Evolution in Modern Thought
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Title: Evolution in Modern Thought
Contributor: J. Arthur Thomson
William Bateson
Célestin Charles Alfred Bouglé
J. B. Bury
Ernst Haeckel
Harald Høffding
C. Lloyd Morgan
Gustav Albert Schwalbe
P. N. Waggett
August Weismann
Release date: August 29, 2007 [eBook #22430]
Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK EVOLUTION IN MODERN THOUGHT ***
Produced by Marilynda Fraser-Cunliffe, Sankar Viswanathan,
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EVOLUTION IN MODERN
THOUGHT
BY HAECKEL, THOMSON, WEISMANN
AND OTHERS
THE MODERN LIBRARY
PUBLISHERS :: :: NEW YORK
* * * * *
CONTENTS
I DARWIN'S PREDECESSORS
J. Arthur Thomson, Professor of Natural History in the University of
Aberdeen
II _The Selection Theory_
August Weismann, Professor of Zoology in the University of
Freiburg (Baden)
III HEREDITY AND VARIATION IN MODERN LIGHTS
W. Bateson, Professor of Biology in the University of Cambridge
IV "THE DESCENT OF MAN"
G. Schwalbe, Professor of Anatomy in the University of Strassburg
V CHARLES DARWIN AS AN ANTHROPOLOGIST
Ernst Haeckel, Professor of Zoology in the University of Jena
VI MENTAL FACTORS IN EVOLUTION
C. Lloyd Morgan, Professor of Psychology at University College,
Bristol
VII THE INFLUENCE OF THE CONCEPTION OF EVOLUTION ON MODERN PHILOSOPHY
H. Hoeffding, Professor of Philosophy in the University of Copenhagen
VIII THE INFLUENCE OF DARWIN UPON RELIGIOUS THOUGHT
Rev. P. H. Waggett
IX DARWINISM AND HISTORY
J. B. Bury, Regious Professor of Modern History in the University
of Cambridge
X DARWINISM AND SOCIOLOGY
C. Bougle, Professor of Social Philosophy in the University of
Toulouse, and Deputy-Professor at the Sorbonne, Paris
* * * * *
EVOLUTION IN MODERN THOUGHT
I
DARWIN'S PREDECESSORS
BY J. ARTHUR THOMSON
_Professor of Natural History in the University of Aberdeen_
In seeking to discover Darwin's relation to his predecessors it is
useful to distinguish the various services which he rendered to the
theory of organic evolution.
(I) As everyone knows, the general idea of the Doctrine of Descent is
that the plants and animals of the present day are the lineal
descendants of ancestors on the whole somewhat simpler, that these
again are descended from yet simpler forms, and so on backwards
towards the literal "Protozoa" and "Protophyta" about which we
unfortunately know nothing. Now no one supposes that Darwin originated
this idea, which in rudiment at least is as old as Aristotle. What
Darwin did was to make it current intellectual coin. He gave it a form
that commended itself to the scientific and public intelligence of the
day, and he won widespread conviction by showing with consummate skill
that it was an effective formula to work with, a key which no lock
refused. In a scholarly, critical, and pre-eminently fair-minded way,
admitting difficulties and removing them, foreseeing objections and
forestalling them, he showed that the doctrine of descent supplied a
modal interpretation of how our present-day fauna and flora have come
to be.
(II) In the second place, Darwin applied the evolution-idea to
particular problems, such as the descent of man, and showed what a
powerful organon it is, introducing order into masses of uncorrelated
facts, interpreting enigmas both of structure and function, both
bodily and mental, and, best of all, stimulating and guiding further
investigation. But here again it cannot be claimed that Darwin was
original. The problem of the descent or ascent of man, and other
particular cases of evolution, had attracted not a few naturalists
before Darwin's day, though no one [except Herbert Spencer in the
psychological domain (1855)] had come near him in precision and
thoroughness of inquiry.
(III) In the third place, Darwin contributed largely to a knowledge of
the factors in the evolution-process, especially by his analysis of
what occurs in the case of domestic animals and cultivated plants, and
by his elaboration of the theory of Natural Selection which Alfred
Russel Wallace independently stated at the same time, and of which
there had been a few previous suggestions of a more or less vague
description. It was here that Darwin's originality was greatest, for
he revealed to naturalists the many different forms--often very
subtle--which natural selection takes, and with the insight of a
disciplined scientific imagination he realised what a mighty engine of
progress it has been and is.
(IV) As an epoch-marking contribution, not only to AEtiology but to
Natural History in the widest sense, we rank the picture which Darwin
gave to the world of the web of life, that is to say, of the
inter-relations and linkages in Nature. For the Biology of the
individual--if that be not a contradiction in terms--no idea is more
fundamental than that of the correlation of organs, but Darwin's most
characteristic contribution was not less fundamental,--it was the idea
of the correlation of organisms. This, again, was not novel; we find
it in the works of naturalists like Christian Conrad Sprengel,
Gilbert White, and Alexander von Humboldt, but the realisation of its
full import was distinctly Darwinian.
_As Regards the General Idea of Organic Evolution_
While it is true, as Prof. H. F. Osborn puts it, that "'Before and
after Darwin' will always be the _ante et post urbem conditam_ of
biological history," it is also true that the general idea of organic
evolution is very ancient. In his admirable sketch _From the Greeks to
Darwin_,[1] Prof. Osborn has shown that several of the ancient
philosophers looked upon Nature as a gradual development and as still
in process of change. In the suggestions of Empedocles, to take the
best instance, there were "four sparks of truth,--first, that the
development of life was a gradual process; second, that plants were
evolved before animals; third, that imperfect forms were gradually
replaced (not succeeded) by perfect forms; fourth, that the natural
cause of the production of perfect forms was the extinction of the
imperfect."[2] But the fundamental idea of one stage giving origin to
another was absent. As the blue AEgean teemed with treasures of beauty
and threw many upon its shores, so did Nature produce like a fertile
artist what had to be rejected as well as what was able to survive,
but the idea of one species emerging out of another was not yet
conceived.
Aristotle's views of Nature[3] seem to have been more definitely
evolutionist than those of his predecessors, in this sense, at least,
that he recognised not only an ascending scale, but a genetic series
from polyp to man and an age-long movement towards perfection. "It is
due to the resistance of matter to form that Nature can only rise by
degrees from lower to higher types." "Nature produces those things
which, being continually moved by a certain principle contained in
themselves, arrive at a certain end."
To discern the outcrop of evolution-doctrine in the long interval
between Aristotle and Bacon seems to be very difficult, and some of
the instances that have been cited strike one as forced. Epicurus and
Lucretius, often called poets of evolution, both pictured animals as
arising directly out of the earth, very much as Milton's lion long
afterwards pawed its way out. Even when we come to Bruno who wrote
that "to the sound of the harp of the Universal Apollo (the World
Spirit), the lower organisms are called by stages to higher, and the
lower stages are connected by intermediate forms with the higher,"
there is great room, as Prof. Osborn points out,[4] for difference of
opinion as to how far he was an evolutionist in our sense of the term.
The awakening of natural science in the sixteenth century brought the
possibility of a concrete evolution theory nearer, and in the early
seventeenth century we find evidences of a new spirit--in the
embryology of Harvey and the classifications of Ray. Besides sober
naturalists there were speculative dreamers in the sixteenth and
seventeenth centuries who had at least got beyond static formulae,
but, as Professor Osborn points out,[5] "it is a very striking fact,
that the basis of our modern methods of studying the Evolution problem
was established not by the early naturalists nor by the speculative
writers, but by the Philosophers." He refers to Bacon, Descartes,
Leibnitz, Hume, Kant, Lessing, Herder, and Schelling. "They alone were
upon the main track of modern thought. It is evident that they were
groping in the dark for a working theory of the Evolution of life, and
it is remarkable that they clearly perceived from the outset that the
point to which observation should be directed was not the past but the
present mutability of species, and further, that this mutability was
simply the variation of individuals on an extended scale."
Bacon seems to have been one of the first to think definitely about
the mutability of species, and he was far ahead of his age in his
suggestion of what we now call a Station of Experimental Evolution.
Leibnitz discusses in so many words how the species of animals may be
changed and how intermediate species may once have linked those that
now seem discontinuous. "All natural orders of beings present but a
single chain".... "All advances by degrees in Nature, and nothing by
leaps." Similar evolutionist statements are to be found in the works
of the other "philosophers," to whom Prof. Osborn refers, who were,
indeed, more scientific than the naturalists of their day. It must be
borne in mind that the general idea of organic evolution--that the
present is the child of the past--is in great part just the idea of
human history projected upon the natural world, differentiated by the
qualification that the continuous "Becoming" has been wrought out by
forces inherent in the organisms themselves and in their environment.
A reference to Kant[6] should come in historical order after Buffon,
with whose writings he was acquainted, but he seems, along with Herder
and Schelling, to be best regarded as the culmination of the
evolutionist philosophers--of those at least who interested themselves
in scientific problems. In a famous passage he speaks of "the
agreement of so many kinds of animals in a certain common plan of
structure" ... an "analogy of forms" which "strengthens the
supposition that they have an actual blood-relationship, due to
derivation from a common parent." He speaks of "the great Family of
creatures, for as a Family we must conceive it, if the above-mentioned
continuous and connected relationship has a real foundation." Prof.
Osborn alludes to the scientific caution which led Kant, biology being
what it was, to refuse to entertain the hope "that a Newton may one
day arise even to make the production of a blade of grass
comprehensible, according to natural laws ordained by no intention."
As Prof. Haeckel finely observes, Darwin rose up as Kant's Newton.[7]
The scientific renaissance brought a wealth of fresh impressions and
some freedom from the tyranny of tradition, and the twofold stimulus
stirred the speculative activity of a great variety of men from old
Claude Duret of Moulins, of whose weird transformism (1609) Dr. Henry
de Varigny[8] gives us a glimpse, to Lorenz Oken (1779-1851) whose
writings are such mixtures of sense and nonsense that some regard him
as a far-seeing prophet and others as a fatuous follower of
intellectual will-o'-the-wisps. Similarly, for De Maillet, Maupertuis,
Diderot, Bonnet, and others, we must agree with Professor Osborn that
they were not actually in the main Evolution movement. Some have been
included in the roll of honour on very slender evidence, Robinet for
instance, whose evolutionism seems to us extremely dubious.[9]
The first naturalist to give a broad and concrete expression to the
evolutionist doctrine of descent was Buffon (1707-1788), but it is
interesting to recall the fact that his contemporary Linnaeus
(1707-1778), protagonist of the counter-doctrine of the fixity of
species,[10] went the length of admitting (in 1762) that new species
might arise by inter-crossing. Buffon's position among the pioneers of
the evolution-doctrine is weakened by his habit of vacillating between
his own conclusions and the orthodoxy of the Sorbonne, but there is no
doubt that he had firm grasp of the general idea of "l'enchainment des
etres."
Erasmus Darwin (1731-1802), probably influenced by Buffon, was another
firm evolutionist, and the outline of his argument in the
_Zoonomia_[11] might serve in part at least to-day. "When we revolve
in our minds the metamorphoses of animals, as from the tadpole to the
frog; secondly, the changes produced by artificial cultivation, as in
the breeds of horses, dogs, and sheep; thirdly, the changes produced
by conditions of climate and of season, as in the sheep of warm
climates being covered with hair instead of wool, and the hares and
partridges of northern climates becoming white in winter: when,
further, we observe the changes of structure produced by habit, as
seen especially in men of different occupations; or the changes
produced by artificial mutilation and prenatal influences, as in the
crossing of species and production of monsters; fourth, when we
observe the essential unity of plan in all warm-blooded animals,--we
are led to conclude that they have been alike produced from a similar
living filament".... "From thus meditating upon the minute portion of
time in which many of the above changes have been produced, would it
be too bold to imagine, in the great length of time since the earth
began to exist, perhaps millions of years before the commencement of
the history of mankind, that all warm-blooded animals have arisen from
one living filament?"... "This idea of the gradual generation of all
things seems to have been as familiar to the ancient philosophers as
to the modern ones, and to have given rise to the beautiful
hieroglyphic figure of the [Greek: proton oon], or first great egg,
produced by night, that is, whose origin is involved in obscurity, and
animated by [Greek: Eros], that is, by Divine Love; from whence
proceeded all things which exist."
Lamarck (1744-1829) seems to have become an evolutionist
independently of Erasmus Darwin's influence, though the parallelism
between them is striking. He probably owed something to Buffon, but he
developed his theory along a different line. Whatever view be held in
regard to that theory there is no doubt that Lamarck was a
thorough-going evolutionist. Professor Haeckel speaks of the
_Philosophie Zoologique_ as "the first connected and thoroughly
logical exposition of the theory of descent."[12]
Besides the three old masters, as we may call them, Buffon, Erasmus
Darwin, and Lamarck, there were other quite convinced pre-Darwinian
evolutionists. The historian of the theory of descent must take
account of Treviranus whose _Biology or Philosophy of Animate Nature_
is full of evolutionary suggestions; of Etienne Geoffroy St. Hilaire,
who in 1830, before the French Academy of Sciences, fought with
Cuvier, the fellow-worker of his youth, an intellectual duel on the
question of descent; of Goethe, one of the founders of morphology and
the greatest poet of Evolution--who, in his eighty-first year, heard
the tidings of Geoffrey St. Hilaire's defeat with an interest which
transcended the political anxieties of the time; and of many others
who had gained with more or less confidence and clearness a new
outlook on Nature. It will be remembered that Darwin refers to
thirty-four more or less evolutionist authors in his Historical
Sketch, and the list might be added to. Especially when we come near
to 1858 do the numbers increase, and one of the most remarkable, as
also most independent champions of the evolution-idea before that date
was Herbert Spencer, who not only marshalled the arguments in a very
forcible way in 1852, but applied the formula in detail in his
_Principles of Psychology_ in 1855.[13]
It is right and proper that we should shake ourselves free from all
creationist appreciations of Darwin, and that we should recognise the
services of pre-Darwinian evolutionists who helped to make the time
ripe, yet one cannot help feeling that the citation of them is apt to
suggest two fallacies. It may suggest that Darwin simply entered into
the labours of his predecessors, whereas, as a matter of fact, he knew
very little about them till after he had been for years at work. To
write, as Samuel Butler did, "Buffon planted, Erasmus Darwin and
Lamarck watered, but it was Mr. Darwin who said 'That fruit is ripe,'
and shook it into his lap" ... seems to us a quite misleading version
of the facts of the case. The second fallacy which the historical
citation is a little apt to suggest is that the filiation of ideas is
a simple problem. On the contrary, the history of an idea, like the
pedigree of an organism, is often very intricate, and the evolution of
the evolution-idea is bound up with the whole progress of the world.
Thus in order to interpret Darwin's clear formulation of the idea of
organic evolution and his convincing presentation of it, we have to do
more than go back to his immediate predecessors, such as Buffon,
Erasmus Darwin, and Lamarck; we have to inquire into the acceptance of
evolutionary conceptions in regard to other orders of facts, such as
the earth and the solar system;[14] we have to realise how the growing
success of scientific interpretation along other lines gave confidence
to those who refused to admit that there was any domain from which
science could be excluded as a trespasser; we have to take account of
the development of philosophical thought, and even of theological and
religious movements; we should also, if we are wise enough, consider
social changes. In short, we must abandon the idea that we can
understand the history of any science as such, without reference to
contemporary evolution in other departments of activity.
While there were many evolutionists before Darwin, few of them were
expert naturalists and few were known outside a small circle; what was
of much more importance was that the genetic view of Nature was
insinuating itself in regard to other than biological orders of facts,
here a little and there a little, and that the scientific spirit had
ripened since the days when Cuvier laughed Lamarck out of court. How
was it that Darwin succeeded where others had failed? Because, in the
first place, he had clear visions--"pensees de la jeunesse, executees
par l'age mur"--which a University curriculum had not made impossible,
which the _Beagle voyage_ made vivid, which an unrivalled British
doggedness made real--visions of the web of life, of the fountain of
change within the organism, of the struggle for existence and its
winnowing, and of the spreading genealogical tree. Because, in the
second place, he put so much grit into the verification of his
visions, putting them to the proof in an argument which is of its
kind--direct demonstration being out of the question--quite
unequalled. Because, in the third place, he broke down the opposition
which the most scientific had felt to the seductive modal formula of
evolution by bringing forward a more plausible theory of the process
than had been previously suggested. Nor can one forget, since
questions of this magnitude are human and not merely academic, that he
wrote so that all men could understand.
_As Regards the Factors of Evolution_
It is admitted by all who are acquainted with the history of biology
that the general idea of organic evolution as expressed in the
Doctrine of Descent was quite familiar to Darwin's grandfather and to
others before and after him, as we have briefly indicated. It must
also be admitted that some of these pioneers of evolutionism did more
than apply the evolution-idea as a modal formula of becoming, they
began to inquire into the factors in the process. Thus there were
pre-Darwinian theories of evolution, and to these we must now briefly
refer.[15]
In all biological thinking we have to work with the categories
Organism--Function--Environment, and theories of evolution may be
classified in relation to these. To some it has always seemed that the
fundamental fact is the living organism,--a creative agent, a striving
will, a changeful Proteus, selecting its environment, adjusting itself
to it, self-differentiating and self-adaptive. The necessity of
recognising the importance of the organism is admitted by all
Darwinians who start with inborn variations, but it is open to
question whether the whole truth of what we might call the Goethian
position is exhausted in the postulate of inherent variability.
To others it has always seemed that the emphasis should be laid on
Function,--on use and disuse, on doing and not doing. Practice makes
perfect; _c'est a force de forger qu'on devient forgeron_. This is one
of the fundamental ideas of Lamarckism; to some extent it met with
Darwin's approval; and it finds many supporters to-day. One of the
ablest of these--Mr. Francis Darwin--has recently given strong reasons
for combining a modernised Lamarckism with what we usually regard as
sound Darwinism.[16]
To others it has always seemed that the emphasis should be laid on the
Environment, which wakes the organism to action, prompts it to change,
makes dints upon it, moulds it, prunes it, and finally, perhaps, kills it.
It is again impossible to doubt that there is truth in this view, for even
if environmentally induced "modifications" be not transmissible,
environmentally induced "variations" are; and even if the direct influence
of the environment be less important than many enthusiastic supporters of
this view--may we call them Buffonians--think, there remains the indirect
influence which Darwinians in part rely on,--the eliminative process. Even
if the extreme view be held that the only form of discriminate elimination
that counts is inter-organismal competition, this might be included under
the rubric of the animate environment.
In many passages Buffon[17] definitely suggested that environmental
influences--especially of climate and food--were directly productive
of changes in organisms, but he did not discuss the question of the
transmissibility of the modifications so induced, and it is difficult
to gather from his inconsistent writings what extent of transformation
he really believed in. Prof. Osborn says of Buffon: "The struggle for
existence, the elimination of the least-perfected species, the contest
between the fecundity of certain species and their constant
destruction, are all clearly expressed in various passages." He quotes
two of these:[18]
"Le cours ordinaire de la nature vivante, est en general toujours
constant, toujours le meme; son mouvement, toujours regulier, roule
sur deux points inebranlables: l'un, la fecondite sans bornes donnee a
toutes les especes; l'autre, les obstacles sans nombre qui reduisent
cette fecondite a une mesure determinee et ne laissent en tout temps
qu'a peu pres la meme quantite d'individus de chaque espece" ... "Les
especes les moins parfaites, les plus delicates, les plus pesantes,
les moins agissantes, les moins armees, etc., ont deja disparu ou
disparaitront.".
Erasmus Darwin[19] had a firm grip of the "idea of the gradual
formation and improvement of the Animal world," and he had his theory
of the process. No sentence is more characteristic than this: "All
animals undergo transformations which are in part produced by their
own exertions, in response to pleasures and pains, and many of these
acquired forms or propensities are transmitted to their posterity."
This is Lamarckism before Lamarck, as his grandson pointed out. His
central idea is that wants stimulate efforts and that these result in
improvements which subsequent generations make better still. He
realised something of the struggle for existence and even pointed out
that this advantageously checks the rapid multiplication. "As Dr.
Krause points out, Darwin just misses the connection between this
struggle and the Survival of the Fittest."[20]
Lamarck[21] (1744-1829) seems to have thought out his theory of
evolution without any knowledge of Erasmus Darwin's which it closely
resembled. The central idea of his theory was the cumulative
inheritance of functional modifications. "Changes in environment bring
about changes in the habits of animals. Changes in their wants
necessarily bring about parallel changes in their habits. If new wants
become constant or very lasting, they form new habits, the new habits
involve the use of new parts, or a different use of old parts, which
results finally in the production of new organs and the modification
of old ones." He differed from Buffon in not attaching importance, as
far as animals are concerned, to the direct influence of the
environment, "for environment can effect no direct change whatever
upon the organisation of animals," but in regard to plants he agreed
with Buffon that external conditions directly moulded them.
Treviranus[22] (1776-1837), whom Huxley ranked beside Lamarck, was on
the whole Buffonian, attaching chief importance to the influence of a
changeful environment both in modifying and in eliminating, but he was
also Goethian, for instance in his idea that species like individuals
pass through periods of growth, full bloom, and decline. "Thus, it is
not only the great catastrophes of Nature which have caused
extinction, but the completion of cycles of existence, out of which
new cycles have begun." A characteristic sentence is quoted by Prof.
Osborn: "In every living being there exists a capability of an endless
variety of form-assumption; each possesses the power to adapt its
organisation to the changes of the outer world, and it is this power,
put into action by the change of the universe, that has raised the
simple zoophytes of the primitive world to continually higher stages
of organisation, and has introduced a countless variety of species
into animate Nature."
Goethe[23] (1749-1832), who knew Buffon's work but not Lamarck's, is
peculiarly interesting as one of the first to use the evolution-idea
as a guiding hypothesis, e.g. in the interpretation of vestigial
structures in man, and to realise that organisms express an attempt to
make a compromise between specific inertia and individual change. He
gave the finest expression that science has yet known--if it has known
it--of the kernel-idea of what is called "bathmism," the idea of an
"inherent growth-force"--and at the same time he held that "the way of
life powerfully reacts upon all form" and that the orderly growth of
form "yields to change from externally acting causes."
Besides Buffon, Erasmus Darwin, Lamarck, Treviranus, and Goethe,
there were other "pioneers of evolution," whose views have been often
discussed and appraised. Etienne Geoffroy Saint-Hilaire (1772-1884),
whose work Goethe so much admired, was on the whole Buffonian,
emphasising the direct action of the changeful _milieu_. "Species vary
with their environment, and existing species have descended by
modification from earlier and somewhat simpler species." He had a
glimpse of the selection idea, and believed in mutations or sudden
leaps--induced in the embryonic condition by external influences. The
complete history of evolution-theories will include many instances of
guesses at truth which were afterwards substantiated, thus the
geographer von Buch (1773-1853) detected the importance of the
Isolation factor on which Wagner, Romanes, Gulick and others have laid
great stress, but we must content ourselves with recalling one other
pioneer, the author of the _Vestiges of Creation_ (1844), a work which
passed through ten editions in nine years and certainly helped to
harrow the soil for Darwin's sowing. As Darwin said, "it did excellent
service in this country in calling attention to the subject, in
removing prejudice, and in thus preparing the ground for the reception
of analogous views."[24] Its author, Robert Chambers (1802-1871) was
in part a Buffonian--maintaining that environment moulded organisms
adaptively, and in part a Goethian--believing in an inherent
progressive impulse which lifted organisms from one grade of
organisation to another.
_As Regards Natural Selection_
The only thinker to whom Darwin was directly indebted, so far as the
theory of Natural Selection is concerned, was Malthus, and we may once
more quote the well-known passage in the Autobiography: "In October,
1838, that is, fifteen months after I had begun my systematic enquiry,
I happened to read for amusement 'Malthus on Population,' and being
well prepared to appreciate the struggle for existence which
everywhere goes on from long-continued observation of the habits of
animals and plants, it at once struck me that under these
circumstances favourable variations would tend to be preserved, and
unfavourable ones to be destroyed. The result of this would be the
formation of new species."[25]
Although Malthus gives no adumbration of the idea of Natural Selection
in his exposition of the eliminative processes which go on in mankind,
the suggestive value of his essay is undeniable, as is strikingly
borne out by the fact that it gave to Alfred Russel Wallace also "the
long-sought clue to the effective agent in the evolution of organic
species."[26] One day in Ternate when he was resting between fits of
fever, something brought to his recollection the work of Malthus which
he had read twelve years before. "I thought of his clear exposition of
'the positive checks to increase'--disease, accidents, war, and
famine--which keep down the population of savage races to so much
lower an average than that of more civilized peoples. It then occurred
to me that these causes or their equivalents are continually acting in
the case of animals also; and as animals usually breed much more
rapidly than does mankind, the destruction every year from these
causes must be enormous in order to keep down the numbers of each
species, since they evidently do not increase regularly from year to
year, as otherwise the world would long ago have been densely crowded
with those that breed most quickly. Vaguely thinking over the enormous
and constant destruction which this implied, it occurred to me to ask
the question, Why do some die and some live? And the answer was
clearly, that on the whole the best fitted live. From the effects of
disease the most healthy escaped; from enemies the strongest, the
swiftest, or the most cunning; from famine the best hunters or those
with the best digestion; and so on. Then it suddenly flashed upon me
that this self-acting process would necessarily _improve the race_,
because in every generation the inferior would inevitably be killed
off and the superior would remain--that is, _the fittest would
survive_."[27] We need not apologise for this long quotation, it is a
tribute to Darwin's magnanimous colleague, the Nestor of the
evolutionist camp,--and it probably indicates the line of thought
which Darwin himself followed. It is interesting also to recall the
fact that in 1852, when Herbert Spencer wrote his famous _Leader_
article on "The Development Hypothesis" in which he argued powerfully
for the thesis that the whole animate world is the result of an
age-long process of natural transformation, he wrote for _The
Westminster Review_ another important essay, "A Theory of Population
deduced from the General Law of Animal Fertility," towards the close
of which he came within an ace of recognising that the struggle for
existence was a factor in organic evolution. At a time when pressure
of population was practically interesting men's minds, Darwin,
Wallace, and Spencer were being independently led from a social
problem to a biological theory. There could be no better illustration,
as Prof. Patrick Geddes has pointed out, of the Comtian thesis that
science is a "social phenomenon."
Therefore, as far more important than any further ferreting out of
vague hints of Natural Selection in books which Darwin never read, we
would indicate by a quotation the view that the central idea in
Darwinism is correlated with contemporary social evolution. "The
substitution of Darwin for Paley as the chief interpreter of the order
of nature is currently regarded as the displacement of an
anthropomorphic view by a purely scientific one: a little reflection,
however, will show that what has actually happened has been merely the
replacement of the anthropomorphism of the eighteenth century by that
of the nineteenth. For the place vacated by Paley's theological and
metaphysical explanation has simply been occupied by that suggested to
Darwin and Wallace by Malthus in terms of the prevalent severity of
industrial competition, and those phenomena of the struggle for
existence which the light of contemporary economic theory has enabled
us to discern, have thus come to be temporarily exalted into a
complete explanation of organic progress."[28] It goes without saying
that the idea suggested by Malthus was developed by Darwin into a
biological theory which was then painstakingly verified by being used
as an interpretative formula, and that the validity of a theory so
established is not affected by what suggested it, but the practical
question which this line of thought raises in the mind is this: if
Biology did thus borrow with such splendid results from social theory,
why should we not more deliberately repeat the experiment?
Darwin was characteristically frank and generous in admitting that the
principle of Natural Selection had been independently recognised by
Dr. W. C. Wells in 1813 and by Mr. Patrick Matthew in 1831, but he had
no knowledge of these anticipations when he published the first
edition of _The Origin of Species_. Wells, whose "Essay on Dew" is
still remembered, read in 1813 before the Royal Society a short paper
entitled "An Account of a White Female, part of whose skin resembles
that of a Negro" (published in 1818). In this communication, as Darwin
said, "he observes, firstly, that all animals tend to vary in some
degree, and, secondly, that agriculturists improve their domesticated
animals by selection; and then, he adds, but what is done in this
latter case 'by art, seems to be done with equal efficacy, though more
slowly, by nature, in the formation of varieties of mankind, fitted
for the country which they inhabit.'"[29] Thus Wells had the clear
idea of survival dependent upon a favourable variation, but he makes
no more use of the idea and applies it only to man. There is not in
the paper the least hint that the author ever thought of generalising
the remarkable sentence quoted above.
Of Mr. Patrick Matthew, who buried his treasure in an appendix to a
work on _Naval Timber and Arboriculture_, Darwin said that "he clearly
saw the full force of the principle of natural selection." In 1860
Darwin wrote--very characteristically--about this to Lyell: "Mr.
Patrick Matthew publishes a long extract from his work on _Naval
Timber and Arboriculture_, published in 1831, in which he briefly but
completely anticipates the theory of Natural Selection. I have ordered
the book, as some passages are rather obscure, but it is certainly, I
think, a complete but not developed anticipation. Erasmus always said
that surely this would be shown to be the case some day. Anyhow, one
may be excused in not having discovered the fact in a work on Naval
Timber."[30]
De Quatrefages and De Varigny have maintained that the botanist Naudin
stated the theory of evolution by natural selection in 1852. He
explains very clearly the process of artificial selection, and says
that in the garden we are following Nature's method. "We do not think
that Nature has made her species in a different fashion from that in
which we proceed ourselves in order to make our variations." But, as
Darwin said, "he does not show how selection acts under nature."
Similarly it must be noted in regard to several pre-Darwinian pictures
of the struggle for existence (such as Herder's, who wrote in 1790
"All is in struggle ... each one for himself" and so on), that a
recognition of this is only the first step in Darwinism.
Profs. E. Perrier and H. F. Osborn have called attention to a
remarkable anticipation of the selection-idea which is to be found in
the speculations of Etienne Geoffroy Saint-Hilaire (1825-1828) on the
evolution of modern Crocodilians from the ancient Teleosaurs. Changing
environment induced changes in the respiratory system and far-reaching
consequences followed. The atmosphere, acting upon the pulmonary
cells, brings about "modifications which are favourable or destructive
('funestes'); these are inherited, and they influence all the rest of
the organisation of the animal because if these modifications lead to
injurious effects the animals which exhibit them perish and are
replaced by others of a somewhat different form, a form changed so as
to be adapted to (a la convenance) the new environment."
Prof. E. B. Poulton[31] has shown that the anthropologist James Cowles
Prichard (1786-1848) must be included even in spite of himself among
the precursors of Darwin. In some passages of the second edition of
his _Researches into the Physical History of Mankind_ (1826), he
certainly talks evolution and anticipates Prof. Weismann in denying
the transmission of acquired characters. He is, however, sadly
self-contradictory and his evolutionism weakens in subsequent
editions--the only ones that Darwin saw. Prof. Poulton finds in
Prichard's work a recognition of the operation of Natural Selection.
"After inquiring how it is that 'these varieties are developed and
preserved in connexion with particular climates and differences of
local situation,' he gives the following very significant answer: 'One
cause which tends to maintain this relation is obvious. Individuals
and families, and even whole colonies perish and disappear in climates
for which they are, by peculiarity of constitution, not adapted. Of
this fact proofs have been already mentioned.'" Mr. Francis Darwin and
Prof. A. C. Seward discuss Prichard's "anticipations" in _More Letters
of Charles Darwin_, Vol. _I._ p. 43, and come to the conclusion that
the evolutionary passages are entirely neutralised by others of an
opposite trend. There is the same difficulty with Buffon.
Hints of the idea of Natural Selection have been detected elsewhere.
James Watt,[32] for instance, has been reported as one of the
anticipators (1851). But we need not prolong the inquiry further,
since Darwin did not know of any anticipations until after he had
published the immortal work of 1859, and since none of those who got
hold of the idea made any use of it. What Darwin did was to follow the
clue which Malthus gave him, to realise, first by genius and
afterwards by patience, how the complex and subtle struggle for
existence works out a natural selection of those organisms which vary
in the direction of fitter adaptation to the conditions of their life.
So much success attended his application of the Selection-formula that
for a time he regarded Natural Selection as almost the sole factor in
evolution, variations being pre-supposed; gradually, however, he came
to recognise that there was some validity in the factors which had
been emphasised by Lamarck and by Buffon, and in his well known
summing up in the sixth edition of the _Origin_ he says of the
transformation of species: "This has been effected chiefly through the
natural selection of numerous successive, slight, favourable
variations; aided in an important manner by the inherited effects of
the use and disuse of parts; and in an unimportant manner, that is, in
relation to adaptive structures, whether past or present, by the
direct action of external conditions, and by variations which seem to
us in our ignorance to arise spontaneously."
To sum up: the idea of organic evolution, older than Aristotle, slowly
developed from the stage of suggestion to the stage of verification,
and the first convincing verification was Darwin's; from being an _a
priori_ anticipation it has become an interpretation of nature, and
Darwin is still the chief interpreter; from being a modal
interpretation it has advanced to the rank of a causal theory, the
most convincing part of which men will never cease to call Darwinism.
FOOTNOTES:
[Footnote 1: _Columbia University Biological Series_, Vol. I. New York
and London, 1894. We must acknowledge our great indebtedness to this
fine piece of work.]
[Footnote 2: _op. cit._ p. 41.]
[Footnote 3: See G. J. Romanes, "Aristotle as a Naturalist,"
_Contemporary Review_, Vol. lix. p. 275, 1891; G. Pouchet, _La
Biologie Aristotelique_, Paris, 1885; E. Zeller, _A History of Greek
Philosophy_, London, 1881, and "Ueber die griechischen Vorgaenger
Darwin's," _Abhandl. Berlin Akad._ 1878, pp. 111-124.]
[Footnote 4: _op. cit._ p. 81.]
[Footnote 5: _op. cit._ p. 87.]
[Footnote 6: See Brock, "Die Stellung Kant's zur Deszendenztheorie,"
_Biol. Centralbl._ viii. 1889, pp. 641-648. Fritz Schultze, _Kant und
Darwin_, Jena, 1875.]
[Footnote 7: Mr. Alfred Russel Wallace writes: "We claim for Darwin
that he is the Newton of natural history, and that, just so surely as
that the discovery and demonstration by Newton of the law of
gravitation established order in place of chaos and laid a sure
foundation for all future study of the starry heavens, so surely has
Darwin, by his discovery of the law of natural selection and his
demonstration of the great principle of the preservation of useful
variations in the struggle for life, not only thrown a flood of light
on the process of development of the whole organic world, but also
established a firm foundation for all future study of nature"
(_Darwinism_, London, 1889, p. 9). See also Prof. Karl Pearson's
_Grammar of Science_ (2nd edit.), London, 1900, p. 32. See Osborn,
_op. cit._ p. 100.]
[Footnote 8: _Experimental Evolution_. London, 1892. Chap. I. p. 14.]
[Footnote 9: See J. Arthur Thomson, _The Science of Life_. London,
1899, Chap. XVI. "Evolution of Evolution Theory."]
[Footnote 10: See Carus Sterne (Ernst Krause), _Die allgemeine
Weltanschauung in ihrer historischen Entwickelung_. Stuttgart, 1889.
Chapter entitled "Bestaendigkeit oder Veraenderlichkeit der
Naturwesen."]
[Footnote 11: _Zoonomia, or the Laws of Organic Life_, 2 vols. London,
1794; Osborn, _op. cit._ p. 145.]
[Footnote 12: See Alpheus S. Packard, _Lamarck, the Founder of
Evolution, His Life and Work, with Translations of his writings on
Organic Evolution_. London, 1901.]
[Footnote 13: See Edward Clodd, _Pioneers of Evolution_, London, p.
161, 1897.]
[Footnote 14: See Chapter ix. "The Genetic View of Nature" in J. T.
Merz's _History of European Thought in the Nineteenth Century_, Vol.
2, Edinburgh and London, 1903.]
[Footnote 15: See Prof. W. A. Locy's _Biology and its Makers_. New
York, 1908. Part II. "The Doctrine of Organic Evolution."]
[Footnote 16: Presidential Address to the British Association meeting
at Dublin in 1908.]
[Footnote 17: See in particular Samuel Butler, _Evolution Old and
New_, London, 1879; J. L. de Lanessan, "Buffon et Darwin," _Revue
Scientifique_, XLIII. pp. 385-391, 425-432, 1889.]
[Footnote 18: _op. cit._ p. 136.]
[Footnote 19: See Ernest Krause and Charles Darwin, _Erasmus Darwin_,
London, 1879.]
[Footnote 20: Osborn, _op. cit._ p. 142.]
[Footnote 21: See E. Perrier, _La Philosophie Zoologique avant
Darwin_, Paris, 1884; A. de Quatrefages, _Darwin et ses Precurseurs
Francais_, Paris, 1870; Packard, _op. cit._; also Claus, _Lamarck als
Begruender der Descendenzlehre_, Wien, 1888; Haeckel, _Natural History
of Creation_, Eng. transl. London, 1879; Lang, _Zur Charakteristik der
Forschungswege von Lamarck und Darwin_, Jena, 1889.]
[Footnote 22: See Huxley's article "Evolution in Biology,"
_Encyclopaedia Britannica_ (9th edit.), 1879, pp. 744-751, and Sully's
article, "Evolution in Philosophy," _ibid._ pp. 751-772.]
[Footnote 23: See Haeckel, _Die Naturanschauung von Darwin, Goethe und
Lamarck_, Jena, 1882.]
[Footnote 24: _Origin of Species_ (6th edit.), p. xvii.]
[Footnote 25: _The Life and Letters of Charles Darwin_, Vol. 1. p. 83.
London, 1887.]
[Footnote 26: A. R. Wallace, _My Life, a Record of Events and
Opinions_, London, 1905, Vol. 1, p. 232.]
[Footnote 27: _My Life_, Vol. 1. p. 361.]
[Footnote 28: P. Geddes. article "Biology." _Chambers's
Encyclopaedia._]
[Footnote 29: _Origin of Species_ (6th edit.), p. xv.]
[Footnote 30: _Life and Letters_, II, p. 301.]
[Footnote 31: _Science Progress_, New Series, Vol. 1. 1897. "A
Remarkable Anticipation of Modern Views on Evolution." See also Chap.
VI. in _Essays on Evolution_, Oxford, 1908.]
[Footnote 32: See Prof. Patrick Geddes's article "Variation and
Selection," _Encyclopaedia Britannica_ (9th edit.) 1888.]
II
THE SELECTION THEORY
BY AUGUST WEISMANN
_Professor of Zoology in the University of Freiburg_ (_Baden_)
I. THE IDEA OF SELECTION
Many and diverse were the discoveries made by Charles Darwin in the
course of a long and strenuous life, but none of them has had so
far-reaching an influence on the science and thought of his time as
the theory of selection. I do not believe that the theory of evolution
would have made its way so easily and so quickly after Darwin took up
the cudgels in favour of it if he had not been able to support it by a
principle which was capable of solving, in a simple manner, the
greatest riddle that living nature presents to us,--I mean the
purposiveness of every living form relative to the conditions of its
life and its marvellously exact adaptation to these.
Everyone knows that Darwin was not alone in discovering the principle
of selection, and that the same idea occurred simultaneously and
independently to Alfred Russel Wallace. At the memorable meeting of
the Linnean Society on 1st July, 1858, two papers were read
(communicated by Lyell and Hooker) both setting forth the same idea of
selection. One was written by Charles Darwin in Kent, the other by
Alfred Wallace in Ternate, in the Malay Archipelago. It was a splendid
proof of the magnanimity of these two investigators, that they thus in
all friendliness and without envy, united in laying their ideas
before a scientific tribunal: their names will always shine side by
side as two of the brightest stars in the scientific sky.
The idea of selection set forth by the two naturalists was at the time
absolutely new, but it was also so simple that Huxley could say of it
later, "How extremely stupid not to have thought of that." As Darwin
was led to the general doctrine of descent, not through the labours of
his predecessors in the early years of the century, but by his own
observations, so it was in regard to the principle of selection. He
was struck by the innumerable cases of adaptation, as, for instance,
that of the woodpeckers and tree-frogs to climbing, or the hooks and
feather-like appendages of seeds, which aid in the distribution of
plants, and he said to himself that an explanation of adaptations was
the first thing to be sought for in attempting to formulate a theory
of evolution.
But since adaptations point to _changes_ which have been undergone by
the ancestral forms of existing species, it is necessary, first of
all, to inquire how far species in general are _variable_. Thus
Darwin's attention was directed in the first place to the phenomenon
of variability, and the use man has made of this, from very early
times, in the breeding of his domesticated animals and cultivated
plants. He inquired carefully how breeders set to work, when they
wished to modify the structure and appearance of a species to their
own ends, and it was soon clear to him that _selection for breeding
purposes_ played the chief part.
But how was it possible that such processes should occur in free
nature? Who is here the breeder, making the selection, choosing out
one individual to bring forth offspring and rejecting others? That was
the problem that for a long time remained a riddle to him.
Darwin himself relates how illumination suddenly came to him. He had
been reading, for his own pleasure, Malthus' book on Population, and,
as he had long known from numerous observations, that every species
gives rise to many more descendants than ever attain to maturity, and
that, therefore, the greater number of the descendants of a species
perish without reproducing, the idea came to him that the decision as
to which member of a species was to perish and which was to attain to
maturity and reproduction might not be a matter of chance, but might
be determined by the constitution of the individuals themselves,
according as they were more or less fitted for survival. With this
idea the foundation of the theory of selection was laid.
In _artificial selection_ the breeder chooses out for pairing only
such individuals as possess the character desired by him in a somewhat
higher degree than the rest of the race. Some of the descendants
inherit this character, often in a still higher degree, and if this
method be pursued throughout several generations, the race is
transformed in respect of that particular character.
_Natural selection_ depends on the same three factors as _artificial
selection_: on _variability_, _inheritance_, and _selection for
breeding_, but this last is here carried out not by a breeder but by
what Darwin called the "struggle for existence." This last factor is
one of the special features of the Darwinian conception of nature.
That there are carnivorous animals which take heavy toll in every
generation of the progeny of the animals on which they prey, and that
there are herbivores which decimate the plants in every generation had
long been known, but it is only since Darwin's time that sufficient
attention has been paid to the facts that, in addition to this regular
destruction, there exists between the members of a species a keen
competition for space and food, which limits multiplication, and that
numerous individuals of each species perish because of unfavourable
climatic conditions. The "struggle for existence," which Darwin
regarded as taking the place of the human breeder in free nature, is
not a direct struggle between carnivores and their prey, but is the
assumed competition for survival between individuals _of the same_
species, of which, on an average, only those survive to reproduce
which have the greatest power of resistance, while the others, less
favourably constituted, perish early. This struggle is so keen, that,
within a limited area, where the conditions of life have long remained
unchanged, of every species, whatever be the degree of fertility, only
two, _on an average_, of the descendants of each pair survive; the
others succumb either to enemies, or to disadvantages of climate, or
to accident. A high degree of fertility is thus not an indication of
the special success of a species, but of the numerous dangers that
have attended its evolution. Of the six young brought forth by a pair
of elephants in the course of their lives only two survive in a given
area; similarly, of the millions of eggs which two thread-worms leave
behind them only two survive. It is thus possible to estimate the
dangers which threaten a species by its ratio of elimination, or,
since this cannot be done directly, by its fertility.
Although a great number of the descendants of each generation fall
victims to accident, among those that remain it is still the greater
or less fitness of the organism that determines the "selection for
breeding purposes," and it would be incomprehensible if, in this
competition, it were not ultimately, that is, on an average, the best
equipped which survive, in the sense of living long enough to
reproduce.
Thus the principle of natural selection is _the selection of the best
for reproduction_, whether the "best" refers to the whole
constitution, to one or more parts of the organism, or to one or more
stages of development. Every organ, every part, every character of an
animal, fertility and intelligence included, must be improved in this
manner, and be gradually brought up in the course of generations to
its highest attainable state of perfection. And not only may
improvement of parts be brought about in this way, but new parts and
organs may arise, since, through the slow and minute steps of
individual or "fluctuating" variations, a part may be added here or
dropped out there, and thus something new is produced.
The principle of selection solved the riddle as to how what was
purposive could conceivably be brought about without the intervention
of a directing power, the riddle which animate nature presents to our
intelligence at every turn, and in face of which the mind of a Kant
could find no way out, for he regarded a solution of it as not to be
hoped for. For, even if we were to assume an evolutionary force that
is continually transforming the most primitive and the simplest forms
of life into ever higher forms, and the homogeneity of primitive times
into the infinite variety of the present, we should still be unable to
infer from this alone how each of the numberless forms adapted to
particular conditions of life should have appeared _precisely at the
right moment in the history of the earth_ to which their adaptations
were appropriate, and precisely at the proper place in which all the
conditions of life to which they were adapted occurred: the
humming-birds at the same time as the flowers; the trichina at the
same time as the pig; the bark-coloured moth at the same time as the
oak, and the wasp-like moth at the same time as the wasp which
protects it. Without processes of selection we should be obliged to
assume a "pre-established harmony" after the famous Leibnitzian model,
by means of which the clock of the evolution of organisms is so
regulated as to strike in exact synchronism with that of the history
of the earth! All forms of life are strictly adapted to the conditions
of their life, and can persist under these conditions alone.
There must therefore be an intrinsic connection between the conditions
and the structural adaptations of the organism, and, _since the
conditions of life cannot be determined by the animal itself, the
adaptations must be called forth by the conditions_.
The selection theory teaches us how this is conceivable, since it
enables us to understand that there is a continual production of what
is non-purposive as well as of what is purposive, but the purposive
alone survives, while the non-purposive perishes in the very act of
arising. This is the old wisdom taught long ago by Empedocles.
II. THE LAMARCKIAN PRINCIPLE
Lamarck, as is well known, formulated a definite theory of evolution
at the beginning of the nineteenth century, exactly fifty years before
the Darwin-Wallace principle of selection was given to the world. This
brilliant investigator also endeavoured to support his theory by
demonstrating forces which might have brought about the
transformations of the organic world in the course of the ages. In
addition to other factors, he laid special emphasis on the increased
or diminished use of the parts of the body, assuming that the
strengthening or weakening which takes place from this cause during
the individual life, could be handed on to the offspring, and thus
intensified and raised to the rank of a specific character. Darwin
also regarded this _Lamarckian principle_, as it is now generally
called, as a factor in evolution, but he was not fully convinced of
the transmissibility of acquired characters.
As I have here to deal only with the theory of selection, I need not
discuss the Lamarckian hypothesis, but I must express my opinion that
there is room for much doubt as to the cooeperation of this principle
in evolution. Not only is it difficult to imagine how the transmission
of functional modifications could take place, but, up to the present
time, notwithstanding the endeavours of many excellent investigators,
not a single actual proof of such inheritance has been brought
forward. Semon's experiments on plants are, according to the botanist
Pfeffer, not to be relied on, and even the recent, beautiful
experiments made by Dr. Kammerer on salamanders, cannot, as I hope to
show elsewhere, be regarded as proof, if only because they do not deal
at all with functional modifications, that is, with modifications
brought about by use, and it is to these _alone_ that the Lamarckian
principle refers.
III. OBJECTIONS TO THE THEORY OF SELECTION
(_a_) _Saltatory evolution_
The Darwinian doctrine of evolution depends essentially on _the
cumulative augmentation_ of minute variations in the direction of
utility. But can such minute variations, which are undoubtedly
continually appearing among the individuals of the same species,
possess any selection-value; can they determine which individuals are
to survive, and which are to succumb; can they be increased by natural
selection till they attain to the highest development of a purposive
variation?
To many this seems so improbable that they have urged a theory of
evolution by leaps from species to species. Koelliker, in 1872,
compared the evolution of species with the processes which we can
observe in the individual life in cases of alternation of generations.
But a polyp only gives rise to a medusa because it has itself arisen
from one, and there can be no question of a medusa ever having arisen
suddenly and _de novo_ from a polyp-bud, if only because both forms
are adapted in their structure as a whole, and in every detail to the
conditions of their life. A sudden origin, in a natural way, of
numerous adaptations is inconceivable. Even the degeneration of a
medusoid from a free-swimming animal to a mere brood-sac (gonophore)
is not sudden and saltatory, but occurs by imperceptible modifications
throughout hundreds of years, as we can learn from the numerous stages
of the process of degeneration persisting at the same time in
different species.
If, then, the degeneration to a simple brood-sac takes place only by
very slow transitions, each stage of which may last for centuries, how
could the much more complex _ascending_ evolution possibly have taken
place by sudden leaps? I regard this argument as capable of further
extension, for wherever in nature we come upon degeneration, it is
taking place by minute steps and with a slowness that makes it not
directly perceptible, and I believe that this in itself justifies us
in concluding that _the same must be true of ascending_ evolution. But
in the latter case the goal can seldom be distinctly recognised while
in cases of degeneration the starting-point of the process can often
be inferred, because several nearly related species may represent
different stages.
In recent years Bateson in particular has championed the idea of
saltatory, or so-called discontinuous evolution, and has collected a
number of cases in which more or less marked variations have suddenly
appeared. These are taken for the most part from among domesticated
animals which have been bred and crossed for a long time, and it is
hardly to be wondered at that their much mixed and much influenced
germ-plasm should, under certain conditions, give rise to remarkable
phenomena, often indeed producing forms which are strongly suggestive
of monstrosities, and which would undoubtedly not survive in free
nature, unprotected by man. I should regard such cases as due to an
intensified germinal selection--though this is to anticipate a
little--and from this point of view it cannot be denied that they have
a special interest. But they seem to me to have no significance as far
as the transformation of species is concerned, if only because of the
extreme rarity of their occurrence.
There are, however, many variations which have appeared in a sudden
and saltatory manner, and some of these Darwin pointed out and
discussed in detail: the copper beech, the weeping trees, the oak with
"fern-like leaves," certain garden-flowers, etc. But none of them have
persisted in free nature, or evolved into permanent types.
On the other hand, wherever enduring types have arisen, we find traces
of a gradual origin by successive stages, even if, at first sight,
their origin may appear to have been sudden. This is the case with
_seasonal Dimorphism_, the first known cases of which exhibited marked
differences between the two generations, the winter and the summer
brood. Take for instance the much discussed and studied form
_Vanessa_ (_Araschnia_) _levana-prorsa_. Here the differences between
the two forms are so great and so apparently disconnected, that one
might almost believe it to be a sudden mutation, were it not that old
transition-stages can be called forth by particular temperatures, and
we know other butterflies, as for instance our Garden Whites, in which
the differences between the two generations are not nearly so marked;
indeed, they are so little apparent that they are scarcely likely to
be noticed except by experts. Thus here again there are small initial
steps, some of which, indeed, must be regarded as adaptations, such as
the green-sprinkled or lightly tinted under-surface which gives them a
deceptive resemblance to parsley or to Cardamine leaves.
Even if saltatory variations do occur, we cannot assume that these
_have ever led to forms which are capable of survival under the
conditions of wild life_. Experience has shown that in plants which
have suddenly varied the power of persistence is diminished.
Korschinsky attributes to them weaknesses of organisation in general;
"they bloom late, ripen few of their seeds, and show great
sensitiveness to cold." These are not the characters which make for
success in the struggle for existence.
We must briefly refer here to the views--much discussed in the last
decade--of H. de Vries, who believes that the roots of transformation
must be sought for in _saltatory variations arising from internal
causes_, and distinguishes such _mutations_, as he has called them,
from ordinary individual variations, in that they breed true, that is,
with strict in-breeding they are handed on pure to the next
generation. I have elsewhere endeavoured to point out the weaknesses
of this theory,[33] and I am the less inclined to return to it here
that it now appears[34] that the far-reaching conclusions drawn by de
Vries from his observations on the Evening Primrose, _Oenothera
lamarckiana_, rest upon a very insecure foundation. The plant from
which de Vries saw numerous "species"--his "mutations"--arise was not,
as he assumed, a _wild species_ that had been introduced to Europe
from America, but was probably a hybrid form which was first
discovered in the Jardin des Plantes in Paris, and which does not
appear to exist anywhere in America as a wild species.
This gives a severe shock to the "Mutation theory," for the other
_actually wild_ species with which de Vries experimented showed no
"mutations" but yielded only negative results.
Thus we come to the conclusion that Darwin[35] was right in regarding
transformations as taking place by minute steps, which, if useful, are
augmented in the course of innumerable generations, because their
possessors more frequently survive in the struggle for existence.
(_b_) _Selection-value of the initial steps_
Is it possible that the insignificant deviations which we know as
"individual variations" can form the beginning of a process of
selection? Can they decide which is to perish and which to survive? To
use a phrase of Romanes, can they have _selection-value_?
Darwin himself answered this question, and brought together many
excellent examples to show that differences, apparently insignificant
because very small, might be of decisive importance for the life of
the possessor. But it is by no means enough to bring forward cases of
this kind, for the question is not merely whether finished adaptations
have selection-value, but whether the first beginnings of these, and
whether the small, I might almost say minimal increments, which have
led up from these beginnings to the perfect adaptation, have also had
selection-value. To this question even one who, like myself, has been
for many years a convinced adherent of the theory of selection, can
only reply: _We must assume so, but we cannot prove it in any case_.
It is not upon demonstrative evidence that we rely when we champion
the doctrine of selection as a scientific truth; we base our argument
on quite other grounds. Undoubtedly there are many apparently
insignificant features, which can nevertheless be shown to be
adaptations--for instance, the thickness of the basin-shaped shell of
the limpets that live among the breakers on the shore. There can be no
doubt that the thickness of these shells, combined with their flat
forms, protects the animals from the force of the waves breaking upon
them,--but how have they become so thick? What proportion of thickness
was sufficient to decide that of two variants of a limpet one should
survive, the other be eliminated? We can say nothing more than that we
infer from the present state of the shell, that it must have varied in
regard to differences in shell-thickness, and that these differences
must have had selection-value,--no proof therefore, but an assumption
which we must show to be convincing.
For a long time the marvellously complex _radiate_ and _lattice-work_
skeletons of Radiolarians were regarded as a mere outflow of "Nature's
infinite wealth of form," as an instance of a purely morphological
character with no biological significance. But recent investigations
have shown that these, too, have an adaptive significance (Haecker).
The same thing has been shown by Schuett in regard to the lowly
unicellular plants, the Peridineae, which abound alike on the surface
of the ocean and in its depths. It has been shown that the long
skeletal processes which grow out from these organisms have
significance not merely as a supporting skeleton, but also as an
extension of the superficial area, which increases the contact with
the water-particles, and prevents the floating organisms from sinking.
It has been established that the processes are considerably shorter in
the colder layers of the ocean, and that they may be twelve times as
long[36] in the warmer layers, thus corresponding to the greater or
smaller amount of friction which takes place in the denser and less
dense layers of the water.
The Peridineae of the warmer ocean layers have thus become long-rayed,
those of the colder layers short-rayed, not through the direct effect
of friction on the protoplasm, but through processes of selection,
which favoured the longer rays in warm water, since they kept the
organism afloat, while those with short rays sank and were eliminated.
If we put the question as to selection-value in this case, and ask how
great the variations in the length of processes must be in order to
possess selection-value; what can we answer except that these
variations must have been minimal, and yet sufficient to prevent too
rapid sinking and consequent elimination? Yet this very case would
give the ideal opportunity for a mathematical calculation of the
minimal selection-value, although of course it is not feasible from
lack of data to carry out the actual calculation.
But even in organisms of more than microscopic size there must
frequently be minute, even microscopic differences which set going the
process of selection, and regulate its progress to the highest
possible perfection.
Many tropical trees possess thick, leathery leaves, as a protection
against the force of the tropical raindrops. The _direct_ influence of
the rain cannot be the cause of this power of resistance, for the
leaves, while they were still thin, would simply have been torn to
pieces. Their toughness must therefore be referred to selection, which
would favour the trees with slightly thicker leaves, though we cannot
calculate with any exactness how great the first stages of increase in
thickness must have been. Our hypothesis receives further support from
the fact that, in many such trees, the leaves are drawn out into a
beak-like prolongation (Stahl and Haberlandt) which facilitates the
rapid falling off of the rain water, and also from the fact that the
leaves, while they are still young, hang limply down in bunches which
offer the least possible resistance to the rain. Thus there are here
three adaptations which can only be interpreted as due to selection.
The initial stages of these adaptations must undoubtedly have had
selection-value.
But even in regard to this case we are reasoning in a circle, not
giving "proofs," and no one who does not wish to believe in the
selection-value of the initial stages can be forced to do so. Among
the many pieces of presumptive evidence a particularly weighty one
seems to me to be _the smallness of the steps of progress_ which we
can observe in certain cases, as for instance in leaf-imitation among
butterflies, and in mimicry generally. The resemblance to a leaf, for
instance of a particular Kallima, seems to us so close as to be
deceptive, and yet we find in another individual, or it may be in many
others, a spot added which increases the resemblance, and which could
not have become fixed unless the increased deceptiveness so produced
had frequently led to the overlooking of its much persecuted
possessor. But if we take the selection-value of the initial stages
for granted, we are confronted with the further question which I
myself formulated many years ago: How does it happen _that the
necessary beginnings of a useful variation are always present_? How
could insects which live upon or among green leaves become all green,
while those that live on bark become brown? How have the desert
animals become yellow and the Arctic animals white? Why were the
necessary variations always present? How could the green locust lay
brown eggs, or the privet caterpillar develop white and lilac-coloured
lines on its green skin?
It is of no use answering to this that the question is wrongly
formulated[37] and that it is the converse that is true; that the
process of selection takes place in accordance with the variations
that present themselves. This proposition is undeniably true, but so
also is another, which apparently negatives it: the variation required
has in the majority of cases actually presented itself. Selection
cannot solve this contradiction; it does not call forth the useful
variation, but simply works upon it. The ultimate reason why one and
the same insect should occur in green and in brown, as often happens
in caterpillars and locusts, lies in the fact that variations towards
brown presented themselves, and so also did variations towards green:
_the kernel of the riddle lies in the varying_, and for the present we
can only say, that small variations in different directions present
themselves in every species. Otherwise so many different kinds of
variations could not have arisen. I have endeavoured to explain this
remarkable fact by means of the intimate processes that must take
place within the germ-plasm, and I shall return to the problem when
dealing with "germinal selection."
We have, however, to make still greater demands on variation, for it
is not enough that the necessary variation should occur in isolated
individuals, because in that case there would be small prospect of its
being preserved, notwithstanding its utility. Darwin at first
believed, that even single variations might lead to transformation of
the species, but later he became convinced that this was impossible,
at least without the cooeperation of other factors, such as isolation
and sexual selection.
In the case of the _green caterpillars with bright longitudinal
stripes_, numerous individuals exhibiting this useful variation must
have been produced to start with. In all higher, that is,
multicellular organisms, the germ-substance is the source of all
transmissible variations, and this germ-plasm is not a simple
substance but is made up of many primary constituents. The question
can therefore be more precisely stated thus: How does it come about
that in so many cases the useful variations present themselves in
numbers just where they are required, the white oblique lines in the
leaf-caterpillar on the under surface of the body, the accompanying
coloured stripes just above them? And, further, how has it come about
that in grass caterpillars, not oblique but longitudinal stripes,
which are more effective for concealment among grass and plants, have
been evolved? And finally, how is it that the same Hawk-moth
caterpillars, which to-day show oblique stripes, possessed
longitudinal stripes in Tertiary times? We can read this fact from the
history of their development, and I have before attempted to show the
biological significance of this change of colour.[38]
For the present I need only draw the conclusion that one and the same
caterpillar may exhibit the initial stages of both, and that it
depends on the manner in which these marking elements are
_intensified_ and _combined_ by natural selection whether whitish
longitudinal or oblique stripes should result. In this case then the
"useful variations" were actually "always there," and we see that in
the same group of Lepidoptera, e.g. species of Sphingidae, evolution
has occurred in both directions according to whether the form lived
among grass or on broad leaves with oblique lateral veins, and we can
observe even now that the species with oblique stripes have
longitudinal stripes when young, that is to say, while the stripes
have no biological significance. The white places in the skin which
gave rise, probably first as small spots, to this protective marking
could be combined in one way or another according to the requirements
of the species. They must therefore either have possessed
selection-value from the first, or, if this was not the case at their
earliest occurrence, there must have been _some other factors_ which
raised them to the point of selection-value. I shall return to this in
discussing germinal selection. But the case may be followed still
farther, and leads us to the same alternative on a still more secure
basis.
Many years ago I observed in caterpillars of _Smerinthus populi_ (the
poplar hawk-moth), which also possess white oblique stripes, that
certain individuals showed _red spots_ above these stripes; these
spots occurred only on certain segments, and never flowed together to
form continuous stripes. In another species (_Smerinthus tiliae_)
similar blood-red spots unite to form a line-like coloured seam in the
last stage of larval life, while in _S. ocellata_ rust-red spots
appear in individual caterpillars, but more rarely than in _S.
populi_, and they show no tendency to flow together.
Thus we have here the origin of a new character, arising from small
beginnings, at least in _S. tiliae_, in which species the coloured
stripes are a normal specific character. In the other species, _S.
populi_ and _S. ocellata_, we find the beginnings of the same
variation, in one more rarely than in the other, and we can imagine
that, in the course of time, in these two species, coloured lines over
the oblique stripes will arise. In any case these spots are the
elements of variation, out of which coloured lines _may_ be evolved,
if they are combined in this direction through the agency of natural
selection. In _S. populi_ the spots are often small, but sometimes it
seems as though several had united to form large spots. Whether a
process of selection in this direction will arise in _S. populi_ and
_S. ocellata_, or whether it is now going on cannot be determined,
since we cannot tell in advance what biological value the marking
might have for these two species. It is conceivable that the spots may
have no selection-value as far as these species are concerned, and may
therefore disappear again in the course of phylogeny, or, on the other
hand, that they may be changed in another direction, for instance
towards imitation of the rust-red fungoid patches on poplar and willow
leaves. In any case we may regard the smallest spots as the initial
stages of variation, the larger as a cumulative summation of these.
Therefore either these initial stages must already possess
selection-value, or, as I said before: _There must be some other
reason for their cumulative summation_. I should like to give one more
example, in which we can infer, though we cannot directly observe, the
initial stages.
All the Holothurians or sea-cucumbers have in the skin calcereous
bodies of different forms, usually thick and irregular, which make the
skin tough and resistant. In a small group of them--the species of
Synapta--the calcareous bodies occur in the form of delicate anchors
of microscopic size. Up till 1897 these anchors, like many other
delicate microscopic structures, were regarded as curiosities, as
natural marvels. But a Swedish observer, Oestergren, has recently
shown that they have a biological significance: they serve the
footless Synapta as auxiliary organs of locomotion, since, when the
body swells up in the act of creeping, they press firmly with their
tips, which are embedded in the skin, against the substratum on which
the animal creeps, and thus prevent slipping backwards. In other
Holothurians this slipping is made impossible by the fixing of the
tube-feet. The anchors act automatically, sinking their tips towards
the ground when the corresponding part of the body thickens, and
returning to the original position at an angle of 45 degrees to the
upper surface when the part becomes thin again. The arms of the anchor
do not lie in the same plane as the shaft, and thus the curve of the
arms forms the outermost part of the anchor, and offers no further
resistance to the gliding of the animal. Every detail of the anchor,
the curved portion, the little teeth at the head, the arms, etc., can
be interpreted in the most beautiful way, above all the form of the
anchor itself, for the two arms prevent it from swaying round to the
side. The position of the anchors, too, is definite and significant;
they lie obliquely to the longitudinal axis of the animal, and
therefore they act alike whether the animal is creeping backwards or
forwards. Moreover, the tips would pierce through the skin if the
anchors lay in the longitudinal direction. Synapta burrows in the
sand; it first pushes in the thin anterior end, and thickens this
again, thus enlarging the hole, then the anterior tentacles displace
more sand, the body is worked in a little farther, and the process
begins anew. In the first act the anchors are passive, but they begin
to take an active share in the forward movement when the body is
contracted again. Frequently the animal retains only the posterior end
buried in the sand, and then the anchors keep it in position, and make
rapid withdrawal possible.
Thus we have in these apparently random forms of the calcereous
bodies, complex adaptations in which every little detail as to
direction, curve, and pointing is exactly determined. That they have
selection-value in their present perfected form is beyond all doubt,
since the animals are enabled by means of them to bore rapidly into
the ground and so to escape from enemies. We do not know what the
initial stages were, but we cannot doubt that the little improvements,
which occurred as variations of the originally simple slimy bodies of
the Holothurians, were preserved because they already possessed
selection-value for the Synaptidae. For such minute microscopic
structures whose form is so delicately adapted to the role they have
to play in the life of the animal, cannot have arisen suddenly and as
a whole, and every new variation of the anchor, that is, in the
direction of the development of the two arms, and every curving of the
shaft which prevented the tips from projecting at the wrong time, in
short, every little adaptation in the modelling of the anchor must
have possessed selection-value. And that such minute changes of form
fall within the sphere of fluctuating variations, that is to say,
_that they occur_ is beyond all doubt.
In many of the Synaptidae the anchors are replaced by calcareous rods
bent in the form of an S, which are said to act in the same way.
Others, such as those of the genus Ankyroderma, have anchors which
project considerably beyond the skin, and, according to Oestergren,
serve "to catch plant-particles and other substances" and so mask the
animal. Thus we see that in the Synaptidae the thick and irregular
calcareous bodies of the Holothurians have been modified and
transformed in various ways in adaptation to the footlessness of these
animals, and to the peculiar conditions of their life, and we must
conclude that the earlier stages of these changes presented themselves
to the processes of selection in the form of microscopic variations.
For it is as impossible to think of any origin other than through
selection in this case as in the case of the toughness, and the
"drip-tips" of tropical leaves. And as these last could not have been
produced directly by the beating of the heavy raindrops upon them, so
the calcareous anchors of Synapta cannot have been produced directly
by the friction of the sand and mud at the bottom of the sea, and,
since they are parts whose function is _passive_ the Lamarckian factor
of use and disuse does not come into question. The conclusion is
unavoidable, that the microscopically small variations of the
calcareous bodies in the ancestral forms have been intensified and
accumulated in a particular direction, till they have led to the
formation of the anchor. Whether this has taken place by the action of
natural selection alone, or whether the laws of variation and the
intimate processes within the germ-plasm have cooeperated will become
clear in the discussion of germinal selection. This whole process of
adaptation has obviously taken place within the time that has elapsed
since this group of sea-cucumbers lost their tube-feet, those
characteristic organs of locomotion which occur in no group except the
Echinoderms, and yet have totally disappeared in the Synaptidae. And
after all what would animals that live in sand and mud do with
tube-feet?
(_c_) _Coadaptation_
Darwin pointed out that one of the essential differences between
artificial and natural selection lies in the fact that the former can
modify only a few characters, usually only one at a time, while Nature
preserves in the struggle for existence all the variations of a
species, at the same time and in a purely mechanical way, if they
possess selection-value.
Herbert Spencer, though himself an adherent of the theory of selection,
declared in the beginning of the nineties that in his opinion the range of
this principle was greatly over-estimated, if the great changes which have
taken place in so many organisms in the course of ages are to be
interpreted as due to this process of selection alone, since no
transformation of any importance can be evolved by itself; it is always
accompanied by a host of secondary changes. He gives the familiar example
of the Giant Stag of the Irish peat, the enormous antlers of which required
not only a much stronger skull cap, but also greater strength of the
sinews, muscles, nerves and bones of the whole anterior half of the animal,
if their mass was not to weigh down the animal altogether. It is
inconceivable, he says, that so many processes of selection should take
place _simultaneously_, and we are therefore forced to fall back on the
Lamarckian factor of the use and disuse of functional parts. And how, he
asks, could natural selection follow two opposite directions of evolution
in different parts of the body at the same time, as for instance in the
case of the kangaroo, in which the forelegs must have become shorter, while
the hind legs and the tail were becoming longer and stronger?
Spencer's main object was to substantiate the validity of the
Lamarckian principle, the cooeperation of which with selection had been
doubted by many. And it does seem as though this principle, if it
operates in nature at all, offers a ready and simple explanation of
all such secondary variations. Not only muscles, but nerves, bones,
sinews, in short all tissues which function actively, increase in
strength in proportion as they are used, and conversely they decrease
when the claims on them diminish. All the parts, therefore, which
depend on the part that varied first, as for instance the enlarged
antlers of the Irish Elk, must have been increased or decreased in
strength, in exact proportion to the claims made upon them,--just as
is actually the case.
But beautiful as this explanation would be, I regard it as untenable,
because it assumes the _transmissibility of functional modifications_
(so-called "acquired" characters), and this is not only
undemonstrable, but is scarcely theoretically conceivable, for the
secondary variations which accompany or follow the first as
correlative variations, occur also in cases in which the animals
concerned are sterile and _therefore cannot transmit anything to their
descendants_. This is true of _worker bees_, and particularly of
_ants_, and I shall here give a brief survey of the present state of
the problem as it appears to me.
Much has been written on both sides of this question since the
published controversy on the subject in the nineties between Herbert
Spencer and myself. I should like to return to the matter in detail,
if the space at my disposal permitted, because it seems to me that the
arguments I advanced at that time are equally cogent to-day,
notwithstanding all the objections that have since been urged against
them. Moreover, the matter is by no means one of subordinate interest;
it is the very kernel of the whole question of the reality and value
of the principle of selection. For if selection alone does not suffice
to explain "_harmonious adaptation_" as I have called Spencer's
_Coadaptation_, and if we require to call in the aid of the Lamarckian
factor it would be questionable whether selection would explain any
adaptations whatever. In this particular case--of worker bees--the
Lamarckian factor may be excluded altogether, for it can be
demonstrated that here at any rate the effects of use and disuse
cannot be transmitted.
But if it be asked why we are unwilling to admit the cooeperation of
the Darwinian factor of selection and the Lamarckian factor, since
this would afford us an easy and satisfactory explanation of the
phenomena, I answer: _Because the Lamarckian principle is fallacious,
and because by accepting it we close the way towards deeper insight_.
It is not a spirit of combativeness or a desire for self-vindication
that induces me to take the field once more against the Lamarckian
principle, it is the conviction that the progress of our knowledge is
being obstructed by the acceptance of this fallacious principle, since
the facile explanation it apparently affords prevents our seeking
after a truer explanation and a deeper analysis.
The workers in the various species of ants are sterile, that is to
say, they take no regular part in the reproduction of the species,
although individuals among them may occasionally lay eggs. In addition
to this they have lost the wings, and the _receptaculum seminis_, and
their compound eyes have degenerated to a few facets. How could this
last change have come about through disuse, since the eyes of workers
are exposed to light in the same way as are those of the sexual
insects and thus in this particular case are not liable to "disuse" at
all? The same is true of the _receptaculum seminis_, which can only
have been disused as far as its glandular portion and its stalk are
concerned, and also of the wings, the nerves tracheae and epidermal
cells of which could not cease to function until the whole wing had
degenerated, for the chitinous skeleton of the wing does not function
at all in the active sense.
But, on the other hand, the workers in all species have undergone
modifications in a positive direction, as, for instance, the greater
development of brain. In many species large workers have evolved,--the
so-called _soldiers_, with enormous jaws and teeth, which defend the
colony,--and in others there are _small_ workers which have taken over
other special functions, such as the rearing of the young Aphides.
This kind of division of the workers into two castes occurs among
several tropical species of ants, but it is also present in the
Italian species, _Colobopsis truncata_. Beautifully as the size of the
jaws could be explained as due to the increased use made of them by
the "soldiers," or the enlarged brain as due to the mental activities
of the workers, the fact of the infertility of these forms is an
insurmountable obstacle to accepting such an explanation. Neither jaws
nor brain can have been evolved on the Lamarckian principle.
The problem of coadaptation is no easier in the case of the ant than
in the case of the Giant Stag. Darwin himself gave a pretty
illustration to show how imposing the difference between the two kinds
of workers in one species would seem if we translated it into human
terms. In regard to the Driver ants (Anomma) we must picture to
ourselves a piece of work, "for instance the building of a house,
being carried on by two kinds of workers, of which one group was five
feet four inches high, the other sixteen feet high."[39]
Although the ant is a small animal as compared with man or with the
Irish Elk, the "soldier" with its relatively enormous jaws is hardly
less heavily burdened than the Elk with its antlers, and in the ant's
case, too, a strengthening of the skeleton, of the muscles, the nerves
of the head, and of the legs must have taken place parallel with the
enlargement of the jaws. _Harmonious adaptation_ (coadaptation) has
here been active in a high degree, and yet these "soldiers" are
sterile! There thus remains nothing for it but to refer all their
adaptations, positive and negative alike, to processes of selection
which have taken place in the rudiments of the workers within the egg
and sperm-cells of their parents. There is no way out of the
difficulty except the one Darwin pointed out. He himself did not find
the solution of the riddle at once. At first he believed that the case
of the workers among social insects presented "the most serious
special difficulty" in the way of his theory of natural selection; and
it was only after it had become clear to him that it was not the
sterile insects themselves but their parents that were selected,
according as they produced more or less well adapted workers, that he
was able to refer to this very case of the conditions among ants "_in
order to show the power of natural selection_."[40] He explains his
view by a simple but interesting illustration. Gardeners have
produced, by means of long continued artificial selection, a variety
of Stock, which bears entirely double, and therefore infertile
flowers.[41] Nevertheless the variety continues to be reproduced from
seed, because, in addition to the double and infertile flowers, the
seeds always produce a certain number of single, fertile blossoms, and
these are used to reproduce the double variety. These single and
fertile plants correspond "to the males and females of an ant-colony,
the infertile plants, which are regularly produced in large numbers,
to the neuter workers of the colony."
This illustration is entirely apt, the only difference between the
two cases consisting in the fact that the variation in the flower is
not a useful, but a disadvantageous one, which can only be preserved
by artificial selection on the part of the gardener, while the
transformations that have taken place parallel with the sterility of
the ants are useful, since they procure for the colony an advantage in
the struggle for existence, and they are therefore preserved by
natural selection. Even the sterility itself in this case is not
disadvantageous, since the fertility of the true females has at the
same time considerably increased. We may therefore regard the sterile
forms of ants, which have gradually been adapted in several directions
to varying functions, _as a certain proof_ that selection really takes
place in the germ-cells of the fathers and mothers of the workers, and
that _special complexes of primordia_ (_ids_) are present in the
workers and in the males and females, and these complexes contain the
primordia of the individual parts (_determinants_). But since all
living entities vary, the determinants must also vary, now in a
favourable, now in an unfavourable direction. If a female produces
eggs, which contain favourably varying determinants in the worker-ids,
then these eggs will give rise to workers modified in the favourable
direction, and if this happens with many females, the colony concerned
will contain a better kind of worker than other colonies.
I digress here in order to give an account of the intimate processes,
which, according to my view, take place within the germ-plasm, and
which I have called "_germinal selection_." These processes are of
importance since they form the roots of variation, which in its turn
is the root of natural selection. I cannot here do more than give a
brief outline of the theory in order to show how the Darwin-Wallace
theory of selection has gained support from it.
With others, I regard the minimal amount of substance which is
contained within the nucleus of the germ-cells, in the form of rods,
bands, or granules, as the _germ-substance_ or _germ-plasm_, and I
call the individual granules _ids_. There is always a multiplicity of
such ids present in the nucleus, either occurring individually, or
united in the form of rods or bands (chromosomes). Each id contains
the primary constituents of a _whole_ individual, so that several ids
are concerned in the development of a new individual.
In every being of complex structure thousands of primary constituents
must go to make up a single id; these I call _determinants_, and I
mean by this name very small individual particles, far below the
limits of microscopic visibility, vital units which feed, grow, and
multiply by division. These determinants control the parts of the
developing embryo,--in what manner need not here concern us. The
determinants differ among themselves, those of a muscle are
differently constituted from those of a nerve-cell or a glandular
cell, etc., and every determinant is in its turn made up of minute
vital units, which I call _biophores_, or the bearers of life.
According to my view, these determinants not only assimilate, like
every other living unit, but they _vary_ in the course of their
growth, as every living unit does; they may vary qualitatively if the
elements of which they are composed vary, they may grow and divide
more or less rapidly, and their variations give rise to
_corresponding_ variations of the organ, cell, or cell-group which
they determine. That they are undergoing ceaseless fluctuations in
regard to size and quality seems to me the inevitable consequence of
their unequal nutrition; for although the germ-cell as a whole usually
receives sufficient nutriment, minute fluctuations in the amount
carried to different parts within the germ-plasm cannot fail to occur.
Now, if a determinant, for instance of a sensory cell, receives for a
considerable time more abundant nutriment than before, it will grow
more rapidly--become bigger, and divide more quickly, and, later, when
the id concerned develops into an embryo, this sensory cell will
become stronger than in the parents, possibly even twice as strong.
This is an instance of a _hereditary individual variation_, arising
from the germ.
The nutritive stream which, according to our hypothesis, favours the
determinant _N_ by chance, that is, for reasons unknown to us, may
remain strong for a considerable time, or may decrease again; but even
in the latter case it is conceivable that the ascending movement of
the determinant may continue, because the strengthened determinant now
_actively_ nourishes itself more abundantly,--that is to say, it
attracts the nutriment to itself, and to a certain extent withdraws it
from its fellow-determinants. In this way, it may--as it seems to
me--get into _permanent upward movement, and attain a degree of
strength from which there is no falling back_. Then positive or
negative selection sets in, favouring the variations which are
advantageous, setting aside those which are disadvantageous.
In a similar manner a _downward_ variation of the determinants may
take place, if its progress be started by a diminished flow of
nutriment. The determinants which are weakened by this diminished flow
will have less affinity for attracting nutriment because of their
diminished strength, and they will assimilate more feebly and grow
more slowly, unless chance streams of nutriment help them to recover
themselves. But, as will presently be shown, a change of direction
cannot take place at _every_ stage of the degenerative process. If a
certain critical stage of downward progress be passed, even favourable
conditions of food-supply will no longer suffice permanently to change
the direction of the variation. Only two cases are conceivable; if the
determinant corresponds to a _useful_ organ, only its removal can
bring back the germ-plasm to its former level; therefore personal
selection removes the id in question, with its determinants, from the
germ-plasm, by causing the elimination of the individual in the
struggle for existence. But there is another conceivable case; the
determinants concerned may be those of an organ which has become
_useless_, and they will then continue unobstructed, but with
exceeding slowness, along the downward path, until the organ becomes
vestigial, and finally disappears altogether.
The fluctuations of the determinants hither and thither may thus be
transformed into a lasting ascending or descending movement; and _this
is the crucial point of these germinal processes_.
This is not a fantastic assumption; we can read it in the fact of the
degeneration of disused parts. _Useless organs are the only ones which
are not helped to ascend again by personal selection, and therefore in
their case alone can we form any idea of how the primary constituents
behave, when they are subject solely to intra-germinal forces_.
The whole determinant system of an id, as I conceive it, is in a state
of continual fluctuation upwards and downwards. In most cases the
fluctuations will counteract one another, because the passive streams
of nutriment soon change, but in many cases the limit from which a
return is possible will be passed, and then the determinants concerned
will continue to vary in the same direction, till they attain positive
or negative selection-value. At this stage personal selection
intervenes and sets aside the variation if it is disadvantageous, or
favours--that is to say, preserves--it if it is advantageous. Only
_the determinant of a useless organ is uninfluenced by personal
selection_, and, as experience shows, it sinks downwards; that is, the
organ that corresponds to it degenerates very slowly but
uninterruptedly till, after what must obviously be an immense stretch
of time, it disappears from the germ-plasm altogether.
Thus we find in the fact of the degeneration of disused parts the
proof that not all the fluctuations of a determinant return to
equilibrium again, but that, when the movement has attained to a
certain strength, it continues _in the same direction_. We have entire
certainty in regard to this as far as the downward progress is
concerned, and we must assume it also in regard to ascending
variations, as the phenomena of artificial selection certainly justify
us in doing. If the Japanese breeders were able to lengthen the
tail-feathers of the cock to six feet, it can only have been because
the determinants of the tail-feathers in the germ-plasm had already
struck out a path of ascending variation, and this movement was taken
advantage of by the breeder, who continually selected for reproduction
the individuals in which the ascending variation was most marked. For
all breeding depends upon the unconscious selection of germinal
variations.
Of course these germinal processes cannot be proved mathematically,
since we cannot actually see the play of forces of the passive
fluctuations and their causes. We cannot say how great these
fluctuations are, and how quickly or slowly, how regularly or
irregularly they change. Nor do we know how far a determinant must be
strengthened by the passive flow of the nutritive stream if it is to
be beyond the danger of unfavourable variations, or how far it must be
weakened passively before it loses the power of recovering itself by
its own strength. It is no more possible to bring forward actual
proofs in this case than it was in regard to the selection-value of
the initial stages of an adaptation. But if we consider that all
heritable variations must have their roots in the germ-plasm, and
further, that when personal selection does not intervene, that is to
say, in the case of parts which have become useless, a degeneration of
the part, and therefore also of its determinant must inevitably take
place; then we must conclude that processes such as I have assumed are
running their course within the germ-plasm, and we can do this with as
much certainty as we were able to infer, from the phenomena of
adaptation, the selection-value of their initial stages. The fact of
the degeneration of disused parts seems to me to afford irrefutable
proof that the fluctuations within the germ-plasm _are the real root
of all hereditary variation_, and the preliminary condition for the
occurrence of the Darwin-Wallace factor of selection. Germinal
selection supplies the stones out of which personal selection builds
her temples and palaces: _adaptations_. The importance for the theory
of the process of degeneration of disused parts cannot be
over-estimated, especially when it occurs in sterile animal forms,
where we are free from the doubt as to the alleged _Lamarckian factor_
which is apt to confuse our ideas in regard to other cases.
If we regard the variation of the many determinants concerned in the
transformation of the female into the sterile worker as having come
about through the gradual transformation of the ids into worker-ids,
we shall see that the germ-plasm of the sexual ants must contain three
kinds of ids, male, female, and worker ids, or if the workers have
diverged into soldiers and nest-builders, then four kinds. We
understand that the worker-ids arose because their determinants struck
out a useful path of variation, whether upward or downward, and that
they continued in this path until the highest attainable degree of
utility of the parts determined was reached. But in addition to the
organs of positive or negative selection-value, there were some which
were indifferent as far as the success and especially the functional
capacity of the workers was concerned: wings, ovarian tubes,
_receptaculum seminis_, a number of the facets of the eye, perhaps
even the whole eye. As to the ovarian tubes it is is possible that
their degeneration was an advantage for the workers, in saving energy,
and if so selection would favour the degeneration; but how could the
presence of eyes diminish the usefulness of the workers to the colony?
or the minute _receptaculum seminis_, or even the wings? These parts
have therefore degenerated _because they were of no further value to
the insect_. But if selection did not influence the setting aside of
these parts because they were neither of advantage nor of disadvantage
to the species, then the Darwinian factor of selection is here
confronted with a puzzle which it cannot solve alone, but which at
once becomes clear when germinal selection is added. For the
determinants of organs that have no further value for the organism,
must, as we have already explained, embark on a gradual course of
retrograde development.
In ants the degeneration has gone so far that there are no
wing-rudiments present in _any_ species, as is the case with so many
butterflies, flies, and locusts, but in the larvae the imaginable
discs of the wings are still laid down. With regard to the ovaries,
degeneration has reached different levels in different species of
ants, as has been shown by the researches of my former pupil,
Elizabeth Bickford. In many species there are twelve ovarian tubes,
and they decrease from that number to one; indeed, in one species no
ovarian tube at all is present. So much at least is certain from what
has been said, that in this case _everything_ depends on the
fluctuations of the elements of the germ-plasm. Germinal selection,
here as elsewhere, presents the variations of the determinants, and
personal selection favours or rejects these, or,--if it be a question
of organs which have become useless,--it does not come into play at
all, and allows the descending variation free course.
It is obvious that even the problem of _coadaptation in sterile
animals_ can thus be satisfactorily explained. If the determinants are
oscillating upwards and downwards in continual fluctuation, and
varying more pronouncedly now in one direction now in the other,
useful variations of every determinant will continually present
themselves anew, and may, in the course of generations, be combined
with one another in various ways. But there is one character of the
determinants that greatly facilitates this complex process of
selection, that, after a certain limit has been reached, they go on
varying in the same direction. From this it follows that development
along a path once struck out may proceed without the continual
intervention of personal selection. This factor only operates, so to
speak, at the beginning, when it selects the determinants which are
varying in the right direction, and again at the end, when it is
necessary to put a check upon further variation. In addition to this,
enormously long periods have been available for all these adaptations,
as the very gradual transition stages between females and workers in
many species plainly show, and thus this process of transformation
loses the marvellous and mysterious character that seemed at the first
glance to invest it, and takes rank, without any straining, among the
other processes of selection. It seems to me that, from the facts that
sterile animal forms can adapt themselves to new vital functions,
their superfluous parts degenerate, and the parts more used adapt
themselves in an ascending direction, those less used in a descending
direction, we must draw the conclusion that harmonious adaptation here
comes about _without the cooeperation of the Lamarckian principle_.
This conclusion once established, however, we have no reason to refer
the thousands of cases of harmonious adaptation, which occur in
exactly the same way among other animals or plants, to a principle,
the _active intervention of which in the transformation of species is
nowhere proved. We do not require it to explain the facts, and
therefore we must not assume it._
The fact of coadaptation, which was supposed to furnish the strongest
argument against the principle of selection, in reality yields the
clearest evidence in favour of it. We _must_ assume it, _because no
other possibility of explanation is open to us, and because these
adaptations actually exist, that is to say, have really taken place_.
With this conviction I attempted, as far back as 1894, when the idea
of germinal selection had not yet occurred to me, to make "harmonious
adaptation" (coadaptation) more easily intelligible in some way or
other, and so I was led to the idea, which was subsequently expounded
in detail by Baldwin, and Lloyd Morgan, and also by Osborn, and Gulick
as _Organic Selection_. It seemed to me that it was not necessary that
all the germinal variations required for secondary variations should
have occurred _simultaneously_, since, for instance, in the case of
the stag, the bones, muscles, sinews, and nerves would be incited by
the increasing heaviness of the antlers to greater activity in _the
individual life_, and so would be strengthened. The antlers can only
have increased in size by very slow degrees, so that the muscles and
bones may have been able to keep pace with their growth in the
individual life, until the requisite germinal variations presented
themselves. In this way a disharmony between the increasing weight of
the antlers and the parts which support and move them would be
avoided, since time would be given for the appropriate germinal
variations to occur, and so to set agoing the _hereditary_ variation
of the muscles, sinews and bones.[42]
I still regard this idea as correct, but I attribute less importance
to "organic selection" than I did at that time, in so far that I do
not believe that it _alone_ could effect complex harmonious
adaptations. Germinal selection now seems to me to play the chief part
in bringing about such adaptations. Something the same is true of the
principle I have called _Panmixia_. As I became more and more
convinced, in the course of years, that the _Lamarckian principle_
ought not to be called in to explain the dwindling of disused parts, I
believed that this process might be simply explained as due to the
cessation of the conservative effect of natural selection. I said to
myself that, from the moment in which a part ceases to be of use,
natural selection withdraws its hand from it, and then it must
inevitably fall from the height of its adaptiveness, because inferior
variants would have as good a chance of persisting as better ones,
since all grades of fitness of the part in question would be mingled
with one another indiscriminately. This is undoubtedly true, as
Romanes pointed out ten years before I did, and this mingling of the
bad with the good probably does bring about a deterioration of the
part concerned. But it cannot account for the steady diminution, which
always occurs when a part is in process of becoming rudimentary, and
which goes on until it ultimately disappears altogether. The process
of dwindling cannot therefore be explained as due to panmixia alone:
we can only find a sufficient explanation in germinal selection.
IV. DERIVATIVES OF THE THEORY OF SELECTION
The impetus in all directions given by Darwin through his theory of
selection has been an immeasurable one, and its influence is still
felt. It falls within the province of the historian of science to
enumerate all the ideas which, in the last quarter of the nineteenth
century, grew out of Darwin's theories, in the endeavour to penetrate
more deeply into the problem of the evolution of the organic world.
Within the narrow limits to which this paper is restricted, I cannot
attempt to discuss any of these.
V. ARGUMENTS FOR THE REALITY OF THE PROCESSES OF SELECTION
(_a_) _Sexual Selection_
Sexual selection goes hand in hand with natural selection. From the
very first I have regarded sexual selection as affording an extremely
important and interesting corroboration of natural selection, but,
singularly enough, it is precisely against this theory that an adverse
judgment has been pronounced in so many quarters, and it is only quite
recently, and probably in proportion as the wealth of facts in proof
of it penetrates into a wider circle, that we seem to be approaching a
more general recognition of this side of the problem of adaptations.
Thus Darwin's words in his preface to the second edition (1874) of his
book, _The Descent of Man and Sexual Selection_, are being justified:
"My conviction as to the operation of natural selection remains
unshaken," and further, "If naturalists were to become more familiar
with the idea of sexual selection, it would, I think, be accepted to a
much greater extent, and already it is fully and favourably accepted
by many competent judges." Darwin was able to speak thus because he
was already acquainted with an immense mass of facts, which, taken
together, yield overwhelming evidence of the validity of the principle
of sexual selection.
_Natural selection_ chooses out for reproduction the individuals that
are best equipped for the struggle for existence, and it does so at
every stage of development; it thus improves the species in all its
stages and forms. _Sexual selection_ operates only on individuals
that are already capable of reproduction, and does so only in relation
to the attainment of reproduction. It arises from the rivalry of one
sex, usually the male, for the possession of the other, usually the
female. Its influence can therefore only _directly_ affect one sex, in
that it equips it better for attaining possession of the other. But
the effect may extend indirectly to the female sex, and thus the whole
species may be modified, without, however, becoming any more capable
of resistance in the struggle for existence, for sexual selection only
gives rise to adaptations which are likely to give their possessor the
victory over rivals in the struggle for possession of the female, and
which are therefore peculiar to the wooing sex: the manifold
"secondary sexual characters." The diversity of these characters is so
great that I cannot here attempt to give anything approaching a
complete treatment of them, but I should like to give a sufficient
number of examples to make the principle itself, in its various modes
of expression, quite clear.
One of the chief preliminary postulates of sexual selection is the
unequal number of individuals in the two sexes, for if every male
immediately finds his mate there can be no competition for the
possession of the female. Darwin has shown that, for the most part,
the inequality between the sexes is due simply to the fact that there
are more males than females, and therefore the males must take some
pains to secure a mate. But the inequality does not always depend on
the numerical preponderance of the males, it is often due to polygamy;
for, if one male claims several females, the number of females in
proportion to the rest of the males will be reduced. Since it is
almost always the males that are the wooers, we must expect to find
the occurrence of secondary sexual characters chiefly among them, and
to find it especially frequent in polygamous species. And this is
actually the case.
If we were to try to guess--without knowing the facts--what means the
male animals make use of to overcome their rivals in the struggle for
the possession of the female, we might name many kinds of means, but
it would be difficult to suggest any which is not actually employed in
some animal group of other. I begin with the mere difference in
strength, through which the male of many animals is so sharply
distinguished from the female, as, for instance, the lion, walrus,
"sea-elephant," and others. Among these the males fight violently for
the possession of the female, who falls to the victor in the combat.
In this simple case no one can doubt the operation of selection, and
there is just as little room for doubt as to the selection-value of
the initial stages of the variation. Differences in bodily strength
are apparent even among human beings, although in their case the
struggle for the possession of the female is no longer decided by
bodily strength alone.
Combats between male animals are often violent and obstinate, and the
employment of the natural weapons of the species in this way has led
to perfecting of these, e.g. the tusks of the boar, the antlers of the
stag, and the enormous, antler-like jaws of the stag-beetle. Here
again it is impossible to doubt that variations in these organs
presented themselves, and that these were considerable enough to be
decisive in combat, and so to lead to the improvement of the weapon.
Among many animals, however, the females at first withdraw from the
males; they are coy, and have to be sought out, and sometimes held by
force. This tracking and grasping of the females by the males has
given rise to many different characters in the latter, as, for
instance, the larger eyes of the male bee, and especially of the males
of the Ephemerids (May-flies), some species of which show, in addition
to the usual compound eyes, large, so-called turban-eyes, so that the
whole head is covered with seeing surfaces. In these species the
females are very greatly in the minority (1-100), and it is easy to
understand that a keen competition for them must take place, and that,
when the insects of both sexes are floating freely in the air, an
unusually wide range of vision will carry with it a decided
advantage. Here again the actual adaptations are in accordance with
the preliminary postulates of the theory. We do not know the stages
through which the eye has passed to its present perfected state, but,
since the number of simple eyes (facets) has become very much greater
in the male than in the female, we may assume that their increase is
due to a gradual duplication of the determinants of the ommatidium in
the germ-plasm, as I have already indicated in regard to sense-organs
in general. In this case, again, the selection-value of the initial
stages hardly admits of doubt; better vision _directly_ secures
reproduction.
In many cases _the organ of smell_ shows a similar improvement. Many
lower Crustaceans (Daphnidae) have better developed organs of smell in
the male sex. The difference is often slight and amounts only to one
or two olfactory filaments, but certain species show a difference of
nearly a hundred of these filaments (Leptodora). The same thing occurs
among insects.
We must briefly consider the clasping or grasping organs which have
developed in the males among many lower Crustaceans, but here natural
selection plays its part along with sexual selection, for the union of
the sexes is an indispensable condition for the maintenance of the
species, and as Darwin himself pointed out, in many cases the two
forms of selection merge into each other. This fact has always seemed
to me to be a proof of natural selection, for, in regard to sexual
selection, it is quite obvious that the victory of the best-equipped
could have brought about the improvement only of the organs concerned,
the factors in the struggle, such as the eye and the olfactory organ.
We come now to the _excitants_; that is, to the group of sexual
characters whose origin through processes of selection has been most
frequently called in question. We may cite the _love-calls_ produced
by many male insects, such as crickets and cicadas. These could only
have arisen in animal groups in which the female did not rapidly flee
from the male, but was inclined to accept his wooing from the first.
Thus, notes like the chirping of the male cricket serve to entice the
females. At first they were merely the signal which showed the
presence of a male in the neighbourhood, and the female was gradually
enticed nearer and nearer by the continued chirping. The male that
could make himself heard to the greatest distance would obtain the
largest following, and would transmit the beginnings, and, later, the
improvement of his voice to the greatest number of descendants. But
sexual excitement in the female became associated with the hearing of
the love-call, and then the sound-producing organ of the male began to
improve, until it attained to the emission of the long-drawn-out soft
notes of the mole-cricket or the maenad-like cry of the cicadas. I
cannot here follow the process of development in detail, but will call
attention to the fact that the original purpose of the voice, the
announcing of the male's presence, became subsidiary, and the exciting
of the female became the chief goal to be aimed at. The loudest
singers awakened the strongest excitement, and the improvement
resulted as a matter of course. I conceive of the origin of bird-song
in a somewhat similar manner, first as a means of enticing, then of
exciting the female.
One more kind of secondary sexual character must here be mentioned:
the odour which emanates from so many animals at the breeding season.
It is possible that this odour also served at first merely to give
notice of the presence of individuals of the other sex, but it soon
became an excitant, and as the individuals which caused the greatest
degree of excitement were preferred, it reached as high a pitch of
perfection as was possible to it. I shall confine myself here to the
comparatively recently discovered fragrance of butterflies. Since
Fritz Mueller found out that certain Brazilian butterflies gave off
fragrance "like a flower," we have become acquainted with many such
cases, and we now know that in all lands, not only many diurnal
Lepidoptera but nocturnal ones also give off a delicate odour, which
is agreeable even to man. The ethereal oil to which this fragrance is
due is secreted by the skin-cells, usually of the wing, as I showed
soon after the discovery of the _scent-scales_. This is the case in
the males; the females have no _special_ scent-scales recognisable as
such by their form, but they must, nevertheless, give off an extremely
delicate fragrance, although our imperfect organ of smell cannot
perceive it, for the males become aware of the presence of a female,
even at night, from a long distance off, and gather round her. We may
therefore conclude, that both sexes have long given forth a very
delicate perfume, which announced their presence to others of the same
species, and that in many species (_not in all_) these small
beginnings become, in the males, particularly strong scent-scales of
characteristic form (lute, brush, or lyre-shaped). At first these
scales were scattered over the surface of the wing, but gradually they
concentrated themselves, and formed broad, velvety bands, or strong,
prominent brushes, and they attained their highest pitch of evolution
when they became enclosed within pits or folds of the skin, which
could be opened to let the delicious fragrance stream forth suddenly
towards the female. Thus in this case also we see that characters, the
original use of which was to bring the sexes together, and so to
maintain the species, have been evolved in the males into means for
exciting the female. And we can hardly doubt, that the females are
most readily enticed to yield to the butterfly that sends out the
strongest fragrance,--that is to say, that excites them to the highest
degree. It is a pity that our organs of smell are not fine enough to
examine the fragrance of male Lepidoptera in general, and to compare
it with other perfumes which attract these insects.[43] As far as we
can perceive them they resemble the fragrance of flowers, but there
are Lepidoptera whose scent suggests musk. A smell of musk is also
given off by several plants: it is a sexual excitant in the
musk-deer, the musk-sheep, and the crocodile.
As far as we know, then, it is perfumes similar to those of flowers
that the male Lepidoptera give off in order to entice their mates and
this is a further indication that animals, like plants, can to a large
extent meet the claims made upon them by life, and produce the
adaptations which are most purposive,--a further proof, too, of my
proposition that the useful variations, so to speak, are _always
there_. The flowers developed the perfumes which entice their
visitors, and the male Lepidoptera developed the perfumes which entice
and excite their mates.
There are many pretty little problems to be solved in this connection,
for there are insects, such as some flies, that are attracted by
smells which are unpleasant to us, like those from decaying flesh and
carrion. But there are also certain flowers, some orchids for
instance, which give forth no very agreeable odour, but one which is
to us repulsive and disgusting; and we should therefore expect that
the males of such insects would give off a smell unpleasant to us, but
there is no case known to me in which this has been demonstrated.
In cases such as we have discussed, it is obvious that there is no
possible explanation except through selection. This brings us to the
last kind of secondary sexual characters, and the one in regard to
which doubt has been most frequently expressed,--decorative colours
and decorative forms, the brilliant plumage of the male pheasant, the
humming-birds, and the bird of Paradise, as well as the bright colours
of many species of butterfly, from the beautiful blue of our little
Lycaenidae to the magnificent azure of the large Morphinae of Brazil.
In a great many cases, though not by any means in all, the male
butterflies are "more beautiful" than the females, and in the Tropics
in particular they shine and glow in the most superb colours. I really
see no reason why we should doubt the power of sexual selection, and I
myself stand wholly on Darwin's side. Even though we certainly cannot
assume that the females exercise a conscious choice of the
"handsomest" mate, and deliberate like the judges in a court of
justice over the perfections of their wooers, we have no reason to
doubt that distinctive forms (decorative feathers), and colours have a
particularly exciting effect upon the female, just as certain odours
have among animals of so many different groups, including the
butterflies. The doubts which existed for a considerable time, as a
result of fallacious experiments, as to whether the colours of flowers
really had any influence in attracting butterflies have now been set
at rest through a series of more careful investigations; we now know
that the colours of flowers are there on account of the butterflies,
as Sprengel first showed, and that the blossoms of Phanerogams are
selected in relation to them, as Darwin pointed out.
Certainly it is not possible to bring forward any convincing proof of
the origin of decorative colours through sexual selection, but there
are many weighty arguments in favour of it, and these form a body of
presumptive evidence so strong that it almost amounts to certainty.
In the first place, there is the analogy with other secondary sexual
characters. If the song of birds and the chirping of the cricket have
been evolved through sexual selection, if the penetrating odours of
male animals,--the crocodile, the musk-deer, the beaver, the
carnivores, and, finally, the flower-like fragrances of the
butterflies have been evolved to their present pitch in this way, why
should decorative colours have arisen in some other way? Why should
the eye be less sensitive to _specifically male_ colours and other
_visible_ signs _enticing to the female_, than the olfactory sense to
specifically male odours, or the sense of hearing to specifically male
sounds? Moreover, the decorative feathers of birds are almost always
spread out and displayed before the female during courtship. I have
elsewhere[44] pointed out that decorative colouring and
sweet-scentedness may replace one another in Lepidoptera as well as in
flowers, for just as some modestly coloured flowers (mignonette and
violet) have often a strong perfume, while strikingly coloured ones
are sometimes quite devoid of fragrance, so we find that the most
beautiful and gaily-coloured of our native Lepidoptera, the species of
Vanessa, have no scent-scales, while these are often markedly
developed in grey nocturnal Lepidoptera. Both attractions may,
however, be combined in butterflies, just as in flowers. Of course, we
cannot explain why both means of attraction should exist in one genus,
and only one of them in another, since we do not know the minutest
details of the conditions of life of the genera concerned. But from
the sporadic distribution of scent-scales in Lepidoptera, and from
their occurrence or absence in nearly related species, we may conclude
that fragrance is a relatively _modern_ acquirement, more recent than
brilliant colouring.
One thing in particular that stamps decorative colouring as a product
of selection is _its gradual intensification_ by the addition of new
spots, which we can quite well observe, because in many cases the
colours have been first acquired by the males, and later transmitted
to the females by inheritance. The scent-scales are never thus
transmitted, probably for the same reason that the decorative colours
of many birds are often not transmitted to the females: because with
these they would be exposed to too great elimination by enemies.
Wallace was the first to point out that in species with concealed
nests the beautiful feathers of the male occurred in the female also,
as in the parrots, for instance, but this is not the case in species
which brood on an exposed nest. In the parrots one can often observe
that the general brilliant colouring of the male is found in the
female, but that certain spots of colour are absent, and these have
probably been acquired comparatively recently by the male and have not
yet been transmitted to the female.
Isolation of the group of individuals which is in process of varying
is undoubtedly of great value in sexual selection, for even a solitary
conspicuous variation will become dominant much sooner in a small
isolated colony, than among a large number of members of a species.
Any one who agrees with me in deriving variations from germinal
selection will regard that process as an essential aid towards
explaining the selection of distinctive courtship-characters, such as
coloured spots, decorative feathers, horny outgrowths in birds and
reptiles, combs, feather-tufts, and the like, since the beginnings of
these would be presented with relative frequency in the struggle
between the determinants within the germ-plasm. The process of
transmission of decorative feathers to the female results, as Darwin
pointed out and illustrated by interesting examples, in the
_colour-transformation of a whole species_, and this process, as the
phyletically older colouring of young birds shows, must, in the course
of thousands of years, have repeated itself several times in a line of
descent.
If we survey the wealth of phenomena presented to us by secondary
sexual characters, we can hardly fail to be convinced of the truth of
the principle of sexual selection. And certainly no one who has
accepted natural selection should reject sexual selection, for, not
only do the two processes rest upon the same basis, but they merge
into one another, so that it is often impossible to say how much of a
particular character depends on one and how much on the other form of
selection.
(_b_) _Natural Selection_
An actual proof of the theory of sexual selection is out of the
question, if only because we cannot tell when a variation attains to
selection-value. It is certain that a delicate sense of smell is of
value to the male moth in his search for the female, but whether the
possession of one additional olfactory hair, or of ten, or of twenty
additional hairs leads to the success of its possessor we are unable
to tell. And we are groping even more in the dark when we discuss the
excitement caused in the female by agreeable perfumes, or by striking
and beautiful colours. That these do make an impression is beyond
doubt; but we can only assume that slight intensifications of them
give any advantage, and we _must_ assume this _since otherwise
secondary sexual characters remain inexplicable_.
The same thing is true in regard to natural selection. It is not
possible to bring forward any actual proof of the selection-value of
the initial stages, and the stages in the increase of variations, as
has been already shown. But the selection-value of a finished
adaptation can in many cases be statistically determined. Cesnola and
Poulton have made valuable experiments in this direction. The former
attached forty-five individuals of the green, and sixty-five of the
brown variety of the praying mantis (_Mantis religiosa_), by a silk
thread to plants, and watched them for seven days. The insects which
were on a surface of a colour Similar to their own remained uneaten,
while twenty-five green insects on brown parts of plants had all
disappeared in eleven days.
The experiments of Poulton and Sanders[45] were made with 600 pupae of
_Vanessa urticae_, the "tortoise-shell butterfly." The pupae were
artificially attached to nettles, tree-trunks, fences, walls, and to
the ground, some at Oxford, some at St. Helens in the Isle of Wight.
In the course of a month 93% of the pupae at Oxford were killed,
chiefly by small birds, while at St. Helens 68% perished. The
experiments showed very clearly that the colour and character of the
surface on which the pupa rests--and thus its own conspicuousness--are
of the greatest importance. At Oxford only the four pupae which were
fastened to nettles emerged; all the rest--on bark, stones and the
like--perished. At St. Helens the elimination was as follows: on
fences where the pupae were conspicuous, 92%; on bark, 66%; on walls,
54%; and among nettles, 57%. These interesting experiments confirm our
views as to protective coloration, and show further, _that the ratio
of elimination in the species is a very high one, and that therefore
selection must be very keen_.
We may say that the process of selection follows as a logical
necessity from the fulfilment of the three preliminary postulates of
the theory: variability, heredity, and the struggle for existence,
with its enormous ratio of elimination in all species. To this we must
add a fourth factor, the _intensification_ of variations which Darwin
established as a fact, and which we are now able to account for
theoretically on the basis of germinal selection. It may be objected
that there is considerable uncertainty about this _logical_ proof,
because of our inability to demonstrate the selection-value of the
initial stages and the individual stages of increase. We have
therefore to fall back on _presumptive evidence_. This is to be found
in _the interpretative value of the theory_. Let us consider this
point in greater detail.
In the first place it is necessary to emphasize what is often
overlooked, namely, that the theory not only explains the
_transformations_ of species, it also explains _their remaining the
same_; in addition to the principle of varying, it contains within
itself that of _persisting_. It is part of the essence of selection,
that it not only causes a part to _vary_ till it has reached its
highest pitch of adaptation, but that it _maintains it at this pitch.
This conserving influence of natural selection_ is of great
importance, and was early recognised by Darwin; it follows naturally
from the principle of the survival of the fittest.
We understand from this how it is that a species which has become
fully adapted to certain conditions of life ceases to vary, but
remains "constant," as long as the conditions of life _for_ it remain
unchanged, whether this be for thousands of years, or for whole
geological epochs. But the most convincing proof of the power of the
principle of selection lies in the innumerable multitude of phenomena
which cannot be explained in any other way. To this category belong
all structures which are only _passively_ of advantage to the
organism, because none of these can have arisen by the alleged
_Lamarckian principle_. These have been so often discussed that we
need do no more than indicate them here. Until quite recently the
sympathetic coloration of animals--for instance, the whiteness of
Arctic animals--was referred, at least in part, to the _direct_
influence of external factors, but the facts can best be explained by
referring them to the processes of selection, for then it is
unnecessary to make the gratuitous assumption that many species are
sensitive to the stimulus of cold and that others are not. The great
majority of Arctic land-animals, mammals and birds, are white, and
this proves that they were all able to present the variation which was
most useful for them. The sable is brown, but it lives in trees, where
the brown colouring protects and conceals it more effectively. The
musk-sheep (_Ovibos moschatus_) is also brown, and contrasts sharply
with the ice and snow, but it is protected from beasts of prey by its
gregarious habit, and therefore it is of advantage to be visible from
as great a distance as possible. That so many species have been able
to give rise to white varieties does not depend on a special
sensitiveness of the skin to the influence of cold, but to the fact
that Mammals and Birds have a general tendency to vary towards white.
Even with us, many birds--starlings, blackbirds, swallows,
etc.--occasionally produce white individuals, but the white variety
does not persist, because it readily falls a victim to the carnivores.
This is true of white fawns, foxes, deer, etc. The whiteness,
therefore, arises from internal causes, and only persists when it is
useful. A great many animals living in a _green environment_ have
become clothed in green, especially insects, caterpillars, and
Mantidae, both persecuted and persecutors.
That it is not the direct effect of the environment which calls forth
the green colour is shown by the many kinds of caterpillar which rest
on leaves and feed on them, but are nevertheless brown. These feed by
night and betake themselves through the day to the trunk of the tree,
and hide in the furrows of the bark. We cannot, however, conclude
from this that they were _unable_ to vary towards green, for there are
Arctic animals which are white only in winter and brown in summer
(Alpine hare, and the ptarmigan of the Alps), and there are also green
leaf-insects which remain green only while they are young and
difficult to see on the leaf, but which become brown again in the last
stage of larval life, when they have outgrown the leaf. They then
conceal themselves by day, sometimes only among withered leaves on the
ground, sometimes in the earth itself. It is interesting that in one
genus, Chaerocampa, one species is brown in the last stage of larval
life, another becomes brown earlier, and in many species the last
stage is not wholly brown, a part remaining green. Whether this is a
case of a double adaptation, or whether the green is being gradually
crowded out by the brown, the fact remains that the same species, even
the same individual, can exhibit both variations. The case is the same
with many of the leaf-like Orthoptera, as, for instance, the praying
mantis (_Mantis religiosa_) which we have already mentioned.
But the best proofs are furnished by those of ten-cited cases in which
the insect bears a deceptive resemblance to another object. We now
know many such cases, such as the numerous imitations of green or
withered leaves, which are brought about in the most diverse ways,
sometimes by mere variations in the form of the insect and in its
colour, sometimes by an elaborate marking, like that which occurs in
the Indian leaf-butterflies, _Kallima inachis_. In the single
butterfly-genus Anaea, in the woods of South America, there are about
a hundred species which are all gaily coloured on the upper surface,
and on the reverse side exhibit the most delicate imitation of the
colouring and pattern of a leaf, generally without any indication of
the leaf-ribs, but extremely deceptive nevertheless. Anyone who has
seen only one such butterfly may doubt whether many of the
insignificant details of the marking can really be of advantage to the
insect. Such details are for instance the apparent holes and splits
in the apparently dry or half-rotten leaf, which are usually due to
the fact that the scales are absent on a circular or oval patch so
that the colourless wing-membrane lies bare, and one can look through
the spot as through a window. Whether the bird which is seeking or
pursuing the butterflies takes these holes for dewdrops, or for the
work of a devouring insect, does not affect the question; the
mirror-like spot undoubtedly increases the general deceptiveness, for
the same thing occurs in many leaf-butterflies, though not in all, and
in some cases it is replaced in quite a peculiar manner. In one
species of Anaea (_A. divina_), the resting butterfly looks exactly
like a leaf out of the outer edge of which a large semi-circular piece
has been eaten, possibly by a caterpillar; but if we look more closely
it is obvious that there is no part of the wing absent, and that the
semi-circular piece is of a clear, pale yellow colour, while the rest
of the wing is of a strongly contrasted dark brown.
But the deceptive resemblance may be caused in quite a different
manner. I have often speculated as to what advantage the brilliant
white C could give to the otherwise dusky-coloured "Comma butterfly"
(_Grapta C. album_). Poulton's recent observations[46] have shown that
this represents the imitation of a crack such as is often seen in dry
leaves, and is very conspicuous because the light shines through it.
The utility obviously lies in presenting to the bird the very familiar
picture of a broken leaf with a clear shining slit, and we may
conclude, from the imitation of such small details, that the birds are
very sharp observers and that the smallest deviation from the usual
arrests their attention and incites them to closer investigation. It
is obvious that such detailed--we might almost say such
subtle--deceptive resemblances could only have come about in the
course of long ages through the acquirement from time to time of
something new which heightened the already existing resemblance.
In face of facts like these there can be no question of chance and no
one has succeeded so far in finding any other explanation to replace
that by selection. For the rest, the apparent leaves are by no means
perfect copies of a leaf; many of them only represent the torn or
broken piece, or the half or two-thirds of a leaf, but then the leaves
themselves frequently do not present themselves to the eye as a whole,
but partially concealed among other leaves. Even those butterflies
which, like the species of Kallima and Anaea, represent the whole of a
leaf with stalk, ribs, apex, and the whole breadth, are not actual
copies which would satisfy a botanist; there is often much wanting. In
Kallima the lateral ribs of the leaf are never all included in the
markings; there are only two or three on the left side and at more
four or five on the right, and in many individuals these are rather
obscure, while in others they are comparatively distinct. This
furnishes us with fresh evidence in favour of their origin through
processes of selection, for a botanically perfect picture could not
arise in this way; there could only be a fixing of such details as
heightened the deceptive resemblance.
Our postulate of origin through selection also enables us to
understand why the leaf-imitation is on the lower surface of the wing
in the diurnal Lepidoptera, and on the upper surface in the nocturnal
forms, corresponding to the attitude of the wings in the resting
position of the two groups.
The strongest of all proofs of the theory, however, is afforded by
cases of true "mimicry," those adaptations discovered by Bates in
1861, consisting in the imitation of one species by another, which
becomes more and more like its model. The model is always a species
that enjoys some special protection from enemies, whether because it
is unpleasant to taste, or because it is in some way dangerous.
It is chiefly among insects and especially among butterflies that we
find the greatest number of such cases. Several of these have been
minutely studied and every detail has been investigated so that it is
difficult to understand how there can still be disbelief in regard to
them. If the many and exact observations which have been carefully
collected and critically discussed for instance by Poulton[47] were
thoroughly studied the arguments which are still frequently urged
against mimicry would be found untenable; we can hardly hope to find
more convincing proof of the actuality of the processes of selection
than these cases put into our hands. The preliminary postulates of the
theory of mimicry have been disputed, for instance, that diurnal
butterflies are persecuted and eaten by birds, but observations
specially directed towards this point in India, Africa, America and
Europe have placed it beyond all doubt. If it were necessary I could
myself furnish an account of my own observations on this point.
In the same way it has been established by experiment and observation
in the field that in all the great regions of distribution there are
butterflies which are rejected by birds and lizards, their chief
enemies, on account of their unpleasant smell or taste. These
butterflies are usually gaily and conspicuously coloured and thus--as
Wallace first interpreted it--are furnished with an easily
recognisable sign: a sign of unpalatableness or _warning colours_. If
they were not thus recognisable easily and from a distance, they would
frequently be pecked at by birds, and then rejected because of their
unpleasant taste; but as it is, the insect-eaters recognise them at
once as unpalatable booty and ignore them. Such _immune_[48] species,
wherever they occur, are imitated by other palatable species, which
thus acquire a certain degree of protection.
It is true that this explanation of the bright, conspicuous colours
is only a hypothesis, but its foundations--unpalatableness, and the
liability of other butterflies to be eaten,--are certain, and its
consequences--the existence of mimetic palatable forms--conform it in
the most convincing manner. Of the many cases now known I select one,
which is especially remarkable, and which has been thoroughly
investigated, _Papilla dardanus_ (_merope_), a large, beautiful,
diurnal butterfly which ranges from Abyssinia throughout the whole of
Africa to the south coast of Cape Colony.
The males of this form are everywhere _almost_ the same in colour and
in form of wings, save for a few variations in the sparse black
markings on the pale yellow ground. But the females occur in several
quite different forms and colourings, and one of these only, the
Abyssinian form, is like the male, while the other three or four are
_mimetic_, that is to say, they copy a butterfly of quite a different
family the Danaids, which are among the _immune_ forms. In each region
the females have thus copied two or three different immune species.
There is much that is interesting to be said in regard to these
species, but it would be out of keeping with the general tenor of this
paper to give details of this very complicated case of polymorphism in
_P. Dardanus_. Anyone who is interested in the matter will find a full
and exact statement of the case in as far as we know it, in Poulton's
_Essays on Evolution_ (pp. 373-375[49]). I need only add that three
different mimetic female forms have been reared from the eggs of a
single female in South Africa. The resemblance of the forms to their
immune models goes so far that even the details of the _local_ forms
of the models are copied by the mimetic species.
It remains to be said that in Madagascar a butterfly,
_Papilio meriones_, occurs, of which both sexes are very similar in
form and markings to the non-mimetic male of _P. dardanus_, so that it
probably represents the ancestor of this latter species.
In face of such facts as these every attempt at another explanation
must fail. Similarly all the other details of the case fulfil the
preliminary postulates of selection, and leave no room for any other
interpretation. That the males do not take on the protective colouring
is easily explained, because they are in general more numerous, and
the females are more important for the preservation of the species,
and must also live longer in order to deposit their eggs. We find the
same state of things in many other species, and in one case (_Elymnias
undularis_) in which the male is also mimetically coloured, it copies
quite a differently coloured immune species from the model followed by
the female. This is quite intelligible when we consider that if there
were _too many_ false immune types, the birds would soon discover that
there were palatable individuals among those with unpalatable warning
colours. Hence the imitation of different immune species by _Papilio
dardanus_!
I regret that lack of space prevents my bringing forward more examples
of mimicry and discussing them fully. But from the case of _Papilio
dardanus_ alone there is much to be learnt which is of the highest
importance for our understanding of transformations. It shows us
chiefly what I once called, somewhat strongly perhaps, _the
omnipotence of natural selection_ in answer to an opponent who had
spoken of its "inadequacy." We here see that one and the same species
is capable of producing four or five different patterns of colouring
and marking; thus the colouring and marking are not, as has often been
supposed, a necessary outcome of the specific nature of the species,
but a true adaptation, which cannot arise as a direct effect of
climatic conditions, but solely through what I may call the sorting
out of the variations produced by the species, according to their
utility. That caterpillars may be either green or brown is already
something more than could have been expected according to the old
conception of species, but that one and the same butterfly should be
now pale yellow, with black; now red with black and pure white; now
deep black with large, pure white spots; and again black with a large
ocheous-yellow spot, and many small white and yellow spots; that in
one sub-species it may be tailed like the ancestral form, and in
another tailless like its Danaid model,--all this shows a far-reaching
capacity for variation and adaptation that we could never have
expected if we did not see the facts before us. How it is possible
that the primary colour-variations should thus be intensified and
combined remains a puzzle even now; we are reminded of the modern
three-colour printing,--perhaps similar combinations of the primary
colours take place in this case; in any case the direction of these
primary variations is determined by the artist whom we know as natural
selection, for there is no other conceivable way in which the model
could affect the butterfly that is becoming more and more like it. The
same climate surrounds all four forms of female; they are subject to
the same conditions of nutrition. Moreover, _Papilio dardanus_ is by
no means the only species of butterfly which exhibits different kinds
of colour-pattern on its wings. Many species of the Asiatic genus
Elymnias have on the upper surface a very good imitation of an immune
Euploeine (Danainae), often with a steel-blue ground-colour, while the
under surface is well concealed when the butterfly is at rest,--thus
there are two kinds of protective coloration each with a different
meaning! The same thing may be observed in many non-mimetic
butterflies, for instance in all our species of Vanessa, in which the
under side shows a grey-brown or brownish-black protective coloration,
but we do not yet know with certainty what may be the biological
significance of the gaily coloured upper surface.
In general it may be said that mimetic butterflies are comparatively
rare species, but there are exceptions, for instance _Limenitis
archippus_ in North America, of which the immune model (_Danaida
plexippus_) also occurs in enormous numbers.
In another mimicry-category the imitators are often more numerous than
the models, namely in the case of the imitation of _dangerous insects_
by harmless species. Bees and wasps are dreaded for their sting, and
they are copied by harmless flies of the genera Eristalis and Syrphus,
and these mimics often occur in swarms about flowering plants without
damage to themselves or to their models; they are feared and are
therefore left unmolested.
In regard also to the _faithfulness of the copy_ the facts are quite
in harmony with the theory, according to which the resemblance must
have arisen and increased _by degrees_. We can recognise this in many
cases, for even now the mimetic species show very _varying degrees of
resemblance_ to their immune model. If we compare, for instance, the
many different imitators of _Danaida chrysippus_ we find that, with
their brownish-yellow ground-colour, and the position and size, and
more or less sharp limitation of their clear marginal spots, they have
reached very different degrees of nearness to their model. Or compare
the female of _Elymnias undularis_ with its model _Danaida genutia_;
there is a general resemblance, but the marking of the Danaida is very
roughly imitated in Elymnias.
Another fact that bears out the theory of mimicry is, that even when
the resemblance in colour-pattern is very great, the _wing-venation_,
which is so constant, and so important in determining the systematic
position of butterflies, is never affected by the variation. The
pursuers of the butterfly have no time to trouble about entomological
intricacies.
I must not pass over a discovery of Poulton's which is of great
theoretical importance--that mimetic butterflies may reach the same
effect by very different means.[50] Thus the glass-like transparency
of the wing of a certain Ithomiine (Methona) and its Pierine mimic
(_Dismorphia orise_) depends on a diminution in the size of the
scales; in the Danaine genus Itune it is due to the fewness of the
scales and in a third imitator, a moth (_Castnia linus var.
heliconoides_) the glass-like appearance of the wing is due neither to
diminution nor to absence of scales, but to their absolute
colourlessness and transparency, and to the fact that they stand
upright. In another moth mimic (Anthomyza) the arrangement of the
transparent scales is normal. Thus it is not some unknown external
influence that has brought about the transparency of the wing in these
five forms, as has sometimes been supposed. Nor is it a hypothetical
_internal_ evolutionary tendency, for all three vary in a different
manner. The cause of this agreement can only lie in selection, which
preserves and intensifies in each species the favourable variations
that present themselves. The great faithfulness of the copy is
astonishing in these cases, for it is not _the whole_ wing which is
transparent; certain markings are black in colour, and these contrast
sharply with the glass-like ground. It is obvious that the pursuers of
these butterflies must be very sharp-sighted, for otherwise the
agreement between the species could never have been pushed so far. The
less the enemies see and observe, the more defective must the
imitation be, and if they had been blind, no visible resemblance
between the species which required protection could ever have arisen.
A seemingly irreconcilable contradiction to the mimicry theory is
presented in the following cases, which were known to Bates, who,
however, never succeeded in bringing them into line with the principle
of mimicry.
In South America there are, as we have already said, many mimics of
the immune Ithomiinae (or as Bates called them Heliconidae). Among
these there occur not merely species which are edible, and thus
require the protection of a disguise, but others which are rejected on
account of their unpalatableness. How could the Ithomiine dress have
developed in their case, and of what use is it, since the species
would in any case be immune? In Eastern Brazil, for instance, there
are four butterflies, which bear a most confusing resemblance to one
another in colour, marking, and form of wing, and all four are
unpalatable to birds. They belong to four different genera and three
sub-families, and we have to inquire: Whence came this resemblance and
what end does it serve? For a long time no satisfactory answer could
be found, but Fritz Mueller,[51] seventeen years after Bates, offered a
solution to the riddle, when he pointed out that young birds could not
have an instinctive knowledge of the unpalatableness of the
Ithomiines, but must learn by experience which species were edible and
which inedible. Thus each young bird must have tasted at least one
individual of each inedible species and discovered its unpalatability,
before it learnt to avoid, and thus to spare the species. But if the
four species resemble each other very closely the bird will regard
them all as of the same kind, and avoid them all. Thus there developed
a process of selection which resulted in the survival of the
Ithomiine-like individuals, and in so great an increase of resemblance
between the four species, that they are difficult to distinguish one
from another even in a collection. The advantage for the four species,
living side by side as they do e.g. in Bahia, lies in the fact that
only one individual from the _mimicry-ring_ ("inedible association")
need be tasted by a young bird, instead of at least four individuals,
as would otherwise be the case. As the number of young birds is great,
this makes a considerable difference in the ratio of elimination. The
four Brazilian species are _Lycorea halia_ (Danainae), _Heliconius
narcaea_ (_eucrate_) (Heliconinae), _Melinaea ethra_, and _Mechanitis
lysimnia_ (Ithomiinae).
These interesting mimicry-rings (trusts), which have much significance
for the theory, have been the subject of numerous and careful
investigations, and at least their essential features are now fully
established. Mueller took for granted, without making any
investigations, that young birds only learn by experience to
distinguish between different kinds of victims. But Lloyd Morgan's[52]
experiments with young birds proved that this is really the case, and
at the same time furnished an additional argument against the
_Lamarckian principle_.
In addition to the mimicry-rings first observed in South America,
others have been described from Tropical India by Moore, and by
Poulton and Dixey from Africa, and we may expect to learn many more
interesting facts in this connection. Here again the preliminary
postulates of the theory are satisfied. And how much more that would
lead to the same conclusion might be added!
As in the case of mimicry many species have come to resemble one
another through processes of selection, so we know whole classes of
phenomena in which plants and animals have become adapted to one
another, and have thus been modified to a considerable degree. I refer
particularly to the relation between flowers and insects. Darwin has
shown that the originally inconspicuous blossoms of the phanerogams
were transformed into flowers through the visits of insects, and that,
conversely, several large orders of insects have been gradually
modified by their association with flowers, especially as regards the
parts of their body actively concerned. Bees and butterflies in
particular have become what they are through their relation to
flowers. In this case again all that is apparently contradictory to
the theory can, on closer investigation, be beautifully interpreted in
corroboration of it. Selection can give rise only to what is of use to
the organism actually concerned, never to what is of use to some other
organism, and we must therefore expect to find that in flowers only
characters of use to _themselves_ have arisen, never characters which
are of use to insects only, and conversely that in the insects
characters useful to them and not merely to the plants would have
originated. For a long time it seemed as if an exception to this rule
existed in the case of the fertilisation of the yucca blossoms by a
little moth, _Pronuba yuccasella_. This little moth has a
sickle-shaped appendage to its mouth-parts which occurs hi no other
Lepidopteron, and which is used for pushing the yellow pollen into the
opening of the pistil, thus fertilising the flower. Thus it appears as
if a new structure, which is useful only to the plant, has arisen in
the insect. But the difficulty is solved as soon as we learn that the
moth lays its eggs in the fruit-buds of the Yucca, and that the
larvae, when they emerge, feed on the developing seeds. In effecting
the fertilisation of the flower the moth is at the same time making
provision for its own offspring, since it is only after fertilisation
that the seeds begin to develop. There is thus nothing to prevent our
referring this structural adaptation in _Pronuba yuccasella_ to
processes of selection, which have gradually transformed the maxillary
palps of the female into the sickle-shaped instrument for collecting
the pollen, and which have at the same time developed in the insect
the instinct to press the pollen into the pistil.
In this domain, then, the theory of selection finds nothing but
corroboration, and it would be impossible to substitute for it any
other explanation, which now that the facts are so well known, could
be regarded as a serious rival to it. That selection is a factor, and
a very powerful factor in the evolution of organisms, can no longer be
doubted. Even although we cannot bring forward formal proofs of it _in
detail_, cannot calculate definitely the size of the variations which
present themselves, and their selection-value, cannot, in short,
reduce the whole process to a mathematical formula, yet we must assume
selection, because it is the only possible explanation applicable to
whole classes of phenomena, and because, on the other hand, it is made
up of factors which we know can be proved actually to exist, and
which, _if_ they exist, must of logical necessity cooeperate in the
manner required by the theory. _We must accept it because the
phenomena of evolution and adaptation must have a natural basis, and
because it is the only possible explanation of them._[53]
Many people are willing to admit that selection explains adaptations,
but they maintain that only a part of the phenomena are thus
explained, because everything does not depend upon adaptation. They
regard adaptation as, so to speak, a special effort on the part of
Nature, which she keeps in readiness to meet particularly difficult
claims of the external world on organisms. But if we look at the
matter more carefully we shall find that adaptations are by no means
exceptional, but that they are present everywhere in such enormous
numbers, that it would be difficult in regard to any structure
whatever, to prove that adaptation had _not_ played a part in its
evolution.
How often has the senseless objection been urged against selection
that it can create nothing, it can only reject. It is true that it
cannot create either the living substance or the variations of it;
both must be given. But in rejecting one thing it preserves another,
intensifies it, combines it, and in this way _creates_ what is new.
_Everything_ in organisms depends on adaptation; that is to say,
everything must be admitted through the narrow door of selection,
otherwise it can take no part in the building up of the whole. But, it
is asked, what of the direct effect of external conditions,
temperature, nutrition, climate and the like? Undoubtedly these can
give rise to variations, but they too must pass through the door of
selection, and if they cannot do this they are rejected, eliminated
from the constitution of the species.
It may, perhaps, be objected that such external influences are often
of a compelling power, and that every animal must submit to them, and
that thus selection has no choice and can neither select nor reject.
There may be such cases; let us assume for instance that the effect
of the cold of the Arctic regions was to make all the mammals become
black; the result would be that they would all be eliminated by
selection, and that no mammals would be able to live there at all. But
in most cases a certain percentage of animals resists these strong
influences, and thus selection secures a foothold on which to work,
eliminating the unfavourable variation, and establishing a useful
colouring, consistent with what is required for the maintenance of the
species.
Everything depends upon adaptation! We have spoken much of adaptation
in colouring, in connection with the examples brought into prominence
by Darwin, because these are conspicuous, easily verified, and at the
same time convincing for the theory of selection. But is it only
desert and polar animals whose colouring is determined through
adaptation? Or the leaf-butterflies, and the mimetic species, or the
terrifying markings, and "warning-colours" and a thousand other kinds
of sympathetic colouring? It is, indeed, never the colouring alone
which makes up the adaptation; the structure of the animal plays a
part, often a very essential part, in the protective disguise, and
thus _many_ variations may cooperate towards _one_ common end. And it
is to be noted that it is by no means only external parts that are
changed; internal parts are _always_ modified at the same time--for
instance, the delicate elements of the nervous system on which depend
the _instinct_ of the insect to hold its wings, when at rest, in a
perfectly definite position, which, in the leaf-butterfly, has the
effect of bringing the two pieces on which the marking occurs on the
anterior and posterior wing into the same direction, and thus
displaying as a whole the fine curve of the midrib on the seeming
leaf. But the wing-holding instinct is not regulated in the same way
in all leaf-butterflies; even our indigenous species of Vanessa, with
their protective ground-colouring, have quite a distinctive way of
holding their wings so that the greater part of the anterior wing is
covered by the posterior when the butterfly is at rest. But the
protective colouring appears on the posterior wing and on the tip of
the anterior, _to precisely the distance to which it is left
uncovered_. This occurs, as Standfuss has shown, in different degrees
in our two most nearly allied species, the uncovered portion being
smaller in _V. urticae_ than in _V. polychloros_. In this case, as in
most leaf-butterflies, the holding of the wing was probably the
primary character; only after that was thoroughly established did the
protective marking develop. In any case, the instinctive manner of
holding the wings is associated with the protective colouring, and
must remain as it is if the latter is to be effective. How greatly
instincts may change, that is to say, may be adapted, is shown by the
case of the Noctuid "shark" moth, _Xylina vetusta_. This form bears a
most deceptive resemblance to a piece of rotten wood, and the
appearance is greatly increased by the modification of the innate
impulse to flight common to so many animals, which has here been
transformed into an almost contrary instinct. This moth does not fly
away from danger, but "feigns death," that is, it draws antennae, legs
and wings close to the body, and remains perfectly motionless. It may
be touched, picked up, and thrown down again, and still it does not
move. This remarkable instinct must surely have developed
simultaneously with the wood-colouring; at all events, both
cooeperating variations are now present, and prove that both the
external and the most minute internal structure have undergone a
process of adaptation.
The case is the same with all structural variations of animal parts,
which are not absolutely insignificant. When the insects acquired
wings they must also have acquired the mechanism with which to move
them--the musculature, and the nervous apparatus necessary for its
automatic regulation. All instincts depend upon compound reflex
mechanisms and are just as indispensable as the parts they have to set
in motion, and all may have arisen through processes of selection if
the reasons which I have elsewhere given for this view are
correct.[54]
Thus there is no lack of adaptations within the organism, and
particularly in its most important and complicated parts, so that we
may say that there is no actively functional organ that has not
undergone a process of adaptation relative to its function and the
requirements of the organism. Not only is every gland structurally
adapted, down to the very minutest histological details, to its
function, but the function is equally minutely adapted to the needs of
the body. Every cell in the mucous lining of the intestine is exactly
regulated in its relation to the different nutritive substances, and
behaves in quite a different way towards the fats, and towards
nitrogenous substances, or peptones.
I have elsewhere called attention to the many adaptations of the whale
to the surrounding medium, and have pointed out--what has long been
known, but is not universally admitted, even now--that in it a great
number of important organs have been transformed in adaptation to the
peculiar conditions of aquatic life, although the ancestors of the
whale must have lived, like other hair-covered mammals, on land. I
cited a number of these transformations--the fish-like form of the
body, the hairlessness of the skin, the transformation of the
fore-limbs to fins, the disappearance of the hind-limbs and the
development of a tail fin, the layer of blubber under the skin, which
affords the protection from cold necessary to a warm-blooded animal,
the disappearance of the ear-muscles and the auditory passages, the
displacement of the external nares to the forehead for the greater
security of the breathing-hole during the brief appearance at the
surface, and certain remarkable changes in the respiratory and
circulatory organs which enable the animal to remain for a long time
under water. I might have added many more, for the list of adaptations
in the whale to aquatic life is by no means exhausted; they are found
in the histological structure and in the minutest combinations in the
nervous system. For it is obvious that a tail-fin must be used in
quite a different way from a tail, which serves as a fly-brush in
hoofed animals, or as an aid to springing in the kangaroo or as a
climbing organ; it will require quite different reflex-mechanisms and
nerve combinations in the motor centres.
I used this example in order to show how unnecessary it is to assume a
special internal evolutionary power for the phylogenesis of species,
for this whole order of whales is, so to speak, _made up of
adaptations_; it deviates in many essential respects from the usual
mammalian type, and all the deviations are adaptations to aquatic
life. But if precisely the most essential features of the organisation
thus depend upon adaptation, what is left for a phyletic force to do,
since it is these essential features of the structure it would have to
determine? There are few people now who believe in a phyletic
evolutionary power, which is not made up of the forces known to
us--adaptation and heredity--but the conviction that _every_ part of
an organism depends upon adaptation has not yet gained a firm footing.
Nevertheless, I must continue to regard this conception as the correct
one, as I have long done.
I may be permitted one more example. The feather of a bird is a
marvellous structure, and no one will deny that as a whole it depends
upon adaptation. But what part of it _does not_ depend upon
adaptation? The hollow quill, the shaft with its hard, thin, light
cortex, and the spongy substance within it, its square section
compared with the round section of the quill, the flat barbs, their
short, hooked barbules which, in the flight-feathers, hook into one
another with just sufficient firmness to resist the pressure of the
air at each wing-beat, the lightness and firmness of the whole
apparatus, the elasticity of the vane, and so on. And yet all this
belongs to an organ which is only passively functional, and therefore
can have nothing to do with the _Lamarckian principle_. Nor can the
feather have arisen through some magical effect of temperature,
moisture, electricity, or specific nutrition, and thus selection is
again our only anchor of safety.
But--it will be objected--the substance of which the feather consists,
this peculiar kind of horny substance, did not first arise through
selection in the course of the evolution of the birds, for it formed
the covering of the scales of their reptilian ancestors. It is quite
true that a similar substance covered the scales of the Reptiles, but
why should it not have arisen among them through selection? Or in what
other way could it have arisen, since scales are also passively useful
parts? It is true that if we are only to call adaptation what has been
acquired by the species we happen to be considering, there would
remain a great deal that could not be referred to selection; but we
are postulating an evolution which has stretched back through aeons,
and in the course of which innumerable adaptations took place, which
had not merely ephemeral persistence in a genus, a family or a class,
but which was continued into whole Phyla of animals, with continual
fresh adaptations to the special conditions of each species, family,
or class, yet with persistence of the fundamental elements. Thus the
feather, once acquired, persisted in all birds, and the vertebral
column, once gained by adaptation in the lowest forms, has persisted
in all the Vertebrates from Amphioxus upwards, although with constant
readaptation to the conditions of each particular group. Thus
everything we can see in animals is adaptation, whether of to-day, or
of yesterday, or of ages long gone by; every kind of cell, whether
glandular, muscular, nervous, epidermic, or skeletal, is adapted to
absolutely definite and specific functions, and every organ which is
composed of these different kinds of cells contains them in the proper
proportions, and in the particular arrangement which best serves the
function of the organ; it is thus adapted to its function.
All parts of the organism are tuned to one another, that is, _they are
adapted to one another_, and in the same way _the organism as a whole
is adapted to the conditions of its life, and it is so at every stage
of its evolution._
But all adaptations _can_ be referred to selection; the only point
that remains doubtful is whether they all _must_ be referred to it.
However that may be, whether the _Lamarckian principle_ is a factor
that has cooeperated with selection in evolution, or whether it is
altogether fallacious, the fact remains, that selection is the cause
of a great part of the phyletic evolution of organisms on our earth.
Those who agree with me in rejecting the _Lamarckian principle_ will
regard selection as the only _guiding_ factor in evolution, which
creates what is new out of the transmissible variations, by ordering
and arranging these, selecting them in relation to their number and
size, as the architect does his building-stones so that a particular
style must result.[55] But the building-stones themselves, the
variations, have their basis in the influences which cause variation
in those vital units which are handed on from one generation to
another, whether, taken together they form the _whole_ organism, as in
Bacteria and other low forms of life, or only a germ-substance, as in
unicellular and multicellular organisms.
FOOTNOTES:
[Footnote 33: _Vortraege ueber Descendenztheorie_, Jena, 1904, II. 269.
Eng. Transl. London, 1904, II. p. 317.]
[Footnote 34: See Poulton, _Essays on Evolution_, Oxford, 1908. pp.
xix-xxii.]
[Footnote 35: _Origin of Species_ (6th edit), pp. 176 _et seq._]
[Footnote 36: Chun, _Reise der Valdivia_, Leipzig, 1904.]
[Footnote 37: Plate, _Selektionsprinzip u. Probleme der Artbildung_
(3rd edit.), Leipzig, 1908.]
[Footnote 38: _Studien zur Descendenz-Theorie_ II., "Die Enstehung der
Zeichnung bei den Schmetterlings-raupen," Leipzig, 1876.]
[Footnote 39: _Origin of Species_ (6th edit.), p. 232.]
[Footnote 40: _Origin of Species_, p. 233; see also edit. 1, p. 242.]
[Footnote 41: _Ibid._ p. 230.]
[Footnote 42: _The Effect of External Influences upon Development_,
Romanes Lecture, Oxford, 1894.]
[Footnote 43: See Poulton, _Essays on Evolution_, 1908, pp. 316, 317.]
[Footnote 44: _The Evolution Theory_, London, 1904, I. p. 219.]
[Footnote 45: _Report of the British Association_ (Bristol, 1898),
London, 1899, pp. 906-909.]
[Footnote 46: _Proc. Ent. Soc._, London, May 6, 1903.]
[Footnote 47: _Essays on Evolution_, 1889-1907, Oxford, 1908,
_passim_, e.g. p. 269.]
[Footnote 48: The expression does not refer to all the enemies of this
butterfly; against ichneumon-flies, for instance, their unpleasant
smell usually gives no protection.]
[Footnote 49: Professor Poulton has corrected some wrong descriptions
which I had unfortunately overlooked in the Plates of my book
_Vortraege ueber Descendenztheorie_, and which refer to _Papilio
dardanus_ (_merope_). These mistakes are of no importance as far as an
understanding of the mimicry-theory is concerned, but I hope shortly
to be able to correct them in a later edition.]
[Footnote 50: _Journ. Linn. Soc. London_ (_Zool._), Vol. xxvi. 1898,
pp. 598-602.]
[Footnote 51: In _Kosmos_, 1879, p. 100.]
[Footnote 52: _Habit and Instinct_, London. 1896.]
[Footnote 53: This has been discussed in many of my earlier works. See
for instance _The All-Sufficiency of Natural Selection, a reply to
Herbert Spencer_, London, 1893.]
[Footnote 54: _The Evolution Theory_, London, 1904, p. 144.]
[Footnote 55: _Variation under Domestication_, 1875, II. pp. 426,
427.]
III
HEREDITY AND VARIATION IN MODERN LIGHTS
BY W. BATESON, M.A., F.R.S.
_Professor of Biology in the University of Cambridge_
Darwin's work has the property of greatness in that it may be admired
from more aspects than one. For some the perception of the principle
of Natural Selection stands out as his most wonderful achievement to
which all the rest is subordinate. Others, among whom I would range
myself, look up to him rather as the first who plainly distinguished,
collected, and comprehensively studied that new class of evidence from
which hereafter a true understanding of the process of Evolution may
be developed. We each prefer our own standpoint of admiration; but I
think that it will be in their wider aspect that his labours will most
command the veneration of posterity.
A treatise written to advance knowledge may be read in two moods. The
reader may keep his mind passive, willing merely to receive the
impress of the writer's thought; or he may read with his attention
strained and alert, asking at every instant how the new knowledge can
be used in a further advance, watching continually for fresh footholds
by which to climb higher still. Of Shelley it has been said that he
was a poet for poets: so Darwin was a naturalist for naturalists. It
is when his writings are used in the critical and more exacting spirit
with which we test the outfit for our own enterprise that we learn
their full value and strength. Whether we glance back and compare his
performance with the efforts of his predecessors, or look forward
along the course which modern research is disclosing, we shall honour
most in him not the rounded merit of finite accomplishment, but the
creative power by which he inaugurated a line of discovery endless in
variety and extension. Let us attempt thus to see his work in true
perspective between the past from which it grew, and the present which
is its consequence. Darwin attacked the problem of Evolution by
reference to facts of three classes: Variation; Heredity; Natural
Selection. His work was not as the laity suppose, a sudden and
unheralded revelation, but the first fruit of a long and hitherto
barren controversy. The occurrence of variation from type, and the
hereditary transmission of such variation had of course been long
familiar to practical men, and inferences as to the possible bearing
of those phenomena on the nature of specific difference had been from
time to time drawn by naturalists. Maupertuis, for example, wrote: "Ce
qui nous reste a examiner, c'est comment d'un seul individu, il a pu
naitre tant d'especes si differentes." And again: "La Nature contient
le fonds de toutes ces varietes: mais le hasard ou l'art les mettent
en oeuvre. C'est ainsi que ceux dont l'industrie s'applique a
satisfaire le gout des curieux, sont, pour ainsi dire, createurs
d'especes nouvelles."[56]
Such passages, of which many (though few so emphatic) can be found in
eighteenth century writers, indicate a true perception of the mode of
Evolution. The speculations hinted at by Buffon,[57] developed by
Erasmus Darwin, and independently proclaimed above all by Lamarck,
gave to the doctrine of descent a wide renown. The uniformitarian
teaching which Lyell deduced from geological observation had gained
acceptance. The facts of geographical distribution[58] had been shown
to be obviously inconsistent with the Mosaic legend. Prichard, and
Lawrence, following the example of Blumenbach, had successfully
demonstrated that the races of Man could be regarded as different
forms of one species, contrary to the opinion up till then received.
These treatises all begin, it is true, with a profound obeisance to
the sons of Noah, but that performed, they continue on strictly modern
lines. The question of the mutability of species was thus prominently
raised.
Those who rate Lamarck no higher than did Huxley in his contemptuous
phrase "_buccinator tantum_," will scarcely deny that the sound of the
trumpet had carried far, or that its note was clear. If then there
were few who had already turned to evolution with positive conviction,
all scientific men must at least have known that such views had been
promulgated; and many must, as Huxley says, have taken up his own
position of "critical expectancy."[59]
Why, then, was it, that Darwin succeeded where the rest had failed?
The cause of that success was twofold. First, and obviously, in the
principle of Natural Selection he had a suggestion which would work.
It might not go the whole way, but it was true as far as it went.
Evolution could thus in great measure be fairly represented as a
consequence of demonstrable processes. Darwin seldom endangers the
mechanism he devised by putting on it strains much greater than it can
bear. He at least was under no illusion as to the omnipotence of
Selection; and he introduces none of the forced pleading which in
recent years has threatened to discredit that principle.
For example, in the latest text of the _Origin_[60] we find him
saying:
"But as my conclusions have lately been much misrepresented,
and 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.'"
But apart from the invention of this reasonable hypothesis, which may
well, as Huxley estimated, "be the guide of biological and
psychological speculation for the next three or four generations,"
Darwin made a more significant and imperishable contribution. Not for
a few generations, but through all ages he should be remembered as the
first who showed clearly that the problems of Heredity and Variation
are soluble by observation, and laid down the course by which we must
proceed to their solution.[61] The moment of inspiration did not come
with the reading of Malthus, but with the opening of the "first
note-book on Transmutation of Species."[62] Evolution is a process of
Variation and Heredity. The older writers, though they had some vague
idea that it must be so, did not study Variation and Heredity. Darwin
did, and so begat not a theory, but a science.
The extent to which this is true, the scientific world is only
beginning to realise. So little was the fact appreciated in Darwin's
own time that the success of his writings was followed by an almost
total cessation of work in that special field. Of the causes which led
to these remarkable consequences I have spoken elsewhere. They
proceeded from circumstances peculiar to the time; but whatever the
causes there is no doubt that this statement of the result is
historically exact, and those who make it their business to collect
facts elucidating the physiology of Heredity and Variation are well
aware that they will find little to reward their quest in the leading
scientific Journals of the Darwinian epoch.
In those thirty years the original stock of evidence current and in
circulation even underwent a process of attrition. As in the story of
the Eastern sage who first wrote the collected learning of the
universe for his sons in a thousand volumes and by successive
compression and burning reduced them to one and from this by further
burning distilled the single ejaculation of the Faith "There is no god
but God and Mohammed is the Prophet of God," which was all his maturer
wisdom deemed essential:--so in the books of that period do we find
the _corpus_ of genetic knowledge dwindle to a few prerogative
instances and these at last to the brief formula of an unquestioned
creed.
And yet in all else that concerns biological science this period was,
in very truth, our Golden Age, when the natural history of the earth
was explored as never before; morphology and embryology were
exhaustively ransacked; the physiology of plants and animals began to
rival chemistry and physics in precision of method and in the rapidity
of its advances; and the foundations of pathology were laid.
In contrast with this immense activity elsewhere the neglect which
befel the special physiology of Descent, or Genetics as we now call
it, is astonishing. This may of course be interpreted as meaning that
the favoured studies seemed to promise a quicker return for effort,
but it would be more true to say that those who chose these other
pursuits did so without making any such comparison; for the idea that
the physiology of Heredity and Variation was a coherent science,
offering possibilities of extraordinary discovery, was not present to
their minds at all. In a word, the existence of such a science was
well nigh forgotten. It is true that in ancillary periodicals, as for
example those that treat of entomology or horticulture, or in the
writings of the already isolated systematists,[63] observations with
this special bearing were from time to time related, but the class of
fact on which Darwin built his conceptions of Heredity and Variation
was not seen in the highways of biology. It formed no part of the
official curriculum of biological students, and found no place among
the subjects which their teachers were investigating.
During this period nevertheless one distinct advance was made, that
with which Weismann's name is prominently connected. In Darwin's
genetic scheme the hereditary transmission of parental experience and
its consequences played a considerable role. Exactly how great that
role was supposed to be, he with his habitual caution refrained from
specifying, for the sufficient reason that he did not know.
Nevertheless much of the process of Evolution, especially that by
which organs have become degenerate and rudimentary, was certainly
attributed by Darwin to such inheritance, though since belief in the
inheritance of acquired characters fell into dispute, the fact has
been a good deal overlooked. The _Origin_ without "use and disuse"
would be a materially different book. A certain vacillation is
discernible in Darwin's utterances on this question, and the fact gave
to the astute Butler an opportunity for his most telling attack. The
discussion which best illustrates the genetic views of the period
arose in regard to the production of the rudimentary condition of the
wings of many beetles in the Madeira group of islands, and by
comparing passages from the _Origin_[64] Butler convicts Darwin of
saying first that this condition was in the main the result of
Selection, with disuse aiding, and in another place that the main
cause of degeneration was disuse, but that Selection had aided. To
Darwin however I think the point would have seemed one of dialetics
merely. To him the one paramount purpose was to show that somehow an
Evolution by means of Variation and Heredity might have brought about
the facts observed, and whether they had come to pass in the one way
or the other was a matter of subordinate concern.
To us moderns the question at issue has a diminished significance. For
over all such debates a change has been brought by Weismann's
challenge for evidence that use and disuse have any transmitted
effects at all. Hitherto the transmission of many acquired
characteristics had seemed to most naturalists so obvious as not to
call for demonstration.[65] Weismann's demand for facts in support of
the main proposition revealed at once that none having real cogency
could be produced. The time-honoured examples were easily shown to be
capable of different explanations. A few certainly remain which cannot
be so summarily dismissed, but--though it is manifestly impossible
here to do justice to such a subject--I think no one will dispute that
these residual and doubtful phenomena, whatever be their true nature,
are not of a kind to help us much in the interpretation of any of
those complex cases of adaptation which on the hypothesis of unguided
Natural Selection are especially difficult to understand. Use and
disuse were invoked expressly to help us over these hard places; but
whatever changes can be induced in offspring by direct treatment of
the parents, they are not of a kind to encourage hope of real
assistance from that quarter. It is not to be denied that through the
collapse of this second line of argument the Selection hypothesis has
had to take an increased and perilous burden. Various ways of meeting
the difficulty have been proposed, but these mostly resolve themselves
into improbable attempts to expand or magnify the powers of Natural
Selection.
Weismann's interpellation, though negative in purpose, has had a
lasting and beneficial effect, for through his thorough demolition of
the old loose and distracting notions of inherited experience, the
ground has been cleared for the construction of a true knowledge of
heredity based on experimental fact.
In another way he made a contribution of a more positive character,
for his elaborate speculations as to the genetic meaning of
cytological appearances have led to a minute investigation of the
visible phenomena occurring in those cell-divisions by which
germ-cells arise. Though the particular views he advocated have very
largely proved incompatible with the observed facts of heredity, yet
we must acknowledge that it was chiefly through the stimulus of
Weismann's ideas that those advances in cytology were made; and though
the doctrine of the continuity of germ-plasm cannot be maintained in
the form originally propounded, it is in the main true and
illuminating.[66] Nevertheless in the present state of knowledge we
are still as a rule quite unable to connect cytological appearances
with any genetic consequences and save in one respect (obviously of
extreme importance--to be spoken of later) the two sets of phenomena
might, for all we can see, be entirely distinct.
I cannot avoid attaching importance to this want of connection between
the nuclear phenomena and the features of bodily organisation. All
attempts to investigate Heredity by cytological means lie under the
disadvantage that it is the nuclear changes which can alone be
effectively observed. Important as they must surely be, I have never
been persuaded that the rest of the cell counts for nothing. What we
know of the behaviour and variability of chromosomes seems in my
opinion quite incompatible with the belief that they alone govern
form, and are the sole agents responsible in heredity.[67]
If, then, progress was to be made in Genetics, work of a different
kind was required. To learn the laws of Heredity and Variation there
is no other way than that which Darwin himself followed, the direct
examination of the phenomena. A beginning could be made by collecting
fortuitous observations of this class, which have often thrown a
suggestive light, but such evidence can be at best but superficial and
some more penetrating instrument of research is required. This can
only be provided by actual experiments in breeding.
The truth of these general considerations was becoming gradually clear
to many of us when in 1900 Mendel's work was rediscovered.
Segregation, a phenomenon of the utmost novelty, was thus revealed.
From that moment not only in the problem of the origin of species, but
in all the great problems of biology a new era began. So unexpected
was the discovery that many naturalists were convinced it was untrue,
and at once proclaimed Mendel's conclusions as either altogether
mistaken, or if true, of very limited application. Many fantastic
notions about the workings of Heredity had been asserted as general
principles before: this was probably only another fancy of the same
class.
Nevertheless those who had a preliminary acquaintance with the facts
of Variation were not wholly unprepared for some such revelation. The
essential deduction from the discovery of segregation was that the
characters of living things are dependent on the presence of definite
elements or factors, which are treated as units in the processes of
Heredity. These factors can thus be recombined in various ways. They
act sometimes separately, and sometimes they interact in conduction
with each other, producing their various effects. All this indicates a
definiteness and specific order in heredity, and therefore in
variation. This order cannot by the nature of the case be dependent on
Natural Selection for its existence, but must be a consequence of the
fundamental chemical and physical nature of living things. The study
of Variation had from the first shown that an orderliness of this kind
was present. The bodies and the properties of livings things are
cosmic, not chaotic. No matter how low in the scale we go, never do we
find the slightest hint of a diminution in that all-pervading
orderliness, nor can we conceive an organism existing for a moment in
any other state. Moreover not only does this order prevail in normal
forms, but again and again it is to be seen in newly-sprung varieties,
which by general consent cannot have been subjected to a prolonged
Selection. The discovery of Mendelian elements admirably coincided
with and at once gave a rationale of these facts. Genetic Variation is
then primarily the consequence of additions to, or omissions from, the
stock of elements which the species contains. The further
investigation of the species-problem must thus proceed by the
analytical method which breeding experiments provide.
In the nine years which have elapsed since Mendel's clue became
generally known, progress has been rapid. We now understand the
process by which a polymorphic race maintains its polymorphism. When a
family consists of dissimilar members, given the numerical proportions
in which these members are occurring, we can represent their
composition symbolically and state what types can be transmitted by
the various members. The difficulty of the "swamping effects of
inter-crossing" is practically at an end. Even the famous puzzle of
sex-limited inheritance is solved, at all events in its more regular
manifestations, and we know now how it is brought about that the
normal sisters of a colour-blind man can transmit the colour-blindness
while his normal brothers cannot transmit it.
We are still only on the fringe of the inquiry. It can be seen
extending and ramifying in many directions. To enumerate these here
would be impossible. A whole new range of possibilities is being
brought into view by study of the inter-relations between the simple
factors. By following up the evidence as to segregation, indications
have been obtained which can only be interpreted as meaning that when
many factors are being simultaneously redistributed among the
germ-cells, certain of them exert what must be described as a
repulsion upon other factors. We cannot surmise whither this discovery
may lead.
In the new light all the old problems wear a fresh aspect. Upon the
question of the nature of Sex, for example, the bearing of Mendelian
evidence is close. Elsewhere I have shown that from several sets of
parallel experiments the conclusion is almost forced upon us that, in
the types investigated, of the two sexes the female is to be regarded
as heterozygous in sex, containing one unpaired dominant element,
while the male is similarly homozygous in the absence of that
element.[68] It is not a little remarkable that on this point--which
is the only one where observations of the nuclear processes of
gameto-genesis have yet been brought into relation with the visible
characteristics of the organisms themselves--there should be
diametrical opposition between the results of breeding experiments and
those derived from cytology.
Those who have followed the researches of the American school will be
aware that, after it had been found in certain insects that the
spermatozoa were of two kinds according as they contained or did not
contain the accessory chromosome, E. B. Wilson succeeded in proving
that the sperms possessing this accessory body were destined to form
_females_ on fertilisation, while sperms without it form males, the
eggs being apparently indifferent. Perhaps the most striking of all
this series of observations is that lately made by T. H. Morgan,[69]
since confirmed by von Baehr, that in a Phylloxeran two kinds of
spermatids are formed, respectively with and without an accessory (in
this case, _double_) chromosome. Of these, only those possessing the
accessory body become functional spermatozoa, the others degenerating.
We have thus an elucidation of the puzzling fact that in these forms
fertilisation results in the formation of _females_ only. How the
males are formed--for of course males are eventually produced by the
parthenogenetic females--we do not know.
If the accessory body is really to be regarded as bearing the factor
for femaleness, then in Mendelian terms female is DD and male is DR.
The eggs are indifferent and the spermatozoa are each male, _or_
female. But according to the evidence derived from a study of the
sex-limited descent of certain features in other animals the
conclusion seems equally clear that in them female must be regarded as
DR and male as RR. The eggs are thus each either male or female and
the spermatozoa are indifferent. How this contradictory evidence is to
be reconciled we do not yet know. The breeding work concerns fowls,
canaries, and the Cu
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