determinant _N_ by chance, that is, for reasons unknown to us, may
remain strong for a considerable time, or may decrease again; but even
in the latter case it is conceivable that the ascending movement of
the determinant may continue, because the strengthened determinant now
_actively_ nourishes itself more abundantly,--that is to say, it
attracts the nutriment to itself, and to a certain extent withdraws it
from its fellow-determinants. In this way, it may--as it seems to
me--get into _permanent upward movement, and attain a degree of
strength from which there is no falling back_. Then positive or
negative selection sets in, favouring the variations which are
advantageous, setting aside those which are disadvantageous.
In a similar manner a _downward_ variation of the determinants may
take place, if its progress be started by a diminished flow of
nutriment. The determinants which are weakened by this diminished flow
will have less affinity for attracting nutriment because of their
diminished strength, and they will assimilate more feebly and grow
more slowly, unless chance streams of nutriment help them to recover
themselves. But, as will presently be shown, a change of direction
cannot take place at _every_ stage of the degenerative process. If a
certain critical stage of downward progress be passed, even favourable
conditions of food-supply will no longer suffice permanently to change
the direction of the variation. Only two cases are conceivable; if the
determinant corresponds to a _useful_ organ, only its removal can
bring back the germ-plasm to its former level; therefore personal
selection removes the id in question, with its determinants, from the
germ-plasm, by causing the elimination of the individual in the
struggle for existence. But there is another conceivable case; the
determinants concerned may be those of an organ which has become
_useless_, and they will then continue unobstructed, but with
exceeding slowness, along the downward path, until the organ becomes
vestigial, and finally disappears altogether.
The fluctuations of the determinants hither and thither may thus be
transformed into a lasting ascending or descending movement; and _this
is the crucial point of these germinal processes_.
This is not a fantastic assumption; we can read it in the fact of the
degeneration of disused parts. _Useless organs are the only ones which
are not helped to ascend again by personal selection, and therefore in
their case alone can we form any idea of how the primary constituents
behave, when they are subject solely to intra-germinal forces_.
The whole determinant system of an id, as I conceive it, is in a state
of continual fluctuation upwards and downwards. In most cases the
fluctuations will counteract one another, because the passive streams
of nutriment soon change, but in many cases the limit from which a
return is possible will be passed, and then the determinants concerned
will continue to vary in the same direction, till they attain positive
or negative selection-value. At this stage personal selection
intervenes and sets aside the variation if it is disadvantageous, or
favours--that is to say, preserves--it if it is advantageous. Only
_the determinant of a useless organ is uninfluenced by personal
selection_, and, as experience shows, it sinks downwards; that is, the
organ that corresponds to it degenerates very slowly but
uninterruptedly till, after what must obviously be an immense stretch
of time, it disappears from the germ-plasm altogether.
Thus we find in the fact of the degeneration of disused parts the
proof that not all the fluctuations of a determinant return to
equilibrium again, but that, when the movement has attained to a
certain strength, it continues _in the same direction_. We have entire
certainty in regard to this as far as the downward progress is
concerned, and we must assume it also in regard to ascending
variations, as the phenomena of artificial selection certainly justify
us in doing. If the Japanese breeders were able to lengthen the
tail-feathers of the cock to six feet, it can only have been because
the determinants of the tail-feathers in the germ-plasm had already
struck out a path of ascending variation, and this movement was taken
advantage of by the breeder, who continually selected for reproduction
the individuals in which the ascending variation was most marked. For
all breeding depends upon the unconscious selection of germinal
variations.
Of course these germinal processes cannot be proved mathematically,
since we cannot actually see the play of forces of the passive
fluctuations and their causes. We cannot say how great these
fluctuations are, and how quickly or slowly, how regularly or
irregularly they change. Nor do we know how far a determinant must be
strengthened by the passive flow of the nutritive stream if it is to
be beyond the danger of unfavourable variations, or how far it must be
weakened passively before it loses the power of recovering itself by
its own strength. It is no more possible to bring forward actual
proofs in this case than it was in regard to the selection-value of
the initial stages of an adaptation. But if we consider that all
heritable variations must have their roots in the germ-plasm, and
further, that when personal selection does not intervene, that is to
say, in the case of parts which have become useless, a degeneration of
the part, and therefore also of its determinant must inevitably take
place; then we must conclude that processes such as I have assumed are
running their course within the germ-plasm, and we can do this with as
much certainty as we were able to infer, from the phenomena of
adaptation, the selection-value of their initial stages. The fact of
the degeneration of disused parts seems to me to afford irrefutable
proof that the fluctuations within the germ-plasm _are the real root
of all hereditary variation_, and the preliminary condition for the
occurrence of the Darwin-Wallace factor of selection. Germinal
selection supplies the stones out of which personal selection builds
her temples and palaces: _adaptations_. The importance for the theory
of the process of degeneration of disused parts cannot be
over-estimated, especially when it occurs in sterile animal forms,
where we are free from the doubt as to the alleged _Lamarckian factor_
which is apt to confuse our ideas in regard to other cases.
If we regard the variation of the many determinants concerned in the
transformation of the female into the sterile worker as having come
about through the gradual transformation of the ids into worker-ids,
we shall see that the germ-plasm of the sexual ants must contain three
kinds of ids, male, female, and worker ids, or if the workers have
diverged into soldiers and nest-builders, then four kinds. We
understand that the worker-ids arose because their determinants struck
out a useful path of variation, whether upward or downward, and that
they continued in this path until the highest attainable degree of
utility of the parts determined was reached. But in addition to the
organs of positive or negative selection-value, there were some which
were indifferent as far as the success and especially the functional
capacity of the workers was concerned: wings, ovarian tubes,
_receptaculum seminis_, a number of the facets of the eye, perhaps
even the whole eye. As to the ovarian tubes it is is possible that
their degeneration was an advantage for the workers, in saving energy,
and if so selection would favour the degeneration; but how could the
presence of eyes diminish the usefulness of the workers to the colony?
or the minute _receptaculum seminis_, or even the wings? These parts
have therefore degenerated _because they were of no further value to
the insect_. But if selection did not influence the setting aside of
these parts because they were neither of advantage nor of disadvantage
to the species, then the Darwinian factor of selection is here
confronted with a puzzle which it cannot solve alone, but which at
once becomes clear when germinal selection is added. For the
determinants of organs that have no further value for the organism,
must, as we have already explained, embark on a gradual course of
retrograde development.
In ants the degeneration has gone so far that there are no
wing-rudiments present in _any_ species, as is the case with so many
butterflies, flies, and locusts, but in the larvae the imaginable
discs of the wings are still laid down. With regard to the ovaries,
degeneration has reached different levels in different species of
ants, as has been shown by the researches of my former pupil,
Elizabeth Bickford. In many species there are twelve ovarian tubes,
and they decrease from that number to one; indeed, in one species no
ovarian tube at all is present. So much at least is certain from what
has been said, that in this case _everything_ depends on the
fluctuations of the elements of the germ-plasm. Germinal selection,
here as elsewhere, presents the variations of the determinants, and
personal selection favours or rejects these, or,--if it be a question
of organs which have become useless,--it does not come into play at
all, and allows the descending variation free course.
It is obvious that even the problem of _coadaptation in sterile
animals_ can thus be satisfactorily explained. If the determinants are
oscillating upwards and downwards in continual fluctuation, and
varying more pronouncedly now in one direction now in the other,
useful variations of every determinant will continually present
themselves anew, and may, in the course of generations, be combined
with one another in various ways. But there is one character of the
determinants that greatly facilitates this complex process of
selection, that, after a certain limit has been reached, they go on
varying in the same direction. From this it follows that development
along a path once struck out may proceed without the continual
intervention of personal selection. This factor only operates, so to
speak, at the beginning, when it selects the determinants which are
varying in the right direction, and again at the end, when it is
necessary to put a check upon further variation. In addition to this,
enormously long periods have been available for all these adaptations,
as the very gradual transition stages between females and workers in
many species plainly show, and thus this process of transformation
loses the marvellous and mysterious character that seemed at the first
glance to invest it, and takes rank, without any straining, among the
other processes of selection. It seems to me that, from the facts that
sterile animal forms can adapt themselves to new vital functions,
their superfluous parts degenerate, and the parts more used adapt
themselves in an ascending direction, those less used in a descending
direction, we must draw the conclusion that harmonious adaptation here
comes about _without the cooeperation of the Lamarckian principle_.
This conclusion once established, however, we have no reason to refer
the thousands of cases of harmonious adaptation, which occur in
exactly the same way among other animals or plants, to a principle,
the _active intervention of which in the transformation of species is
nowhere proved. We do not require it to explain the facts, and
therefore we must not assume it._
The fact of coadaptation, which was supposed to furnish the strongest
argument against the principle of selection, in reality yields the
clearest evidence in favour of it. We _must_ assume it, _because no
other possibility of explanation is open to us, and because these
adaptations actually exist, that is to say, have really taken place_.
With this conviction I attempted, as far back as 1894, when the idea
of germinal selection had not yet occurred to me, to make "harmonious
adaptation" (coadaptation) more easily intelligible in some way or
other, and so I was led to the idea, which was subsequently expounded
in detail by Baldwin, and Lloyd Morgan, and also by Osborn, and Gulick
as _Organic Selection_. It seemed to me that it was not necessary that
all the germinal variations required for secondary variations should
have occurred _simultaneously_, since, for instance, in the case of
the stag, the bones, muscles, sinews, and nerves would be incited by
the increasing heaviness of the antlers to greater activity in _the
individual life_, and so would be strengthened. The antlers can only
have increased in size by very slow degrees, so that the muscles and
bones may have been able to keep pace with their growth in the
individual life, until the requisite germinal variations presented
themselves. In this way a disharmony between the increasing weight of
the antlers and the parts which support and move them would be
avoided, since time would be given for the appropriate germinal
variations to occur, and so to set agoing the _hereditary_ variation
of the muscles, sinews and bones.[42]
I still regard this idea as correct, but I attribute less importance
to "organic selection" than I did at that time, in so far that I do
not believe that it _alone_ could effect complex harmonious
adaptations. Germinal selection now seems to me to play the chief part
in bringing about such adaptations. Something the same is true of the
principle I have called _Panmixia_. As I became more and more
convinced, in the course of years, that the _Lamarckian principle_
ought not to be called in to explain the dwindling of disused parts, I
believed that this process might be simply explained as due to the
cessation of the conservative effect of natural selection. I said to
myself that, from the moment in which a part ceases to be of use,
natural selection withdraws its hand from it, and then it must
inevitably fall from the height of its adaptiveness, because inferior
variants would have as good a chance of persisting as better ones,
since all grades of fitness of the part in question would be mingled
with one another indiscriminately. This is undoubtedly true, as
Romanes pointed out ten years before I did, and this mingling of the
bad with the good probably does bring about a deterioration of the
part concerned. But it cannot account for the steady diminution, which
always occurs when a part is in process of becoming rudimentary, and
which goes on until it ultimately disappears altogether. The process
of dwindling cannot therefore be explained as due to panmixia alone:
we can only find a sufficient explanation in germinal selection.
IV. DERIVATIVES OF THE THEORY OF SELECTION
The impetus in all directions given by Darwin through his theory of
selection has been an immeasurable one, and its influence is still
felt. It falls within the province of the historian of science to
enumerate all the ideas which, in the last quarter of the nineteenth
century, grew out of Darwin's theories, in the endeavour to penetrate
more deeply into the problem of the evolution of the organic world.
Within the narrow limits to which this paper is restricted, I cannot
attempt to discuss any of these.
V. ARGUMENTS FOR THE REALITY OF THE PROCESSES OF SELECTION
(_a_) _Sexual Selection_
Sexual selection goes hand in hand with natural selection. From the
very first I have regarded sexual selection as affording an extremely
important and interesting corroboration of natural selection, but,
singularly enough, it is precisely against this theory that an adverse
judgment has been pronounced in so many quarters, and it is only quite
recently, and probably in proportion as the wealth of facts in proof
of it penetrates into a wider circle, that we seem to be approaching a
more general recognition of this side of the problem of adaptations.
Thus Darwin's words in his preface to the second edition (1874) of his
book, _The Descent of Man and Sexual Selection_, are being justified:
"My conviction as to the operation of natural selection remains
unshaken," and further, "If naturalists were to become more familiar
with the idea of sexual selection, it would, I think, be accepted to a
much greater extent, and already it is fully and favourably accepted
by many competent judges." Darwin was able to speak thus because he
was already acquainted with an immense mass of facts, which, taken
together, yield overwhelming evidence of the validity of the principle
of sexual selection.
_Natural selection_ chooses out for reproduction the individuals that
are best equipped for the struggle for existence, and it does so at
every stage of development; it thus improves the species in all its
stages and forms. _Sexual selection_ operates only on individuals
that are already capable of reproduction, and does so only in relation
to the attainment of reproduction. It arises from the rivalry of one
sex, usually the male, for the possession of the other, usually the
female. Its influence can therefore only _directly_ affect one sex, in
that it equips it better for attaining possession of the other. But
the effect may extend indirectly to the female sex, and thus the whole
species may be modified, without, however, becoming any more capable
of resistance in the struggle for existence, for sexual selection only
gives rise to adaptations which are likely to give their possessor the
victory over rivals in the struggle for possession of the female, and
which are therefore peculiar to the wooing sex: the manifold
"secondary sexual characters." The diversity of these characters is so
great that I cannot here attempt to give anything approaching a
complete treatment of them, but I should like to give a sufficient
number of examples to make the principle itself, in its various modes
of expression, quite clear.
One of the chief preliminary postulates of sexual selection is the
unequal number of individuals in the two sexes, for if every male
immediately finds his mate there can be no competition for the
possession of the female. Darwin has shown that, for the most part,
the inequality between the sexes is due simply to the fact that there
are more males than females, and therefore the males must take some
pains to secure a mate. But the inequality does not always depend on
the numerical preponderance of the males, it is often due to polygamy;
for, if one male claims several females, the number of females in
proportion to the rest of the males will be reduced. Since it is
almost always the males that are the wooers, we must expect to find
the occurrence of secondary sexual characters chiefly among them, and
to find it especially frequent in polygamous species. And this is
actually the case.
If we were to try to guess--without knowing the facts--what means the
male animals make use of to overcome their rivals in the struggle for
the possession of the female, we might name many kinds of means, but
it would be difficult to suggest any which is not actually employed in
some animal group of other. I begin with the mere difference in
strength, through which the male of many animals is so sharply
distinguished from the female, as, for instance, the lion, walrus,
"sea-elephant," and others. Among these the males fight violently for
the possession of the female, who falls to the victor in the combat.
In this simple case no one can doubt the operation of selection, and
there is just as little room for doubt as to the selection-value of
the initial stages of the variation. Differences in bodily strength
are apparent even among human beings, although in their case the
struggle for the possession of the female is no longer decided by
bodily strength alone.
Combats between male animals are often violent and obstinate, and the
employment of the natural weapons of the species in this way has led
to perfecting of these, e.g. the tusks of the boar, the antlers of the
stag, and the enormous, antler-like jaws of the stag-beetle. Here
again it is impossible to doubt that variations in these organs
presented themselves, and that these were considerable enough to be
decisive in combat, and so to lead to the improvement of the weapon.
Among many animals, however, the females at first withdraw from the
males; they are coy, and have to be sought out, and sometimes held by
force. This tracking and grasping of the females by the males has
given rise to many different characters in the latter, as, for
instance, the larger eyes of the male bee, and especially of the males
of the Ephemerids (May-flies), some species of which show, in addition
to the usual compound eyes, large, so-called turban-eyes, so that the
whole head is covered with seeing surfaces. In these species the
females are very greatly in the minority (1-100), and it is easy to
understand that a keen competition for them must take place, and that,
when the insects of both sexes are floating freely in the air, an
unusually wide range of vision will carry with it a decided
advantage. Here again the actual adaptations are in accordance with
the preliminary postulates of the theory. We do not know the stages
through which the eye has passed to its present perfected state, but,
since the number of simple eyes (facets) has become very much greater
in the male than in the female, we may assume that their increase is
due to a gradual duplication of the determinants of the ommatidium in
the germ-plasm, as I have already indicated in regard to sense-organs
in general. In this case, again, the selection-value of the initial
stages hardly admits of doubt; better vision _directly_ secures
reproduction.
In many cases _the organ of smell_ shows a similar improvement. Many
lower Crustaceans (Daphnidae) have better developed organs of smell in
the male sex. The difference is often slight and amounts only to one
or two olfactory filaments, but certain species show a difference of
nearly a hundred of these filaments (Leptodora). The same thing occurs
among insects.
We must briefly consider the clasping or grasping organs which have
developed in the males among many lower Crustaceans, but here natural
selection plays its part along with sexual selection, for the union of
the sexes is an indispensable condition for the maintenance of the
species, and as Darwin himself pointed out, in many cases the two
forms of selection merge into each other. This fact has always seemed
to me to be a proof of natural selection, for, in regard to sexual
selection, it is quite obvious that the victory of the best-equipped
could have brought about the improvement only of the organs concerned,
the factors in the struggle, such as the eye and the olfactory organ.
We come now to the _excitants_; that is, to the group of sexual
characters whose origin through processes of selection has been most
frequently called in question. We may cite the _love-calls_ produced
by many male insects, such as crickets and cicadas. These could only
have arisen in animal groups in which the female did not rapidly flee
from the male, but was inclined to accept his wooing from the first.
Thus, notes like the chirping of the male cricket serve to entice the
females. At first they were merely the signal which showed the
presence of a male in the neighbourhood, and the female was gradually
enticed nearer and nearer by the continued chirping. The male that
could make himself heard to the greatest distance would obtain the
largest following, and would transmit the beginnings, and, later, the
improvement of his voice to the greatest number of descendants. But
sexual excitement in the female became associated with the hearing of
the love-call, and then the sound-producing organ of the male began to
improve, until it attained to the emission of the long-drawn-out soft
notes of the mole-cricket or the maenad-like cry of the cicadas. I
cannot here follow the process of development in detail, but will call
attention to the fact that the original purpose of the voice, the
announcing of the male's presence, became subsidiary, and the exciting
of the female became the chief goal to be aimed at. The loudest
singers awakened the strongest excitement, and the improvement
resulted as a matter of course. I conceive of the origin of bird-song
in a somewhat similar manner, first as a means of enticing, then of
exciting the female.