The Making of Species
by Douglas Dewar · English · Gutenberg #44063 · 653,372 chars · public domain (US)
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Title: The Making of Species
Author: Douglas Dewar
Frank Finn
Release date: October 29, 2013 [eBook #44063]
Most recently updated: October 23, 2024
Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK THE MAKING OF SPECIES ***
[Illustration: HEX CURASSOW FEEDING YOUNG BIRD, WITH PLUMAGE OF THE
GLOBOSE CURASSOW]
THE MAKING
OF SPECIES
BY DOUGLAS DEWAR, B.A. (Cantab), I.C.S., F.Z.S.
AND FRANK FINN, B.A. (Oxon), F.Z.S., M.B.O.U.
WITH FIFTEEN ILLUSTRATIONS
LONDON: JOHN LANE THE BODLEY HEAD
NEW YORK: JOHN LANE COMPANY MCMIX
_Turnbull & Spears, Printers, Edinburgh_
PREFACE
Post-Darwinian books on evolution fall naturally into four classes. I.
Those which preach Wallaceism, as, for example, Wallace’s _Darwinism_,
Poulton’s _Essays on Evolution_, and the voluminous works of Weismann.
II. Those advocating Lamarckism. Cope’s _Factors of Evolution_ and the
writings of Haeckel belong to this class. III. The writings of De Vries,
forming a group by themselves. They advocate the theory that species
spring suddenly into being; that new species arise by mutations from
pre-existing species. IV. The large number of books of a more judicial
nature, books written by men who decline to subscribe to any of the above
three creeds. Excellent examples of such works are Kellog’s _Darwinism
To-Day_, Lock’s _Recent Progress in the Study of Variation, Heredity, and
Evolution_, and T. H. Morgan’s _Evolution and Adaptation_.
All four classes are characterised by defects.
Books of the two first classes exhibit the faults of ardent partisanship.
They formulate creeds, and, as Huxley truly remarked, “Science commits
suicide when it adopts a creed.” The books which come under the third
category have the defects of extreme youth. De Vries has discovered a new
principle, and it is but natural that he should exaggerate its
importance, and see in it more than it contains. But, as time wears on,
these faults will disappear, and the theory of mutations will assume its
true form and fall into its proper place, which is somewhere between the
dustbin, to which Wallaceians would relegate it, and the exalted pinnacle
on to which De Vries would elevate it.
In the present state of our knowledge, books of Class IV. are the most
useful to the student, since they are unbiassed, and contain a judicial
summing-up of the evidence for and against the various evolutionary
theories which now occupy the field. Their chief defect is that they are
almost entirely destructive. They shatter the faith of the reader, but
offer nothing in place of that which they have destroyed. T. H. Morgan’s
_Evolution and Adaptation_, however, contains much constructive matter,
and so is the most valuable work of this class in existence.
Zoological science stands in urgent need of constructive books on
evolution—books with leanings towards neither Wallaceism, nor Lamarckism,
nor De Vriesism; books which shall set forth facts of all kinds,
concealing none, not even those which do not admit of explanation in the
present state of our knowledge.—It has been our aim to produce a book of
this description.
We have endeavoured to demonstrate that neither pure Lamarckism nor pure
Wallaceism affords a satisfactory explanation of the various phenomena of
the organic world. We have further, while recognising the very great
value of the work of De Vries, tried to show that that eminent botanist
has allowed his enthusiasm to carry him a little too far into the realm
of speculation. We have followed up the exposure of the weak points of
the theories, which at present occupy the field, with certain
suggestions, which, we believe, throw new light on many biological
problems.
Our aim in writing this book has been twofold. In the first place we have
attempted to place before the general public in simple language a true
statement of the present position of biological science. In the second
place, we have endeavoured to furnish the scientific men of the day with
food for reflection.
Even as the British nation seems to be slowly but surely losing, through
its conservatism, the commercial supremacy it had the good fortune to
gain last century, so is it losing, through the unwillingness of many of
our scientific men to keep abreast of the times, that scientific
supremacy which we gained in the middle of last century by the labours of
Charles Darwin and Alfred Russell Wallace. To-day it is not among
Englishmen, but among Americans and Continentals, that we have to look
for advanced scientific ideas.
Even as the Ultra-Cobdenites believe that Free Trade is a panacea for all
economic ills, so do most English men of science believe that natural
selection offers the key to every zoological problem. Both are living in
a fool’s paradise. Another reason why Great Britain is losing her
scientific supremacy is that too little attention is paid to bionomics,
or the study of live animals. Morphology, or the science of dead
organisms, receives more than its due share of attention. It is in the
open, not in the museum or the dissecting-room, that nature can best be
studied. Far be it from us to deprecate the study of morphology. We wish
merely to insist upon the fact, that the leaders of biological science
must of necessity be those naturalists who go to the tropics and other
parts of the earth where nature can be studied under the most favourable
conditions, and those who conduct scientific breeding experiments.
Natural selection—the idea which has revolutionised modern biological
science—came, not to professors, but to a couple of field-naturalists who
were pursuing their researches in tropical countries. It is absurd to
expect those who stay at home and gain most of their knowledge
second-hand to be the pioneers of biological science.
We fear that this book will come as a rude shock to many scientific men.
By way of consolation we may remind such that they will find themselves
in much the same position as that occupied by theologians immediately
after the appearance of the _Origin of Species_.
At that time theological thought was cramped by dogma. But the clergy
have since reconsidered their position, they have modified their views,
and thus kept abreast of the times. Meanwhile scientific men have lagged
behind. The blight of dogma has seized hold of them. They have adopted a
creed to which all must subscribe or be condemned as heretics. Huxley
said that the adoption of a creed was tantamount to suicide. We are
endeavouring to save biology in England from committing suicide, to save
it from the hands of those into which it has fallen.
We would emphasise that it is not Darwinism we are attacking, but that
which is erroneously called Neo-Darwinism. Neo-Darwinism is a
pathological growth on Darwinism, which, we fear, can be removed only by
a surgical operation.
Darwin, himself, protested in vain against the length to which some of
his followers were pushing his theory. On p. 657 of the new edition of
the _Origin of Species_ he wrote: “As my conclusions have lately been
much misrepresented, and as it has been stated that I attribute the
modification of species exclusively to natural selection, I may be
permitted to remark that in the first edition of this work, and
subsequently, I placed in a most conspicuous position—namely, at the
close of the Introduction—the following words: ‘I am convinced that
natural selection has been the main but not the exclusive means of
modification.’ This has been of no avail. Great is the power of steady
misrepresentation; but the history of science shows that this power does
not long endure.”
Notwithstanding this protest the Wallaceians continue on their course,
and give to the world a spurious Darwinism. It is our belief that were
Darwin alive to-day his sympathies would be with us, and not with those
who call themselves his followers. It was one of Darwin’s strong points
that he never avoided facts. If new facts came to light which were
incompatible with a theory of his, he promptly modified his theory. Since
his death a number of new facts have come to light which, in our opinion,
plainly indicate that the theory of natural selection as enunciated by
Darwin needs considerable modification.
We have in this book set forth certain of these facts and indicated the
directions in which the Darwinian theory seems to require modification.
This volume originated as the result of several conversations we, the
joint authors, had last summer. We discovered that we had a great many
ideas in common on the subject of evolution. This seemed strange, seeing
that our education had not been on the same lines. One of us took a
degree in natural science at Cambridge, and subsequently entered His
Majesty’s Indian Civil Service, but continued his zoological studies in
India as a hobby. The other, a naturalist from childhood, nevertheless
took a classical degree at Oxford, then received a technical zoological
training, adopted zoology as a profession, and held for some years a
position in the Natural History Museum at Calcutta.
Our conversations revealed that we were both of opinion that biology is
in an unhealthy condition, especially in England, and that the science
sorely needs some fresh impetus. Neither of us had the time to attempt,
single-handed, to give the required impetus, but as one of us happened to
be home on eighteen months’ leave, we thought we might undertake the task
in collaboration.
We felt that we might collaborate the more successfully because the large
number of facts collected by the one of us form the necessary complement
to the philosophical studies of the other.
We have endeavoured, so far as possible, to avoid technical terms, and
have made a special point of quoting, wherever practicable, familiar
animals as examples, in order that the work may make its appeal not only
to the zoologist but to the general reader.
It may, perhaps, be urged against us that we have quoted too freely from
popular writings, including those of which we are the authors. Our reply
to this is that the study of bionomics, the science of living animals,
occupies so small a place in English scientific literature that we have
been compelled to have recourse to popular works for many of our facts;
and we would, moreover, point out that a popular work is not necessarily
inaccurate in its information.
In conclusion, we would warn the reader against the danger of confounding
Inference with Fact. The failure to distinguish between the two has
vitiated much of the work of the Wallaceian school of biologists.
Facts are always to be accepted. Inferences should be scrutinised with
the utmost care.
In making our deductions, we have endeavoured to act without bias. We
shall, therefore, welcome any new facts, be they consistent with, or
opposed to, our inferences.
D. D.
F. F.
CONTENTS
PAGE
CHAPTER I 1
Rise of the Theory of Natural Selection and its Subsequent Development
Pre-Darwinian Evolutionists—Causes which led to the speedy triumph
of the theory of Natural Selection—Nature of the opposition which
Darwin had to overcome—Post-Darwinian biology—Usually accepted
classification of present-day biologists as Neo-Lamarckians and
Neo-Darwinians is faulty—Biologists fall into three classes rather
than two—Neo-Lamarckism: its defects—Wallaceism: its
defects—Neo-Darwinism distinguished from Neo-Lamarckism and
Wallaceism—Neo-Darwinism realises the strength and weakness of the
theory of Natural Selection, recognises the complexity of the
problems which biologists are endeavouring to solve.
CHAPTER II 30
Some of the more Important Objections to the Theory of Natural Selection
Brief statement of Theory—Objections to the Theory fall into two
classes—Those which strike at the root of the Theory—Those which
deny the all-sufficiency of Natural Selection—Objections which
strike at root of Theory are based on misconception—Objections to
Wallaceism—The Theory fails to explain the origin of
Variations—Natural Selection called on to explain too much—Unable
to explain beginnings of new organs—The Theory of change of
function—The co-ordination of variations—The fertility of races of
domesticated animals—Missing links—Swamping effects of
intercrossing—Small variations cannot have a survival value—Races
inhabiting same area—Excessive specialisation—Chance and Natural
Selection—Struggle for existence most severe among young
animals—Natural Selection fails to explain mimicry and other
phenomena of colour—Conclusion, that scarcely an organism exists
which does not possess some feature inexplicable on the theory of
Natural Selection as held by Wallace and his followers.
CHAPTER III 52
Variation
The assumption of Darwin and Wallace that variations are haphazard
in origin and indefinite in direction—If these assumptions be not
correct Natural Selection ceases to be the fundamental factor in
evolution—Darwin’s views regarding variation underwent
modification—He eventually recognised the distinction between
definite and indefinite variations, and between continuous and
discontinuous variations—Darwin attached but little importance to
either definite or discontinuous variations—Darwin’s views on the
causes of variations—Criticism of Darwin’s views—Variations appear
to occur along certain definite lines—There seems to be a limit to
the extent to which fluctuating variations can be accumulated—De
Vries’ experiments—Bateson on “discontinuous variation”—Views held
by De Vries—Distinction between continuous and discontinuous
variations—The work of De Vries—Advantages enjoyed by the botanist
in experimenting on the making of species—Difficulties encountered
by the animal breeder—Mutations among animals—The distinction
between germinal and somatic variations—The latter, though not
transmitted to offspring, are often of considerable value to their
possessor in the struggle for existence.
CHAPTER IV 111
Hybridism
The alleged sterility of hybrids a stumbling-block to
evolutionists—Huxley’s views—Wallace on the sterility of
hybrids—Darwin on the same—Wallace’s theory that the infertility of
hybrids has been caused by Natural Selection so as to prevent the
evils of intercrossing—Crosses between distinct species not
necessarily infertile—Fertile crosses between species of
plants—Sterile plant hybrids—Fertile mammalian hybrids—Fertile bird
hybrids—Fertile hybrids among amphibia—Limits of
hybridisation—Multiple hybrids—Characters of hybrids—Hybridism does
not appear to have exercised much effect on the origin of new
species.
CHAPTER V 133
Inheritance
Phenomena which a complete theory of inheritance must explain—In
the present state of our knowledge it is not possible to formulate
a complete theory of inheritance—Different kinds of
inheritance—Mendel’s experiments and theory—The value and
importance of Mendelism has been exaggerated—Dominance sometimes
imperfect—Behaviour of the nucleus of the sexual
cell—Chromosomes—Experiments of Delage and Loeb—Those of Cuénot on
mice and Castle on guinea pigs—Suggested modification of the
generally-accepted Mendelian formulæ—Unit characters—Biological
isomerism—Biological molecules—Interpretation of the phenomena of
variation and heredity on the conception of biological
molecules—Correlation—Summary of the conception of biological
molecules.
CHAPTER VI 170
The Colouration of Organisms
The theory of protective colouration has been carried to absurd
lengths—It will not bear close scrutiny—Cryptic colouring—Sematic
colours—Pseudo-sematic colours—Batesian and Müllerian
mimicry—Conditions necessary for mimicry—Examples—Recognition
markings—The theory of obliterative colouration—Criticism of the
theory—Objections to the theory of cryptic colouring—Whiteness of
the Arctic fauna is exaggerated—Illustrative tables—Pelagic
organisms—Objectors to the Neo-Darwinian theories of colouration
are to be found among field naturalists—G. A. B. Dewar, Gadow,
Robinson, F. C. Selous quoted—Colours of birds’ eggs—Warning
colouration—Objections to the theory—Eisig’s theory—So-called
intimidating attitudes of animals—Mimicry—The case for the
theory—The case against the theory—“False mimicry”—Theory of
recognition colours—The theory refuted—Colours of flowers and
fruits—Neo-Darwinian explanations—Objections—Kay Robinson’s
theory—Conclusion that Neo-Darwinian theories are untenable—Some
suggestions regarding the colouration of animals—Through the
diversity of colouring of organisms something like order runs—The
connection between biological molecules and colour—Tylor on colour
patterns in animals—Bonhote’s theory of pœcilomeres—Summary of
conclusions arrived at.
CHAPTER VII 297
Sexual Dimorphism
Meaning of the term—Fatal to Wallaceism—Sexual Selection—The law of
battle—Female preference—Mutual Selection—Finn’s
experiments—Objections to the theory of Sexual Selection—Wallace’s
explanation of sexual dimorphism stated and shown to be
unsatisfactory—The explanation of Thomson and Geddes shown to be
inadequate—Stolzmann’s theory stated and criticised—Neo-Lamarckian
explanation of sexual dimorphism stated and criticised—Some
features of sexual dimorphism—Dissimilarity of the sexes probably
arises as a sudden mutation—The four kinds of mutations—Sexual
dimorphism having shown itself, Natural Selection determines
whether or not the organisms which display it shall survive.
CHAPTER VIII 345
The Factors of Evolution
Variation along definite lines and Natural Selection are undoubtedly
important factors of evolution—Whether or not sexual selection is a
factor we are not yet in a position to decide—_Modus operandi_ of
Natural Selection—Correlation an important factor—Examples of
correlation—Correlation is a subject that requires close
study—Isolation a factor in evolution—Discriminate
isolation—Indiscriminate isolation—Is the latter a factor?—Romanes’
views—Criticism of these—Indiscriminate isolation shown to be a
factor—Summary of the methods in which new species arise—Natural
Selection does not make species—It merely decides which of certain
ready-made forms shall survive—Natural Selection compared to a
competitive examination and to a medical board—We are yet in
darkness as to the fundamental causes of the Origin of Species—In
experiment and observation rather than speculation lies the hope of
discovering the nature of these causes.
Footnotes 389
Index 389
LIST OF ILLUSTRATIONS
FACING PAGE
Heck’s Curassow feeding Young Bird, which has the Plumage of the Hens
of the Globose Curassow, its Father’s Species _Frontispiece_
_By permission of the Avicultural Society._
A Turbit belonging to Mr H. P. Scatliff 92
_From “The Modern Turbit,” published by “The Feathered World,” London._
Yellow-Rumped and Chestnut-Breasted Finches, with Specimens in
Transitional State 98
On the left, the yellow-rumped finch; on the right, the
chestnut-breasted; birds in state of change in the middle.
_By permission of the Avicultural Society._
Male Amherst Pheasant 122
The chief colours of this species (_Chrysolophus amherstiæ_) are
white and metallic green, so that it is very different in
appearance from its near ally the gold pheasant.
Harlequin Quail (_Coturnix delegorguei_) 124
_By permission of the Avicultural Society._
Rain Quail (_Coturnix coromandelica_) 124
The markings on the throats of these quails are of the type usually
put down as “recognition marks,” but as the Harlequin Quail is
African and the Rain Quail Indian, the two species cannot possibly
interbreed. The pattern, then, can have no “recognition”
significance.
_By permission of the Avicultural Society._
Bouru Friar-Bird 222
Like most of the group to which it belongs, this honey-eater
(_Tropidorhynchus bouruensis_) is a soberly coloured bird, but is
noisy, active, and aggressive.
_By permission of Messrs Hutchinson & Co._
Bouru Oriole 222
This “mimicking” oriole (_Oriolus bouruensis_) is of the same tone
of colour as its supposed model the Friar-bird of the same island.
_By permission of Messrs Hutchinson & Co._
King-Crow or Drongo 232
This very conspicuous black bird (_Dicrurus ater_), ranging from
Africa to China, is a striking feature of the landscape wherever it
occurs.
_By permission of Messrs Hutchinson & Co._
Drongo-Cuckoo 232
The fork of the tail in this bird is unique among cuckoos, but is
nevertheless much less developed than in the supposed model, and
may be an adaptation for evolutions in flight, as such tails
usually appear to be.
_By permission of Messrs Hutchinson & Co._
Shikra Hawk 236
The upper surface of the tail, not shown in this drawing, exactly
corresponds with that of the cuckoo “mimic.”
_By permission of Messrs Hutchinson & Co._
Hawk-Cuckoo 236
This species (_Hierococcyx varius_) is commonly known in India as
the “Brain-fever bird.”
_By permission of Messrs Hutchinson & Co._
Brazilian Troupial 284
This species (_Icterus vulgaris_) is that most frequently seen in
captivity; the pattern of colour is found in several other allied
forms.
_By permission of Messrs Hutchinson & Co._
Indian Black-Headed Oriole 284
Several other orioles besides this (_O. melanocephalus_) have the
black head.
_By permission of Messrs Hutchinson & Co._
Queen Whydah 298
This species (_Tetraenura regia_) is a typical example of seasonal
sexual dimorphism, the male being long-tailed and conspicuously
coloured only during the breeding season, and at other times
resembling the sparrow-like female.
_By permission of the Foreign Bird Club._
Courtship of Skylark 314
Illustrating display by a species with no decorative colouring or
sex difference.
THE MAKING OF SPECIES
CHAPTER I
RISE OF THE THEORY OF NATURAL SELECTION AND ITS SUBSEQUENT DEVELOPMENT
Pre-Darwinian Evolutionists—Causes which led to the speedy triumph of
the theory of Natural Selection—Nature of the opposition which Darwin
had to overcome—Post-Darwinian biology—Usually accepted classification
of present-day biologists as Neo-Lamarckians and Neo-Darwinians is
faulty—Biologists fall into three classes rather than
two—Neo-Lamarckism: its defects—Wallaceism: its defects—Neo-Darwinism
distinguished from Neo-Lamarckism and Wallaceism—Neo-Darwinism realises
the strength and weakness of the theory of Natural Selection,
recognises the complexity of the problems which biologists are
endeavouring to solve.
Darwinism and evolution are not interchangeable terms. On this fact it is
impossible to lay too much emphasis. Charles Darwin was not the
originator of the theory of evolution, nor even the first to advocate it
in modern times. The idea that all existing things have been produced by
natural causes from some primordial material is as old as Aristotle. It
was lost sight of in the mental stagnation of the Middle Ages. In that
dark period zoological science was completely submerged. It was not until
men shook off the mental lethargy that had held them for many generations
that serious attention was paid to biology. From the moment when men
began to apply scientific methods to that branch of knowledge the idea of
evolution found supporters.
Buffon suggested that species are not fixed, but may be gradually changed
by natural causes into different species.
Goethe was a thorough-going evolutionist; he asserted that all animals
were probably descended from a common original type.
Lamarck was the first evolutionist who sought to show the means whereby
evolution has been effected. He tried to prove that the efforts of
animals are the causes of variation; that these efforts originate changes
in form during the life of the individual which are transmitted to its
offspring.
St Hilaire was another evolutionist who endeavoured to explain how
evolution had occurred. He believed that the transformations of animals
are effected by changes in their environment. These hypotheses were
considered, and rightly considered, insufficient to explain anything like
general evolution, so that the idea failed for a time to make headway.
Strength of Darwin’s Position
As knowledge grew, as facts accumulated, the belief in evolution became
more widespread. Hutton, Lyell, Spencer, and Huxley were all convinced
that evolution had occurred, but they could not explain how it had
occurred.
Thus, by the middle of last century, all that was needed to make
evolution an article of scientific belief was the discovery of a method
whereby it could be effected. This Darwin and Wallace were able to
furnish in the shape of the theory of natural selection. The discovery
was made independently, but Darwin being the older man, the more
influential, and the one who had gone the more deeply and carefully into
the matter, gained the lion’s share of the credit of the discovery. The
theory of natural selection is universally known as the Darwinian theory,
notwithstanding the fact that Darwin, unlike Wallace, always recognised
that natural selection is not the sole determining factor in organic
evolution.
From the moment of the enunciation of his great hypothesis, Darwin’s
position was an exceedingly strong one. Everything was in his favour.
As we have seen, the theory was enunciated at the psychological moment,
at the time when zoological science was ripe for it. Most of the leading
zoologists were evolutionists at heart, and were only too ready to accept
any theory which afforded a plausible explanation of what they believed
to have occurred.
Hence the rapturous welcome accorded to the theory of natural selection
by the more progressive biologists.
Another point in Darwin’s favour was the delightful simplicity of his
hypothesis. Nothing could be more enticingly probable. It is based on the
unassailable facts of variation, heredity, and the tendency of animals to
multiply in numbers. Everybody knows that the breeder can fix varieties
by careful breeding. Darwin had simply to show that there is in nature
something to take the part played among domesticated animals by the human
breeder. This he was able to do. As the numbers of species remain
stationary, it is evident that only a small portion of the animals that
are born can reach maturity. A child can see that the individuals most
likely to survive are those best adapted to the circumstances of their
life. Even as the breeder weeds out of his stock the creatures not suited
to his purpose, so in nature do the unfit perish in the everlasting
struggle for existence.
In nature there is a selection corresponding to that of the breeder.
It is useless to deny the existence of this selection in nature, this
natural selection. The only disputable point is whether such selection
can do all that Darwin demanded of it.
The man in the street, then, was able to comprehend the theory of natural
selection. This was greatly in its favour. Men are usually well disposed
towards doctrines which they can readily understand.
The nineteenth century was a superficial age. It liked simplicity in all
things. If Darwin could show that natural selection was capable of
producing one species, men were not only ready but eager to believe that
it could explain the whole of organic evolution.
The simplicity of the Darwinian theory has its evil side. It has
undoubtedly tended to make modern biologists superficial in their
methods. It has, indeed, stimulated the imagination of men of science;
but the stimulation has not in all cases been a healthy one.
So far from adhering to the sound rule laid down by Pasteur, “never
advance anything that cannot be proved in a simple and decisive manner,”
many modern naturalists allow their imagination to run riot, and so
formulate ill-considered theories, and build up hypotheses on the most
insecure foundations. “A tiny islet of truth,” writes Archdale Reid, “is
discovered, on which are built tremendous and totally illegitimate
hypotheses.”
Another source of Darwin’s strength was the vast store of knowledge he
had accumulated. For twenty years he had been steadily amassing facts in
support of his hypothesis. He enunciated no crude theory, he indulged in
no wild speculations. He was content to marshal a great array of facts,
and to draw logical conclusions therefrom. He was as cautious in his
deductions as he was careful of his facts. He thus stood head and
shoulders above the biologists of his day. He was a giant among pigmies.
So well equipped was he that those who attempted to oppose him found
themselves in the position of men, armed with bows and arrows, who seek
to storm a fortress defended by maxim guns.
Nor was this all. The majority of the best biologists of his time did not
attempt to oppose him. They were, as we have seen, ready to receive with
open arms any hypothesis which seemed to explain how evolution had
occurred. Some of them perceived that there were weak points in the
Darwinian theory, but they preferred not to expose these; they were
rather disposed to make the best of the hypothesis. It had so many merits
that it seemed to them but reasonable to suppose that subsequent
investigation would prove that the defects were apparent rather than
real.
Opponents of Darwin
We hear much of the “magnitude of the prejudices” which Darwin had to
overcome, and of the mighty battle which Darwin and his lieutenant Huxley
had to fight before the theory of the origin of species by natural
selection obtained acceptance. We venture to say that statements such as
these are misleading. We think we may safely assert that scarcely ever
has a theory which fundamentally changed the prevailing scientific
beliefs met with less opposition. It would have been a good thing for
zoology had Darwin not obtained so easy a victory.
Sir Richard Owen, a distinguished anatomist, certainly attacked the
doctrine in no unmeasured terms, but his attack was anonymous and so
cannot be considered very formidable. Far more important was the
opposition of Dr St George Mivart, whose worth as a biologist has never
been properly appreciated. His most important work, entitled the _Genesis
of Species_, might be read with profit even now by many of our modern
Darwinians.
For some time after the publication of the _Origin of Species_ Mivart
appears to be almost the only man of science fully alive to the weak
points of the Darwinian theory. The great majority seem to have been
dazzled by its brilliancy.
The main attack on Darwinism was conducted by the theologians and their
allies, who considered it to be subversive of the Mosaic account of the
Creation. Now, when one whose scientific knowledge is, to say the best of
it, not extensive, attacks a man who has studied his subject
dispassionately for years, and invariably expresses himself with extreme
caution, the onslaught can have but one result—the attacker will be
repulsed with heavy loss, and the onlookers will have a higher opinion of
his valour than of his common sense.
The theologians were in the unfortunate position of warriors who do not
know what it is against which they are fighting; they confounded natural
selection with evolution, and directed the main force of their attack
against the latter, under the impression that they were fighting the
Darwinian theory.
It was the misfortune of those theologians that it is possible to prove
that evolution, or, at any rate, some evolution has occurred; they thus
kicked against the pricks with disastrous results to themselves. When
this attack had been repulsed men believed that the theory of natural
selection had been demonstrated, that it was as much a law of nature as
that of gravitation. What had really happened was that the fact of
evolution had been proved, and the theory of natural selection obtained
the credit. Men thought that Darwinism was evolution. Had the theologians
admitted evolution but denied the ability of natural selection to explain
it, the Darwinian theory, in all probability, would not have gained the
ascendency which it now enjoys.
Evolution and Natural Selection
To us who are able to look back dispassionately upon the biological
warfare of the last century, Darwin’s opponents—or the majority of
them—appear very foolish. We must, however, bear in mind that at the time
of the publication of the _Origin of Species_ both natural selection and
evolution were comparatively unknown ideas. Darwin had to fight for both.
He had to prove evolution as well as natural selection. Many of the facts
adduced by him supported both. It is, therefore, not altogether
surprising that many of his opponents failed to distinguish between them.
A glance at the _Origin of Species_ will suffice to show how considerable
is the portion of the book that deals with the evidence in favour of
evolution rather than of natural selection.
Of the fourteen chapters which make up the book no fewer than nine are
devoted to proving that evolution has occurred. It has been truly said,
that for every one fact biologists have found in support of the special
theory of natural selection they have found ten facts supporting the
doctrine of evolution. Darwin, then, was in the position of a skilled
barrister who has a plausible case and who knows the ins and outs of his
brief, while his opponents stood in the shoes of inexperienced counsel
who had but recently received their brief, and who had not had the time
to master the details thereof. In such circumstances it is not difficult
to predict which way the verdict of the jury will go.
Darwin, moreover, had a charming personality. Never was a man with a
theory less dogmatic. Never was the holder of a theory more careful of
the expressions he used. Never was a scientific man more ready to give
ear to his opponents, to meet them half way, and, where necessary, to
compromise. Darwin was not afraid of facts, and was always ready to alter
his views when they appeared to be opposed to facts. The average
scientific man of to-day makes facts fit his theory; if they refuse to
fit it he ignores or denies them.
Darwin continually modified his views; when he found himself in a tight
place he did not hesitate to resort to Lamarckian factors, such as the
inheritance of the effects of use and disuse and of the effects of
environment. He conceded that natural selection was insufficient to
account for all the phenomena of organic evolution, and advanced the
theory of sexual selection in order to account for facts which the major
hypothesis seemed to him incapable of explaining.
Darwin, moreover, having ample private means, was not obliged to work for
a living, and was therefore able to devote the whole of his time to
research. The advantages of such a position cannot be over-estimated,
and, perhaps, have not been sufficiently taken into account in
apportioning the praise between Darwin and Wallace for their great
discovery.
Huxley
To all these factors in Darwin’s favour we must add his good fortune in
possessing so able a lieutenant as Huxley.
Huxley was an ardent evolutionist, an able writer, and a brilliant
debater. A man of his mental calibre was able, like a clever barrister,
to make out a plausible case for any theory which he chose to take up.
While nominally a strong supporter of the Darwinian theory, he was in
reality fighting for the doctrine of descent. Had _any_ plausible theory
of evolution been enunciated, Huxley would undoubtedly have fought for it
equally earnestly.
A firm believer in evolution, Huxley was, as Professor Poulton says,
confronted by two difficulties,—first, the insufficiency of the evidence
of evolution, and, secondly, the absence of any explanation of how the
phenomenon had occurred. The _Origin of Species_ solved both these
difficulties. It adduced much weighty evidence in favour of evolution,
and suggested a _modus operandi_. Small wonder, then, that Huxley became
a champion of Darwinism. But, as Poulton writes, on page 202 of _Essays
on Evolution_, “while natural selection thus enabled Huxley freely to
accept evolution, he was by no means fully satisfied with it.” “He never
committed himself to a full belief in natural selection, and even
contemplated the possibility of its ultimate disappearance.” To use
Huxley’s own words: “Whether the particular shape which the doctrine of
evolution, as applied to the organic world, took in Darwin’s hands, would
prove to be final or not, was, to me, a matter of indifference.”
The result of the fortuitous combination of the circumstances which we
have set forth was that in a surprisingly short time the theory of
natural selection came to be regarded as a law of nature on a par with
the laws of gravitation. Thus, paradoxical though it seems, practical
certainty was given to a hitherto uncertain doctrine by the addition of a
still more uncertain theory.
“At once,” writes Waggett, “the theory of development leapt from the
position of an obscure guess to that of a fully-equipped theory and
almost a certainty.”
Darwin thus became a dictator whose authority none durst question. A
crowd of slavish adherents gathered round him, a herd of men to whom he
seemed an absolutely unquestionable authority. Darwinism became a creed
to which all must subscribe. It still retains this position in the
popular mind.
Growing Opposition to Darwinism
The ease with which the theory of natural selection gained supremacy was,
as we have already said, a misfortune to biological science. It produced
for a time a considerable mental stagnation among zoologists. Since
Darwin’s day the science has not made the progress that might reasonably
have been expected, because the theory has so captivated the minds of the
majority of biologists that they see everything through Darwinian
spectacles. The wish has been in many cases the father to the
observation. Zoologists are ever on the lookout for the action of natural
selection, and in consequence frequently imagine they see it where it
does not exist. Many naturalists, consciously or unconsciously, stretch
facts to make them fit the Darwinian theory. Those facts which refuse to
be so distorted are, if not actively ignored or suppressed, overlooked as
throwing no light upon the doctrine. This is no exaggeration. A perusal
of almost any popular book dealing with zoological theory leaves the
impression that there is nothing left to be explained in the living
world, that there is no door leading to the secret chambers of nature to
which natural selection is not an “open sesame.”
But the triumph of natural selection has not been so complete as its more
enthusiastic supporters would have us believe. Some there are who have
never admitted the all-sufficiency of natural selection. In the British
Isles these have never been numerous. In the United States of America and
on the Continent they are more abundant. The tendency seems to be for
them to increase in numbers. Hence the recent lamentations of Dr Wallace
and Sir E. Ray Lankester. Modern biologists are commonly supposed to fall
into two schools of thought—the Neo-Darwinian and the Neo-Lamarckian.
The former are the larger body, and pin their faith absolutely to natural
selection. They deny the inheritance of acquired characters, and preach
the all-sufficiency of natural selection to explain the varied phenomena
of nature. The Neo-Lamarckians do not admit the omnipotency of natural
selection. Some of them allow it no virtue. Others regard it as a force
which keeps variation within fixed limits, which says to each organism,
“thus far shalt thou vary and no farther.” This school lays great stress
on the inheritance of acquired characters, especially on the inheritance
of the effects of use and disuse.
The above statement of the recent developments of Darwinism is
incomplete, for it fails to include those who occupy a middle position.
If it be possible to classify a large number of men of which scarcely any
two hold identical views, it is into three, rather than two, classes that
they must be divided.
Speaking broadly, evolutionists of to-day may be said to represent three
distinct lines of thought. For the sake of classification we may speak of
them as falling into three schools, which we may term the Neo-Lamarckian,
the Wallaceian, and the Neo-Darwinian, according as their views incline
towards those held by Lamarck, Wallace, or Darwin.
The Neo-Lamarckian School
As adherents of the Neo-Lamarckian school, we cite Cope, Spencer, Orr,
Eimer, Naegeli, Henslow, Cunningham, Haeckel, Korchinsky, and a number of
others. It may almost be said of these Neo-Lamarckians that each holds a
totally distinct theory of evolution. So heterogeneous are their views
that it is difficult to find a single article common to the evolutionary
belief of all. It is commonly asserted that all Neo-Lamarckians are
agreed, firstly, that acquired characters are transmissible; and,
secondly, that such transmission is an important factor in the production
of new species. This assertion is certainly true of the great bulk of
Neo-Lamarckians, but it does not appear to hold in the case of those who
believe that evolution is the result of some unknown inner force. So far
as we can see, a belief in the inheritance of acquired characters is not
necessary to the theories of orthogenesis held by Naegeli and Korchinsky.
For that reason it would possibly be more correct to place those who hold
such views in a fourth school. Since, however, a number of undoubted
Neo-Lamarckians, as, for example, Cope, believe in an inner growth-force,
it is convenient to regard Naegeli as a Neo-Lamarckian. His views need
not detain us long. Those who wish to study them in detail will find them
in his _Mechanisch-physiologische Theorie der Abstammungslehre_.
Naegeli believes that there is inherent in protoplasm a growth-force,
which makes each organism in itself a force making towards progressive
evolution. He holds that animals and plants would have become much as
they are now even if no struggle for existence had taken place. “To the
believers in this kind of . . . orthogenesis,” writes Kellog (_Darwinism
To-day_, p. 278), “organic evolution has been, and is now, ruled by
unknown inner forces inherent in organisms, and has been independent of
the influence of the outer world. The lines of evolution are immanent,
unchangeable, and ever slowly stretch toward some ideal goal.” It is easy
to enunciate such a theory, impossible to prove it, and difficult to
disprove it.
It seems to us that the fact that, so soon as organisms are removed from
the struggle for existence, they tend to degenerate, is a sufficient
reason for refusing to accept theories of the description put forth by
Naegeli. More truly Lamarckian is Eimer’s theory of orthogenesis,
according to which it is the environment which determines the direction
which variation takes; and the variations which are induced by the
environment are transmitted to the offspring.
Orr’s Views
Spencer and Orr preach nearly pure Lamarckism. The former, while fully
recognising the importance of natural selection, considered that
sufficient weight has not been given to the effects of use and disuse, or
to the direct action of the environment in determining or modifying
organisms.
The similarity of the views of Orr and Lamarck is best seen by comparing
their respective explanations of the long neck of the giraffe. Lamarck
thought that this was the direct result of continual stretching. The
animal continually strains its neck in the search for food, hence it
grows longer as the individual grows older, and this elongated neck has
been transmitted to the offspring. Orr writes, on page 164 of his
_Development and Heredity_: “The giraffe seems to present the most
remarkable illustration of the lengthening of the bones as the result of
the frequent repetition of such shocks. As is well known, this animal
feeds on the foliage of trees. From the earliest youth of the species,
and the earliest youth of each individual, it must have been stretching
upwards for food, and, as is the custom of such quadrupeds, it must have
constantly raised itself off its forefeet, and, as it dropped, must have
received a shock that made itself felt from the hoofs through the legs
and vertical neck to the head. In the hind legs the shock would not be
felt. It is impossible to imagine that an animal which, during the
greater part of every day of its life (both its individual and racial
life), performed motions so uniform and constant, would not be peculiarly
specialised as a result. The forces acting upon such an animal are widely
different from the forces acting upon an animal which eats the grass at
its feet like an ox, or one which must run and climb like a goat or a
deer, and the resultant modifications of growth in the several cases must
also be different. The principle of increased growth in the direction of
the shock, resulting from superabundant repair of the momentary
compression, explains how the giraffe acquired the phenomenal length of
the bones of its forelegs and neck; and the absence of the shock in the
hind-quarters shows why they remained undeveloped and absurdly
disproportionate to the rest of the body.”
Inheritance of Acquired Characters
It seems to us that a fatal objection to all these Neo-Lamarckian
theories of evolution is that they are based on the assumption that
acquired characters are inherited, whereas all the evidence goes to show
that such characters are not inherited. In these days, when scientific
knowledge is so widely diffused, it is scarcely necessary to say that all
the characteristics which an organism displays are either congenital or
inborn, or acquired by the organism during its lifetime. Thus a man may
have naturally a large biceps muscle, and this is a congenital character;
or he may by constant exercise develop or greatly increase the size of
the biceps. The large biceps, in so far as it has been increased by
exercise, is said to be an acquired character, for it was not inherited
by its possessor, but acquired by him in his lifetime. We must bear in
mind that the period in the life history of an organism at which a
character appears, is not necessarily a test as to whether it is
congenital or acquired, for a great many congenital characters, such as a
man’s beard, do not appear until some years after birth. As we have seen,
the Neo-Lamarckians believe that it is possible for an organism to
transmit to its offspring characters which it has acquired during the
course of its existence. But, as we have already said, the evidence goes
to show that such characters are not inherited. For example, the tail of
the young fox-terrier is not shorter than that of other breeds of dogs,
notwithstanding the fact that its ancestors have for generations had the
greater portion of their caudal appendage removed shortly after birth.
We do not propose to discuss at any great length the vexed question of
the inheritance of acquired characters, for the simple reason that the
Neo-Lamarckians have not brought forward a single instance which
indubitably proves that such characters are inherited.
Mr J. T. Cunningham, in a paper of great value and interest, entitled
“The Heredity of Secondary Sexual Characters in relation to Hormones: a
Theory of the Heredity of Somatogenic Characters,” which appeared in vol.
xxvi., No. 3, of the _Archiv für Entwicklungsmechanik des Organismen_,
states: “The dogma that acquired characters cannot be inherited . . . is
founded not so much on evidence, or the absence of evidence, as on _a
priori_ reasoning, on the supposed difficulty or impossibility of
conceiving a means by which such inheritance could be effected.” Such
appears certainly to be true of some zoologists, but we trust that Mr
Cunningham will do us the justice to believe that our opinion that the
inheritance of acquired characters does not play an important part in the
evolution of, at any rate, the higher animals, is based, not on the
ground of _a priori_ reasoning, but on facts. All the evidence seems to
show that such characteristics are not inherited. If, as Mr Cunningham
thinks, all secondary sexual characters are due to the inheritance of the
effects of use, etc., how is it that no Neo-Lamarckian is able to bring
forward a clear case of the inheritance of a well-defined acquired
character? If such characteristics are habitually inherited, countless
examples should be forthcoming. Fanciers in their endeavours are
constantly “doctoring” the animals they keep for show purposes; and it
seems to us certain that if acquired characters are inherited, breeders
would long ago have discovered this and acted upon the discovery. If
Neo-Darwinians are charged with refusing to believe that acquired
characters are inherited because they “cannot conceive the means by which
it could be effected,” may it not be said with equal justice that many
Neo-Lamarckians believe that acquired characters are inherited, not on
evidence thereof, but because if such characters are not inherited it is
very difficult to account for many of the phenomena presented by the
organic world?
In many of the lower animals, as, for example, the hydra, the germinal
material is diffused through the organism, so that a complete individual
can be developed from a small portion of the creature. In such
circumstances it seems not improbable that the external environment may
act directly on the germinal substance, and induce changes in it which
may perhaps be transmitted to the offspring. If this be so, it would seem
that some acquired characters may be inherited in such organisms. Very
many plants can be propagated from cuttings, buds, etc., so that we might
reasonably expect some acquired characters to be hereditary in them. The
majority of botanists appear to hold Lamarckian views; but on the
evidence at present available, it is doubtful whether such views are the
correct ones.
Plants are so plastic, so protean, so sensitive to their environment that
their external structure appears to be determined by the external
conditions in which they find themselves quite as much as by their
inherited tendencies. In this respect they differ very considerably from
the higher animals. The peacock, for example, presents the same outward
appearance[1] whether bred and reared in Asia or Europe, in a hot or
cold, a damp or a dry climate. The same plant, on the other hand, differs
greatly in outward appearance according as it is grown in a dry or a damp
soil, a hot or a cold country. In his recent book _The Heredity of
Acquired Characters in Plants_, the Rev. G. Henslow cites several
examples of the celerity with which plants react to their environment. On
page 32 he writes: “The following is an experiment I made with the common
rest-harrow (_Ononis spinosa, L._) growing wild in a very dry situation
by a roadside. I collected some seeds, and also took cuttings. These I
planted in a garden border, keeping this well moist with a hand-light
over it, and a saucer of water, so that the air should be thoroughly
moist as well. Its natural conditions were thus completely reversed. They
all grew vigorously. The new branches of the first year’s growth bore
spines, proving their hereditary character, but instead of their being
long and stout, they were not an inch long, and like needles. This proved
the spines to be a hereditary feature. In the second year there were none
at all; moreover, the plants blossomed, and, taken altogether, there was
no appreciable difference from _O. repens, L._”
From this experiment Professor Henslow draws the inference that acquired
characters tend to be inherited in plants. In our opinion the experiment
affords strong evidence against the Lamarckian doctrine. Here we have a
plant which has, perhaps, for thousands of generations developed spines
owing to its dry environment. If acquired characters are inherited we
should have expected this spiny character to have become fixed and
persisted under changed conditions, for some generations at any rate. But
what do we find? By the second year the thorns have entirely disappeared.
All the years during which the plant was exposed to a dry environment
have left no stamp upon it. The fact that the new branches of the first
year’s growth bore small spines is not, as Professor Henslow asserts,
proof of their hereditary character. It merely shows that the initial
stimulus to their development occurred while the plant was still in its
dry surroundings.
In the same way all other so-called proofs of the heredity of acquired
characters break down when critically examined.
In our opinion “not proven” is the proper verdict on the question of the
possibility of the inheritance of acquired characters in the higher
animals. One thing is certain, and that is that acquired characters are
not commonly inherited in those organisms in which there is a sharp
distinction between the germinal and the somatic cells.
It is nothing short of a misfortune that Haeckel’s _History of Creation_,
which seems to be so widely read in England, should be built on a
fallacious foundation. It seems to us that this work is calculated to
mislead rather than to teach.
Our attitude is not quite that of the Wallaceian school, which denies the
possibility of the inheritance of acquired characters. In practice,
however, the attitude we adopt is as fatal to Lamarckism in all its forms
as the dogmatic assertions of the Wallaceians. It matters not whether
acquired characters are very rarely or never inherited. In either case
their inheritance cannot have played an important part in evolution. All
those theories which rely on use-inheritance as a factor in evolution are
therefore in our opinion worthless, being opposed to facts. Our attitude,
then, is that the inheritance of acquired characteristics, if it does
occur, is so rare as to be a negligible quantity in organic evolution.
We may add that the position which we occupy will not be affected even if
the Lamarckians do succeed eventually in proving that some acquired
characters are really inherited. Such proof would merely help to
elucidate some of the problems which confront the biologist. Thus the
question of the inheritance of acquired characters, while full of
interest, has no very important bearing on the question of the making of
species.
The Wallaceian School
The Wallaceians hold the doctrines which have been set forth above as
those of the Neo-Darwinian school. It is incorrect to call those who pin
their faith to the all-sufficiency of natural selection Neo-Darwinians,
because Darwin at no time believed that natural selection explained
everything. Darwin moreover was a Lamarckian to the extent that he was
inclined to think that acquired characteristics could be inherited. His
theory of inheritance by gemmules involved the assumption that such
characters are inherited. It is Wallace who out-Darwins Darwin, who
preaches the all-sufficiency of natural selection. For this reason we dub
the school which holds this article of belief, and to which Weismann,
Poulton, and apparently Ray Lankester belong, the Wallaceian school.
Weismann has put forth a theory of inheritance, that of the continuity of
the germ plasm, which makes this inheritance a physical impossibility. We
believe that the Wallaceians have erred as far from the truth as the
Lamarckians have, because, as we shall show hereafter, a great many of
the organs and structures displayed by organisms cannot be explained on
the natural selection hypothesis. Those who pin their faith to this,
needlessly increase the difficulty of the problem which they have to
face.
There remains the third school, to which we belong, and of which Bateson,
De Vries, Kellog and T. H. Morgan appear to be adherents. This school
steers a course between the Scylla of use-inheritance and the Charybdis
of the all-sufficiency of natural selection. It may seem surprising to
some that we should class De Vries as a Neo-Darwinian, seeing that he is
the originator of the theory of evolution by means of mutations, which we
shall discuss in Chapter III. of this work. As a matter of fact the
theory of mutations should be regarded, not as opposed to the theory of
Darwin, but as a theory engrafted upon it. De Vries himself writes:—“My
work claims to be in full accord with the principles laid down by
Darwin.” Similarly Hubrecht writes in the _Contemporary Review_ for
November 1908: “Paradoxical as it may sound, I am willing to show that my
colleague, Hugo de Vries, of Amsterdam, who a few years ago grafted his
_Mutations Theorie_ on the thriving and very healthy plant of Darwinism,
is a much more staunch Darwinian than either Dr Wallace himself, or the
two great authorities in biological science whom he mentions, Sir William
Thistleton Dyer and Professor Poulton.”
Complexity of the Problem
Having classified ourselves, it remains for us (the authors of the
present work) to define our position more precisely. Like Darwin we
welcome all factors which appear to be capable of effecting evolution. We
have no axe to grind in the shape of a pet hypothesis, and consequently
our passions are not roused when men come forward with new ideas
seemingly opposed to some which already occupy the field. We recognise
the extreme complexity of the problems that confront us. We look facts in
the face and decline to ignore any, no matter how ill they fit in with
existing theories. We recognise the strength and the weakness of the
Darwinian theory. We see plainly that it has the defect of the period in
which it was enunciated. The eighteenth century was the age of
cocksureness, the age in which all phenomena were thought to be capable
of simple explanation.
This is well exemplified by the doctrines of the Manchester school as
regards political and economic science. The whole art of legislation was
thought to be summed up in the words _laissez faire_. The whole sphere of
legitimate government was asserted to be the keeping of order and the
enforcing of contracts. Experience has demonstrated that a State guided
solely by these principles is wretchedly governed. A large proportion of
recent Acts of Parliament limits the freedom of contract. Such
limitations are necessary in the case of contracts between the weak and
the strong. Similarly the earlier economists considered political economy
a very simple affair. They asserted that men are actuated by but one
motive—the love of money. All their men were economic men, men devoid of
all attributes save an intense love of gold. Experience has shown that
these premises are not correct. Love of family, pride of race, caste
prejudices are more or less deeply implanted in men, so that they are
rarely actuated solely by the love of money.
The Aim of the Biologist
Thus it is that the political economy of to-day as set forth by Marshall
is far more complex and less dogmatic than that of Ricardo or Adam Smith.
Similarly the political philosophy of Sidgwick is very different to that
of Herbert Spencer. So is it with the theory of organic evolution. The
theory of natural selection is no more able to explain all the varied
phenomena of nature than is Ricardo’s assumption that all men are
actuated solely by the love of money capable of accounting for the
multifarious existing economic phenomena. Even as the love of wealth is
an important motive of human actions, so is natural selection an
important factor in evolution. But even as the majority of human actions
are the resultant of a variety of motives, so are the majority of
existing organisms the resultant of a complex system of forces. Even as
it is the duty of the economist to discover the various motives which
lead to human actions, so is it the duty of the biologist to bring to
light the factors which are operative in the making of species.
CHAPTER II
SOME OF THE MORE IMPORTANT OBJECTIONS TO THE THEORY OF NATURAL SELECTION
Brief statement of Theory—Objections to the Theory fall into two
classes—Those which strike at the root of the Theory—Those which deny
the all-sufficiency of Natural Selection—Objections which strike at
root of Theory are based on misconception—Objections to Wallaceism—The
Theory fails to explain the origin of Variations—Natural Selection
called on to explain too much—Unable to explain beginnings of new
organs—The Theory of change of function—The co-ordination of
variations—The fertility of races of domesticated animals—Missing
links—Swamping effects of intercrossing—Small variations cannot have a
survival value—Races inhabiting same area—Excessive
specialisation—Chance and Natural Selection—Struggle for existence most
severe among young animals—Natural Selection fails to explain mimicry
and other phenomena of colour—Conclusion, that scarcely an organism
exists which does not possess some feature inexplicable on the theory
of Natural Selection as held by Wallace and his followers.
“The burden of proof is on him who asserts” is a rule of evidence which
the man of science should apply as rigidly as does the lawyer.
It is therefore incumbent upon us to prove our assertion that the theory
of natural selection does not afford an adequate explanation of all the
varied phenomena observed in the organic world.
Theory of Natural Selection
The theory of natural selection is so generally understood, that to set
it forth in detail in this place would be quite superfluous.
Darwin, it will be remembered, based his great hypothesis on the
following observed facts:—
1. No two individuals of a species are exactly alike. This is sometimes
called the law of variation.
2. All creatures tend in a general way to resemble their parents in
appearance more closely than they resemble individuals not related to
them. This may be termed the law of heredity.
3. Each pair of organisms produces in the course of a lifetime, on an
average, many more than two young ones.
4. On an average the total number of each species remains stationary.
From (3) and (4) follows the doctrine of Malthus, namely, that many more
individuals are born than can reach maturity.
Darwin applied this doctrine to the whole of the animal and the vegetable
kingdoms.
In his introduction to _The Origin of Species_ he writes:—“As many more
individuals of each species are born than can possibly survive; and as,
consequently, there is a frequently recurring struggle for existence, it
follows that any being, if it vary, however slightly, in any manner
profitable to itself, under the complex and sometimes varying conditions
of life, will have a better chance of surviving, and thus be naturally
selected. From the strong principle of inheritance, any selected variety
will tend to propagate its new and modified form.”
In other words, the struggle for existence amongst all organic beings
throughout the world, which inevitably follows from the high geometrical
ratio of their increase, results in the survival of the fittest, that is
to say, of those best adapted to cope with their enemies and to secure
their food. Since organisms are thus naturally selected in nature, we may
speak of a natural selection which acts in much the same way as the human
breeder does. Darwin’s theory, then, is that all the variety of organisms
which now exist have been evolved from one or more forms by this process
of natural selection.
Various Anti-Darwinian Views
The objections which have been urged against the theory of natural
selection fall into two classes.
I. Those which strike at its root, which either deny that there is any
natural selection, or declare that it is not capable of producing a new
species.
II. Those which are directed against the all-sufficiency of natural
selection to account for organic evolution.
Those of the first class need not detain us long, although among those
who formulate them are to be found some eminent men of science.
Delage alleges that selection is powerless to form species, its function
is, according to him, limited to the suppression of variations radically
bad, and to the maintaining of a species in its normal character. It is
thus an inimical factor in evolution, a retarder rather than an
accelerator of species-change. It merely acts by preserving the type at
the expense of the variants, and so acts as a brake on evolution.
Korschinsky, while possibly not denying that selection occurs in nature,
declares that its influence on evolution is _nil_, or, if it has any
influence, that it is a hindering one.
Eimer similarly denies any capacity on the part of natural selection to
create species.
Pfeffer urges a very different objection. He says that if such a force as
natural selection existed it would transform species much more rapidly
than it does!
Now, in order that the above objections can carry any weight, one of two
sets of conditions must be fulfilled.
Either all organisms must be perfectly adapted to their environment, and
this environment must never change, or there must be inherent in each
species a kind of growth-force which impels the species to develop in
certain fixed directions. In either of these circumstances natural
selection will be an inhibitory force, for if the normal organism is
perfectly adapted to its environment, all variations from the type must
be unfavourable, and natural selection will weed out the individuals that
display them. No careful student of nature can maintain, either that all
animals are perfectly adapted to their environment, or that this never
changes. Hence those who deny that natural selection is a factor in the
making of species, assume the second set of conditions, that species
develop in certain fixed directions, being impelled either by internal or
external forces. How far these ideas are founded on fact we shall
endeavour to determine when speaking of variation. It must suffice at
present to say that even if any of these views of orthogenesis be
established, natural selection will have, so to speak, a casting vote, it
will decide which series of species developing along preordained lines
shall survive and which shall not survive.
Thus we reach by a different line of argument the conclusion we arrived
at in the last chapter: namely, there is no room for doubt that natural
selection is a factor in the making of species.
We must now pass on to the second class of objections, those which are
urged against the all-sufficiency of natural selection. So numerous are
these that it is not feasible to consider them all. A brief notice of the
more important ones should suffice to satisfy any unbiassed person;
firstly, that natural selection is an important factor in evolution;
secondly, that the position taken up by Wallace and his followers, that
natural selection, acting on minute variations, is the one and only
factor in organic evolution, is untenable.
Darwinism does not explain Variation
1. It has been urged that the Darwinian theory makes no attempt to
explain variation, and that, until we know what it is that causes
variations, we are not in a position to explain evolution. This of course
is quite true, but the objection is scarcely a fair one, since, as we
have seen, Darwin freely admitted that his theory made no attempt to
explain the origin of variations. It is not reasonable to object to a
theory because it fails to explain phenomena with which it expressly
states that it is not concerned. On the other hand, the objection is one
that must be reckoned with, for, as we shall see, it makes a great
difference to the importance of natural selection as a factor in
evolution if variations appear indiscriminately in all directions, as
Darwin tacitly assumed they do, or whether, as some biologists believe,
they are determinate in direction, being the result of a growth-force
inherent in all organisms.
2. Very similar to the above-mentioned objection is that which points out
that it is a long journey from Amoeba to man. It is difficult to believe
that this long course of development from the simple to the complex is
due to the action of a blind force, to the survival of those whose
fortuitous variations happen to be best adapted to the environment. The
result seems out of all proportion to the cause. There must be some
potent force inherent in protoplasm, or behind organisms, impelling them
upwards. This objection is as difficult to refute as it is to establish.
It is purely speculative.
3. A very serious objection to the Darwinian theory is that the
beginnings of new organs cannot be explained by the action of natural
selection on fortuitous minute variations, and natural selection can act
on an organ only when that organ has attained sufficient size to be of
practical utility to its possessor. When once an organ has come into
being it is not difficult to understand how it can be improved, modified
and developed by natural selection. But how can we explain the origin of
an organ such as a limb by the action of natural selection on minute
variations?
Theory of Change of Function
The theory of the change of function goes some way towards meeting the
difficulty, for by means of it we are able to understand how certain
organs, as, for example, the lung of air-breathing animals, might have
come into existence. This is said to have been developed from the
swimming-bladder of fishes. This bladder is, to use the words of Milnes
Marshall, “a closed sac lying just underneath the vertebral column. In
many fish it acquires a connection by a duct with some part of the
alimentary canal. It then becomes an accessory breathing organ,
especially in those fish which are capable of living out of water for a
time, _e.g._ the _Protopterus_ of America. An interesting series of
modifications exists connecting the air-bladder with the lung of the
higher vertebrates, which is undoubtedly the same organ.”
This theory, however, does not seem adequate to explain the origin of all
organs. It does not explain, for example, how limbs developed in a
limbless organism. Wallace tried to avoid the difficulty by asserting
that it is unreasonable to ask a new theory that it shall reveal to us
exactly what took place in remote geological ages and how it took place.
To this the obvious reply is, firstly, that we ought not to give
unqualified acceptance to any theory of evolution until it does afford us
such explanations, and, secondly, that the theory of the origin of
species by means of natural selection is no longer a new one.
Latterly, however, Wallace appears to have given up all hope of being
able to account for the origin of new organs by means of natural
selection, for he states on page 431 of the issue of the _Fortnightly
Review_ for March 1909: “It follows—not as a theory but as a fact—that
whenever an advantageous variation is needed, it can only consist in an
increase or decrease of some power or faculty already existing.” Now, in
order for an increase or decrease to occur, there must be something in
existence to be increased or diminished. Wallace, it is true, speaks here
only of powers and faculties; but it can scarcely be supposed that he
believes that variations as to structure are intrinsically different from
those relating to powers and faculties.
4. Herbert Spencer urges, as an objection to the theory of natural
selection, that favourable variations in one organ are likely to be
counterbalanced by unfavourable variations in some other organ. He
maintains that the chances are enormous against the occurrence of the
“many coincident and co-ordinated variations” that are necessary to
create a life or death determining advantage.
This objection was urged by a writer in the _Edinburgh Review_ in January
1909, and even by Wallace himself in the _Fortnightly Review_ last March
against the mutation theory. This objection, strong though it appears on
paper, exists only in the imagination of the objector.
Those who urge it display a misunderstanding of the manner in which
natural selection acts, and ignorance of the phenomenon of the
correlation of organs.
Correlation
Natural selection deals with an organism as a whole. Its effect is to
permit those creatures to survive which, taken as a whole, are best
adapted to their environment.
Physiologists insist with ever-increasing emphasis that there is more or
less correlation and inter-connection between the various parts of an
organism.
The several organs of an animal are not so many isolated units. It is
impossible to act on one organ without affecting some or all of the
others.
Variations in a given direction of one organ are usually accompanied by
correlated variations in some of the other organs. If strength be of
paramount importance to an animal, natural selection will tend to
preserve those individuals which exhibit strength to a marked degree, and
this exhibition of strength may be accompanied by other peculiarities,
such as short legs or a certain colour, so that natural selection will
indirectly tend to produce individuals with short legs and having the
colour in question, and it may happen that this particular colour is one
that renders the animal more conspicuous than the normal colour does.
Nevertheless, on account of the all-needful strength which accompanies
it, those animals so coloured may survive while those of a more
protective hue perish. Thus, paradoxical though it seems, natural
selection may indirectly be responsible for characteristics which in
themselves are injurious to the individual. This is probably the case as
regards the decorative plumage of some male birds. The phenomenon of
correlation was recognised by Darwin, and has, we believe, played an
important part in the making of species. We shall deal more fully with
the subject in a later chapter.
5. An oft-urged objection to the theory of natural selection, and one
which weighed very strongly with Huxley, is that breeders have hitherto
not succeeded in breeding a variety which is infertile with the parent
species. If, Huxley asked, breeders cannot produce such a thing, how can
we say we consider it proved that natural selection produces new species
in nature? This objection, however, loses much of its force in view of
the fact that many perfectly distinct species are quite fertile when bred
together. We shall recur to this in Chapter IV.
6. The fact that palæontology has hitherto failed to yield links
connecting many existing species is a classical objection to the theory
of the origin of species by gradual evolution.
Missing Links
Wallace states this objection as follows, on page 376 of his _Darwinism_:
“Many of the gaps that still remain are so vast that it seems incredible
to these writers that they could ever have been filled up by a close
succession of species, since these must have been spread over so many
ages, and have existed in such numbers, that it seems impossible to
account for their total absence from deposits in which great numbers of
species belonging to other groups are preserved and have been
discovered.”
Wallace’s reply is to the effect that in the case of many species
palæontology affords abundant evidence of the gradual change of one
species into another, the foot of the horse being a well-known case. The
genealogy of this noble quadruped can be traced from the Eocene four-toed
_Orohippus_, through the _Mesohippus_, the _Miohippus_, the
_Protohippus_, and the _Pliohippus_, until we reach the one-toed _Equus_.
Wallace further points out that in order that the fossil of any organism
may be preserved, the “concurrence of a number of favourable conditions”
is required, and against this the chances are enormous. Lastly, he urges
the imperfection of our knowledge of the things that lie embedded in the
earth’s crust.
The objection based on the lack of “missing links” loses some of its
force if we accept the theory that species sometimes arise as sports.
Thus, suppose a species with well-developed horns produces as a mutation
a hornless variety, which eventually replaces the horned form, we should
look in vain for any forms intermediate between the parent and the
daughter species. On the other hand, it is significant that just where
the links are most needed they are missing. For example, the splint bones
of the horse, taken in conjunction with the feet of existing tapirs,
which have four toes in front and three behind, would have led us to
infer, without the help of the geological record, that the horse was a
descendant of a polydactyle ancestor. When, however, we come to the
origin of birds, bats, and whales, palæontology fails to give us any
assistance, so that we are in the dark as to the origin of such really
important modifications.
7. The swamping effects of inter-crossing is an objection which has been
repeatedly urged against the Darwinian theory.
This objection is not so serious as it appears at first sight. Darwin and
Wallace maintain, firstly, that natural selection acts by eliminating all
individuals except those which present favourable variations. The
favoured few alone survive and mate with one another, so that there is
here no question of the swamping effects of inter-crossing, none but
well-adapted individuals being left to mate with one another.
The objection gains greater force when directed against the theory that
evolution proceeds by sudden jumps. But in this connection we must bear
in mind that the experiments of Mendel and his followers have
demonstrated that some of the offspring of crosses may resemble their
pure ancestors and breed true _inter se_. Nor is this all.
Recurrent Mutations
Experience shows that where a mutation, or sport, or discontinuous
variation occurs, it frequently repeats itself; for example, the
black-winged sport of the peafowl has occurred several times over and in
different flocks of birds. The sport or mutation must have a definite
cause. There must be something within the organism, something in the
generative cells, which causes the mutation to arise; and hence, on _a
priori_ grounds, we should expect the same mutation to arise about the
same time in many individuals. It seems legitimate to infer that things
have been quietly working up to a climax. When this is reached there
results a mutation. Therefore we should expect sudden mutations to appear
simultaneously in a number of individuals. To this important subject we
shall return.
8. An almost insuperable objection to the theory that species have
originated by the action of natural selection on minute variations, is
that such small differences cannot be of a life-or-death value, or, as it
is usually called, a survival value to their possessor. But if evolution
is the result of the preservation by natural selection of such slight
variations, it is absolutely necessary that each of these should possess
a survival value.
As D. Dewar has pointed out, on page 704 of vol. ii. of _The Albany
Review_, it is only when the beast of prey and its victim are evenly
matched as regards fleetness and power of endurance that small variations
in these qualities can have a survival value. But in the rough and tumble
of the struggle for existence the victim and its foe are but rarely
well-matched. Take as an example the case of a flycatcher. “This bird,”
writes D. Dewar, “will sometimes take three or four insects in the course
of one flight; all are captured with the same ease, although the length
of wing in each victim varies. So great is the superiority of the bird
that it does not notice the difference in the flying powers of its puny
quarry.” It is unnecessary to labour this point.
9. Species or varieties differing considerably in colour may exist side
by side, as the hooded and carrion crows, the white and dark breasted
forms of the Arctic skua, the pale and dark forms of the fulmar petrel,
the grey and rufous forms of the American scops owl (_Megascops asio_).
It is true that preponderance of one form or another in certain districts
points to some advantage possessed by one over the other, but, for all we
know, it may be due to heredity, and in any case the co-existence of the
two types in part of their range, or at certain seasons, shows that
selection is not at all rigorous.
The same argument applies to the co-existence of very
differently-coloured species with generally similar habits, such as that
of the jaguar and puma in South America, and the five very
differently-coloured flycatchers in the Nilgiri Hills.
Leaf-butterflies
In short, there is abundant evidence to show that considerable
differences in colour do not appear to have any effect on the chances of
survival in the struggle for existence of those that display them. Yet
this is precisely what the supporters of the Darwinian hypothesis cannot
afford to admit, for they then find it impossible to account for the
origin of such a form as _Kallima_, the leaf-butterfly, by the action of
natural selection. As most people are aware, this creature displays a
remarkable resemblance to a decaying leaf. “These butterflies” (there are
several species which show the marvellous imitation), writes Kellog, on
page 53 of _Darwinism To-day_, “have the under sides of both fore and
hind wings so coloured and streaked that when apposed over the back in
the manner common to butterflies at rest, the four wings combine to
resemble with absurd fidelity a dead leaf still attached by a short
petiole to the twig or branch. I say absurd, for it seems to me the
resemblance is over-refined. Here for safety’s sake it is no question of
mimicking some one particular kind of other organism or inanimate thing
in nature which birds do not molest. It is simply to produce the effect
of a dead leaf on a branch. Leaf-shape and general dead-leaf
colour-scheme are necessary for this illusion. But are these following
things necessary? namely, an extra-ordinarily faithful representation of
mid-rib and lateral veins, even to faint microscopically-tapering vein
tips; a perfect short petiole produced by the apposed ‘tails’ of the
hind-wings; a concealment of the head of the butterfly so that it shall
not mar the outlines of the lateral margin of the leaf; and finally,
delicate little flakes of purplish or yellowish brown to mimic spots of
decay and fungus-attacked spots in the leaf! And, as culmination, a tiny
circular clear spot in the fore-wings (terminal part of the leaf) which
shall represent a worm-eaten hole, or a piercing of the dry leaf by
flying splinter, or the complete decay of a little spot due to fungus
growth! A general and sufficient seeming of a dead leaf, object of no
bird’s active interest, yes, but not a dead leaf modelled with the
fidelity of the waxworkers in the modern natural history museums. When
natural selection has got Kallima along to that highly desirable stage
when it was so like a dead leaf in general seeming that every bird
sweeping by saw it only as a brown leaf clinging precariously to a
half-stripped branch, it was natural selection’s bounden duty, in
conformance to its obligations to its makers, to stop the further
modelling of Kallima and just hold it up to its hardly won advantage. But
what happens? Kallima continues its way, specifically and absurdly
dead-leafwards, until to-day it is a much too fragile thing to be
otherwise than very gingerly handled by its rather anxious
foster-parents, the Neo-Darwinian selectionists.” It is obvious that if
natural selection has produced so highly specialised an organism as the
dead-leaf butterfly, every minute variation must be of value and have
been seized upon by natural selection.
A Dilemma
Thus the Wallaceians are on the horns of a dilemma. If they assert, as
they appear to do, that every infinitesimal variation has a survival
value, they find it difficult to explain the existence, side by side of
such forms as the hooded and carrion crows, to say why in some species of
bird both sexes assume a conspicuous nuptial plumage at the very time
when they stand most in need of protective coloration, why the cock
paradise flycatcher is chestnut for the first two years of his life and
then turns as white as snow. If, on the other hand, the Wallaceians
assert that small variations are unimportant and have no survival value,
they are, as Kellog points out, in trouble over the close and detailed
resemblance which the Kallima butterflies bear to dead leaves.
10. An objection to the Darwinian theory which has been advanced by Conn,
Henslow, D. Dewar, and others, is that the selection theory fails to take
into account the effects of chance. “If,” writes D. Dewar on page 707 of
_The Albany Review_, vol. ii., “the struggle for existence were of the
nature of a race at a well-regulated athletic meeting, where the
competitors are given a fair start, where there is no difference in the
conditions to which the various runners are subjected, then indeed would
every variation tell. I would rather liken the struggle for existence to
the rush to get out of a crowded theatre, poorly provided with exits,
when an alarm of fire is given. The people to escape are not necessarily
the strongest of those present. Propinquity to a door may be a more
valuable asset than strength.”
Or again, we may take the imaginary case of some antelopes being pursued
by wolves. The chase, being prolonged, brings the antelopes to a locality
with which they are not familiar. The foremost of the herd, the most
swift, and therefore the individual which should stand the best chance of
survival, suddenly finds himself on soft boggy ground, which, owing to
the depth to which his feet sink into the soil, seriously impedes his
progress. His fellow antelopes, now outdistanced, seeing his predicament,
take another course and soon leave him behind, to fall an easy prey to
his foes. Here we have a case of the perishing of the most fit as regards
the important point of speed.
The Effects of Chance
Writing of plants, Professor Henslow says, on page 16 of _The Heredity of
Acquired Characters in Plants_: “As the whole of the animal kingdom
ultimately lives upon the vegetable, plants must supply the entire
quantity of food supplied, not to add innumerable vegetable parasites as
well, for both young and old. Myriads of germinating seeds perish
accordingly, being destroyed by slugs and other mollusca, and ‘mildews,’
etc. But far more seeds and spores—about 50,000,000 of these it is
calculated can be borne in a single male-fern—never germinate at all.
They fall where the conditions of life are unfavourable and perish. This
misfortune is not due to any inadaptiveness in themselves, but to the
surrounding conditions which will not let them germinate. Thus thousands
of acorns and other fruits, as of elder, drop upon the ground in and by
our hedges, road-sides, copses, and elsewhere; but scarcely any or even
no seedlings are to be seen round the trees.”
Every year thousands of birds perish in the great migratory flight,
others succumb in a cyclone, a fierce tropical storm, a prolonged
drought, a severe frost. Here death overtakes multitudes, all that dwell
in a locality, the weak and the strong, the swift and the slow alike.
This objection may be met by saying that in the long run it is the
fittest that will survive. This is true. The objection is nevertheless of
importance in showing how exceedingly uncertain must be the action of
natural selection if it have but small variations upon which to work. In
such circumstances the mills of natural selection may grind surely, but
they must grind very slowly.
11. We must bear in mind that the struggle for existence is most severe
among young animals, among creatures that are not fully developed. Nature
pays no attention to potentialities. The weak go to the wall in the
conflict, even though, if allowed time, they might develop into prodigies
of strength.
Moreover, and this is an important point, death in the case of young
creatures overtakes broods and families rather than individuals.
The above-cited objections to the theory that species have originated by
the action of natural selection on minute variations, are mostly of a
general nature; let us now notice briefly a few more concrete objections.
We shall not devote much space to these in the present chapter, since we
shall be continually confronted with them when dealing with the subject
of animal colouring.
The Origin of Mimicry
12. Natural selection, as we shall see, fails to account for the origin
of what is known as protective mimicry. Some insects look like inanimate
objects, others resemble other insects which are believed or known to be
unpalatable. Those creatures displaying this resemblance to other objects
or creatures, and deriving profit therefrom, are said to “mimic” the
objects or creatures they copy. They are also called “Mimics.” It is easy
to understand the profit that these mimics derive from their mimicry.
When once the disguise has been assumed we can comprehend how natural
selection will tend to improve it by eliminating those that mimic badly;
but it seems to us that the theory fails utterly to account for the
origin of the likeness.
13. Similarly, the Neo-Darwinian theory fails to explain the colours of
the eggs of birds laid in open nests, why, for example, the eggs of the
accentor or hedge-sparrow are blue and those of the doves are white.
14. The theory fails to give a satisfactory explanation of the phenomena
of sexual dimorphism. Why, for example, in some species of doves and
ducks, the sexes are alike, while in other species with similar habits
they differ in appearance.
15. It fails to explain why the rook is black and why the jackdaw has a
grey neck.
These and many other objections we shall deal with more fully in the
chapter on animal colouration. It must suffice here to mention them, and
to say that our experience teaches us that scarcely a single species of
bird or beast exists which does not display some characteristic which is
inexplicable on the theory that natural selection, acting on small
variations, is the one and only cause of organic evolution.
CHAPTER III
VARIATION
The assumption of Darwin and Wallace that variations are haphazard in
origin and indefinite in direction—If these assumptions be not correct
Natural Selection ceases to be the fundamental factor in
evolution—Darwin’s views regarding variation underwent modification—He
eventually recognised the distinction between definite and indefinite
variations, and between continuous and discontinuous variations—Darwin
attached but little importance to either definite or discontinuous
variations—Darwin’s views on the causes of variations—Criticism of
Darwin’s views—Variations appear to occur along certain definite
lines—There seems to be a limit to the extent to which fluctuating
variations can be accumulated—De Vries’ experiments—Bateson on
“discontinuous variation”—Views held by De Vries—Distinction between
continuous and discontinuous variations—The work of De Vries—Advantages
enjoyed by the botanist in experimenting on the making of
species—Difficulties encountered by the animal breeder—Mutations among
animals—The distinction between germinal and somatic variations—The
latter, though not transmitted to offspring, are often of considerable
value to their possessor in the struggle for existence.
Nature of Variation
As we have already seen, the Darwinian theory, unlike that of Lamarck,
does not attempt to explain the origin of variations. It is content with
the fact that variations do occur.
Although Darwin did not try to explain how it is that variation occurs,
and was very guarded in the expressions he used concerning it, he assumed
that variations are indefinite in variety and occur indiscriminately in
all directions, as the following quotations from the _Origin of Species_
will show: “But the number and diversity of inheritable deviations of
structure . . . are endless” (page 14, ed. 1902). “The variations are
supposed to be extremely slight, but of the most diversified nature.” “I
have hitherto sometimes spoken as if the variations so common and
multiform with organic beings under domestication, and in a lesser degree
to those under nature, were due to chance. This, of course, is a wholly
incorrect expression, but it serves to acknowledge plainly our ignorance
of the cause of each particular variation” (page 164).
Wallace is far less guarded in his expressions. On page 82 of his
_Darwinism_ he speaks of “the constant and large amount of variation of
every part in all directions . . . which must afford an ample supply of
favourable variations whenever required.”
The double assumption that variations are for all practical purposes
haphazard in origin and indefinite in direction is necessary if natural
selection is to be the main factor in evolution. For if variations be not
haphazard, if they are definite, if there be a directive force behind
them, like fate behind the classical gods, then selection is not the
fundamental cause of evolution. It can at most effect, not the origin of
species, but the survival of certain species which have arisen as the
result of some other force. Its position is changed; it is no longer a
cause of the origin of new organisms, but a sieve determining which of
certain ready-made forms shall survive. Evidently, then, we shall not be
able to fully understand the evolutionary process until we have
discovered how it is that variations are caused. In other words, we must
go considerably farther than Darwin attempted to do.
Before proceeding to inquire into the true nature of variations, it
behoves us to set forth briefly the ideas of Darwin on the subject. We
shall then be in a position to see how much progress has been made since
the days of that great biologist.
It is not at all easy to discover exactly what were Darwin’s views on the
subject of variation. A perusal of his works reveals contradictions, and
gives one the impression that he himself scarcely knew his own mind upon
the subject. This should not be a matter for surprise.
We must remember that Darwin had to do pioneer work, that he had to deal
with altogether new conceptions. Such being the case, his ideas were of
necessity somewhat hazy; they underwent considerable modification as
fresh facts came to his knowledge.
Definite and Indefinite Variability
Towards the end of his life Darwin recognised that variability is of two
kinds—definite and indefinite. Indefinite variation is indiscriminate
variation in all directions around a mean, variation which obeys what we
may perhaps call the law of chance. Definite variation is variation in a
determinate direction—variation chiefly on one side of the mean. Darwin
believed that these determinate variations were caused by external
forces, and that they are inherited. He thus accepted Lamarckian factors.
“Each of the endless variations,” he writes, “which we see in the plumage
of our fowls, must have had some efficient cause, and if the same causes
were to act uniformly during a long series of generations on many
individuals, all probably would be modified in the same direction.”
But Darwin was always of opinion that this definite variability, this
variability in one direction as the result of some fixed cause, is far
less important, from an evolutionary point of view, than indefinite
variability, that it is the exception rather than the rule, that the
usual result of changed conditions is to let loose a flood of indefinite
variability, that it is almost exclusively upon this that natural
selection acts.
Darwin also recognised that variations differ in degree, even as they do
in kind. He perceived that some variations are much more pronounced than
others. He recognised the distinction between what are now known as
continuous and discontinuous variations. The former are slight departures
from the normal; the latter are considerable deviations from the mean or
mode; great jumps, as it were, taken by nature, as, for example, the pea
and the rose combs of fowls, which were derived from the normal single
comb.
Monstrosities
“At long intervals of time,” wrote Darwin, “out of millions of
individuals reared in the same country and fed on nearly the same food,
deviations of structure so strongly pronounced as to deserve to be called
monstrosities arise, but monstrosities cannot be separated by any
distinct line from slighter variations.” Therefore it is evident that he
regarded the difference between continuous and discontinuous variations
as not one of kind, but merely of degree. To the discontinuous variations
Darwin attached very little importance from an evolutionary point of
view. He looked upon them as something abnormal.
“It may be doubted,” he wrote, “whether such sudden and considerable
deviations of structure such as we occasionally see in our domestic
productions, more especially with plants, are ever permanently propagated
in a state of nature. Almost every part of every organic being is so
beautifully related to its complex conditions of life that it seems as
improbable that any part should have been suddenly produced perfect, as
that a complex machine should have been invented by a man in a perfect
state. Under domestication monstrosities sometimes occur which resemble
normal structures in widely different animals. Thus pigs have
occasionally been born with a sort of proboscis, and if any wild species
of the same genus had naturally possessed a proboscis, it might have been
argued that this had appeared as a monstrosity; but I have as yet failed
to find, after diligent search, cases of monstrosities resembling normal
structures in nearly allied forms, and these alone bear on the question.
If monstrous forms of this kind ever do appear in a state of nature and
are capable of reproduction (which is not always the case), as they occur
rarely and singly, their preservation would depend on unusually
favourable circumstances. They would, also, during the first and
succeeding generations cross with the ordinary form, and thus their
abnormal character would almost inevitably be lost.” But, in a later
edition of the _Origin of Species_, Darwin seems to contradict the above
assertion: “It should not, however, be overlooked that certain rather
strongly marked variations, which no one would rank as mere individual
differences, frequently recur owing to a similar organisation being
similarly acted on—of which fact numerous instances could be given with
our domestic productions. In such cases, if the varying individual did
not actually transmit to its offspring its newly acquired character, it
would undoubtedly transmit to them, as long as the existing conditions
remained the same, a still stronger tendency to vary in the same manner.
There can also be little doubt that the tendency to vary in the same
manner has often been so strong that all the individuals of the same
species have been similarly modified without the aid of any form of
selection. Or only a third, fifth, or tenth part of the individuals may
have been thus affected, of which fact several instances could be given.
Thus Graba estimates that about one-fifth of the guillemots in the Faroe
islands consist of a variety so well marked, that it was formerly ranked
as a distinct species under the name _Uria lacrymans_. In cases of this
kind, if the variation were of a beneficial nature, the original form
would soon be supplanted by the modified form, through the survival of
the fittest.” Here we seem to have a plain statement of the origin of new
forms by mutation.
Minute Variations
Again, we read (page 34): “Some variations useful to him (_i.e._ man)
have probably arisen suddenly, or by one step; many botanists, for
instance, believe that the fuller’s teasel, with its hooks, which cannot
be rivalled by any mechanical contrivance, is only a variety of the wild
Dipsacus; and this amount of change may have suddenly arisen in a
seedling. This is known to be the case with the turnspit dog.”[2] But, as
we have already said, Darwin at no time attached much importance to these
jumps made by nature as a factor in evolution. He pinned his faith to the
minute, indefinite variations which he believed could be piled up, one
upon another, so that, if allowed sufficient time, either nature or the
human breeder could, by a continued selection of these minute variations,
call into being any kind of organism. The importance of selection, he
writes, “consists in the great effect produced by the accumulation in one
direction, during successive generations, of differences absolutely
inappreciable by an uneducated eye” (page 36). On page 132 he writes: “I
can see no limit to the amount of change, to the beauty and complexity of
the coadaptations between all organic beings . . . which may have been
effected[3] in the long course of time by nature’s power of selection.”
He expressly states, on page 149, that he sees no reason to limit the
process to the formation of genera alone.
Although the theory of natural selection does not attempt to explain the
causes of variation, Darwin paid some attention to the subject. He
believed that both internal and external causes contribute to variation,
that variations tend to be inherited whether the result of causes within
the organism or outside it. He believed that the inherited effect of use
and disuse was a cause of variation, and cited, as examples, the lighter
wing-bones and heavier leg-bones of the domestic duck and the drooping
ears of some domestic animals. He supposed that animals showed a greater
tendency to vary when under domestication than when in their natural
state, attributing the supposed greater variability to the excess of food
received, and the changed conditions of the life of domestic animals.
Nevertheless, he was fully alive to the fact that “nearly similar
variations sometimes arise under, as far as we can judge, dissimilar
conditions; and, on the other hand, dissimilar variations arise under
conditions which appear to be nearly uniform.” In other words, the nature
of organisms appeared to Darwin to be a more important factor in the
origin of variations than external conditions. Evidence of this is
afforded by the fact that some animals are more variable than others.
Finally, he frankly admitted how great was his ignorance of the causes of
variability. Variability is, he stated, governed by unknown laws which
are infinitely complex.
Lines of Variation
It will be convenient to deal with each of Darwin’s main ideas on
variation separately, and to consider to what extent they seem to require
modification in the light of later research.
Firstly, Darwin believed that variations arise in what appears to be a
haphazard manner, that they occur in all directions, and seem to be
governed by the same laws as chance. It is our belief that we are now in
a position to make more definite statements regarding variation than
Darwin was able to.
Biologists can now assert definitely that variations do not always occur
equally in all directions. The results of many years of the efforts of
practical breeders demonstrate this. These men have not been able to
produce a green horse, a pigeon with alternate black and white feathers
in the tail, or a cat with a trunk, for the simple reason that the
organisms upon which they operated do not happen to have varied in the
required direction. It may perhaps be objected that breeders have no
desire to produce such forms; had they wished to do so, they would
probably have succeeded. To this objection we may reply that they have
not managed to produce many organisms, which would be highly desirable
from a breeder’s point of view, as, for example, a blue rose, hens that
lay brown eggs but do not become broody at certain seasons of the year,
or a cat that cannot scratch.
As Mivart well says, on page 118 of his _Genesis of Species_, “Not only
does it appear that there are barriers which oppose change in certain
directions, but that there are positive tendencies to development along
certain special lines. In a bird which has been kept and studied like the
pigeon, it is difficult to believe that any remarkable spontaneous
variations would pass unnoticed by breeders, or that they would not have
been attended to and developed by some fancier or other. On the
hypothesis of indefinite variability, it is then hard to say why pigeons
with bills like toucans, or with certain feathers lengthened like those
of trogons, or those of birds of paradise, have never been produced.”
There are certain lines along which variation seems never to occur. Take
the case of the tail of a bird. Variable though this organ be, there are
certain kinds of tail that are seen neither in wild species nor
domesticated races. A caudal appendage, of which the feathers are
alternately coloured, occurs neither in wild species nor in artificial
breeds. For some reason or other, variations in this direction do not
occur. Similarly, with the exception of one or two of the “Noddy” terns,
whenever a bird has any of its tail feathers considerably longer than the
others, it is always the outer pair or the middle pair that are so
elongated. It would thus appear that variations in which the other
feathers are especially lengthened do not usually occur. The fact that
they are elongated in two or three wild species is the more significant,
because it shows that there is apparently nothing inimical to the welfare
of a species in having, say, the third pair of tail feathers from the
middle exceptionally prolonged.
Breeders’ Boasts
This is a most important point, and one which seems to be ignored by the
majority of scientific men, who appear to be misled by the boastful talk
of certain successful breeders. Thus, on page 29 of the _Origin of
Species_, Darwin quotes, with approval, Youatt’s description of selection
as “the magician’s wand, by means of which he may summon into life
whatever form and mould he pleases.” Darwin further cites Sir John
Sebright as saying, with regard to pigeons, that he would “produce any
given feather in three years, but it would take him six years to obtain
head and beak.”
If it were possible absolutely to originate anything by selection,
horticulturists would almost certainly ere this have produced a pure
black flower. The fact that not a single mammal exists, either in nature
or under domestication, with scarlet, blue, or green in its hair, appears
to show that, for some reason or other, mammals never vary in any of
these directions.
The fact that so few animals have developed prehensile tails seems to
indicate that variation does not often occur in that direction, for
obviously a prehensile tail is of the very greatest utility to its
possessor; so that there can be little room for doubt that it would be
seized upon and preserved by natural selection, whenever it occurred.
As E. H. Aitken very truly says, “so early and useful an invention
should, one would think, have been spread widely in after time; but there
appears to be some difficulty in developing muscles at the thin end of a
long tail, for the animals that have turned it into a grasping organ are
few and are widely scattered. Examples are the chameleon among lizards,
our own little harvest mouse, and, pre-eminent among all, the American
monkeys” (_Strand Magazine_, Nov. 1908).
Even as there are many variations which seem never to occur in nature, so
are there others which occur so frequently that they may be looked for in
any species. Albinistic forms appear now and again in almost every
species of mammal or bird; while melanistic sports, although not so
common, are not by any means rare.
Every complete manual on poultry gives for each breed a note of the
faults which constantly appear, and which the fancier has to watch
carefully for and guard against. The fact that these “faults” occur so
frequently in each breed shows how strong is the tendency to vary in
certain definite directions. It is true that some of these faults are in
the nature of reversions, as, for example, the appearance of red hackles
in the cocks of black breeds of poultry. On the other hand, some
certainly are not reversions, such as the appearance of a white ring in
the neck of the female of the Rouen duck, which should resemble the
Mallard as regards the plumage of the neck. Again, the tendency of Buff
Orpingtons to assume white in the wings and tail must be regarded as a
variation which is not in the nature of a reversion. In short, the
efforts of all breeders are largely directed to fighting against the
tendencies which animals display towards variation in certain directions.
Albinistic Variations
This tendency to vary in the direction of whiteness may account for many
of the white markings which occur in nature, as, for example, the white
tails of the Sea Eagle (_Haliaetus albicilla_) the Nicobar Pigeon
(_Caloenas nicobarica_), and many hornbills. Provided that such
variations are not too great a handicap to their possessors in the
struggle for existence, natural selection will allow them to persist.
It was the belief of Linnæus, based on experience, that every blue or
red-coloured flower is likely to produce a white variety, hence he held
that it is not safe to trust to colour for the identification of a
botanical species.
On the other hand, white flowers are not likely to produce red varieties,
and we believe we may positively assert that they never produce a blue
sport. Similarly, white animals appear not to give rise to colour
varieties.
We are never surprised to find that an ordinary upright plant produces as
a sport or mutation a pendulous, or fastigiate form. These aberrant
varieties, be it noted, occur in species which belong to quite different
orders.
De Vries points out that laciniated leaves appear in such widely
separated trees and shrubs as the walnut, the beech, the hazel-nut, and
the turnip.
Another example of the definiteness of variation is furnished by what
Grant Allen calls the “Law of Progressive Colouration” of flowers.
On pp. 20, 21 of _The Colours of Flowers_, he writes, “All flowers, as we
know, easily sport a little in colour. But the question is, do their
changes tend to follow any regular and definite order? Is there any
reason to believe that the modification runs from any one colour toward
any other? Apparently there is. . . . All flowers, it would seem, were in
their earliest form yellow; then some of them became white; after that a
few of them grew to be red or purple; and finally a comparatively small
number acquired the various shades of lilac, mauve, violet, or blue.”
Over-development
So among animals there are many colour patterns and structures that
appear in widely different genera, as, for example, the magpie colouring
in birds. With this phenomenon we shall deal more fully when speaking of
animal colouration. There is certainly no small amount of evidence which
seems to indicate that, from some cause or other, an impetus has been
given to certain organs to develop along definite lines. The reduction of
the number of digits in several mammalian families which are not nearly
related is a case in point. This phenomenon is, as Cope points out,
observed in Marsupials, Rodents, Insectivores, Carnivores, and Ungulates.
He, being a Lamarckian, ascribes this to the inherited effects of use.
Wallaceians attribute it solely to the action of natural selection. The
assumption of a growth-force or tendency for the development of one digit
at the expense of the others, would explain the phenomenon equally well.
And it is significant that many palæontologists are believers in some
kind of a growth-force. In the case of certain extinct animals we seem to
have examples of the over-development of organs. “Palæontology,” writes
Kellog on p. 275 of his _Darwinism To-day_, “reveals to us the one-time
existence of animals, of groups of animals, and of lines of descent,
which have had characteristics which led to extinction. The unwieldiness
of the giant Cretaceous reptiles, the fixed habit of life of the
crinoids, the coiling of the ammonities and the nautili, the gigantic
antlers of the Irish stag—all these are examples of development along
disadvantageous lines, or to disadvantageous degrees. The statistical
studies of variation have made known numerous cases where the slight, as
yet non-significant (in a life-and-death struggle) variation in pattern
of insects, in dimensions of parts, in relative proportions of
superficial non-active areas, are not fortuitous, that is, do not occur
scattered evenly about a mean or mode according to the law of error, but
show an obvious and consistent tendency to occur along certain lines, to
accumulate in certain directions.”
It seems to us that the only proper attitude to adopt in the present
state of our knowledge is, not to call in to our aid an unknown
growth-force, but simply to say that there is evidence to show that
variations frequently occur along certain definite lines only.
Speed of Racehorses
Darwin’s second assumption was that there is no limit to which variations
may be accumulated in any direction; that by adding one minute variation
to another through countless generations new species, new genera, new
families may arise. This assumption, if applied to continuous or
fluctuating variations, seems opposed to facts. All the evidence
available goes to show that there is a definite limit to which minute
variations can be accumulated in any given direction. No one has
succeeded in breeding a dog as large as a horse, or a pigeon with a beak
as long as that of a snipe. In the case of racehorses, which have been
selected so carefully through a long period of time, we seem to have
reached the limit of speed which can be attained by the multiplication of
insignificant variations. We do not wish to dogmatise, but we believe
that of late years there has not been any material increase in the speed
of our racehorses.
Mr S. Sidney says, on page 174 of _Cassell’s Book of the Horse_: “As far
as form went (_pace_ Admiral Rous), the British racehorse had reached
perfection in 1770, when ‘Eclipse’ was six years old.” He quotes the
measurements of the skeleton of “Eclipse” in the Museum of the Royal
College of Surgeons as evidence of this. All the efforts of breeders,
then, have failed appreciably to improve the form of the British
racehorse in the course of over a century and a quarter.
Experiments of De Vries
De Vries has made some important experiments with a view to determining
whether or not there is a limit to the amount of change which can be
induced by the selection of fluctuating or continuous variations as
opposed to mutations. “I accidentally found,” he writes, on page 345 of
_Species and Varieties: their Origin by Mutation_, “two individuals of
the ‘five-leaved’ race (of clover); by transplanting them into my garden
I have isolated them and kept them free from cross-fertilisation with the
ordinary type. Moreover, I brought them under such conditions as are
necessary for the full development of their character; and last, but not
least, I have tried to improve their character as far as possible by a
very rigid and careful selection. . . . By this method I brought my
strain within two years up to an average of nearly 90 per cent. of the
seedlings with a divided primary leaf (such seedlings averaging five
leaves in the adult). . . . This condition was reached by the sixth
generation in the year 1894, and has since proved to be the limit, the
figures remaining practically the same through all the succeeding
generations. . . . I have cultivated a new generation of this race nearly
every year since 1894, using always the strictest selection. This has led
to a uniform type, but has not been adequate to produce further
improvement.” Similarly, De Vries found in the bulbous buttercup
(_Ranunculus bulbosus_) a strain varying largely in the number of petals;
therefore he tried by means of continuous selection of those flowers
having the largest number of petals to produce a double flower, but was
not able to do so. He succeeded in evolving a strain with an average
number of nine petals, some individuals having as many as twenty or
thirty; but even by breeding only from these last he could not increase
the average number of petals in any generation beyond nine. This was the
limit to be obtained by the most rigorous selection of fluctuating
variations.
Selection, based on fluctuating variation, does not, asserts De Vries,
conduce to the production of improved races. “Only temporary
ameliorations are obtained, and the selection must be made in the same
manner every year. Moreover, the improvement is very limited, and does
not give any promise of further increase.” Notwithstanding prolonged
efforts, horticulturists have not yet succeeded in breeding a biennial
race of either beetroots or carrots that does not continually give rise
to useless annual forms. Writing of the beet, De Vries says useless
annual varieties “are sure to return each year. They are ineradicable.
Every individual is in the possession of this latent quality, and liable
to convert it into activity as soon as the circumstances provoke its
appearance, as is proved by the increase of annuals in the early
sowings”—that is to say, in circumstances favourable to the annual
variety.
It will be urged perhaps that these experiments, which seem to show that
there is a limit to which a species can be modified by the accumulation
of fluctuating variations, cannot have been properly carried out, because
all the various breeds of pigeons and other domestic animals clearly show
that extraordinary differences not only can, but have actually been
produced by the selection of such variations. This objection is based
upon the assumption that breeders have in the past dealt only with
fluctuating variations. This assumption does not appear to be justified.
It is exceedingly probable that most, if not all, the varieties of
domesticated animals have originated in mutations. Take, for instance,
the modern turbit pigeon; this has been derived from the old Court-bec,
described and figured over two centuries ago by Aldrovandus.
De Vries goes so far as to assert that the various races of pears are all
mutations; that each distinct flavour is a mutation, and that it is
impossible to produce a new flavour by selecting fluctuating variations.
Thus it would appear that in every case of the production of a new breed
a mutation has occurred which has attracted the fancy of some breeder,
and he has seized upon this and perpetuated it.
All the evidence available tends to show that there is a limit—and one
which is quickly reached—to the amount of change that can be produced by
the selection of fluctuating or continuous variations. We, therefore,
seem driven to the belief that evolution is based on the kind of
variation which Professor Bateson terms “discontinuous variation” and
Professor De Vries calls “mutation.”
Bateson on Variation
As long ago as 1894 Bateson published his _Materials for the Study of
Variation_, in which he set forth a large number of cases of
discontinuous variation which he had collected. He pointed out that
species are discontinuous, that they are sharply separated one from
another, whereas “environments often shade into one another and form a
continuous series.” How, then, he asked, if variations are minute and
continuous, have these discontinuous species arisen? May not variation
prove to be discontinuous, and thus make it clear why species are
discontinuous?
On page 15 of the above-cited work we find: “The preliminary question,
then, of the degree of continuity with which the process of evolution
occurs has never been decided. In the absence of such a decision, there
has nevertheless been a common assumption, either tacit or expressed,
that the process is a continuous one. The immense consequence of a
knowledge of the truth as to this will appear from a consideration of the
gratuitous difficulties which have been introduced by this assumption.
Chief among these is the difficulty which has been raised in connection
with the building up of new organs in their initial and imperfect stages,
the mode of transformation of organs, and, generally, the selection and
perpetuation of minute variations. Assuming, then, that variations are
minute, we are met by this familiar difficulty. We know that certain
devices and mechanisms are useful to their possessors; but from our
knowledge of natural history we are led to think that their usefulness is
consequent on the degree of perfection in which they exist, and that if
they were at all imperfect, they would not be useful. Now it is clear
that in any continuous process of evolution such stages of imperfection
must occur, and the objection has been raised that natural selection
cannot protect such imperfect mechanisms so as to lift them into
perfection. Of the objections which have been brought against the theory
of natural selection this is by far the most serious.”
Bateson further pointed out that chemical compounds are not continuous,
that they do not merge gradually each into the next, and suggested that
we might expect a similar phenomenon in the organic world.
Elsewhere he says: “Let the believer in the efficacy of selection
operating on continuous fluctuations try to breed a white or a black rat
from a pure strain of black-and-white rats, by choosing for breeding the
whitest or the blackest; or to raise a dwarf sweet pea from a tall race
by choosing the shortest. It will not work. Variation leads and selection
follows.”
Work of Bateson and De Vries
But Bateson’s views fell upon stony ground, because zoologists are mostly
men of theory and not practical breeders. They laboured under the
delusion that mutations or “sports” are rare in nature, and that when
these do happen to occur they must of necessity be swamped by
inter-crossing.
However, the discovery of the Abbé Mendel’s account of his experiments on
breeding mongrel sweet peas has opened the eyes of many zoologists, so
that they have at last learned what practical breeders have known for
untold years—namely, that sports have a way of perpetuating themselves.
Moreover, Mendel was able to give a theoretical explanation of his
discoveries, with the result that the believers in discontinuous
variation have largely increased in number of late.
While we are unable to see eye to eye with Professor Bateson in all
things, we gladly recognise the immense value of his work. Had his
statements in 1894 received the attention they merited, zoological theory
would to-day be considerably more advanced than it actually is.
Professor De Vries has gone farther than Bateson, having engrafted upon
the Darwinian hypothesis the theory of mutations. He has done no small
amount of experimental work, and has undoubtedly thrown much new light on
the ways in which species arise. He is purely a botanist, so that he
argues only from plants. Nevertheless, we believe that some of his
conclusions are applicable to animals. We are far from accepting his
theory of mutations _in toto_. We are, however, convinced that he, like
Bateson, is on the right track. There can be no doubt that a great many
new forms have originated suddenly, by jumps, and not by imperceptibly
slow degrees. Before giving a list of the names of some of the races,
both plant and animal, which appear to have come into existence suddenly,
it will be of advantage to consider for a little some of the more
important conceptions of De Vries.
Varieties and Elementary Species
That eminent botanist, as we have already seen, insists on the
distinction between fluctuating variations and mutations. The former
correspond, for all practical purposes, to the continuous variations of
Bateson, and the latter seem to be equivalent to his discontinuous
variations.
According to De Vries, all plants display fluctuating variation, but only
a small percentage exhibit the phenomenon of mutation. The most daring of
his conceptions is, that the history of every species is made up of
alternating periods of inactivity, when only fluctuating variations
occur, and of activity when “swarms of species” are produced by mutation,
and of these only a few at the most survive; natural selection, which De
Vries likens to a sieve, determining which shall live and which shall
perish.
As we have seen, De Vries does not believe that new species can arise by
the accumulation of fluctuating variations. By means of these the race
may be greatly improved, but nothing more can be accomplished. These
variations follow Quetelet’s law, which says that, for biological
phenomena, deviations from the average comply with the same laws as the
deviations from the average in any other case, if ruled by chance alone.
Very different in character are mutations. By means of these, new forms,
quite unlike the parent species, suddenly spring into being. Mutations
are said by De Vries to be of two kinds—those that produce varieties and
those which result in new elementary species.
According to De Vries, those species of plants which are in a state of
mutation (he refers to the species of the systematic botanists) are of a
composite nature, being made up of a collection of varieties and
elementary species. His conception of a variety is a plant that differs
from the parent plant in the loss or suppression of one or more
characters, while an elementary species differs from the parent form in
the possession of some new and additional character. But we will allow
him to speak for himself: “We can consider (page 141 _Species and
Varieties_) the following as the principal difference between elementary
species and varieties: that the first arise by the acquisition of
entirely new characters, and the latter by the loss of existing
qualities, or by the gain of such peculiarities as may already be seen in
other allied species. If we suppose elementary species and varieties
originated by sudden leaps and bounds, or mutations, then the elementary
species have mutated in the line of progression, some varieties have
mutated in the line of retrogression, while others have diverged from the
parental types in a line of digression or in the way of repetition. . . .
The system (of the vegetable kingdom) is built up of species; varieties
are only local and lateral, never of real importance for the whole
structure.”
De Vries asserts that these elementary species, when once they arise,
breed true, and show little or no tendency to revert to the ancestral
form. We can, says De Vries, ascertain only by experiment which plants
are in the mutating state and which are not. The great majority, however,
are not at present in the mutating state.
Mutations
The distinction between fluctuating variation and mutation has been
roughly illustrated by the case of a solid block of wood having a number
of facets, on one of which it stands. If the block be tilted slightly it
will, when the force that has tilted it is removed, return to its old
position. Such a gentle tilt may be compared to a fluctuating variation
in an organism. If, however, the block be tilted to such an angle that
when left to itself the block does not return to its old position, but
tips over and comes to rest on another facet, we have a representation of
the kind of change indicated by a mutation.
The analogy is far from perfect, for it makes it appear that the smallest
mutation must of necessity involve a departure from the normal type more
considerable than that of the largest fluctuating variation. Now,
although mutations ordinarily consist in considerable deviations from the
mean or mode of the type, while continuous variations are usually minute
deviations, it sometimes happens that the extreme fluctuations are more
considerable than some mutations. Hence “fluctuating” describes this
latter kind of variation more accurately than “continuous” does.
The test, then, of a mutation is not so much the amount of deviation as
the degree in which it is inherited. Mutations show no tendency to a
gradual return to the mean of the parent species; fluctuating variations
do display such a tendency. A mutation consists, as M. E. East says, in
the production of a new mode or centre for linear fluctuation; it is, as
it were, a shifting of the centre of gravity; the centre about which
those fluctuations which we call continuous variations occur.
As it is of considerable importance thoroughly to grasp the true nature
of mutations or discontinuous variations, and as some writers do not
appear to realise wherein lies the essential difference between the two
kinds of variation, we will, at the risk of appearing tedious, give a
further illustration. Let A be a species of bird of which the average
length of the wing is 20 inches, and let us suppose that individuals
belonging to that species occur in which the length of the wing varies as
much as 3 inches each side of the mean; thus it is possible to find
individuals of this species with a wing as short as 17 inches, or as long
as 23 inches. Let B be another species of which the average length of the
wing is 17 inches, and let us suppose that a 3-inch variation on each
side of the mean be found to occur. Individuals belonging to species B
will occur which have a wing as short as 14 inches, or as long as 20
inches. Thus some individuals of the short-winged species will have
longer wings than certain individuals of the long-winged species.
Similarly, certain individuals of a species which display a mutation may
show less deviation from the mean than some individuals showing a very
pronounced fluctuating variation. In other words, even as by measuring
the length of wing in the above example it was not always possible to say
whether a given individual belonged to species A or B, so is it not
always possible to say by looking at an individual that shows a
considerable departure from the mean whether that departure is due to a
mutation or a fluctuating variation.
Law of Regression
It is only by watching the effect of the peculiarity on the offspring of
its possessor that we are able to determine the nature of the variation.
Where the peculiarity is due to a fluctuating variation the offspring
will display the peculiarity in a diminished degree; but if the
peculiarity be due to a mutation, the offspring are likely to display it
in as marked a degree as the parent.
Fritz Müller and Galton conducted independently enquiries into the amount
of the regression shown by the progeny of parents which have deviated
from the average by fluctuating variation.
Müller experimented with Indian corn; Galton with the sweet pea.
Each found that where the deviation of the parents is represented by the
figure 5, that of their offspring is usually 2, that is to say, the
deviation they display is, on the average, less than half that of their
parents.
Applying this rule to the hypothetical case given above, if two
individuals of species A having a length of wing of 20 inches be bred
together, their offspring will, on an average, have a length of wing of
20 inches, since neither parents showed any deviation from the mean. On
the other hand, the offspring of 20-inch-wing individuals of species B
would show, on an average, a length of wing of only about 18¼ inches.
They tend to return to that mode from which their parents had departed.
But suppose that the deviation of the parents in this case had been due,
not to fluctuating variation, but to a mutation; this would mean that,
owing to some internal change in the egg that produced each parent, 20
inches became the normal length of wing; that the normal length of wing
had suddenly shifted from 17 inches to 20 inches.
The result of this would be that their offspring would have on an average
a wing-length of 20 inches instead of 18¼ inches, that the centre of
variation as regards length of wing had suddenly shifted from 17 to 20,
that, in future, all fluctuating variations would occur on either side of
20 inches, instead of on either side of 17 inches as heretofore.
Thus a variation is a fluctuating one or a mutation according as it does
or does not obey Galton’s Law of Regression.
De Vries’s Dictum
De Vries says that it is of the essence of mutations that they are
completely inherited. This statement, although substantially true, fails
to take into consideration the factor of fluctuating variation. For
example, in the above instance if the two individuals of species B had
mutated into forms with a 20-inch wing, their offspring will nevertheless
vary _inter se_, some of them will have wings shorter than 20 inches and
others wings more than 20 inches in length. But the average wing-length
of the offspring of the two mutating individuals will be 20 inches.
So much, then, for the practical difference between a mutation and a
fluctuating variation. In Chapter V. we shall discuss the possible causes
of the difference. By way of anticipation we may say that the suggestion
we shall make is that a mutation is due to some rearrangement in the
particles which represent that part of the organism in the fertilised
egg, whereas a fluctuating variation is caused by variations in the
particles themselves.
De Vries, it should be noted, bases his theory largely on experimental
evidence. His dictum is “the origin of species is an object of
experimental observation.” He has, we consider, proved conclusively that
among plants mutations sometimes occur, and, further, that in a mutating
plant the same mutation tends to occur again and again. This latter is a
most important fact, because it goes some way towards overcoming the
difficulty urged by Darwin that isolated sports must be swamped by
continual crossing with the normal type. If mutations arise in swarms, as
De Vries asserts they do, then any particular mutation is likely, sooner
or later, to cross with a similar mutation and so be able to perpetuate
itself.
Mutating Plants
The classical example of a mutating plant is the evening primrose of the
species _Oenothera lamarckiana_. This is described by De Vries as a
stately plant, with a stout stem, attaining often a height of 1.6 metres
or more. The flowers are large and of a bright yellow colour, attracting
immediate attention, even from a distance. “This striking species,” he
writes, in _Species and Varieties_ (p. 525), “was found in a locality
near Hilversum, in the vicinity of Amsterdam, where it grew in some
thousands of individuals. Ordinarily biennial, it produces rosettes in
the first, and stems in the second year. Both the stems and the rosettes
were seen to be highly variable, and soon distinct varieties could be
distinguished among them.
“The first discovery of this locality was made in 1886. Afterwards I
visited it many times, often weekly or even daily, and always at least
once a year up to the present time. This stately plant showed the
long-sought peculiarity of producing a number of new species every year.
Some of them were observed directly in the field, either as stems or
rosettes. The latter could be transplanted into my garden for further
observation, and the stems yielded seeds to be sown under like control.
Others were too weak to live a sufficiently long time in the field. They
were discovered by sowing seed from indifferent plants of the wild
locality in the garden. A third and last method of getting still more new
species from the original strain was the repetition of the sowing
process, by saving and sowing the seed which ripened on the introduced
plants. These various methods have led to the discovery of over a dozen
new types, never previously observed or described.” Some of these De
Vries regards as varieties, in the sense in which he uses the words;
others, he maintains, are real progressive species, some of which are
strong and healthy, others weaker and apparently not destined to be
successful. All these types proved absolutely constant from seed.
“Hundreds of thousands of seedlings may have arisen, but they always come
true and never revert to the original _O. lamarckiana_ type. But some of
them, however, are, like their parent form, liable to mutations.” The
case of the evening primrose is by no means an isolated one. De Vries
cites several other instances of plants in a mutating state. “The common
poppy,” he says (p. 189), “varies in height, in colour of foliage and
flowers; the last are often double or laciniated. It may have white or
bluish seeds, the capsules may open themselves or remain closed, and so
on. But every single variety is absolutely constant, and never runs into
another when the flowers are artificially pollinated and the visits of
insects excluded.” Similarly the garden carnation sometimes gives rise to
the wheat-ear form. “In this variety,” writes De Vries (p. 228), “the
flower is suppressed, and the loss is attended by a corresponding
increase in the number of pairs of bracts. This malformation results in
square spikes, or somewhat elongated heads, consisting only of the
greenish bracts. As there are no flowers, the variety is quite sterile,
and, as it is not regarded by horticulturists as an improvement on the
ordinary bright carnations, it is seldom multiplied by layering.
Notwithstanding this it appears from time to time, and has been seen in
different countries and at different periods, and what is of great
importance for us, in different strains of carnations. Though sterile,
and obviously dying out as often as it springs into existence, it is
nearly two centuries old. It was described in the beginning of the
eighteenth century by Volckamer, and afterwards by Jaeger, De Candolle,
Weber, Masters, Magnus, and many other botanists. I have had it twice at
different times and from different growers.” Similarly, the long-headed
green dahlia arose twice over some years ago in the nursery of Messrs
Zocher & Co.
Further, the peloric Toad-flax (_Linaria vulgaris peloria_) is, De Vries
informs us, “known to have originated from the ordinary type at different
times and in different countries under more or less divergent
conditions.” And, as this variety is wholly barren, it must in each
instance have had an independent origin. Lastly, the purple beech seems
to be a mutation which has originated at least three times over.
Mutation Theory Criticised
Every one interested in biological theory should read both _Species and
Varieties_ and _Plant Breeding_ by De Vries, works which are of
incalculable value to the horticulturist and agriculturist as well as to
the biologist.
While not wishing to detract in any way from the truly splendid work done
by De Vries, we feel constrained to bring several charges against him.
Firstly, he suffers from the complaint that seizes nine out of ten
originators of new theories. He pushes his theory to extreme lengths; he
allows his imagination to run away with him. We do not think that on the
evidence available he is justified in asserting that every species passes
through alternating periods of comparative quiescence and periods in
which it throws off, as mutations, swarms of elementary species. He is
justified in asserting that discontinuous variation is by no means an
uncommon phenomenon, but further than this it does not seem safe to go at
present.
Secondly, he ought to lay more stress on the fact that _Oenothera
lamarckiana_ is a plant which does not appear to be known in the wild
state, and that it is therefore possibly a hybrid plant, and the
so-called elementary species which it gives off may be merely the
varieties out of which it has been built up. Boulenger and Bailey have
both studied this plant, and they have not been able to witness all the
mutations of which De Vries speaks, so that the former says, “The fact
that _Oenothera lamarckiana_ was originally described from a garden
flower, grown in the Paris _Jardin des Plantes_, and that, in spite of
diligent search, it has not been discovered wild anywhere in America,
favours the probability that it was produced by crossing various forms of
the polymorphic _Oenothera biennis_, which had been previously introduced
in Europe.”
Definition of a Species
It has further been objected that, even if these various forms which
Lamarck’s evening primrose throws off are true mutations, they ought not
to be called new species, for they do not differ sufficiently from the
parent species to deserve the name of new species. The reply to this
criticism is that De Vries asserts that mutations produce new elementary
species, which are not the same things as new species in the ordinary
sense of the term. Most Linnæan species differ from one another to a far
greater extent than do elementary species. It seems to us quite plain
that new species arise, not by a single mutation, but by two or three
successive mutations which occur in various parts of an organism.
First arises a well-marked variety, by a single mutation. Subsequent
mutations follow, so that a distinct race is produced. And, finally,
fresh mutations occur, so that a new species is eventually produced.
What De Vries calls an elementary species the majority of systematists
would call a well-marked variety.
We may take this opportunity of remarking that the definition of a
species is one on which naturalists seem unable to agree.
So vast is the field of biology, that now-a-days biologists are compelled
to specialise to some extent. Thus we have botanists, ornithologists,
those who devote themselves to the study of mammals, those who confine
themselves to reptiles, or insects, or fishes, or crustaceans, or
bacteria, etc.
Now each class of systematists has its own particular criterion of what
constitutes a species. Ornithologists do not seem very exacting. Most of
them appear to consider a constant difference of colour sufficient for
the formation into a species of the birds that display such a variation.
Those who study reptiles, on the other hand, do not allow that a mere
difference in colour is sufficient to promote its possessor to specific
rank. Into these nice questions we cannot enter. For our purpose a
species is a group of individuals that differ from all other individuals
in displaying certain well-marked and tolerably constant characters,
which they transmit to their offspring.
Our contention, then, is that new species, in the ordinarily-accepted use
of the term, do not arise as a rule by one sudden bound (although they
may sometimes do so), but are the result of the accumulation of several
mutations or discontinuous variations. Some of these mutations are
exceedingly well marked, while others are so small as to be
indistinguishable from the more extreme fluctuating variations. Before
passing on to consider some cases of well-marked mutations which have
occurred among animals and plants, we should like to take this
opportunity of pointing out that as regards experiments in evolution the
botanist is far more favourably situated than the zoologist.
The botanist is able to reproduce many species vegetatively, _e.g._ by
cuttings, and is thus easily able to multiply examples of mutation. He
can also reproduce the great majority of plants by self-fertilisation,
and so experiences no difficulty in “fixing” a new form. Again, plants
are far easier to control than animals; as a rule they can be
transplanted without any impairment of their capacity for breeding.
Moreover, they produce a greater number of offspring than the most
prolific of the higher animals. The animal breeder is thus at an obvious
disadvantage as compared with the horticulturist. It is only with great
difficulty that he can fix the mutations which appear in his stock.
“Scatliff Strain” of Turbit
The history of the production of the “Scatliff strain” of turbit affords
a good example of the kind of difficulties that confront the breeder.
Pigeon fanciers require that the ideal turbit shall have, among other
things, an unbroken “sweep,” that is to say the line of the profile from
the tip of the beak to the back of the head should be the arc of a
circle. As a rule this line is broken by the overgrowth of the wattle at
the base of the beak. Mr Scatliff, however, has succeeded in breeding a
strain which possesses the required description of profile.
“In the year 1895,” writes Mr H. P. Scatliff on page 25 of _The Modern
Turbit_, “I visited Mr Houghton’s lofts and purchased three or four extra
stout and short-beaked stock birds. . . . The following year I mated one
of these to one of my own black hens, and reared one of the most
successful show birds ever bred, viz. ‘Champion Ladybird,’ a black hen. .
. . Most of the leading judges and many turbit breeders remarked upon
this hen’s wonderful profile, which seemed to improve as she got older
instead of getting worse, as is usual in rather coarse-wattled birds. I,
too, had remarked this, and it opened my eyes to a point in turbit
breeding which I had never heard mentioned by any turbit judges or
breeders, and which I believe I am now pointing out for the first time in
print, viz. that the feathers over her beak wattle which formed her front
_grew from the top and right to the front of her wattle, and not from
slightly behind_, as in almost every other turbit of her day; thus, as
the wattle developed and grew coarser, the front became more developed,
and made her head larger without in any way spoiling the sweep of the
profile.
“The same year ‘Ladybird’ was bred I bred eight others from the same
pair, and with one exception all turned out to be hens. There was only
one other hen, however (a dun), that had this same point, but in a lesser
degree than ‘Ladybird,’ and from these two hens nearly all my blacks, and
several of my blues are descended.”
[Illustration: A TURBIT BELONGING TO MR. H. P. SCATLIFF]
Mr Scatliff, having “spotted” this point, looked about him for another
bird having the peculiarity, with the object, if possible, of fixing the
same in his strain. He discovered this point in a pigeon belonging to Mr
Johnston of Hull, and purchased the bird for £20. But it died in the
following spring without producing for Mr Scatliff a single young one.
The next year Scatliff found that a bird belonging to a Mr Brannam had
the required peculiarity and so purchased him for £20. But that cock,
too, died before anything was bred from him. Nothing daunted, Scatliff
found that another of Brannam’s cocks displayed the same peculiarity, so
purchased him in 1899 for £15, but he also died before the year was out.
Meanwhile Scatliff had, by mating up “Ladybird” with the most likely of
his own cocks, succeeded in producing one or two young cocks with the
desired point. By breeding these with their mother “Ladybird” and their
offspring again with “Ladybird,” Scatliff eventually succeeded in
breeding some turbits, both blacks and duns, with the required
peculiarity fully developed, but not before he had spent a further sum of
£55 on two other cocks, both of which died before they could be mated
with the famous “Ladybird.” However, amid all his misfortunes, Scatliff
informs us that he bought one bird, by name “Amazement,” which did assist
him in fixing his strain. Thus Scatliff spent considerably over £100 in
purchases, and took eight years fixing the peculiarity in question. Had
“Ladybird” been a flower, the peculiarity could probably have been fixed
in one generation by self-fertilisation.
This furnishes an excellent example of the trouble which breeders will
take, and the expense to which they will go in order to produce a desired
result. Nevertheless, it appears to be the fashion for scientific men to
decry the work of the breeder.
Let us now pass on to consider the cases of mutations which are known to
have occurred among animals.
Mutations among Animals
Some instances of great and sudden variation in domesticated animals have
become classical, and been detailed in almost every work on evolution.
These are, firstly, the celebrated hornless Paraguay cattle. This
hornless breed, or rather the ancestor of the breed, arose quite
suddenly.
Many domestic horned breeds of animals, especially sheep and goats, throw
off hornless sports. Were a hornless breed of buffalo found in nature, it
would undoubtedly be ranked a new species, and the Wallaceians would
doubtless exercise much ingenuity in explaining how natural selection had
brought about the gradual disappearance of the horns; and
palæontologists, being baffled in their search for intermediaries between
the hornless species and their horned ancestors, would complain of the
imperfection of the geological record.
It may, perhaps, be argued that this hornless mutation was a direct
result of the unnatural conditions to which the Paraguay cattle were
subjected, it may be asserted that since there are no species of hornless
cattle in nature, such mutations have never occurred under natural
conditions, and hence the Paraguay cattle prove nothing. As a matter of
fact, we know that in nature a great many mutations occur which are not
perpetuated because not beneficial to the species. A hornless individual
in the wild state would stand but little chance in fighting for females
against his horned brethren. We must keep clearly in mind that the theory
of mutation does not seek to abolish natural selection; it merely affords
that force something substantial to work upon.
The second classical example of a leap taken by nature is furnished by
the Franqueiro breed of long-horned cattle in Brazil. These furnish us
with an example of a mutation in the other direction. Then there is the
Niata or bull-dog breed of cattle, which are also South American. These
instances would seem to indicate that cattle are what De Vries would call
“in a mutating state” in that part of the world.
The other classical examples of great and sudden variations are the Ancon
sheep of Massachusetts, the Mauchamp breed of Merino sheep, the tufted
turkeys, and the long-haired race of guinea-pigs.
The “wonder horses,” whose manes and tails grow to an extraordinary
length, so as to trail on the ground, may perhaps be cited as a race
which originated in a sudden mutation. They are all descendants of a
single individual, Linus I., whose mane and tail were respectively
eighteen and twenty-one feet long. But in this case it is important to
note that the parents and grandparents of Linus I. had exceptionally long
hair.
Mutations among Birds
Coming now to birds we find several undoubted examples of mutations, or
new forms which have come suddenly into being.
The black-winged peafowl, whose peculiarities were commented on by
Darwin, afford a striking example of this phenomenon. These birds breed
true when mated together, and are known to have arisen from common
peafowl in no less than nine instances. The cocks have the wings (except
the primary quills), black glossed with blue and green, and have the
thighs black, whereas, in the ordinary peacock, the same part of the wing
is nearly all mottled black and pale buff, and the thighs are drab. The
black-winged hen, on the other hand, is nearly white, but has a black
tail and black speckling on the upper surface of the body, while her
primary quills are cinnamon coloured as in male peafowl, not drab as in
the normal hens. The young are white when hatched, the young cock
gradually assuming the dark colour as he matures.
This mutation, which, in one case quoted by Darwin, increased among a
flock of peafowl until the black-winged supplanted the ordinary kind, is
so distinct in appearance in all stages that it was formerly supposed to
be a true species (_Pavo nigripennis_), of which the wild habitat was
unknown.
The Golden Pheasant (_Chrysolophus pictus_) produces, in domestication,
the dark-throated form (_C. obscurus_), in which the cock has the throat
sooty-black instead of buff, and the scapulars or shoulder feathers black
instead of red. Moreover, the two middle-tail-feathers are barred with
black and brown like the lateral ones, while in the ordinary form they
are spotted with brown on a black ground. The hens have a chocolate-brown
ground-colour instead of yellow-ochre as in the normal type. The chicks
are likewise darker.
The common duck, in domestication, when coloured like the wild mallard,
sometimes produces a form in which the chocolate breast and white collar
of the drake are absent, the pencilled grey of the abdomen reaching up to
the green neck. In this mutation the duck has the head uniformly speckled
black and brown, and lacks the light eye-brow and cheek-stripes found in
the normal duck. Both sexes have the bar on the wing dull black instead
of metallic blue.
The ducklings which ultimately bear this plumage are sooty-black
throughout, not black and yellow like normal ones.
The phenomenon of mutation is not confined to animals in a state of
domestication. The common Little Owl of Europe (_Athene noctua_) has
produced the mutation _A. chiaradiæ_ in the wild state. In this the
irides are dark, instead of yellow as in the normal type, and the plumage
of the back of the wings is longitudinally streaked with white instead of
barred. Several examples of this form were found, along with normal
young, in the nest of one particular pair of little owls in Italy, but
the whole family were foolishly exterminated by local ornithologists.
The reed bunting (_Emberiza schœniclus_) exists in two distinct forms—one
having a much stouter bill than the other (_E. pyrrhuloides_). This
probably is an example of a mutation.
The rare yellow-rumped Finch (_Munia flaviprymna_), of Australia, has
displayed a tendency to change into the allied and far commoner
chestnut-breasted Finch (_M. castaneithorax_) during the lifetime of the
individual (_Avicultural Magazine_, 1907). Conversely, the male of the
common Red-billed Weaver (_Quelea quelea_) of Africa has been found in
its old age to assume the characters of the comparatively rare _Q.
russi_, its black throat becoming pale buff as in that form.
Everyone is familiar with the chequered variety of the common blue-rock
pigeon, in which the wings are regularly mottled with black instead of
being barred. This form sometimes occurs among wild birds, so that it has
been described as a distinct species. It is important to note that there
are red, dun, and silver chequers as well as blue ones.
[Illustration: YELLOW-RUMPED AND CHESTNUT-BREASTED FINCHES, WITH
TRANSITIONAL SPECIMENS]
A well-marked mutation which appears regularly in nature is the
red-headed variety of the beautiful Gouldian Finch (_Pöephila mirabilis_)
of North Australia. Normally the head of the cock is black, but in about
ten per cent. of the individuals the cock has a crimson head, while that
of the hen is dull crimson and black.
Mutations which occur with such regularity are certainly rare. On the
other hand, there are certain mutations which we may expect to see appear
in any species of plant or animal.
Albinistic forms are a case in point, and less frequently we see white
varieties which are not pure albinos, because the eye retains some at
least of the normal pigment. As examples, we may cite white dogs, cats,
fowls, horses, ducks, geese, and Java sparrows among domesticated
animals, and the white forms of the Amazonian dolphin and of the giant
Petrel of the South seas (_Ossifraga gigantea_) among wild creatures.
In a white mutation the eye may lose all its pigment, and then we have a
true albino. Such forms on account of their imperfect vision cannot
survive in a state of nature, hence no wild pink-eyed species are known.
Or the eye may display a partial loss of pigment, as, for example, in the
white domestic forms of the common goose, the Chinese goose, and the
Muscovy duck. Finn saw a case in which the eyes of a pink-eyed rabbit
changed after death into this type of eye—that is, with the pupil black
and the iris blue. It is to be observed that this kind of eye sometimes
occurs in coloured horses, rabbits, and dogs. Finally, we have white
mutations in which the eye loses none of the pigment. These are abundant
in nature, and probably most of the white species of birds—as, for
example, some egrets, swans, etc.—arose in this way.[4] Pure white
species are comparatively uncommon in nature, because, except in
snow-clad regions, white creatures are easily seen by their adversaries.
Most white birds are of considerable size, and well able to look after
themselves.
Similarly black mutations occur frequently among animals, both under
domestication and in a state of nature. All are familiar with black dogs,
cats, horses, fowls, ducks, pigeons. Black mutations, however, do not
occur nearly so frequently as white ones. So far as we are aware no black
mutation has been recorded among canaries, geese, guinea-fowl, ferrets,
Java sparrows or doves, all of which produce white mutations.
On the other hand, in the wild state black species occur more frequently
than normal-eyed white forms. This is probably because such creatures are
less conspicuous than white ones. As examples of black mutations which
occur in nature, we may cite black leopards, water rats, squirrels,
foxes, barking deer (_Cervulus muntjac_), hawk-eagles, harriers, peppered
moth (_Amphidasys betularia_), etc.
That many black species have arisen as sudden mutations from
lighter-coloured animals seems tolerably certain from the facts that in
Malacca the black leopard forms a local race; that some of the Gibbon
apes are as often black as light coloured; that the American black bear
is sometimes brown, while the other bears, when not brown, are almost
invariably black.
Color Mutations
Not uncommon, although rarer than black or melanistic forms, are reddish
or chestnut varieties. These occur both among tame and wild animals.
Among domesticated creatures, sandy cats, “red” pigeons, buff fowls,
chestnut horses, red guinea pigs afford examples of this mutation. Among
wild animals many of the species of squirrel, not naturally red, produce
red mutations; and some of the grey owls—as, for example, the Indian race
of the Scops (_Scops giu_)—throw off a red or chestnut form. As everyone
knows, some species are normally red.
Green or olive species not unfrequently throw off yellow mutations. As
examples of these we may cite yellow canaries, yellow budgerigars
(_Melopsittacus undulatus_), goldfish, golden tench, and the golden form
of the common carp among captive animals; and among animals in a state of
nature, yellow forms have been recorded of the rose-ringed Paroquet
(_Palæornis torquatus_), the green woodpecker, the pike, and the eel.
These lutinistic forms usually have normally coloured eyes. Sometimes,
but only very rarely, these yellow forms throw off white sports—as, for
example, the “silver” form of the goldfish. Finn has seen a white variety
of the common carp. White canaries are excessively rare, while white
budgerigars are unknown.
It is worthy of note that entirely yellow species of birds and fish are
unknown. We would suggest that the explanation of this is that yellowness
is correlated with some physical characteristic unfavourable to an
organism exposed to the struggle for existence; hence individuals which
are yellow are not permitted to survive. In some species of moths
individuals occur in which the parts normally red are yellow. According
to Bateson, a chalk pit at Madingly, near Cambridge, has long been known
to collectors as a habitat of a yellow-marked form of the six-spot Burnet
Moth (_Zygæna filipendulæ_). These lutinistic forms are not confined to
one genus of Butterflies. Moreover, in the Pin-tailed Nonpareil Finch
(_Eythrura prasina_) of the Eastern Archipelago the red tail and other
red parts of the plumage are not infrequently replaced by yellow in wild
individuals of either sex and of any age. In the blue-fronted Amazon
parrot (_Chrysotis æstiva_)—a most variable bird—the normally red edge of
the pinion is sometimes yellow. Bateson, in his _Materials for the Study
of Variation_, gives other examples of this kind of variation.
Mutations among Invertebrates
As further instances of mutations among animals which have been observed
in nature, we may mention the _valezina_ form of the female of the
Silver-washed Fritillary Butterfly (_Argynnis paphia_) and the _helice_
form of the female Clouded-yellow Butterfly (_Colias edusa_).
The common jelly-fish is an organism which frequently throws off sports,
and some zoologists are of opinion that the medusoid _Pseudoclytia
pentata_ arose by a discontinuous variation from _Epenthesis folleata_ or
a closely allied form. Thomson discusses this particular case at some
length on pages 87-89 of his _Heredity_, and gives it as his opinion that
the evidence in favour of this latter having arisen as a mutation is
“exceedingly strong.”
Mutating Species
It is our belief that many species of birds which occur in nature have
been derived from other species which still exist, but as no one has ever
seen the mutation take place, we cannot furnish any proof thereof. We
merely rely on the fact that the species in question differ so slightly
from one another that there seems every likelihood that they have
suddenly arisen and managed to establish themselves alongside of the
parent species.
The Curassows, _Crax grayi_, _C. hecki_, each of which is only known by a
very few specimens, appear to be mutations of the female of the globose
Curassow, _Crax globicera_. The fact that when a female _hecki_ bred in
the London Zoological Gardens with a male _globicera_, the solitary young
one which lived to grow up was a pure _globicera_, renders the assumption
almost certain.
The Chamba Monaul (_Lophophorus chambanus_) seems to be a mutation of the
male of the common Monaul or Impeyan Pheasant (_Lophophorus impeyanus_),
the common species of the Himalayas.
The Three-coloured Mannikin (_Munia malacca_) of South India is probably
simply a white-bellied form of the widely-ranging Black-headed Mannikin
(_M. atricapilla_), which has the abdomen chestnut like the back.
Intermediate wild-caught forms have been recorded.
The African Cordon-bleu (_Estrelda phœnicotis_) and Blue-bellied Waxbill
(_E. cyanogastra_) would also seem to be mutations, as almost the only
difference between them lies in the fact that the male of the former has
a crimson cheek-patch, which is wanting in the latter.
The Ringed Finch (_Stictoptera annulosa_) of Java, and Bicheno’s Finch
(_S. bichenovii_) of Australia, only differ in the former having the rump
black, while in the latter it is white, and this difference appears to be
of the nature of a mutation.
So, it might be urged, is the pure white breast of the male Upland Goose
(_Chloëphaga magellanica_), which part, in the very similar _C. dispar_,
is barred as in the females, the latter form being probably the ancestor.
The differences between the silver-grey-necked Crowned Crane of the Cape
(_Balearica chrysopelargus_) and the dark-necked species of West Africa
(_B. regulorum_) seem also to be not more than could be accounted for by
mutation.
Peculiar forms, such as a rabbit with a convoluted brain or a mouse with
a peculiar pattern of molar teeth, have been come upon by anatomists.
The above-cited mutations are all very considerable ones, and we do not
profess to have mentioned a tenth part of those which have actually been
recorded.
We trust that we have collected and set forth sufficient evidence to show
that the phenomenon of discontinuous variation is a very general one, and
this would seem to tell against the hypothesis of De Vries that species
pass through alternate periods of comparative stability and periods when
swarms of mutations appear. We think it more probable that all species
throw off at greater or less intervals discontinuous variations, and that
it is upon these that natural selection acts.
We further hope that we have succeeded in making clear what we believe to
be the very sharp distinction between continuous and discontinuous
variations, even when the latter are inconsiderable, as frequently
happens.
Somatic and Germinal Variations
Before leaving the subject of variation it is necessary to notice the
distinction, which Weismann was the first to emphasise, between somatic
and germinal variations.
Every adult organism must be regarded as the result of two sets of
forces; inherited tendencies or internal forces, and the action of
environment or external forces. The differences which the various members
of a family show are due in part to the initial differences in the
germinal material of which they are composed, and in part to the
differences of their environment. The former differences are the result
of what we may call germinal variations, and the latter the result of
somatic variations. It is scarcely ever possible to say of any particular
variation that it is a germinal or a somatic one, because even before
birth a developing organism has been subjected to environmental
influences. One of a litter may have received more nourishment than the
others. Nevertheless, any marked variation which appears at birth is
probably largely germinal. According to Weismann and the majority of
zoologists, there is a fundamental difference between these germinal and
somatic variations, in that the former tend to be inherited, while the
latter are never inherited. Weismann believes that very early in the
formation of the embryo the cells which will form the generative organs
of the developing organism are separated off from those cells which will
go to build up the body, and become as much isolated from them as if they
were contained in a hermetically-sealed flask, so that they remain
totally unaffected by any changes which the environment effects in the
somatic cells. Therefore, says Weismann, acquired characters cannot be
inherited.
While the majority of zoologists believe that acquired characters are not
inherited, probably not many will go so far as Weismann and declare that
the environment cannot exercise _any_ effect whatever on the germ cells.
Somatic Variations
Even though acquired characters or variations are not inherited, it does
not follow that they do not play an important part in evolution. Acquired
variations are the result of the way in which an organism reacts to its
environment. If an organism is unable to react to its environment it must
inevitably perish. If it is able to react, it matters not, so far as the
chances of survival of the organism are concerned, whether the adaptation
is the result of a congenital variation or a somatic one. This will be
rendered clear by a hypothetical example. Let us suppose that a certain
mammal is forced, owing to the intensity of the struggle for existence,
to migrate into the Arctic regions. Let us further suppose that this
organism is preyed upon by some creature that hunts by sight rather than
by scent. Let us yet further imagine that this predacious species is
swifter than our animal, on which it preys. It is obvious that, other
things being equal, the more closely the creature preyed upon assimilates
to its surroundings the more likely is it to escape the observation of
its foes, and so to survive and give birth to offspring. Now suppose that
the glare from the snow-covered ground bleaches its coat. This whitening
of the fur is a somatic variation, one which is induced by the
environment. Such an animal will be as difficult to see, if the bleaching
is such as to render it snow-white, as if its whiteness were due to a
germinal variation. Thus, as regards its chances of survival, it matters
not whether its whiteness be the result of germinal or somatic variation.
But if the whiteness is due to a somatic variation, its offspring will
show no tendency to inherit the variation; they will have in turn to
undergo the bleaching process. If, on the other hand, the whiteness is
due to a germinal variation, the offspring will tend to inherit this
peculiarity and to be born white. In such a case, it is unlikely that the
fur of an organism which is naturally coloured will be completely
bleached by the snow, and, even if it be, the bleaching process will take
time, meanwhile the creature will be comparatively conspicuous. So that
those which are naturally whiter than the average, that is to say, those
in which the tendency to whiteness appears as a germinal variation, will
be less conspicuous than those which tend to be the ordinary colour. Thus
the former will enjoy a better chance of survival, and will be likely to
transmit their whiteness to their offspring in so far as it is due to a
germinal or congenital variation.
Thus, although none of the whiteness due to somatic variations is
transmitted to the offspring, such variations are of considerable
importance to the species, as they enable it to survive and allow time
for the germinal variations in the required direction to appear.
That this case need not be purely hypothetical is shown by the fact that
dun domestic pigeons, which are of an earthy-brown colour when fresh
moulted, soon fade in the sun to a dull creamy hue. Thus a coloration
adapted to an ordinary soil could soon be suited to a desert environment.
The ruddy sheldrake also, normally a bright chestnut-coloured bird, and
one that haunts exposed sunny places, in many cases fades very much,
becoming almost straw-coloured.
Many variations which organisms display are of a mixed kind, being in
part the result of inner forces and in part due to the action of the
environment. In so far as they are due to this latter they do not appear
to be inherited.
Thus, although we cannot say of many variations whether they are
germinal, or somatic, or of a mixed kind, it is of great importance to
keep continually in mind the fundamental differences between the two
kinds.
Some somatic variations are due to the direct action of the environment;
they are merely the expression of the manner in which an organism
responds to external stimuli.
What is the cause of germinal variations? This is a question to which we
are not yet in a position to give a satisfactory answer.
The attempt to explain their origin plunges us into the realm of theory.
This doubtless is a realm full of fascination, but it is an unexplored
region of extreme darkness, in which, we believe, it is scarcely possible
to take the right road until more of the light of fact has been shed upon
it.
In the chapter dealing with inheritance we shall indicate the lines along
which it is likely that future progress will be made.
CHAPTER IV
HYBRIDISM
The alleged sterility of hybrids a stumbling-block to
evolutionists—Huxley’s views—Wallace on the sterility of hybrids—Darwin
on the same—Wallace’s theory that the infertility of hybrids has been
caused by Natural Selection so as to prevent the evils of
intercrossing—Crosses between distinct species not necessarily
infertile—Fertile crosses between species of plants—Sterile plant
hybrids—Fertile mammalian hybrids—Fertile bird hybrids—Fertile hybrids
among amphibia—Limits of hybridisation—Multiple hybrids—Characters of
hybrids—Hybridism does not appear to have exercised much effect on the
origin of new species.
The alleged sterility of the hybrids produced by crossing different
species has long proved a great stumbling-block to evolutionists. Huxley,
in particular, felt the force of this objection to the Darwinian theory.
If the hybrids between natural species are sterile, while those of all
the varieties which the breeder has produced are perfectly fertile, it is
obviously quite useless for evolutionists to point with pride to the
results obtained by the breeder, and to declare that his products differ
from one another to a greater extent than do many well-recognised
species.
“After much consideration, and with no bias against Mr Darwin’s views,”
wrote Huxley to the _Westminster Review_ in 1860, “it is our clear
conviction that, as the evidence now stands, it is not absolutely proven
that a group of animals having all the characters exhibited by species in
nature, has ever been originated by selection, whether natural or
artificial. Groups having the morphological nature of species, distinct
and permanent races, in fact, have been so produced over and over again;
but there is no positive evidence at present that any group of animals
has, by variation and selective breeding, given rise to another group
which was in the least degree infertile with the first. Mr Darwin is
perfectly aware of this weak point, and brings forward a multitude of
ingenious and important arguments to diminish the force of the objection.
We admit the value of these arguments to the fullest extent; nay, we will
go so far as to express our belief that experiments, conducted by a
skilful physiologist, would very probably obtain the desired production
of mutually more or less infertile breeds from a common stock in a
comparatively few years; but still, as the case stands at present, this
little ‘rift within the lute’ is not to be disguised or overlooked.”
Alleged Sterility of Hybrids
Similarly Wallace writes, at the beginning of chapter vii. of his
_Darwinism_: “One of the greatest, or perhaps we may say the greatest, of
all the difficulties in the way of accepting the theory of natural
selection as a complete explanation of the origin of species, has been
the remarkable difference between varieties and species in respect of
fertility when crossed. Generally speaking, it may be said that the
varieties of any one species, however different they may be in external
appearance, are perfectly fertile when crossed, and their mongrel
offspring are equally fertile when bred among themselves; while distinct
species, on the other hand, however closely they may resemble one another
externally, are usually infertile when crossed, and their hybrid
offspring absolutely sterile. This used to be considered a fixed law of
nature, constituting the absolute test and criterion of a species as
distinct from a variety; and so long as it was believed that species were
separate creations, or at all events had an origin quite distinct from
that of varieties, this law could have no exceptions, because if any two
species had been found to be fertile when crossed and their hybrid
offspring to be also fertile, this fact would have been held to prove
them to be not species but varieties. On the other hand, if two varieties
had been found to be infertile, or their mongrel offspring to be sterile,
then it would have been said—These are not varieties, but true species.
Thus the old theory led inevitably to reasoning in a circle, and what
might be only a rather common fact was elevated into a law which had no
exceptions.”
Thus the sterility of hybrids was a zoological bogey which had to be
demolished. The plan of campaign adopted by Darwin and Wallace was,
firstly, to try to disprove the assertion that the hybrids between
different species are always sterile, and secondly, to find a reason for
the alleged sterility of these hybrids.
Fertile Hybrids
Darwin succeeded in obtaining some examples of crosses between botanical
species which were said to be fertile. These he quotes in chapter viii.
of _The Origin of Species_. As regards animals, he met with less success.
“Although,” he writes, “I do not know of any thoroughly
well-authenticated cases of perfectly fertile hybrid animals, I have some
reason to believe that the hybrids from _Cervulus vaginalis_ and
_reevesii_, and from _Phasianus colchicus_ and _P. torquatus_ and with
_P. versicolor_ are perfectly fertile. There is no doubt that these three
pheasants, namely, the common, the true ring-necked, and the Japan,
intercross, and are becoming blended together in the woods of several
parts of England. The hybrids from the common and Chinese geese (_A.
cygnoides_), species which are so different that they are generally
ranked in distinct genera, have often been bred in this country with
either pure parent, and in one single instance they have bred _inter se_.
This was effected by Mr Eyton, who raised two hybrids from the same
parents but from different hatches; and from these two birds he raised no
less than eight hybrids (grandchildren of the pure geese) from one nest.
In India, however, these cross-bred geese must be far more fertile; for I
am assured by two eminently capable judges, namely, Mr Blyth and Captain
Hutton, that whole flocks of these crossed geese are kept in various
parts of the country; and as they are kept for profit, where neither pure
parent species exists, they must certainly be highly fertile.[5] . . . So
again there is reason to believe that our European and the humped Indian
cattle are quite fertile together; and from facts communicated to me by
Mr Blyth, I think they must be considered as distinct species.”
Darwin does not seem to have been very satisfied with the evidence he had
collected, for he said: “Finally, looking to all the ascertained facts on
the intercrossing of plants and animals, it may be concluded that some
degree of sterility, both in first crosses and in hybrids, is an
extremely general result; but that it cannot, under our present state of
knowledge, be considered as absolutely universal.”
Similarly Wallace writes: “Nevertheless, the fact remains that most
species which have hitherto been crossed produce sterile hybrids, as in
the well-known case of the mule; while almost all domestic varieties,
when crossed, produce offspring which are perfectly fertile among
themselves.”
Darwin resorted to much ingenious argument in his attempt to explain what
he believed to be the almost universal sterility of hybrids, as opposed
to mongrels or crosses between varieties. He pointed out that changed
conditions tend to produce sterility, as is evidenced by the fact that
many creatures refuse to breed in confinement, and believed that the
crossing of distinct wild species produced a similar effect on the sexual
organs. He expressed his belief that the early death of the embryos is a
very frequent cause of sterility in first crosses.
Wallace thus summarises Darwin’s conclusions as to the cause of the
sterility of hybrids: “The sterility or infertility of species with each
other, whether manifested in the difficulty of obtaining first crosses
between them or in the sterility of the hybrids thus obtained, is not a
constant or necessary result of species difference, but is incidental on
unknown peculiarities of the reproductive system. These peculiarities
constantly tend to arise under changed conditions owing to the extreme
susceptibility of that system, and they are usually correlated with
variations of form or of colour. Hence, as fixed differences of form and
colour, slowly gained by natural selection in adaptation to changed
conditions, are what essentially characterise distinct species, some
amount of infertility between species is the usual result.”
A Biological Bogey
But Wallace has not been content to let the matter remain where Darwin
left it. He has boldly tried to make an ally of this bogey of the
infertility of hybrids. On page 179 of _Darwinism_ he argues, most
ingeniously, that the sterility of hybrids has been actually produced by
natural selection to prevent the evils of the intercrossing of allied
species. We will not reproduce his argument for the simple reason that it
is now well-known, or should be well-known, that hybrids between allied
species are by no means always sterile. The doctrine of the infertility
of hybrids seems to have been founded on the fact that the hybrids best
known to breeders, namely the cross between the ass and the horse, and
those between the canary and other finches, are sterile.
Fertile Crosses between Species of Plants
In the case of plants the number of fertile hybrids between species is so
large that we cannot attempt to enumerate them. De Vries cites several
instances in Lecture IX of his _Species and Varieties: Their Origin by
Mutation_.
One of these—the hybrid between the purple and the yellow species of
Lucerne which is known to botanists as _Medicago media_ is, writes De
Vries, “cultivated in some parts of Germany on a large scale, as it is
more productive than the ordinary lucerne.” Other examples of perfectly
fertile plant hybrids cited by De Vries are the crosses between _Anemone
magellanica_ and _A. sylvestris_, between _Salix alba_ and _Salix
pentandra_, between _Rhododendron hirsutum_ and _R. ferrugineum_.
He gives an instance of a hybrid—_Ægilops speltæformis_, which, though
fertile, is not so fertile as a normal species would be. It is worthy of
note that Burbank of California has obtained a hybrid between the
blackberry and the raspberry, which is not only fertile, but quite
popular as producing a novel fruit.
Sterile Plant Hybrids
De Vries does not cite nearly so many examples of sterile hybrids,
presumably because they are not so easy to find. He mentions the sterile
“Gordon’s currant,” which is considered to be a hybrid between the
Californian and the Missouri species. He also gives _Cytisus adami_ as an
absolutely sterile hybrid, this being a cross between two species of
Labernum—the common and the purple.
In the case of animals the known hybrids are so much less numerous that
we are able to furnish a list which may be taken as fairly exhaustive.
Fertile Mammalian Hybrids
Taking the mammals first, we find that, in addition to those cited by
Darwin, there are several recorded cases of crosses between well-defined
species which are fertile.
There is the hybrid between the brown bear and the polar bear, which is
perfectly fertile. In the London Zoological Gardens there is a specimen
of this hybrid, also one of this individual’s offspring by a pure polar
bear.
The stoat has been crossed with the domestic ferret, a descendant of the
polecat, a very distinct species; the resulting hybrids have nevertheless
proved fertile.
The bull American bison produces with the domestic cow hybrids known as
“cataloes,” which are fertile. The reverse cross of the domestic bull
with the bison cow does not, however, succeed at all, which reminds us of
what happens in the case of finch-hybrids.
Bird fanciers when crossing the canary with wild species of finch, almost
invariably use a hen canary as the female parent, because domesticated
female animals breed more readily than do captive wild ones.
The domestic yak breeds frequently in the Himalayas with the perfectly
distinct zebu or humped cow of India, and the hybrids are fertile. Yet
the zebu and the Indian buffalo, living constantly side by side in the
plains of India, never interbreed at all.
Among wild ruminants of this hollow-horned family, the Himalayan Argali
(_Ovis ammon_) ram, a giant sheep of the size of a donkey, has been known
to appropriate a herd of ewes of the Urial (_O. vignei_), a very distinct
species of the size of a domestic sheep. Many hybrids were born, and
these, in turn, bred with the pure urials of the herd.
In our parks the little Sika deer of Japan (_Cervus sika_), a species
about the size of the fallow-deer, with an even more marked seasonal
change of colouration and antlers having only three tines, breeds with
the red deer, and the hybrids are fertile.
In certain parts of Asia Minor the natives cross the female one-humped
camel with the male of the bactrian or two-humped species. The hybrids
(which are one-humped) will breed with the pure species; but, although
the hybrids are strong and useful, the three-quarter bred beasts are
apparently of little value.
Fertile Bird Hybrids
Coming to birds, we are confronted by a longer list of fertile hybrids.
This is the natural outcome of the fact that a greater number of bird
species have been kept in captivity.
The oldest known fertile hybrid is that between the common and Chinese
geese above cited, but many others have since been recorded. Even among
birds so seldom bred, comparatively, as the parrot family, a fertile
hybrid has been produced, that between the Australian Rosella Parrakeet
(_Platycercus eximius_) and Pennant’s Parrakeet (_P. elegans_). The
hybrid was first described as a distinct species, the Red-mantled
Parrakeet (_P. erythropeplus_). These two parrakeets, though nearly
allied, are very distinct; Pennant’s being coloured red, blue, and black,
with a distinct young plumage of uniform dull green; the rosella in
addition to the above colours displays much yellow and some white and
green. It is, moreover, considerably smaller and has no distinct youthful
dress.
The Amherst Pheasant (_Chrysolophus amherstiæ_) and the Gold Pheasant
(_C. pictus_) have long been known as producing hybrids which are fertile
either _inter se_ or with the parents. Here the species are still more
distinct; not only are the leading colours of the Amherst white and
green, instead of red and gold, but it is a bigger bird with a larger
tail and smaller crest, and a bare patch round the eyes.
The Pintail Duck (_Dafila acuta_) and the Mallard or Wild Duck and its
domestic descendants (_Anas boscas_), when bred together, produce hybrids
which have been proved fertile between themselves and with the pure
pintail. Any sportsman or frequenter of our parks can see for himself the
distinctness of the species concerned.
The Pied Wagtail (_Motacilla lugubris_) and the Grey Wagtail (_M.
melanope_) have produced hybrids in aviaries, which have proved fertile.
The two species are distinct in every way, as all British ornithologists
know.
The Cut-throat Finch (_Amadina fasciata_) and Red-headed Finch (_A.
erythrocephala_) of Africa have hybridised in aviaries, and the produce
has proved fertile. The red-headed finch, among other differences, is far
larger than the cut-throat, and the males have the head all red, not
merely a throat-band of that colour.
The Japanese Greenfinch (_Ligurinus sinicus_) which is not green, but
brown and grey, with bolder yellow wing- and tail-markings than our
larger European greenfinch, has produced fertile hybrids with this latter
bird.
[Illustration: MALE AMHERST PHEASANT]
The Red Dove of India (_Oenopopilia tranquebarica_) has produced hybrids
with the tame Collared Dove (_T. risorius_) and these have bred again
when paired with the red species. _O. tranquebarica_, although presenting
a general similarity to the collared dove, is truly distinct, being much
smaller, with a shorter tail, and displaying a marked sex-difference (the
male only being red, and the female drab). Its voice is also utterly
unlike the well-known penetrating and musical _coo_ of the Collared Dove.
There is a large class of fertile wild hybrids produced between forms
differing only in colour, such as those between the Hooded Crow (_Corvus
cornix_) and Carrion Crow (_Corvus corone_), the various species of
_Molpastes_ bulbuls, and the Indian Roller (_Coracias indica_) and
Burmese Roller (_C. affinis_). Indeed, it may be said that wherever two
such colour-species meet they hybridize and become more or less fused.
In this connection sportsmen, as mentioned by Darwin, performed
unconsciously a most interesting experiment when, more than a century
ago, they introduced largely into their coverts the Chinese Ring-necked
Pheasant (_Phasianus torquatus_) and the Japanese _P. versicolor_. So
freely has the former bred with the common species already present there
(_Phasianus colchicus_) that nowadays nearly all our English pheasants
show traces of the cross in the shape of white feathers on the neck, or
the green tinge of the plumage of the lower back. The influence of the
Japanese Green Pheasant (_P. versicolor_) has been very slight.
It is, of course, open to anyone to assert that such crosses are not true
hybrids, as the species are not fully distinct, but mere
colour-mutations.
The fact of the intermingling, however, is a fatal blow to the theory of
recognition marks, since it demonstrates that merely distinctive
colouring is not a preventative of cross-breeding. To this matter we
shall return later.
Fertile Hybrids among Amphibia
Our Crested Newt (_Molge cristata_) and the Continental Marbled Newt (_M.
marmorata_) interbreed in France, in the wild state, and the resulting
hybrid was at first described as a distinct species, under the name of
_Molge blasii_. These two newts differ greatly in appearance. In the
Marbled Newt the colouration is brilliant green and black above, and
shows no orange below, thus differing much from that of the Crested Newt,
which is black above and mottled with orange beneath, while the crest of
the breeding-male of this species lacks the notches which are so
conspicuous in that of the Crested Newt.
[Illustration: HARLEQUIN QUAIL (Coturnix delegorguei)]
[Illustration: RAIN QUAIL (Coturnix coromandelica)]
Insects
Among insects, M. de Quatrefages states that the hybrid progeny of the
silk-moths _Bombyx cynthia_ and _B. arrindia_ are fertile for eight
generations when bred _inter se_.
Limits to the Possibilities of Hybridisation
Hybrids can apparently only be produced between species of the same
natural family. The stories of cat-rabbits, deer-ponies, fowl-ducks, and
similar distant crosses invariably break down on close examination. A
belief in such remote crosses characterized the ancient “bestiaries,” and
still lingers, as witness the falsely-reputed crosses alluded to above.
This belief has no doubt arisen from the fact that the domestic breeds of
dogs, fowls, etc., are popularly confounded with truly distinct species.
Mongrels are well known to be readily produced, and hence the notion
arises that hybrids between the most widely-separated species are
possible.
In practice, the most remote cross of which authenticated specimens exist
is that between the red grouse and the domestic fowl (bantam cock). It is
true that the grouse are commonly ranked by ornithologists as a family
distinct (_Tetraonidae_) from that of the pheasants and partridges
(_Phasianidae_), to which the fowl belongs; but the relationship is
admittedly very close, and we doubt if general zoologists would
countenance the maintenance of the families as distinct. Ornithologists
are notoriously apt to over-rate small differences when drawing up a
classification. It would be therefore safe to say, in the present state
of our knowledge, that species belonging to different natural families
cannot hybridize.
In some cases multiple hybrids have been produced. Thus, at the London
Zoological Gardens, many years ago, a hybrid between the Gayal of India
(_Bos frontalis_) and the Indian humped cow mentioned above was put to an
American bison, and produced a double hybrid calf.
M. G. Rogeron of Angers bred many hybrids from a male pochard and a duck
bred from a Mallard and a Gadwall.
More recently, Mr J. L. Bonhote has succeeded in combining the blood of
five wild species of ducks in one individual.
Mr J. T. Newman has also bred turtle-doves containing the blood of three
distinct species.
A cross, which usually results in sterile offspring, may in very rare
cases produce a fertile individual; thus, Mr A. Suchetet once succeeded
in obtaining a three-quarter-bred bird from the not uncommon hybrid of
the tame pigeon and tame collared dove (_Turtur risorius_), which is
usually barren, by pairing it with a dove; but the bird thus produced,
when again paired with a dove, was itself sterile. Some of the cases here
given seem to encourage Darwin’s view that domestication tends to
eliminate sterility; but it is doubtful if this can be upheld. The hybrid
between the Muscovy duck (_Cairina moschata_) and common duck is usually,
at all events, sterile, like that between the pigeon and dove; yet all
these birds have been long domesticated. The hybrid between the fowl and
the guinea-fowl is likewise barren, nor has the long domestication of the
horse and ass lessened the sterility of the mule.
Characters of Hybrids
Some facts may be noted respecting the characters of hybrids. In the
first place, it is important to notice that the characters of the hybrid
vary according to the sexes of the species concerned; thus, the “hinny,”
which is bred from a horse and a she-ass, is a different animal from the
true “mule,” which is bred from the jackass and mare, and is inferior to
it.
Similarly, Mr G. E. Weston, a great authority on British cage-birds and
their hybrids, informs us that when hybrids are bred from a male canary
and a hen goldfinch or siskin—contrary to the almost universal practice
of using the hen canary for crossing—the progeny are inferior in size and
colour to the hybrids obtained in the ordinary way.
Hybrids, in animals at all events, differ from crosses between mutations
or colour-variations in not exhibiting the phenomenon of alternative
inheritance; they do not follow one parent or the other exclusively, but
always exhibit some blending of the characters of both, which is, after
all, what might have been expected, since well-defined species usually
differ in more than one character.
Thus, the cross between the Amherst and gold pheasants chiefly resembles
the latter, but has the ruff white as in the Amherst, while the crest,
though in form it resembles that of the gold species, is not yellow as in
that species, nor red as in the Amherst, but of an intermediate tint,
brilliant orange.
The mule between the horse and ass, as all know, combines the shapes of
the
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