplasm of the
general substance of the cell becomes constricted, and division
occurs, so as to include a group of chromatin loops in each of the
two fission products. Each of these then rearranges itself together
with the associated chromatin into a nucleus such as was present in
the mother cell to commence with (I)[13].
[13] Ray Lankester, _Encyclop. Brit._, 9th ed., Vol. XIX, pp. 832-3.
Since the above was published, however, further progress has been made.
In particular it has been found that the chromatin fibres pass from
phase D to phase F by a process of longitudinal splitting (Fig. 37 _g_,
_h_; Fig. 38, VI, VII)--which is a point of great importance for
Weismann's theory of heredity,--and that the protoplasm outside the
nucleus seems to take as important a part in the karyokinetic process as
does the nuclear substance. For the so-called "attraction-spheres" (Fig.
38 II _a_, III, III _a_, VIII to XII), which were at first supposed to
be of subordinate importance in the process as a whole, are now known to
take an exceedingly active part in it (see especially IX to XI). Lastly,
it may be added that there is a growing consensus of authoritative
opinion, that the chromatin fibres are the seats of the material of
heredity, or, in other words, that they contain those essential elements
of the cell which endow the daughter-cells with their distinctive
characters. Therefore, where the parent-cell is an ovum, it follows from
this view that all hereditary qualities of the future organism are
potentially present in the ultra-microscopical structure of the
chromatin fibres.
[Illustration: FIG. 37.--Study of successive changes taking place in
the nucleus of an epithelium cell, preparatory to division of the
cell. (From _Quain's Anatomy_, after Flemming.) _a_, resting cell,
showing the nuclear network; _b_, first stage of division, the
chromatoplasm transformed into a skein of closely contorted
filaments; _c_ to _f_, further stages in the growth and looping
arrangement of the filaments; _g_, stellate phase, or aster; _h_,
completion of the splitting of the filaments, already begun in _f_
and _g_; _i_, _j_, _k_, successive stages in separation of the
filaments into two groups; _l_, the final result of this (diaster);
_m_ to _q_, stages in the division of the whole cell into two,
showing increasing contortion of the filaments, until they reach the
resting stage at _q_].
[Illustration: FIG. 38.--Formation and conjugation of the pronuclei
in _Ascaris megalocephala_. (From _Quain's Anatomy_, after E. von
Beneden.) _f_, female pronucleus; _m_, male pronucleus; _p_, one of
the polar bodies.
I. The second polar body has just been extruded; both male and
female pronuclei contain two chromatin particles; those of the male
pronucleus are becoming transformed into a skein. II. The chromatin
in both pronuclei now forms into a skein.
II _a_. The skeins are more distinct. Two attraction (or
protoplasmic) spheres, each with a central particle united with a
small spindle of achromatic fibres, have made their appearance in
the general substance of the egg close to the mutually approaching
pronuclei. The male pronucleus has the remains of the body of the
spermatozoön adhering to it.
III. Only the female pronucleus is shown in this figure. The skein
is contracted and thickened. The attraction-spheres are near one
side of the ovum, and are connected with its periphery by a cone of
fibres forming a polar circle, _p.c._; _e.c._, equatorial circle.
III _a_. The pronuclei have come into contact, and the
spindle-system is now arranged across their common axis.
IV. Contraction of the skein, and formation of two U-or V-shaped
chromatin fibres in each pronucleus.
V. The V-shaped chromatin filaments are now quite distinct: the male
and female pronuclei are in close contact.]
[Illustration: (38 continued)
VI., VII. The V-shaped filaments are splitting longitudinally; their
structure of fine granules of chromatin is apparent in VII., which
is more highly magnified. The conjugation of the pronuclei is
apparently complete in VII. The attraction-spheres and achromatic
spindle, although present, are not depicted in IV., V., VI., and
VII.
VIII. Equatorial arrangement of the four chromatin loops in the
middle of the now segmenting ovum: the achromatic substance forming
a spindle-shaped system of granules with fibres radiating from the
poles of the spindle (attraction-spheres); the chromatin forms an
equatorial plate. (Compare Fig. 36 G.)
IX. Shows diagrammatically the commencing separation of the
chromatin fibres of the conjugated nuclei, and the system of fibres
radiating from the attraction-spheres. (Compare again Fig. 36 G.)
_p.c._, polar circle; _e.c._, equatorial circle; _c.c._, central
particle.
X. Further separation of the chromatin filaments. Each of the
central particles of the attraction-spheres has divided into two.
XI. The chromatin fibres are becoming developed into the skeins of
the two daughter-nuclei. These are still united by fibres of
achromatin. The general protoplasm of the ovum is becoming divided.
XII. The two daughter-nuclei exhibit a chromatin network. Each of
the attraction-spheres has divided into two, which are joined by
fibres of achromatin, and connected with the periphery of the cell
in the same way as in the original or parent sphere, III.]
As I shall have more to say about these processes in the next volume,
when we shall see the important part which they bear in Weismann's
theory of heredity, it is with a double purpose that I here introduce
these yet further illustrations of them upon a somewhat larger scale.
The present purpose is merely that of showing, more clearly than
hitherto, the great complexity of these processes on the one hand, and,
on the other, the general similarity which they display in egg-cells and
in tissue-cells. But as in relation to this purpose the illustrations
speak for themselves, I may now pass on at once to the history of
embryonic development, which follows fertilization of the ovum.
* * * * *
We have seen that when the new nucleus of the fertilized ovum (which is
formed by a coalescence of the male pronucleus with the female) has
completed its karyokinetic processes, it is divided into two equal
parts; that these are disposed at opposite poles of the ovum; and that
the whole contents of the ovum are thereupon likewise divided into two
equal parts, with the result that there are now two nucleated cells
within the spherical wall of the ovum where before there had only been
one. Moreover, we have also seen that a precisely similar series of
events repeat themselves in each of these two cells, thus giving rise to
four cells (see Fig. 29). It must now be added that such duplication is
continued time after time, as shown in the accompanying illustrations
(Figs. 39, 40).
[Illustration: FIG. 39.--Segmentation of ovum. (After Häckel.)
Successive stages are marked by the letters A, B, C. D represents
several stages in advance of C.]
[Illustration: FIG. 40.--The contents of an ovum in an advanced
stage of segmentation, drawn in perspective. (After Häckel.)]
All this, it will be noticed, is a case of cell-multiplication, which
differs from that which takes place in the unicellular organisms only in
its being _invariably_ preceded (as far as we know) by karyokinesis, and
in the resulting cells being all confined within a common envelope, and
so in not being free to separate. Nevertheless, from what has already
been said, it will also be noticed that this feature makes all the
difference between a Metazoön and a Protozoön; so that already the ovum
presents the distinguishing character of a Metazoön.
I have dealt thus at considerable length upon the processes whereby the
originally unicellular ovum and spermatozoön become converted into the
multicellular germ, because I do not know of any other exposition of the
argument from Embryology where this, the first stage of the argument,
has been adequately treated. Yet it is evident that the fact of all the
processes above described being so similar in the case of sexual (or
metazoal) reproduction among the innumerable organisms where it occurs,
constitutes in itself a strong argument in favour of evolution. For the
mechanism of fertilization, and all the processes which even thus far we
have seen to follow therefrom, are hereby shown to be not only highly
complex, but likewise highly specialized. Therefore, the remarkable
similarity which they present throughout the whole animal kingdom--not
to speak of the vegetable--is expressive of organic continuity, rather
than of absolute discontinuity in every case, as the theory of special
creation must necessarily suppose. And it is evident that this argument
is strong in proportion to the uniformity, the specialization, and the
complexity of the processes in question.
Having occupied so much space with supplying what appear to me the
deficiencies in previous expositions of the argument from Embryology, I
can now afford to take only a very general view of the more important
features of this argument as they are successively furnished by all the
later stages of individual development. But this is of little
consequence, seeing that from the point at which we have now arrived
previous expositions of the argument are both good and numerous. The
following then is to be regarded as a mere sketch of the evidences of
phyletic (or ancestral) evolution, which are so abundantly furnished by
all the subsequent phases of ontogenetic (or individual) evolution.
The multicellular body which is formed by the series of segmentations
above described is at first a sphere of cells (Fig. 40). Soon, however,
a watery fluid gathers in the centre, and progressively pushes the cells
towards the circumference, until they there constitute a single layer.
The ovum, therefore, is now in the form of a hollow sphere containing
fluid, confined within a continuous wall of cells (Fig. 41 A). The next
thing that happens is a pitting in of one portion of the sphere (B). The
pit becomes deeper and deeper, until there is a complete invagination of
this part of the sphere--the cells which constitute it being
progressively pushed inwards until they come into contact with those at
the opposite pole of the ovum. Consequently, instead of a hollow sphere
of cells, the ovum now becomes an open sac, the walls of which are
composed of a double layer of cells (C). The ovum is now what has been
called a gastrula; and it is of importance to observe that probably all
the Metazoa pass through this stage. At any rate it has been found to
occur in all the main divisions of the animal kingdom, as a glance at
the accompanying figures will serve to show (Fig. 42)[14]. Moreover many
of the lower kinds of Metazoa never pass beyond it; but are all their
lives nothing else than gastrulæ, wherein the orifice becomes the mouth
of the animal, the internal or invaginated layer of cells the stomach,
and the outer layer the skin. So that if we take a child's india-rubber
ball, of the hollow kind with a hole in it, and push in one side with
our fingers till internal contact is established all round, by then
holding the indented side downwards we should get a very fair anatomical
model of a gastræa form, such as is presented by the adult condition of
many of the most primitive Metazoa--especially the lower
_Coelenterata_. The preceding figures represent two other such forms
in nature, the first locomotive and transitory, the second fixed and
permanent (Figs. 43, 44).
[14] In most vertebrated animals this process of gastrulation has
been more or less superseded by another, which is called
delamination; but it scarcely seems necessary for our present
purposes to describe the latter. For not only does it eventually
lead to the same result as gastrulation--i. e. the converting of the
ovum into a double-walled sac,--but there is good evidence among the
lower Vertebrata of its being preceded by gastrulation; so that,
even as to the higher Vertebrata, embryologists are pretty well
agreed that delamination has been but a later development of, or
possibly improvement upon, gastrulation.
[Illustration: FIG. 41.--Formation of the gastrula of _Amphioxus_.
(After Kowalevsky.) A, wall of the ovum, composed of a single layer
of cells; B, a stage in the process of gastrulation; C, completion
of the process; S, original or segmentation cavity of ovum; _al_,
alimentary cavity of gastrula; _ect_, outer layer of cells; _ent_,
inner layer of cells; _b_, orifice, constituting the mouth in
permanent forms.]
[Illustration: FIG. 42.--Gastrulation. A, Gastrula of a Zoophyte
(_Gastrophysema_). (After Häckel.) B, Gastrula of a Worm
(_Sagitta_). (After Kowalevsky.) C, Gastrula of an Echinoderm
(_Uraster_). (After A. Agassiz.) D, Gastrula of an Arthropod
(_Nauplius_). (After Häckel.) E, Gastrula of a Mollusk (_Limnæus_).
(After Rabl.) F, Gastrula of a Vertebrate (_Amphioxus_). (After
Kowalevsky.) In all, _d_, indicates the intestinal cavity; _o_, the
primitive mouth; _s_, the cleavage-cavity; _i_, the endoderm, or
intestinal layer; _e_, the ectoderm or skin-layer.]
[Illustration: FIG. 43.--Gastrula of a Chalk Sponge. (After Häckel.)
A, External view. B, Longitudinal section. _g_, digestive cavities;
_o_, mouth; _i_, endoderm; _e_, ectoderm.]
[Illustration: FIG. 44.--_Prophysema primordiale_, an extant
gastræa-form. (After Häckel.) (A). External view of the whole
animal, attached by its foot to seaweed. (B). Longitudinal section
of the same. The digestive cavity (_d_) opens at its upper end in
the mouth (_m_). Among the cells of the endoderm (_g_) lie amoeboid
egg-cells of large size (_e_). The ectoderm (_h_) is encrusted with
grains of sand, above the sponge spicules.]
Here, then, we leave the lower forms of Metazoa in their condition of
permanent gastrulæ. They differ from the transitory stage of other
Metazoa only in being enormously larger (owing to greatly further
_growth_, without any further _development_ as to matters of fundamental
importance), and in having sundry tentacles and other organs added later
on to meet their special requirements. The point to remember is, that in
all cases a gastrula is an open sac composed of two layers of cells--the
outer layer being called the ectoderm, and the inner the endoderm. They
have also been called the animal layer and the vegetative layer, because
it is the outer layer (ectoderm) that gives rise to all the organs of
sensation and movement--viz. the skin, the nervous system, and the
muscular system; while it is the inner layer (endoderm) that gives rise
to all the organs of nutrition and reproduction. It is desirable only
further to explain that gastrulation does not take place in all the
Metazoa after exactly the same plan. In different lines of descent
various and often considerable modifications of the original and most
simple plan have been introduced; but I will not burden the present
exposition by describing these modifications[15]. It is enough for us
that they always end in the formation of the two primary layers of
ectoderm and endoderm.
[15] The most extreme of them is that which is mentioned in the last
foot-note.
The next stage of differentiation is common to all the Metazoa, except
those lowest forms which, as we have just seen, remain permanently as
large gastrulæ, with sundry specialized additions in the way of
tentacles, &c. This stage of differentiation consists in the formation
of either a pouch or an additional layer between the ectoderm and the
endoderm, which is called the mesoderm. It is probably in most cases
derived from the endoderm, but the exact mode of its derivation is still
somewhat obscure. Sometimes it has the appearance of itself constituting
two layers; but it is needless to go into these details; for in any case
the ultimate result is the same--viz. that of converting the Metazoön
into the form of a tube, the walls of which are composed of concentric
layers of cells. The outermost layer afterwards gives rise to the
epidermis with its various appendages, and also to the central nervous
system with its organs of special sense. The median layer gives rise to
the voluntary muscles, bones, cartilages, &c., the nutritive systems of
the blood, the chyle, the lymph, and the muscular tube of the intestine.
Lastly, the innermost layer developes into the epithelium lining of the
intestine, with its various appendages of liver, lungs, intestinal
glands, &c.
I have just said that this three or four layered stage is shared by all
the Metazoa, except those very lowest forms--such as sponges and
jelly-fish--which do not pass on to it. But from this point the
developmental histories of all the main branches of the Metazoa
diverge--the Vermes, the Echinodermata, the Mollusca, the Articulata,
and the Vertebrata, each taking a different road in their subsequent
evolution. I will therefore confine attention to only one of these
several roads or methods, namely, that which is followed by the
Vertebrata--observing merely that, if space permitted, the same
principles of progressive though diverging histories of evolution would
equally well admit of being traced in all the other sub-kingdoms which
have just been named.
In order to trace these principles in the case of the Vertebrata, it is
desirable first of all to obtain an idea of the anatomical features
which most essentially distinguish the sub-kingdom as a whole. The
following, then, is what may be termed the ideal plan of vertebrate
organization, as given by Prof. Häckel. First, occupying the major axis
of body we perceive the primitive vertebral column. The parts lying
above this axis are those which have been developed from the ectoderm
and mesoderm--viz. voluntary muscles, central nervous system, and organs
of special sense. The parts lying below this axis are for the most part
those which have been developed from the endoderm--namely, the
digestive tract with its glandular appendages, the circulating system
and the respiratory system. In transverse section, therefore, the ideal
vertebrate consists of a solid axis, with a small tube occupied by the
nervous system above, and a large tube, or body-cavity, below. This
body-cavity contains the viscera, breathing organs, and heart, with its
prolongations into the main blood-vessels of the organism. Lastly, on
either side of the central axis are to be found large masses of
muscle--two on the dorsal and two on the ventral. As yet, however, there
are no limbs, nor even any bony skeleton, for the primitive vertebral
column is hitherto unossified cartilage. This ideal animal, therefore,
is to all appearance as much like a worm as a fish, and swims by means
of a lateral undulation of its whole body, assisted, perhaps, by a
dorsal fin formed out of skin.
[Illustration: FIG. 45.--Ideal primitive vertebrate, seen from the
left side. (After Häckel.) _na_, nose; _au_, eye; _g_, ear; _md_,
mouth; _ks_, gill-openings; _x_, notochord; _mr_, spinal tube; _kg_,
gill-vessels; _k_, gill-intestine; _hz_, heart; _ms_, muscles; _ma_,
stomach; _v_, intestinal vein; _c_, body-cavity; _a_, aorta; _l_,
liver; _d_, small intestine; _e_, ovary; _h_, testes; _n_, kidney
canal; _af_, anus; _lh_, true or leather-skin; _oh_, outer-skin
(epidermis); _f_, skin-fold, acting as a fin.]
[Illustration: FIG. 46.--The same in transverse section through the
ovaries; lettering as in the preceding Fig.]
[Illustration: FIG. 47.--_Amphioxus lanceolatus_. (After Häckel.)
_a_, anus; _au_, eye; _b_, ventral muscles; _c_, body-cavity; _ch_,
notochord; _d_, intestine; _do_ and _du_, dorsal and ventral walls
of intestine; _f_, fin-seam; _h_, skin; _k_, gills; _ka_,
gill-artery; _lb_, liver; _lv_, liver-vein; _m 1_, brain-bladder; _m
2_, spinal marrow; _mg_, stomach; _o_, mouth; _p_, ventral pore;
_r_, dorsal muscle; _s_, tail-fin; _t_, aorta; _v_, intestinal vein;
_x_, boundary between gill-intestine and stomach-intestine; _y_,
hypobranchial groove.]
Now I should not have presented this ideal representation of a primitive
vertebrate--for I have very little faith in the "scientific use of the
imagination" where it aspires to discharge the functions of a Creator in
the manufacture of archetypal forms--I say I should not have presented
this ideal representative of a primitive vertebrate, were it not that
the ideal is actually realized in a still existing animal. For there
still survives what must be an immensely archaic form of vertebrate,
whose anatomy is almost identical with that of the imaginary type which
has just been described. I allude, of course, to _Amphioxus_, which is
by far the most primitive or generalized type of vertebrated animal
hitherto discovered. Indeed, we may say that this remarkable creature is
almost as nearly allied to a worm as it is to a fish. For it has no
specialized head, and therefore no skull, brain, or jaws: it is
destitute alike of limbs, of a centralized heart, of developed liver,
kidneys, and, in short, of most of the organs which belong to the other
Vertebrata. It presents, however, a rudimentary backbone, in the form of
what is called a notochord. Now a primitive dorsal axis of this kind
occurs at a very early period of embryonic life in all vertebrated
animals; but, with the exception of _Amphioxus_, in all other existing
Vertebrata this structure is not itself destined to become the permanent
or bony vertebral column. On the contrary, it gives way to, or is
replaced by, this permanent bony structure at a later stage of
development. Consequently, it is very suggestive that so distinctively
embryonic a structure as this temporary cartilaginous axis of all the
other known Vertebrata should be found actually persisting to the
present day as the permanent axis of _Amphioxus_. In many other
respects, likewise, the early embryonic history of other Vertebrata
refers us to the permanent condition of _Amphioxus_. In particular, we
must notice that the wall of the neck is always perforated by what in
_Amphioxus_ are the gill-openings, and that the blood-vessels as they
proceed from the heart are always distributed in the form of what are
called gill-arches, adapted to convey the blood round or through the
gills for the purpose of aeration. In all existing fish and other
gill-breathing Vertebrata, this arrangement is permanent. It is
likewise met with in a peculiar kind of worm, called _Balanoglossus_--a
creature so peculiar, indeed, that it has been constituted by Gegenbaur
a class all by itself. We can see by the wood-cuts that it presents a
series of gill-slits, like the homologous parts of the fishes with which
it is compared--i. e. fishes of a comparatively low type of
organization, which dates from a time before the development of external
gills. (Figs. 48, 49, 50.) Now, as I have already said, these
gill-_slits_ are supported internally by the gill-_arches_, or the
blood-vessels which convey the blood to be oxygenized in the branchial
apparatus (see below, Figs. 51, 52, 53); and the whole arrangement is
developed from the anterior part of the intestine--as is likewise the
respiratory mechanism of all the gill-breathing Vertebrata. That so
close a parallel to this peculiar mechanism should be met with in a
worm, is a strong additional piece of evidence pointing to the
derivation of the Vertebrata from the Vermes.
[Illustration: FIG. 48.--_Balanoglossus_. (After A. Agassiz.) _r_,
proboscis; _h_, collar; _k_, gill-slits; _d_, digestive posterior
intestine; _v_, intestinal vessel; _a_, anus.]
[Illustration: FIG. 49.--A large Sea-lamprey (_Petromyzon marinus_),
much reduced in size. (After Cuvier and Häckel.) A series of seven
gill-slits are visible.]
[Illustration: FIG. 50--Adult Shark (_Carcharias melanopterus_).
(After Cuvier and Häckel.)]
[Illustration: FIG. 51.--Diagram of heart and gill-arches of a fish.
(After Owen.)]
[Illustration: FIG. 52.--One gill-arch, with branchial fringe
attached. (After Owen.) H, Heart.]
[Illustration: FIG. 53.--Diagram of heart and gill-arches in a
lizard. (After Owen.) The gill-arches, _a a' a''_, and _b b' b''_,
are called aortic arches in air-breathing vertebrata.]
Well, I have just said that in all the gill-breathing Vertebrata, this
mechanism of gill-slits and vascular gill-arches in the front part of
the intestinal tract is permanent. But in the air-breathing Vertebrata
such an arrangement would obviously be of no use. Consequently, the
gill-slits in the sides of the neck (see Figs. 16 and 57, 58), and the
gill-arches of the large blood-vessels (Figs. 54, 55, 56), are here
exhibited only as transitory phases of development. But as such they
occur in all air-breathing Vertebrata. And, as if to make the homologies
as striking as possible, at the time when the gill-slits and the
gill-arches are developed in the embryonic young of air-breathing
Vertebrata, the heart is constructed upon the fish-like type. That is to
say, it is placed far forwards, and, from having been a simple tube as
in Worms, is now divided into two chambers, as in Fish. Later on it
becomes progressively pushed further back between the developing lungs,
while it progressively acquires the three cavities distinctive of
Amphibia, and finally the four cavities belonging only to the complete
double circulation of Birds and Mammals. Moreover, it has now been
satisfactorily shown that the lungs of air-breathing Vertebrata, which
are thus destined to supersede the function of gills, are themselves the
modified swim-bladder or float, which belongs to Fish. Consequently, all
these progressive modifications in the important organs of circulation
and respiration in the air-breathing Vertebrata, together make up as
complete a history of their aquatic pedigree as it would be possible for
the most exacting critic to require.
[Illustration: FIG. 54.--Ideal diagram, of primitive gill-or
aortic-arches. (After Rathke.) H, outline of heart. The arrows show
the course of the blood.]
[Illustration: FIG. 55.--The same, modified for a bird. (After Le
Conte.) The dark lines show the aortic arches which persist. A,
aorta; _p_, pulmonary arches; SC, S'C', sub-clavian; C, C',
carotids.]
[Illustration: FIG. 56.--The same, modified for a mammal. (After Le
Conte.)]
[Illustration: FIG. 57.--A series of embryos at three comparable and
progressive stages of development (marked I, II, III), representing
each of the classes of vertebrated animals below the Mammalia (After
Häckel.)]
[Illustration: FIG. 58.--Another series of embryos, also at three
comparable and progressive stages of development (marked I, II,
III), representing four different divisions of the class Mammalia.
(After Häckel.)]
If space permitted, it would be easy to present abundance of additional
evidence to the same effect from the development of the skeleton, the
skull, the brain, the sense-organs, and, in short, of every constituent
part of the vertebrate organization. Even without any anatomical
dissection, the similarity of all vertebrated embryos at comparable
stages of development admits of being strikingly shown, if we merely
place the embryos one beside the other. Here, for instance, are the
embryos of a fish, a salamander, a tortoise, a bird, and four different
mammals. In each case three comparable stages of development are
represented. Now, if we read the series horizontally, we can see that
there is very little difference between the eight animals at the
earliest of the three stages represented--all having fish-like tails,
gill-slits, and so on. In the next stage further differentiation has
taken place, but it will be observed that the limbs are still so
rudimentary that even in the case of Man they are considerably shorter
than the tail. But in the third stage the distinctive characters are
well marked.
* * * * *
So much then for an outline sketch of the main features in the embryonic
history of the Vertebrata. But it must be remembered that the science of
comparative embryology extends to each of the other three great branches
of the tree of life, where these take their origin, through the worms,
from the still lower, or gastræa, forms. And in each of these three
great branches--namely, the Echinodermata, the Mollusca, and the
Arthropoda--we have a repetition of just the same kind of evidence in
favour of continuous descent, with adaptive modification in sundry
lines, as that which I have thus briefly sketched in the case of the
Vertebrata. The roads are different, but the method of travelling is the
same. Moreover, when the embryology of the Worms is closely studied, the
origin of these different roads admits of being clearly traced. So that
when all this mass of evidence is taken together, we cannot wonder that
evolutionists should now regard the science of comparative embryology as
the principal witness to their theory.
CHAPTER V.
PALÆONTOLOGY.
The present Chapter will be devoted to a consideration of the evidence
of organic evolution which has been furnished by the researches of
geologists. On account of its direct or historical nature, this branch
of evidence is popularly regarded as the most important--so much so,
indeed, that in the opinion of most educated persons the whole doctrine
of organic evolution must stand or fall according to the so-called
"testimony of the rocks." Now, without at all denying the peculiar
importance of this line of evidence, I must begin by remarking that it
does not present the denominating importance which popular judgment
assigns to it. For although popular judgment is right in regarding the
testimony of the rocks as of the nature of a history, this judgment, as
a rule, is very inadequately acquainted with the great imperfections of
that history. Knowing in a general way what magnificent advances the
science of geology has made during the present century, the public mind
is more or less imbued with the notion, that because we now possess a
tolerably complete record of the chronological succession of geological
formations, we must therefore possess a correspondingly complete record
of the chronological succession of the forms of life which from time to
time have peopled the globe. Now in one sense this notion is partly
true, but in another sense it is profoundly false. It is partly true if
we have regard only to those larger divisions of the vegetable or animal
kingdoms which naturalists designate by the terms classes and orders.
But the notion becomes progressively more untrue when it is applied to
families and genera, while it is most of all untrue when applied to
species. That this must be so may be rendered apparent by two
considerations.
In the first place, it does not follow that because we have a tolerably
complete record of the succession of geological formations, we have
therefore any correspondingly complete record of their fossiliferous
contents. The work of determining the relative ages of the rocks does
not require that every cubic mile of the earth's surface should be
separately examined, in order to find all the different fossils which it
may contain. Were this the case, we should hitherto have made but very
small progress in our reading of the testimony of the rocks. The
relative ages of the rocks are determined by broad comparative surveys
over extensive areas; and although the identification of widely
separated deposits is often greatly assisted by a study of their
fossiliferous contents, the mere pricking of a continent here and there
is all that is required for this purpose. Hence, the accuracy of our
information touching the relative ages of geological strata does not
depend upon--and, therefore, does not betoken--any equivalent accuracy
of knowledge touching the fossiliferous material which these strata may
at the present time actually contain. And, as we well know, the
opportunities which the geologist has of discovering fossils are
extremely limited, if we consider these opportunities in relation to the
area of geological formations. The larger portion of the earth's surface
is buried beneath the sea; and much the larger portion of the
fossiliferous deposits on shore are no less hopelessly buried beneath
the land. Therefore it is only upon the fractional portion of the
earth's surface which at the present time happens to be actually exposed
to his view that the geologist is able to prosecute his search for
fossils. But even here how miserably inadequate this search has hitherto
been! With the exception of a scratch or two in the continents of Asia
and America, together with a somewhat larger number of similar scratches
over the continent of Europe, even that comparatively small portion of
the earth's surface which is available for the purpose has been hitherto
quite unexplored by the palæontologist. How enormously rich a store of
material remains to be unearthed by the future scratchings of this
surface, we may dimly surmise from the astonishing world of bygone life
which is now being revealed in the newly discovered fossiliferous
deposits on the continent of America.
But, besides all this, we must remember, in the second place, that all
the fossiliferous deposits in the world, even if they could be
thoroughly explored, would still prove highly imperfect, considered as a
history of extinct forms of life. In order that many of these forms
should have been preserved as fossils, it is necessary that they should
have died upon a surface neither too hard nor too soft to admit of their
leaving an impression; that this surface should afterwards have
hardened sufficiently to retain the impression; that it should then have
been protected from the erosion of water, as well as from the
disintegrating influence of the air; and yet that it should not have
sunk far enough beneath the surface to have come within the no less
disintegrating influence of subterranean heat. Remembering thus, as a
general rule, how many conditions require to have met before a fossil
can have been both formed and preserved, we must conclude that the
geological record is probably as imperfect in itself as are our
opportunities of reading even the little that has been recorded. If we
speak of it as a history of the succession of life upon the planet, we
must allow, on the one hand, that it is a history which merits the name
of a "chapter of accidents"; and, on the other hand, that during the
whole course of its compilation pages were being destroyed as fast as
others were being formed, while even of those that remain it is only a
word, a line, or at most a short paragraph here and there, that we are
permitted to see. With so fragmentary a record as this to study, I do
not think it is too much to say that no conclusions can be fairly based
upon it, merely from the absence of testimony. Only if the testimony
were positively opposed to the theory of descent, could any argument be
fairly raised against that theory on the grounds of this testimony. In
other words, if any of the fossils hitherto discovered prove the order
of succession to have been incompatible with the theory of genetic
descent, then the record may fairly be adduced in argument, because we
should then be in possession of definite information of a positive
kind, instead of a mere absence of information of any kind. But if the
adverse argument reaches only to the extent of maintaining that the
geological record does not furnish us with so complete a series of
"connecting links" as we might have expected, then, I think, the
argument is futile. Even in the case of human histories, written with
the intentional purpose of conveying information, it is an unsafe thing
to infer the non-occurrence of an event from a mere silence of the
historian--and this especially in matters of comparatively small detail,
such as would correspond (in the present analogy) to the occurrence of
_species_ and _genera_ as connecting links. And, of course, if the
history had only come down to us in fragments, no one would attach any
importance at all to what might have been only the _apparent_ silence of
the historian.
In view, then, of the unfortunate imperfection of the geological record
_per se_, as well as of the no less unfortunate limitation of our means
of reading even so much of the record as has come down to us, I conclude
that this record can only be fairly used in two ways. It may fairly be
examined for positive testimony against the theory of descent, or for
proof of the presence of organic remains of a high order of development
in a low level of strata. And it may be fairly examined for negative
testimony, or for the absence of connecting links, if the search be
confined to the larger taxonomic divisions of the fauna and flora of the
world. The more minute these divisions, the more restricted must have
been the areas of their origin, and hence the less likelihood of their
having been preserved in the fossil state, or of our finding them even
if they have been. Therefore, if the theory of evolution is true, we
ought not to expect from the geological record a full history of
_specific_ changes in any but at most a comparatively small number of
instances, where local circumstances happen to have been favourable for
the writing and preservation of such a history. But we might reasonably
expect to find a general concurrence of geological testimony to the
larger fact--namely, of there having been throughout all geological time
a uniform progression as regards the larger taxonomic divisions. And, as
I will next proceed to show, this is, in a general way, what we do find,
although not altogether without some important exceptions, with which I
shall deal in an Appendix.
There is no _positive_ proof _against_ the theory of descent to be drawn
from a study of palæontology, or proof of the presence of any kind of
fossils in strata where the fact of their presence is incompatible with
the theory of evolution. On the other hand, there is an enormous body of
uniform evidence to prove two general facts of the highest importance in
the present connexion. The first of these general facts is, that an
increase in the diversity of types both of plants and animals has been
constant and progressive from the earliest to the latest times, as we
should anticipate that it must have been on the theory of descent in
ever-ramifying lines of pedigree. And the second general fact is, that
through all these branching lines of ever-multiplying types, from the
first appearance of each of them to their latest known conditions, there
is overwhelming evidence of one great law of organic nature--the law of
gradual advance from the general to the special, from the low to the
high, from the simple to the complex.
Now, the importance of these large and general facts in the present
connexion must be at once apparent; but it may perhaps be rendered more
so if we try to imagine how the case would have stood supposing
geological investigation to have yielded in this matter an opposite
result, or even so much as an equivocal result. If it had yielded an
opposite result, if the lower geological formations were found to
contain as many, as diverse, and as highly organized types as the later
geological formations, clearly there would have been no room at all for
any theory of progressive evolution. And, by parity of reasoning, in
whatever degree such a state of matters were found to prevail, in that
degree would the theory in question have been discredited. But seeing
that these opposite principles do not prevail in any (relatively
speaking) considerable degree[16], we have so far positive testimony of
the largest and most massive character in favour of this theory. For
while all these large and general facts are very much what they ought to
be according to this theory, they cannot be held to lend any support at
all to the rival theory. In other words, it is clearly no essential part
of the theory of special creation that species should everywhere exhibit
this gradual multiplication as to number, coupled with a gradual
diversification and general elevation of types, in all the growing
branches of the tree of life. No one could adopt seriously the jocular
lines of Burns, to the effect that the Creator required to practise his
prentice hand on lower types before advancing to the formation of
higher. Yet, without some such assumption, it would be impossible to
explain, on the theory of independent creations, why there should have
been this gradual advance from the few to the many, from the general to
the special, from the low to the high.
[16] For objections which may be brought against this and similar
statements, see the Appendix.
+---+--------------------------+---------------------------------------
| |_Epochs and Formations._ |_Faunal Characters._
|C | |
|a |--------------------------+---------------------------------------
|i |POST-PLIOCENE. |Man. Mammalia principally of living
|n | Glacial Period. | species. Mollusca exclusively recent.
|o | +---------------------------------------
|z |PLIOCENE, 3,000 feet. |Mammalia principally of recent genera
|o | | --living species rare. Mollusca very
|i | | modern.
|c | +---------------------------------------
| |MIOCENE, 4,000 ft. |Mammalia principally of living
|o | | families; extinct genera numerous;
|r | | species all extinct. Mollusca largely
| |OLIGOCENE, 8,000 ft. | of recent species.
|T | +---------------------------------------
|e |EOCENE, 10,000 ft. |Mammalia with numerous extinct families
|r | | and orders; all the species and
|i | | most of the genera extinct. Modern
|a | | type Shell-Fish.
|r | |
|y | |
+---|--------------------------+---------------------------------------
| |LARAMIE, 4,000 ft. |Passage beds.
| |--------------------------+---------------------------------------
|M |CRETACEOUS, 12,000 ft. |Dinosaurian (bird-like) Reptiles;
|e | Chalk. | Pterodactyls (flying Reptiles);
|s | | toothed Birds; earliest Snake; bony
|o | | Fishes; Crocodiles; Turtles;
|z | | Ammonites.
|o | +---------------------------------------
|i |JURASSIC, 6,000 ft. |Earliest Birds; giant Reptiles
|c | Oolite. | (Ichthyosaurs, Dinosaurs,
| | Lias. | Pterodactyls); Ammonites; Clam- and
|o | | Snail-Shells very abundant; decline
|r | | of Brachiopods; Butterfly.
| | +---------------------------------------
|S |TRIAS, 5,000 ft. |First Mammalian (Marsupial); 2-gilled
|e | New Red Sandstone. | Cephalopods (Cuttle-Fishes,
|c | | Belemnites); reptilian Foot-Prints.
|o | |
|n | |
|d | |
|a | |
|r | |
|y | |
|---|--------------------------+---------------------------------------
|P |PERMIAN, 5,000 ft. |Earliest true Reptiles.
|a | +---------------------------------------
|l |CARBONIFEROUS, 26,000 ft. |Earliest Amphibian (Labyrinthodont);
|e | | extinction of Trilobites; first
|o | Coal. | Cray-fish; Beetles; Cockroaches;
|z | | Centipedes; Spiders.
|o | +---------------------------------------
|i |DEVONIAN, 18,000 ft. |Cartilaginous and Ganoid Fishes;
|c | Old Red Sandstone. | earliest and (snail) and freshwater
| | | Shells; Shell-Fish abundant; decline
|o | | of Trilobites; May-flies; Crab.
|r | +---------------------------------------
| |SILURIAN, 33,000 ft. |Earliest Fish; the first Air-Breathers
|P | | (Insect, Scorpion); Brachiopods and
|r | | 4-gilled Cephalopods very abundant;
|i | | Trilobites; Corals; Graptolites.
|m | +---------------------------------------
|a |CAMBRIAN, 24,000 ft. |Trilobites; Brachiopod Mollusks.
|r | |
|y | |
|---|--------------------------+---------------------------------------
|A |ARCHÆEAN, 30,000 ft. |
|z | Huronian. |Eozoön, (probably not a fossil).
|o | Laurentian. |
|i |--------------------------+---------------------------------------
|c |PRIMEVAL. |Non-sedimentary.
+------------------------------+--------------------------------------+
I submit, then, that so far as the largest and most general principles
in the matter of palæontology are concerned, we have about as strong and
massive a body of evidence as we could reasonably expect this branch of
science to yield; for it is at once enormous in amount and positive in
character. Therefore, if I do not further enlarge upon the evidence
which we here have, as it were _en masse_, it is only because I do not
feel that any words could add to its obvious significance. It may best
be allowed to speak for itself in the millions of facts which are
condensed in this tabular statement of the order of succession of all
the known forms of animal life, as presented by the eminent
palæontologist, Professor Cope[17].
[17] For difficulties and objections, see Appendix.
Or, taking a still more general survey, this tabular statement may be
still further condensed, and presented in a diagrammatic form, as it has
been by another eminent American palæontologist, Prof. Le Conte, in his
excellent little treatise on _Evolution and its Relations to Religious
Thought_. The following is his diagrammatic representation, with his
remarks thereon.
When each ruling class declined in importance, it did not perish,
but continued in a subordinate position. Thus, the whole organic
kingdom became not only higher and higher in its highest forms, but
also more and more complex in its structure and in the interaction
of its correlated parts. The whole process and its result is
roughly represented in the accompanying diagram, in which A B
represents the course of geological time, and the curve, the rise,
culmination, and decline of successive dominate classes.
[Illustration: FIG. 59.--Diagram of Geological Succession of the
Classes of the Animal Kingdom. (After Le Conte.)]
I will here leave the evidence which is thus yielded by the most general
principles that have been established by the science of palæontology;
and I will devote the rest of this chapter to a detailed consideration
of a few highly special lines of evidence. By thus suddenly passing from
one extreme to the other, I hope to convey the best idea that can be
conveyed within a brief compass of the minuteness, as well as the
extent, of the testimony which is furnished by the rocks.
* * * * *
When Darwin first published his _Origin of Species_, adverse critics
fastened upon the "missing-link" argument as the strongest that they
could bring against the theory of descent. Although Darwin had himself
strongly insisted on the imperfection of the geological record, and the
consequent precariousness of any negative conclusions raised upon it,
these critics maintained that he was making too great a demand upon the
argument from ignorance--that, even allowing for the imperfection of the
record, they would certainly have expected at least a few cases of
testimony to _specific_ transmutation. For, they urged in effect,
looking to the enormous profusion of the extinct species on the one
hand, and to the immense number of known fossils on the other, it was
incredible that no satisfactory instances of specific transmutation
should ever have been brought to light, if such transmutation had ever
occurred in the universal manner which the theory was bound to suppose.
But since Darwin first published his great work palæontologists have
been very active in discovering and exploring fossiliferous beds in
sundry parts of the world; and the result of their labours has been to
supply so many of the previously missing links that the voice of
competent criticism in this matter has now been well-nigh silenced.
Indeed, the material thus furnished to an advocate of evolution at the
present time is so abundant that his principal difficulty is to select
his samples. I think, however, that the most satisfactory result will be
gained if I restrict my exposition to a minute account of some few
series of connecting links, rather than if I were to take a more general
survey of a larger number. I will, therefore, confine the survey to the
animal kingdom, and there mention only some of the cases which have
yielded well-detailed proof of continuous differentiation.
It is obvious that the parts of animals most likely to have been
preserved in such a continuous series of fossils as the present line of
evidence requires, would have been the hard parts. These are horns,
bones, teeth, and shells. Therefore I will consider each of these four
classes of structures separately.
* * * * *
Horns wherever they occur, are found to be of high importance for
purposes of classification. They are restricted to the Ruminants, and
appear under three different forms or types--namely solid, as in
antelopes; hollow, as in sheep; and deciduous, as in deer. Now, in each
of these divisions we have a tolerably complete palæontological history
of the evolution of horns. The early ruminants were altogether hornless
(Fig. 60). Then, in the middle Miocene, the first antelopes appeared
with tiny horns, which progressively increased in size among the
ever-multiplying species of antelopes until the present day. But it is
in the deer tribe that we meet with even better evidence touching the
progressive evolution of horns; because here not only size, but shape,
is concerned. For deer's horns, or antlers, are arborescent; and hence
in their case we have an opportunity of reading the history, not only of
a progressive growth in size, but also of an increasing development of
form. Among the older members of the tribe, in the lower Miocene, there
are no horns at all. In the mid-Miocene we meet with two-pronged horns
(_Cervus dicrocerus_, Figs. 61, 62, 1/5 nat. size). Next, in the upper
Miocene (_C. matheronis_, Fig. 63, 1/8 nat. size), and extending into
the Pliocene (_C. pardinensis_, Fig. 64, 1/18 nat. size), we meet with
three-pronged horns. Then, in the Pliocene we find also four-pronged
horns (_C. issiodorensis_, Fig. 65, 1/16 nat. size), leading us to
five-pronged (_C. tetraceros_). Lastly, in the Forest-bed of Norfolk we
meet with arborescent horns (_C. Sedgwickii_, Fig. 66, 1/35 nat. size).
The life-history of existing stags furnishes a parallel development
(Fig. 67), beginning with a single horn (which has not yet been found
palæontologically), going on to two prongs, three prongs, four prongs,
and afterwards branching.
[Illustration: FIG. 60.--Skull of _Oreodon Culbertsoni_. (After
Leidy.)]
[Illustration: FIGS. 61-66. The series is reduced from Gaudry's
illustrations, after Farge, Croizet, Jobert and Boyd Dawkins.]
[Illustration: FIG. 67.--Successive stages in the development of an
existing Deer's Antlers. (After Gaudry, but a better illustration
has already been given on p. 100.)]
* * * * *
Coming now to bones, we have a singularly complete record of transition
from one type or pattern of structure to another in the phylogenetic
history of tails. This has been so clearly and so tersely conveyed by
Prof. Le Conte, that I cannot do better than quote his statement.
It has long been noticed that there are among fishes two styles of
tail-fins. These are the even-lobed, or homocercal (Fig. 68), and
the uneven-lobed, or heterocercal (Fig. 69). The one is
characteristic of ordinary fishes (teleosts), the other of sharks
and some other orders. In structure the difference is even more
fundamental than in form. In the former style the backbone stops
abruptly in a series of short, enlarged joints, and thence sends
off rays to form the tail-fin (Fig. 68); in the latter the
backbone runs through the fin to its very point, growing slenderer
by degrees, and giving off rays above and below from each joint,
but the rays on the lower side are much longer (Fig. 69). This type
of fin is, therefore, _vertebrated_, the other _non-vertebrated_.
Figs. 68 and 69 show these two types in form and structure. But
there is still another type found only in the lowest and most
generalized forms of fishes. In these the tail-fin is vertebrated
and yet symmetrical. This type is shown in Fig. 70.
[Illustration: FIG. 68.--Homocercal Tail, showing (A) external form
and (B) internal structure.]
[Illustration: FIG. 69.--Heterocercal Tail, showing (A) external
form and (B) internal structure.]
[Illustration: FIG. 70.--Vertebrated but symmetrical fin
(diphycercal), showing (A) external form and (B) internal
structure.]
Now, in the development of a teleost fish (Fig. 68), as has been
shown by Alexander Agassiz, the tail-fin is first like Fig. 70;
then becomes heterocercal, like Fig. 69; and, finally, becomes
homocercal like Fig. 68. Why so? Not because there is any special
advantage in this succession of forms; for the changes take place
either in the egg or else in very early embryonic states. The
answer is found in the fact that _this is the order of change in
the phylogenetic series_. The earliest fish-tails were either like
Fig. 69 or Fig. 70; never like Fig. 68. The earliest of all were
almost certainly like Fig. 70; then they became like Fig. 69; and,
finally, only much later in geological history (Jurassic or
Cretaceous), they became like Fig. 68. This order of change is
still retained in the embryonic development of the last introduced
and most specialized order of existing fishes. The family history
is repeated in the individual history.
Similar changes have taken place in the form and structure of
birds' tails. The earliest bird known--the Jurassic
_Archæopteryx_--had a long reptilian tail of twenty-one joints,
each joint bearing a feather on each side, right and left (Fig.
71): [see also Fig. 73]. In the typical modern bird, on the
contrary, the tail-joints are diminished in number, shortened up,
and enlarged, and give out long feathers, fan-like, to form the
so-called tail (Fig. 72). The _Archæopteryx'_ tail is
_vertebrated_, the typical bird's _non-vertebrated_. This
shortening up of the tail did not take place at once, but
gradually. The Cretaceous birds, intermediate in time, had tails
intermediate in structure. The _Hesperornis_ of Marsh had twelve
joints. At first--in Jurassic strata--the tail is fully a half of
the whole vertebral column. It then gradually shortens up until it
becomes the aborted organ of typical modern birds. Now, in
embryonic development, the tail of the modern typical bird _passes
through all these stages_. At first the tail is nearly one half the
whole vertebral column; then, as development goes on, while the
rest of the body grows, the growth of the tail stops, and thus
finally becomes the aborted organ we now find. The ontogeny still
passes through the stages of the phylogeny. The same is true of all
tailless animals.
[Illustration: FIG. 71.--Tail of _Archæopteryx_. A indicates origin
of simply-jointed tail.]
[Illustration: FIG. 72.--Tail of modern Bird. The numerals indicate
the foreshortened, enlarged, and consolidated joints; _f_, terminal
segment of the vertebral column; D, shafts of feathers.]
[Illustration: FIG. 73.--_Archæopteryx macura_, restored, 1/2 nat.
size. (After Flower.) The section of the tail is copied from Owen,
nat. size.]
The extinct _Archæopteryx_ above alluded to presents throughout its
whole organization a most interesting assemblage of "generalized
characters." For example, its teeth, and its still unreduced digits of
the wings (which, like those of the feet, are covered with scales),
refer us, with almost as much force as does the vertebrated tail, to the
Sauropsidian type--or the trunk from which birds and reptiles have
diverged.
We will next consider the palæontological evidence which we now
possess of the evolution of mammalian limbs, with special reference to
the hoofed animals, where this line of evidence happens to be most
complete.
I may best begin by describing the bones as these occur in the sundry
branches of the mammalian type now living. As we shall presently see,
the modifications which the limbs have undergone in these sundry
branches chiefly consist in the suppression of some parts and the
exaggerated development of others. But, by comparing all mammalian limbs
together, it is easy to obtain a generalized type of mammalian limb,
which in actual life is perhaps most nearly conformed to in the case of
bears. I will therefore choose the bear for the purpose of briefly
expounding the bones of mammalian limbs in general--merely asking it to
be understood, that although in the case of many other mammalia some of
these bones may be dwindled or altogether absent, while others may be
greatly exaggerated as to relative size, in no case do any _additional_
bones appear.
On looking, then, at the skeleton of a bear (Fig. 74), the first thing
to observe is that there is a perfect serial homology between the bones
of the hind legs and of the fore legs. The thigh-bone, or femur,
corresponds to the shoulder-bone, or humerus; the two shank bones (tibia
and fibula) correspond to the two arm-bones (radius and ulna); the many
little ankle-bones (tarsals) correspond to the many little wrist-bones
(carpals); the foot-bones (meta-tarsals) correspond to the hand-bones
(meta-carpals); and, lastly, the bones of each of the toes correspond to
those of each of the fingers.
[Illustration: FIG. 74.--Skeleton of Polar Bear, drawn from nature
(_Brit. Mus._).]
The next thing to observe is, that the disposition of bones in the case
of the bear is such that the animal walks in the way that has been
called plantigrade. That is to say, all the bones of the fingers, as
well as those of the toes, feet, and ankles, rest upon the ground, or
help to constitute the "soles." Our own feet are constructed on a
closely similar pattern. But in the majority of living mammalian forms
this is not the case. For the majority of mammals are what has been
called digitigrade. That is to say, the bones of the limb are so
disposed that both the foot and hand bones, and therefore also the ankle
and wrist, are removed from the ground altogether, so that the animal
walks exclusively upon its toes and fingers--as in the case of this
skeleton (Fig. 75), which is the skeleton of a lion. The next figures
display a series of limbs, showing the progressive passage of a
completely plantigrade into a highly digitigrade type--the curved lines
of connexion serving to indicate the homologous bones (Figs. 76, 77).
[Illustration: _Fig_. 75.--Skeleton of Lion. (After Huxley.)]
[Illustration: FIG. 76.--Anterior limb of Man, Dog, Hog, Sheep, and
Horse. (After Le Conte.) _Sc_, shoulder-blade; _c_, coracoid; _a_,
_b_, bones of fore-arm; 5, bones of the wrist; 6, bones of the hand;
7, bones of the fingers.]
[Illustration: FIG. 77.--Posterior limb of Man, Monkey, Dog, Sheep
and Horse. (After Le Conte.) 1, Hip-joint; 2, thigh-bone; 3,
knee-joint; 4, bones of leg; 5, ankle-joint; 6, bones of foot; 7,
bones of toes.]
I will now proceed to detail the history of mammalian limbs, as this has
been recorded for us in fossil remains.
The most generalized or primitive types of limb hitherto discovered in
any vertebrated animal above the class of fishes, are those which are
met with in some of the extinct aquatic reptiles. Here, for instance, is
a diagram of the left hind limb of _Baptanodon discus_ (Fig. 78). It has
six rows of little symmetrical bones springing from a leg-like origin.
But the whole structure resembles the fin of a fish about as nearly as
it does the leg of a mammal. For not only are there six rows of bones,
instead of five, suggestive of the numerous rays which characterise the
fin of a fish; but the structure as a whole, having been covered over
with blubber and skin, was throughout flexible and unjointed--thus in
function, even more than in structure, resembling a fin. In this
respect, also, it must have resembled the paddle of a whale (see Fig.
79); but of course the great difference will be noted, that the paddle
of a whale reveals the dwindled though still clearly typical bones of a
true mammalian limb; so that although in outward form and function these
two paddles are alike, their inward structure clearly shows that while
the one testifies to the absence of evolution, the other testifies to
the presence of degeneration. If the paddle of _Baptanodon_ had occurred
in a whale, or the paddle of a whale had occurred in _Baptanodon_,
either fact would in itself have been well-nigh destructive of the whole
theory of evolution.
[Illustration: FIG. 78.--A, posterior limb of _Baptanodon discus_.
(After Marsh.) F, thigh-bone; I to VI, undifferentiated bones of the
leg and foot. B, anterior limb of _Chelydra serpentina_. (After
Gegenbaur.) U and R, bones of the fore-arm; I to V, fully
differentiated bones of the hand, following those of the wrist.]
[Illustration: FIG. 79.--Paddle of a Whale.]
Such, then, is the most generalized as it is the most ancient type of
vertebrate limb above the class of fishes. Obviously it is a type
suited only to aquatic life. Consequently, when aquatic Vertebrata began
to become terrestrial, the type would have needed modification in order
to serve for terrestrial locomotion. In particular, it would have needed
to gain in consolidation and in firmness, which means that it would have
needed also to become jointed. Accordingly, we find that this archaic
type gave place in land-reptiles to the exigencies of these
requirements. Here for example is a diagram, copied from Gegenbaur, of
the right fore-foot of _Chelydra serpentina_ (Fig. 78). As compared with
the homologous limb of its purely aquatic predecessor, there is to be
noticed the disappearance of one of the six rows of small bones, a
confluence of some of the remainder in the other five rows, a
duplication of the arm-bone into a radius and ulna, in order to admit of
jointed rotation of the hand, and a general disposition of the small
bones below these arm-bones, which clearly foreshadows the joint of the
wrist. Indeed, in this fore-foot of _Chelydra_, a child could trace all
the principal homologies of the mammalian counterpart, growing, like the
next stage in a dissolving view, out of the primitive paddle of
_Baptanodon_--namely, first the radius and ulna, next the carpals, then
the meta-carpals, and, lastly, the three phalanges in each of the five
digits.
Such a type of foot no doubt admirably meets the requirements of slow
reptilian locomotion over swampy ground. But for anything like rapid
locomotion over hard and uneven ground, greater modifications would be
needed. Such modifications, however, need not be other in kind: it is
enough that they should continue in the same line of advance, so as to
reach a higher degree of firmness, combined with better joints.
Accordingly we find that this took place, not indeed among reptiles,
whose habits of cold-blooded life have not changed, but among their
warm-blooded descendants, the mammals. Moreover, when we examine the
whole mammalian series, we find that the required modifications must
have taken place in slightly different ways in three lines of descent
simultaneously. We have first the plantigrade and digitigrade
modifications already mentioned (pp. 178, 179) Of these the plantigrade
walking entailed least change, because most resembling the ancestral or
lizard-like mode of progression. All that was here needed was a general
improvement as to relative lengths of bones, with greater consolidation
and greater flexibility of joints. Therefore I need not say anything
more about the plantigrade division. But the digitigrade modification
necessitated a change of structural plan, to the extent of raising the
wrist and ankle joints off the ground, so as to make the quadruped walk
on its fingers and toes. We meet with an interesting case of this
transition in the existing hare, which while at rest supports itself on
the whole hind foot after the manner of a plantigrade animal, but when
running does so upon the ends of its toes, after the manner of a
digitigrade animal.
It is of importance for us to note that this transition from the
original plantigrade to the more recent digitigrade type, has been
carried out on two slightly different plans in two different lines of
mammalian descent. The hoofed mammals--which are all digitigrade--are
sub-classified as artiodactyls and perissodactyls, i. e. even-toed and
odd-toed. Now, whether an animal has an even or an odd number of toes
may seem a curiously artificial distinction on which to found so
important a classification of the mammalian group. But if we look at the
matter from a less empirical and more intelligent point of view, we
shall see that the alternative of having an even or an odd number of
toes carries with it alternative consequences of a practically important
kind to any animal of the digitigrade type. For suppose an aboriginal
five-toed animal, walking on the ends of its five toes, to be called
upon to resign some of his toes. If he is left with an even number, it
must be two or four; and in either case the animal would gain the
firmest support by so disposing his toes as to admit of the axis of his
foot passing between an equal number of them--whether it be one or two
toes on each side. On the other hand, if our early mammal were called
upon to retain an odd number of toes, he would gain best support by
adjusting matters so that the axis of his foot should be coincident with
his middle toe, whether this were his only toe, or whether he had one on
either side of it. This consideration shows that the classification into
even-toed and odd-toed is not so artificial as it no doubt at first
sight appears. Let us, then, consider the stages in the evolution of
both these types of feet.
Going back to the reptile _Chelydra_, it will be observed that the axis
of the foot passes down the middle toe, which is therefore supported by
two toes on either side (Fig. 78). It may also be noticed that the wrist
or ankle bones do not interlock, either with one another or with the
bones of the hand or foot below them. This, of course, would give a
weak foot, suited to slow progression over marshy ground--which, as we
have seen, was no doubt the origin of the mammalian plantigrade foot.
Here, for instance, to all intents and purposes, is a similar type of
foot, which belonged to a very early mammal, antecedent to the elephant
series, the horse series, the rhinoceros, the hog, and, in short, all
the known hoofed mammalia (Fig. 80). It was presumably an inhabitant of
swampy ground, slow in its movements, and low in its intelligence.
[Illustration: FIG. 80.--Fossil skeleton of _Phenacodus primavus_.
(After Cope.)]
But now, as we have seen, for more rapid progression on hard uneven
ground, a stronger and better jointed foot would be needed. Therefore we
find the bones of the wrist and ankle beginning to interlock, both among
themselves and also with those of the foot and hand immediately below
them. Such a stage of evolution is still apparent in the now existing
elephant. (See Fig. 81.)
[Illustration: FIG. 81.--Bones of the foot of four different forms
of the perissodactyl type, showing gradual reduction in the number
of digits, coupled with a greater consolidation of the bones above
the digits. The series reads from right to left. Drawn from nature
(_Brit. Mus._).]
Next, however, a still stronger foot was made by the still further
interlocking of the wrist and ankle bones, so that both the first and
second rows of them were thus fitted into each other, as well as into
the bones of the hand and foot beneath. This further modification is
clearly traceable in some of the earlier perissodactyls, and occurs in
the majority at the present time. Compare, for example, the greater
interlocking and consolidation of these small bones in the Rhinoceros as
contrasted with the Elephant (Fig. 81). Moreover, simultaneously with
these consolidating improvements in the mechanism of the wrist and ankle
joints, or possibly at a somewhat later period, a reduction in the
number of digits began to take place. This was a continuation of the
policy of consolidating the foot, analogous to the dropping out of the
sixth row of small bones in the paddle of _Baptanodon_. (Fig. 78.) In
the pentadactyl plantigrade foot of the early mammals, the first digit,
being the shortest, was the first to leave the ground, to dwindle,
and finally to disappear. More work being thus thrown on the remaining
four, they were strengthened by interlocking with the wrist (or ankle)
bones above them, as just mentioned; and also by being brought closer
together.
[Illustration: FIG. 82.--Bones of the foot of four different forms
of the artiodactyl type, showing gradual reduction of the number of
digits, coupled with a greater consolidation of the bones above the
digits. The series reads from right to left. Drawn from nature
(_Brit. Mus._).]
The changes which followed I will render in the words of Professor
Marsh.
Two kinds of reduction began. One leading to the existing
perissodactyl foot, and the other, apparently later, resulting in
the artiodactyl type. In the former the axis of the foot remained
in the middle of the third digit, as in the pentadactyl foot. [See
Fig. 81.] In the latter, it shifted to the outer side of this
digit, or between the third and fourth toe. [See Fig. 82.]
In the further reduction of the perissodactyl foot, the fifth
digit, being shorter than the remaining three, next left the
ground, and gradually disappeared. [Fig. 81 B.] Of the three
remaining toes, the middle or axial one was the longest, and
retaining its supremacy as greater strength and speed were
required, finally assumed the chief support of the foot [Fig. 81
C], while the outer digits left the ground, ceased to be of use,
and were lost, except as splint-bones [Fig. 81 D]. The feet of the
existing horse shows the best example of this reduction in the
Perissodactyls, as it is the most specialized known in the
Ungulates [Fig. 81 D].
In the artiodactyl foot, the reduction resulted in the gradual
diminution of the two outer of the four remaining toes, the third
and fourth doing all the work, and thus increasing in size and
power. The fifth digit, for the same reasons as in the
perissodactyl foot, first left the ground and became smaller. Next,
the second soon followed, and these two gradually ceased to be
functional, [and eventually disappeared altogether, as shown in the
accompanying drawing of the feet of still existing animals, Fig. 82
B, C, D].
The limb of the modern race-horse is a nearly perfect piece of
machinery, especially adapted to great speed on dry, level ground.
The limb of an antelope, or deer, is likewise well fitted for
rapid motion on a plain, but the foot itself is adapted to rough
mountain work as well, and it is to this advantage, in part, that
the Artiodactyls owe their present supremacy. The plantigrade
pentadactyl foot of the primitive Ungulate--and even the
perissodactyl foot that succeeded it--both belong to the past humid
period of the world's history. As the surface of the earth slowly
dried up, in the gradual desiccation still in progress, new types
of feet became a necessity, and the horse, antelope, and camel were
gradually developed, to meet the altered conditions.
The best instance of such progressive modifications in the case of
perissodactyl feet is furnished by the fossil pedigree of the existing
horse, because here, within the limits of the same continuous family
line, we have presented the entire series of modifications.
There are now known over thirty species of horse-like creatures,
beginning from the size of a fox, then progressively increasing in bulk,
and all standing in linear series in structure as in time. Confining
attention to the teeth and feet, it will be seen from the wood-cut on
page 189 that the former grow progressively longer in their sockets, and
also more complex in the patterns of their crowns. On the other hand,
the latter exhibit a gradual diminution of their lateral toes, together
with a gradual strengthening of the middle one. (See Fig. 83.) So that
in the particular case of the horse-ancestry we have a practically
complete chain of what only a few years ago were "missing links." And
this now practically completed chain shows us the entire history of what
happens to be the most peculiar, or highly specialized, limb in the
whole mammalian class--namely, that of the existing horse. Of the other
two wood-cuts, the former (Fig. 84) shows the skeleton of a very early
and highly generalized ancestor, while the other is a partial
restoration of a much more recent and specialized one. (Fig. 85.)
[Illustration: FIG. 83.--Feet and teeth in fossil pedigree of the
Horse. (After Marsh.) _a_, bones of the fore-foot; _b_, bones of the
hind-foot; _c_, radius and ulna; _d_, tibia and fibula; _e_, roots
of a tooth; _f_ and _g_, crowns of upper and lower molar teeth.]
[Illustration: FIG. 84.--_Palæotherium_. (Lower Tertiary of Paris
Basin.)]
[Illustration: FIG. 85.--_Hipparion_. (New World Pliocene.)]
On the other hand, progressive modifications of the artiodactyl feet may
be traced geologically up to the different stages presented by living
ruminants, in some of which it has proceeded further than in others. For
instance, if we compare the pig, the deer, and the camel (Fig. 82), we
immediately perceive that the dwindling of the two rudimentary digits
has proceeded much further in the case of the deer than in that of the
pig, and yet not so far as in that of the camel, seeing that here they
have wholly disappeared. Moreover, complementary differences are to be
observed in the degree of consolidation presented by the two useful
digits. For while in the pig the two foot-bones are still clearly
distinguishable throughout their entire length, in the deer, and still
more in the camel, their union is more complete, so that they go to
constitute a single bone, whose double or compound character is
indicated externally only by a slight bifurcation at the base.
Nevertheless, if we examine the state of matters in the unborn young of
these animals, we find that the two bones in question are still
separated throughout their length, and thus precisely resemble what used
to be their permanent condition in some of the now fossil species of
hoofed mammalia.
Turning next from bones of the limb to other parts of the mammalian
skeleton, let us briefly consider the evidence of evolution that is here
likewise presented by the vertebral column, the skull, and the teeth.
As regards the vertebral column, if we examine this structure in any of
the existing hoofed animals, we find that the bony processes called
zygapophyses, which belong to each of the constituent vertebræ, are so
arranged that the anterior pair belonging to each vertebra interlocks
with the posterior pair belonging to the next vertebra. In this way the
whole series of vertebræ are connected together in the form of a chain,
which, while admitting of considerable movement laterally, is everywhere
guarded against dislocation. But if we examine the skeletons of any
ungulates from the lower Eocene deposits, we find that in no case is
there any such arrangement to secure interlocking. In all the hoofed
mammals of this period the zygapophyses are flat. Now, from this flat
condition to the present condition of full interlocking we obtain a
complete series of connecting links. In the middle Miocene period we
find a group of hoofed animals in which the articulation begins by a
slight rounding of the previously flat surfaces: later on this rounding
progressively increases, until eventually we get the complete
interlocking of the present time.
As regards teeth, and still confining attention to the hoofed mammals,
we find that low down in the geological series the teeth present on
their grinding surfaces only three simple tubercles. Later on a fourth
tubercle is added, and later still there is developed that complicated
system of ridges and furrows which is characteristic of these teeth at
the present time, and which was produced by manifold and various
involutions of the three or four simple tubercles of Eocene and lower
Miocene times. In other words, the principle of gradual improvement in
the construction of teeth, which has already been depicted as regards
the particular case of the Horse-family (Fig. 83), is no less apparent
in the pedigree of all the other mammalia, wherever the palæontological
history is sufficiently intact to serve as a record at all.
Lastly, as regards the skull, casts of the interior show that all the
earlier mammals had small brains with comparatively smooth or
unconvoluted surfaces; and that as time went on the mammalian brain
gradually advanced in size and complexity. Indeed so small were the
cerebral hemispheres of the primitive mammals that they did not overlap
the cerebellum, while their smoothness must have been such as in this
respect to have resembled the brain of a bird or reptile. This, of
course, is just as it ought to be, if the brain, which the skull has to
accommodate, has been gradually evolved into larger and larger
proportions in respect of its cerebral hemispheres, or the upper masses
of it which constitute the seat of intelligence. Thus, if we look at the
above series of wood-cuts, which represents the comparative structure of
the brain in the existing classes of the Vertebrata, we can immediately
understand why the fossil skulls of Mammalia should present a gradual
increase in size and furrowing, so as to accommodate the general
increase of the brain in both these respects between the level marked
"maml" and that marked "man," in the last of the diagrams. (Fig. 87.)
[Illustration: FIG. 86.--Comparative series of Brains. (After Le
Conte.) The series reads from above downwards, and represents
diagrammatically the brain of a Fish, a Reptile, a Bird, a Mammal,
and a Man. In each case the letter A marks a side view, and the
letter B a top view. The small italics throughout signify the
following homologous parts: _m_, medulla; _cb_, cerebellum; _op_,
optic lobes; _cr_, cerebrum and thalamus; _ol_, olfactory lobes. The
series shows a progressive consolidation and enlargement of the
brain in general, and of the cerebrum and cerebellum in particular,
which likewise exhibit continually advancing structure in respect of
convolution. In the case of Man, these two parts of the brain have
grown to so great a size that they conceal all the other parts from
the superficial points of view represented in the diagram.]
[Illustration: FIG. 87.--Ideal section through all the above stages.
(After Le Conte.])
The tabular statement on the following diagram, which I borrow from
Prof. Cope, will serve at a glance to reveal the combined significance
of so many lines of evidence, united within the limits of the same group
of animals.
To give only one special illustration of the principle of evolution as
regards the skull, here is one of the most recent instances that has
occurred of the discovery of a missing link, or connecting form (see
Fig. 88). The fossil (B), which was found in New Jersey, stands in an
intermediate position between the stag and the elk. In the stag (A) the
skull is high, showing but little of that anterior attenuation which is
such a distinctive feature of the skull of the elk (C). The nasal bones
(N) of the former, again, are remarkably long when compared with the
similar bones of the latter, and the premaxillaries (PMX), instead of
being projected forward along the horizontal plane of the base of the
skull, are deflected sharply downward. In all these points, it will be
seen, the newly discovered form (_Cervalces_) holds an intermediate
position (B). "The skull exhibits a partial attenuation anteriorly,
the premaxillaries are directed about equally downward and forward, and
the nasal bones are measurably contracted in size. The horns likewise
furnish characters which further serve to establish this dual
relationship[18]."
[18] Heilprin, _Geological Evidences of Evolution_, pp. 73-4 (1888).
[Illustration: FIG. 88.--Skulls of--A, Canadian Stag; B, _Cervalces
Americanus_; and C, Elk. (After Heilprin.)]
Formation.
|No. of toes
| |Feet
| | |Astragalus.
| | | |Carpus and tarsus.
| | | | |Ulno-radius.
| | | | | |Superior molars.
| | | | | | |Zygapophyses.
| | | | | | | |Brain.
| | | | | | | |
Pliocene.
|1-1, 2-2
| |Digitigrade. (Plantigrade.)
| | |Grooved. (Flat.)
| | | |Interlocking. (Opposite.)
| | | | |Faceted.
| | | | | |4-tubercles, crested and cemented.
| | | | | | |Doubly involute. Singly involute.
| | | | | | | |Hemispheres larger, convoluted.
| | | | | | | |
Upper Miocene. (Loup Fork.)
|3-3, 4-4, (5-5)
| |
| | |
| | | |
| | | | |
| | | | | |
| | | | | | |
| | | | | | | |
| | | | | | | |
Middle. (John Day.)
|2-2, 3-3, 4-4
| |Digitigrade.
| | |Grooved.
| | | |Interlocking.
| | | | |Faceted. Smooth
| | | | | |4-tubercles, and crested
| | | | | | |Singly involute. Double involute.
| | | | | | | |Hemispheres larger, convoluted.
| | | | | | | |
Lower (White River.)
|3-3, 4-3
| |Digitigrade. Plantigrade.
| | |Grooved.
| | | |Interlocking.
| | | | |Smooth. Faceted.
| | | | | |4-tubercles, and crested
| | | | | | |? Singly involute.
| | | | | | | |Hemispheres small, and largeer.
| | | | | | | |
Eocene. Upper (Bridger.)
|3-3, 4-3, 4-5
| |(Digitigrade.) Plantigrade.
| | |Grooved. (Flat.)
| | | |Opposite. Interlocking.
| | | | |Smooth.
| | | | | |4-tubercles. 3-tubercles, and crested.
| | | | | | |Singly involute. Plane
| | | | | | | |Hemispheres small.
| | | | | | | |
Middle. (Wasatch.)
|4-3, 4-5, 5-5
| |Plantigrade. (Digitigrade.)
| | |Flat. (Grooved.)
| | | |Opposite. Interlocking.
| | | | |Smooth.
| | | | | |4-tubercles. 3-tubercles, a few crested.
| | | | | | |Plane. Singly involute.
| | | | | | | |Hemispheres small;
| | | | | | | |mesencephalon sometimes exposed
| | | | | | | |
Lower (Puerco.)
|5-5
| |Plantigrade.
| | |Flat.
| | | |Opposite.
| | | | |Smooth.
| | | | | |3-tubercles. (4-tubercles), none crested.
| | | | | | |Plane.
| | | | | | | |Mesencephalon exposed;
| | | | | | | |hemisphere small and smoother.
| | | | | | | |
The evidence, then, which is furnished by all parts of the vertebral
skeleton--whether we have regard to Fishes, Reptiles, Birds, or
Mammals--is cumulative and consistent. Nowhere do we meet with any
deviation or ambiguity, while everywhere we encounter similar proofs of
continuous transformation--proofs which vary only with the varying
amount of material which happens to be at our disposal, being most
numerous and detailed in those cases where the greatest number of fossil
forms has been preserved by the geological record. Here, therefore, we
may leave the vertebral skeleton; and, having presented a sample of the
evidence as yielded by horns and bones, I will conclude by glancing with
similar brevity at the case of shells--which, as before remarked,
constitute the only other sufficiently hard or permanent material to
yield unbroken evidence touching the fossil ancestry of animals.
Of course it will be understood that I am everywhere giving merely
samples of the now superabundant evidence which is yielded by
palæontology; and, as this chapter is already a long one, I must content
myself with citing only the case of mollusk-shells, although shells of
other classes might be made to yield highly important additions to the
testimony. Moreover, even as regards the one division of mollusk-shells,
I can afford to quote only a very few cases. These, however, are in my
opinion the strongest single pieces of evidence in favour of
transmutation which have thus far been brought to light.
Near the village of Steinheim, in Würtemberg, there is an ancient
lake-basin, dating from Tertiary times. The lake has long ago dried up;
but its aqueous deposits are extraordinarily rich in fossil shells,
especially of different species of the genus _Planorbis_. The following
is an authoritative _résumé_ of the facts.
As the deposits seem to have been continuous for ages, and the
fossil shells very abundant, this seemed to be an excellent
opportunity to test the theory of derivation. With this end in
view, they have been made the subject of exhaustive study by
Hilgendorf in 1866, and by Hyatt in 1880. In passing from the
lowest to the highest strata the species change greatly and many
times, the extreme forms being so different that, were it not for
the intermediate forms, they would be called not only different
species, but different genera. And yet the gradations are so
insensible that the whole series is nothing less than a
demonstration, in this case at least, of origin of species by
derivation with modifications. The accompanying plate of successive
forms (Fig. 89), which we take from Prof. Hyatt's admirable memoir,
will show this better than any mere verbal explanation. It will be
observed that, commencing with four slight varieties--probably
sexually isolated varieties--of one species, each series shows a
gradual transformation as we go upward in the strata--i. e. onward
in time. Series I branches into three sub-series, in two of which
the change of form is extreme. Series IV is remarkable for great
increase in size as well as change in form. In the plate we give
only selected stages, but in the fuller plates of the memoir, and
still more in the shells themselves, the subtilest gradations are
found[19].
[19] Le Conte, _loc. cit._, pp. 236-7.
[Illustration: FIG. 89.--Transmutations of _Planorbis_. (After
Hyatt.)]
Here is another and more recently observed case of transmutation in the
case of mollusks.
The recent species, _Strombus accipitrinus_, still inhabits the coasts
of Florida. Its extinct prototype, _S. Leidy_, was discovered a few
years ago by Prof. Heilprin in the Pliocene formations of the interior
of Florida. The peculiar shape of the wing, and tuberculation of the
whorl, are thus proved to have grown but of a previously more conical
form of shell.
[Illustration: FIG. 90.--Transformation of _Strombus_. (After
Heilprin.) 1, 1_a_, _Strombus Leidy_ (1, typical), Pliocene; 2,
2_a_, _Strombus accipitrinus_ (2_a_ typical) Recent.]
Lastly, attention may here again be directed to the very instructive
series of shells which has already been shown in a previous chapter, and
which serves to illustrate the successive geological forms of _Paludina_
from the Tertiary beds of Slavonia, as depicted by Prof. Neumayr of
Vienna. (Fig. 1, p. 19.)
CHAPTER VI.
GEOGRAPHICAL DISTRIBUTION.
The argument from geology is the argument from the distribution of
species in time. I will next take the argument from the distribution of
species in space--that is, the present geographical distribution of
plants and animals.
Seeing that the theory of descent with adaptive modification implies
slow and gradual change of one species into another, and progressively
still more slow and gradual changes of one genus, family, or order into
another genus, family, or order, we should expect on this theory that
the organic types living on any given geographical area would be found
to resemble or to differ from organic types living elsewhere, according
as the area is connected with or disconnected from other geographical
areas. For instance, the large continental islands of Australia and New
Zealand are widely disconnected from all other lands of the world, and
deep sea soundings show that they have probably been thus disconnected,
either since the time of their origin, or, at the least, through immense
geological epochs. The theory of evolution, therefore, would expect to
find two general facts with regard to the inhabitants of these islands.
First, that the inhabitants should form, as it were, little worlds of
their own, more or less unlike the inhabitants of any other parts of the
globe. And next, that some of these inhabitants should present us with
independent information touching archaic forms of life. For it is
manifestly most improbable that the course of evolutionary history
should have run exactly parallel in the case of these isolated oceanic
continents and in continents elsewhere. Australia and New Zealand,
therefore, ought to present a very large number, not only of peculiar
species and genera, but even of families, and possibly of orders. Now
this is just what Australia and New Zealand do present. The case of the
dog being doubtful, there is an absence of all mammalian life, except
that of one of the oldest and least highly developed orders, the
Marsupials. There even occurs a unique order, still lower in the scale
of organization--so low, in fact, that it deserves to be regarded as but
nascent mammalian: I mean, of course, the Monotremata. As regards Birds,
we have the peculiar wingless forms alluded to in a previous chapter
(viz. that on Morphology); and, without waiting to go into details, it
is notorious that the faunas of Australia and New Zealand are not only
highly peculiar, but also suggestively archaic. Therefore, in both the
respects above mentioned, the anticipations of our theory are fully
borne out. But as it would take too long to consider, even cursorily,
the faunas and floras of these immense islands, I here allude to them
only for the sake of illustration. In order to present the argument from
geographical distribution within reasonable limits, I think it is best
to restrict our examination to smaller areas; for these will better
admit of brief and yet adequate consideration. But of course it will be
understood that the less isolated the region, and the shorter the time
that it has been isolated, the smaller amount of peculiarity should we
expect to meet with on the part of its present inhabitants. Or,
conversely stated, the longer and the greater the isolation, the more
peculiarity of species would our theory expect to find. The object of
the present chapter will be to show that these, and other cognate
expectations, are fully realized by facts; but, before proceeding to do
this, I must say a few words on the antecedent standing of the argument.
Where the question is, as at present, between the rival theories of
special creation and gradual transmutation, it may at first sight well
appear that no test can be at once so crucial and so easily applied as
this of comparing the species of one geographical area with those of
another, in order to see whether there is any constant correlation
between differences of type and degrees of separation. But a little
further thought is enough to show that the test is not quite so simple
or so absolute--that it is a test to be applied in a large and general
way over the surface of the whole earth, rather than one to be relied
upon as exclusively rigid in every special case.
In the first place, there is the obvious consideration that lands or
seas which are discontinuous now may not always have been so, or not for
long enough to admit of the effects of separation having been exerted to
any considerable extent upon their inhabitants. Next, there is the
scarcely less important consideration, that although land areas may long
have been separated from one another by extensive tracts of ocean,
birds and insects may more or less easily have been able to fly from
one to the other; while even non-flying animals and plants may often
have been transported by floating ice or timber, wind or water currents,
and sundry other means of dispersal. Again, there is the important
influence of climate to be taken into account. We know from geological
evidence that in the course of geological time the self-same continents
have been submitted to enormous changes of temperature--varying in fact
from polar cold to almost tropical heat; and as it is manifestly
impossible that forms of life suited to one of these climates could have
survived during the other, we can here perceive a further and most
potent cause interfering with the test of geographical distribution as
indiscriminately applied in all cases. When the elephant and
hippopotamus were flourishing in England amid the luxuriant vegetation
which these large animals require, it is evident that scarcely any one
species of either the fauna or the flora of this country can have been
the same as it was when its African climate gave place to that of
Greenland. Therefore, as Mr. Wallace observes, "If glacial epochs in
temperate lands and mild climates near the poles have, as now believed
by men of eminence, occurred several times over in the past history of
the earth, the effects of such great and repeated changes both on
migration, modification, and extinction of species, must have been of
overwhelming importance--of more importance perhaps than even the
geological changes of sea and land."
But although for these, and certain other less important reasons which I
need not wait to detail, we must conclude that the evidence from
geographical distribution is not to be regarded as a crucial test
between the rival theories of creation and evolution in all cases
indiscriminately, I must next remark that it is undoubtedly one of the
strongest lines of evidence which we possess. When we once remember
that, according to the general theory of evolution itself, the present
geographical distribution of plants and animals is "the visible outcome
or residual product of the whole past history of the earth," and,
therefore, that of the conditions determining the characters of life
inhabiting this and that particular area continuity or discontinuity
with other areas is but one,--when we remember this, we find that no
further reservation has to be made: all the facts of geographical
distribution speak with one consent in favour of the naturalistic
theory.
* * * * *
The first of these facts which I shall adduce is, that although the
geographical range of any given species is, as a rule, continuous, such
is far from being always the case. Very many species have more or less
discontinuous ranges--the mountain-hare, for instance, extending from
the Arctic regions over the greater portion of Europe to the Ural
Mountains and the Caucasus, and yet over all this enormous tract
appearing only in isolated or discontinuous patches, where there happen
to be either mountain ranges or climates cold enough to suit its nature.
Now, in all such cases of discontinuity in the range of a species the
theory of evolution has a simple explanation to offer--namely, either
that some representatives of the species have at some former period been
able to migrate from one region to the other, or else that at one time
the species occupied the whole of the range in question, but afterwards
became broken up as geographical, climatic, or other changes rendered
parts of the area unfit for the species to inhabit. Thus, for instance,
it is easy to understand that during the last cold epoch the
mountain-hare would have had a continuous range; but that as the Arctic
climate gradually receded to polar regions, the species would be able to
survive in southern latitudes only on mountain ranges, and thus would
become broken up into many discontinuous patches, corresponding with
these ranges. In the same way we can explain the occurrence of Arctic
vegetation on the Alps and Pyrenees--namely, as left behind by the
retreat of the Arctic climate at the close of the glacial period.
But now, on the other hand, the theory of special creation cannot so
well afford to render this obvious explanation of discontinuity. In the
case of the Arctic flora of the Alps, for instance, although it is true
that much of this vegetation is of an Arctic type, it is not true that
the species are all identical with those which occur in the Arctic
regions. Therefore the theory of special creation would here have to
assume that, although the now common species were left behind on the
Alps by the retreat of glaciation northwards, the peculiar Alpine
species were afterwards created separately upon the Alps, and yet
created with such close affinities to the pre-existing species as to be
included with them under the same genera. Looking to the absurdity of
this supposition, as well as of others which I need not wait to mention,
certain advocates of special creation have sought to take refuge in
another hypothesis--namely, that species which present a markedly
discontinuous range may have had a corresponding number of different
centres of creation, the same specific type having been turned down, so
to speak, on widely separated areas. But to me it seems that this
explanation presents even greater difficulty than the other. If it is
difficult to say why the Divinity should have chosen to create new
species of plants on the Alps on so precisely the same pattern as the
old, much more would it be difficult to say why, in addition to these
new species, he should also have created again the old species which he
had already placed in the Arctic regions.
* * * * *
So much, then, for discontinuity of distribution. The next general fact
to be adduced is, that there is no constant correlation between habitats
and animals or plants suited to live upon them. Of course all the
animals and plants living upon any given area are well suited to live
upon that area; for otherwise they could not be there. But the point now
is, that besides the area on which they do live, there are usually many
other areas in different parts of the globe where they might have lived
equally well--as is proved by the fact that when transported by man they
thrive as well, or even better, than in their native country. Therefore,
upon the supposition that all species were separately created in the
countries where they are respectively found, we must conclude that they
were created in only some of the places where they might equally well
have lived. Probably there is at most but a small percentage either of
plants or animals which would not thrive in some place, or places, on
the earth's surface other than that in which they occur; and hence we
must say that one of the objects of special creation--if this be the
true theory--was that of depositing species in only some among the
several parts of the earth's surface equally well suited to support
them.
Now, I do not contend that this fact in itself raises any difficulty
against the theory of special creation. But I do think that a very
serious difficulty is raised when to this fact we add another--namely,
that on every biological region we encounter species related to other
species in genera, and usually also genera related to other genera in
families. For if each of all the constituent species of a genus, and
even of a family, were separately created, we must hence conclude that
in depositing them there was an unaccountable design manifested to make
areas of distribution correspond to the natural affinities of their
inhabitants. For example, the humming-birds are geographically
restricted to America, and number 120 genera, comprising over 400
species. Hence, if this betokens 400 separate acts of creation, it
cannot possibly have been due to chance that they were all performed on
the same continent: it must have been design which led to every species
of this large family of birds having been deposited in one geographical
area. Or, to take a case where only the species of a single genus are
concerned. The rats and mice proper constitute a genus which comprises
altogether more than 100 species, and they are all exclusively
restricted to the Old World. In the New World they are represented by
another genus comprising about 70 species, which resemble their Old
World cousins in form and habits; but differ from them in dentition and
other such minor points. Now, the question is,--Why should all the 100
species have been separately created on one side of the Atlantic with
one pattern of dentition, and all the 70 species on the other side with
another pattern? What has the Atlantic Ocean got to do with any
"archetypal plan" of rats' teeth?
Or again, to recur to Australia, why should all the mammalian forms of
life be restricted to the one group of Marsupials, when we know that not
only the Rodents, such as the rabbit, but all other orders of mammals,
would thrive there equally well. And similarly, of course, in countless
other instances. Everywhere we meet with this same correlation between
areas of distribution and affinities of classification.
Now, it is at once manifest how completely this general fact harmonizes
with the theory of evolution. If the 400 species of humming-birds, for
instance, are all modified descendants of common ancestors, and if none
of their constituent individuals have ever been large enough to make
their way across the oceans which practically isolate their territory
from all other tropical and sub-tropical regions of the globe, then we
can understand why it is that all the 400 species occupy the same
continent. But on the special-creation theory we can see no reason why
the 400 species should all have been deposited in America. And, as
already observed, we must remember that this correlation between a
geographically restricted habitat and the zoological or botanical
affinities of its inhabitants, is repeated over and over and over again
in the faunas and floras of the world, so that merely to enumerate the
instances would require a separate chapter.
Furthermore, the general argument thus presented in favour of descent
with continuous modification admits of being enormously strengthened by
three different classes of additional facts.
The first is, that the correlation in question--namely, that between a
geographically restricted habitat and the zoological or botanical
affinities of its inhabitants--is not limited to the now existing
species, but extends also to the extinct. That is to say, the dead
species are allied to the living species, as we should expect that they
must be, if the latter are modified descendants of the former. On the
alternative theory, however, we have to suppose that the policy of
maintaining a correlation between geographical restriction and natural
affinity extends very much further back than even the existing species
of plants and animals; indeed we must suppose that a practically
infinite number of additional acts of separate creation were governed by
the same policy, in the case of long lines of species long since
extinct.
Thus far, then, the only answer which an advocate of special creation
can adduce is, that for some reason unknown to us such a policy may have
been more wise than it appears: it may have served some inscrutable
purpose that allied products of distinct acts of creation should all be
kept together on the same areas. Well, in answer to this unjustifiable
appeal to the argument from ignorance, I will adduce the second of the
three considerations. This is, that in cases where the geographical
areas are not restricted the policy in question fails. In other words,
where the inhabitants of an area are free to migrate to other areas, the
policy of correlating affinity with distribution is most significantly
forgotten. In this case species wander away from their native homes, and
the course of their wanderings is marked by the origination of new
species springing up en route. Now, is it reasonable to suppose that the
mere circumstance of some members of a species being able to leave their
native home should furnish any occasion for creating new and allied
species upon the tracts over which they travel, or the territories to
which they go? When the 400 existing species of humming-birds have all
been created on the same continent for some reason supposed to be
unknown, why should this reason give way before the accident of any
means of migration being furnished to humming-birds, so that they should
be able to visit, say the continents of Africa and Asia, there gain a
footing beside the sun-birds, and henceforth determine a new centre for
the separate creation of additional species of humming-birds peculiar to
the Old World--as has happened in the case of the majority of species
which, unlike the humming-birds, have been at any time free to migrate
from their original homes?
Lastly, my third consideration is, that the supposed policy in question
does not extend to affinities which are wider than those between species
and genera--more rarely to families, scarcely ever to orders, and never
to classes. In other words, nature shows a double correlation in her
geographical distribution of organic types:--first, that which we have
already considered between geographical restriction and natural
affinity among inhabitants of the same areas; second, another of a more
detailed character between _degrees_ of geographical restriction and
_degrees_ of natural affinity. The more distant the affinity, the more
general is the extension. This, of course, is what we should expect on
the theory of descent with modification, because the more distant the
affinity, and therefore, _ex hypothesi_, the larger and the older the
original group of organisms, the greater must be the chance of
dispersal. The 400 species of humming-birds may well be unable to
migrate from their native continent; but it would indeed have been an
unaccountable fact if no other species of all the class of birds had
ever been able to have crossed the Atlantic Ocean. Thus, on the theory
of evolution, we can well understand the second correlation now before
us--namely, between remoteness of affinity and generality of
dispersal,--so that there is no considerable portion of the habitable
globe without representatives of all the classes of animals, few
portions without representatives of all the orders, but many portions
without many of the families, innumerable portions without innumerable
genera, and, of course, all portions without the great majority of
species. Now, while this general correlation thus obviously supports the
theory of natural descent with progressive modification, it makes
directly against the opposite theory of special creation. For we have
recently seen that when we restrict our view to the case of species and
genera, the theory of special creation is obliged to suppose that for
some inscrutable reason the Deity had regard to systematic affinity
while determining on what large areas to create his species[20]. But
now we see that he must be held to have neglected this inscrutable
reason (whatever it was) when he passed beyond the range of genera--and
this always in proportion to the remoteness of systematic affinity on
the part of the species concerned.
[20] I say "_large_ areas" for the sake of argument; but the same
correlation between distribution and affinity extends likewise to
_small_ areas where only _small_ differences of affinity are
concerned. Thus, for instance, speaking of smaller areas, Moritz
Wagner says:--"The broader and more rapid the river, the higher and
more regular the mountain-chain, the calmer and more extensive the
sea, the more considerable, as a general rule, will be the taxonomic
separation between the populations"; and he shows that, in
correlation with such differences in the _degrees_ of separation,
are the _degrees_ of diversification--i. e., the _numbers_ of
species, and even of varieties, which these topographical barriers
determine.
I cannot well conceive a _reductio ad absurdum_ more complete than this.
But, having now presented these most general facts of geographical
distribution in their relation to the issue before us, we may next
proceed to consider a few illustrations of them in detail, for in this
way I think that their overwhelming weight may become yet more
abundantly apparent.
* * * * *
It will assist us in dealing with these detailed illustrations if we
begin by considering the means of dispersal of organisms from one place
to another. Of course the most ordinary means is that of continuous
wandering, or emigration; but where geographical barriers of any kind
have to be surmounted, organisms may only be able to pass them by more
exceptional and accidental means. The principal barriers of a
geographical kind are oceans, rivers, mountain-chains, and
desert-tracts, in the case of terrestrial organisms; and, in the case
of aquatic organisms, the presence of land. But it is to be observed
that, as regards marine organisms, any considerable difference in the
temperature of the water may constitute a barrier as effectual as the
presence of land; and also that, in the case of all shallow-water
faunas, a tract of deep ocean constitutes almost as complete a barrier
as it does to terrestrial faunas.
Now, the means whereby barriers admit of being accidentally or
occasionally surmounted are, of course, various; and they differ in the
case of different organisms. Birds, bats, and insects, on account of
their powers of flight, are particularly apt to be blown out great
distances to sea, and hence of all animals are most likely to become the
involuntary colonists of distant shores. Floating timber serves to
convey seeds and eggs of small animals over great distances; and Darwin
has shown that many kinds of seeds are able of themselves to float for
more than a month in sea-water without losing their powers of
germination. For instance, out of 87 kinds, 64 germinated after an
immersion of 28 days, and a few survived an immersion of 137 days. As a
result of all his experiments he concludes, that the seeds of at least
ten per cent. of the species of plants of any country might be floated
by sea-currents during 28 days, without losing their powers of
germination; and this, at the average rate of flow of several Atlantic
currents, would serve to transport the seeds to a distance of at least
900 miles. Again, he proved that even seeds which are quickly destroyed
by contact with sea-water admit of being successfully transported during
30 days, if they be contained within the crop of a dead bird. He also
proved that living birds are most active agents in the work of
dissemination, and this not only by taking seeds into their crops
(where, so long as they remain, the seeds are uninjured), but likewise
by carrying seeds (and even young mollusks) attached to their feet and
feathers. In the course of these experiments he found that a small
cup-full of mud, which he gathered from the edges of three ponds in
February, was so charged with seeds that when sown in the ground these
few ounces of mud yielded no less than 537 plants, belonging to many
different species. It is therefore evident what opportunities are thus
afforded for the transportation of seeds on the feet and bills of
wading-birds. Lastly, floating ice is well known to act as a carrier of
any kind of life which may prove able to survive this mode of transit.
Such being the nature of geographical barriers, and the means that
organisms of various kinds may occasionally have of overcoming them, I
will now give a few detailed illustrations of the argument from
geographical distribution, as previously presented in its general form.
To begin with aquatic animals. As Darwin remarks, "the marine
inhabitants of the Eastern and Western shores of South America are very
distinct; with extremely few shells, crustacea, or echinodermata in
common." Again, westward of the shores of America, a wide space of open
ocean extends, which, as we have seen, furnishes as effectual a barrier
as does the land to any emigration of shallow-water animals. Now, as
soon as this reach of deep water is passed, we meet in the eastern
islands of the Pacific with another and totally distinct fauna. "So that
three marine faunas range northward and southward in parallel lines not
far from each other, under corresponding climates": they are, however,
"separated from each other by impassable barriers, either of land or
open sea": and it is in exact coincidence with the course of these
barriers that we find so remarkable a differentiation of the faunas[21].
Obviously, therefore, it is impossible to suggest that this correlation
is accidental. Altogether many thousands of species are involved, and
within this comparatively limited area they are sharply marked off into
three groups as to their natural affinities, and into three groups as to
their several basins. Hence, if all these species were separately
created, there is no escape from the conclusion that for some reason or
another the act of creation was governed by the presence of these
barriers, so that species deposited on the Eastern shores of South
America were formed with one set of natural affinities, while species
deposited on the Western shore were formed with another set; and
similarly with regard to the third set of species in the third basin,
which, extending over a whole hemisphere to the coast of Africa without
any further barrier, nowhere presents, over this vast area, any other
case of a distinct marine fauna. But what conceivable reason can there
have been thus to consult these geographical barriers in the original
creation of specific types? Even if such a case stood alone, it would
be strongly suggestive of error on the part of the special creation
theory. But let us take another case, this time from fresh-water faunas.
[21] The only exception is in the case of the fish on each side of
the isthmus of panama, where about 30 per cent, of the species are
identical. But it is possible enough that at some previous time this
narrow isthmus may have been even narrower than at present, if not
actually open. At all events, the fact that this partial exception
occurs just where the land-barrier is so narrow, is more suggestive
of migration than of independent creation.
Although the geographical distribution of fresh-water fish and
fresh-water shells is often surprisingly extensive and apparently
capricious, this may be explained by the means of dispersal being here
so varied--not only aquatic birds, floods, and whirlwinds, but also
geographical changes of water-shed having all assisted in the process.
Moreover, in some cases it is possible that the habits of more widely
distributed fresh-water fish may have originally been wholly or partly
marine--which, of course, would explain the existing discontinuity of
their existing fresh-water distribution. But, be this as it may (and it
is not a question that affects the issue between special creation and
gradual evolution, since it is only a question as to how a given species
has been dispersed from its original home, whether or not in that home
it was specially created), the point I desire to bring forward is, that
where we find a barrier to the emigration of fresh-water forms which is
more formidable than a thousand miles of ocean--a barrier over which
neither water-fowl nor whirlwinds are likely to pass, and which is above
the reach of any geological changes of water-shed,--where we find such a
barrier, we always find a marked difference in the fresh-water faunas on
either side of it. The kind of barrier to which I allude is a high
mountain-chain. It may be only a few miles wide; yet it exercises a
greater influence on the diversification of specific types, where
fresh-water faunas are concerned, than almost any other. But why should
this be the case on any intelligible theory of special creation? Why, in
the depositing of species of newly created fresh-water fish, should the
presence of an impassable mountain-chain have determined so uniformly a
difference of specific affinity on either side of it? The question, so
far as I can see, does not admit of an answer from any reasonable
opponent.
* * * * *
Turning now from aquatic organisms to terrestrial, the body of facts
from which to draw is so large, that I think the space at my disposal
may be best utilized by confining attention to a single division of
them--that, namely, which is furnished by the zoological study of
oceanic islands.
In the comparatively limited--but in itself extensive--class of facts
thus presented, we have a particularly fair and cogent test as between
the alternative theories of evolution and creation. For where we meet
with a volcanic island, hundreds of miles from any other land, and
rising abruptly from an ocean of enormous depth, we may be quite sure
that such an island can never have formed part of a now submerged
continent. In other words, we may be quite sure that it always has been
what it now is--an oceanic peak, separated from all other land by
hundreds of miles of sea, and therefore an area supplied by nature for
the purpose, as it were, of testing the rival theories of creation and
evolution. For, let us ask, upon these tiny insular specks of land what
kind of life should we expect to find? To this question the theories of
special creation and of gradual evolution would agree in giving the
same answer up to a certain point. For both theories would agree in
supposing that these islands would, at all events in large part, derive
their inhabitants from accidental or occasional arrivals of wind-blown
or water-floated organisms from other countries--especially, of course,
from the countries least remote. But, after agreeing upon this point,
the two theories must part company in their anticipations. The
special-creation theory can have no reason to suppose that a small
volcanic island in the midst of a great ocean should be chosen as the
theatre of any extraordinary creative activity, or for any particularly
rich manufacture of peculiar species to be found nowhere else in the
world. On the other hand, the evolution theory would expect to find that
such habitats are stocked with more or less peculiar species. For it
would expect that when any organisms chanced to reach a wholly isolated
refuge of this kind, their descendants should forthwith have started
upon an independent course of evolutionary history. Protected from
intercrossing with any members of their parent species elsewhere, and
exposed to considerable changes in their conditions of life, it would
indeed be fatal to the general theory of evolution if these descendants,
during the course of many generations, were not to undergo appreciable
change. It has happened on two or three occasions that European rats
have been accidentally imported by ships upon some of these islands, and
even already it is observed that their descendants have undergone a
slight change of appearance, so as to constitute them what naturalists
call local varieties. The change, of course, is but slight, because the
time allowed for it has been so short. But the longer the time that a
colony of a species is thus completely isolated under changed conditions
of life the greater, according to the evolution theory, should we expect
the change to become. Therefore, in all cases where we happen to know,
from independent evidence of a geological kind, that an oceanic island
is of very ancient formation, the evolution theory would expect to
encounter a great wealth of peculiar species. On the other hand, as I
have just observed, the special-creation theory can have no reason to
suppose that there should be any correlation between the age of an
oceanic island and the number of peculiar species which it may be found
to contain.
Therefore, having considered the principles of geographical distribution
from the widest or most general point of view, we shall pass to the
opposite extreme, and consider exhaustively, or in the utmost possible
detail, the facts of such distribution where the conditions are best
suited to this purpose--that is, as I have already said, upon oceanic
islands, which may be metaphorically regarded as having been formed by
nature for the particular purpose of supplying naturalists with a
crucial test between the theories of creation and evolution. The
material upon which my analysis is to be based will be derived from the
most recent works upon geographical distribution--especially from the
magnificent contributions to this department of science which we owe to
the labours of Mr. Wallace. Indeed, all that follows may be regarded as
a condensed filtrate of the facts which he has collected. Even as thus
restricted, however, our subject-matter would be too extensive to be
dealt with on the present occasion, were we to attempt an exhaustive
analysis of the floras and faunas of all oceanic islands upon the face
of the globe. Therefore, what I propose to do is to select for such
exhaustive analysis a few of what may be termed the most oceanic of
oceanic islands--that is to say, those oceanic islands which are most
widely separated from mainlands, and which, therefore, furnish the most
unquestionable of test cases as between the theories of special creation
and genetic descent.
* * * * *
_Azores._--A group of volcanic islands, nine in number, about 900 miles
from the coast of Portugal, and surrounded by ocean depths of 1,800 to
2,500 fathoms. There is geological evidence that the origin of the group
dates back at least as far as Miocene times. There is a total absence of
all terrestrial Vertebrata, other than those which are known to have
been introduced by man. Flying animals, on the other hand, are abundant;
namely, 53 species of birds, one species of bat, a few species of
butterflies, moths, and hymenoptera, with 74 species of indigenous
beetles. All these animals are unmodified European species, with the
exception of one bird and many of the beetles. Of the 74 indigenous
species of the latter, 36 are not found in Europe; but 19 are natives of
Madeira or the Canaries, and 3 are American, doubtless transplanted by
drift-wood. The remaining 14 species occur nowhere else in the world,
though for the most part they are allied to other European species.
There are 69 known species of land-shells, of which 37 are European, and
32 peculiar, though all allied to European forms. Lastly, there are 480
known species of plants, of which 40 are peculiar, though allied to
European species.
_Bermudas._--A small volcanic group of islands, 700 miles from North
Carolina. Although there are about 100 islands in the group, their total
area does not exceed 50 square miles. The group is surrounded by water
varying in depth from 2,500 to 3,800 fathoms. The only terrestrial
Vertebrate (unless the rats and mice are indigenous) is a lizard allied
to an American form, but specifically distinct from it, and therefore a
solitary species which does not occur anywhere else in the world. None
of the birds or bats are peculiar, any more than in the case of the
Azores; but, as in that case, a large percentage of the land-shells are
so--namely, at least one quarter of the whole. Neither the botany nor
the entomology of this group has been worked out; but I have said enough
to show how remarkably parallel are the cases of these two volcanic
groups of islands situated in different hemispheres, but at about the
same distance from large continents. In both there is an extraordinary
paucity of terrestrial vertebrata, and of any peculiar species of bird
or beast. On the other hand, there is in both a marvellous wealth of
peculiar species of insects and land-shells. Now these correlations are
all abundantly intelligible. It is a difficult matter for any
terrestrial animal to cross 900, or even 700, miles of ocean: therefore
only one lizard has succeeded in doing so in one of the two parallel
cases; and, living cut off from intercrossing with its parent form, the
descendants of that lizard have become modified so as to constitute a
peculiar species. But it is more easy for large flying animals to cross
those distances of ocean: consequently, there is only one instance of a
peculiar species of bird or bat--namely, a bull-finch in the Azores,
which, being a small land-bird, is not likely ever to have had any other
visitors from its original parent species coming over from Europe to
keep up the original breed. Lastly, it is very much more easy for
insects and land-mollusca to be conveyed to such islands by wind and
floating timber than it is for terrestrial mammals, or even than it is
for small birds and bats; but yet such means of transit are not
sufficiently sure to admit of much recruiting from the mainland for the
purpose of keeping up the specific types. Consequently, the insects and
the land-shells present a much greater proportion of peculiar
species--namely, one half and one fourth of the land-shells in the one
case, and one eighth of the beetles in the other. All these
correlations, I say, are abundantly intelligible on the theory of
evolution; but who shall explain, on the opposite theory, why orders of
beetles and land-mollusca should have been chosen from among all other
animals for such superabundant creation on oceanic islands, so that in
the Azores alone we find no less than 32 of the one and 14 of the other?
And, in this connexion, I may again allude to the peculiar species of
beetles in the island of Madeira. Here there are an enormous number of
peculiar species, though they are nearly all related to, or included
under the same genera as, beetles on the neighbouring continent. Now, as
we have previously seen, no less than 200 of these species have lost the
use of their wings. Evolutionists explain this remarkable fact by their
general laws of degeneration under disuse, and the operation of natural
selection, as will be shown later on; but it is not so easy for special
creationists to explain why this enormous number of peculiar species of
beetles should have been deposited on Madeira, all allied to beetles on
the nearest continent, and nearly all deprived of the use of their
wings. And similarly, of course, with all the peculiar species of the
Bermudas and the Azores. For who will explain, on the theory of
independent creation, why all the peculiar species, both of animals and
plants, which occur on the Bermudas should so unmistakably present
American affinities, while those which occur on the Azores no less
unmistakably present European affinities? But to proceed to other, and
still more remarkable, cases.
_The Galapagos Islands._--This archipelago is of volcanic origin,
situated under the equator between 500 and 600 miles from the West Coast
of South America. The depth of the ocean around them varies from 2,000
to 3,000 fathoms or more. This group is of particular interest, from the
fact that it was the study of its fauna which first suggested to
Darwin's mind the theory of evolution. I will, therefore, begin by
quoting a short passage from his writings upon the zoological relations
of this particular fauna.
Here almost every product of the land and of the water bears the
unmistakeable stamp of the American continent. There are twenty-six
land birds; of these, twenty-one, or perhaps twenty-three, are
ranked as distinct species, and would commonly be assumed to have
been here created; yet the close affinity of most of these birds to
American species is manifest in every character, in their habits,
gestures, and tones of voice. So it is with the other animals, and
with a large proportion of the plants, as shown by Dr. Hooker in
his admirable Flora of this archipelago. The naturalist, looking at
the inhabitants of these volcanic islands in the Pacific, distant
several hundred miles from the continent, feels that he is standing
on American land. Why should this be so? Why should the species
which are supposed to have been created in the Galapagos
Archipelago, and nowhere else, bear so plainly the stamp of
affinity to those created in America? There is nothing in the
conditions of life, in the geological nature of the islands, in
their height or climate, or in the proportions in which the several
classes are associated together, which closely resembles the
conditions of the South American coast; in fact, there is a
considerable dissimilarity in all these respects. On the other
hand, there is a considerable degree of resemblance in the volcanic
nature of the soil, in the climate, height, and size of the
islands, between the Galapagos and Cape de Verde Archipelagoes; but
what an entire and absolute difference in their inhabitants! The
inhabitants of the Cape de Verde Islands are related to those of
Africa, like those of the Galapagos to America. Facts such as these
admit of no sort of explanation on the ordinary view of independent
creation; whereas on the view here maintained, it is obvious that
the Galapagos Islands would be likely to receive colonists from
America, and the Cape de Verde Islands from Africa; such colonists
would be liable to modification--the principle of inheritance still
betraying their original birthplace[22].
[22] _origin of species_, pp. 353-4.
The following is a synopsis of the fauna and flora of this archipelago,
so far as at present known. The only terrestrial vertebrates are two
peculiar species of land-tortoise, and one extinct species; five species
of lizards, all peculiar--two of them so much so as to constitute a
peculiar genus;--and two species of snakes, both closely allied to South
American forms. Of birds there are 57 species, of which no less than 38
are peculiar; and all the non-peculiar species, except one, belong to
aquatic tribes. The true land birds are represented by 31 species, of
which all, except one, are peculiar; while more than half of them go to
constitute peculiar genera. Moreover, while they are all unquestionably
allied to South American forms, they present a beautiful series of
gradations, "from perfect identity with the continental species, to
genera so distinct that it is difficult to determine with what forms
they are most nearly allied; and it is interesting to note that this
diversity bears a distinct relation to the probabilities of, and
facilities for, migration to the islands. The excessively abundant
rice-bird, which breeds in Canada, and swarms over the whole United
States, migrating to the West Indies and South America, visiting the
distant Bermudas almost every year, and extending its range as far as
Paraquay, is the only species of land-bird which remains completely
unchanged in the Galapagos; and we may therefore conclude that some
stragglers of the migrating host reach the islands sufficiently often to
keep up the purity of the breed[23]." Again, of the thirty peculiar
land-birds, it is observable that the more they differ from any other
species or genera on the South American continent, the more certainly
are they found to have their nearest relations among those South
American forms which have the more restricted range, and are therefore
the least likely to have found their way to the islands with any
frequency.
[23] Wallace, _Island Life_, pp. 271-2.
The insect fauna of the Galapagos islands is scanty, and chiefly
composed of beetles. These number 35 species, which are nearly all
peculiar, and in some cases go to constitute peculiar genera. The same
remarks apply to the twenty species of land-shells. Lastly, of the total
number of flowering plants (332 species) more than one half (174
species) are peculiar. It is observable in the case of these peculiar
species of plants--as also of the peculiar species of birds--that many
of them are restricted to single islands. It is also observable that,
with regard both to the fauna and flora, the Galapagos Islands as a
whole are very much richer in peculiar species than either the Azores or
Bermudas, notwithstanding that both the latter are considerably more
remote from their nearest continents. This difference, which at first
sight appears to make against the evolutionary interpretation, really
tends to confirm it. For the Galapagos Islands are situated in a calm
region of the globe, unvisited by those periodic storms and hurricanes
which sweep over the North Atlantic, and which every year convey some
straggling birds, insects, seeds, &c., to the Azores and Bermudas.
Notwithstanding their somewhat greater isolation geographically,
therefore, the Azores and Bermudas are really less isolated biologically
than are the Galapagos Islands; and hence the less degree of peculiarity
on the part of their endemic species. But, on the theory of special
creation, it is impossible to understand why there should be any such
correlation between the prevalence of gales and a comparative inertness
of creative activity. And, as we have seen, it is equally impossible on
this theory to understand why there should be a further correlation
between the _degree_ of peculiarity on the part of the isolated
species, and the degree in which their nearest allies on the mainland
are there confined to narrow ranges, and therefore less likely to keep
up any biological communication with the islands.
_St. Helena._--A small volcanic island, ten miles long by eight wide,
situated in mid-ocean, 1100 miles from Africa, and 1800 from South
America. It is very mountainous and rugged, bounded for the most part by
precipices, rising from ocean depths of 17,000 feet, to a height above
the sea-level of nearly 3,000. When first discovered it was richly
clothed with forests; but these were all destroyed by human agency
during the 16th, 17th and 18th centuries. The records of civilization
present no more lamentable instance of this kind of destruction. From a
merely pecuniary point of view the abolition of these primeval forests
has proved an irreparable loss; but from a scientific point of view the
loss is incalculable. These forests served to harbour countless forms of
life, which extended at least from the Miocene age, and which, having
found there an ocean refuge, survived as the last remnants of a remote
geological epoch. In those days, as Mr. Wallace observes, St. Helena
must have formed a kind of natural museum or vivarium of archaic species
of all classes, the interest of which we can now only surmise from the
few remnants of those remnants, which are still left among the more
inaccessible portions of the mountain peaks and crater edges. These
remnants of remnants are as follows.
There is a total absence of all indigenous mammals, reptiles,
fresh-water fish, and true land-birds. There is, however, a species of
plover, allied to one in South Africa; but it is specifically distinct,
and therefore peculiar to the island. The insect life, on the other
hand, is abundant. Of beetles no less than 129 species are believed to
be aboriginal, and, with one single exception, the whole number are
peculiar to the island. "But in addition to this large amount of
specific peculiarity (perhaps unequalled anywhere else in the world),
the beetles of this island are remarkable for their generic isolation,
and for the altogether exceptional proportion in which the great
divisions of the order are represented. The species belong to 39 genera,
of which no less than 25 are peculiar to the island; and many of these
are such isolated forms that it is impossible to find their allies in
any particular country[24]." More than two-thirds of all the species
belong to the group of weevils--a circumstance which serves to explain
the great wealth of beetle-population, the weevils being beetles which
live in wood, and St. Helena having been originally a densely wooded
island. This circumstance is also in accordance with the view that the
peculiar insect fauna has been in large part evolved from ancestors
which reached the island by means of floating timber; for, of course, no
explanation can be suggested why special creation of this highly
peculiar insect fauna should have run so disproportionately into the
production of weevils. About two-thirds of the whole number of beetles,
or over 80 species, show no close affinity with any existing insects,
while the remaining third have some relations, though often very remote,
with European and African forms. That this high degree of peculiarity
is due to high antiquity is further indicated, according to our theory,
by the large number of species which some of the types comprise. Thus,
the 54 species of _Cossonidæ_ may be referred to three types; the 11
species of _Bembidium_ form a group by themselves; and the _Heteromera_
form two groups. "Now, each of these types may well be descended from a
single species, which originally reached the island from some other
land; and the great variety of generic and specific forms into which
some of them have diverged is an indication, and to some extent a
measure, of the remoteness of their origin[25]." But, on the
counter-supposition that all these 128 peculiar species were separately
created to occupy this particular island, it is surely unaccountable
that they should thus present such an arborescence of natural affinities
amongst themselves.
[24] Wallace, _Island Life_, p. 287.
[25] Wallace, _Island Life_, p. 287.
Passing over the rest of the insect fauna, which has not yet been
sufficiently worked out, we next find that there are only 20 species of
indigenous land-shells--which is not surprising when we remember by what
enormous reaches of ocean the island is surrounded. Of these 20 species
no less than 13 have become extinct, three are allied to European
species, while the rest are so highly peculiar as to have no near allies
in any other part of the globe. So that the land-shells tell exactly the
same story as the insects.
Lastly, the plants likewise tell the same story. The truly indigenous
flowering plants are about 50 in number, besides 26 ferns. Forty of the
former and ten of the latter are peculiar to the island, and, as Sir
Joseph Hooker tells us, "cannot be regarded as very close specific
allies of any other plants at all" Seventeen of them belong to peculiar
genera, and the others all differ so markedly as species from their
congeners, that not one comes under the category of being an insular
form of a continental species. So that with respect to its plants no
less than with respect to its animals, we find that the island of St.
Helena constitutes a little world of unique species, allied among
themselves, but diverging so much from all other known forms that in
many cases they constitute unique genera.
_Sandwich Islands._--These are an extensive group of islands, larger
than any we have hitherto considered--the largest of the group being
about the size of Devonshire. The entire archipelago is volcanic, with
mountains rising to a height of nearly 14,000 feet. The group is
situated in the middle of the North Pacific, at a distance of
considerably over 2,000 miles from any other land, and surrounded by
enormous ocean depths. The only terrestrial vertebrata are two lizards,
one of which constitutes a peculiar genus. There are 24 aquatic birds,
five of which are peculiar; four birds of prey, two of which are
peculiar; and 16 land-birds, all of which are peculiar. Moreover, these
16 land-birds constitute no less than 10 peculiar genera, and even one
peculiar family of five genera. This is an amount of peculiarity far
exceeding that of any other islands, and, of course, corresponds with
the great isolation of this archipelago. The only other animals which
have here been carefully studied are the land-shells, and these tell the
same story as the birds. For there are no less than 400 species which
are all, without any exception, peculiar; while about three-quarters of
them go to constitute peculiar genera. Again, of the plants, 620 species
are believed to be endemic; and of these 377 are peculiar, yielding no
less than 39 peculiar genera.
* * * * *
Prejudice apart, I think we must all now agree that it is needless to
continue further this line of proof. I have chosen the smallest and most
isolated islands for the purposes of our present argument, first because
these furnish the most crucial kind of test, and next because they best
admit of being dealt with in a short space. But, if necessary, a vast
amount of additional material could be furnished, not only from other
small oceanic islands, but still more from the largest islands of the
world, such as Australia and New Zealand. However, after the detailed
inventories which have now been given in the case of some of the smaller
islands most remote from mainlands, we may well be prepared to accept it
as a general law, that _wherever_ there is evidence of land-areas having
been for a long time separated from other land-areas, there we meet with
a more or less extraordinary profusion of unique species, often running
up into unique genera. And, in point of fact, so far as naturalists have
hitherto been able to ascertain, _there is no exception to this general
law in any region of the globe_. Moreover, there is everywhere a
constant correlation between the _degree_ of this peculiarity on the
part of the fauna and flora, and the _time_ during which they have been
isolated. Thus, for instance, among the islands which I have called
into evidence, those that are at once the most isolated and give
independent proofs of the highest antiquity, are the Galapagos Islands,
the Sandwich Islands, and St. Helena. Now, if we apply the method of
tabular analysis to these three cases, we obtain the following most
astonishing results. For the sake of simplicity I will omit the
enumeration of peculiar genera, and confine attention to peculiar
species. Moreover, I will consider only terrestrial animals; for, as we
have already seen, aquatic animals are so much more likely to reach
oceanic islands that they do not furnish nearly so fair a test of the
evolutionary hypothesis.
PECULIAR SPECIES.
+------------+---------+----------+-----------+----------+----------+
| | Shells. | Insects. | Reptiles. | Birds. | Mammals. |
| +---------+----------+-----------+----------+----------+
| Sandwich. | 400 | ? | 2 | 16 | 0 |
| Galapagos | 15 | 35 | 10 | 30 | 0 |
| St. Helena | 20 | 128 | 0 | 1 | 0 |
+------------+---------+----------+-----------+----------+----------+
| Totals. | 435 | 163 | 12 | 47 | 0 |
+------------+---------+----------+-----------+----------+----------+
NON-PECULIAR SPECIES.
+------------+---------+----------+-----------+----------+----------+
| | Shells. | Insects. | Reptiles. | Birds. | Mammals. |
| +---------+----------+-----------+----------+----------+
| Sandwich. | 0 | ? | 0 | 0 | 0 |
| Galapagos | ? | ? | 0 | 1 | 0 |
| St. Helena | 0 | ? | 0 | 0 | 0 |
+------------+---------+----------+-----------+----------+----------+
| Totals. | 0 | ? | 0 | 1 | 0 |
+------------+---------+----------+-----------+----------+----------+
From this synopsis we perceive that out of a total of 658 species of
terrestrial animals known to inhabit these three oceanic territories,
all are peculiar, with the exception of a single land-bird which is
found in the Galapagos Islands. This is the rice-bird, so very abundant
on the American continent that its representatives must not unfrequently
become the involuntary colonists of the Archipelago. There are, however,
a few species of non-peculiar insects inhabiting the Sandwich and
Galapagos Islands, the exact number of which is doubtful, and on this
account are not here quoted. But at most they would be represented by
units, and therefore do not affect the general result. Lastly, the
remarkable fact will be noted, that there is no single representative of
the mammalian class in any of these islands.
If we turn next to consider the case of plants, we obtain the following
result:--
+-----------+-----------+--------------+
| |_Peculiar |_Non-peculiar |
| | Species._ | Species._ |
+-----------|-----------|--------------+
|Sandwich | 377 | 243 |
|Galapagos | 174 | 158 |
|St. Helena | 50 | 26 |
| | ---- | ---- |
| Totals | 601 | 427 |
+-----------+-----------+--------------+
So that by adding together peculiar species both of land-animals and
plants, we find that on these three limited areas alone there are 1258
forms of life which occur nowhere else upon the globe--not to speak of
the peculiar aquatic species, nor of the presumably large number of
peculiar species of all kinds not hitherto discovered in these
imperfectly explored regions.
Now let us compare these facts with those which are presented by the
faunas and floras of islands less remote from continents, and known
from independent geological evidence to be of comparatively recent
origin--that is, to have been separated from their adjacent mainlands in
comparatively recent times, and therefore as islands to be comparatively
young. The British Isles furnish as good an instance as could be chosen,
for they together comprise over 1000 islands of various sizes, which are
nowhere separated from one another by deep seas, and in the opinion of
geologists were all continuous with the European continent since the
glacial period.
BRITISH ISLES.
NON-PECULIAR SPECIES.
+---------+---------+----------+-----------+---------+---------+
| | Land | | Reptiles | Land | Land |
| Plants. | Shells. | Insects. | and | Birds. | Mammals.|
| | | | Amphibia. | | |
+---------+---------+----------+-----------+---------+---------+
| 1462 | 83 | 12,551 | 13 | 130 | 40 |
+---------+---------+----------+-----------+---------+---------+
PECULIAR SPECIES.
+---------+---------+----------+-----------+---------+---------+
| | Land | | Reptiles | Land | Land |
| Plants. | Shells. | Insects. | and | Birds. | Mammals.|
| | | | Amphibia. | | |
+---------+---------+----------+-----------+---------+---------+
| 46 | 4 | 149 | 0 | 1 | 0 |
+---------+---------+----------+-----------+---------+---------+
Total Peculiar Plants 46
Total Peculiar Animals 154
----
Grand Total 200
I have drawn up this table in the most liberal manner possible,
including as peculiar species forms which many naturalists regard as
merely local varieties. But, even as thus interpreted, how wonderful is
the contrast between the 1000 islands of Great Britain and the single
volcanic rock of St. Helena, where almost all the animals and about half
the plants are peculiar, instead of about 1/80 of the animals, and 1/30
of the plants. Of course, if no peculiar species of any kind had
occurred in the British Isles, advocates of special creation might have
argued that it was, so to speak, needless for the Divinity to have added
any new species to those European forms which fully populated the
islands at the time when they were separated from the continent. But, as
the matter stands, advocates of special creation must face the fact that
a certain small number of new and peculiar species have been formed on
the British Isles; and, therefore, that creative activity has not been
wholly suspended in their case. Why, then, has it been so meagre in this
case of a thousand islands, when it has proved so profuse in the case of
all single islands more remote from mainlands, and presenting a higher
antiquity? Or why should the Divinity have thus appeared so uniformly to
consult these merely accidental circumstances of space and time in the
depositing of his unique specific types? Do not such facts rather speak
with irresistible force in favour of the view, that while all ancient
and solitary islands have had time enough, and separation enough, to
admit of distinct histories of evolution having been written in their
living inhabitants, no one of the thousand islands of Great Britain has
had either time enough, or separation enough, to have admitted of more
than some of the first pages of such a history having been commenced?
But this allusion to Great Britain introduces us to another point. It
will have been observed that, unlike oceanic islands remote from
mainlands, Great Britain is well furnished both with reptiles (including
amphibia) and mammals. For there is no instance of any oceanic island
situated at more than 300 miles from a continent where any single
species of the whole class of mammals is to be found, excepting species
of the only order which is able to fly--namely, the bats. And the same
has to be said of frogs, toads, and newts, whose spawn is quickly killed
by contact with sea-water, and therefore could never have reached remote
islands in a living state. Hence, on evolutionary principles; it is
quite intelligible why oceanic islands should not present any species of
mammals or batrachians--peculiar or otherwise,--save such species of
mammals as are able to fly. But on the theory of special creation we can
assign no reason why, notwithstanding the extraordinary profusion of
unique types of other kinds which we have seen to occur on oceanic
islands, the Deity should have made this curious exception to the
detriment of all frogs, toads, newts, and mammals, save only such as are
able to fly. Or, if any one should go so far to save a desperate
hypothesis as to maintain that there must have been some hidden reason
why batrachians and quadrupeds were not specially created on oceanic
islands, I may mention another small--but in this relation a most
significant--fact. This is that on some of these islands there occur
certain peculiar species of plants, the seeds of which are provided with
numerous tiny hooks, obviously and beautifully adapted--like those on
the seeds of allied plants elsewhere--to catch the wool or hair of
moving quadrupeds, and so to further their own dissemination. But, as we
have just seen, there are no quadrupeds in the islands to meet these
beautiful adaptations on the part of the plants; so that special
creationists must resort to the almost impious supposition that in these
cases the Deity has only carried out half his plan, in that while he
made an elaborate provision for these uniquely created species of
plants, which depended for its efficiency on the presence of quadrupeds,
he nevertheless neglected to place any quadrupeds on the islands where
he had placed the plants. Such one-sided attempts at adaptation surely
resolve the thesis of special creation to a _reductio ad absurdum_; and
hence the only reasonable interpretation of them is, that while the
seeds of allied or ancestral plants were able to float to the islands,
no quadrupeds were ever able over so great a distance to swim.
* * * * *
Although much more evidence might still be given under the head of
geographical distribution, I must now close with a brief summary of the
main points that have been adduced.
After certain preliminary considerations, I began by noticing that the
theory of evolution has a much more intelligible account to give than
has its rival of the facts of discontinuous distribution--the Alpine
flora, for instance, being allied to the Arctic, not because the same
species were separately created in both places, but because during the
glacial period these species extended all over Europe, and were left
behind on the Alps as the Arctic flora receded northwards--which was
sufficiently long ago to explain why some of the Alpine species are
unique, though closely allied to Arctic forms.
Next we saw that, although living things are always adapted to the
climates under which they live (since otherwise they could not live
there at all), it is equally true that, as a rule, besides the area on
which they do live, there are many other areas in different parts of the
globe where they might have lived equally well. Consequently we must
conclude that, if all species were separately created, many species were
severally created on only one among a number of areas where they might
equally well have thrived. Now, although this conclusion in itself may
not seem opposed to the theory of special creation, a most serious
difficulty is raised when it is taken in connexion with another fact of
an equally general kind. This is, that on every biological region we
encounter chains of allied species constituting allied genera, families,
and so on; while we scarcely ever meet with allied species in different
biological regions, notwithstanding that their climates may be similar,
and, consequently, just as well suited to maintain some of the allied
species. Hence we must further conclude, if all species were separately
created, that in the work of creation some unaccountable regard was paid
to making areas of distribution correspond to degrees of structural
affinity. A great many species of the rat genus were created in the Old
World, and a great many species of another, though allied, genus were
created in the New World: yet no reason can be assigned why no one
species of the Old World series should not just as well have been
deposited in the New World, and _vice versa_. On the other hand, the
theory of evolution may claim as direct evidence in its support all the
innumerable cases such as these--cases, indeed, so innumerable that, as
Mr. Wallace remarks, it may be taken as a law of nature that "every
species has come into existence coincident both in space and time with a
pre-existing and closely allied species." A general law which, while in
itself most strongly suggestive of evolution, is surely impossible to
reconcile with any reasonable theory of special creation. Furthermore,
this law extends backwards through all geological time, with the result
that the extinct species which now occur only as fossils on any given
geological area, resemble the species still living upon that area, as we
should expect that they must, if the former were the natural progenitors
of the latter. On the other hand, if they were not the natural
progenitors, but all the species, both living and extinct, were the
supernatural and therefore independent creations which the rival theory
would suppose, then no reason can be given why the extinct species
should thus resemble the living--any more than why the living species
should resemble one another. For, as we have seen, there are almost
always many other habitats on other parts of the globe, where any
members of any given group of species might equally well have been
deposited; and this, of course, applies to geological no less than to
historical time. Yet throughout all time we meet with this most
suggestive correlation between continuity of a geographical area and
structural affinity between the forms of life which have lived, or are
still living, upon that area.
Similarly, we find the further, and no less suggestive, correlation
between the birth of new species and the immediate pre-existence of
closely allied species on the same area--or, at most, on closely
contiguous areas.
Where a continuous area has long been circumscribed by barriers of any
kind, which prevent the animals from wandering beyond it, then we find
that all the species, both extinct and living, constitute more or less a
world of their own; while, on the other hand, where the animals are free
to migrate from one area to another, the course of their migrations is
marked by the origination of new species springing up _en route_, and
serving to connect the older, or metropolitan, forms with the younger,
or colonising, forms in the way of a graduated series. This principle,
however, admits of being traced only in certain cases of species
belonging to the same genus, of genera belonging to the same family, or,
at most, of families belonging to the same order. In other words, the
more general the structural affinity, the more general is the
geographical extension--as we should expect to be the case on the theory
of descent with branching modifications, seeing that the larger, the
older, and the more diverse the group of organisms compared, the greater
must be their chances of dispersal.
These general considerations led us to contemplate more in detail the
correlation between structural affinity and barriers to free migration.
Such barriers, of course, differ in the cases of different organisms.
Marine organisms are stopped by land, unsuitable temperature, or
unsuitable depths; fresh-water organisms by sea and by mountain-chains;
terrestrial organisms chiefly by water. Now it is a matter of fact
which admits of no dispute, that in each of these cases we meet with a
direct correlation between the kind of barrier and the kind of organisms
whose structural affinities are affected thereby. Where we have to do
with marine organisms, barriers such as the Isthmus of Panama and the
varying depth of the Western Pacific determine three very distinct
faunas, ranging north and south in closely parallel lines, and under
corresponding climates. Where we have to do with fresh-water organisms,
we find that a mountain-chain only a few miles wide has more influence
in determining differences of organic type on either side of it than is
exercised by even thousands of miles of a continuous land-area, if this
be uninterrupted by any mountains high enough to prevent water-fowl,
whirlwinds, &c., from dispersing the ova. Again, where we have to do
with terrestrial organisms, the most effectual barriers are wide reaches
of ocean; and, accordingly, we find that these exercise an enormous
influence on the modification of terrestrial types. Moreover, we find
that the _more_ terrestrial an organism, or the _greater_ the difficulty
it has in traversing a wide reach of ocean, the _greater_ is the
modifying influence of such a barrier upon that type. In oceanic
islands, for example, many of the plants and aquatic birds usually
belong to the same species as those which occur on the nearest
mainlands, and where there are any specific differences, these but
rarely run up to generic differences. But the land-birds, insects, and
reptiles which are found on such islands are nearly always specifically,
and very often generically, distinct from those on the nearest
mainland--although invariably allied with sufficient closeness to leave
no manner of doubt as to their affinities with the fauna of that
mainland. Lastly, no amphibians and no mammals (except bats) are ever
found on any oceanic islands. Yet, as we have seen, on the theory of
special creation, these islands must all be taken to have been the
theatres of the most extraordinary creative activity, so that on only
three of them we found no less than 1258 unique species, whereof 657
were unique species of land animals, to be set against one single
species known to occur elsewhere. Nevertheless, notwithstanding this
prodigious expenditure of creative energy in the case of land-birds,
land-shells, insects, and reptiles, no single new amphibian, or no
single new mammal, has been created on any single oceanic island, if we
except the only kind of mammal that is able to fly, and the ancestors of
which, like those of the land-birds and insects, might therefore have
reached the islands ages ago. Moreover, with regard to mammals, even in
cases where allied forms occur on either side of a sea-channel, it is
found to be a general rule that if the channel is shallow, the species
on either side of it are much more closely related than if it be
deep--and this irrespective of its width. Therefore we can only
conclude, in the words of Darwin--"As the amount of modification which
animals of all kinds undergo partly depends on lapse of time, and as the
islands which are separated from each other or from the mainland by
shallow channels are more likely to have been continuously united within
a recent period than islands separated by deeper channels, we can
understand how it is that a relation exists between the depth of the sea
separating two mammalian faunas, and the degree of their affinity--a
relation which is quite inexplicable on the theory of independent acts
of creation."
* * * * *
Looking to all these general principles of geographical distribution,
and remembering the sundry points of smaller detail relating to oceanic
islands which I will not wait to recapitulate, to my mind it seems that
there is no escape from the following conclusion, with which I will
bring my brief epitome of the evidence to a close. The conclusion to
which, I submit, all the evidence leads is, that if the doctrine of
special creation is taken to be true, then it must be further taken that
the one and only principle which has been consistently followed in the
geographical deposition of species, is that of so depositing them as to
make it everywhere appear that they were not thus deposited at all, but
came into existence where they now occur by way of genetic descent with
perpetual migration and correlative modification. On no other principle,
so far as I can see, would it be possible to account for the fact that
"every species has come into existence coincident both in space and time
with a pre-existing and closely allied species," together with the
carefully graduated regard to physical barriers which the Creator must
have displayed while depositing his newly formed species on either sides
of them--everywhere making _degrees_ of structural affinity correspond
to _degrees_ of geographical continuity, and _degrees_ of structural
difference correspond to _degrees_ of geographical separation, whether
by mountain-chains in the case of fresh-water faunas, by land and by
deep sea in the case of marine faunas, or by reaches of ocean in the
case of terrestrial faunas--stocking oceanic islands with an enormous
profusion of peculiar species all allied to those on the nearest
mainlands, yet everywhere avoiding the creation upon them of any
amphibian or mammal, except an occasional bat. We are familiar with the
doctrine that God is a God who hideth himself; here, however, it seems
to me, we should have but a thinly-veiled insinuation, not merely that
in his works he is hidden, but that in these works he is untrue. Than
which I cannot conceive a stronger condemnation of the theory which it
has been my object fairly to represent and dispassionately to
criticise.
SECTION II
_SELECTION_
CHAPTER VII.
THE THEORY OF NATURAL SELECTION.
Thus far we have been considering the main evidences of organic
evolution considered as a fact. We now enter a new field, namely, the
evidences which thus far have been brought to light touching the causes
of organic evolution considered as a process.
As was pointed out in the opening chapter, this is obviously the
methodical course to follow: we must have some reasonable assurance that
a fact is a fact before we endeavour to explain it. Nevertheless, it is
not necessary that we should actually demonstrate a fact to be a fact
before we endeavour to explain it. Even if we have but a reasonable
presumption as to its probability, we may find it well worth while to
consider its explanation; for by so doing we may obtain additional
evidence of the fact itself. And this because, if it really is a fact,
and if we hit upon the right explanation of it, by proving the
explanation probable, we may thereby greatly increase our evidence of
the fact. In the very case before us, for example, the evidence of
evolution as a fact has from the first been largely derived from testing
Darwin's theory concerning its method. It was this theoretical
explanation of its method which first set him seriously to enquire into
the evidences of evolution as a fact; and ever since he published his
results, the evidences which he adduced in favour of natural selection
as a method have constituted some of the strongest reasons which
scientific men have felt for accepting evolution as a fact. Of course
the evidence in favour of this fact has gone on steadily growing, quite
independently of the assistance which was thus so largely lent to it by
the distinctively Darwinian theory of its method; and, indeed, so much
has this been the case, that in the present treatise we have been able
to consider such direct evidence of the fact itself, without any
reference at all to the indirect or accessory evidence which is derived
from that of natural selection as a method. From which it follows that
in most of what I am about to say in subsequent chapters on the
evidences of natural selection as a method, there will be furnished a
large addition to the evidences which have already been detailed of
evolution as a fact. But, as a matter of systematic treatment, I have
thought it desirable to keep these two branches of our subject separate.
Which means that I have made the evidences of evolution as a fact to
stand independently on their own feet--feet which in my opinion are
amply strong enough to bear any weight of adverse criticism that can be
placed upon them.
Our position, then, is this. On the foundation of the previous chapters,
I will henceforth assume that we all accept organic evolution as a fact,
without requiring any of the accessory evidence which is gained by
independent proof of natural selection as a method. But in making this
assumption--namely, that we are all now firmly persuaded of the fact of
evolution--I do not imagine that such is really the case. I make the
assumption for the purposes of systematic exposition, and in order that
different parts of the subject may be kept distinct. I confess it does
appear to me remarkable that there should still be a doubt in any
educated mind touching the general fact of evolution; while it becomes
to me unaccountable that such should be the case with a few still living
men of science, who cannot be accused of being ignorant of the evidences
which have now been accumulated. But in whatever measure we may
severally have been convinced--or remained unconvinced--on this matter,
for the purposes of exposition I must hereafter assume that we are all
agreed to the extent of regarding the process of evolution as, at least,
sufficiently probable to justify enquiry touching its causes on
supposition of its truth.
Now, the causes of evolution have been set forth in a variety of
different hypotheses, only the chief of which need be mentioned here.
Historically speaking the first of these was that which was put forward
by Erasmus Darwin, Lamarck, and Herbert Spencer. It consists in putting
together the following facts and inferences.
We know that, in the lifetime of the individual, increased use of
structures leads to an increase of their functional efficiency; while,
on the other hand, disuse leads to atrophy. The arms of a blacksmith,
and the legs of a mountaineer, are familiar illustrations of the first
principle: our hospital wards are full of illustrations of the second.
Again, we know that the characters of parents are transmitted to their
progeny by means of heredity. Now the hypothesis in question consists
in supposing that if any particular organs in a species are habitually
used for performing any particular action, they must undergo a
structural improvement which would more and more adapt them to the
performance of that action; for in each generation constant use would
better and better adapt the structures to the discharge of their
functions, and they would then be bequeathed to the next generation in
this their improved form by heredity. So that, for instance, if there
had been a thousand generations of blacksmiths, we might expect the sons
of the last of them to inherit unusually strong arms, even if these
young men had themselves taken to some other trade not requiring any
special use of their arms. Similarly, if there had been a thousand
generations of men who used their arms but slightly, we should expect
their descendants to show but a puny development of the upper
extremities. Now let us apply all this to the animal kingdom in general.
The giraffe, for instance, is a ruminant whose entire frame has been
adapted to support an enormously long neck, which is of use to the
animal in reaching the foliage of trees. The ancestors of the giraffe,
having had ordinary necks, were supposed by Lamarck to have gradually
increased the length of them, through many successive generations, by
constantly stretching to reach high foliage; and he further supposed
that, when the neck became so long as to require for its support special
changes in the general form of the animal as a whole, these special
changes would have brought about the dwindling of other parts from which
so much activity was no longer required--the general result being that
the whole organization of the animal became more and more adapted to
browsing on high foliage. And so in the cases of other animals, Lamarck
believed that the adaptation of their forms to their habits could be
explained by this simple hypothesis that the habits created the forms,
through the effects of use and disuse, coupled with heredity.
Such is what is ordinarily known as Lamarck's theory of evolution. We
may as well remember, however, that it really constitutes only one part
of his theory; for besides this hypothesis of the cumulative inheritance
of functionally-produced modifications--to which we may add the
inherited effects of any direct action exercised by surrounding
conditions of life,--Lamarck believed in some transcendental principle
tending to produce gradual improvement in pre-determined lines of
advance. Therefore it would really be more correct to designate the
former hypothesis by the name either of Erasmus Darwin, or, still
better, of Herbert Spencer. Nevertheless, in order to avoid confusion, I
will follow established custom, and subsequently speak of this
hypothesis as the Lamarckian hypothesis--understanding, however, that in
employing this designation I am not referring to any part or factor of
Lamarck's general theory of evolution other than the one which has just
been described--namely, the hypothesis of the cumulative transmission of
functionally-produced, or otherwise "acquired," modifications.
This, then, was the earliest hypothesis touching the causes of organic
evolution. But we may at once perceive that it is insufficient to
explain all that stands to be explained. In the first place, it refers
in chief part only to the higher animals, which are actuated to effort
by intelligence. Its explanatory power in the case of most
invertebrata--as well as in that of all plants--is extremely limited,
inasmuch as these organisms can never be moved to a greater or less use
of their several parts by any discriminating volition, such as that
which leads to the continued straining of a giraffe's neck for the
purpose of reaching foliage. In the second place, even among the higher
animals there are numberless tissues and organs which unquestionably
present a high degree of adaptive evolution, but which nevertheless
cannot be supposed to have fallen within the influence of Lamarckian
principles. Of such are the shells of crustacea, tortoises, &c., which
although undoubtedly of great use to the animals presenting them, cannot
ever have been _used_ in the sense required by Lamarck's hypothesis, i.
e. actively exercised, so as to increase a flow of nutrition to the
part. Lastly, in the third place, the validity of Lamarck's hypothesis
in any case whatsoever has of late years become a matter of serious
question, as will be fully shown and discussed in the next volume.
Meanwhile it is enough to observe that, on account of all these reasons,
the theory of Lamarck, even if it be supposed to present any truth at
all, is clearly insufficient as a full or complete theory of organic
evolution.
* * * * *
In historical order the next theory that was arrived at was the theory
of natural selection, simultaneously published by Darwin and Wallace on
July 1st, 1858.
If we may estimate the importance of an idea by the change of thought
which it effects, this idea of natural selection is unquestionably the
most important idea that has ever been conceived by the mind of man.
Yet the wonder is that it should not have been hit upon long before. Or
rather, I should say, the wonder is that its immense and immeasurable
importance should not have been previously recognised. For, since the
publication of this idea by Darwin and Wallace, it has been found that
its main features had already occurred to at least two other
minds--namely, Dr. Wells in 1813, and Mr. Patrick Matthew in 1831. But
neither of these writers perceived that in the few scattered sentences
which they had written upon the subject they had struck the key-note of
organic nature, and resolved one of the principal chords of the
universe. Still more remarkable is the fact that Mr. Herbert
Spencer--notwithstanding his great powers of abstract thought and his
great devotion of those powers to the theory of evolution, when as yet
this theory was scorned by science--still more remarkable, I say, is the
fact that Mr. Herbert Spencer should have missed what now appears so
obvious an idea. But most remarkable of all is the fact that Dr.
Whewell, with all his stores of information on the history of the
inductive sciences, and with all his acumen on the matter of scientific
method, should not only have conceived the idea of natural selection,
but expressly stated it as a logically possible explanation of the
origin of species, and yet have so stated it merely for the purpose of
dismissing it with contempt[26]. This, I think, is most remarkable,
because it serves to prove how very far men's minds at that time must
have been from entertaining, as in any way antecedently probable, the
doctrine of transmutation. In order to show this I will here quote one
passage from the writings of Whewell, and another from a distinguished
French naturalist referred to by him.
[26] For quotations, see Note A.
In 1846 Whewell wrote:--
Not only is the doctrine of the transmutation of species in itself
disproved by the best physiological reasonings, but the additional
assumptions which are requisite to enable its advocates to apply it
to the explanation of the geological and other phenomena of the
earth, are altogether gratuitous and fantastical[27].
[27] whewell, _indications of the creator_, 2nd ed., 1846.
Then he quotes with approval the following opinion:--
Against this hypothesis, which, up to the present time, I regard as
purely gratuitous, and likely to turn geologists out of the sound
and excellent road in which they now are, I willingly raise my
voice, with the most absolute conviction of being in the right[28].
[28] de blainville, _compte rendu_, 1837.
And, after displaying the proof rendered by Lyell of uniformitarianism
in geology, and cordially subscribing thereto, Whewell adds:--
We are led by our reasonings to this view, that the present order
of things was commenced by an act of creative power entirely
different to any agency which has been exerted since. None of the
influences which have modified the present races of animals and
plants since they were placed in their habitations on the earth's
surface can have had any efficacy in producing them at first. We
are necessarily driven to assume, as the beginning of the present
cycle of organic nature, an event not included in the course of
nature[29].
[29] Whewell, _ibid._, p. 162.
So much, then, for the state of the most enlightened and representative
opinions on the question of evolution before the publication of
Darwin's work; and so much, likewise, for the only reasonable
suggestions as to the causes of evolution which up to that time had been
put forward, even by those few individuals who entertained any belief in
evolution as a fact. It was the theory of natural selection that changed
all this, and created a revolution in the thought of our time, the
magnitude of which in many of its far-reaching consequences we are not
even yet in a position to appreciate; but the action of which has
already wrought a transformation in general philosophy, as well as in
the more special science of biology, that is without a parallel in the
history of mankind.
* * * * *
Although every one is now more or less well acquainted with the theory
of natural selection, it is necessary, for the sake of completeness,
that I should state the theory; and I will do so in full detail.
It is a matter of observable fact that all plants and animals are
perpetually engaged in what Darwin calls a "struggle for existence."
That is to say, in every generation of every species a great many more
individuals are born than can possibly survive; so that there is in
consequence a perpetual battle for life going on among all the
constituent individuals of any given generation. Now, in this struggle
for existence, which individuals will be victorious and live? Assuredly
those which are best fitted to live, in whatever respect, or respects,
their superiority of fitness may consist. Hence it follows that Nature,
so to speak, _selects_ the best individuals out of each generation to
live. And not only so; but as these favoured individuals transmit their
favourable qualities to their offspring, according to the fixed laws of
heredity, it further follows that the individuals composing each
successive generation have a general tendency to be better suited to
their surroundings than were their forefathers. And this follows, not
merely because in every generation it is only the "flower of the flock"
that is allowed to breed, but also because, if in any generation some
new and beneficial qualities happen to arise as slight variations from
the ancestral type, they will (other things permitting) be seized upon
by natural selection, and, being transmitted by heredity to subsequent
generations, will be added to the previously existing type. Thus the
best idea of the whole process will be gained by comparing it with the
closely analogous process whereby gardeners, fanciers, and
cattle-breeders create their wonderful productions; for just as these
men, by always "_selecting_" their best individuals to breed from,
slowly but continuously improve their stock, so Nature, by a similar
process of "_selection_" slowly but continuously makes the various
species of plants and animals better and better suited to the conditions
of their life.
Now, if this process of continuously adapting organisms to their
environment takes place in nature at all, there is no reason why we
should set any limits on the extent to which it is able to go, up to the
point at which a complete and perfect adaptation is achieved. Therefore
we might suppose that all species would eventually reach this condition
of perfect harmony with their environment, and then remain fixed. And
so, according to the theory, they would, if the environment were itself
unchanging. But forasmuch as the environment (i. e. the sum total of the
external conditions of life) of almost every organic type alters more
or less from century to century--whether from astronomical, geological,
and geographical changes, or from the immigrations and emigrations of
other species living on contiguous areas, and so on--it follows that the
process of natural selection need never reach a terminal phase. And
forasmuch as natural selection may thus continue, _ad infinitum_, slowly
to alter a specific type in adaptation to a gradually changing
environment, if in any case the alteration thus effected is sufficient
in amount to lead naturalists to name the result as a distinct species,
it follows that natural selection has transmuted one specific type into
another. Similarly, by a continuation of the process, specific types
would become transmuted into generic, generic into family types, and so
on. Thus the process is supposed to go on throughout all the countless
forms of life continuously and simultaneously--the world of organic
types being thus regarded as in a state of perpetual, though gradual,
flux.
* * * * *
Now, the first thing we have to notice about this theory is, that in all
its main elements it is merely a statement of observable facts. It is an
observable fact that in all species of plants and animals a very much
larger number of individuals are born than can possibly survive. Thus,
for example, it has been calculated that if the progeny of a single pair
of elephants--which are the slowest breeding of animals--were all
allowed to reach maturity and propagate, in 750 years there would be
living 19,000,000 descendants. Again, in the case of vegetables, if a
species of annual plant produces only two seeds a year, if these in
successive years were all allowed to reproduce their kind, in twenty
years there would be 11,000,000 plants from a single ancestor. Yet we
know that nearly all animals and plants produce many more young at a
time than in either of these two supposed cases. Indeed, as individuals
of many kinds of plants, and not a few kinds of animals, produce every
year several thousand young, we may make a rough estimate and say, that
over organic nature as a whole probably not one in a thousand young are
allowed to survive to the age of reproduction. How tremendous,
therefore, must be the struggle for existence! It is thought a terrible
thing in battle when one half the whole number of combatants perish. But
what are we to think of a battle for life where only one in a thousand
survives?
This, then, is the first fact. The second is the fact so long ago
recognised, that the battle is to the strong, the race to the swift. The
thousandth individual which does survive in the battle for
existence--which does win the race for life--is, without question, one
of the individuals best fitted to do so; that is to say, best fitted to
the conditions of its existence considered as a whole. Nature is,
therefore, always picking out, or selecting, such individuals to live
and to breed.
The third fact is, that the individuals so selected transmit their
favourable qualities to their offspring by heredity. There is no doubt
about this fact, so far as we are concerned with it. For although, as I
have already hinted, considerable doubt has of late years been cast upon
Lamarck's doctrine of the hereditary transmission of _acquired_
characters, it remains as impossible as ever it was to question the
hereditary transmission of what are called _congenital_ characters. And
this is all that Darwin's theory necessarily requires.
The fourth fact is, that although heredity as a whole produces a
wonderfully exact copy of the parent in the child, there is never a
precise reduplication. Of all the millions of human beings upon the face
of the earth, no one is so like another that we cannot see some
difference; the resemblance is everywhere specific, nowhere individual.
Now this same remark applies to all specific types. The only reason why
we notice individual differences in the case of the human type more than
we do in the case of any other types, is because our attention is here
more incessantly focussed upon these differences. We are compelled to
notice them in the case of our own species, however small they may
appear to a naturalist, because, unless we do so, we should not
recognise the members of our own family, or be able to distinguish
between a man whom we know is ready to do us an important service, and
another man whom we know is ready to cut our throats. But our common
mother Nature is able thus to distinguish between all her children. Her
eyes are much more ready to detect small individual peculiarities than
are the eyes of any naturalist. No slight variations in the cast of
feature or disposition of parts, no minute difference in the arrangement
of microscopical cells, can escape her ever vigilant attention. And,
consequently, when among all the innumerable multitudes of individual
variations any one arises which--no matter in how slight a degree--gives
to that individual a better chance of success in the struggle for life,
Nature chooses that individual to survive, and so to perpetuate the
improvement in his or her progeny.
Now I say that all these several component parts of Darwinian doctrine
are not matters of theory, but matters of fact. The only element of
theory in his doctrine of evolution by natural selection has reference
to the degree in which these observable facts, when thus brought
together, are adequate to account for the process of evolution.
* * * * *
So much, then, as a statement of the theory of natural selection. But
from this statement--i. e. from the theory of natural selection
itself--there follow certain matters of general principle which it is
important to bear in mind. These, therefore, I shall here proceed to
mention.
First of all, it is evident that the theory is applicable as an
explanation of organic changes in specific types only in so far as these
changes are of _use_, or so far as such changes endow the species with
better chances of success in the general struggle for existence. This is
the only sense in which I shall always employ the terms use, utility,
service, benefit, and so forth--that is to say, in the sense of
life-preserving.
* * * * *
Next, it must be clearly understood that the life which it is the
object, so to speak, of natural selection to preserve, is primarily the
life of the _species_; not that of the _individual_. Natural selection
preserves the life of the individual only in so far as this is conducive
to that of the species. Wherever the life-interests of the individual
clash with those of the species, that individual is sacrificed in favour
of others who happen better to subserve the interests of the species.
For example, in all organisms a greater or less amount of vigour is
wasted, so far as individual interests are concerned, in the formation
and the nourishment of progeny. In the great majority of plants and
animals an enormous amount of physiological energy is thus expended.
Look at the roe or the milt of a herring, for instance, and see what a
huge drain has been made upon the individual for the sake of its
species. Again, all unselfish instincts have been developed for the sake
of the species, and usually against the interests of the individual. An
ant which will allow her head to be slowly drawn from her body rather
than relinquish her hold upon a pupa, is clearly acting in response to
an instinct which has been developed for the benefit of the hive, though
fatal to the individual. And, in a lesser degree, the parental
instincts, wherever they occur, are more or less detrimental to the
interests of the individual, though correspondingly essential to those
of the race.
These illustrations will serve to show that natural selection always
works primarily for the life-interests of the species--and, indeed, only
works for those of the individual at all in so far as the latter happen
to coincide with the former. Or, otherwise stated, the object of natural
selection is always that of producing and maintaining specific types in
the highest degree of efficiency, no matter what may become of the
constituent individuals. Which is a striking republication by Science of
a general truth previously stated by Poetry:--
So careful of the type she seems,
So careless of the single life.
Tennyson thus noted the fact, and a few years later Darwin supplied the
explanation.
But of course in many, if not in the majority of cases, anything that
adds to the life-sustaining power of the single life thereby ministers
also to the life-sustaining power of the type; and thus we can
understand why all mechanisms and instincts which minister to the single
life have been developed--namely, because the life of the species is
made up of the lives of all its constituent individuals. It is only
where the interests of the one clash with those of the other that
natural selection works against the individual. So long as the interests
are coincident, it works in favour of both.
Natural selection, then, is a theory which seeks to explain by natural
causes the occurrence of every kind of adaptation which is to be met
with in organic nature, on the assumption that adaptations of every kind
have primary reference to the preservation of species, and therefore
also, as a general rule, to the preservation of their constituent
individuals. And from this it follows that where it is for the benefit
of a species to change its type, natural selection will effect that
change, thus leading to a specific transmutation, or the evolution of a
new species. In such cases the old species may or may not become
extinct. If the transmutation affects the species as a whole, or
throughout its entire range, of course _that_ particular type becomes
extinct, although it does so by becoming changed into a still more
suitable type in the course of successive generations. If, on the other
hand, the transmutation affects only a part of the original species, or
not throughout its entire range, then the other parts of that species
may survive for any number of ages as they originally were. In the one
case there is a ladder-like transmutation of species in time; in the
other case a possibly tree-like multiplication of species in space. But
whether the evolution of species be thus serial in time or divergent in
space, the object of natural selection, so to speak, is in either case
the same--namely, that of preserving all types which prove best suited
to the conditions of their existence.
* * * * *
Once more, the term "struggle for existence" must be understood to
comprehend, not only a competition for life among contemporary
individuals of the same species, but likewise a struggle by all such
individuals taken collectively for the continuance of their own specific
type. Thus, on the one hand, while there is a perpetual civil war being
waged between members of the same species, on the other hand there is a
foreign war being waged by the species as a whole against its world as a
whole. Hence it follows that natural selection does not secure survival
of the fittest as regards individuals only, but also survival of the
fittest as regards types. This is a most important point to remember,
because, as a general rule, these two different causes produce exactly
opposite effects. Success in the civil war, where each is fighting
against all, is determined by _individual_ fitness and _self-reliance_.
But success in the foreign war is determined by what may be termed
_tribal_ fitness and _mutual dependence_. For example, among social
insects the struggle for existence is quite as great between different
tribes or communities, as it is between different individuals of the
same community; and thus we can understand the extraordinary degree in
which not only co-operative instincts, but also largely intelligent
social habits, have here been developed[30]. Similarly, in the case of
mankind, we can understand the still more extraordinary development of
these things--culminating in the moral sense. I have heard a sermon,
preached at one of the meetings of the British Association, entirely
devoted to arguing that the moral sense could not have been evolved by
natural selection, seeing that the altruism which this sense involves is
the very opposite of selfishness, which alone ought to have been the
product of survival of the fittest in a struggle for life. And, of
course, this argument would have been perfectly sound had Darwin limited
the struggle for existence to individuals, without extending it to
communities. But if the preacher had ever read Darwin's works he would
have found that, when thus extended, the principle of natural selection
is bound to work in favour of the co-operative instincts in the case of
so highly social an animal as man; and that of these instincts
conscience is the highest imaginable exhibition.
[30] For cases, see _Animal Intelligence_, in the chapters on Ants
and Bees; and, for discussion of principles, _Mental Evolution in
Animals_, in the chapters on Instinct.
What I have called tribal fitness--in contradistinction to individual
fitness--begins with the family, developes in the community (herd, hive,
clan, &c.), and usually ends with the limits of the species. On the one
hand, however, it is but seldom that it extends so far as to embrace the
entire species; while, on the other hand, it may in some cases, and as
it were sporadically, extend beyond the species. In these latter cases
members of different species mutually assist one another, whether in the
way of what is called symbiosis, or in a variety of other ways which I
need not wait to mention. For the only point which I now desire to make
clear is, that all cases of mutual aid or co-operation, whether within
or beyond the limits of species, are cases which fall under the
explanatory sweep of the Darwinian theory[31].
[31] Prince Kropotkin in the _Nineteenth Century_ (Feb. 1888, Apr.
1891) has adduced a large and interesting body of facts, showing the
great prevalence of the principle of co-operation in organic
nature.
* * * * *
Another important point to notice is, that it constitutes no part of the
theory of natural selection to suppose that survival of the fittest must
invariably lead to _improvement_ of type, in the sense of superior
organization. On the contrary, if from change of habits or conditions of
life an organic type ceases to have any use for previously useful
organs, natural selection will not only allow these organs in successive
generations to deteriorate--by no longer placing any selective premium
upon their maintenance--but may even proceed to assist the agencies
engaged in their destruction. For, being now useless, they may become
even deleterious, by absorbing nutriment, causing weight, occupying
space, &c., without conferring any compensating benefit. Thus we can
understand why it is that parasites, for example, present the phenomena
of what is called _degeneration_, i. e. showing by their whole structure
that they have descended from a possibly very much higher type of
organization than that which they now exhibit. Having for innumerable
generations ceased to require their legs, their eyes, and so forth, all
such organs of high elaboration have either disappeared or become
vestigial, leaving the parasite as a more or less effete representative
of its ancestry.
These facts of degeneration, as we have previously seen, are of very
general occurrence, and it is evident that their importance in the field
of organic evolution as a whole has been very great. Moreover, it ought
to be particularly observed that, as just indicated, the facts may be
due either to a passive _cessation_ of selection, or to an active
_reversal_ of it. Or, more correctly, these facts are probably _always_
due to the cessation of selection, although in most cases where species
in a state of nature are concerned, the process of degeneration has been
both hastened and intensified by the super-added influence of the
reversal of selection. In the next volume I shall have occasion to recur
to this distinction, when it will be seen that it is one of no small
importance to the general theory of descent.
* * * * *
We may now proceed to consider certain misconceptions of the Darwinian
theory which are largely, not to say generally, prevalent among
supporters of the theory. These misconceptions, therefore, differ from
those which fall to be considered in the next chapter, i. e.
misconceptions which constitute grounds of objection to the theory.
* * * * *
Of all the errors connected with the theory of natural selection,
perhaps the one most frequently met with--especially among supporters of
the theory--is that of employing the theory to explain all cases of
phyletic modification (or inherited change of type) indiscriminately,
without waiting to consider whether in particular cases its application
is so much as logically possible. The term "natural selection" thus
becomes a magic word, or Sesame, at the utterance of which every closed
door is supposed to be immediately opened. Be it observed, I am not here
alluding to that merely blind faith in natural selection, which of late
years has begun dogmatically to force this principle as the sole cause
of organic evolution in every case where it is _logically possible_ that
the principle can have come into play. Such a blind faith, indeed, I
hold to be highly inimical, not only to the progress of biological
science, but even to the true interests of the natural selection theory
itself. As to this I shall have a good deal to say in the next volume.
Here, however, the point is, that the theory in question is often
invoked in cases where it is not even logically possible that it can
apply, and therefore in cases where its application betokens, not merely
an error of judgment or extravagance of dogmatism, but a fallacy of
reasoning in the nature of a logical contradiction. Almost any number of
examples might be given; but one will suffice to illustrate what is
meant. And I choose it from the writings of one of the authors of the
selection theory itself, in order to show how easy it is to be cheated
by this mere juggling with a phrase--for of course I do not doubt that a
moment's thought would have shown the writer the untenability of his
statement.
In his most recent work Mr. Wallace advances an interesting hypothesis
to the effect that differences of colour between allied species, which
are apparently too slight to serve any other purpose, may act as
"recognition marks," whereby the opposite sexes are enabled at once to
distinguish between members of their own and of closely resembling
species. Of course this hypothesis can only apply to the higher animals;
but the point here is that, supposing it to hold for them, Mr. Wallace
proceeds to argue thus:--Recognition marks "have in all probability been
acquired in the process of differentiation for the purpose of checking
the intercrossing of allied forms," because "one of the first needs of a
new species would be to keep separate from its nearest allies, and this
could be more readily done by some easily seen external mark[32]."
Now, it is clearly not so much as logically possible that these
recognition-marks (supposing them to be such) can have been acquired
by natural selection, "for the purpose of checking intercrossing of
allied forms." For the theory of natural selection, from its own
essential nature as a theory, is logically exclusive of the supposition
that survival of the fittest ever provides changes in anticipation of
future uses. Or, otherwise stated, it involves a contradiction of the
theory itself to say that the colour-changes in question were originated
by natural selection, in order to meet "one of the _first_ needs of a
_new_ species," or for the purpose of _subsequently_ preventing
intercrossing with allied forms. If it had been said that these
colour-differentiations were originated by some cause other than natural
selection (or, if by natural selection, still with regard to some
_previous_, instead of _prophetic_, "purpose"), and, when so "acquired,"
_then_ began to serve the "purpose" assigned, the argument would not
have involved the fallacy which we are now considering. But, as it
stands, the argument reverts to the teleology of pre-Darwinian days--or
the hypothesis of a "purpose" in the literal sense which sees the end
from the beginning, instead of a "purpose" in the metaphorical sense of
an adaptation that is evolved by the very modifications which subserve
it[33].
[32] _Darwinism_, pp. 218 and 227.
[33] Since the above was written Prof. Lloyd Morgan has published a
closely similar notice of the passage in question. "This language,"
he says, "seems to savour of teleology (that pitfall of the
evolutionist). The cart is put before the horse. The
recognition-marks were, I believe, not produced to prevent
intercrossing, but intercrossing has been prevented because of
preferential mating between individuals possessing special
recognition-marks. To miss this point is to miss an important
segregation-factor."--(_Animal Life and Intelligence_, p. 103.)
Again, on pp. 184-9, he furnishes an excellent discussion on the
whole subject of the fallacy alluded to in the text, and gives
illustrative quotations from other prominent Darwinians. I should
like to add that Darwin himself has nowhere fallen into this, or any
of the other fallacies, which are mentioned in the text.
* * * * *
Another very prevalent, and more deliberate, fallacy connected with the
theory of natural selection is, _that it follows deductively from the
theory itself_ that the principle of natural selection must be the sole
means of modification in all cases where modification is of an
_adaptive_ kind,--with the consequence that no other principle can ever
have been concerned in the production of structures or instincts which
are of any use to their possessors. Whether or not natural selection
actually has been the sole means of adaptive modification in the race,
as distinguished from the individual, is a question of biological
fact[34]; but it involves a grave error of reasoning to suppose that
this question can be answered deductively from the theory of natural
selection itself, as I shall show at some length in the next volume.
[34] Of course adaptive modifications produced in the individual
lifetime, and not _inherited_, do not concern the question at all.
In this and the following paragraphs, therefore, "adaptations,"
"adaptive modifications," &c., refer exclusively to such as are
hereditary, i. e. phyletic.
* * * * *
A still more extravagant, and a still more unaccountable fallacy is the
one which represents it as following deductively from the theory of
natural selection itself, that all _hereditary_ characters are
"necessarily" due to natural selection. In other words, not only all
adaptive, but likewise all non-adaptive hereditary characters, it is
said, _must_ be due to natural selection. For non-adaptive characters
are taken to be due to "correlation of growth," in connexion with some
of the adaptive ones--natural selection being thus the _indirect_ means
of producing the former _wherever_ they may occur, on account of its
being the _direct_ and the _only_ means of producing the latter. Thus it
is deduced from the theory of natural selection itself,--1st, that the
principle of natural selection is the only possible cause of adaptive
modification: 2nd, that non-adaptive modifications can only occur in the
race as correlated appendages to the adaptive: 3rd, that, consequently,
natural selection is the only possible cause of modification, whether
adaptive or non-adaptive. Here again, therefore, we must observe that
none of these sweeping generalizations can possibly be justified by
deductive reasoning from the theory of natural selection itself. Any
attempt at such deductive reasoning must necessarily end in circular
reasoning, as I shall likewise show in the second volume, where this
whole "question of utility" will be thoroughly dealt with.
* * * * *
Once more, there is an important oversight very generally committed by
the followers of Darwin. For even those who avoid the fallacies above
mentioned often fail to perceive, that natural selection can only begin
to operate if the _degree_ of adaptation is already given as
sufficiently high to count for something in the struggle for
_existence_. Any adaptations which fall below this level of importance
cannot possibly have been produced by survival of the fittest. Yet the
followers of Darwin habitually speak of adaptative characters, which _in
their own opinion_ are subservient merely to comfort or convenience, as
having been produced by such means. Clearly this is illogical; for it
belongs to the essence of Darwin's theory to suppose, that natural
selection can have no jurisdiction beyond the line where structures or
instincts already present a sufficient degree of adaptational value to
increase, in some measure, the expectation of life on the part of their
possessors. We cannot speak of adaptations as due to natural selection,
without thereby affirming that they present what I have elsewhere termed
a "selection value."
* * * * *
Lastly, as a mere matter of logical definition, it is well-nigh
self-evident that the theory of natural selection is a theory of the
origin, and cumulative development, of _adaptations_, whether these be
distinctive of species, or of genera, orders, families, classes, and
sub-kingdoms. It is only when the adaptations happen to be distinctive
of the first (or lowest) of these taxonomic divisions, that the theory
which accounts for _these_ adaptations accounts also for the forms which
present them,--i. e. becomes _also_ a theory of the origin of species.
This, however, is clearly but an accident of particular cases; and,
therefore, even in them the theory is _primarily_ a theory of
adaptations, while it is but secondarily a theory of the species which
present them. Or, otherwise stated, the theory is no more a theory of
the origin of species than it is of the origin of genera, families, and
the rest; while, on the other hand, it is _everywhere_ a theory of the
adaptive modifications whereby each of these taxonomic divisions has
been differentiated as such. Yet, sufficiently obvious as the accuracy
of this definition must appear to any one who dispassionately considers
it, several naturalists of high standing have denounced it in violent
terms. I shall therefore have to recur to the subject at somewhat
greater length hereafter. At present it is enough merely to mention the
matter, as furnishing another and a curious illustration of the not
infrequent weakness of logical perception on the part of minds well
gifted with the faculty of observation. It may be added, however, that
the definition in question is in no way hostile to the one which is
virtually given by Darwin in the title of his great work. _The Origin of
Species by means of Natural Selection_ is beyond doubt the best title
that could have been given, because at the time when the work was
published the _fact_, no less than the _method_, of organic evolution
had to be established; and hence the most important thing to be done at
that time was to prove the transmutation of species. But now that this
has been done to the satisfaction of naturalists in general, it is as I
have said, curious to find some of them denouncing a wider definition of
the principle of natural selection, merely because the narrower (or
included) definition is invested with the charm of verbal
associations[35].
[35] The question as to whether natural selection has been the only
principle concerned in the origination of species, is quite distinct
from that as to the accuracy of the above definition.
* * * * *
So much for fallacies and misconceptions touching Darwin's theory, which
are but too frequently met with in the writings of its supporters. We
must now pass on to mention some of the still greater fallacies and
misconceptions which are prevalent in the writings of its opponents.
And, in order to do this thoroughly, I shall begin by devoting the
remainder of the present chapter to a consideration of the antecedent
standing of the two theories of natural selection and supernatural
design. This having been done, in the succeeding chapters I shall deal
with the evidences for, and the objections against, the former theory.
* * * * *
Beginning, then, with the antecedent standing of these alternative
theories, the first thing to be noticed is, that they are both concerned
with the same subject-matter, which it is their common object to
explain. Moreover, this subject-matter is clearly and sharply divisible
into two great classes of facts in organic nature--namely, those of
Adaptation and those of Beauty. Darwin's theory of descent explains the
former by his doctrine of natural selection, and the latter by his
doctrine of sexual selection. In the first instance, therefore, I shall
have to deal only with the facts of adaptation, leaving for subsequent
consideration the facts of beauty.
Innumerable cases of the adaptation of organisms to their surroundings
being the facts which now stand before us to be explained either by
natural selection or by supernatural intention, we may first consider a
statement which is frequently met with--namely, that even if all such
cases of adaptation were proved to be fully explicable by the theory of
descent, this would constitute no disproof of the theory of design: all
the cases of adaptation, it is argued, might still be due to design,
even though they admit of being hypothetically accounted for by the
theory of descent. I have heard an eminent Professor tell his class that
the many instances of mechanical adaptation discovered and described by
Darwin as occurring in orchids, seemed to him to furnish better proof of
supernatural contrivance than of natural causes; and another eminent
Professor has informed me that, although he had read the _Origin of
Species_ with care, he could see in it no evidence of natural selection
which might not equally well have been adduced in favour of intelligent
design. But here we meet with a radical misconception of the whole
logical attitude of science. For, be it observed, this exception _in
limine_ to the evidence which we are about to consider does not question
that natural selection _may_ be able to do all that Darwin ascribes to
it. The objection is urged against his interpretation of the facts
merely on the ground that these facts might _equally well_ be ascribed
to intelligent design. And so undoubtedly they might, if we were all
simple enough to adopt a supernatural explanation whenever a natural one
is found sufficient to account for the facts. Once admit the irrational
principle that we may assume the operation of higher causes where the
operation of lower ones is sufficient to explain the observed phenomena,
and all our science and all our philosophy are scattered to the winds.
For the law of logic which Sir William Hamilton called the law of
parsimony--or the law which forbids us to assume the operation of higher
causes when lower ones are found sufficient to explain the observed
effects--this law constitutes the only barrier between science and
superstition. It is always possible to give a hypothetical explanation
of any phenomenon whatsoever, by referring it immediately to the
intelligence of some supernatural agent; so that the only difference
between the logic of science and the logic of superstition consists in
science recognising a validity in the law of parsimony which
superstition disregards. Therefore one can have no hesitation in saying
that this way of looking at the evidence in favour of natural selection
is not a scientific or a reasonable way of looking at it, but a purely
superstitious way. Let us take, as an illustration, a perfectly parallel
case. When Kepler was unable to explain by any known causes the paths
described by the planets, he resorted to a supernatural explanation, and
supposed that every planet was guided in its movements by some presiding
angel. But when Newton supplied a beautifully simple physical
explanation, all persons with a scientific habit of mind at once
abandoned the metaphysical one. Now, to be consistent, the
above-mentioned Professors, and all who think with them, ought still to
adhere to Kepler's hypothesis in preference to Newton's explanation;
for, excepting the law of parsimony, there is certainly no other
logical objection to the statement, that the movements of the planets
afford as good evidence of the influence of guiding angels as they do of
the influence of gravitation.
So much, then, for the illogical position that, granting the evidence in
favour of natural descent and supernatural design to be equal and
parallel, we should hesitate in our choice between the two theories.
But, of course, if the evidence is supposed _not_ to be equal and
parallel--i. e. if it is supposed that the theory of natural selection
is not so good a theory whereby to explain the facts of adaptation as is
that of supernatural design,--then the objection is no longer the one
which we are considering. It is quite another objection, and one which
is not _prima facie_ absurd. Therefore let us state clearly the distinct
question which thus arises.
Innumerable cases of adaptation of organisms to their environments are
the observed facts for which an explanation is required. To supply this
explanation, two, and only two, hypotheses are in the field. Of these
two hypotheses one is intelligent design manifested directly in special
creation; the other is natural causation operating through countless
ages of the past. Now, the adaptations in question involve an
innumerable multitude of special mechanisms, in most cases even within
the limits of any one given species; but when we consider the sum of all
these mechanisms presented by organic nature as a whole, the mind must
indeed be dull which does not feel astounded. For, be it further
observed, these mechanical contrivances[36] are, for the most part, no
merely simple arrangements, which might reasonably be supposed due,
like the phenomena of crystallization, to comparatively simple physical
causes. On the contrary, they everywhere and habitually exhibit so
deep-laid, so intricate, and often so remote an adaptation of means to
ends, that no machinery of human contrivance can properly be said to
equal their perfection from a mechanical point of view. Therefore,
without question, the hypothesis which first of all they suggest--or
suggest most readily--is the hypothesis of design. And this hypothesis
becomes virtually the only hypothesis possible, if it be assumed--as it
generally was assumed by natural theologians of the past,--that all
species of plants and animals were introduced into the world _suddenly_.
For it is quite inconceivable that any known cause, other than
intelligent design, could be competent to turn out instantaneously any
one of these intricate pieces of machinery, already adapted to the
performance of its special function. But, on the other hand, if there is
any evidence to show that one species becomes slowly transformed into
another--or that one set of adaptations becomes slowly changed into
another set as changing circumstances require,--then it becomes quite
possible to imagine that a strictly natural causation may have had
something to do with the matter. And this suggestion becomes greatly
more probable when we discover, from geological evidence and
embryological research, that in the history both of races and of
individuals the various mechanisms in question have themselves had a
history--beginning in the forms of most uniformity and simplicity,
gradually advancing to forms more varied and complex, nowhere exhibiting
any interruptions in their upward progress, until the world of organic
machinery as we now have it is seen to have been but the last phase of a
long and gradual growth, the ultimate roots of which are to be found in
the soil of undifferentiated protoplasm.
[36] It is often objected to Darwin's terminology, that it embraces
such words as "contrivance," "purpose," &c., which are strictly
applicable only to the processes or the products of thought. But
when it is understood that they are used in a neutral or
metaphorical sense, I cannot see that any harm arises from their
use.
Lastly, when there is supplied to us the suggestion of natural selection
as a cause presumably adequate to account for this continuous growth in
the number, the intricacy, and the perfection of such mechanisms, it is
only the most unphilosophical mind that can refuse to pause as between
the older hypothesis of design and the newer hypothesis of descent.
Thus it is clear that the _a priori_ standing of the rival hypotheses of
naturalism and supernaturalism in the case of all these pieces of
organic machinery, is profoundly affected by the question whether they
came into existence suddenly, or whether they did so gradually. For, if
they all came into existence suddenly, the fact would constitute
well-nigh positive proof in favour of supernaturalism, or creation by
design; whereas, if they all came into existence gradually, this fact
would in itself constitute presumptive evidence in favour of naturalism,
or of development by natural causes. And, as shown in the previous
chapters, the proof that all species of plants and animals came into
existence gradually--or the proof of evolution as a fact--is simply
overwhelming.
From a still more general point of view I may state the case in another
way, by borrowing and somewhat expanding an illustration which, I
believe, was first used by Professor Huxley. If, when the tide is out,
we see lying upon the shore a long line of detached sea-weed, marking
the level which is reached by full tide, we should be free to conclude
that the separation of the sea-weed from the sand and the stones was due
to the intelligent work of some one who intended to collect the sea-weed
for manure, or for any other purpose. But, on the other hand, we might
explain the fact by a purely physical cause--namely, the separation by
the sea-waves of the sea-weed from the sand and stones, in virtue of its
lower specific gravity. Now, thus far the fact would be explained
equally well by either hypothesis; and this fact would be the fact of
_selection_. But whether we yielded our assent to the one explanation or
to the other would depend upon a due consideration of all collateral
circumstances. The sea-weed might not be of a kind that is of any use to
man; there might be too great a quantity of it to admit of our supposing
that it had been collected by man; the fact that it was all deposited on
the high-water-mark would in itself be highly suggestive of the agency
of the sea; and so forth. Thus, in such a case any reasonable observer
would decide in favour of the physical explanation, or against the
teleological one.
Now the question whether organic evolution has been caused by physical
agencies or by intelligent design is in precisely the same predicament.
There can be no logical doubt that, theoretically at all events, the
physical agencies which the present chapter is concerned with, and which
are conveniently summed up in the term natural selection, are as
competent to produce these so-called mechanical contrivances, and the
other cases of adaptation which are to be met with in organic nature,
as intelligent design could be. Hence, our choice as between these two
hypotheses must be governed by a study of all collateral circumstances;
that is to say, by a study of the evidences in favour of the physical
explanation. To this study, therefore, we shall now address ourselves,
in the course of the following chapters.
CHAPTER VIII.
EVIDENCES OF THE THEORY OF NATURAL SELECTION.
I will now proceed to state the main arguments in favour of the theory
of natural selection, and then, in the following chapter, the main
objections which have been urged against it.
In my opinion, the main arguments in favour of the theory are three in
number.
First, it is a matter of observation that the struggle for existence in
nature does lead to the extermination of forms less fitted for the
struggle, and thus makes room for forms more fitted. This general fact
may be best observed in cases where an exotic species proves itself
better fitted to inhabit a new country than is some endemic species
which it exterminates. In Great Britain, for example, the so-called
common rat is a comparatively recent importation from Norway, and it has
so completely supplanted the original British rat, that it is now
extremely difficult to procure a single specimen of the latter: the
native black rat has been all but exterminated by the foreign brown rat.
The same thing is constantly found in the case of imported species of
plants. I have seen the river at Cambridge so choked with the inordinate
propagation of a species of water-weed which had been introduced from
America, that considerable expense had to be incurred in order to clear
the river for traffic. In New Zealand the same thing has happened with
the European water-cress, and in Australia with the common rabbit. So it
is doubtless true, as one of the natives is said to have philosophically
remarked, "the white man's rat has driven away our rat, the European fly
drives away our fly, his clover kills our grass, and so will the Maoris
disappear before the white man himself." Innumerable other cases to the
same effect might be quoted; and they all go to establish the fact that
forms less fitted to survive succumb in their competition with forms
better fitted.
* * * * *
Secondly, there is a general consideration of the largest possible
significance in the present connexion--namely, that among all the
millions of structures and instincts which are so invariably, and for
the most part so wonderfully, adapted to the needs of the species
presenting them, we cannot find a single instance, either in the
vegetable or animal kingdom, of a structure or an instinct which is
developed for the exclusive benefit of another species. Now this great
and general fact is to my mind a fact of the most enormous, not to say
overwhelming, significance. The theory of natural selection has now been
before the world for more than thirty years, and during that time it had
stood a fire of criticism such as was never encountered by any
scientific theory before. From the first Darwin invited this criticism
to adduce any single instance, either in the vegetable or animal
kingdom, of a structure or an instinct which should unquestionably be
proved to be of exclusive use to any species other than the one
presenting it. He even went so far as to say that if any one such
instance could be shown he would surrender his whole theory on the
strength of it--so assured had he become, by his own prolonged
researches, that natural selection was the true agent in the production
of adaptive structures, and, as such, could never have permitted such a
structure to occur in one species for the benefit of another. Now, as
this invitation has been before the world for so many years, and has not
yet been answered by any naturalist, we may by this time be pretty
confident that it never will be answered. How tremendous, then, is the
significance of this fact in its testimony to Darwin's theory! The
number of animal and vegetable species, both living and extinct, is to
be reckoned by millions, and every one of these species presents on an
average hundreds of adaptive structures,--at least one of which in many,
possibly in most, if not actually in all cases, is peculiar to the
species that presents it. In other words, there are millions of adaptive
structures (not to speak of instincts) which are peculiar to the species
presenting them, and also many more which are the common property of
allied species: yet, notwithstanding this inconceivable profusion of
adaptive structures in organic nature, there is no single instance that
has been pointed out of the occurrence of such a structure save for the
benefit of the species that presents it. Therefore, I say that this
immensely large and general fact speaks with literally immeasurable
force in favour of natural selection, as at all events one of the main
causes of organic evolution. For the fact is precisely what we should
expect if this theory is true, while upon no other theory can its
universality and invariability be rendered intelligible. On the
beneficent design theory, for instance, it is inexplicable that no
species should ever be found to present a structure or an instinct
having primary reference to the welfare of another species, when, _ex
hypothesi_, such an endless amount of thought has been displayed in the
creation of structures and instincts having primary reference to the
species which present them. For how magnificent a display of divine
beneficence would organic nature have afforded, if all--or even
some--species had been so inter-related as to have ministered to each
others wants. Organic species might then have been likened to a
countless multitude of voices, all singing in one great harmonious
psalm. But, as it is, we see absolutely no vestige of such
co-ordination: every species is for itself, and for itself alone--an
outcome of the always and everywhere fiercely raging struggle for life.
In order that the force of this argument may not be misapprehended, it
is necessary to bear in mind that it is in no way affected by cases
where a structure or an instinct is of primary benefit to its possessor,
and then becomes of secondary benefit to some other species on account
of the latter being able in some way or another to utilise its action.
Of course organic nature is full of cases of this kind; but they only go
to show the readiness which all species display to utilise for
themselves everything that can be turned to good account in their own
environments, and so, among other things, the structures and instincts
of other animals. For instance, it would be no answer to Darwin's
challenge if any one were to point to a hermit-crab inhabiting the
cast-off shell of a mollusk; because the shell was primarily of use to
the mollusk itself, and, so far as the mollusk is concerned, the fact of
its shell being afterwards of a secondary use to the crab is quite
immaterial. What Darwin's challenge requires is, that some structure or
instinct should be shown which is not merely of such secondary or
accidental benefit to another species, but clearly adapted to the needs
of that other species in the first instance--such, for example, as would
be the case if the tail of a rattle-snake were of no use to its
possessor, while serving to warn other animals of the proximity of a
dangerous creature; or, in the case of instincts, if it were true that a
pilot-fish accompanies a shark for the purpose of helping the shark to
discover food. Both these instances have been alleged; but both have
been shown untenable. And so it has proved of all the other cases which
thus far have been put forward.
Perhaps the most remarkable of all the allegations which ever have been
put forward in this connexion are those that were current with regard to
instincts before the publication of Darwin's work. These allegations are
the most remarkable, because they serve to show, in a degree which I do
not believe could be shown anywhere else, the warping power of
preconceived ideas. A short time ago I happened to come across the 8th
edition of the _Encyclopædia Britannica_, and turned up the article on
"Instinct" there, in order to see what amount of change had been wrought
with regard to our views on this subject by the work of Darwin--the 8th
edition of the _Encyclopædia Britannica_ having been published shortly
before _The Origin of Species by means of Natural Selection_. I cannot
wait to give any lengthy quotations from this representative exponent of
scientific opinion upon the subject at that time; but its general drift
may be appreciated if I transcribe merely the short concluding
paragraph, wherein he sums up his general results. Here he says:--
It thus only remains for us to regard instinct as a mental faculty,
_sui generis_, the gift of God to the lower animals, that man in
his own person, and by them, might be relieved from the meanest
drudgery of nature.
Now, here we have the most extraordinary illustration that is imaginable
of the obscuring influence of a preconceived idea. Because he started
with the belief that instincts _must_ have been implanted in animals for
the benefit of man, this writer, even when writing a purely scientific
essay, was completely blinded to the largest, the most obvious, and the
most important of the facts which the phenomena of instinct display.
For, as a matter of fact, among all the many thousands of instincts
which are known to occur in animals, there is no single one that can be
pointed to as having any special reference to man; while, on the other
hand, it is equally impossible to point to one which does not refer to
the welfare of the animal presenting it. Indeed, when the point is
suggested, it seems to me surprising how few in number are the instincts
of animals which have proved to be so much as of secondary or accidental
benefit to man, in the same way as skins, furs, and a whole host of
other animal products are thus of secondary use to him. Therefore, this
writer not only failed to perceive the most obvious truth that every
instinct, without any single exception, has reference to the animal
which presents it; but he also conceived a purely fictitious inversion
of this truth, and wrote an essay to prove a statement which all the
instincts in the animal kingdom unite in contradicting.
This example will serve to show, in a striking manner, not only the
distance that we have travelled in our interpretation of organic nature
between two successive editions of the _Encyclopædia Britannica_, but
also the amount of verification which this fact furnishes to the theory
of natural selection. For, inasmuch as it belongs to the very essence of
this theory that all adaptive characters (whether instinctive or
structural) must have reference to their own possessors, we find
overpowering verification furnished to the theory by the fact now before
us--namely, that immediately prior to the enunciation of this theory,
the truth that all adaptive characters have reference only to the
species which present them was not perceived. In other words, it was the
testing of this theory by the facts of nature that _revealed_ to
naturalists the general law which the theory, as it were, predicted--the
general law that all adaptive characters have primary reference to the
species which present them. And when we remember that this is a kind of
verification which is furnished by millions of separate cases, the whole
mass of it taken together is, as I have before said, overwhelming.
It is somewhat remarkable that the enormous importance of this argument
in favour of natural selection as a prime factor of organic evolution
has not received the attention which it deserves. Even Darwin himself,
with his characteristic reserve, has not presented its incalculable
significance; nor do I know any of his followers who have made any
approach to an adequate use of it in their advocacy of his views. In
preparing the present chapter, therefore, I have been particularly
careful not to pitch too high my own estimate of its evidential value.
That is to say, I have considered, both in the domain of structures and
of instincts, what instances admit of being possibly adduced _per
contra_, or as standing outside the general law that adaptive structures
and instincts are of primary use only to their possessors. In the result
I can only think of two such instances. These, therefore, I will now
dispose of.
The first was pointed out, and has been fully discussed, by Darwin
himself. Certain species of ants are fond of a sweet fluid that is
secreted by aphides, and they even keep the aphides as we keep cows for
the purpose of profiting by their "milk." Now the point is, that the use
of this sweet secretion to the aphis itself has not yet been made out.
Of course, if it is of no use to the aphis, it would furnish a case
which completely meets Darwin's own challenge. But, even if this
supposition did not stand out of analogy with all the other facts of
organic nature, most of us would probably deem it prudent to hold that
the secretion must primarily be of some use to the aphis itself,
although the matter has not been sufficiently investigated to inform us
of what this use is. For, in any case, the secretion is not of any vital
importance to the ants which feed upon it: and I think but few impartial
minds would go so far to save an hypothesis as to maintain, that the
Divinity had imposed this drain upon the internal resources of one
species of insect for the sole purpose of supplying a luxury to
another. On the whole, it seems most probable that the fluid is of the
nature of an excretion, serving to carry off waste products. Such, at
all events, was the opinion at which Darwin himself arrived, as a result
of observing the facts anew, and in relation to his theory.
* * * * *
The other instance to which I have alluded as seeming at first sight
likely to answer Darwin's challenge is the formation of vegetable galls.
The great number and variety of galls agree in presenting a more or less
elaborate structure, which is not only foreign to any of the uses of
plant-life, but singularly and specially adapted to those of the
insect-life which they shelter. Yet they are produced by a growth of the
plant itself, when suitably stimulated by the insects' inoculation--or,
according to recent observations, by emanations from the bodies of the
larvæ which develop from the eggs deposited in the plant by the insect.
Now, without question, this is a most remarkable fact; and if there were
many more of the like kind to be met with in organic nature, we might
seriously consider whether the formation of galls should not be held to
make against the ubiquitous agency of natural selection. But inasmuch as
the formation of galls stands out as an exception to the otherwise
universal rule of every species for itself, and for itself alone, we are
justified in regarding this one apparent exception with extreme
suspicion. Indeed, I think we are justified in regarding the peculiar
pathological effect produced in the plant by the secretions of the
insect as having been in the first instance accidentally beneficial to
the insects. Thus, if any other effect than that of a growing tumour had
been produced in the first instance, or if the needs of the insect
progeny had not been such as to have derived profit from being enclosed
in such a tumour, then, of course, the inoculating instinct of these
animals could not have been developed by natural selection. But, given
these two conditions, and it appears to me there is nothing very much
more remarkable about an accidental correlation between the effects of a
parasitic larva on a plant and the needs of that parasite, than there is
between the similarly accidental correlation between a hydated parasite
and the nutrition furnished to it by the tissues of a warm-blooded
animal. Doubtless the case of galls is somewhat more remarkable,
inasmuch as the morbid growth of the plant has more concern in the
correlation--being, in many instances, a more specialized structure on
the part of a host than occurs anywhere else, either in the animal or
vegetable world. But here I may suggest that although natural selection
cannot have acted upon the plant directly, so as to have produced galls
ever better and better adapted to the needs of the insect, it may have
so acted upon the plants indirectly _though the insects_. For it may
very well have been that natural selection would ever tend to preserve
those individual insects, the quality of whose emanations tended to
produce the form of galls best suited to nourish the insect progeny; and
thus the character of these pathological growths may have become ever
better and better adapted to the needs of the insects. Lastly, looking
to the enormous number of relations and inter-relations between all
organic species, it is scarcely to be wondered at that even so
extraordinary an instance of correlation as this should have arisen thus
by accident, and then have been perfected by such an _indirect_ agency
of natural selection as is here suggested[37].
[37] Note B.
* * * * *
The third general class of facts which tell so immensely in favour of
natural selection as an important cause of organic evolution, are those
of domestication. The art of the horticulturist, the fancier, the
cattle-breeder, &c., consists in producing greater and greater
deviations from a given wild type of plant or animal, in any particular
direction that may be desired for purposes either of use or of beauty.
Cultivated cereals, fruits, and flowers are known to have been all
derived from wild species; and, of course, the same applies to all our
domesticated varieties of animals. Yet if we compare a cabbage rose with
a wild rose, a golden pippin apple with a crab, a toy terrier with any
species of wild dog, not to mention any number of other instances, there
can be no question that, if such differences had appeared in nature, the
organisms presenting them would have been entitled to rank as distinct
species--or even, in many cases, as distinct genera. Yet we know, as a
matter of fact, that all these differences have been produced by a
process of artificial selection, or pairing, which has been continuously
practised by horticulturists and breeders through a number of
generations. It is the business of these men to note the individual
organisms which show most variation in the directions required, and then
to propagate from these individuals, in order that the progeny shall
inherit the qualities desired. The results thus become cumulative from
generation to generation, until we now have an astonishing manifestation
of useful qualities on the one hand, and of beautiful qualities on the
other, according as the organisms have been thus bred for purposes of
use or for those of beauty.
Now it is immediately obvious that in these cases the process of
artificial selection is precisely analogous to that of natural selection
(and of sexual selection which will be considered later on), in all
respects save one: the utility or the beauty which it is the aim of
artificial selection continually to enhance, is utility or beauty in
relation to the requirements or to the tastes of man; whereas the
utility or the beauty which is produced by natural selection and sexual
selection has reference only to the requirements or the tastes of the
organisms themselves. But, with the exception of this one point of
difference, the processes and the products are identical in kind.
Persevering selection by man is thus proved to be capable of creating
what are virtually new specific types, and this in any required
direction. Hence, when we remember how severe is the struggle for
existence in nature, it becomes impossible to doubt that selection by
nature is able to do at least as much as artificial selection in the way
of thus creating new types out of old ones. Artificial selection,
indeed, notwithstanding the many and marvellous results which it has
accomplished, can only be regarded as but a feeble imitation of natural
selection, which must act with so much greater vigilance and through
such immensely greater periods of time. In a word, the proved
capabilities of artificial selection furnish, in its best conceivable
form, what is called an argument _a fortiori_ in favour of natural
selection.
Or, to put it in another way, it may be said that for thousands of years
mankind has been engaged in making a gigantic experiment to test, as it
were by anticipation, the theory of natural selection. For, although
this prolonged experiment has been carried on without any such intention
on the part of the experimenters, it is none the less an experiment in
the sense that its results now furnish an overwhelming verification of
Mr. Darwin's theory. That is to say, they furnish overwhelming proof of
the efficacy of the selective principle in the modification of organic
types, when once this principle is brought steadily and continuously to
bear upon a sufficiently long series of generations.
In order to furnish ocular evidence of the value of this line of
verification, I have had the following series of drawings prepared.
Another and equally striking series might be made of the products of
artificial selection in the case of plants; but it seems to me that the
case of animals is more than sufficient for the purpose just stated.
Perhaps it is desirable to add that considerable care has been bestowed
upon the execution of these portraits; and that in every case the latter
have been taken from the most typical specimens of the artificial
variety depicted. Those of them which have not been drawn directly from
life are taken from the most authoritative sources; and, before being
submitted to the engraver, they were all examined by the best judges in
each department. In none of the groups, however, have I aimed at an
exhaustive representation of all the varieties: I have merely introduced
representatives of as many as the page would in each case accommodate.
[Illustration: FIG. 91.--Pigeons. Drawn from life (prize
specimens).]
[Illustration: FIG. 92.--Pigeons, continued. Drawn from life (prize
specimens).]
[Illustration: FIG. 93.--Fowls. Drawn from life (prize specimens).]
[Illustration: FIG. 94.--Fowls, continued. Drawn from life (prize
specimens).]
[Illustration: FIG. 95.--Pair of Japanese Fowls, long-tailed breed.
Drawn from stuffed specimens in the British Museum.]
[Illustration: FIG. 96.--Canaries. Drawn from life (prize
specimens).]
[Illustration: FIG. 97.--Sebastopol, or Frizzled Goose. Drawn from a
photograph.]
[Illustration: FIG. 98.--The Dingo, or wild dog of Australia, 1/10
nat. size. Drawn from life (_Zoological Gardens_).]
[Illustration: FIG. 99.--Dogs. Drawn from life (prize specimens).]
[Illustration: FIG. 100.--Dogs, continued. Drawn from life (prize
specimens).]
[Illustration: FIG. 101.--The Hairless Dog of Japan, 1/10 nat. size.
Drawn from a photograph kindly lent for the purpose by the
proprietor.]
[Illustration: FIG. 102.--The skull of a Bull-dog compared with that
of a Deerhound. Drawn from nature.]
[Illustration: FIG. 103. Rabbits. Drawn from life (prize
specimens).]
[Illustration: FIG. 104.--Horses. Drawn from life (prize
specimens).]
[Illustration: FIG. 105.--Sheep. The illustrations are confined to
British breeds. Drawn from life (prize specimens).]
[Illustration: FIG. 106.--Cattle. The illustrations are confined to
British breeds. Drawn from life (prize specimens).]
[Illustration: FIG. 107.--Wild Boar contrasted with a modern
Domesticated Pig. Drawn from life (_Zoological Gardens_, and prize
specimen).]
The exigencies of space have prevented, in some of the groups, strict
adherence to a uniform scale--with the result that contrasts between
different breeds in respect of size are not adequately rendered. This
remark applies especially to the dogs; for although the artist has
endeavoured to draw them in perspective, unless the distance between
those in the foreground and those in the background is understood to be
more considerable than it appears, an inadequate idea is given of the
relative differences of size. The most instructive of the groups, I
think, is that of the Canaries; because the many and great changes in
different directions must in this case have been produced by artificial
selection in so comparatively short a time--the first mention of this
bird that I can find being by Gesner, in the sixteenth century.
* * * * *
Now, it is surely unquestionable that in these typical proofs of the
efficacy of artificial selection in the modification of specific types,
we have the strongest conceivable testimony to the power of natural
selection in the same direction. For it thus appears that wherever
mankind has had occasion to operate by selection for a sufficiently long
time--that is to say, on whatever species of plant or animal he chooses
thus to operate for the purpose of modifying the type in any required
direction,--the results are always more or less the same: he finds that
all specific types lend themselves to continuous deflection in any
particulars of structure, colour, &c., that he may desire to modify.
Nevertheless, to this parallel between the known effects of artificial
selection, and the inferred effects of natural selection, two objections
have been urged. The first is, that in the case of artificial selection
the selecting agent is a voluntary intelligence, while in the case of
natural selection the selecting agent is Nature herself; and whether or
not there is any counterpart of man's voluntary intelligence in nature
is a question with which Darwinism has nothing to do. Therefore, it is
alleged, the analogy between natural selection and artificial selection
fails _ab initio_, or at the fountain-head of the causes which are taken
by the analogy to be respectively involved.
The second objection to the analogy is, that the products of artificial
selection, closely as they may resemble natural species in all other
respects, nevertheless present one conspicuous and highly important
point of difference: they rarely, if ever, present the physiological
character of mutual infertility, which is a character of extremely
general occurrence in the case of natural species, even when these are
most nearly allied.
I will deal with these two objections in the next chapter, where I shall
be concerned with the meeting of all the objections which have ever been
urged against the theory of natural selection. Meanwhile I am engaged
only in presenting the general arguments which support the theory, and
therefore mention these objections to one of them merely _en passant_.
And I do so in order to pledge myself effectually to dispose of them
later on, so that for the purposes of my present argument both these
objections may be provisionally regarded as non-existent; which means,
in other words, that we may provisionally regard the analogy between
artificial selection and natural selection as everywhere logically
intact.
* * * * *
To sum up, then, the results of the foregoing exposition thus far, what
I hold to be the three principal, or most general, arguments in favour
of the theory of natural selection, are as follows.
First, there is the _a priori_ consideration that, if on independent
grounds we believe in the theory of evolution at all, it becomes obvious
that natural selection _must_ have had _some_ part in the process. For
no one can deny the potent facts of heredity, variability, the struggle
for existence, and survival of the fittest. But to admit these facts is
to admit natural selection as a principle which must be, at any rate,
one of the factors of organic evolution, supposing such evolution to
have taken place. Next, when we turn from these _a priori_
considerations, which thus show that natural selection _must_ have been
concerned to some extent in the process of evolution, we find in organic
nature evidence _a posteriori_ of the extent to which this principle
_has_ been thus concerned. For we find that among all the countless
millions of adaptive structures which are to be met with in organic
nature, it is an invariable rule that they exist in relation to the
needs of the particular species which present them: they never have any
primary reference to the needs of other species. And as this
extraordinarily large and general fact is exactly what the theory of
natural selection would expect, the theory is verified by the fact in an
extraordinarily cogent manner. In other words, the fact goes to prove
that in _all_ cases where adaptive structures or instincts are
concerned, natural selection must have been either the sole cause at
work, or, at the least, an influence controlling the operation of all
other causes.
Lastly, an actually experimental verification of the theory has been
furnished on a gigantic scale by the operations of breeders, fanciers,
and horticulturists. For these men, by their process of selective
accumulation, have empirically proved what immense changes of type may
thus be brought about; and so have verified by anticipation, and in a
most striking manner, the theory of natural selection--which, as now so
fully explained, is nothing more than a theory of cumulative
modifications by means of selective breeding.
So much, then, by way of generalities. But perhaps the proof of natural
selection as an agency of the first importance in the transmutation of
species may be best brought home to us by considering a few of its
applications in detail. I will therefore devote the rest of the present
chapter to considering a few cases of this kind.
There are so many large fields from which such special illustrations may
be supplied, that it is difficult to decide which of them to draw upon.
For instance, the innumerable, always interesting, and often astonishing
adaptations on the part of flowers to the fertilising agency of insects,
has alone given rise to an extensive literature since the time when
Darwin himself was led to investigate the subject by the guidance of his
own theory. The same may be said of the structures and movements of
climbing plants, and in short, of all the other departments of natural
history where the theory of natural selection has led to the study of
the phenomena of adaptation. For in all these cases the theory of
natural selection, which first led to their discovery, still remains the
only scientific theory by which they can be explained. But among all the
possible fields from which evidences of this kind may be drawn, I think
the best is that which may be generically termed defensive colouring. To
this field, therefore, I will restrict myself. But, even so, the cases
to be mentioned are but mere samples taken from different divisions of
this field; and therefore it must be understood at the outset that they
could easily be multiplied a hundred-fold.
_Protective Colouring._
A vast number of animals are rendered more or less inconspicuous by
resembling the colours of the surfaces on which they habitually rest.
Such, for example, are grouse, partridges, rabbits, &c. Moreover, there
are many cases in which, if the needs of the creature be such that it
must habitually frequent surfaces of different colours, it has acquired
the power of changing its colour accordingly--e. g. cuttle-fish,
flat-fish, frogs, chameleons, &c. The physiological mechanism whereby
these adaptive changes of colour are produced differs in different
animals; but it is needless for our purposes to go into this part of the
subject. Again, there are yet other cases where protective colouring
which is admirably suited to conceal an animal through one part of the
year, would become highly conspicuous during another part of it--namely,
when the ground is covered with snow. Accordingly, in these cases the
animals change their colour in the winter months to a snowy white:
witness stoats, mountain hares, ptarmigan, &c. (Fig. 108.)
[Illustration: FIG. 108.--Seasonal changes of colour in Ptarmigan
(_Lagopus mutus_). Drawn from stuffed specimens in the British
Museum, 1/6 nat. size, with appropriate surroundings supplied.]
Now, it is sufficiently obvious that in all these classes of cases the
concealment from enemies or prey which is thus secured is of advantage
to the animals concerned; and, therefore, that in the theory of natural
selection we have a satisfactory theory whereby to explain it. And this
cannot be said of any other theory of adaptive mechanisms in nature that
has ever been propounded. The so-called Lamarckian theory, for instance,
cannot be brought to bear upon the facts at all; and on the theory of
special creation it is unintelligible why the phenomena of protective
colouring should be of such general occurrence. For, in as far as
protective colouring is of advantage to the species which present it, it
is of corresponding disadvantage to those other species against the
predatory nature of which it acts as a defence. And, of course, the
same applies to yet other species, if they serve as prey. Moreover,
the more minutely this subject is investigated in all its details,
the more exactly is it found to harmonise with the naturalistic
interpretation[38].
[38] Were it not that some of Darwin's critics have overlooked the
very point wherein the great value of protective colouring as
evidence of natural selection consists, it would be needless to
observe that it does so in the _minuteness_ of the protective
resemblance which in so many cases is presented. Of course where the
resemblance is only very general, the phenomena might be ascribed to
mere coincidence, of which the instincts of the animal have taken
advantage. But in the measure that the resemblance becomes minutely
detailed, the supposition of mere coincidence is excluded, and the
agency of some specially adaptive cause demonstrated. Again, it is
almost needless to say, no real difficulty is presented (as has been
alleged) by the cases above quoted of seasonal imitations, on the
ground that natural selection could not act alternately on the same
individual. Natural selection is not supposed to act alternately on
the same individual. It is supposed to act always in the same
manner, and if, as in the case of a regularly recurring change in
the colours of the environment, correspondingly recurrent changes
are required to appear in the colours of the animals, natural
selection sets its premium upon those individuals the constitutions
of which best lend themselves to seasonal changes of the needful
kind--probably under the influence of stimuli supplied by the
changes of external conditions (temperature, moisture, &c.).
In the first place, we always find a complete correspondence between
imitative colouring and instinctive endowment. If a caterpillar exactly
resembles the colour of a twig, it also presents the instinct of
habitually reposing in the attitude which makes it most resemble a
twig--standing out from the branch on which it rests at the same angle
as is presented by the real twigs of the tree on which it lives.
Here, again, is a bird protectively coloured so as to resemble stones
upon the rough ground where it habitually lives; and the drawing shows
the attitude in which the bird instinctively reposes, so as still
further to increase its resemblance to a stone. (Fig. 109.)
[Illustration: FIG. 109.--_Oedicnemus crepitans_, showing the
instinctive attitude of concealment. Drawn from a stuffed specimen
in the British Museum, 1/6 nat. size, with appropriate surroundings
supplied.
To take only one other instance, hares and rabbits, like grouse and
partridges--or like the plover just alluded to,--instinctively crouch
upon those surfaces the colours of which they resemble; and I have often
remarked that if, on account of any individual peculiarity of
coloration, the animal is not able thus to secure concealment, it
nevertheless exhibits the instinct of crouching which is of benefit to
all its kind, although, from the accident of its own abnormal colouring,
this instinct is then actually detrimental to the animal itself. For
example, every sportsman must have noticed that the somewhat rare
melanic variety of the common rabbit will crouch as steadily as the
normal brownish-gray type, notwithstanding that, owing to its abnormal
colour, a "nigger-rabbit" thus renders itself the most conspicuous
object in the landscape. In all such cases, of course, there has been a
deviation from the normal type in respect of colour, with the result
that the inherited instinct is no longer in tune with the other
endowments of the animal. Such a variation of colour, therefore, will
tend to be suppressed by natural selection; while any variations which
may bring the animal still more closely to resemble its habitual
surroundings will be preserved. Thus we can understand the truly
wonderful extent to which this principle of protective colouring has
been carried in many cases where the need of it has been most urgent.
Not only colour, but structure, may be profoundly modified for the
purposes of protective concealment. Thus, caterpillars which resemble
twigs do so not only in respect of colour, but also of shape; and this
even down to the most minute details in cases where the adaptation is
most complete: certain butterflies and leaf-insects so precisely
resemble the leaves upon which, or among which, they live, that it is
almost impossible to detect them in the foliage--not only the colour,
the shape, and the venation being all exactly imitated, but in some
cases even the defects to which the leaves are liable, in the way of
fungoid growths, &c. There are other insects which with similar
exactness resemble moss, lichens, and so forth. A species of fish
secures a complete resemblance to bunches of sea-weed by a frond-like
modification of all its appendages, and so on through many other
instances. Now, in all such cases where there is so precise an
imitation, both in colour and structure, it seems impossible to suggest
any other explanation of the facts than the one which is supplied by
Mr. Darwin's theory--namely, that the more perfect the resemblance is
caused to become through the continuous influence of natural selection
always picking out the best imitations, the more highly discriminative
becomes the perception of those enemies against the depredations of
which this peculiar kind of protection is developed; so that, in virtue
of this action and re-action, eventually we have a degree of imitation
which renders it almost impossible for a naturalist to detect the animal
when living in its natural environment.
[Illustration: FIG. 110.--Imitative forms and colours in insects.
Drawn from nature (_R. Coll. Surg. Mus._).]
_Warning Colours._
In strange and glaring contrast to all these cases of protective
colouring, stand other cases of conspicuous colouring. Thus, for
example, although there are numberless species of caterpillars which
present in an astonishing degree the phenomena of protective colouring,
there are numberless other species which not only fail to present these
phenomena in any degree, but actually go to the opposite extreme of
presenting colours which appear to have been developed for the sake of
their conspicuousness. At all events, these caterpillars are usually the
most conspicuous objects in their surroundings, and therefore in the
early days of Darwinism they were regarded by Darwin himself as
presenting a formidable difficulty in the way of his theory. To Mr.
Wallace belongs the merit of having cleared up this difficulty in an
extraordinarily successful manner. He virtually reasoned thus. If the
_raison d'être_ of protective colouring be that of concealing agreeably
flavoured caterpillars from the eye-sight of birds, may not the _raison
d'être_ of conspicuous colouring be that of protecting disagreeably
flavoured caterpillars from any possibility of being mistaken by birds?
Should this be the case, of course the more conspicuous the colouring
the better would it be for the caterpillars presenting it. Now as soon
as this suggestion was acted upon experimentally, it was found to be
borne out by facts. Birds could not be induced to eat caterpillars of
the kinds in question; and there is now no longer any doubt that their
conspicuous colouring is correlated with their distastefulness to birds,
in the same way as the inconspicuous or imitative colouring of other
caterpillars is correlated with their tastefulness to birds. Here then
is yet another instance, added to those already given, of the
verification yielded to the theory of natural selection by its proved
competency as a guide to facts in nature; for assuredly this particular
class of facts would never have been suspected but for its suggestive
agency.
As in the case of protective imitation, so in this case of warning
conspicuousness, not only colour, but structure may be greatly modified
for the purpose of securing immunity from attack. Here, of course, the
object is to assume, as far as possible, a touch-me-not appearance; so
that, although destitute of any real means of offence, the creatures in
question present a fictitiously dangerous aspect. As the
Devil's-coach-horse turns up his stingless tail when threatened by an
enemy, so in numberless ways do many harmless animals of all classes
pretend to be formidable. But the point now is that these instincts of
self-defence are often helped out by structural modifications,
expressly and exclusively adapted to this end. For example, what a
remarkable series of protective adjustments occurs in the life-history
of the Puss Moth--culminating with so comical an instance of the
particular device now under consideration as the following. I quote the
facts from Mr. E. B. Poulton's admirable book on _The Colours of
Animals_ (pp. 269-271).
[Illustration: FIG. 111.--The larva of Puss Moth (_C. vinula_)
when undisturbed; full-fed; natural size.]
The larva of the Puss Moth (Cerura vinula) is very common upon
poplar and willow. The circular dome-like eggs are laid, either
singly or in little groups of two or three, upon the upper side of
the leaf, and being of a reddish colour strongly suggest the
appearance of little galls, or the results of some other injury to
the leaf. The youngest larvæ are black, and also rest upon the
upper surface of the leaf, resembling the dark patches which are
commonly seen in this position. As the larva grows, the apparent
black patch would cover too large a space, and would lead to
detection if it still occupied the whole surface of the body. The
latter gains a green ground-colour which harmonises with the leaf,
while the dark marking is chiefly confined to the back. As growth
proceeds the relative amount of green increases, and the dark mark
is thus prevented from attaining a size which would render it too
conspicuous. In the last stage of growth the green larva becomes
very large, and usually rests on the twigs of its food-plant (Fig.
111). The dark colour is still present on the back but is softened
to a purplish tint, which tends to be replaced by a combination of
white and green in many of the largest larvæ. Such a larva is well
concealed by General Protective Resemblance, and one may search a
long time before finding it, although assured of its presence from
the stripped branches of the food-plant and the fæces on the ground
beneath.
[Illustration: FIG. 112.--The larva of Puss Moth in its terrifying
attitude after being disturbed; full-fed; natural size.]
As soon as a large larva is discovered and disturbed it withdraws
its head into the first body-ring, inflating the margin, which is
of a bright red colour. There are two intensely black spots on this
margin in the appropriate position for eyes, and the whole
appearance is that of a large flat face extending to the outer edge
of the red margin (see Fig. 112). The effect is an intensely
exaggerated caricature of a vertebrate face, which is probably
alarming to the vertebrate enemies of the caterpillar. The
terrifying effect is therefore mimetic. The movements entirely
depend on tactile impressions: when touched ever so lightly a
healthy larva immediately assumes the terrifying attitude, and
turns so as to present its full face towards the enemy; if touched
on the other side or on the back it instantly turns its face in the
appropriate direction. The effect is also greatly strengthened by
two pink whips which are swiftly protruded from the prongs of the
fork in which the body terminates. The prongs represent the last
pair of larval legs which have been greatly modified from their
ordinary shape and use. The end of the body is at the same time
curved forward over the back (generally much further than in Fig.
112), so that the pink filaments are brandished above the head.
_Mimicry._
Lastly, these facts as to imitative and conspicuous colouring lead on to
the yet more remarkable facts of what is called mimicry. By mimicry is
meant the imitation in form and colour of one species by another, in
order that the imitating species may be mistaken for the imitated, and
thus participate in some advantage which the latter enjoys. For
instance, if, as in the case of the conspicuously-coloured caterpillars,
it is of advantage to an ill-savoured species that it should hold out a
warning to enemies, clearly it may be of no less advantage to a
well-savoured species that it should borrow this flag, and thus be
mistaken for its ill-savoured neighbour. Now, the extent to which this
device of mimicry is carried is highly remarkable, not only in respect
of the number of its cases, but also in respect of the astonishing
accuracy which in most of these cases is exhibited by the imitation.
There need be little or virtually no zoological affinity between the
imitating and the imitated forms; that is to say, in some cases the
zoological affinity is not closer than ordinal, and therefore cannot
possibly be ascribed to kinship. Like all the other branches of the
general subject of protective resemblance in form or colouring, this
branch has already been so largely illustrated by previous writers,
that, as in the previous cases, I need only give one or two examples.
Those which I choose are chosen on account of the colours concerned not
being highly varied or brilliant, and therefore lending themselves to
less ineffectual treatment by wood-engraving than is the case where
attempts are made to render by this means even more remarkable
instances. (Figs. 113, 114, 115.)
[Illustration: FIG. 113.--Three cases of mimicry. Drawn from nature:
first two pairs nat. size, last pair 2/3 (_R. Coll. Surg. Mus._).]
[Illustration: FIG. 114.--Two further cases of mimicry; flies
resembling a wasp in the one and a bee in the other. Drawn from
nature: nat. size (_R. Coll. Surg. Mus._).]
[Illustration: FIG. 115.--A case of mimicry where a non-venomous
species of snake resembles a venomous one. Drawn from nature: 1/3
nat. size (_R. Coll. Surg. Mus._).]
It is surely apparent, without further comment, that it is impossible to
imagine stronger evidence in favour of natural selection as a true cause
in nature, than is furnished by this culminating fact in the matter of
protective resemblance, whereby it is shown that a species of one
genus, family, or even order, will accurately mimic the appearance of a
species belonging to another genus, family, or order, so as to deceive
its natural enemies into mistaking it for a creature of so totally
different a kind. And it must be added that while this fact of mimicry
is of extraordinarily frequent occurrence, there can be no possibility
of our mistaking its purpose. For the fact is never observable except in
the case of species which occupy the same area or district.
Such being what appears to me the only reasonable view of the matter, I
will now conclude this chapter on the evidences of natural selection as
at all events the main factor of organic evolution, by simply adding
illustrations of two further cases of mimicry, which are perhaps even
more remarkable than any of the foregoing examples. The first of the two
(Fig. 115) speaks for itself. The second will be rendered intelligible
by the following few words of explanation.
There are certain ants of the Amazons which present the curious instinct
of cutting off leaves from trees, and carrying them like banners over
their heads to the hive, as represented in Fig. 116, B, where one ant is
shown without a leaf, and the others each with a leaf. Their object in
thus collecting leaves is probably that of growing a fungus upon the
"soil" which is furnished by the leaves when decomposing. But, be this
as it may[39], the only point we are now concerned with is the
appearance which these ants present when engaged in their habitual
operation of carrying leaves. For it has been recently observed by Mr.
W. L. Sclater, that in the localities where these hymenopterous insects
occur, there occurs also a _homopterous_ insect which mimics the ant,
leaf and all, in a wonderfully deceptive manner. The leaf is imitated by
the thin flattened body of the insect, "which in its dorsal aspect is so
compressed laterally that it is no thicker than a leaf, and terminates
in a sharp jagged edge." The colour is exactly the same as that of a
leaf, and the brown legs show themselves beneath the green body in just
the same way as those of the ant show themselves beneath the leaf. So
that both the form and the colouring of the homopterous insect has been
brought to resemble, with singular exactness, those belonging to a
different order of insect, when the latter is engaged in its peculiar
avocation. A glance at the figure is enough to show the means employed
and the result attained. In A, an ant and its mimic are represented as
about 2-1/2 times their natural size, and both proceeding in the same
direction. It ought to be mentioned, however, that in reality the margin
of the leaf is seldom allowed to retain its natural serrations as here
depicted: the ants usually gnaw the edge of the real leaf, so that the
margin of the false one bears an even closer resemblance to it than the
illustration represents. B is a drawing from life of a group of five
ants carrying leaves, and their mimic walking beside them[40].
[39] For a full account of this instinct and its probable purpose,
see _Animal Intelligence_, pp. 93-6.
[40] Both drawings are reproduced from Mr. Poulton's paper upon the
subject (_Proc. Zool. Soc._, June 16, 1891).
[Illustration: FIG. 116 PROTECTIVE MIMICRY]
CHAPTER IX.
CRITICISMS OF THE THEORY OF NATURAL SELECTION.
I will now proceed to consider the various objections and difficulties
which have hitherto been advanced against the theory of natural
selection.
Very early in the day Owen hurled the weight of his authority against
the new theory, and this with a strength of onslaught which was only
equalled by its want of judgment. Indeed, it is painfully apparent that
he failed to apprehend the fundamental principles of the Darwinian
theory. For he says:--
Natural Selection is an explanation of the process [of
transmutation] of the same kind and value as that which has been
proffered of the mystery of "secretion." For example, a particular
mass of matter in a living animal takes certain elements out of the
blood, and rejects them as "bile." Attributes were given to the
liver which can only be predicated of the whole animal; the
"appetency" of the liver, it was said, was for the elements of
bile, and "biliosity," or the "hepatic sensation," guided the gland
to their secretion. Such figurative language, I need not say,
explains absolutely nothing of the nature of bilification[41].
[41] _Anatomy of Vertebrates_, vol. iii. p. 794.
Assuredly, it was needless for Owen to say that figurative language of
this kind explains nothing; but it was little less than puerile in him
to see no more in the theory of natural selection than such a mere
figure of speech. To say that the liver selects the elements of bile, or
that nature selects specific types, may both be equally unmeaning
re-statements of facts; but when it is explained that the term natural
selection, unlike that of "hepatic sensation," is used as a shorthand
expression for a whole group of well-known natural causes--struggle,
variation, survival, heredity,--then it becomes evidence of an almost
childish want of thought to affirm that the expression is figurative and
nothing more. The doctrine of natural selection may be a huge mistake;
but, if so, this is not because it consists of any unmeaning metaphor:
it can only be because the combination of natural causes which it
suggests is not of the same adequacy in fact as it is taken to be in
theory.
Owen further objected that the struggle for existence could only act as
a cause of the extinction of species, not of their origination--a view
of the case which again shows on his part a complete failure to grasp
the conception of Darwinism. Acting alone, the struggle for existence
could only cause extermination; but acting together with variation,
survival, and heredity, it may very well--for anything that Owen, or
others who followed in this line of criticism, show to the
contrary--have produced every species of plant and animal that has ever
appeared upon the face of the earth.
Another and closely allied objection is, that the theory of natural
selection "personifies an abstraction." Or, as the Duke of Argyll states
it, the theory is "essentially the image of mechanical necessity
concealed under the clothes, and parading in the mask, of mental
purpose. The word 'natural' suggests Matter, and the physical forces.
The word 'selection' suggests Mind, and the powers of choice." This,
however, is a mere quarrelling about words. Darwin called the principle
which he had discovered by the name natural selection in order to mark
the analogy between it and artificial selection. No doubt in this
analogy there is not necessarily supposed to be in nature any
counterpart to the mind of the breeder, nor, therefore, to his powers of
intelligent choice. But there is no need to limit the term _selection_
(_se_ and _lego_, Gr. [Greek: legô]) to powers of intelligent choice. As
previously remarked, a bank of sea-weed on the sea-shore may be said to
have been selected by the waves from all the surrounding sand and
stones. Similarly, we may say that grain is selected from chaff by the
wind in the process of winnowing corn. Or, if it be thought that there
is any ambiguity involved in such a use of the term in the case of
"Natural Selection," there is no objection to employing the phrase which
has been coined by Mr. Spencer as its equivalent--namely, "Survival of
the Fittest." The point of the theory is, that those organisms which are
best suited to their surroundings are allowed to live and to propagate,
while those which are less suited are eliminated; and whether we call
this process a process of selection, or call it by any other name, is
clearly immaterial.
A material question is raised only when it is asked whether the process
is one that can be ascribed to causation strictly natural. It is often
denied that such is the case, on the ground that natural selection does
not originate the variations which it favours, but depends upon the
variations being supplied by some other means. For, it is said, all that
natural selection does is to preserve the suitable variations _after
they have arisen_. Natural selection does not _cause_ these suitable
variations; and therefore, it is argued, Darwin and his followers are
profoundly mistaken in representing the principle as one which
_produces_ adaptations. Now, although this objection has been put
forward by some of the most intelligent minds in our generation, it
appears to me to betoken some extraordinary failure to appreciate the
very essence of Darwinian doctrine. No doubt it is perfectly true that
natural selection does not produce variations of any kind, whether
beneficial or otherwise. But if it be granted that variations of many
kinds are occurring in every generation, and that natural selection is
competent to preserve the more favourable among them, then it appears to
me unquestionable that this principle of selection deserves to be
regarded as, in the full sense of the word, a natural cause. The
variations being expressly regarded by the theory as more or less
promiscuous[42], survival of the fittest becomes the winnowing fan,
whose function it is to eliminate all the less fit in each generation,
in order to preserve the good grain, out of which to constitute the next
generation. And as this process is supposed to be continuous through
successive generations, its action is supposed to be cumulative, till
from the eye of a worm there is gradually developed the eye of an eagle.
Therefore it follows from these suppositions (which are not disputed by
the present objection), that if it had not been for the process of
selection, such development would never have been begun; and that in the
exact measure of its efficiency will the development proceed. But any
agency without the operation of which a result cannot take place may
properly be designated the cause of that result: it is the agency which,
in co-operation with all the other agencies in the cosmos, produces that
result.
[42] The degree in which variability is indefinite, or, on the
contrary, determinate, is a question which is not yet ripe for
decision--nor even, in my opinion, for discussion. But I may here
state the following general principles with regard to it.
(1) It is evident that up to some point or another variations _must_
be pre-determined in definite lines. Men do not gather grapes from
thorns, figs from thistles, nor even moss-roses from sweet-briars.
In other words, "the nature of the organism" in all cases
necessitates the limiting of variations within certain bounds.
(2) But when the question is as to what these bounds may be, we can
only answer in a general way that, according to the general theory
of evolution, they must be such as are imposed by heredity, coupled
with the degree to which external conditions of life (and possibly
also use-inheritance) are capable, in given cases, of modifying
congenital characters. These are the only causes which the theory of
descent can consistently recognise as producing variations in
determinate directions.
(3) Inasmuch as variation presupposes the existence of parts that
vary, and inasmuch as the variation of parts can only be in the
alternative directions of increase or decrease around an average, it
follows that, in the first instance at all events, every variation,
if determinate, must be so only in one or other of these two
opposite directions.
(4) In as far as variations are summated in successive generations,
so as eventually to give rise to new structures, organs, mechanisms,
&c., natural selection is theoretically competent to explain the
facts, without our having to postulate the operation of unknown
causes producing variations in determinate lines,--or not further
than is stated in paragraphs 1 and 2.
(5) Nevertheless, it does not follow that there are not such other
unknown causes; and, if there are, of course the importance of
natural selection as a cause of adaptive modification would be
limited in proportion to their number and the extent of their
operation. But it is for those who, like the late Professors Asa
Gray and Nägeli, maintain the existence of such causes, to
substantiate their belief by indicating them.
Take any analogous case. The selective agency of specific gravity which
is utilised in gold-washing does not create the original differences
between gold-dust and dust of all other kinds. But these differences
being presented by as many different bodies in nature, the gold-washer
takes advantage of the selective agency in question, and, by using it as
a cause of segregation, is enabled to separate the gold from all the
earths with which it may happen to be mixed. So far as the objects of
the gold-washer are concerned, it is immaterial with what other earths
the gold-dust may happen to be mixed. For although gold-dust may occur
in intimate association with earths of various kinds in various
proportions, and although in each case the particular admixture which
occurs must have been due to definite causes, these things, in relation
to the selective process of the washer, are what is called accidental:
that is to say, they have nothing to do with the causative action of the
selective process. Now, in precisely the same sense Darwin calls the
multitudinous variations of plants and animals accidental. By so calling
them he expressly says he does not suppose them to be accidental in the
sense of not all being due to definite causes. But they are accidental
in relation to the sifting process of natural selection: all that they
have to do is to furnish the promiscuous material on which this sifting
process acts.
Or let us take an even closer analogy. The power of selective breeding
by man is so wonderful, that in the course of successive generations all
kinds of peculiarities as to size, shape, colour, special appendages or
abortions, &c., can be produced at pleasure, as we saw in the last
chapter. Now all the promiscuous variations which are supplied to the
breeder, and out of which, by selecting only those that are suited to
his purpose, he is able to produce the required result--all those
promiscuous variations, in relation to that purpose, are accidental.
Therefore the selective agency of the breeder deserves to be regarded as
the cause of that which it produces, or of that which could not have
been produced but for the operation of such agency. But where is the
difference between artificial and natural selection in this respect?
And, if there is no difference, is not natural selection as much
entitled to be regarded as a true cause of the origin of natural
species, as artificial selection is to be regarded as a true cause of
our domesticated races? Here, as in the case of the previous
illustration, if there be any ambiguity in speaking of variations as
accidental, it arises from the incorrect or undefined manner in which
the term "accidental" is used by Darwin's critics. In its original and
philosophically-correct usage, the term "accident" signifies a property
or quality not essential to our conception of a substance: hence, it has
come to mean anything that happens as a result of unforeseen causes--or,
lastly, that which is causeless. But, as we know that nothing can happen
without causes of some kind, the term "accident" is divested of real
meaning when it is used in the last of these senses. Yet this is the
sense that is sought to be placed upon it by the objection which we are
considering. If the objectors will but understand the term in its
correct philosophical sense--or in the only sense in which it presents
any meaning at all,--they will see that Darwinians are both logically
and historically justified in employing the word "accidental" as the
word which serves most properly to convey the meaning that they
intend--namely, variations due to causes accidental to the struggle for
existence. Similarly, when it is said that variations are "spontaneous,"
or even "fortuitous," nothing further is meant than that we do not know
the causes which lead to them, and that, so far as the principle of
selection is concerned, it is immaterial what these causes may be. Or,
to revert to our former illustration, the various weights of different
kinds of earths are no doubt all due to definite causes; but, in
relation to the selective action of the gold-washer, all the different
weights of whatever kinds of earth he may happen to include in his
washing-apparatus are, _strictly speaking_, accidental. And as at
different washings he meets with different proportions of heavy earths
with light ones, and as these "variations" are immaterial to him, he may
colloquially speak of them as "fortuitous," or due to "chance," even
though he knows that at each washing they must have been determined by
definite causes.
More adequately to deal with this merely formal objection, however,
would involve more logic-chopping than is desirable on the present
occasion. But I have already dealt with it fully elsewhere,--viz. in
_The Contemporary Review_ for June, 1888, to which therefore I may refer
any one who is interested in dialectics of this kind[43].
[43] Within the last few months this objection has been presented
anew by Mr. D. Syme, whose book _On the Modification of Organisms_
exhibits a curious combination of shrewd criticisms with almost
ludicrous misunderstandings. One of the latter it is necessary to
state, because it pervades the quotation which I am about to supply.
He everywhere compares "natural selection" with "the struggle for
existence," uses them as convertible terms, and while absurdly
stating that "Darwin defines natural selection as the struggle for
existence," complains of "the liability of error, both on his own
part and on the part of his readers," which arises from his not
having everywhere adhered to this definition! (p. 8).
"Darwin has put forth two distinct and contradictory theories of the
functions of natural selection. According to the one theory natural
selection is selective or preservative, and nothing more. According
to the other theory natural selection creates the variations(!) ...
It certainly seems absurd to speak of natural selection, or the
struggle for existence, as selective or preservative, for the
struggle for existence does not preserve at all, not even the fit
variations, as both the fit and the unfit struggle for existence,
the unfit naturally more than the fit, and the fit are preserved,
not in consequence of the struggle, but in consequence of their
fitness. Suppose two varieties of the same species are driven, by an
increase of their numbers, to seek for subsistence in a colder
region than they have been accustomed to, and that one of these
varieties had a hardier constitution than the other; and let us
suppose that the former withstood the severe climate better than the
latter, and consequently survived, while the other perished. In this
case the hardier survived, not because of the struggle, but because
it had a constitution better adapted to the climate. I wish to
ascertain if a certain metal in my possession is gold or some baser
metal, and I apply the usual test; but the mere fact of my testing
this metal would not make it gold or any other kind of metal."
I have thought it worth while to quote this passage for the sake of
showing the extraordinary confusion of mind which still prevails on
the part of Darwin's critics, even with reference to the very
fundamental parts of his theory. For, as I have said, the writer of
this passage shows himself a shrewd critic in some other parts of
his essay, where he is not engaged especially on the theory of
natural selection.
I will now pass on to consider another misconception of the Darwinian
theory, which is very prevalent in the public mind. It is virtually
asked, If some species are supposed to have been improved by natural
selection, why have not all species been similarly improved? Why should
not all invertebrated animals have risen into vertebrated? Or why
should not all monkeys have become men?
The answers are manifold. In the first place, it by no means follows
that because an advance in organization has proved itself of benefit in
the case of one form of life, therefore any or every other form would
have been similarly benefited by a similar advance. The business of
natural selection is to bring this and that form of life into the
closest harmony with its environment that all the conditions of the case
permit. Sometimes it will happen that the harmony will admit of being
improved by an improvement of organization. But just as often it will
happen that it will be best secured by leaving matters as they are. If,
therefore, an organism has already been brought into a tolerably full
degree of harmony with its environment, natural selection will not try
to change it so long as the environment remains unchanged; and this, no
doubt, is the reason why some species have survived through enormous
periods of geological time without having undergone any change. Again,
as we saw in a previous chapter, there are yet other cases where, on
account of some change in the environment or even in the habits of the
organisms themselves, adaption will be best secured by an active
_reversal_ of natural selection, with the result of causing
_degeneration_.
But, it is sometimes further urged, there are cases where we cannot
doubt that improvement of organization would have been of benefit to
species; and yet such improvement has not taken place--as, for instance,
in the case all monkeys not turning into men. Here, however, we must
remember that the operation of natural selection in any case depends
upon a variety of highly complex conditions; and, therefore, that the
fact of all those conditions having been satisfied in one instance is no
reason for concluding that they must also have been satisfied in other
instances. Take, for example, the case of monkeys passing into men. The
wonder to me appears to be that this improvement should have taken place
in even one line of descent; not that, having taken place in one line,
it should not also have taken place in other lines. For how enormously
complex must have been the conditions--physical, anatomical,
physiological, psychological, sociological--which by their happy
conjunction first began to raise the inarticulate cries of an ape into
the rational speech of a man. Therefore, the more that we appreciate the
superiority of a man to an ape, the less ought we to countenance this
supposed objection to Darwin's theory--namely, that natural selection
has not effected the change in more than one line of descent.
Even in the case of two races of mankind where one has risen higher in
the scale of civilization than another, it is now generally impossible
to assign the particular causes of the difference; much more, then, must
this be impossible in the case of still more remote conditions which
have led to the divergence of species. The requisite variations may not
have arisen in the one line of descent which did arise in the other; or
if they did arise in both, some counterbalancing disadvantages may have
attended their initial development in the one case which did not obtain
in the other. In short, where so exceedingly complex a play of
conditions are concerned, the only wonder would be if two different
lines of descent _had_ happened to present two independent and yet
perfectly parallel lines of history.
These general considerations would apply equally to the great majority
of other cases where some types have made great advances upon others,
notwithstanding that we can see no reason why the latter should not in
this respect have imitated the former. But there is yet a further
consideration which must be taken into account. The struggle for
existence is always most keen between closely allied species, because,
from the similarity of their forms, habits, needs, &c., they are in
closest competition. Therefore it often happens that the mere fact of
one species having made an advance upon others of itself precludes the
others from making any similar advance: the field, so to speak, has
already been occupied as regards that particular improvement, and where
the struggle for existence is concerned possession is emphatically nine
points of the law. For example, to return to the case of apes becoming
men, the fact of one rational species having been already evolved (even
if the rational faculty were at first but dimly nascent) must make an
enormous change in the conditions as regards the possibility of any
other such species being subsequently evolved--unless, of course, it be
by way of descent from the rational one. Or, as Sir Charles Lyell has
well put it, two rational species can never _coexist_ on the globe,
although the descendants of one rational species may in time become
_transformed_ into another single rational species[44].
[44] _Principles of Geology_, vol. ii. p. 487 (11th ed.).
In view of such considerations, another and exactly opposite objection
has sometimes been urged--viz. that we ought never to find inferior
forms of organization in company with superior, because in the struggle
for existence the latter ought to have exterminated the former. Or, to
quote the most recent expression of this view, "in every locality there
would only be one species, and that the most highly organized; and thus
a few superior races would partition the earth amongst them to the
entire exclusion of the innumerable varieties, species, genera, and
orders which now inhabit it[45]." Of course to this statement it would
be sufficient to enquire, On what would these few supremely organized
species subsist? Unless manna fell from heaven for their especial
benefit, it would appear that such forms could under no circumstances be
the most improved forms; in exterminating others on such a scale as
this, they would themselves be quickly, and very literally, improved off
the face of the earth. But even when the statement is not made in so
extravagant a form as this, it must necessarily be futile as an
objection unless it has first been shown that we know exactly all the
conditions of the complex struggle for existence between the higher and
lower forms in question. And this it is impossible that we ever can
know. The mere fact that one form has been changed in virtue of this
struggle must in many cases of itself determine a change in the
conditions of the struggle. Again, the other and closely allied forms
(and these furnish the best grounds for the objection) may also have
undergone defensive changes, although these may be less conspicuous to
our observation, or perhaps less suggestive of "improvement" to our
imperfect means of judging. Lastly, not to continue citing an endless
number of such considerations, there is the broad fact that it is only
to those cases where, for some reason or another, the lower forms have
not been exposed to a struggle of fatal intensity, that the objection
applies. But we know that in millions of other cases the lower (i. e.
less fitted) forms _have_ succumbed, and therefore I do not see that the
objection has any ground to stand upon. That there is a general tendency
for lower forms to yield their places to higher is shown by the gradual
advance of organization throughout geological time; for if _all_ the
inferior forms had survived, the earth could not have contained them,
unless she had been continually growing into something like the size of
Jupiter. And if it be asked why any of the inferior forms have survived,
the answer has already been given, as above.
[45] Syme, on the _Modification of Organisms_, p. 46.
There is only one other remark to be made in this connexion. Mr. Syme
chooses two cases as illustrations of the supposed difficulty. These are
sufficiently diverse--viz. Foraminifera and Man. Touching the former,
there is nothing that need be added to the general answer just given.
But with regard to the latter it must be observed that the dominion of
natural selection as between different races of mankind is greatly
restricted by the presence of rationality. Competition in the human
species is more concerned with wits and ideas than with nails and teeth;
and therefore the "struggle" between man and man is not so much for
actual _being_, as for _well-being_. Consequently, in regard to the
present objection, the human species furnishes the worst example that
could have been chosen.
* * * * *
Hitherto I have been considering objections which arise from
misapprehensions of Darwin's theory. I will now go on to consider a
logically sound objection, which nevertheless is equally futile,
because, although it does not depend on any misapprehension of the
theory, it is not itself supported by fact.
The objection is the same as that which we have already considered in
relation to the general theory of descent--namely, that similar organs
or structures are to be met with in widely different branches of the
tree of life. Now this would be an objection fatal to the theory of
natural selection, supposing these organs or structures in the cases
compared are not merely analogous, but also homologous. For it would be
incredible that in two totally different lines of descent one and the
same structure should have been built up independently by two parallel
series of variations, and that in these two lines of descent it should
always and independently have ministered to the same function. On the
other hand, there would be nothing against the theory of natural
selection in the fact that two structures, _not_ homologous, should come
by independent variation in two different lines of descent to be adapted
to perform the same function. For it belongs to the very essence of the
theory of natural selection that a useful function should be secured by
favourable variations of whatever structural material may happen to be
presented by different organic types. Flying, for instance, is a very
useful function, and it has been developed independently in at least
four different lines of descent--namely, the insects, reptiles, birds,
and mammals. Now if in all, or indeed in any, of these four cases the
wings had been developed on the same anatomical pattern, so as not only
to present the analogical resemblance which it is necessary that they
should present in order to discharge their common function of flying,
but likewise an homologous or structural resemblance, showing that they
had been formed on the same anatomical "plan,"--if such has been the
case, I say, the theory of natural selection would certainly be
destroyed.
Now it has been alleged by competent naturalists that there are several
such cases in organic nature. We have already noticed in a previous
chapter (pp. 58, 59), that Mr. Mivart has instanced the eye of the
cuttle-fish as not only analogous to, but also homologous with, the eye
of a true fish--that is to say, the eye of a mollusk with the eye of a
vertebrate. And he has also instanced the remarkable resemblance of a
shrew to a mouse--that is, of an insectivorous mammal to a rodent--not
to mention other cases. In the chapter alluded to these instances of
homology, alleged to occur in different branches of the tree of life,
were considered with reference to the process of organic evolution as a
fact: they are now being considered with reference to the agency of
natural selection as a method. And just as in the former case it was
shown, that if any such alleged instances could be proved, the proof
would be fatal to the general theory of organic evolution by physical
causes, so in the present case, if this could be proved, it would be
equally fatal to the more special theory of natural selection. But, as
we have before seen, no single case of this kind has ever been made
out; and, therefore, not only does this supposed objection fall to the
ground, but in so doing it furnishes an additional argument in favour of
natural selection. For in the earlier chapter just alluded to I showed
that this great and general fact of our nowhere being able to find two
homologous structures in different branches of the tree of life, was the
strongest possible testimony in favour of the theory of evolution. And,
by parity of reasoning, I now adduce it as equally strong evidence of
natural selection having been the cause of _adaptive_ structures,
independently developed in all the different lines of descent. For the
alternative is between adaptations having been caused by natural
selection or by supernatural design. Now, if adaptations were caused by
natural selection, we can very well understand why they should never be
homologous in different lines of descent, even in cases where they have
been brought to be so closely analogous as to have deceived so good a
naturalist as Mr. Mivart. Indeed, as I have already observed, so well
can we understand this, that any single instance to the contrary would
be sufficient to destroy the theory of natural selection _in toto_,
unless the structure be one of a very simple type. But on the other
hand, it is impossible to suggest any rational explanation why, if all
adaptations are due to supernatural design, such scrupulous care should
have been taken never to allow homologous adaptations to occur in
different divisions of the animal or vegetable kingdoms. Why, for
instance, should the eye of a cuttle-fish _not_ have been constructed on
the same ideal pattern as that of vertebrate? Or why, among the
thousands of vertebrated species, should no one of their eyes be
constructed on the ideal pattern that was devised for the cuttle-fish?
Of course it may be answered that perhaps there was some hidden reason
why the design should never have allowed an adaptation which it had
devised for one division of organic nature to appear in another--even in
cases where the new design necessitated the closest possible resemblance
in everything else, save in the matter of anatomical homology.
Undoubtedly such may have been the case--or rather such _must_ have been
the case--if the theory of special design is true. But where the
question is as to the truth of this theory, I think there can be no
doubt that its rival gains an enormous advantage by being able to
_explain_ why the facts are such as they are instead of being obliged to
take refuge in hypothetical possibilities of a confessedly
unsubstantiated and apparently unsubstantial kind.
Therefore, as far as this objection to the theory of natural selection
is concerned--or the allegation that homologous structures occur in
different divisions of organic nature--not only does it fall to the
ground, but positively becomes itself converted into one of the
strongest arguments in favour of the theory. As soon as the allegation
is found to be baseless, the very fact that it cannot be brought to bear
upon any one of all the millions of adaptive structures in organic
nature becomes a fact of vast significance on the opposite side.
* * * * *
The next difficulty to which I shall allude is that of explaining by the
theory of natural selection the preservation of the first beginnings of
structures which are then useless, though afterwards, when more fully
developed, they become useful. For it belongs to the very essence of the
theory of natural selection, that a structure must be supposed already
useful before it can come under the influence of natural selection:
therefore the theory seems incapable of explaining the origin and
conservation of _incipient_ organs, or organs which are not yet
sufficiently developed to be of any service to the organisms presenting
them.
This objection is one that has been advanced by all the critics of
Darwinism; but has been presented with most ability and force by the
Duke of Argyll. I will therefore state it in his words.
If the doctrine of evolution be true--that is to say, if all
organic creatures have been developed by ordinary generation from
parents--then it follows of necessity that the primæval germs must
have contained potentially the whole succeeding series. Moreover,
if that series has been developed gradually and very slowly, it
follows, also as a matter of necessity, that every modification of
structure must have been functionless at first, when it began to
appear.... Things cannot be selected until they have first been
produced. Nor can any structure be selected by utility in the
struggle for existence until it has not only been produced, but has
been so far perfected as to actually be used.
The Duke proceeds to argue that all adaptive structures must therefore
originally have been due to special design: in the earlier stages of
their development they must all have been what he calls "prophetic
germs." Not yet themselves of any use, and therefore not yet capable of
being improved by natural selection, both in their origin and in the
first stages (at all events) of their development, they must be
regarded as intentionally preparatory to the various uses which they
subsequently acquire.
Now this argument, forcible as it appears at first sight, is really at
fault both in its premiss and in its conclusion. By which I mean that,
in the first place the premiss is not true, and, in the next place, that
even if it were, the conclusion would not necessarily follow. The
premiss is, "that every modification of structure must have been
functionless at first, when it began to appear;" and the conclusion is,
that, _quâ_ functionless, such a modification cannot have been caused by
natural selection. I will consider these two points separately.
First as to the premiss, it is not true that every modification of
structure must necessarily be functionless when it first begins to
appear. There are two very good reasons why such should not be the case
in all instances, even if it should be the case in some. For, as a
matter of observable fact, a very large proportional number of incipient
organs are useful from the very moment of their inception. Take, for
example, what is perhaps the most wonderful instance of refined
mechanism in nature--the eye of a vertebrated animal. Comparative
anatomy and embryology combine to testify that this organ had its origin
in modifications of the endings of the ordinary nerves of the skin. Now
it is evident that from the very first any modification of a cutaneous
nerve whereby it was rendered able, in however small a degree, to be
differently affected by light and by darkness would be of benefit to the
creature presenting it; for the creature would thus be able to seek the
one and shun the other according to the requirements of its life. And
being thus useful from the very moment of its inception, it would
afterwards be gradually improved as variations of more and more utility
presented themselves, until not only would finer and finer degrees of
difference between light and shade become perceptible, but even the
outlines of solid bodies would begin to be appreciated. And so on, stage
by stage, till from an ordinary nerve-ending in the skin is evolved the
eye of an eagle.
Moreover, in this particular instance there is very good reason to
suppose that the modification of the cutaneous nerves in question began
by a progressive increase in their sensitiveness to temperature.
Wherever dark pigment happened to be deposited in the skin--and we know
that in all animals it is apt to be deposited in points and patches, as
it were by accident, or without any "prophecy" as to future uses,--the
cutaneous nerves in its vicinity would be better able to appreciate the
difference between sun and shade in respect of temperature, even though
as yet there were no change at all in these cutaneous nerves tending to
make them responsive to light. Now it is easy to see how, from such a
purely accidental beginning, natural selection would have had from the
first sufficient material to act upon. It being of advantage to a lowly
creature that it should distinguish with more and more delicacy, or with
more and more rapidity, between light and darkness by means of its
thermal sensations, the pigment spots in the skin would be rendered
permanent by natural selection, while the nerves in that region would by
the same agency be rendered more and more specialized as organs adapted
to perceive changes of temperature, until from the stage of responding
to the thermal rays of the non-luminous spectrum alone, they become
capable of responding also to luminous.
So much, then, for the first consideration which serves to invalidate
the Duke's premiss. The second consideration is, that very often an
organ which began by being useful for the performance of one function,
after having been fully developed for the performance of that function,
finds itself, so to speak, accidentally fitted to the performance of
some other and even more important function, which it thereupon begins
to discharge, and so to undergo a new course of adaptive development. In
such cases, and so far as the new function is concerned, the difficulty
touching the first inception of an organ does not apply; for here the
organ has already been built up by natural selection for one purpose,
before it begins to discharge the other. As an example of such a case we
may take the lung of an air-breathing animal. Originally the lung was a
swim-bladder, or float, and as such it was of use to the aquatic
ancestors of terrestrial animals. But as these ancestors gradually
became more and more amphibious in their habits, the swim-bladder began
more and more to discharge the function of a lung, and so to take a
wholly new point of departure as regards its developmental history. But
clearly there is here no difficulty with regard to the inception of its
new function, because the organ was already well developed for one
purpose before it began to serve another. Or, to take only one
additional example, there are few structures in the animal kingdom so
remarkable in respect of adaptation as is the wing of a bird or a bat;
and at first sight it might well appear that a wing could be of no
conceivable use until it had already acquired enormous proportional
dimensions, as well as an immense amount of special elaboration as to
its general form, size of muscle, amount of blood-supply, and so on.
For, obviously, not until it had attained all these things could it even
begin to raise the animal in the air. But observe how fallacious is this
argument. Although it is perfectly true that a wing could be of no use
_as a wing_ until sufficiently developed to serve the purpose of flight,
this is merely to say that until it has become a wing it is no use as a
wing. It does not, however, follow that on this account it was of no
prior use for any other purpose. The first modifications of the
fore-limb which ended in its becoming an organ of flight may very well
have been due to adapting it as an organ for increased rapidity of
locomotion of other kinds--whether on land as in the case of its now
degenerated form in the ostrich, or in water as in the case of the
expanded fins of fish. Indeed, we may see the actual process of
transition from the one function to the other in the case of
"flying-fish." Here the progressive expansion of the pectoral fins must
certainly have been always of use for continuously promoting rapidity of
locomotion through water; and thus natural selection may have
continuously increased their development until they now begin to serve
also as wings for carrying the animal a short distance through air.
Again, in the case of the so-called flying squirrels we find the limbs
united to the body by means of large extensions of the skin, so-that
when jumping from one tree to another the animal is able to sustain
itself through a long distance in the air by merely spreading out its
limbs, and thus allowing the skin-extensions to act after the manner of
a parachute. Here, of course, we have not yet got a wing, any more than
we have in the case of the flying-fish; but we have the foundations laid
for the possible development of a future wing, upon a somewhat similar
plan as that which has been so wonderfully perfected in the case of
bats. And through all the stages of progressive expansion which the skin
of the squirrel has undergone, the expansion has been of use, even
though it has not yet so much as begun to acquire the distinctive
functions of a wing. Here, then, there is obviously nothing "prophetic"
in the matter, any more than there was in the case of the swim-bladder
and the lung, or in that of the nerve-ending and the eye. In short, it
is the business of natural selection to secure the highest available
degree of adaptation for the time being; and, in doing this, it not
unfrequently happens that an extreme development of a structure in one
direction (produced by natural selection for the sake of better and
better adapting the structure to perform some particular function) ends
by beginning to adapt it to the performance of some other function. And,
whenever this happens to be the case, natural selection forthwith begins
to act upon the structure, so to speak, from a new point of departure.
So much, then, for the Duke's premiss--namely, that "every modification
of structure _must_ have been functionless _at first_, when it began to
appear." This premiss is clearly opposed to observable fact. But now,
the second position is that, even if this were not so, the Duke's
conclusion would not follow. This conclusion, it will be remembered, is,
that if incipient structures are useless, it necessarily follows that
natural selection can have had no part whatever in their inception. Now,
this is a conclusion which does not "necessarily" follow. Even if it be
granted that there are structures which in their first beginnings are
not of any use at all for any purpose, it is still possible that they
may owe their origin to natural selection--not indeed directly, but
indirectly. This possibility arises from the occurrence in nature of a
principle which has been called the Correlation of Growth.
Mr. Darwin, who has paid more attention to this matter than any other
writer, has shown, in considerable detail, that all the parts of any
given organism are so intimately bound together, or so mutually
dependent upon each other, that when one part is caused to change by
means of natural selection, some other parts are very likely to undergo
modification as a consequence. For example, there are several kinds of
domesticated pigeons and fowls, which grow peculiar wing-like feathers
on the feet. These are quite unlike all the other feathers in the
animal, except those of the wing, to which they bear a very remarkable
resemblance. Mr. Darwin records the case of a bantam where these
wing-like feathers were nine inches in length, and I have myself seen a
pigeon where they reproduced upon the feet a close imitation of the
different kinds of feathers which occupy homologous positions in the
wing--primaries, secondaries, and tertiaries all being distinctly
repeated in their proper anatomical relations. Furthermore, in this
case, as in most cases where such wing-feathers occur upon the feet, the
third and fourth toes were partly united by skin; and, as is well known,
in the wing of a bird the third and fourth digits are completely united
by skin; "so that in feather-footed pigeons, not only does the exterior
surface support a row of long feathers, like wing-feathers [which, as
just stated, may in some cases be obviously differentiated into
primaries, secondaries and tertiaries], but the very same digits which
in the wing are completely united by skin become partially united by
skin in the feet; and thus by the law of correlated variation of
homologous parts, we can understand the curious connexion of feathered
legs and membrane between the two outer toes[46]." The illustration is
drawn from the specimen to which I have referred.
[46] _Variation of Plants and Animals_, vol. ii. p. 315.
[Illustration: FIG. 117.--Feather-footed pigeon. Drawn from nature.]
Many similar instances of the same law are to be met with throughout
organic nature; and it is evident that in this principle we find a
conceivable explanation of the origin of such adaptive structures as
could not have been originated by natural selection acting directly upon
themselves: they may have been originated by natural selection
developing other adaptive structures elsewhere in the organism, the
gradual evolution of which has entailed the production of these by
correlation of growth. And, if so, when once started in this way, these
structures, because thus accidentally useful, will now themselves come
under the _direct_ action of natural selection, and so have their
further evolution determined with or without the correlated association
which first led to their inception.
Of course it must be understood that in thus applying the principle of
correlated growth, to explain the origin of adaptive structures where it
is impossible to explain such origin by natural selection having from
the first acted directly upon these structures themselves, Darwinists
do not suppose that in all--or even in most--cases of correlated growth
the correlated structures are of use. On the contrary, it is well known
that structures due to correlated growth are, as a rule, useless. Being
only the by-products of adaptive changes going on elsewhere, in any
given case the chances are against these correlated effects being
themselves of any utilitarian significance; and, therefore, as a matter
of fact, correlated growths appear to be usually meaningless from the
point of view of adaptation. Still, on the doctrine of chances, it is
to be expected that sometimes a change of structure which has thus been
indirectly produced by correlation of growth might happen to prove
useful for some purpose or another; and in as many cases as such
indirectly produced structures do prove useful, they will straightway
begin to be improved by the direct action of natural selection. In all
such cases, therefore, we should have an explanation of the _origin_ of
such a structure, which is the only point that we are now considering.
I think, then, that all this effectually disposes of the doctrine of
"prophetic germs." But, before leaving the subject, I should like to
make one further statement of greater generality than any which I have
hitherto advanced. This statement is, that we must remember how large a
stock of meaningless structures are always being produced in the course
of specific transmutations, not only by correlation of growth, which we
have just been considering, but also by the direct action of external
conditions, together with the constant play of all the many and complex
forces internal to organisms themselves. In other words, important as
the principle of correlation undoubtedly is, we must remember that even
this is very far from being the only principle which is concerned in the
origination of structures that may or may not chance to be useful.
Therefore, it is not only natural selection when operating indirectly
through the correlation of growth that is competent to produce new
structures without reference to utility. In all the complex action and
reaction of internal and external forces, new variations are perpetually
arising without any reference to utility, either present or future.
Among all this multitude of promiscuous variations, the chances must be
that some percentage will prove of some service, either from the first
moment of their appearance, or else after they have undergone some
amount of development. Such development prior to utility may be due,
either to correlation of growth, to the structure having previously
performed some other function, as already explained, or else to a
continued operation of the causes which were concerned in the first
appearance of originally useless characters. In a series of chapters
which will be devoted to the whole question of utility in the next
volume, I shall hope to give very good reasons for concluding that
useless characters are not only of highly frequent occurrence, but are
due to a variety of other causes besides correlation of growth. And, if
so, the possibility of originally useless characters happening in some
cases to become, by increased development, useful characters, is
correspondingly increased. Among a hundred varietal or specific
characters which are directly produced in as many different species by a
change of climate, for example, some five or six may be _potentially_
useful: that is to say, characters thus adventitiously produced in an
incipient form may only require to be further developed by a continuance
of the same causes as first originated them, in order that some
percentage of the whole number shall become of some degree of use. Those
professed followers of Darwin, therefore, who without any reason--or, as
it appears to me, against all reason--deny the possibility of useless
specific characters in any case or in any degree (unless correlated with
useful characters), are playing into the hands of Darwin's critics by
indirectly countenancing the difficulty which we are now considering.
For, if correlation of growth is unreasonably supposed to be the only
possible cause of the origin of incipient structures which are not
useful from the first moment of their inception, clearly the field is
greatly narrowed as regards the occurrence of incipient characters
sufficient in amount--and, still more, in constancy of appearance and
persistency of transmission--to admit of furnishing material for the
working of natural selection. But in the measure that incipient
characters--whether varietal or specific--are recognised as not always
or "necessarily" useful from the moment of their inception, and yet
capable of being developed to a certain extent by the causes which first
led to their occurrence, in that measure is this line of criticism
closed. For of all the variations which thus occur, it is only those
which afterwards prove of any use that are laid hold upon and wrought up
by natural selection into adaptive structures, or working organs. And,
therefore, what we see in organic nature is the net outcome of the
development of all the happy chances. So it comes that the appearance
presented by organic nature as a whole is that of a continual fulfilment
of structural prophecies, when, in point of fact, if we had a similar
record of all the other variations it would be seen that possibly not
one such prophecy in a thousand is ever destined to be fulfilled.
* * * * *
Here, then, I feel justified in finally taking leave of the difficulty
from the uselessness of incipient organs, as this difficulty has been
presented, in varying degrees of emphasis, by the Duke of Argyll, Mr.
Mivart, Professors Nägeli, Bronn, Broca, Eimer, and, indeed, by all
other writers who have hitherto advanced it. For, as thus presented, I
think I have shown that it admits of being adequately met. But now, I
must confess, to me individually it does appear that behind this
erroneous presentation of the difficulty there lies another question,
which is deserving of much more serious attention. For although it
admits of being easily shown--as I have just shown--that the difficulty
as ordinarily presented fails on account of its extravagance, the
question remains whether, if stated with more moderation, a real
difficulty might not be found to remain.
My quarrel with the conclusion, like my quarrel with the premiss, is due
to its universality. By saying in the premiss that _all_ incipient
organs are _necessarily_ useless at the time of their inception, these
writers admit of being controverted by fact; and by saying in the
conclusion that, _if_ all incipient organs are useless, it necessarily
follows that in _no_ case can natural selection have been the cause of
building up an organ until it becomes useful, they admit of being
controverted by logic. For, even if the premiss were true in
fact--namely, that all incipient organs are useless at the time of their
inception,--it would not necessarily follow that in no case could
natural selection build up a useless structure into a useful one;
because, although it is true that in no case can natural selection do
this by acting on a useless structure _directly_, it may do so by acting
on the useless structure _indirectly_, through its direct action on some
other part of the organism with which the useless structure happens to
be correlated. Moreover, as I believe, and will subsequently endeavour
to prove, there is abundant evidence to show that incipient characters
are often developed to a large extent by causes other than natural
selection (or apart from any reference to utility), with the result that
some of them thus happen to become of use, when, of course, the supposed
difficulty is at an end.
But although it is thus easy to dispose of both the propositions in
question, on account of their universality, stated more carefully they
would require, as I have said, more careful consideration. Thus, if it
had been said that some incipient organs are _presumably_ useless at the
time of their inception, and that in _some of these cases_ it is
difficult, or impossible, to conceive how the principle of correlation,
or any other principle hitherto suggested, can apply--then the question
would have been raised from the sphere of logical discussion to that of
biological fact. And the new question thus raised would have to be
debated, no longer on the ground of general or abstract principles, but
on that of special or concrete cases. Now until within the last year or
two it has not been easy to find such a special or concrete case--that
is to say, a case which can be pointed to as apparently excluding the
possibility of natural selection having had anything to do with the
genesis of an unquestionably adaptive structure. But eventually such a
case has arisen, and the Duke of Argyll has not been slow in perceiving
its importance. This case is the electric organ in the tail of the
skate. No sooner had Professor Cossar Ewart published an abstract of his
first paper on this subject, than the Duke seized upon it as a case for
which, as he said, he had long been waiting--namely, the case of an
_adaptive_ organ the genesis of which _could not possibly_ be attributed
to natural selection, and must therefore be attributed to supernatural
design. Now, I do not deny that he is here in possession of an admirable
case--a case, indeed, so admirable that it almost seems to have been
specially designed for the discomfiture of Darwinians. Therefore, in
order to do it full justice, I will show that it is even more formidable
than the Duke of Argyll has represented.
Electric organs are known to occur in several widely different kinds of
fish--such as the _Gymnotus_ and _Torpedo_. Wherever these organs do
occur, they perform the function of electric batteries in storing and
discharging electricity in the form of more or less powerful shocks.
Here, then, we have a function which is of obvious use to the fish for
purposes both of offence and defence. These organs are everywhere
composed of a transformation of muscular, together with an enormous
development of nervous tissue; but inasmuch as they occupy different
positions, and are also in other respects dissimilar in the different
zoological groups of fishes where they occur, no difficulty can be
alleged as to these analogous organs being likewise homologous in
different divisions of the aquatic vertebrata.
Now, in the particular case of the skate, the organ is situated in the
tail, where it is of a spindle-like form, measuring, in a large fish,
about two feet in length by about an inch in diameter at the middle of
the spindle. Although its structure is throughout as complex and perfect
as that of the electric organ in _Gymnotus_ or _Torpedo_, its smaller
size does not admit of its generating a sufficient amount of
electricity to yield a discharge that can be felt by the hand.
Nevertheless, that it does discharge under suitable stimulation has been
proved by Professor Burdon Sanderson by means of a telephone; for he
found that every time he stimulated the animal its electrical discharge
was rendered audible by the telephone. Here, then, the difficulty
arises. For of what conceivable use is such an organ to its possessor?
We can scarcely suppose that any aquatic animal is more sensitive to
electric shocks than is the human hand; and even if such were the case,
a discharge of so feeble a kind taking place in water would be
short-circuited in the immediate vicinity of the skate itself. So there
can be no doubt that such weak discharges as the skate is able to
deliver must be wholly imperceptible alike to prey and to enemies. Yet
for the delivery of such discharges there is provided an organ of such
high peculiarity and huge complexity, that, regarded as a piece of
living mechanism, it deserves to rank as at once the most extremely
specialized and the most highly elaborated structure in the whole animal
kingdom. Thousands of separately formed elements are ranged in row after
row, all electrically insulated one from another, and packed away into
the smallest possible space, with the obvious end, or purpose, of
conspiring together for the simultaneous delivery of an electric shock.
Nevertheless, the shock when delivered is, as we have just seen, too
slight to be of any conceivable use to the skate. Therefore it appears
impossible to suggest how this astonishing structure--much more
astonishing, in my opinion, than the human eye or the human hand--can
ever have been begun, or afterwards developed, by means of natural
selection. For if it be not even yet of any conceivable use to its
possessor, clearly thus far survival of the fittest can have had nothing
to do with its formation. On the other hand, seeing that electric organs
when of larger size, as in the _Gymnotus_ and _Torpedo_, are of obvious
use to their possessors, the facts of the case, so far as the skate is
concerned, assuredly do appear to sanction the doctrine of "prophetic
germs." The organ in the skate seems to be on its way towards becoming
such an organ as we meet with in these other animals; and, therefore,
unless we can show that it is now, and in all previous stages of its
evolution has throughout been, of use to the skate, the facts do present
a serious difficulty to the theory of natural selection, while they
readily lend themselves to the interpretation of a disposing or
fore-ordaining mind, which knows how to construct an electric battery by
thus transforming muscular tissue into electric tissue, and is now
actually in process of constructing such an apparatus for the
prospective benefit of future creatures.
Should it be suggested that possibly the electric organ of the skate may
be in process of degeneration, and therefore that it is now the
practically functionless remnant of an organ which in the ancestors of
the skate was of larger size and functional use--against so obvious a
suggestion there lie the whole results of Professor Ewart's
investigations, which go to indicate that the organ is here not in a
stage of degeneration, but of evolution. For instance, in _Raia
radiata_, it does not begin to be formed out of the muscular tissue
until some time after the animal has left the egg-capsule, and assumed
all the normal proportions (though not yet the size) of the adult
creature. The organ, therefore, is one of the very latest to appear in
the ontogeny of _R. radiata_; and, moreover, it does not attain its full
_development_ (i. e. not merely _growth_, but transforming of muscular
fibres into electrical elements) till the fish attains maturity. Read in
the light of embryology, these facts prove, (1) that the electric organ
of _R. radiata_ must be one of the very latest products of the
animal's phylogeny; and, (2) that as yet, at all events, it has not
begun to degenerate. But, if not, it must either be at a stand-still, or
it must be in course of further evolution; and, whichever of these
alternatives we adopt, the difficulty of accounting for its present
condition remains. In this connexion also it is worth while to remark
that the electric organ, even after it has attained its full
_development_, continues its _growth_ with the growth of the fish, and
this in a much higher ratio, either than the tail alone, or the whole
animal. Lastly, Prof. Burdon Sanderson finds that _section for section_
the organ in the skate is as efficient as it is in _Torpedo_. It is
evident that these facts also point to the skate's organ being in course
of phylogenetic evolution.
[Illustration: Fig. 118.--_Raia radiata_, representing the life size
of the youngest individual in which muscle fibres have been found
developing into electric cells].
[Illustration: Fig. 119.--Electric organ of the skate. The left-hand
drawing (i) represents the entire organ (natural size) of a
full-grown _r. radiata_. This is a small skate, which rarely exceeds
50 centms. in length; but in the large _r. batis_, the organ may
exceed two feet in length. The other drawings represent single
muscle-fibres in successive stages of transition. In the first of
the series (ii) the motor plate, and the nerves connected with it,
have already been considerably enlarged. In the other three
specimens, the fibre becomes more and more club-like, and eventually
cup-like. These changes of shape are expressive of great changes of
structure, as may be seen in the last of the series (v), where the
shallow cup is seen in partial section. The electric plate lines the
concavity of the cup, and is richly supplied with nerves (only a few
of which are represented in the last drawing); the thick walls of
the cup are composed of muscular fibres, the striation of which is
distinctly visible.]
[Illustration: Fig. 120.--Electric cells of _raia radiata_. The
drawing on the left represents one of the clubs magnified, as in the
preceding wood-cut. The drawing on the right represents a number of
these clubs, less highly magnified, _in situ_.]
Again, it cannot be answered that the principle of correlation may be
drawn upon in mitigation of the difficulty. The structure of the
electric organ is far too elaborate, far too specialized, and far too
obviously directed to a particular end, to admit of our conceivably
supposing it due to any accidental correlation with structural changes
going on elsewhere. Even as regards the initial changes of
muscle-elements into electrical-elements, I do not think the principle
of correlation can be reasonably adduced by way of explanation; for, as
shown in the illustrations, even this initial change is most
extraordinarily peculiar, elaborate, and specialized. But, be this as it
may, I am perfectly certain that the principle of correlation cannot
possibly be adduced to explain the subsequent _association of these
electrical elements into an electric battery_, actuated by a special
nervous mechanism of enormous size and elaboration--unless of course,
the progress of such a structure were assumed to have been throughout of
some utility. Under this supposition, however, the principle of
correlation would be forsaken in favour of that of natural selection;
and we should again be in the presence of the same difficulty as that
with which we started.
But now, and further, if we do thus abandon correlation in favour of
natural selection, and therefore if for the sake of saving an hypothesis
we assume that the organ as it now stands _must_ be of some use to the
existing skate, we should still have to face the question--Of what
conceivable use can those initial stages of its formation have been,
when first the muscle-elements began to be changed into the very
different electrical-elements, and when therefore they became useless as
muscles while not yet capable of performing even so much of the
electrical function as they now perform?
Lastly, we must remember that not only have we here the most highly
specialized, the most complex, and altogether the most elaboratively
adaptive organ in the animal kingdom; but also that in the formation of
this structure there has been needed an altogether unparalleled
expenditure of the most physiologically expensive of all
materials--namely, nervous tissue. Whether estimated by volume or by
weight, the quantity of nervous tissue which is consumed in the electric
organ of the skate is in excess of all the rest of the nervous system
put together. It is needless to say that nowhere else in the animal
kingdom--except, of course, in other electric fishes--is there any
approach to so enormous a development of nervous tissue for the
discharge of a special function. Therefore, as nervous tissue is,
physiologically speaking, the most valuable of all materials, we are
forced to conclude that natural selection ought strongly to have
_opposed_ the evolution of such organs, unless from the first moment of
their inception, and throughout the whole course of their development,
they were of some such paramount importance as biologically to justify
so unexampled an expenditure. Yet this paramount importance does not
admit of being so much as surmised, even where the organ has already
attained the size and degree of elaboration which it presents in the
skate.
In view of all these considerations taken together, I freely confess
that the difficulty presented by this case appears to me of a magnitude
and importance altogether unequalled by that of any other single
case--or any series of cases--which has hitherto been encountered by the
theory of natural selection. So that, if there were many other cases of
the like kind to be met with in nature, I should myself at once allow
that the theory of natural selection would have to be discarded. But
inasmuch as this particular case stands so far entirely by itself, and
therefore out of analogy with thousands, or even millions, of other
cases throughout the whole range of organic nature, I am constrained to
feel it more probable that the electric organ of the skate will some day
admit of being marshalled under the general law of natural selection--in
just the same way as proved to be the case with the conspicuous
colouring of those caterpillars, which, as explained in the last
chapter, at one time seemed to constitute a serious difficulty to the
theory, and yet, through a better knowledge of all the relations
involved, has now come to constitute one of the strongest witnesses in
its favour.
* * * * *
I have now stated all the objections of any importance which have
hitherto been brought against the theory of natural selection, excepting
three, which I left to be dealt with together because they form a
logically connected group. With a brief consideration of these,
therefore, I will bring this chapter to a close.
The three objections to which I allude are, (1) that a large
proportional number of specific, as well as of higher taxonomic
characters, are seemingly useless characters, and therefore do not lend
themselves to explanation by the Darwinian theory; (2) that the most
general of all specific characters--viz. cross-infertility between
allied species--cannot possibly be due to natural selection, as is
demonstrated by Darwin himself; (3) that the swamping effects of free
intercrossing must always render impossible by natural selection alone
any evolution of species in divergent (as distinguished from serial)
lines of change.
These three objections have been urged from time to time by not a few of
the most eminent botanists and zoologists of our century; and from one
point of view I cannot myself have the smallest doubt that the
objections thus advanced are not only valid in themselves, but also by
far the most formidable objections which the theory of natural selection
has encountered. From another point of view, however, I am equally
convinced that they all admit of absolute annihilation. This strong
antithesis arises, as I have said, from differences of standpoint, or
from differences in the view which we take of the theory of natural
selection itself. If we understand this theory to set forth natural
selection as the sole cause of organic evolution, then all the above
objections to the theory are not merely, as already stated, valid and
formidable, but as I will now add, logically insurmountable. On the
other hand, if we take theory to consist merely in setting forth natural
selection as a factor of organic evolution, even although we believe it
to have been the chief factor or principal cause, all the three
objections in question necessarily vanish. For in this case, even if it
be satisfactorily proved that the theory of natural selection is unable
to explain the three classes of facts above mentioned, the theory is not
thereby affected: facts of each and all of these classes may be
consistently left by the theory to be explained by causes other than
natural selection--whether these be so far capable or incapable of
hypothetical formulation. Thus it is evident that whether the three
objections above named are to be regarded as logically insurmountable by
the theory, or as logically non-existent in respect to it, depends
simply upon the manner in which the theory itself is stated.
In the next volume a great deal more will have to be said upon these
matters--especially with regard to the causes other than natural
selection which in my opinion are capable of explaining these so-called
"difficulties." In the present connexion, however, all I have attempted
to show is, that, whatever may be thought touching the supplementary
theories whereby I shall endeavour to explain the facts of inutility,
cross-sterility, and non-occurrence of free intercrossing, no one of
these facts is entitled to rank as an objection against the theory of
natural selection, unless we understand this theory to claim an
exclusive prerogative in the field of organic evolution. This, as we
have previously seen, is what Mr. Wallace does claim for it; while on
the other hand, Mr. Darwin expressly--and even vehemently--repudiates
the claim: from which it follows that all the three main objections
against the theory of natural selection are objections which vitally
affect the theory only as it has been stated and upheld by Wallace. As
the theory has been stated and upheld by Darwin, all these objections
are irrelevant. This is a fact which I had not myself perceived at the
time when I mentioned these objections in a paper entitled
_Physiological Selection_, which was published in 1886. The discussions
to which that paper gave rise, however, led me to consider these matters
more closely; and further study of Darwin's writings, with these matters
specially in view, has led me to see that none of the objections in
question are relevant to his theory, as distinguished from that of Mr.
Wallace. This, I acknowledge, I ought to have perceived before I
published the paper just alluded to; but in those days I had had no
occasion to follow out the differences between Darwin and Wallace to all
their consequences, and therefore adopted the prevalent view that their
theories of evolution were virtually identical. Now, however, I have
endeavoured to make it clear that the points wherein they differ involve
the important consequences above set forth. All these the most
formidable objections against the theory of natural selection arise
simply and solely from what I conceive to be the erroneous manner in
which the theory has been presented by Darwin's distinguished colleague.
* * * * *
I have now considered, as impartially as I can, all the main criticisms
and objections which have been brought against the theory of natural
selection; and the result is to show that, neither singly nor
collectively, are they entitled to much weight. On the other hand, as
we have seen in the preceding chapter, there is a vast accumulation of
evidence in favour of the theory. Hence, it is no wonder that the theory
has now been accepted by all naturalists, with scarcely any one notable
exception, as at any rate the best working hypothesis which has ever
been propounded whereby to explain the facts of organic evolution.
Moreover, in the opinion of those most competent to judge, the theory is
entitled to be regarded as something very much more than a working
hypothesis: it is held to be virtually a completed induction, or, in
other words, the proved exhibition of a general law, whereby the
causation of organic evolution admits of being in large part--if not
altogether--explained.
Now, whether or not we subscribe to this latter conclusion ought, I
think, to depend upon what we mean by an explanation in the case which
is before us. If we mean only that, given the large class of known facts
and unknown causes which are conveniently summarized under the terms
Heredity and Variability, then the further facts of Struggle and
Survival serve, in some considerable degree or another, to account for
the phenomena of adaptive evolution, I cannot see any room to question
that the evidence is sufficient to prove the statement. But it is clear
that by taking for granted these great facts of Heredity and
Variability, we have assumed the larger part of the problem as a whole.
Or, more correctly, by thus generalizing, in a merely verbal form, all
the unknown causes which are concerned in these two great factors of the
process in question, we are not so much as attempting to explain the
precedent causation which serves as a condition to the process. Much
more than half the battle would already have been won, had Darwin's
predecessors been able to explain the causes of Heredity and Variation;
hence it is but a very partial victory which we have hitherto gained in
our recent discovery of the effects of Struggle and Survival.
Yet partial though it be in relation to the whole battle, in itself, or
considered absolutely, there can be no reasonable doubt that it
constitutes the greatest single victory which has ever been gained by
the science of Biology. For this very reason, however, it behoves us to
consider all the more carefully the extent to which it goes. But my
discussion of this matter must be relegated to the next volume, where I
hope to give abundant proof of the soundness of Darwin's judgment as
conveyed in the words:--"I am convinced that natural selection has been
the main, but not the exclusive, means of modification."
CHAPTER X.
THE THEORY OF SEXUAL SELECTION, AND CONCLUDING REMARKS.
Although the explanatory value of the Darwinian theory of natural
selection is, as we have now seen, incalculably great, it nevertheless
does not meet those phenomena of organic nature which perhaps more than
any other attract the general attention, as well as the general
admiration, of mankind: I mean all that class of phenomena which go to
constitute the Beautiful. Whatever value beauty as such may have, it
clearly has not a life-preserving value. The gorgeous plumage of a
peacock, for instance, is of no advantage to the peacock in his struggle
for life, and therefore cannot be attributed to the agency of natural
selection. Now this fact of beauty in organic structures is a fact of
wide generality--almost as wide, indeed, as is the fact of their
utility. Mr. Darwin, therefore, suggested another hypothesis whereby to
render a scientific explanation of this fact. Just as by his theory of
natural selection he sought to explain the major fact of utility, so did
he endeavour to explain the minor fact of beauty by a theory of what he
termed Sexual Selection.
It is a matter of observation that the higher animals do not pair
indiscriminately; but that the members of either sex prefer those
individuals of the opposite sex which are to them most attractive. It is
important to understand _in limine_ that nobody has ever attempted to
challenge this statement. In other words, it is an unquestionable fact
that among many of the higher animals there literally and habitually
occurs a _sexual selection_; and this fact is not a matter of inference,
but, as I have said, a matter of observation. The inference only begins
where, from this observable fact, it is argued,--1st, that the sexual
selection has reference to an æsthetic taste on the part of the animals
themselves; and 2nd, that, supposing the selection to be determined by
such a taste, the cause thus given is adequate to explain the phenomena
of beauty which are presented by these animals. I will consider these
two points separately.
From the evidence which Darwin has collected, it appears to me
impossible to doubt that an æsthetic sense is displayed by many birds,
and not a few mammals. This of course does not necessarily imply that
the _standards_ of such a sense are the same as our own; nor does it
necessarily imply that there is any constant relation between such a
sense and high levels of intelligence in other respects. In point of
fact, such is certainly not the case, because the best evidence that we
have of an æsthetic sense in animals is derived from birds, and not from
mammals. The most cogent cases to quote in this connexion are those of
the numerous species of birds which habitually adorn their nests with
gaily coloured feathers, wool, cotton, or any other gaudy materials
which they may find lying about the woods and fields. In many cases a
marked preference is shown for particular objects--as, for instance, in
the case of the Syrian nut-hatch, which chooses the iridescent wings of
insects, or that of the great crested fly-catcher, which similarly
chooses the cast-off skins of snakes. But no doubt the most remarkable
of these cases is that of the baya-bird of Asia, which after having
completed its bottle-shaped and chambered nest[47], studs it over with
small lumps of clay, both inside and out, upon which the cock-bird
sticks fire-flies, apparently for the sole purpose of securing a
brilliantly decorative effect. Other birds, such as the hammer-head of
Africa, adorn the surroundings of their nests (which are built upon the
ground) with shells, bones, pieces of broken glass and earthenware, or
any objects of a bright and conspicuous character which they may happen
to find. The most consummate artists in this respect are, however, the
bower-birds; for the species of this family construct elaborate
play-houses in the form of arched tunnels, built of twigs upon the
ground. Through and around such a tunnel they chase one another; and it
is always observable that not only is the floor paved with a great
collection of shells, bones, coloured stones, and any other brilliant
objects which they are able to carry in their beaks, but also that the
walls are decorated with the most gaudy articles which the birds can
find. There is one genus, in Papua, which even goes so far as to provide
the theatre with a surrounding garden. A level piece of ground is
selected as a site for the building. The latter is about two feet high,
and constructed round the growing stalk of a shrub, which therefore
serves as a central pillar to which the frame-work of the roof is
attached. Twigs are woven into this frame-work until the whole is
rendered rain-proof. The tent thus erected is about nine feet in
circumference at its base, and presents a large arch as an entrance. The
central pillar is banked up with moss at its base, and a gallery is
built round the interior of the edifice. This gallery is decorated with
flowers, fruits, fungi, &c. These are also spread over the garden, which
covers about the same area as the play-house. The flowers are said to
be removed when they fade, while fresh ones are gathered to supply their
places. Thus the garden is always kept bright with flowers, as well as
with the brilliant green of mosses, which are collected and distributed
in patches, resembling tiny lawns.
[47] The chambers are three in number. The two upper ones are
occupied respectively by the male and the sitting female. The lower
one serves as a general living room when the young are hatched.
[Illustration: FIG. 121.--The Garden Bower-bird (_Amblyornis
inornata_). Reduced from _Gould's Birds of New Guinea_ to 1/4 nat.
size.]
Now these sundry cases alone seem to prove a high degree of the æsthetic
sense as occurring among birds; for, it is needless to say, none of the
facts just mentioned can be due to natural selection, seeing that they
have no reference to utility, or the preservation of life. But if an
æsthetic sense occurs in birds, we should expect, on _a priori_ grounds,
that it would probably be exercised with reference to the personal
appearance of the sexes. And this expectation is fully realized. For it
is an observable fact that in most species of birds where the males are
remarkable for the brilliancy of their plumage, not only is this
brilliancy most remarkable during the pairing season, but at this season
also the male birds take elaborate pains to display their charms before
the females. Then it is that the peacock erects his tail to strut round
and round the hens, taking care always to present to them a front view,
where the coloration is most gorgeous. And the same is true of all other
gaily coloured male birds. During the pairing season they actively
compete with one another in exhibiting their attractiveness to the
females; and in many cases there are added all sorts of extraordinary
antics in the way of dancings and crowings. Again, in the case of all
song-birds, the object of the singing is to please the females; and for
this purpose the males rival one another to the best of their musical
ability.
Thus there can be no question that the courtship of birds is a highly
elaborate business, in which the males do their best to surpass one
another in charming the females. Obviously the inference is that the
males do not take all this trouble for nothing; but that the females
give their consent to pair with the males whose personal appearance, or
whose voice, proves to be the most attractive. But, if so, the young of
the male bird who is thus _selected_ will inherit his superior beauty;
and thus, in successive generations, a continuous advance will be made
in the beauty of plumage or of song, as the case may be,--both the
origin and development of beauty in the animal world being thus supposed
due to the æsthetic taste of animals themselves.
Such is the theory of sexual selection in its main outlines; and with
regard to it we must begin by noting two things which are of most
importance. In the first place, it is a theory wholly and completely
distinct from the theory of natural selection; so that any truth or
error in the one does not in the least affect the other. The second
point is, that there is not so great a wealth of evidence in favour of
sexual selection as there is in favour of natural selection; and,
therefore, that while all naturalists nowadays accept natural selection
as _a_ (whether or not _the_) cause of adaptive, useful, or
life-preserving structures, there is no such universal--but only a very
general--agreement with reference to sexual selection as a cause of
decorative, beautiful, or life-embellishing structures. Nevertheless,
the evidence in favour of sexual selection is both large in amount and
massive in weight.
Our consideration of this evidence will bring us to the second division
of our subject, as previously marked out for discussion--namely,
granting that an æsthetic sense occurs in certain large divisions of the
animal kingdom, what is the proof that such a sense is a cause of the
beauty which is presented by the animals in question?
Before proceeding to state this proof, however, it is desirable to
observe that under the theory of sexual selection Darwin has included
two essentially different classes of facts. For besides the large class
of facts to which I have thus far been alluding,--i. e. the cases where
two sexes of the same species differ from one another in respect of
ornamentation,--there is another class of facts equally important,
namely, the cases where the two sexes of the same species differ from
one another in respect of size, strength, and the possession of natural
weapons, such as spurs, horns, &c. In most of these cases it is the
males which are thus superiorly endowed; and it is a matter of
observation that in all cases where they are so endowed they use their
superior strength and natural weapons for fighting together, in order to
secure possession of the females. Hence results what Mr. Darwin has
called the Law of Battle between males of the same species; and this law
of battle he includes under his theory of sexual selection. But it is
evident that the principle which is operative in the law of battle
differs from the principle which is concerned in the form of sexual
selection that has to do with embellishment, and consequent charm. The
law of battle, in fact, more nearly approaches the law of natural
selection; seeing that it expresses the natural advantages of brute
force in the struggling of rival animals, and so frequently results in
_death of the less fitted_, as distinguished from a mere failure to
propagate. Now against this doctrine of the law of battle, and the
consequences to which it leads in the superior fighting powers of male
animals, no objection has been raised in any quarter. It is only with
regard to the other aspect of the theory of sexual selection--or that
which is concerned with the superior embellishment of male animals--that
any difference of opinion obtains. I will now proceed to give the main
arguments on both sides of this question, beginning with a _résumé_ of
the evidences in favour of sexual selection.
In the first place, the fact that secondary sexual characters of the
embellishing kind are so generally restricted to the male sex in itself
seems to constitute very cogent proof that, in some way or another, such
characters are connected with the part which is played by the male in
the act of propagation. Moreover, secondary sexual characters of this
kind are of quite as general occurrence as are those of the other kind
which have to do with rivalry in battle; and the former are usually of
the more elaborate description. Therefore, as there is no doubt that
secondary sexual characters of the one order have an immediate purpose
to serve in the act of propagation, we are by this close analogy
confirmed in our surmise that secondary sexual characters of the other,
and still more elaborate, order are likewise so concerned. Moreover,
this view of their meaning becomes still further strengthened when we
take into consideration the following facts. Namely, (_a_) secondary
sexual characters of the embellishing kind are, as a rule, developed
only at maturity; and most frequently during only a part of the year,
which is invariably the breeding season: (_b_) they are always more or
less seriously affected by emasculation: (_c_) they are always, and
only, displayed in perfection during the act of courtship: (_d_) then,
however, they are displayed with the most elaborate pains; yet always,
and only, before the females: (_e_) they appear, at all events in many
cases, to have the effect of charming the females into a performance of
the sexual act; while it is certain that in many cases, both among
quadrupeds and birds, individuals of the one sex are capable of feeling
a strong antipathy against, or a strong preference for, certain
individuals of the opposite sex.
Such are the main lines of evidence in favour of the theory of sexual
selection. And although it is enough that some of them should be merely
stated as above in order that their immense significance should become
apparent, in the case of others a bare statement is not sufficient for
this purpose. More especially is this the case as regards the enormous
profusion, variety, and elaboration of sexually-embellishing characters
which occur in birds and mammals--not to mention several divisions of
Arthropoda; together with the extraordinary amount of trouble which, in
a no less extraordinary number of different ways, is taken by the male
animals to display their embellishments before the females. And even in
many cases where to our eyes there is no particular embellishment to
display, the process of courtship consists in such an elaborate
performance of dancings, struttings, and attitudinizings that it is
scarcely possible to doubt their object is to incite the opposite sex.
Here, for instance, is a series of drawings illustrating the courtship
of spiders. I choose this case as an example, partly because it is the
one which has been published most recently, and partly because it is of
particular interest as occurring so low down in the zoological scale. I
am indebted to the kindness of Mr. and Mrs. Peckham for permission to
reproduce these few selected drawings from their very admirable work,
which is published by the Natural History Society of Wisconsin, U.S. It
is evident at a glance that all these elaborate, and to our eyes
ludicrous, performances are more suggestive of incitation than of any
other imaginable purpose. And this view of the matter is strongly
corroborated by the fact that it is the most brightly coloured parts of
the male spiders which are most obtruded upon the notice of the female
by these peculiar attitudes--in just the same way as is invariably the
case in the analogous phenomena of courtship among birds, insects, &c.
[Illustration: FIG. 122.--Courtship of Spiders. A few examples of
some of the attitudes adopted by different species of males when
approaching their females. (After Peckham.)]
[Illustration: FIG. 123.--Courtship of Spiders. Continued from Fig.
122, similarly showing some of the attitudes of approach adopted by
males of yet other different species. (After Peckham.)]
But so great is the mass of material which Darwin has collected in proof
of all the points mentioned in the foregoing paragraph, that to attempt
anything in the way of an epitome would really be to damage its
evidential force. Therefore I deem it best simply to refer to it as it
stands in his _Descent of Man_, concluding, as he concludes,--"This
surprising uniformity in the laws regulating the differences between the
sexes in so many and such widely separated classes is intelligible if we
admit the action throughout all the higher divisions of the animal
kingdom of one common cause, namely, sexual selection"; while, as he
might well have added, it is difficult to imagine that all the large
classes of facts which an admission of this common cause serves to
explain, can ever admit of being rendered intelligible by any other
theory.
We may next proceed to consider the objections which have been brought
against the theory of sexual selection. And this is virtually the same
thing as saying that we may now consider Mr. Wallace's views upon the
subject.
Reserving for subsequent consideration the most general of these
objections--namely, that at best the theory can only apply to the more
intelligent animals, and so must necessarily fail to explain the
phenomena of beauty in the less intelligent, or in the non-intelligent,
as well as in all species of plants--we may take _seriatim_ the other
objections which, in the opinion of Mr. Wallace, are sufficient to
dispose of the theory even as regards the higher animals.
In the first place, he argues that the principal cause of the greater
brilliancy of male animals in general, and of male birds in particular,
is that they do not so much stand in need of protection arising from
concealment as is the case with their respective females. Consequently
natural selection is not so active in repressing brilliancy of colour in
the males, or, which amounts to the same thing, is more active in
"repressing in the female those bright colours which are normally
produced in both sexes by general laws."
Next, he argues that not only does natural selection thus exercise a
negative influence in passively permitting more heightened colour to
appear in the males, but even exercises a positive influence in actively
promoting its development in the males, while, at the same time,
actively repressing its appearance in the females. For heightened
colour, he says, is correlated with health and vigour; and as there can
be no doubt that healthy and vigorous birds best provide for their
young, natural selection, by always placing its premium on health and
vigour in the males, thus also incidentally promotes, through correlated
growth, their superior coloration.
Again, with regard to the display which is practised by male birds, and
which constitutes the strongest of all Mr. Darwin's arguments in favour
of sexual selection, Mr. Wallace points out that there is no evidence of
the females being in any way affected thereby. On the other hand, he
argues that this display may be due merely to general excitement; and he
lays stress upon the more special fact that moveable feathers are
habitually erected under the influence of anger and rivalry, in order to
make the bird look more formidable in the eyes of antagonists.
Furthermore, he adduces the consideration that, even if the females are
in any way affected by colour and its display on the part of the males,
and if, therefore, sexual selection be conceded a true principle in
theory, still we must remember that, as a matter of fact, it can only
operate in so far as it is allowed to operate by natural selection. Now,
according to Mr. Wallace, natural selection must wholly neutralize any
such supposed influence of sexual selection. For, unless the survivors
in the general struggle for existence happen to be those which are also
the most highly ornamented, natural selection must neutralize and
destroy any influence that may be exerted by female selection. But
obviously the chances against the otherwise best fitted males happening
to be likewise the most highly ornamented must be many to one, unless,
as Wallace supposes, there is some correlation between embellishment and
general perfection, in which case, as he points out, the theory of
sexual selection lapses altogether, and becomes but a special case of
natural selection.
Once more, Mr. Wallace argues that the evidence collected by Mr. Darwin
himself proves that each bird finds a mate under any circumstances--a
general fact which in itself must quite neutralize any effect of sexual
selection of colour or ornament, since the less highly coloured birds
would be at no disadvantage as regards the leaving of healthy progeny.
Lastly, he urges the high improbability that through thousands of
generations all the females of any particular species--possibly spread
over an enormous area--should uniformly and always have displayed
exactly the same taste with respect to every detail of colour to be
presented by the males.
Now, without any question, we have here a most powerful array of
objections against the theory of sexual selection. Each of them is ably
developed by Mr. Wallace himself in his work on _Tropical Nature_; and
although I have here space only to state them in the most abbreviated of
possible forms, I think it will be apparent how formidable these
objections appear. Unfortunately the work in which they are mainly
presented was published several years after the second edition of the
_Descent of Man_, so that Mr. Darwin never had a suitable opportunity of
replying. But, if he had had such an opportunity, as far as I can judge
it seems that his reply would have been more or less as follows.
In the first place, Mr. Wallace fails to distinguish between brilliancy
and ornamentation--or between colour as merely "heightened," and as
distinctively decorative. Yet there is obviously the greatest possible
difference between these two things. We may readily enough admit that a
mere heightening of already existing coloration is likely enough--at all
events in many cases--to accompany a general increase of vigour, and
therefore that natural selection, by promoting the latter, may also
incidentally promote the former, in cases where brilliancy is not a
source of danger. But clearly this is a widely different thing from
showing that not only a _general brilliancy of colour_, but also _the
particular disposition of colours_, in the form of ornamental patterns,
can thus be accounted for by natural selection. Indeed, it is expressly
in order to account for the occurrence of such ornamental patterns that
Mr. Darwin constructed his theory of sexual selection; and therefore, by
thus virtually ignoring the only facts which that theory endeavours to
explain, Mr. Wallace is not really criticizing the theory at all. By
representing that the theory has to do only with brilliancy of colour,
as distinguished from disposition of colours, he is going off upon a
false issue which has never really been raised[48]. Look, for example,
at a peacock's tail. No doubt it is sufficiently brilliant; but far more
remarkable than its brilliancy is its elaborate pattern on the one hand,
and its enormous size on the other. There is no conceivable reason why
mere _brilliancy of colour_, as an accidental concomitant of general
vigour, should have run into so extraordinary, so elaborate, and so
beautiful a _design of colours_. Moreover, this design is only unfolded
when the tail is erected, and the tail is not erected in battle (as Mr.
Wallace's theory of the erectile function in feathers would require),
but in courtship; obviously, therefore, the purpose of the pattern, so
to speak, is correlated with the act of courtship--it being only then,
in fact, that the general purpose of the whole structure, as well as the
more special purpose of the pattern, becomes revealed. Lastly, the fact
of this whole structure being so large, entailing not only a great
amount of physiological material in its production, but also of
physiological energy in carrying about such a weight, as well as of
increased danger from impeding locomotion and inviting capture--all this
is obviously incompatible with the supposition of the peacock's tail
having been produced by natural selection. And such a case does not
stand alone. There are multitudes of other instances of ornamental
structures imposing a drain upon the vital energies of their possessors,
without conferring any compensating benefit from a utilitarian point of
view. Now, in all these cases, without any exception, such structures
are ornamental structures which present a plain and obvious reference to
the relationship of the sexes. Therefore it becomes almost impossible to
doubt--first, that they exist for the sake of ornament; and next, that
the ornament exists on account of that relationship. If such structures
were due merely to a superabundance of energy, as Mr. Wallace supposes,
not only ought they to have been kept down by the economizing influence
of natural selection; but we can see no reason, either why they should
be so highly ornamental on the one hand, or so exclusively related to
the sexual relationship on the other.
[48] Note C.
Finally, we must take notice of the fact that where peculiar
_structures_ are concerned for purposes of display in courtship, the
_elaboration_ of these structures is often no less remarkable than that
of patterns where colours are thus concerned. Take, for example, the
case of the Bell-bird, which I select from an innumerable number of
instances that might be mentioned because, while giving a verbal
description of this animal, Darwin does not supply a pictorial
representation thereof. The bird, which lives in South America, has a
very loud and peculiar call, that can be heard at a distance of two or
three miles. The female is dusky-green; but the adult male is a
beautiful white, excepting the extraordinary structure with which we are
at present concerned. This is a tube about three inches long, which
rises from the base of the beak. It is jet black, and dotted over with
small downy feathers. The tube is closed at the top, but its cavity
communicates with the palate, and thus the whole admits of being
inflated from within, when, of course, it stands erect as represented in
one of the two drawings. When not thus inflated, it hangs down, as
shown in the second figure, which represents the plumage of a young
male. (Fig. 124.)
[Illustration: FIG. 124.--The Bell-bird (_Chasmorhynchus niveus_,
1/4 natural size). Drawn from nature (_R. Coll. Surg. Mus._). In the
drawing of the adult male the ornamental appendage is represented in
its inflated condition, during courtship; in the drawing of the
young male it is shown in its flaccid condition.]
In another species of the genus there are three of these appendages--the
two additional ones being mounted on the corners of the mouth. (Fig.
125.) In all species of the genus (four in number) the tubes are
inflated during courtship, and therefore perform the function of sexual
embellishments. Now the point to which I wish to draw attention is, that
so specialized and morphologically elaborate a structure cannot be
regarded as merely adventitious. It must have been developed by some
definite cause, acting through a long series of generations. And as no
other function can be assigned to it than that of charming the female
when it is erected in courtship, the peculiarity of form and mechanism
which it presents--like the elaboration of patterns in cases where
colour only is concerned--virtually compels us to recognise in sexual
selection the only conceivable cause of its production.
[Illustration: FIG. 125.--_C. tricarunculatus_, 1/4 natural size.
Copied from the _Ibis_. The ornamental appendages of the male are
represented in a partly inflated condition.]
For these reasons I think that Mr. Wallace's main objection falls to the
ground. Passing on to his subsidiary objections, I do not see much
weight in his merely negative difficulty as to there being an absence of
evidence upon hen birds being charmed by the plumage, or the voice, of
their consorts. For, on the one hand, it is not very safe to infer what
sentiments may be in the mind of a hen; and, on the other hand, it is
impossible to conceive what motive can be in the mind of a cock, other
than that of making himself attractive, when he performs his various
antics, displays his ornamental plumes, or sings his melodious songs.
Considerations somewhat analogous apply to the difficulty of supposing
so much similarity and constancy of taste on the part of female animals
as Mr. Darwin's theory undoubtedly requires. Although we know very
little about the psychology of the lower animals, we do observe in many
cases that small details of mental organization are often wonderfully
constant and uniform throughout all members of a species, even where it
is impossible to suggest any utility as a cause.
Again, as regards the objection that each bird finds a mate under any
circumstances, we have here an obvious begging of the whole question.
That every feathered Jack should find a feathered Jill is perhaps what
we might have antecedently expected; but when we meet with innumerable
instances of ornamental plumes, melodious songs, and the rest, as so
many witnesses to a process of sexual selection having always been in
operation, it becomes irrational to exclude such evidence on account of
our antecedent prepossessions.
There remains the objection that the principles of natural selection
must necessarily swallow up those of sexual selection. And this
consideration, I doubt not, lies at the root of all Mr. Wallace's
opposition to the supplementary theory of sexual selection. He is
self-consistent in refusing to entertain the evidence of sexual
selection, on the ground of his antecedent persuasion that in the great
drama of evolution there is no possible standing-ground for any other
actor than that which appears in the person of natural selection. But
here, again, we must refuse to allow any merely antecedent presumption
to blind our eyes to the actual evidence of other agencies having
co-operated with natural selection in producing the observed results.
And, as regards the particular case now before us, I think I have shown,
as far as space will permit, that in the phenomena of decorative
colouring (as distinguished from merely brilliant colouring), of
melodious song (as distinguished from merely tuneless cries), of
enormous arborescent antlers (as distinguished from merely offensive
weapons), and so forth--I say that in all these phenomena we have
phenomena which cannot possibly be explained by the theory of natural
selection; and, further, that if they are to be explained at all, this
can only be done, so far as we can at present see, by Mr. Darwin's
supplementary theory of sexual selection.
I have now briefly answered all Mr. Wallace's objections to this
supplementary theory, and, as previously remarked, I feel pretty
confident that, at all events in the main, the answer is such as Mr.
Darwin would himself have supplied, had there been a third edition of
his work upon the subject. At all events, be this as it may, we are
happily in possession of unquestionable evidence that he believed all
Mr. Wallace's objections to admit of fully satisfactory answers. For his
very last words to science--read only a few hours before his death at a
meeting of the Zoological Society--were:
I may perhaps be here permitted to say that, after having carefully
weighed, to the best of my ability, the various arguments which
have been advanced against the principle of sexual selection, I
remain firmly convinced of its truth[49].
[49] Since the above exposition of the theory of sexual selection
was written, Mr. Poulton has published his work on the _Colours of
Animals_. He there reproduces some of the illustrations which occur
in Mr. and Mrs. Peckham's work on _Sexual Selection in Spiders_, and
furnishes appropriate descriptions. Therefore, while retaining the
illustrations, I have withdrawn my own descriptions.
Mr. Poulton has also in his book supplied a _résumé_ of the
arguments for and against the theory of sexual selection in general.
Of course in nearly all respects this corresponds with the _résumé_
which is given in the foregoing pages; but I have left the latter as
it was originally written, because all the critical part is
reproduced _verbatim_ from a review of Mr. Wallace's _Darwinism_, of
a date still earlier than that of Mr. Poulton's book--viz.
_Contemporary Review_, August, 1889.
_Concluding Remarks._
I will now conclude this chapter, and with it the present volume, by
offering a few general remarks on what may be termed the philosophical
relations of Darwinian doctrine to the facts of adaptation on the one
hand, and to those of beauty on the other. Of course we are all aware
that before the days of this doctrine the facts of adaptation in organic
nature were taken to constitute the clearest possible evidence of
special design, on account of the wonderful mechanisms which they
everywhere displayed; while the facts of beauty were taken as
constituting no less conclusive evidence of the quality of such special
design as beneficent, not to say artistic. But now that the Darwinian
doctrine appears to have explained scientifically the former class of
facts by its theory of natural selection, and the latter class of facts
by its theory of sexual selection, we may fitly conclude this brief
exposition of the doctrine as a whole by considering what influence such
naturalistic explanations may fairly be taken to exercise upon the
older, or super-naturalistic, interpretations.
To begin with the facts of adaptation, we must first of all observe that
the Darwinian doctrine is immediately concerned with these facts only in
so far as they occur in organic nature. With the adaptations--if they
can properly be so called--which occur in all the rest of nature, and
which go to constitute the Cosmos as a whole so wondrous a spectacle of
universal law and perfect order, this doctrine is but indirectly
concerned. Nevertheless, it is of course fundamentally concerned with
them to the extent that it seeks to bring the phenomena of organic
nature into line with those of inorganic; and therefore to show that
whatever view we may severally take as to the kind of causation which is
energizing in the latter we must now extend to the former. This is
usually expressed by saying that the theory of evolution by natural
selection is a mechanical theory. It endeavours to comprise all the
facts of adaptation in organic nature under the same category of
explanation as those which occur in inorganic nature--that is to say,
under the category of physical, or ascertainable, causation. Indeed,
unless the theory has succeeded in doing this, it has not succeeded in
doing anything--beyond making a great noise in the world. If Mr. Darwin
has not discovered a new mechanical cause in the selection principle,
his labour has been worse than in vain.
Now, without unduly repeating what has already been said in Chapter
VIII, I may remark that, whatever we may each think of the measure of
success which has thus far attended the theory of natural selection in
explaining the facts of adaptation, we ought all to agree that,
considered as a matter of general reasoning, the theory does certainly
refer to a _vera causa_ of a strictly physical kind; and, therefore,
that no exception can be taken to the theory in this respect on grounds
of _logic_. If the theory in this respect is to be attacked at all, it
can only be on grounds of _fact_--namely, by arguing that the cause does
not occur in nature, or that, if it does, its importance has been
exaggerated by the theory. Even, however, if the latter proposition
should ever be proved, we may now be virtually certain that the only
result would be the relegation of all the residual phenomena of
adaptation to other causes of the physical order--whether known or
unknown. Hence, as far as the matter of _principle_ is concerned, we may
definitely conclude that the great naturalistic movement of our century
has already brought all the phenomena of adaptation in organic nature
under precisely the same category of mechanical causation, as similar
movements in previous centuries have brought all the known phenomena of
inorganic nature: the only question that remains for solution is the
strictly _scientific_ question touching the particular causes of the
mechanical order which have been at work.
So much, then, for the phenomena of adaptation. Turning next to those of
beauty, we have already seen that the theory of sexual selection stands
to these in precisely the same relation as the theory of natural
selection does to those of adaptation. In other words, it supplies a
physical explanation of them; because, as far as our present purposes
are concerned, it may be taken for granted, or for the sake of argument,
that inasmuch as psychological elements enter into the question the
cerebral basis which they demand involves a physical side.
There is, moreover, this further point of resemblance between the two
theories: neither of them has any reference to inorganic nature.
Therefore, with the charm or the loveliness of landscapes, of earth and
sea and sky, of pebbles, crystals, and so forth, we have at present
nothing to do. How it is that so many inanimate objects are invested
with beauty--why it is that beauty attaches to architecture, music,
poetry, and many other things--these are questions which do not
specially concern the biologist. If they are ever to receive any
satisfactory explanation in terms of natural causation, this must be
furnished at the hands of the psychologist. It may be possible for him
to show, more satisfactorily than hitherto, that all beauty, whenever
and wherever it occurs, is literally "in the eyes of the beholder"; or
that objectively considered, there is no such thing as beauty. It may
be--and in my opinion it probably is--purely an affair of the percipient
mind itself, depending on the association of ideas with pleasure-giving
objects. This association may well lead to a liking for such objects,
and so to the formation of what is known as æsthetic feeling with regard
to them. Moreover, beauty of inanimate nature must be an affair of the
percipient mind itself, unless there be a creating intelligence with
organs of sense and ideals of beauty similar to our own. And, apart from
any deeper considerations, this latter possibility is scarcely entitled
to be regarded as a probability, looking to the immense diversities in
those ideals among different races of mankind. But, be this as it may,
the scientific problem which is presented by the fact of æsthetic
feeling, even if it is ever to be satisfactorily solved, is a problem
which, as already remarked, must be dealt with by psychologists. As
biologists we have simply to accept this feeling as a fact, and to
consider how, out of such a feeling as a cause, the beauty of organic
nature may have followed as an effect.
Now we have already seen how the theory of sexual selection supposes
this to have happened. But against this theory a formidable objection
arises, and one which I have thought it best to reserve for treatment in
this place, because it serves to show the principal difference between
Mr. Darwin's two great generalizations, considered as generalizations in
the way of mechanical theory. For while the theory of natural selection
extends equally throughout the whole range of organic nature, the theory
of sexual selection has but a comparatively restricted scope, which,
moreover, is but vaguely defined. For it is obvious that the theory can
only apply to living organisms which are sufficiently intelligent to
admit of our reasonably accrediting them with æsthetic taste--namely, in
effect, the higher animals. And just as this consideration greatly
restricts the possible scope of the theory, as compared with that of
natural selection, so does it render undefined the zoological limits
within which it can be reasonably employed. Lastly, this necessarily
undefined, and yet most important limitation exposes the theory to the
objection just alluded to, and which I shall now mention.
The theory, as we have just seen, is necessarily restricted in its
application to the higher animals. Yet the facts which it is designed to
explain are not thus restricted. For beauty is by no means restricted to
the higher animals. The whole of the vegetable world, and the whole of
the animal world at least as high up in the scale as the insects, must
be taken as incapable of æsthetic feeling. Therefore, the extreme
beauty of flowers, sea-anemones, corals, and so forth, cannot possibly
be ascribed to sexual selection.
Now, with regard to this difficulty, we must begin by excluding the case
of the vegetable kingdom as irrelevant. For it has been rendered highly
probable--if not actually proved--by Darwin and others, that the beauty
of flowers and of fruits is in large part due to natural selection. It
is to the advantage of flowering plants that their organs of
fructification should be rendered conspicuous--and in many cases also
odoriferous,--in order to attract the insects on which the process of
fertilization depends. Similarly, it is to the advantage of all plants
which have brightly coloured fruits that these should be conspicuous for
the purpose of attracting birds, which eat the fruits and so disseminate
the seed. Hence all the gay colours and varied forms, both of flowers
and fruits, have been thus adequately explained as due to natural
causes, working for the welfare, as distinguished from the beauty, of
the plants. For even the distribution of colours on flowers, or the
beautiful patterns which so many of them present, are found to be useful
in guiding insects to the organs of fructification.
Again, the green colouring of leaves, which lends so much beauty to the
vegetable world, has likewise been shown to be of vital importance to
the physiology of plant-life; and, therefore, may also be ascribed to
natural selection. Thus, there remains only the forms of plants other
than the flowers. But the forms of leaves have also in many cases been
shown to be governed by principles of utility; and the same is to be
said of the branching structure which is so characteristic of trees and
shrubs, since this is the form most effectual for spreading out the
leaves to the light and air. Here, then, we likewise find that the cause
determining plant beauty is natural selection; and so we may conclude
that the only reason why the forms of trees which are thus determined by
utility appeal to us as beautiful, is because we are accustomed to these
the most ordinary forms. Our ideas having been always, as it were,
moulded upon these forms, æsthetic feeling becomes attached to them by
the principle of association. At any rate, it is certain that when we
contemplate almost any forms of plant-structure which, for special
reasons of utility, differ widely from these (to us) more habitual
forms, the result is not suggestive of beauty. Many of the tropical and
un-tree-like plants--such as the cactus tribe--strike us as odd and
quaint, not as beautiful. Be this however as it may, I trust I have said
enough to prove that in the vegetable world, at all events, the
attainment of beauty cannot be held to have been an object aimed at, so
to speak, for its own sake. Even if, for the purposes of argument, we
were to suppose that all the forms and colours in the vegetable world
are due to special design, there could be no doubt that the purpose of
this design has been in chief part a utilitarian purpose; it has not
aimed at beauty exclusively for its own sake. For most of such beauty as
we here perceive is plainly due to the means adopted for the attainment
of life-preserving ends, which, of course, is a metaphorical way of
saying that it is probably due to natural selection[50].
[50] The beauty of autumnal tints in fading leaves may possibly be
adduced _per contra_. But here we have to remember that it is only
some kinds of leaves which thus become beautiful when fading, while,
even as regards those that do, it is not remarkable that their
chlorophyll should, as it were, accidentally assume brilliant tints
while breaking down into lower grades of chemical constitution. The
case, in fact, is exactly parallel to those in the animal kingdom
which are considered in the ensuing paragraphs.
Turning, then, to the animal kingdom below the level of insects, here we
are bound to confess that the beauty which so often meets us cannot
reasonably be ascribed either to natural or to sexual selection. Not to
sexual selection for the reasons already given; the animals in question
are neither sufficiently intelligent to possess any æsthetic taste, nor,
as a matter of fact, do we observe that they exercise any choice in
pairing. Not to natural selection, because we cannot here, as in the
case of vegetables, point to any benefit as generally arising from
bright colours and beautiful forms. On the principles of naturalism,
therefore, we are driven to conclude that the beauty here is purely
adventitious, or accidental. Nor need we be afraid to make this
admission, if only we take a sufficiently wide view of the facts. For,
when we do take such a view, we find that beauty here is by no means of
invariable, or even of general, occurrence. There is no loveliness about
an oyster or a lob-worm; parasites, as a rule, are positively ugly, and
they constitute a good half of all animal species. The truth seems to
be, when we look attentively at the matter, that in all cases where
beauty does occur in these lower forms of animal life, its presence is
owing to one of two things--either to the radiate form, or to the bright
tints. Now, seeing that the radiate form is of such general occurrence
among these lower animals--appearing over and over again, with the
utmost insistence, even among groups widely separated from one another
by the latest results of scientific classification--seeing this, it
becomes impossible to doubt that the radiate form is due to some
morphological reasons of wide generality. Whether these reasons be
connected with the internal laws of growth, or to the external
conditions of environment, I do not pretend to suggest. But I feel safe
in saying that it cannot possibly be due to any design to secure beauty
for its own sake. The very generality of the radiate form is in itself
enough to suggest that it must have some physical, as distinguished from
an æsthetic, explanation; for, if the attainment of beauty had here been
the object, surely it might have been even more effectually accomplished
by adopting a greater variety of typical forms--as, for instance, in the
case of flowers.
Coming then, lastly, to the case of brilliant tints in the lower
animals, Mr. Darwin has soundly argued that there is nothing forced or
improbable in the supposition that organic compounds, presenting as they
do such highly complex and such varied chemical constitutions, should
often present brilliant colouring _incidentally_. Considered merely as
colouring, there is nothing in the world more magnificent than arterial
blood; yet here the colouring is of purely utilitarian significance. It
is of the first importance in the chemistry of respiration; but is
surely without any meaning from an æsthetic point of view. For the
colour of the cheeks, and of the flesh generally, in the _white_ races
of mankind, could have been produced quite as effectually by the use of
pigment--as in the case of certain monkeys. Now the fact that in the
case of blood, as in that of many other highly coloured fluids and
solids throughout the animal kingdom, the colour is _concealed_, is
surely sufficient proof that the colour, if regarded from an æsthetic
point of view, is accidental. Therefore, when, as in other cases, such
colouring occurs upon the surface, and thus becomes apparent, are we not
irresistibly led to conclude that its _exhibition_ in such cases is
likewise accidental, so far as any question of æsthetic design is
concerned?
I have now briefly glanced at all the main facts of organic nature with
reference to beauty; and, as a result, I think it is impossible to
resist the general conclusion, that in organic nature beauty does not
exist as an end _per se_. All cases where beauty can be pointed to in
organic nature are seemingly due--either to natural selection, acting
without reference to beauty, but to utility; to sexual selection, acting
with reference to the taste of animals; or else to sheer accident. And
if this general conclusion should be held to need any special
verification, is it not to be found in the numberless cases where
organic nature not only fails to be beautiful, but reveals itself as the
reverse. Not again to refer to the case of parasites, what can be more
unshapely than a hippopotamus, or more generally repulsive than a
crocodile? If it be said that these are exceptions, and that the forms
of animals as a rule are graceful, the answer--even apart from
parasites--is obvious. In all cases where the habits of life are such as
to render rapid locomotion a matter of utilitarian necessity, the
outlines of an animal _must_ be graceful--else, whether the locomotion
be terrestrial, aerial, or aquatic, it must fail to be swift. Hence it
is only in such cases as that of the hippopotamus, rhinoceros,
elephant, crocodile, and so forth, where natural selection has had no
concern in developing speed, that the accompanying accident of
gracefulness can be allowed to disappear. But if beauty in organic
nature had been in itself what may be termed an artistic object on the
part of a divine Creator, it is absurd to suggest that his design in
this matter should only have been allowed to appear where we are able to
detect other and very good reasons for its appearance.
* * * * *
Thus, whether we look to the facts of adaptation or to those of beauty,
everywhere throughout organic nature we meet with abundant evidence of
natural causation, while nowhere do we meet with any independent
evidence of supernatural design. But, having led up to this conclusion,
and having thus stated it as honestly as I can, I should like to finish
by further stating what, in my opinion is its logical bearing upon the
more fundamental tenets of religious thought.
As I have already observed at the commencement of this brief exposition,
prior to the Darwinian theory of organic evolution, the theologian was
prone to point to the realm of organic nature as furnishing a peculiarly
rich and virtually endless store of facts, all combining in their
testimony to the wisdom and the beneficence of the Deity. Innumerable
adaptations of structures to functions appeared to yield convincing
evidence in favour of design; the beauty so profusely shed by living
forms appeared to yield evidence, no less convincing, of that design as
beneficent. But both these sources of evidence have now, as it were,
been tapped at their fountain-head: the adaptation and the beauty are
alike receiving their explanation at the hands of a purely mechanical
philosophy. Nay, even the personality of man himself is assailed; and
this not only in the features which he shares with the lower animals,
but also in his god-like attributes of reason, thought, and conscience.
All nature has thus been transformed before the view of the present
generation in a manner and to an extent that has never before been
possible: and inasmuch as the change which has taken place has taken
place in the direction of naturalism, and this to the extent of
rendering the mechanical interpretation of nature universal, it is no
wonder if the religious mind has suddenly awakened to a new and a
terrible force in the words of its traditional enemy--Where is now thy
God?
This is not the place to discuss the bearings of science on
religion[51]; but I think it is a place where one may properly point out
the limits within which no such bearings obtain. Now, from what has just
been said, it will be apparent that I am not going to minimise the
change which has been wrought. On the contrary, I believe it is only
stupidity or affectation which can deny that the change in question is
more deep and broad than any single previous change in the whole history
of human thought. It is a fundamental, a cosmical, a world-transforming
change. Nevertheless, in my opinion, it is a change of a non-theistic,
as distinguished from an a-theistic, kind. It has rendered impossible
the appearance in literature of any future Paley, Bell, or Chalmers; but
it has done nothing in the way of negativing that belief in a Supreme
Being which it was the object of these authors to substantiate. If it
has demonstrated the futility of their proof, it has furnished nothing
in the way of disproof. It has shown, indeed, that their line of
argument was misjudged when they thus sought to separate organic nature
from inorganic as a theatre for the special or peculiar display of
supernatural design; but further than this it has not shown anything.
The change in question therefore, although greater in degree, is the
same in kind as all its predecessors: like all previous advances in
cosmological theory which have been wrought by the advance of science,
this latest and greatest advance has been that of revealing the
constitution of nature, or the method of causation, as everywhere the
same. But it is evident that this change, vast and to all appearance
final though it be, must end within the limits of natural causation
itself. The whole world of life and mind may now have been annexed to
that of matter and energy as together constituting one magnificent
dominion, which is everywhere subject to the same rule, or method of
government. But the ulterior and ultimate question touching the nature
of this government as mental or non-mental, personal or impersonal,
remains exactly where it was. Indeed, this is a question which cannot be
affected by _any_ advance of science, further than science has proved
herself able to dispose of erroneous arguments based upon ignorance of
nature. For while the sphere of science is necessarily restricted to
that of natural causation which it is her office to explore, the
question touching the _nature of this natural causation_ is one which as
necessarily lies without the whole sphere of such causation itself:
therefore it lies beyond any possible intrusion by science. And not only
so. But if the nature of natural causation be that of the highest order
of known existence, then, although we must evidently be incapable of
conceiving what such a Mind is, at least we seem capable of judging what
in many respects it is not. It cannot be more than one; it cannot be
limited either in space or time; it cannot be other than at least as
self-consistent as its manifestations in nature are invariable. Now,
from the latter deduction there arises a point of first-rate importance
in the present connexion. For if the so-called First Cause be
intelligent, and therefore all secondary causes but the expression of a
supreme Will, in as far as such a Will is self-consistent, the operation
of all natural causes must be uniform,--with the result that, as seen by
us, this operation must needs appear to be what we call mechanical. The
more unvarying the Will, the more unvarying must be this expression
thereof; so that, if the former be absolutely self-consistent, the
latter cannot fail to be as reasonably interpreted by the theory of
mindless necessity, as by that of ubiquitous intention. Such being, as
it appears to me, the pure logic of the matter, the proof of organic
evolution amounts to nothing more than the proof of a natural process.
What mode of being is ultimately concerned in this process--or in what
it is that this process ultimately consists--is a question upon which
science is as voiceless as speculation is vociferous.
[51] The best treatise on this subject is Prof. Le Conte's
_Evolution and its Relation to Religious Thought_ (Appleton & Co.
1888).
But, it may still be urged, surely the principle of natural selection
(with its terrible basis in the struggle for existence) and the
principle of sexual selection (with its consequence in denying beauty
to be an end in itself) demonstrate that, _if_ there be design in
nature, such design at all events cannot be beneficent. To this,
however, I should again reply that, just as touching the major question
of design itself, so as touching this minor question of the quality of
such design as beneficent, I do not see how the matter has been much
affected by a discovery of the principles before us. For we did not need
a Darwin to tell us that the whole creation groaneth and travaileth
together in pain. The most that in this connexion Darwin can fairly be
said to have done is to have estimated in a more careful and precise
manner than any of his predecessors, the range and the severity of this
travail. And if it be true that the result of what may be called his
scientific analysis of nature in respect of suffering is to have shown
the law of suffering even more severe, more ubiquitous, and more
necessary than it had ever been shown before, we must remember at the
same time how he has proved, more rigidly than was ever proved before,
that suffering is a condition to improvement--struggle for life being
the _raison d'être_ of higher life, and this not only in the physical
sphere, but also in the mental and moral.
Lastly, if it be said that the _choice_ of such a method, whereby
improvement is only secured at the cost of suffering, indicates a kind
of callousness on the part of an intelligent Being supposed to be
omnipotent, I confess that such does appear to me a legitimate
conclusion--subject, however, to the reservation that higher knowledge
might displace it. For, as far as matters are now actually presented to
the unbiased contemplation of a human mind, this provisional inference
appears to me unavoidable--namely, that if the world of sentient life be
due to an Omnipotent Designer, the aim or motive of the design must have
been that of securing a continuous advance of animal improvement,
without any regard at all to animal suffering. For I own it does not
seem to me compatible with a fair and honest exercise of our reason to
set the sum of animal happiness over against the sum of animal misery,
and then to allege that, in so far as the former tends to balance--or to
over-balance--the latter, thus far is the moral character of the design
as a whole vindicated. Even if it could be shown that the sum of
happiness in the brute creation considerably preponderates over that of
unhappiness--which is the customary argument of theistic apologists,--we
should still remain without evidence as to this state of matters having
formed any essential part of the design. On the other hand, we should
still be in possession of seemingly good evidence to the contrary. For
it is clearly a condition to progress by survival of the fittest, that
as soon as organisms become sentient selection must be exercised with
reference to sentiency; and this means that, if further progress is to
take place, states of sentiency _must_ become so organized with
reference to habitual experience of the race, that pleasures and pains
shall answer respectively to states of agreement and disagreement with
the sentient creature's environment. Those animals which found pleasure
in what was deleterious to life would not survive, while those which
found pleasure in what was beneficial to life would survive; and so
eventually, in every species of animal, states of sentiency as agreeable
or disagreeable must approximately correspond with what is good for the
species or bad for the species. Indeed, we may legitimately surmise that
the reason why sentiency (and, _a fortiori_, conscious volition) has
ever appeared upon the scene at all, has been because it
furnishes--through this continuously selected adjustment of states of
sentiency to states of the sentient organism--so admirable a means of
securing rapid, and often refined, adjustments by the organism to the
habitual conditions of its life[52]. But, if so, not only is this state
of matters a _condition_ to progress in the future; it is further, and
equally, a _consequence_ of progress in the past.
[52] See _Mental Evolution in Animals_, pp. 110-111.
However, be this as it may, from all that has gone before does it not
become apparent that pleasure or happiness on the one hand, and pain or
misery on the other, must be present in sentient nature? And so long as
they are both seen to be equally necessary under the process of
evolution by natural selection, we have clearly no more reason to regard
the pleasure than the pain as an object of the supposed design. Rather
must we see in both one and the same condition to progress under the
method of natural causation which is before us; and therefore I cannot
perceive that it makes much difference--so far as the argument for
beneficence is concerned--whether the pleasures of animals outweigh
their pains, or _vice versâ_.
Upon the whole, then, it seems to me that such evidence as we have is
against rather than in favour of the inference, that if design be
operative in animate nature it has reference to animal enjoyment or
well-being, as distinguished from animal improvement or evolution. And
if this result should be found distasteful to the religious mind--if it
be felt that there is no desire to save the evidences of design unless
they serve at the same time to testify to the nature of that design as
beneficent,--I must once more observe that the difficulty thus presented
to theism is not a difficulty of modern creation. On the contrary, it
has always constituted the fundamental difficulty with which natural
theologians have had to contend. The external world appears, in this
respect, to be at variance with our moral sense; and when the antagonism
is brought home to the religious mind, it must ever be with a shock of
terrified surprise. It has been newly brought home to us by the
generalizations of Darwin; and therefore, as I said at the beginning,
the religious thought of our generation has been more than ever
staggered by the question--Where is now thy God? But I have endeavoured
to show that the logical standing of the case has not been materially
changed; and when this cry of Reason pierces the heart of Faith, it
remains for Faith to answer now, as she has always answered before--and
answered with that trust which is at once her beauty and her
life--Verily thou art a God that hidest thyself.
_APPENDIX AND NOTES_
APPENDIX TO CHAPTER V.
ON OBJECTIONS WHICH HAVE BEEN BROUGHT AGAINST THE THEORY OF ORGANIC
EVOLUTION ON GROUNDS OF PALÆONTOLOGY.
While stating in the text, and in a necessarily general way, the
evidence which is yielded by palæontology to the theory of organic
evolution, I have been desirous of not overstating it. Therefore, in the
earlier paragraphs of the chapter, which deal with the most general
heads of such evidence, I introduced certain qualifying phrases; and I
will now give the reasons which led me to do so.
Of all the five biological sciences which have been called into
evidence--viz. those of Classification, Morphology, Embryology,
Palæontology, and Geographical Distribution--it is in the case of
palæontology alone that any important or professional opinions still
continue to be unsatisfied. Therefore, in order that justice may be done
to this line of dissent, I have thought it better to deal with the
matter in a separate Appendix, rather than to hurry it over in the text.
And, as all the difficulties or objections which have been advanced
against the theory of evolution on grounds of palæontology must vary, as
to their strength, with the estimate which is taken touching the degree
of imperfection of the geological record, I will begin by adding a few
paragraphs to what has already been said in the text upon this subject.
First, then, as to the difficulties in the way of fossils being formed
at all. We have already noticed in the text that it is only the more or
less hard parts of organisms which under any circumstances can be
fossilized; and even the hardest parts quickly disintegrate if not
protected from the weather on land, or from the water on the sea-bottom.
Moreover, as Darwin says, "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 a 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." Next, as regards littoral animals, he shows the
difficulty which they must have in becoming fossils, and gives a
striking example in several of the existing species of a sub-family of
cirripedes (_Chthamalinæ_), "which coat the rocks all over the world in
infinite numbers," yet, with the exception of one species which inhabits
deep water, no vestige of any of them has been found in any tertiary
formation, although it is known that the genus _Chthamalus_ existed
through the Chalk period. Lastly, "with respect to the terrestrial
productions which lived through the secondary and palæozoic periods, it
is superfluous to state 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 one exception; while,
"in regard to mammiferous remains, a glance at the historical table in
Lyell's Manual will bring home the truth, how accidental and rare has
been 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 palæozoic formations."
But perhaps of even more importance than all these known causes which
prevent the formation of fossils, is the existence of unknown causes
which make for the same result. For example, the Flysch-formation is a
formation of several thousand feet in thickness (as much as 6000 in some
places), and it extends for at least 300 miles from Vienna to
Switzerland; moreover, it consists of shale and sandstone. Therefore,
alike in respect of time, space, and character, it is just such a
formation as we should expect to find highly rich in fossils; yet,
"although this great mass has been most carefully searched, no fossils,
except a few vegetable remains, have been found."
So much then for the difficulty, so to speak, which nature experiences
in the manufacture of fossils. Probably not one per cent. of the species
of animals which have inhabited the earth has left a single individual
as a fossil, whereby to record its past existence.
But of even more importance than this difficulty of making fossils in
the first instance, is the difficulty of preserving them when they are
made. The vast majority of fossils have been formed under water, and a
large proportional number of these--whether the animals were marine,
terrestrial, or inhabitants of fresh water--have been formed in
sedimentary deposits either of sand, gravel, or other porous material.
Now, where such deposits have been afterwards raised into the air for
any considerable time--and this has been more or less the case with all
deposits which are available for exploration--their fossiliferous
contents will have been, as a general rule, dissolved by the percolation
of rain-water charged with carbonic acid. Similarly, sea-water has
recently been found to be a surprisingly strong solvent of calcareous
material: hence, Saturn-like, the ocean devours her own progeny as far
as shells and bones of all kinds are concerned--and this to an extent of
which we have probably no adequate conception.
Of still greater destructive influence, however, than these solvent
agencies in earth and sea, are the erosive agencies of both. Any one who
watches the pounding of the waves upon the shore; who then observes the
effect of it upon the rocks broken into shingle, and on the shingle
reduced to sand; who, looking behind him at the cliffs, sees there the
evidence of the gradual advance of this all-pulverising power--an
advance so gradual that no yard of it is accomplished until within that
yard the "white teeth" have eaten well into the "bowels of the earth";
who then reflects that this process is going on simultaneously over
hundreds of thousands of miles of coast-lines throughout the world; and
who finally extends his mental vision from space to time, by trying
dimly to imagine what this ever-roaring monster must have consumed
during the hundreds of millions of years that slowly rising and slowly
sinking continents have exposed their whole areas to her jaws; whoever
thus observes and thus reflects must be a dull man, if he does not begin
to feel that in the presence of such a destroyer as this we have no
reason to wonder at a frequent silence in the testimony of the rocks.
But although the erosive agency of the sea is thus so inconceivably
great, it is positively small if compared with erosive agencies on land.
The constant action of rain, wind, and running water, in wearing down
the surfaces of all lands into "the dust of continents to be"; the
disintegrating effects on all but the very hardest rocks of winter
frosts alternating with summer heats; the grinding power of ice in
periods of glaciation; and last, but not least, the wholesale melting up
of sedimentary formations whenever these have sunk for any considerable
distance beneath the earth's surface:--all these agencies taken together
constitute so prodigious a sum of energies combined through
immeasureable ages in their common work of destruction, that when we
try to realise what it must amount to, we can scarcely fail to wonder,
not that the geological record is highly imperfect, but that so much of
the record has survived as we find to have been the case. And, if we add
to these erosive and solvent agencies on land the erosive and solvent
agencies of the sea, we may almost begin to wonder that anything
deserving the name of a geological record is in existence at all.
That such estimates of the destructive powers of nature are not mere
matters of speculative reasoning may be amply shown by stating one
single fact, which, like so many others where the present subject is
concerned, we owe to the generalizations of Darwin. Plutonic rocks,
being those which have emerged from subterranean heat of melting
intensity, must clearly at some time or another have lain beneath the
whole thickness of sedimentary deposits, which at that time occupied any
part of the earth's surface where we now find the Plutonic rocks exposed
to view. Or, in other words, wherever we now find Plutonic rocks at the
surface of the earth, we must conclude that all the sedimentary rocks by
which they were covered when in a molten state have since been entirely
destroyed; several vertical miles of the only kinds of rocks in which
fossils can possibly occur must in all such cases have been abolished
_in toto_. Now, in many parts of the world metamorphic rocks--which have
thus gradually risen from Plutonic depths, while miles of various other
rock-formations have been removed from their now exposed surfaces--cover
immense areas, and therefore testify by their present horizontal range,
no less than by their previously vertical depth, to the enormous scale
on which a total destruction has taken place of everything that once lay
above them. For instance, the granitic region of Parime is at least
nineteen times the size of Switzerland; a similar region south of the
Amazon is probably larger than France, Spain, Italy, and Great Britain
all put together; and, more remarkable still, over the area of the
United States and Canada, granitic rocks exceed in the proportion of 19
to 12-1/2 the whole of the newer Palæozoic formations. Lastly, after
giving these examples, Darwin adds the important consideration, that "in
many regions the metamorphic and granitic rocks would be found much more
widely extended than they appear to be, if all the sedimentary beds were
removed which rest unconformably on them, and which could not have
formed part of the original mantle under which they were crystallized."
The above is a brief condensation of the already condensed statement
which Darwin has given of the imperfection of the geological record; but
I think it is enough to show, in a general way, how precarious must be
the nature of any objections to the theory of evolution which are
founded merely upon the silence of palæontology in cases where, if the
record were anything like complete, we should be entitled to expect from
it some positive information. But, as we have seen in the text,
imperfect though the record be, in as far as it furnishes positive
information at all, this is well-nigh uniformly in favour of the theory;
and therefore, even on grounds of palæontology alone, it appears to me
that Darwin is much too liberal where he concludes his discussion by
saying,--"Those who believe that the geological record is in any degree
perfect, will undoubtedly at once reject the theory." If in any measure
reasonable, such persons ought rather to examine their title to such a
belief; and even if they disregard the consensus of testimony which is
yielded by all the biological sciences to the theory of evolution, they
ought at least to hold their judgment in suspense until they shall have
not only set against the apparently negative testimony which is yielded
by geology its unquestionably positive testimony, but also well
considered the causes which may--or rather must--have so gravely
impaired the geological record.
However, be this as it may, I will now pass on to consider the
difficulties and objections which have been brought against the theory
on grounds of palæontology.
These may be classified under four heads. First, the absence of varietal
links between allied species; second, the sudden appearance of whole
groups of species--not only as genera and families, but even sometimes
as orders and classes--without any forms leading up to them; third, the
occurrence of highly organized types at much lower levels of geological
strata than an evolutionist would antecedently expect; and, fourth, the
absence of fossils of any kind lower down than the Cambrian strata.
Now all these objections depend on estimates of the imperfection of the
geological record much lower than that which is formed by Darwin.
Therefore I have arranged the objections in their order of difficulty in
this respect, or in the order that requires successively increasing
estimates of the imperfection of the record, if they are to be
successively answered.
I think that the first of them has been already answered in the text, by
showing that even a very moderate estimate of the imperfection of the
record is enough to explain why intermediate _varieties_, connecting
allied _species_, are but comparatively seldom met with. Moreover it was
shown that in some cases, where shells are concerned, remarkably
well-connected series of such varieties have been met with. And the same
applies to species and genera in certain other cases, as in the equine
family.
But no doubt a greater difficulty arises where whole groups of species
and genera, or even families and orders, appear to arise suddenly,
without anything leading up to them. Even this the second difficulty,
however, admits of being fully met, when we remember that in very many
cases it has been proved, quite apart from the theory of descent, that
superjacent formations have been separated from one another by wide
intervals of time. And even although it often happens that intermediate
deposits which are absent in one part of the world are present in
another, we have no right to assume that such is always the case.
Besides, even if it were, we should have no right further to assume that
the faunas of widely separated geographical areas were identical during
the time represented by the intermediate formation. Yet, unless they
were identical, we should not expect the fossils of the intermediate
formation, where extant, to yield evidence of what the fossils would
have been in this same formation elsewhere, had it not been there
destroyed. Now, as a matter of fact, "geological formations of each
region are almost invariably intermittent"; and although in many cases a
more or less continuous record of past forms of life can be obtained by
comparing the fossils of one region and formation with those of another
region and adjacent formations, it is evident (from what we know of the
present geographical distribution of plants and animals) that not a few
cases there must have been where the interruption of the record in one
region cannot be made good by thus interpolating the fossils of another
region. And we must remember it is by selecting the cases where this
cannot be done that the objection before us is made to appear
formidable. In other words, _unless_ whole groups of new species which
are unknown in formation A appear suddenly in formation C of one region
(X), where the intermediate formation B is absent; and _unless_ in some
other region (Y), where B is present, the fossiliferous contents of B
fail to supply the fossil ancestry of the new species in A (X); _unless_
such a state of matters is found to obtain, the objection before us has
nothing to say. But at best this is negative evidence; and, in order to
consider it fairly, we ought to set against it the cases where an
interposition of fossils found in B (Y) _does_ furnish the fossil
ancestry of what would _otherwise_ have been an abrupt appearance of
whole groups of new species in A (X). Now such cases are neither few
nor unimportant, and therefore they deprive the objection of the force
it would have had if the selected cases to the contrary were the general
rule.
In addition to these considerations, the following, some of which are of
a more special kind, appear to me so important that I will quote them
almost _in extenso_.
We continually 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 archipelagoes of Europe and the United States. We do not
make due allowance for the intervals of time which have elapsed
between our consecutive formations,--longer perhaps in many cases
than the time required for the accumulation of each formation.
These intervals will have given time for the multiplication of
species from some one parent form; and, in the succeeding
formation, such groups of species will appear as if suddenly
created.
I may here recall a remark formerly made, namely, that it might
require a long succession of ages, to adapt an organism to some new
and peculiar line of life, for instance, to fly through the air;
and consequently that the transitional form would often long remain
confined to some one region; but that, when this adaptation had
once been effected, and a few species had thus acquired a great
advantage over other organisms, a comparatively short time would be
necessary to produce many divergent forms, which would spread
rapidly and widely throughout the world....
In geological treatises, published not many years ago, mammals were
always spoken of as having abruptly come in at the commencement of
the tertiary series. And now one of the richest known accumulations
of fossil mammals belongs to the middle of the secondary series;
and true mammals have been discovered in the new red sandstone at
nearly the commencement of this great series. Cuvier used to urge
that no monkey occurred in any tertiary stratum; but now extinct
species have been discovered in India, South America, and in Europe
as far back as the miocene stage. Had it not been for the rare
accident of the preservation of footsteps in the new red sandstone
of the United States, who would have ventured to suppose that, no
less than at least thirty kinds of bird-like animals, some of
gigantic size, existed during that period? Not a fragment of bone
has been discovered in these beds. Not long ago palæontologists
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 green-sand. And still more recently that
strange bird, the Archeopteryx ... 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 number of existing and extinct tertiary
species; from the extraordinary abundance of the individuals of
many species all over the world from the Arctic regions to the
equator, inhabiting various zones of depths from the upper tidal
limits to 50 fathoms; from the perfect manner in which specimens
are preserved in the oldest tertiary beds; from the ease with which
even a fragment of a valve can be recognized; from all these
circumstances, I inferred that had sessile cirripedes existed
during the secondary periods, they would certainly have been
preserved and discovered; and as not one species had then been
discovered in beds of this age, I concluded that this great group
had been suddenly developed at the commencement of the tertiary
series. This was a sore trouble to me, adding as I thought one more
instance of the abrupt appearance of a great group of species. But
my work had hardly been published, when a skilful palæontologist,
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 sessile cirripede was a Chthamalus, a very
common, large, and ubiquitous genus, of which not one specimen has
as yet been found even in any tertiary stratum. Still more
recently, a Pyrgoma, a member of a distinct sub-family of sessile
cirripedes, has been discovered by Mr. Woodward in the upper chalk;
so that we now have abundant evidence of the existence of this
group of animals during the secondary period.
The case most frequently insisted on by palæontologists of the
apparently sudden appearance of a whole group of species, is that
of the teleostean fishes, low down, according to Agassiz, in the
Chalk period. This group includes the large majority of existing
species. But certain Jurassic and Triassic forms are now commonly
admitted to be teleostean; and even some palæozoic forms have been
thus classed by one high authority. If the teleosteans had really
appeared suddenly in the northern hemisphere, the fact would have
been highly remarkable; but it would not have formed an insuperable
difficulty, unless it could likewise have been shown that at the
same period the species were suddenly and simultaneously developed
in other quarters of the world. It is almost superfluous to remark
that hardly any fossil fish are known from south of the equator;
and by running through Pictet's Palæontology it will be seen that
very few species are known from several formations in Europe. Some
few families of fish now have a confined range; the teleostean fish
might formerly have had a similarly confined range, and after
having been largely developed in some one sea, might 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 multiplied; and here they would remain
confined, until some of the species became adapted to a cooler
climate, and were enabled to double the southern capes of Africa or
Australia, and thus reach other and distant seas.
From these considerations, from our ignorance of the geology of
other countries beyond the confines of Europe and the United
States; and from the revolution in our palæontological 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 some one barren point in Australia, and
then to discuss the number and range of its productions[53].
[53] _Origin of Species_, 282-5.
In view of all the foregoing facts and considerations, it appears to me
that the second difficulty on our list is completely answered. Indeed,
even on a moderate estimate of the imperfection of the geological
record, the wonder would have been if many cases had _not_ occurred
where groups of species present the fictitious appearance of having been
suddenly and simultaneously created in the particular formations where
their remains now happen to be observable.
Turning next to the third objection, there cannot be any question that
every here and there in the geological series animals occur of a much
higher grade zoologically than the theory of evolution would have
expected to find in the strata where they are found. At any rate,
speaking for myself, I should not have antecedently expected to meet
with such highly differentiated insects as butterflies and dragonflies
in the middle of the Secondaries: still less should I have expected to
encounter beetles, cockroaches, spiders, and May-flies in the upper and
middle Primaries--not to mention an insect and a scorpion even in the
lower. And I think the same remark applies to a whole sub-kingdom in the
case of Vertebrata. For although it is only the lowest class of the
sub-kingdom which, so far as we positively know, was represented in the
Devonian and Silurian formations, we must remember, on the one hand,
that even a cartilaginous or ganoid fish belongs to the highest
sub-kingdom of the animal series; and, on the other hand, that such
animals are thus proved to have abounded in the very lowest strata where
there is good evidence of there having been any forms of life at all.
Lastly, the fact that Marsupials occur in the Trias, coupled with the
fact that the still existing Monotremata are what may be termed animated
fossils, referring us by their lowly type of organization to some period
enormously more remote,--these facts render it practically certain that
some members of this very highest class of the highest sub-kingdom must
have existed far back in the Primaries.
These things, I say, I should not have expected to find, and I think all
other evolutionists ought to be prepared to make the same
acknowledgment. But as these things have been found, the only possible
way of accounting for them on evolutionary principles is by supposing
that the geological record is even more imperfect than we needed to
suppose in order to meet the previous objections. I cannot see, however,
why evolutionists should be afraid to make this acknowledgment. For I do
not know any reason which would lead us to suppose that there is any
common measure between the distances marked on our tables of geological
formations, and the times which those distances severally represent. Let
the reader turn to the table on page 163, and then let him say why the
30,000 feet of so-called Azoic rocks may not represent a greater
duration of time than does the thickness of all the Primary rocks above
them put together. For my own part I believe that this is probably the
case, looking to the enormous ages during which these very early
formations must have been exposed to destructive agencies of all kinds,
now at one time and now at another, in different parts of the world.
And, of course, we are without any means of surmising what ranges of
time are represented by the so-called Primeval rocks, for the simple
reason that they are non-sedimentary, and non-sedimentary rocks cannot
be expected to contain fossils.
But, it will be answered, the 30,000 feet of Azoic rocks, lying above
the Primeval, _are_ sedimentary to some extent: they are not all
completely metamorphic: yet they are all destitute of fossils. This is
the fourth and last difficulty which has to be met, and it can only be
met by the considerations which have been advanced by Lyell and Darwin.
The former says:--
The total absence of any trace of fossils has inclined many
geologists to attribute the origin of the most ancient strata to an
azoic period, or one antecedent to the existence of organic beings.
Admitting, they say, the obliteration, in some cases, of fossils by
plutonic action, we might still expect that traces of them would
oftener be found in certain ancient systems of slate, which can
scarcely be said to have assumed a crystalline structure. But in
urging this argument it seems to be forgotten that there are
stratified formations of enormous thickness, and of various ages,
some of them even of tertiary date, and which we know were formed
after the earth had become the abode of living creatures, which
are, nevertheless, in some districts, entirely destitute of all
vestiges of organic bodies[54].
[54] _Elements of Geology_, p. 587.
He then proceeds to mention sundry causes (in addition to plutonic
action) which are adequate to destroy the fossiliferous contents of
stratified rocks, and to show that these may well have produced enormous
destruction of organic remains in these oldest of known formations.
Darwin's view is that, during the vast ages of time now under
consideration, it is probable that the distribution of sea and land over
the earth's surface has not been uniformly the same, even as regards
oceans and continents. Now, if this were the case, "it might well happen
that strata which had subsided some miles nearer to the centre of the
earth, and which had been pressed on by an enormous weight of
superincumbent water, might have undergone far more metamorphic action
than strata which have always remained nearer to the surface. The
immense areas in some parts of the world, for instance in South America,
of naked metamorphic rocks, which must have been heated under great
pressure, have always seemed to me to require some special 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 condition[55]." The probability of this view
he sustains by certain general considerations, as well as particular
facts touching the geology of oceanic islands, &c.
[55] _Origin of Species_, p. 289.
On the whole, then, it seems to me but reasonable to conclude, with
regard to all four objections in question, as Darwin concludes with
regard to them:--
For my part, following out Lyell's metaphor, I look at the
geological record as a history of the world imperfectly kept,
written in a changing dialect; of this history we possess the last
volume alone, relating only to two or three countries. Of this
volume, only here and there a short chapter has been preserved; and
of each page only here and there a few lines. Each word of the
slowly-changing language, more or less different in the successive
chapters, may represent the forms of life, which are entombed in
our consecutive formations, and which falsely appear to us to have
been abruptly introduced. On this view, the difficulties above
discussed are greatly diminished, or even disappear[56].
[56] _Ibid._
As far as I can see, the only reasonable exception that can be taken to
this general view of the whole matter, is one which has been taken from
the side of astronomical physics.
Put briefly, it is alleged by one of the highest authorities in this
branch of science, that there cannot have been any such enormous reaches
of unrecorded time as would be implied by the supposition of there
having been a lost history of organic evolution before the Cambrian
period. The grounds of this allegation I am not qualified to examine;
but in a general way I agree with Prof. Huxley in feeling that, from the
very nature of the case, they are necessarily precarious,--and this in
so high a degree that any conclusions raised on such premises are not
entitled to be deemed formidable[57].
[57] See _Lay Sermons_, Lecture on Geological Reform.
* * * * *
Turning now to plants, the principal and the ablest opponent of the
theory of evolution is here unquestionably Mr. Carruthers[58]. The
difficulties which he adduces may be classified under three heads, as
follows:--
[58] See especially the following Presidential addresses:--Geol.
Assoc. Nov. 1876; Section D. Brit. Assoc., 1886; Lin. Soc., 1890.
1. There is no evidence of change in specific forms of existing plants.
Not only are the numerous species of plants which have been found in
Egyptian mummies indistinguishable from their successors of to-day; but,
what is of far more importance, a large number of our own indigenous
plants grew in Great Britain during the glacial period (including under
this term the warm periods between those of successive glaciations), and
in no one case does it appear that any modification of specific type has
occurred. This fact is particularly remarkable as regards leaves,
because on the one hand they are the organs of plants which are most
prone to vary, while on the other hand they are likewise the organs
which lend themselves most perfectly to the process of fossilization, so
that all details of their structure can be minutely observed in the
fossil state. Yet the interval since the glacial period, although not a
long one geologically speaking, is certainly what may be called an
appreciable portion of time in the history of Dicotyledonous plants
since their first appearance in the Cretaceous epoch. Again, if we
extend this kind of enquiry so as to include the world as a whole, a
number of other species of plants dating from the glacial epoch are
found to tell the same story--notwithstanding that, in the opinion of
Mr. Carruthers, they must all have undergone many changes of environment
while advancing before, and retreating after, successive glaciations in
different parts of the globe. Or, to quote his own words:--"The various
physical conditions which of necessity affected these {41} species in
their diffusion over such large areas of the earth's surface in the
course of, say, 250,000 years, should have led to the production of many
varieties; but the uniform testimony of the remains of this considerable
pre-glacial flora, as far as the materials admit of a comparison, is
that no appreciable change has taken place."
2. There is no appearance of generalized forms among the earliest plants
with which we are acquainted. For example, in the first dry land
flora--the Devonian--we have representatives of the _Filices_,
_Equisetaceæ_, and _Lycopodiaceæ_, all as highly specialized as their
living representatives, and exhibiting the differential characters of
these closely related groups. Moreover, these plants were even more
highly organized than their existing descendants in regard to their
vegetative structure, and in some cases also in regard to their
reproductive organs. So likewise the Gymnosperms of that time show in
their fossil state the same highly organized woody structure as their
living representatives.
3. Similarly, and more generally, the Dicotyledonous plants, which first
appear in the Cretaceous rocks, appear there suddenly, without any forms
leading up to them--notwithstanding that "we know very well the
extensive flora of the underlying Wealden." Moreover, we have all the
three great divisions of the Dicotyledons appearing together, and so
highly differentiated that all the species are referred to existing
genera, with the exception of a very few imperfectly preserved, and
therefore uncertain fragments.
Such being the facts, we may begin by noticing that, even at first
sight, they present different degrees of difficulty. Thus, I cannot see
that there is much difficulty with regard to those in class 2. Only if
we were to take the popular (and very erroneous) view of organic
evolution as a process which is always and everywhere bound to promote
the specialization of organic types--only then ought we to see any real
difficulty in the absence of generalized types preceding these existing
types. Of course we may wonder why still lower down in the geological
series we do not meet with more generalized (or ancestral) types; but
this is the difficulty number 3, which we now proceed to examine.
Concerning the other two difficulties, then, the only possible way of
meeting that as to the absence of any parent forms lower down in the
geological series is by falling back--as in the analogous case of
animals--upon the imperfection of the geological record. Although it is
certainly remarkable that we should not encounter any forms serving to
connect the Dicotyledonous plants of the Chalk with the lower forms of
the underlying Wealden, we must again remember that difficulties thus
depending on the absence of any corroborative record, are by no means
equivalent to what would have arisen in the presence of an adverse
record--such, for instance, as would have been exhibited had the floras
of the Wealden and the Chalk been inverted. But, as the case actually
stands, the mere fact that Dicotyledonous plants, where they first
occur, are found to have been already differentiated into their three
main divisions, is in itself sufficient evidence, on the general theory
of evolution, that there must be a break in the record as hitherto known
between the Wealden and the Chalk. Nor is it easy to see how the
opponents of this theory can prove their negative by furnishing evidence
to the contrary. And although such might justly be deemed an unfair way
of putting the matter, were this the only case where the geological
record is in evidence, it is not so when we remember that there are
numberless other cases where the geological record does testify to
connecting links in a most satisfactory manner. For in view of this
consideration the burden of proof is thrown upon those who point to
particular cases where there is thus a conspicuous absence of
transitional forms--the burden, namely, of proving that such cases are
not due merely to a break in the record. Besides, the break in the
record as regards this particular case may be apparent rather than real.
For I suppose there is no greater authority on the pure geology of the
subject than Sir Charles Lyell, and this is what he says of the
particular case in question. "If the passage seem at present to be
somewhat sudden from the flora of the Lower or Neocomian to that of the
Upper Cretaceous period, the abruptness of the change will probably
disappear when we are better acquainted with the fossil vegetation of
the uppermost tracts of the Neocomian and that of the lowest strata of
the Gault, or true Cretaceous series[59]."
[59] _Elements of Geology_, p. 280.
Lastly, the fact of the flora of the glacial epoch not having exhibited
any modifications during the long residence of some of its specific
types in Great Britain and elsewhere, is a fact of some importance to
the general theory of evolution, since it shows a higher degree of
stability on the part of these specific types than might perhaps have
been expected, supposing the theory to be true. But I do not see that
this constitutes a difficulty against the theory, when we have so many
other cases of proved transmutation to set against it. For instance, not
to go further afield than this very glacial flora itself, it will be
remembered that in an earlier chapter I selected it as furnishing
specially cogent proof of the transmutation of species. What, then, is
the explanation of so extraordinary a difference between Mr. Carruthers'
views and my own upon this point? I believe the explanation to be that
he does not take a sufficiently wide survey of the facts.
To begin with, it seems to me that he exaggerates the vicissitudes to
which the species of plants that he calls into evidence have been
exposed while advancing before, and retreating after, the ice. Rather do
I agree with Darwin that "they would 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 indicated in
this volume, these forms will not have been liable to much
modification[60]." But, be this matter of opinion as it may, a much
better test is afforded by those numerous cases all the world over,
where arctic species have been left stranded on alpine areas by the
retreat of glaciation; because here there is no room for differences of
opinion as to a "change of environment" having taken place. Not to speak
of climatic differences between arctic and alpine stations, consider
merely the changes which must have taken place in the relations of the
thus isolated species to each other, as well as to those of all the
foreign plants, insects, &c., with which they have long been thrown into
close association. If in _such_ cases no variation or transmutation had
taken place since the glacial epoch, then indeed there would have been a
difficulty of some magnitude. But, by parity of reasoning, whatever
degree of difficulty would have been thus presented is not merely
discharged, but converted into at least an equal degree of
corroboration, when it is found that under such circumstances, in
whatever part of the world they have occurred, some considerable amount
of variation and transmutation has always taken place,--and this in the
animals as well as in the plants. For instance, again to quote Darwin,
"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 distinct yet closely allied
species representing each other on the several ranges[61]." Lastly, if
instead of considering the case of alpine floras, we take the much
larger case of the Old and New World as a whole, we meet with much
larger proofs of the same general facts. For, "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 one great region with the
native American productions, and would have had to compete with them;
and, in the other great region, with those of the Old World.
Consequently we have here everything favourable for much
modification,--for far more modification than with the Alpine
productions left isolated, within a much more recent period, on the
several mountain ranges and on the arctic lands of Europe and N.
America. Hence it has come, that when we compare the now living
productions of the temperate regions of the New and Old Worlds, we find
very few identical species; but we find in every class many forms, which
some naturalists rank as geographical races, and others as distinct
species; and a host of closely allied or representative forms which are
ranked by all naturalists as specifically distinct[62]."
[60] _Origin of Species_, p. 332.
[61] _Origin of Species_, p. 332.
[62] _Ibid_. pp. 333-4.
In view then of all the above considerations--and especially those
quoted from Darwin--it appears to me that far from raising any
difficulty against the theory of evolution, the facts adduced by Mr.
Carruthers make in favour of it. For when once these facts are taken in
connection with the others above mentioned, they serve to complete the
correspondence between degrees of modification with degrees of time on
the one hand, and with degrees of evolution, of change of environment,
&c., on the other. Or, in the words of Le Conte, when dealing with this
very subject, "It is impossible to conceive a more beautiful
illustration of the principles we have been trying to enforce[63]."
[63] _Evolution and its Relation to Religious Thought_, p. 194.
NOTE A TO PAGE 257.
The passages in Dr. Whewell's writings, to which allusion is here made,
are somewhat too long to be quoted in the text. But as I think they
deserved to be given, I will here reprint a letter which I wrote to
_Nature_ in March, 1888.
In his essay on the _Reception of the Origin of Species_, Prof.
Huxley writes:--
"It is interesting to observe that the possibility of a fifth
alternative, in addition to the four he has stated, has not dawned
upon Dr. Whewell's mind" (_Life and Lectures of Charles Darwin_,
vol. ii, p. 195).
And again, in the article _Science_, supplied to _The Reign of
Queen Victoria_, he says:--
"Whewell had not the slightest suspicion of Darwin's main theorem,
even as a logical possibility" (p 365).
Now, although it is true that no indication of such a logical
possibility is to be met with in the _History of the Inductive
Sciences_, there are several passages in the _Bridgewater Treatise_
which show a glimmering idea of such a possibility. Of these the
following are, perhaps, worth quoting. Speaking of the adaptation
of the period of flowering to the length of a year, he says:--
"Now such an adjustment must surely be accepted as a proof of
design, exercised in the formation of the world. Why should the
solar year be so long and no longer? or, this being such a length,
why should the vegetable cycle be exactly of the same length? Can
this be chance?... And, if not by chance, how otherwise could such
a coincidence occur than by an intentional adjustment of these two
things to one another; by a selection of such an organization in
plants as would fit them to the earth on which they were to grow;
by an adaptation of construction to conditions; of the scale of
construction to the scale of conditions? It cannot be accepted as
an explanation of this fact in the economy of plants, that it is
necessary to their existence; that no plants could possibly have
subsisted, and come down to us, except those which were thus
suited to their place on the earth. This is true; but it does not
at all remove the necessity of recurring to design as the origin of
the construction by which the existence and continuance of plants
is made possible. A watch could not go unless there were the most
exact adjustment in the forms and positions of its wheels; yet no
one would accept it as an explanation of the origin of such forms
and positions that the watch would not go if these were other than
they were. If the objector were to suppose that plants were
originally fitted to years of various lengths, and that such only
have survived to the present time as had a cycle of a length equal
to our present year, or one which could be accommodated to it, we
should reply that the assumption is too gratuitous and extravagant
to require much consideration."
Again, with regard to "the diurnal period," he adds:--
"Any supposition that the astronomical cycle has occasioned the
physiological one, that the structure of plants has been brought to
be what it is by the action of external causes, or that such plants
as could not accommodate themselves to the existing day have
perished, would be not only an arbitrary and baseless assumption,
but, moreover, useless for the purposes of explanation which it
professes, as we have noticed of a similar supposition with respect
to the annual cycle."
Of course these passages in no way make against Mr. Huxley's
allusions to Dr. Whewell's writings in proof that, until the
publication of the _Origin of Species_, the "main theorem" of this
work had not dawned on any other mind, save that of Mr. Wallace.
But these passages show, even more emphatically than total silence
with regard to the principle of survival could have done, the real
distance which at that time separated the minds of thinking men
from all that was wrapped up in this principle. For they show that
Dr. Whewell, even after he had obtained a glimpse of the principle
"as a logical possibility," only saw in it an "arbitrary and
baseless assumption." Moreover, the passages show a remarkable
juxtaposition of the very terms in which the theory of natural
selection was afterwards formulated. Indeed, if we strike out the
one word "intentional" (which conveys the preconceived idea of the
writer, and thus prevented him from doing justice to any
naturalistic view), all the following parts of the above quotations
might be supposed to have been written by a Darwinian. "If not by
chance, how otherwise could such a coincidence occur, than by an
_adjustment_ of these two things to one another; by a _selection_
of such an organization in plants as would _fit_ them to the earth
on which they were to grow; by an adaptation of _construction_ to
_conditions_; of the _scale_ of construction to the _scale_ of
conditions?" Yet he immediately goes on to say: "If the objector
were to suppose that plants were originally _fitted_ to years of
various lengths, and that such only have _survived_ to the present
time ... _as could be accommodated to it_ (i. e. the actual cycle),
we should reply that the assumption is too gratuitous and
extravagant to require much consideration." Was there ever a more
curious exhibition of failure to perceive the importance of a
"logical possibility"? And this at the very time when another mind
was bestowing twenty years of labour on its "consideration."
NOTE B TO PAGE 295.
Since these remarks were delivered in my lectures as here printed, Mr.
Mivart has alluded to the subject in the following and precisely
opposite sense:--
Many of the more noteworthy instincts lead us from manifestations
of purpose directed to the maintenance of the individual, to no
less plain manifestations of a purpose directed to the preservation
of the race. But a careful study of the interrelations and
interdependencies which exist between the various orders of
creatures inhabiting this planet shows us yet a more noteworthy
teleology--the existence of whole orders of such creatures being
directed to the service of other orders in various degrees of
subordination and augmentation respectively. This study reveals to
us, as a fact, the enchainment of all the various orders of
creatures in a hierarchy of activities, in harmony with what we
might expect to find in a world the outcome of a First Cause
possessed of intelligence and will[64].
[64] _On Truth_, p. 493.
Having read this much, a Darwinian is naturally led to expect that Mr.
Mivart is about to offer some examples of instincts or structures
exemplifying what in the margin he calls the "Hierarchy of
Ministrations." Yet the only facts he proceeds to adduce are the
sufficiently obvious facts, that the inorganic world existed before the
organic, plants before herbivorous animals, these before carnivorous,
and so on: that is to say, everywhere the conditions to the occurrence
of any given stage of evolution preceded such occurrence, as it is
obvious that they must, if, as of course it is not denied, the
possibility of such occurrence depended on the precedence of such
conditions. Now, it is surely obvious that such a "hierarchy of
ministrations" as this, far from telling against the theory of natural
selection, is the very thing which tells most in its favour. The fact
that animals, for instance, only appeared upon the earth after there
were plants for them to feed upon, is clearly a necessity of the case,
whether or not there was any design in the matter. Such "ministrations,"
therefore, as plant-organisms yield to animal-organisms is just the kind
of ministration that the theory of natural selection requires. Thus far,
then, both the theories--natural selection and super-natural
design--have an equal right to appropriate the facts. But now, if in no
one instance can it be shown that the ministration of plant-life to
animal-life is of such a kind as to subserve the interests of
animal-life without at the same time subserving those of the plant-life
itself, then the fact makes wholly in favour of the naturalistic
explanation of such ministration as appears. If any plants had presented
any characters pointing prospectively to needs of animals without
primarily ministering to their own, then, indeed, there would have been
no room for the theory of natural selection. But as this can nowhere be
alleged, the theory of natural selection finds all the facts to be
exactly as it requires them to be: such ministration as plants yield to
animals becomes so much evidence of natural selection having slowly
formed the animals to appropriate the nutrition which the plants had
previously gathered--and gathered under the previous influence of
natural selection acting on themselves entirely for their own sakes.
Therefore I say it is painfully manifest that "the enchainment of all
the various orders of creatures in a hierarchy of activities," is _not_
"in harmony with what we might expect to find in a world the outcome of
a First Cause possessed of intelligence and [beneficent] will." So far
as any argument from such "enchainment" reaches, it makes entirely
against the view which Mr. Mivart is advocating. In point of fact, there
is a total absence of any such "ministration" by one "order of
creatures" to the needs of any other order, as the beneficent design
theory would necessarily expect; while such ministration as actually
does obtain is exactly and universally the kind which the naturalistic
theory requires.
Again, quite independently, and still more recently, Mr. Mivart alluded
in _Nature_ (vol. xli, p. 41) to the difficulty which the apparently
exceptional case of gall-formation presents to the theory of natural
selection. Therefore I supplied (vol. xli, p. 80) the suggestion given
in the text, viz. that although it appears impossible that the sometimes
remarkably elaborate and adaptive structures of galls can be due to
natural selection acting directly on the plants themselves--seeing that
the adaptation has reference to the needs of their parasites--it is
quite possible that the phenomena may be due to natural selection acting
indirectly on the plants, by always preserving those individual insects
(and larvae) the character of whose secretions is such as will best
induce the particular shapes of galls that are required. Several other
correspondents took part in the discussion, and most of them accepted
the above explanation. Mr. T. D. A. Cockerell, however, advanced another
and very ingenious hypothesis, showing that there is certainly one
conceivable way in which natural selection might have produced all the
phenomena of gall-formation by acting directly on the plants
themselves[65]. Subsequently Mr. Cockerell published another paper upon
the subject, stating his views at greater length. The following is the
substance of his theory as there presented:--
[65] _Nature_, vol. xli, p. 344.
Doubtless there were internal plant-feeding larvae before there
were galls: and, indeed, we have geological evidence that boring
insects date very far back indeed. The primitive internal feeders,
then, were miners in the roots, stems, twigs, or leaves, such as
occur very commonly at the present day. These miners are
excessively harmful to plant-life, and form a class of the most
destructive insect-pests known to the farmer: they frequently cause
the death of the whole or part of the plant attacked. Now, we may
suppose that the secretions of certain of these insects caused a
swelling to appear where the larvae lived, and on this excrescence
the larvae fed. It is easy to see that the greater the excrescence,
and the greater the tendency of the larvae to feed upon it, instead
of destroying the vital tissues, the smaller is the amount of harm
to the plant. Now the continued life and vitality of the plant is
beneficial to the larvae, and the larger or more perfect the gall,
the greater the amount of available food. Hence natural selection
will have preserved and accumulated the gall-forming tendencies, as
not only beneficial to the larvae, but as a means whereby the
larvae can feed with least harm to the plant. So far from being
developed for the exclusive benefit of the larvae, it is easy to
see that, allowing a tendency to gall-formation, natural selection
would have developed galls exclusively for the benefit of the
plants, so that they might suffer a minimum of harm from the
unavoidable attacks of insects.
But here it may be questioned--have we proof that internal feeders
tend to form galls? In answer to this I would point out that
gall-formation is a peculiar feature, and cannot be expected to
arise in every group of internal feeders. But I think we can afford
sufficient proof that wherever it has arisen it has been preserved;
and further, that even the highly complex forms of galls are
evolved from forms so simple that we hesitate to call them galls at
all[66].
[66] _Entomologist_, March, 1890.
The paper then proceeds to give a number of individual cases. No doubt
the principal objection to which Mr. Cockerell's hypothesis is open is
one that was pointed out by Herr Wetterhan, viz. "the much greater
facility afforded to the indirect action through insects, by the
enormously more rapid succession of generations with the latter than
with many of their vegetable hosts--oaks above all[67]." This
difficulty, however, Mr. Cockerell believes maybe surmounted by the
consideration that a growing plant need not be regarded as a single
individual, but rather as an assemblage of such[68].
[67] _Nature_, vol. xli, p. 394.
[68] _Ibid._ vol. xli, pp. 559-560.
NOTE C TO PAGE 394.
The only remarks that Mr. Wallace has to offer on the _pattern of
colours_, as distinguished from a mere _brilliancy of colour_, are added
as an afterthought suggested to him by the late Mr. Alfred Tylor's book
on _Colouration of Animals and Plants_ (1886). But, in the first place,
it appears to me that Mr. Wallace has formed an altogether extravagant
estimate of the value of this work. For the object of the work is to
show, "that diversified colouration follows the chief lines of
structure, and changes at points, such as the joints, where function
changes." Now, in publishing this generalization, Mr. Tylor--who was not
a naturalist--took only a very limited view of the facts. When applied
to the animal kingdom as a whole, the theory is worthless; and even
within the limits of mammals, birds, and insects--which are the classes
to which Mr. Tylor mainly applies it--there are vastly more facts to
negative than to support it. This may be at once made apparent by the
following brief quotation from Prof. Lloyd Morgan:--
It can hardly be maintained that the theory affords us any adequate
explanation of the _specific_ colour-tints of the humming-birds, or
the pheasants, or the Papilionidae among butterflies. If, as Mr.
Wallace argues, the immense tufts of golden plumage in the bird of
paradise owe their origin to the fact that they are attached just
above the point where the arteries and nerves for the supply of the
pectoral muscles leave the interior of the body--and the
physiological rationale is not altogether obvious,--are there no
other birds in which similar arteries and nerves are found in a
similar position? Why have these no similar tufts? And why, in the
birds of paradise themselves, does it require four years ere these
nervous and arterial influences take effect upon the plumage?
Finally, one would inquire how the colour is determined and held
constant in each species. The difficulty of the Tylor-Wallace view,
even as a matter of origin, is especially great in those numerous
cases in which the colour is determined by delicate lines, thin
plates, or thin films of air or fluid. Mr. Poulton, who takes a
similar line of argument in his _Colours of Animals_ (p. 326), lays
special stress on the production of _white_ (pp. 201-202).
As regards the latter point, it may be noticed that not in any part of
his writings, so far as I can find, does Mr. Wallace allude to the
highly important fact of colours in animals being so largely due to
these purely physical causes. Everywhere he argues as if colours were
universally due to pigments; and in my opinion this unaccountable
oversight is the gravest defect in Mr. Wallace's treatment both of the
facts and the philosophy of colouration in the animal kingdom. For
instance, as regards the particular case of sexual colouration, the
oversight has prevented him from perceiving that his theory of
"brilliancy" as due to "a surplus of vital energy," is not so much as
logically possible in what must constitute at least one good half of the
facts to which he applies it--unless he shows that there is some
connection between vital energy and the development of striations,
imprisonment of air-bubbles, &c. But any such connection--so
essentially important for his theory--he does not even attempt to show.
Lastly, and quite apart from these remarkable oversights, even if Mr.
Tylor's hypothesis were as reasonable and well-sustained as it is
fanciful and inadequate, still it could not apply to _sexual_
colouration: it could apply only to colouration as affected by
physiological functions common to both sexes. Yet it is in order to
furnish a "preferable substitute" for Mr. Darwin's theory of _sexual_
colouration, that Mr. Wallace adduces the hypothesis in question as one
of "great weight"! In this matter, therefore, I entirely agree with
Poulton and Lloyd Morgan.
INDEX
A.
Accident, Darwin's use of the word, 334-340;
beauty due to, 408, 409.
Achromatin, 126-134.
Acquired characters, _see_ Characters.
_Acræa eurita_, 328.
Adaptation, facts of, in relation to theory of natural selection,
401-403, 411.
Adaptive characters, _see_ Characters.
Æsthetic sense in animals, 380-385;
_see_ Beautiful.
Agassiz, Prof. A., on fauna of the Mammoth cave, 70.
Alpine plants, 209, 210, 440-442.
_Amauris niavius_, 328.
_Amblyornis inornata_, 381-383.
_Amphioxus_, 137, 138, 145, 146.
Analogy, 38, 50-65, 176, 177, 347-350.
Anthropoid, _see_ Apes.
Antlers, 98-100, 167-169.
Ants, co-operative instincts of, 268;
leaf-cutting, 332;
keeping aphides, 292.
Ape, eye of, 75;
_appendix vermiformis_ of, 84-86.
Apes, ears of, compared with those of man, 88;
muscles of, 77, 82, 83;
feet of, 77, 78;
tail of, compared with that of man, 82-84;
hair of, compared with that of man, 89-91;
teeth of, compared with those of man, 92-94;
flattening of tibiæ of, 95, 96.
Aphides, 292.
_Appendix vermiformis_ of man compared with that of orang, 84-86.
_Apteryx_, 68, 69.
_Archæopteryx_, 171-173.
Arctic plants, 209, 210, 440-442.
Argyll, Duke of, on natural selection, 334-362.
Aristotle, his idea of scientific method, 1;
on classification, 23, 24.
Arm, distribution of hair on, in man and apes, 89-92.
_Arthropoda_, embryology of, 155.
Artificial selection, analogy of, to natural selection, 295-314;
pictorial representations of products of, 298-312.
Artiodactyls, 182-191.
Association, principle of, in æsthetics, 404-407.
Aster, 129-133.
Attraction-spheres, 128, 132, 133.
Australia, fauna of, 204, 205;
thriving of exotic species in, 286;
portrait of wild dog of, 304.
Azores, 224, 225.
B.
Bacon, Lord, on scientific method, 2.
_Balanoglossus_, 147, 148.
_Baptanodon discus_, posterior limb of, 179-181.
Barriers, in relation to geographical distribution, 216-224.
Bats, 56, 224, 226, 240.
Battle, law of, 385, 386.
Baya-bird, 381.
Bear, skeleton of, 174;
feet of, 178.
Beautiful, the, sense of, in animals, 380-385;
standards of, 380-404;
Darwin's explanation of, in organic nature, 379-411;
facts of, in inorganic nature in relation to Darwin's theory of, in
organic, 404;
often determined by natural selection, 406, 407;
absent in many plants and animals, 408;
in nature often accidental, 409-411;
does not exist in organic nature as an end _per se_, 410, 411.
Bees, co-operative instincts of, 268.
Beetles, wingless, 68-70;
on oceanic islands, 224, 226, 229, 232.
Bell, Dr., on natural theology, 412.
Bell-bird, 396-398.
_Bembidium_, 233.
Bermudas, 225-227.
Biology, ideas of method in, 1-9.
Birds, ovum of, 124;
embryology of, 151-155;
paleontology of, 163-165, 172, 173;
brain of, 194-197;
as carriers of seed, eggs, and small organisms, 217, 218;
distribution of, 224-240;
æsthetic sense of, 380-385;
courtship of, 380-385.
_Birgus latro_, 62-65.
Blood, colour of arterial, 409.
Boar, _see_ Pig.
_Bombus lapidarius_, 331.
Bower-birds, play-houses of, 381-383.
Boyd-Dawkins, on flattening of early human tibiæ, 96.
Brain, palæontology of, 194-197.
British Isles, _see_ Islands.
Broca, 363.
Bronn, 363.
Budding, _see_ Germination.
Burdon-Sanderson, Prof., on electric organ of skate, 366.
Butler, Bishop, on argument from ignorance, 41.
Butterflies, defensive colouring of, 321-329.
C.
Cæsalpino, on classification, 24.
Calf, embryology of, 153.
Camel, foot of, 187-191.
Canadian stag, 196, 198, 199.
Canaries, portraits of, 303;
first mentioned by Gesner, 312, 313.
Cape de Verde Archipelagoes, fauna of, 228.
_Carcharias melanopterus_, 149.
Carruthers, on evolution, 436-442.
Caterpillars, colours and forms of, 319, 322-326.
Cattle, portraits of, 311.
Causation, natural, 402, 413, 414.
Caves, faunas of dark, 70-72.
Cell, physiological, and properties of the, 104-134.
_Cerura vinula_, 325, 326.
_Cervalces Americanus_, 196, 198, 199.
_Cervus dicrocerus_, _issiodorensis_, _matheronis_, _pardinensis_,
_Sedgwickii_, _tetraceros_, 168.
Chalmers, Dr., on natural theology, 412.
Chameleons, 317.
Characters, as adaptive, 273-276, 286-293, 349;
as specific, 274-276, 286-295;
as congenital and acquired, 274-276.
_Chasmorhynchus niveus_, and _C. tricarunculatus_, 396-398.
_Chelydra serpentina_, anterior limb of, 179-181.
Chick, embryology of, 153.
Chimpanzee, _see_ Apes.
Chlorophyll, 408.
_Chondracanthus cornutus_, 122.
Cirripedes, 430.
Classification, 23-49;
of organic nature by Genesis and Leviticus, 23;
artificial and natural, 24-26;
empirical rules of, 33-40;
Darwin on, 35, 36, 39,40;
form of, a nexus or tree, 29-32;
of organic forms like that of languages, 32;
single characters in relation to, 37;
aggregates of characters in relation to, 35-37;
adaptive and non-adaptive characters in relation to, 34, 35, 38, 39;
chains of affinities in relation to, 39-40;
biological differs from astronomical, 43.
Cockerell, on vegetable galls, 447, 448.
Colours, of plants and animals in relation to the theory of natural
selection, 317-332;
in relation to the theory of sexual selection, 391, 392, 394-396,
408-410, 448-450.
Colouring, _see_ Recognition marks, Protective, Seasonal, Warning, and
Mimicry.
Congenital characters, _see_ Characters.
Conjugation, of Protozoa, 115-117.
Continuity, principle of, in nature, 15-21.
Contrivance, Darwin's use of the word, 281.
Co-operation, mutual, of species alleged, 445-448.
Co-operative instincts, due to natural selection, 267, 269.
Cope, Professor, his table of geological formations, 163, 164;
his table of palæontological development of feet, vertebral column, and
brain, 197.
Correlation of growth, 357-362.
_Cossonidæ_, 233.
Courtship, _see_ Sexual Selection.
Crabs, 62-65, 139.
Cuttle-fish, 317.
Cuvier, on method in natural history, 3-4;
on monkeys, 429.
Cyst, _see_ Encystation.
D.
Darwin, Charles, his influence on ideas of method, 1-9;
on classification, 35, 36, 39, 40;
on vestigial characters in man, 77, 86, 87, 92;
on imperfection of geological record, 165, and Appendix;
on means of dispersal, 216, 218;
on geographical distribution, 218, 219;
on fauna of the Galapagos Archipelago, 227, 228;
on natural selection, 252, 253, 255, 256, 286, 375, 376;
his use of such words as 'accident,' 'fortuitous,' 'purpose,'
'contrivance,' &c., 281, 334-340;
on sexual selection, 379-400.
Darwin, Erasmus, his theory of evolution, 253.
De Blainville, on the theory of descent, 258.
De Candolle, on classification, 34.
Deer, 98, 99, 167-169, 187, 191, 196, 198, 199.
Degeneration, 269, 270, 342.
Delamination, 139.
_Diadema euryta_, 330.
Diaster, 129-133.
Dingo, _see_ Dog.
_Dinornis_, 60, 61.
_Diptera_ mimicking _Hymenoptera_, 329.
Dog, dentition of, 39;
Dingo, 304;
domesticated varieties of, 305, 307;
hairless, 307;
skulls of, 307.
Duck, logger-headed, 68.
Dugong, eye of, 75.
E.
Eagle, eye of, 75.
Ear, of whales, 65;
vestigial features of human, 76, 86-89;
of man and apes compared, 88.
Eaton, Rev. A. E., on wingless insects, 70.
_Echinodermata_, 125-127, 138, 155.
Ectoderm, 137-142.
Egg, _see_ Ovum.
Eimer, 363.
_Elaps fulvius_ imitated by non-venomous snakes, 330.
Electric organs, 365-373.
Elephant, foot of, 185, 186;
rate of propagation of, 261, 262.
Elk, 196-198, 199.
Embryo, human, _see_ Man.
Embryogeny, _see_ Ontogeny.
Embryology, 98-155.
Embryos, comparative series of, 152, 153.
_Encyclopædia Britannica_, eighth ed., on instinct, 289-291.
Encystation of Protozoa, 115.
Endoderm, 137-142.
Equatorial plate, 129.
_Equus_, _see_ Horse.
_Erythrolamprus venustissimus_, 330.
Evolution, organic, fact of, Section I;
Method of, Section II;
ideas upon, prior to Darwin, 253-258;
divergent, 266, 267.
Ewart, Professor Cossar, on electric organ of skate, 364, 367.
Existence, _see_ Struggle for.
Eye, of octopus, 57, 58, 347-350;
absence of, in dark cave animals, 70-72;
nictitating membrane of, 74, 75;
development of, from cutaneous nerve-ending, 352-354.
F.
Feet, 51-59, 66, 77-80, 174-192, 197.
Fertilization of ova, 127, 128;
of flowers by insects, 406.
Fish, embryology of, 143-155;
palæontology of, 163, 165, 169-171;
brain of, 194-197;
distribution of, 224-246;
flying, 355.
Fission, reproduction by, 106, 107.
Flat fish, 317.
Float, _see_ Swim-bladder.
Flowers, fertilization of, by insects, 406.
Fly, imitating a wasp, 329.
Flying-fish, and squirrels, 355.
_Foraminifera_, 346.
Forbes, H. O., on scapulo-coracoid bones of _Dinornis_, 60.
Fortuitous, Darwin's use of the word, 340.
Fossils, _see_ Palæontology.
Frogs, 317.
G.
Galapagos Islands, 227-231, 236, 237.
_Galeus_, eye of, 75.
Galls, vegetable, 293-295, 446-448.
Gastræa, 137-140.
_Gastrophysema_, 138.
Gastrulation, 137, 140.
Gegenbaur, 147, 181.
Gemmation, reproduction by, 106, 107, 110, 111.
Generalization, 5.
Generalized types, 33.
Genesis, classification of organic nature in, 23.
Genial tubercle, 96.
Geographical distribution, 204-248;
_see_ Glacial period, Barriers Transport of organisms, Oceanic
islands, &c.
Geology, record of imperfect, 156-160, and Appendix;
_see_ Palæontology.
Germs, prophetic, 272, 351-362.
Gesner, on classification, 24;
on canaries, 313.
Gill-arches, 146, 147, 150, 151.
Gill-slits, 146, 147, 150-153.
Gills, of young salamanders, 102;
origin of, in embryo, 144;
of fish, 150, 152.
Giraffe, neck of, in relation to Lamarck's theory, 254.
Glacial periods, effects of, on distribution of plants and animals,
209, 210, and Appendix.
Goose, Frizzled, portrait of, 304.
Gorilla, _see_ Apes.
Gray, Professor Asa, 337
Great-toe, in man and apes, 79-81.
Grouse, 317-319
Growth, correlation of, 357, 362.
_Gymnotus_, 365, 367.
H.
Häckel, on analogy between species and languages, 32;
on reproduction as discontinuous growth, 105, 106;
his ideal primitive vertebrate, 143, 144.
Hair, vestigial characters of, in man, 89-92.
Hales, 3.
Haller, 3.
Hamilton, Sir William, 272.
Hands, 51-55, 66, 80-82, 174-192.
Hare, 318, 319.
Hartmann, on flattening of early human tibiæ, 96.
Harvey, on Lord Bacon's writings, 2.
Heart, development of, 154.
Heilprin, on skulls of deer, 198, 199;
on fossil shells, 201, 202.
Hen, ovum of, 122.
Heredity, in relation to classification, 28-31;
in relation to embryology, 98-102;
chromatin-fibres in relation to, 134;
in relation to theories of organic evolution, 253-255, 260-264, 377.
Hermit-crabs, 62-65, 288, 289.
_Heteromera_, 233.
Hilgendorf, on shells of _Planorbis_, 201.
_Hipparion_, 191, 192.
Hippopotamus, foot of, 187.
Hog, _see_ Pig.
Homology, 38, 50-65, 176, 177, 347-350, 357-359.
Homopterous insect, imitating leaf-cutting ants, 331, 332.
Hooker, Sir Joseph, on flora of St. Helena, 234.
Horns, 98-100, 167-169.
Horse, eye of, 75;
limb-bones of, 176, 177, 186, 188-192;
teeth of, 189-191;
portraits of domesticated breeds of, 309.
Human, _see_ Man.
_Humerus_, perforations of, in quadrumana and man, 94, 95.
Humming-birds, restricted to the New World, 211.
Hunter, 3;
on ear of whale, 65.
Huxley, Prof., on mechanical selection, 283;
on age of the earth, 435, 436;
on Dr. Whewell, 243.
Hyatt, on shells of _Planorbis_, 201.
_Hydra_, 111, 122.
_Hyrax_, foot of, 185, 186.
I.
Ignorance, argument from, 41, 42, 49.
Illative Sense, 6.
Imitative colours, 317-323, 326-332.
Infant, feet of, 78, 79;
grasping power of, 81.
Infertility, inter-specific, in relation to natural selection, 374-376.
Insects, wingless, 68-70;
in primary formations, 163, Appendix;
on oceanic islands, 224-238;
in relation to galls, 293-295, 446-448;
defensive colouring of, 321-332;
fertilizing flowers, 406.
Instincts, always of primary use to species presenting them, 286-293.
Intercrossing, in relation to natural selection, 374-376.
Inutility of specific characters, in relation to natural selection,
374-376.
Islands, oceanic, 224-237;
British, 238-241.
J.
Japan, hairless dog of, 101.
Jelly-fish, 119, 120.
K.
_Kallima_, 323.
Karyokinesis, 112-114, 128-134.
Kepler, 272.
Kerguelen Island, flightless insects of, 70.
Kropotkin, Prince, on co-operative instincts, 269.
L.
_Lagopus mutus_, 317, 318.
Lamarck, his method in natural history, 4;
his theory of evolution, 253-256.
Lamprey, 148.
Languages, classification of, resembles that of organic forms, 32.
Lankester, E. Ray, on karyokinesis, 129, 130.
Leaf insect, 322.
Le Conte, on geological succession of animal classes, 164, 165;
on types of tails, 169-173;
on fossil shells of _Planorbis_, 201;
his work on the relation of the theory of evolution to religious
thought, 412.
_Leptalis_, 328.
_Leuculmis echinus_, 122.
Leviticus, classification of organic nature in, 23.
Life, origin of, 15.
Linnæus, on method in natural history, 3;
on classification, 26, 35-40.
Lion, skeleton of, 175;
feet of, 178.
Lizard, heart and gill-arches of, 150.
Lloyd Morgan, 273, 449, 450.
Lungs, development of, 154, 354.
Lyell, Sir Charles, on classification, 32;
on uniformitarianism, 258;
on rational species, 344;
on geological record, 420, 435, 439.
M.
Madeira, wingless beetles of, 68-70;
peculiar beetles of, 226, 227.
Mammals, ovum of, 120-124;
embryology of, 151-155;
palæontology of, 163, 165, 167, 180-199;
limbs of, 174-178, 182-199;
brain of, 194-199;
of Australia and New Zealand, 204, 205;
distribution of, on islands, 224-240.
Mammoth cave, fauna of, 70-72.
Man, nictitating membrane of, 75;
vestigial muscles of, 76, 77, 82, 83;
tail of, compared with that of apes, 82-84;
hair of, compared with that of apes, 89-92;
teeth of, compared with those of apes, 92-94;
perforation of humerus of, 94, 95;
flattening of ancient tibiæ of, 95, 96;
embryology of, 119, 153;
hand of, 54;
arm of, 90, 91;
limb-bones of, 176, 177;
palæontology of, 163, 165;
brain of, 194, 195;
Mr. Syme on, 346, 347.
Marsh, on palæontology of the horse, 188-190.
Matthew, Patrick, on natural selection, 257.
Mesoderm, 142.
_Mesohippus_, 189, 192.
Metaphyta, 104, 105.
Metazoa, 104.
Method, ideas of, in natural history, 1-9;
of organic evolution, 252-261.
Meyer, Professor Ludwig, on helix of the human ear, 86.
Mimicry, 320-322.
Ministration, mutual, of species alleged, 445, 446.
_Miohippus_, 189.
Mivart, St. George, on eye of octopus, 57, 58, 348, 349;
on incipient organs, 362;
on mutual ministration of species, 445, 446.
Mollusca, shells of, 19, 199-203;
eye of, 57, 58;
embryology of, 155;
palæontology of, 163, 165.
Monkeys, why all, do not become men, 342-344.
_Monotremata_, 205.
Morgan, _see_ Lloyd Morgan.
Morphology, 50-97.
Mule, portrait of, 309.
Multicellular organisms, 104.
Multiplication, _see_ Reproduction.
N.
Nägeli, Prof., 337, 367.
Natural History, ideas of method in, 1-9.
Natural, interpretations as opposed to super-natural, 13-15;
causation, 13-15.
Natural selection, 252-378, 401-410;
Wells, Matthew, and Whewell on, 257, 258, 443-445;
statement of theory of, 256-284;
of evidences of, 285-332;
of criticisms of, 333-378;
relation of theory of, to religious thought, 401-410;
preserves types, 264-267;
cessation and reversal of, 270, 342;
errors touching theory of, 270-284, 332-364;
definition of, 275-376;
antecedent standing of theory of, 277-284;
Prof. Owen on, 333, 334;
Duke of Argyll on, 334-362;
Mr. Syme on, 340, 341, 345;
need not always make for improvement, 341-347;
homology and analogy in relation to, 347-350;
often determines beauty, 406, 407;
in relation to the formation of galls, 293-295. 446-448.
Nature, organic, 17;
inorganic, 1, 17, 18.
_Nauplius_, 138.
Neumayr, 19.
New Zealand, fauna of, 68, 204, 205;
thriving of exotic species in, 286.
Newman, on the Illative Sense, 6.
Newton, his idea of scientific method, 6.
Nictitating membrane, 74, 75.
Notochord, 146.
Novum Organon, the, on scientific method, 2.
Nucleus, 105, 112-134.
Nucleus-spindle, 129.
Nut-hatch, Syrian, ornamented nests of, 381.
O.
Objective methods, 6.
Oceanic islands, _see_ Islands.
Octopus, eye of, 57, 58, 348-350.
_[OE]dicnemus crepitans_, 320.
Ontogeny, as recapitulation of phylogeny, 98-104.
Orang Outang, _see_ Apes.
_Oredon Culbertsoni_, 167.
Origin of Species, the, influence exercised by, on ideas of method, 1-9
_Orohippus_, 189.
Otaria, eye of, 75.
Ovum, 113-142;
human, 120-133;
amoeboid movements of young, 121-123;
segmentation of, 134, 135.
Owen, on ear of whale, 65;
on natural selection, 333, 334.
Owl, eye of, 75.
P.
Paddle, _see_ Whale, and _Baptanodon discus_.
_Pagurus bernhardus_, 64.
Pain, in relation to the theory of evolution, 417.
Palæontology, 159-203;
general testimony of, 156-165;
testimony of, in particular cases, 165-203;
consideration of objections to theory of evolution founded on grounds
of, 156-165, and Appendix.
_Palæotherium_, 190, 191.
Paley, on natural theology, 98, 412.
_Paludina_, successive forms of, 19.
Panama, Isthmus of, 219.
_Panniculus carnosis_, 77.
_Papilio merope_, 330.
Parasites, of animals, devoid of beauty, 408.
Parsimony, law of, 272.
Parthenogenesis, 119.
Partridges, 319.
Peacock, tail of, 378;
courtship of, 383.
Peckham, Mr. and Mrs., on courtship of spiders, 388-390.
Perissodactyls, 182-192.
_Petromyzon marinus_, 148.
_Phenacodus primævus_, 184, 185.
Phylogeny, _see_ Ontogeny.
Physiological selection, 376.
Pig, embryology of, 153;
feet of, 176, 187;
portraits of wild and domesticated, 312.
Pigeons, portraits of, 298, 299;
feather-footed, 359.
Pilot fish, 289.
_Planorbis_, transmutations of, 200, 201.
Pleasure and pain, in relation to the theory of evolution, 417.
_Plica semilunaris_, 75.
_Pliohippus_, 189.
Polar bear, skeleton of, 174;
feet of, 178.
Polar bodies, 125, 126.
Polar star, 129.
Polyps, 114.
Porpoises, 24, 25, 50.
Poulton, E. B., on warning colours, 325, 326;
on mimicry, 331, 332;
sexual selection, 400, 401, 449, 450.
Poultry, portraits of, 300-302.
Pronucleus, 126-128.
Prophetic types, 272, 351-362.
_Prophysema primordiale_, 140.
Protective colouring, 317-323.
_Protohippus_, 189.
Protozoa, 104.
Ptarmigan, 317, 318.
_Pterodactyl_, wing of, 56.
Purpose, Darwin's use of the word, 281, 340.
Puss moth, larva of, 325, 326.
Python, 66, 67.
Q.
Quadrumana, muscles of, 76, 82, 83;
perforations of humeri of, 94, 95;
hair on phalanges of, 91.
R.
Rabbit, embryology of, 153;
multiplication of, in Australia, 286;
portraits of wild and domesticated breeds of, 308;
protective colouring of, 319, 320.
Radiate form, beauty of, 408, 409.
_Raia radiata_, and _batis_, 367-371.
Rats, species of, restricted to Old and New Worlds, 212;
British and Norwegian, 285, 286.
Rattle-snake, tail of, 289.
Recognition marks, 271-273.
Religion, in relation to Darwinism, 401-418.
Reproduction, different methods of, 106-117;
essence of sexual, 110;
foreshadowing of sexual in unicellular organisms, 115-117.
Reptiles, wing of flying, 56;
rudimentary limbs of, 67;
nictitating membrane of, 75;
branchial arches of, 150;
embryology of, 152;
palæontology of, 163, 165, 178-180;
brain of, 194-197;
distribution of, 224-240.
_Rhinoceros_, foot of, 186.
Robinson, Dr L., on grasping power of an infant's hands, 80-82.
Rudimentary organs, 65-97.
Ruminants, palæontology of, 167, 168.
S.
Sacrum of man, compared with that of apes, 82-84.
_Sagitta_, 138.
Salamander, young of terrestrial, living in water, 102;
embryology of, 152.
Sandwich Islands, 234-237.
Science, method of, 1-9.
Sclater, W. L., on a case of mimicry, 331, 332.
Scorpion in Silurian formation, 163.
Sea, lamprey, 148;
destructive agency of the, 423, 424.
Seal, 51, 52, 75.
Seasonal changes of colour, 317-319.
Selection, value, 275;
by physical processes, 282, 283, 335. _See also_ Natural selection,
Artificial selection, Sexual selection, Physiological selection.
Sentiency, in relation to the theory of evolution, 417.
Sex, difference of, restricted to Metazoa and Metaphyta, 105.
Sexual reproduction, _see_ Reproduction.
Sexual selection, theory of, 277, 378-410;
statement and evidences of, 379-391;
criticisms of, 391-400;
includes law of battle with that of charming, 385, 386;
in relation to religious thought, 411-418;
Tylor's theory substituted for, by Wallace, 449, 450.
Shark, eye of, 75;
man-eating, 149;
and pilot-fish, 289.
Sheep, limb-bones of, 176, 177;
portraits of, 310.
Shells, of crabs, 62-64;
palæontology of mollusks, 199-203;
land on oceanic islands, 224-240.
Silliman's Journal, on fauna of the Mammoth Cave, 70.
Skate, electric organ of, 364-373.
Skull, palæontology of, 194-199;
of bull-dog compared with that of deer-hound, 307.
Slavonia, Tertiary deposits of, 18, 19.
Species, not eternal, but either created or evolved, 13;
named as such through absence of intermediate forms, 18-20;
groups of, in classification, 20,
and appearing suddenly in geological formations, 427-432, 437-440;
origin of, coincide in space and time with pre-existing and allied
species, 22;
geographical distribution of, 204-248;
extinct and living allied on same areas, 213;
life of, preserved by natural selection, 264-270;
not room for more than one rational, 344;
characters of, 274-276, 286-295, 374-376;
inter-sterility of allied, 374-376;
mutual ministration of alleged, 445, 446.
Specific characters, _see_ Characters.
Speculation, method of, 3-9.
Spencer, Herbert, on reproduction as discontinuous growth, 105, 106;
on use-inheritance, 253-256;
his failure to conceive the idea of natural selection, 257.
Spermatozoa, 123, 126-128.
Spiders, in primary formations, 163;
courtship of, 388, 389.
Sponges, 122, 139, 140.
Spontaneous, Darwin's use of the term, 340.
Spores, 115.
Squirrels, flying, 355.
Sterility, _see_ Infertility.
St. Helena, 231-234, 236-237.
St. Hilaire, 4.
Stick-insect, 322.
Stoat, 318.
_Strombus accipilrinus_, 201.
_Strombus Leidy_, 201.
Struggle for existence, 259-270.
Subjective, methods, 6.
Survival of the fittest, 335. _See also_ Natural selection.
Swim-bladder of fish, 154, 354.
Symbiosis, 269.
Syme, David, on the theory of natural selection, 340, 341.
T.
Tail, types of, in fish and birds, 169-173.
Tasmanian wolf, dentition of, 39.
Teeth, of Tasmanian wolf, 39;
molar, of man, compared with those of apes, 92-94;
palæontology of horses', 189-191.
Temperature, sense of, probable origin of that of sight, 353, 354.
Tennyson, 266.
Tibiæ, flattening of, 95, 96.
Tissue-cells, _see_ Cell.
Toes, 79, 80; _see also_ Feet.
Tomes, C. S., on molar teeth of man and apes, 94.
_Torpedo_, 365, 367.
Tortoise, embryology of, 152, 154.
_Toxopneustes variegatus_, and _T. lividus_, 122.
Transport of organisms, means of, 207, 216-218.
Tribal fitness, as distinguished from individual, 267-269.
Trout, ovum of, 122.
Turtle, eye of, 75.
Tylor, Alfred, on colouration of animals, 448-450.
Type, preserved by natural selection, 264-269;
improvement of, by natural selection, 269, 270;
prophetic, 272, 351-362.
Types, as simple and generalized, 33.
U.
Unicellular organisms, 104.
_Uraster_, 138.
Utility, of specific characters, 274, 275;
of incipient characters, 351-363;
of electric organs, 365-373.
V.
Variation, in relation to natural selection, 263, 335-340, 377.
Verification, 6-9.
Vertebral column, embryology of 145, 146;
palæontology of, 192, 193.
Vertebrated animal, ideal primitive, 143, 144;
embryology of, 143, 155.
_Vespa vulgaris_, 331.
Vestigial organs, 65-97.
_Volucella inans_, and _V. bombylans_, 329.
W.
Wagner, Moritz, on geographical distribution, 216.
Wallace, A. R., on origin of species as coincident in time and space
with pre-existing and allied species, 22;
on wingless insects, 70;
on absence of hair from human back, and function of on arms of
orang, 89;
on geographical distribution, 207, 231, 232, 233, 243;
on natural selection, 256;
on recognition marks, 271-273;
on alleged deductive consequences of the natural selection theory,
273-276;
his theory of warning colours, 323, 324;
on sexual selection, 391-400, 450;
his principal defect in treating of animal colouration, 449, 450.
Warning colours, 323-326.
Wasp, imitated by a fly, 329.
Water-cress, multiplication of, in New Zealand, 286.
Weevils, on St. Helena, 232.
Weismann, his theory of heredity, 130, 134.
Wells, Dr., on natural selection, 257.
Wetterhan, Prof., on vegetable galls, 448.
Whales, 38, 50, 53, 54, 65, 180.
Whewell, on natural selection, 257, 258, 443-445.
Wings, 54-56, 60, 61, 68-70, 355.
Wolf, Tasmanian, dentition of, 34.
Wood, John, on vestigial muscles in man, 77.
Woodward, on fossil cirripedes, 431.
Woolner, on the human ear, 86.
Worms, embryology of, 155.
Wyman, Prof., on the great toe of human embryo, 79, 80.
Z.
_Zona pellucida_, 121.
+--------------------------------------------------------------+
| |
| Transcriber’s Notes and Errata |
| |
| The following words were found in both hyphenated and |
| unhyphenated forms in the text. The number of instances is |
| given in parentheses after each word. |
| |
| |deer-hound (2) |deerhound (1) | |
| |fresh-water (13) |freshwater (1) | |
| |inter-relations (1) |interrelations (1) | |
| |re-action (1) |reaction (1) | |
| |sea-weed (7) |seaweed (1) | |
| |super-natural (2) |supernatural (24) | |
| |wood-cut (3) |woodcut (3) | |
| |wood-cuts (4) |woodcuts (1) | |
| |
| There were 9 instances of 'larvae' and 3 instances of |
| 'larvæ'. |
| |
| The following typographical errors were corrected: |
| |
| |Error |Correction | |
| | | | |
| |arboresent |arborescent | |
| |the |The | |
| |dicussion |discussion | |
| |
| In the index, the page entry for "Lyell, Sir Charles ... on |
| geological record" was changed from '420' to '422'. |
| |
| Also, the page entry for "Natural selection ... definition |
| of" was changed from '275-376' to '275-276'. |
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*** End of this LibraryBlog Digital Book "Darwin, and After Darwin (Vol. 1 and 3, of 3) - An Exposition of the Darwinian Theory and a Discussion of - Post-Darwinian Questions" ***
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