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Title: Evolution in Modern Thought

Author: Ernst Haeckel
        J. Arthur Thomson
        August Weismann

Release Date: August 29, 2007 [EBook #22430]

Language: English

Character set encoding: ISO-8859-1

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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. Höffding, 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. Bouglé, 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 Ætiology 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 Ægean 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 Linnæus
(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'enchaînment des
êtres."

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: prôton ôon], or first great egg,
produced by night, that is, whose origin is involved in obscurity, and
animated by [Greek: Erôs], 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--"pensées de la jeunesse, executées
par l'âge mûr"--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 à 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 général toujours
constant, toujours le même; son mouvement, toujours régulier, roule
sur deux points inébranlables: l'un, la fécondité sans bornes donnée à
toutes les espèces; l'autre, les obstacles sans nombre qui réduisent
cette fécondité à une mesure déterminée et ne laissent en tout temps
qu'à peu près la même quantité d'individus de chaque espèce" ... "Les
espèces les moins parfaites, les plus délicates, les plus pesantes,
les moins agissantes, les moins armées, etc., ont déjà disparu ou
disparaîtront.".

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. Étienne 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 Étienne 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 (à 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 Aristotélique_, Paris, 1885; E. Zeller, _A History of Greek
Philosophy_, London, 1881, and "Ueber die griechischen Vorgänger
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 "Beständigkeit oder Veränderlichkeit 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 Précurseurs
Français_, Paris, 1870; Packard, _op. cit._; also Claus, _Lamarck als
Begründer 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 coöperation 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. Kölliker, 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 (Häcker).
The same thing has been shown by Schütt 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 coöperation 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 rôle 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 coöperated 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 coöperation 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 coöperation 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 coöperation 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 Müller 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 Lepid