(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 adaptation. 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 or 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 Muller 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 became, 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. (See Poulton, "Essays
on Evolution", 1908, pages 316, 317.) 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 ("The Evolution Theory",
London, 1904, I. page 219.) pointed out that decorative colouring and
sweet-scentedness may replace one another in Lepidoptera as well as
in flowers, for just as some modestly coloured flowers (mignonette and
violet) have often a strong perfume, while strikingly coloured ones are
sometimes quite devoid of fragrance, so we find that the most beautiful
and gaily-coloured of our native Lepidoptera, the species of Vanessa,
have no scent-scales, while these are often markedly developed in grey
nocturnal Lepidoptera. Both attractions may, however, be combined in
butterflies, just as in flowers. Of course, we cannot explain why both
means of attraction should exist in one genus, and only one of them in
another, since we do not know the minutest details of the conditions
of life of the genera concerned. But from the sporadic distribution of
scent-scales in Lepidoptera, and from their occurrence or absence in
nearly related species, we may conclude that fragrance is a relatively
MODERN acquirement, more recent than brilliant colouring.
One thing in particular that stamps decorative colouring as a product of
selection is ITS GRADUAL INTENSIFICATION by the addition of new spots,
which we can quite well observe, because in many cases the colours have
been first acquired by the males, and later transmitted to the females
by inheritance. The scent-scales are never thus transmitted, probably
for the same reason that the decorative colours of many birds are often
not transmitted to the females: because with these they would be exposed
to too great elimination by enemies. Wallace was the first to point out
that in species with concealed nests the beautiful feathers of the male
occurred in the female also, as in the parrots, for instance, but
this is not the case in species which brood on an exposed nest. In the
parrots one can often observe that the general brilliant colouring of
the male is found in the female, but that certain spots of colour are
absent, and these have probably been acquired comparatively recently by
the male and have not yet been transmitted to the female.
Isolation of the group of individuals which is in process of varying
is undoubtedly of great value in sexual selection, for even a solitary
conspicuous variation will become dominant much sooner in a small
isolated colony, than among a large number of members of a species.
Anyone who agrees with me in deriving variations from germinal selection
will regard that process as an essential aid towards explaining the
selection of distinctive courtship-characters, such as coloured spots,
decorative feathers, horny outgrowths in birds and reptiles, combs,
feather-tufts, and the like, since the beginnings of these would
be presented with relative frequency in the struggle between the
determinants within the germ-plasm. The process of transmission of
decorative feathers to the female results, as Darwin pointed out and
illustrated by interesting examples, in the COLOUR-TRANSFORMATION OF A
WHOLE SPECIES, and this process, as the phyletically older colouring
of young birds shows, must, in the course of thousands of years, have
repeated itself several times in a line of descent.
If we survey the wealth of phenomena presented to us by secondary sexual
characters, we can hardly fail to be convinced of the truth of the
principle of sexual selection. And certainly no one who has accepted
natural selection should reject sexual selection, for, not only do the
two processes rest upon the same basis, but they merge into one another,
so that it is often impossible to say how much of a particular character
depends on one and how much on the other form of selection.
(b) NATURAL SELECTION.
An actual proof of the theory of sexual selection is out of the
question, if only because we cannot tell when a variation attains to
selection-value. It is certain that a delicate sense of smell is of
value to the male moth in his search for the female, but whether the
possession of one additional olfactory hair, or of ten, or of twenty
additional hairs leads to the success of its possessor we are unable
to tell. And we are groping even more in the dark when we discuss the
excitement caused in the female by agreeable perfumes, or by striking
and beautiful colours. That these do make an impression is beyond doubt;
but we can only assume that slight intensifications of them give any
advantage, and we MUST assume this SINCE OTHERWISE SECONDARY SEXUAL
CHARACTERS REMAIN INEXPLICABLE.
The same thing is true in regard to natural selection. It is not
possible to bring forward any actual proof of the selection-value of
the initial stages, and the stages in the increase of variations, as has
been already shown. But the selection-value of a finished adaptation can
in many cases be statistically determined. Cesnola and Poulton have made
valuable experiments in this direction. The former attached forty-five
individuals of the green, and sixty-five of the brown variety of the
praying mantis (Mantis religiosa), by a silk thread to plants, and
watched them for seventeen days. The insects which were on a surface of
a colour similar to their own remained uneaten, while twenty-five green
insects on brown parts of plants had all disappeared in eleven days.
The experiments of Poulton and Sanders ("Report of the British
Association" (Bristol, 1898), London, 1899, pages 906-909.) were made
with 600 pupae of Vanessa urticae, the "tortoise-shell butterfly." The
pupae were artificially attached to nettles, tree-trunks, fences, walls,
and to the ground, some at Oxford, some at St Helens in the Isle of
Wight. In the course of a month 93 per cent of the pupae at Oxford were
killed, chiefly by small birds, while at St Helens 68 per cent perished.
The experiments showed very clearly that the colour and character of the
surface on which the pupa rests--and thus its own conspicuousness--are
of the greatest importance. At Oxford only the four pupae which were
fastened to nettles emerged; all the rest--on bark, stones and the
like--perished. At St Helens the elimination was as follows: on fences
where the pupae were conspicuous, 92 per cent; on bark, 66 per cent; on
walls, 54 per cent; and among nettles, 57 per cent. These interesting
experiments confirm our views as to protective coloration, and show
further, THAT THE RATIO OF ELIMINATION IN THE SPECIES IS A VERY HIGH
ONE, AND THAT THEREFORE SELECTION MUST BE VERY KEEN.
We may say that the process of selection follows as a logical necessity
from the fulfilment of the three preliminary postulates of the theory:
variability, heredity, and the struggle for existence, with its enormous
ratio of elimination in all species. To this we must add a fourth
factor, the INTENSIFICATION of variations which Darwin established as
a fact, and which we are now able to account for theoretically on
the basis of germinal selection. It may be objected that there is
considerable uncertainty about this LOGICAL proof, because of our
inability to demonstrate the selection-value of the initial stages and
the individual stages of increase. We have therefore to fall back on
PRESUMPTIVE EVIDENCE. This is to be found in THE INTERPRETATIVE VALUE OF
THE THEORY. Let us consider this point in greater detail.
In the first place, it is necessary to emphasise what is often
overlooked, namely, that the theory not only explains the
TRANSFORMATIONS of species, it also explains THEIR REMAINING THE SAME;
in addition to the principle of varying, it contains within itself that
of PERSISTING. It is part of the essence of selection, that it not
only causes a part to VARY till it has reached its highest pitch of
adaptation, but that it MAINTAINS IT AT THIS PITCH. THIS CONSERVING
INFLUENCE OF NATURAL SELECTION is of great importance, and was early
recognised by Darwin; it follows naturally from the principle of the
survival of the fittest.
We understand from this how it is that a species which has become
fully adapted to certain conditions of life ceases to vary, but remains
"constant," as long as the conditions of life FOR IT remain unchanged,
whether this be for thousands of years, or for whole geological epochs.
But the most convincing proof of the power of the principle of selection
lies in the innumerable multitude of phenomena which cannot be explained
in any other way. To this category belong all structures which are only
PASSIVELY of advantage to the organism, because none of these can have
arisen by the alleged LAMARCKIAN PRINCIPLE. These have been so often
discussed that we need do no more than indicate them here. Until quite
recently the sympathetic coloration of animals--for instance, the
whiteness of Arctic animals--was referred, at least in part, to
the DIRECT influence of external factors, but the facts can best be
explained by referring them to the processes of selection, for then it
is unnecessary to make the gratuitous assumption that many species are
sensitive to the stimulus of cold and that others are not. The great
majority of Arctic land-animals, mammals and birds, are white, and this
proves that they were all able to present the variation which was most
useful for them. The sable is brown, but it lives in trees, where
the brown colouring protects and conceals it more effectively. The
musk-sheep (Ovibos moschatus) is also brown, and contrasts sharply
with the ice and snow, but it is protected from beasts of prey by its
gregarious habit, and therefore it is of advantage to be visible from
as great a distance as possible. That so many species have been able to
give rise to white varieties does not depend on a special sensitiveness
of the skin to the influence of cold, but to the fact that Mammals and
Birds have a general tendency to vary towards white. Even with us, many
birds--starlings, blackbirds, swallows, etc.--occasionally produce white
individuals, but the white variety does not persist, because it readily
falls a victim to the carnivores. This is true of white fawns, foxes,
deer, etc. The whiteness, therefore, arises from internal causes, and
only persists when it is useful. A great many animals living in a
GREEN ENVIRONMENT have become clothed in green, especially insects,
caterpillars, and Mantidae, both persecuted and persecutors.
That it is not the direct effect of the environment which calls forth
the green colour is shown by the many kinds of caterpillar which rest on
leaves and feed on them, but are nevertheless brown. These feed by night
and betake themselves through the day to the trunk of the tree, and hide
in the furrows of the bark. We cannot, however, conclude from this that
they were UNABLE to vary towards green, for there are Arctic animals
which are white only in winter and brown in summer (Alpine hare, and
the ptarmigan of the Alps), and there are also green leaf-insects which
remain green only while they are young and difficult to see on the leaf,
but which become brown again in the last stage of larval life, when they
have outgrown the leaf. They then conceal themselves by day, sometimes
only among withered leaves on the ground, sometimes in the earth itself.
It is interesting that in one genus, Chaerocampa, one species is brown
in the last stage of larval life, another becomes brown earlier, and in
many species the last stage is not wholly brown, a part remaining green.
Whether this is a case of a double adaptation, or whether the green is
being gradually crowded out by the brown, the fact remains that the same
species, even the same individual, can exhibit both variations. The case
is the same with many of the leaf-like Orthoptera, as, for instance, the
praying mantis (Mantis religiosa) which we have already mentioned.
But the best proofs are furnished by those often-cited cases in which
the insect bears a deceptive resemblance to another object. We now know
many such cases, such as the numerous imitations of green or withered
leaves, which are brought about in the most diverse ways, sometimes by
mere variations in the form of the insect and in its colour, sometimes
by an elaborate marking, like that which occurs in the Indian
leaf-butterflies, Kallima inachis. In the single butterfly-genus Anaea,
in the woods of South America, there are about a hundred species which
are all gaily coloured on the upper surface, and on the reverse side
exhibit the most delicate imitation of the colouring and pattern of a
leaf, generally without any indication of the leaf-ribs, but extremely
deceptive nevertheless. Anyone who has seen only one such butterfly
may doubt whether many of the insignificant details of the marking can
really be of advantage to the insect. Such details are for instance the
apparent holes and splits in the apparently dry or half-rotten leaf,
which are usually due to the fact that the scales are absent on a
circular or oval patch so that the colourless wing-membrane lies bare,
and one can look through the spot as through a window. Whether the
bird which is seeking or pursuing the butterflies takes these holes for
dewdrops, or for the work of a devouring insect, does not affect
the question; the mirror-like spot undoubtedly increases the general
deceptiveness, for the same thing occurs in many leaf-butterflies,
though not in all, and in some cases it is replaced in quite a peculiar
manner. In one species of Anaea (A. divina), the resting butterfly looks
exactly like a leaf out of the outer edge of which a large semicircular
piece has been eaten, possibly by a caterpillar; but if we look more
closely it is obvious that there is no part of the wing absent, and that
the semicircular piece is of a clear, pale yellow colour, while the rest
of the wing is of a strongly contrasted dark brown.
But the deceptive resemblance may be caused in quite a different manner.
I have often speculated as to what advantage the brilliant white C
could give to the otherwise dusky-coloured "Comma butterfly" (Grapta C.
album). Poulton's recent observations ("Proc. Ent. Soc"., London, May 6,
1903.) have shown that this represents the imitation of a crack such as
is often seen in dry leaves, and is very conspicuous because the light
shines through it.
The utility obviously lies in presenting to the bird the very familiar
picture of a broken leaf with a clear shining slit, and we may conclude,
from the imitation of such small details, that the birds are very sharp
observers and that the smallest deviation from the usual arrests their
attention and incites them to closer investigation. It is obvious that
such detailed--we might almost say such subtle--deceptive resemblances
could only have come about in the course of long ages through the
acquirement from time to time of something new which heightened the
already existing resemblance.
In face of facts like these there can be no question of chance, and no
one has succeeded so far in finding any other explanation to replace
that by selection. For the rest, the apparent leaves are by no means
perfect copies of a leaf; many of them only represent the torn or
broken piece, or the half or two-thirds of a leaf, but then the leaves
themselves frequently do not present themselves to the eye as a whole,
but partially concealed among other leaves. Even those butterflies
which, like the species of Kallima and Anaea, represent the whole of
a leaf with stalk, ribs, apex, and the whole breadth, are not actual
copies which would satisfy a botanist; there is often much wanting.
In Kallima the lateral ribs of the leaf are never all included in the
markings; there are only two or three on the left side and at most four
or five on the right, and in many individuals these are rather obscure,
while in others they are comparatively distinct. This furnishes us with
fresh evidence in favour of their origin through processes of selection,
for a botanically perfect picture could not arise in this way; there
could only be a fixing of such details as heightened the deceptive
resemblance.
Our postulate of origin through selection also enables us to understand
why the leaf-imitation is on the lower surface of the wing in the
diurnal Lepidoptera, and on the upper surface in the nocturnal forms,
corresponding to the attitude of the wings in the resting position of
the two groups.
The strongest of all proofs of the theory, however, is afforded by
cases of true "mimicry," those adaptations discovered by Bates in 1861,
consisting in the imitation of one species by another, which becomes
more and more like its model. The model is always a species that enjoys
some special protection from enemies, whether because it is unpleasant
to taste, or because it is in some way dangerous.
It is chiefly among insects and especially among butterflies that we
find the greatest number of such cases. Several of these have been
minutely studied, and every detail has been investigated, so that it is
difficult to understand how there can still be disbelief in regard
to them. If the many and exact observations which have been carefully
collected and critically discussed, for instance by Poulton ("Essays
on Evolution", 1889-1907, Oxford, 1908, passim, e.g. page 269.) were
thoroughly studied, the arguments which are still frequently urged
against mimicry would be found untenable; we can hardly hope to find
more convincing proof of the actuality of the processes of selection
than these cases put into our hands. The preliminary postulates of
the theory of mimicry have been disputed, for instance, that diurnal
butterflies are persecuted and eaten by birds, but observations
specially directed towards this point in India, Africa, America and
Europe have placed it beyond all doubt. If it were necessary I could
myself furnish an account of my own observations on this point.
In the same way it has been established by experiment and observation
in the field that in all the great regions of distribution there
are butterflies which are rejected by birds and lizards, their
chief enemies, on account of their unpleasant smell or taste. These
butterflies are usually gaily and conspicuously coloured and thus--as
Wallace first interpreted it--are furnished with an easily recognisable
sign: a sign of unpalatableness or WARNING COLOURS. If they were not
thus recognisable easily and from a distance, they would frequently be
pecked at by birds, and then rejected because of their unpleasant taste;
but as it is, the insect-eaters recognise them at once as unpalatable
booty and ignore them. Such IMMUNE (The expression does not refer to all
the enemies of this butterfly; against ichneumon-flies, for instance,
their unpleasant smell usually gives no protection.) species, wherever
they occur, are imitated by other palatable species, which thus acquire
a certain degree of protection.
It is true that this explanation of the bright, conspicuous colours
is only a hypothesis, but its foundations,--unpalatableness, and the
liability of other butterflies to be eaten,--are certain, and its
consequences--the existence of mimetic palatable forms--confirm it in
the most convincing manner. Of the many cases now known I select
one, which is especially remarkable, and which has been thoroughly
investigated, Papilio dardanus (merope), a large, beautiful, diurnal
butterfly which ranges from Abyssinia throughout the whole of Africa to
the south coast of Cape Colony.
The males of this form are everywhere ALMOST the same in colour and in
form of wings, save for a few variations in the sparse black markings on
the pale yellow ground. But the females occur in several quite different
forms and colourings, and one of these only, the Abyssinian form, is
like the male, while the other three or four are MIMETIC, that is to
say, they copy a butterfly of quite a different family the Danaids,
which are among the IMMUNE forms. In each region the females have thus
copied two or three different immune species. There is much that is
interesting to be said in regard to these species, but it would be out
of keeping with the general tenor of this paper to give details of this
very complicated case of polymorphism in P. dardanus. Anyone who is
interested in the matter will find a full and exact statement of the
case in as far as we know it, in Poulton's "Essays on Evolution" (pages
373-375). (Professor Poulton has corrected some wrong descriptions which
I had unfortunately overlooked in the Plates of my book "Vortrage uber
Descendenztheorie", and which refer to Papilio dardanus (merope).
These mistakes are of no importance as far as and understanding of the
mimicry-theory is concerned, but I hope shortly to be able to correct
them in a later edition.) I need only add that three different mimetic
female forms have been reared from the eggs of a single female in South
Africa. The resemblance of these forms to their immune models goes so
far that even the details of the LOCAL forms of the models are copied by
the mimetic species.
It remains to be said that in Madagascar a butterfly, Papilio meriones,
occurs, of which both sexes are very similar in form and markings to
the non-mimetic male of P. dardanus, so that it probably represents the
ancestor of this latter species.
In face of such facts as these every attempt at another explanation must
fail. Similarly all the other details of the case fulfil the preliminary
postulates of selection, and leave no room for any other interpretation.
That the males do not take on the protective colouring is easily
explained, because they are in general more numerous, and the females
are more important for the preservation of the species, and must also
live longer in order to deposit their eggs. We find the same state of
things in many other species, and in one case (Elymnias undularis)
in which the male is also mimetically coloured, it copies quite a
differently coloured immune species from the model followed by the
female. This is quite intelligible when we consider that if there were
TOO MANY false immune types, the birds would soon discover that there
were palatable individuals among those with unpalatable warning colours.
Hence the imitation of different immune species by Papilio dardanus!
I regret that lack of space prevents my bringing forward more examples
of mimicry and discussing them fully. But from the case of Papilio
dardanus alone there is much to be learnt which is of the highest
importance for our understanding of transformations. It shows us chiefly
what I once called, somewhat strongly perhaps, THE OMNIPOTENCE OF
NATURAL SELECTION in answer to an opponent who had spoken of its
"inadequacy." We here see that one and the same species is capable of
producing four or five different patterns of colouring and marking;
thus the colouring and marking are not, as has often been supposed,
a necessary outcome of the specific nature of the species, but a
true adaptation, which cannot arise as a direct effect of climatic
conditions, but solely through what I may call the sorting out of the
variations produced by the species, according to their utility. That
caterpillars may be either green or brown is already something more than
could have been expected according to the old conception of species, but
that one and the same butterfly should be now pale yellow, with black;
now red with black and pure white; now deep black with large, pure white
spots; and again black with a large ochreous-yellow spot, and many small
white and yellow spots; that in one sub-species it may be tailed like
the ancestral form, and in another tailless like its Danaid model,--all
this shows a far-reaching capacity for variation and adaptation that
wide never have expected if we did not see the facts before us. How
it is possible that the primary colour-variations should thus be
intensified and combined remains a puzzle even now; we are reminded of
the modern three-colour printing,--perhaps similar combinations of the
primary colours take place in this case; in any case the direction of
these primary variations is determined by the artist whom we know as
natural selection, for there is no other conceivable way in which the
model could affect the butterfly that is becoming more and more like it.
The same climate surrounds all four forms of female; they are subject
to the same conditions of nutrition. Moreover, Papilio dardanus is by
no means the only species of butterfly which exhibits different kinds of
colour-pattern on its wings. Many species of the Asiatic genus Elymnias
have on the upper surface a very good imitation of an immune Euploeine
(Danainae), often with a steel-blue ground-colour, while the under
surface is well concealed when the butterfly is at rest,--thus there are
two kinds of protective coloration each with a different meaning! The
same thing may be observed in many non-mimetic butterflies, for
instance in all our species of Vanessa, in which the under side shows
a grey-brown or brownish-black protective coloration, but we do not yet
know with certainty what may be the biological significance of the gaily
coloured upper surface.
In general it may be said that mimetic butterflies are comparatively
rare species, but there are exceptions, for instance Limenitis archippus
in North America, of which the immune model (Danaida plexippus) also
occurs in enormous numbers.
In another mimicry-category the imitators are often more numerous than
the models, namely in the case of the imitation of DANGEROUS INSECTS by
harmless species. Bees and wasps are dreaded for their sting, and they
are copied by harmless flies of the genera Eristalis and Syrphus, and
these mimics often occur in swarms about flowering plants without damage
to themselves or to their models; they are feared and are therefore left
unmolested.
In regard also to the FAITHFULNESS OF THE COPY the facts are quite in
harmony with the theory, according to which the resemblance must have
arisen and increased BY DEGREES. We can recognise this in many
cases, for even now the mimetic species show very VARYING DEGREES OF
RESEMBLANCE to their immune model. If we compare, for instance, the
many different imitators of Danaida chrysippus we find that, with their
brownish-yellow ground-colour, and the position and size, and more or
less sharp limitation of their clear marginal spots, they have reached
very different degrees of nearness to their model. Or compare the female
of Elymnias undularis with its model Danaida genutia; there is a general
resemblance, but the marking of the Danaida is very roughly imitated in
Elymnias.
Another fact that bears out the theory of mimicry is, that even when the
resemblance in colour-pattern is very great, the WING-VENATION, which is
so constant, and so important in determining the systematic position
of butterflies, is never affected by the variation. The pursuers of the
butterfly have no time to trouble about entomological intricacies.
I must not pass over a discovery of Poulton's which is of great
theoretical importance--that mimetic butterflies may reach the same
effect by very different means. ("Journ. Linn. Soc. London (Zool.)",
Vol. XXVI. 1898, pages 598-602.) Thus the glass-like transparency of the
wing of a certain Ithomiine (Methona) and its Pierine mimic (Dismorphia
orise) depends on a diminution in the size of the scales; in the Danaine
genus Ituna it is due to the fewness of the scales, and in a third
imitator, a moth (Castnia linus var. heliconoides) the glass-like
appearance of the wing is due neither to diminution nor to absence of
scales, but to their absolute colourlessness and transparency, and to
the fact that they stand upright. In another moth mimic (Anthomyza) the
arrangement of the transparent scales is normal. Thus it is not some
unknown external influence that has brought about the transparency of
the wing in these five forms, as has sometimes been supposed. Nor is it
a hypothetical INTERNAL evolutionary tendency, for all three vary in a
different manner. The cause of this agreement can only lie in selection,
which preserves and intensifies in each species the favourable
variations that present themselves. The great faithfulness of the copy
is astonishing in these cases, for it is not THE WHOLE wing which is
transparent; certain markings are black in colour, and these contrast
sharply with the glass-like ground. It is obvious that the pursuers
of these butterflies must be very sharp-sighted, for otherwise the
agreement between the species could never have been pushed so far. The
less the enemies see and observe, the more defective must the imitation
be, and if they had been blind, no visible resemblance between the
species which required protection could ever have arisen.
A seemingly irreconcilable contradiction to the mimicry theory is
presented in the following cases, which were known to Bates, who,
however, never succeeded in bringing them into line with the principle
of mimicry.
In South America there are, as we have already said, many mimics of the
immune Ithomiinae (or as Bates called them Heliconidae). Among these
there occur not merely species which are edible, and thus require the
protection of a disguise, but others which are rejected on account of
their unpalatableness. How could the Ithomiine dress have developed in
their case, and of what use is it, since the species would in any case
be immune? In Eastern Brazil, for instance, there are four butterflies,
which bear a most confusing resemblance to one another in colour,
marking, and form of wing, and all four are unpalatable to birds. They
belong to four different genera and three sub-families, and we have to
inquire: Whence came this resemblance and what end does it serve? For
a long time no satisfactory answer could be found, but Fritz Muller
(In "Kosmos", 1879, page 100.), seventeen years after Bates, offered a
solution to the riddle, when he pointed out that young birds could not
have an instinctive knowledge of the unpalatableness of the Ithomiines,
but must learn by experience which species were edible and which
inedible. Thus each young bird must have tasted at least one individual
of each inedible species and discovered its unpalatability, before it
learnt to avoid, and thus to spare the species. But if the four species
resemble each other very closely the bird will regard them all as of
the same kind, and avoid them all. Thus there developed a process
of selection which resulted in the survival of the Ithomiine-like
individuals, and in so great an increase of resemblance between the four
species, that they are difficult to distinguish one from another even in
a collection. The advantage for the four species, living side by side
as they do e.g. in Bahia, lies in the fact that only one individual
from the MIMICRY-RING ("inedible association") need be tasted by a young
bird, instead of at least four individuals, as would otherwise be the
case. As the number of young birds is great, this makes a considerable
difference in the ratio of elimination.
These interesting mimicry-rings (trusts), which have much significance
for the theory, have been the subject of numerous and careful
investigations, and at least their essential features are now fully
established. Muller took for granted, without making any investigations,
that young birds only learn by experience to distinguish between
different kinds of victims. But Lloyd Morgan's ("Habit and Instinct",
London, 1896.) experiments with young birds proved that this is really
the case, and at the same time furnished an additional argument against
the LAMARCKIAN PRINCIPLE.
In addition to the mimicry-rings first observed in South America, others
have been described from Tropical India by Moore, and by Poulton and
Dixey from Africa, and we may expect to learn many more interesting
facts in this connection. Here again the preliminary postulates of the
theory are satisfied. And how much more that would lead to the same
conclusion might be added!
As in the case of mimicry many species have come to resemble one another
through processes of selection, so we know whole classes of phenomena
in which plants and animals have become adapted to one another, and have
thus been modified to a considerable degree. I refer particularly to the
relation between flowers and insects; but as there is an article on "The
Biology of Flowers" in this volume, I need not discuss the subject, but
will confine myself to pointing out the significance of these remarkable
cases for the theory of selection. Darwin has shown that the originally
inconspicuous blossoms of the phanerogams were transformed into flowers
through the visits of insects, and that, conversely, several large
orders of insects have been gradually modified by their association
with flowers, especially as regards the parts of their body actively
concerned. Bees and butterflies in particular have become what they
are through their relation to flowers. In this case again all that is
apparently contradictory to the theory can, on closer investigation, be
beautifully interpreted in corroboration of it. Selection can give rise
only to what is of use to the organism actually concerned, never to what
is of use to some other organism, and we must therefore expect to find
that in flowers only characters of use to THEMSELVES have arisen, never
characters which are of use to insects only, and conversely that in the
insects characters useful to them and not merely to the plants would
have originated. For a long time it seemed as if an exception to this
rule existed in the case of the fertilisation of the yucca blossoms by
a little moth, Pronuba yuccasella. This little moth has a sickle-shaped
appendage to its mouth-parts which occurs in no other Lepidopteron,
and which is used for pushing the yellow pollen into the opening of
the pistil, thus fertilising the flower. Thus it appears as if a new
structure, which is useful only to the plant, has arisen in the insect.
But the difficulty is solved as soon as we learn that the moth lays
its eggs in the fruit-buds of the Yucca, and that the larvae, when they
emerge, feed on the developing seeds. In effecting the fertilisation
of the flower the moth is at the same time making provision for its own
offspring, since it is only after fertilisation that the seeds begin to
develop. There is thus nothing to prevent our referring this structural
adaptation in Pronuba yuccasella to processes of selection, which
have gradually transformed the maxillary palps of the female into the
sickle-shaped instrument for collecting the pollen, and which have at
the same time developed in the insect the instinct to press the pollen
into the pistil.
In this domain, then, the theory of selection finds nothing but
corroboration, and it would be impossible to substitute for it any
other explanation, which, now that the facts are so well known, could
be regarded as a serious rival to it. That selection is a factor, and
a very powerful factor in the evolution of organisms, can no longer be
doubted. Even although we cannot bring forward formal proofs of it IN
DETAIL, cannot calculate definitely the size of the variations which
present themselves, and their selection-value, cannot, in short,
reduce the whole process to a mathematical formula, yet we must assume
selection, because it is the only possible explanation applicable to
whole classes of phenomena, and because, on the other hand, it is made
up of factors which we know can be proved actually to exist, and
which, IF they exist, must of logical necessity cooperate in the manner
required by the theory. WE MUST ACCEPT IT BECAUSE THE PHENOMENA OF
EVOLUTION AND ADAPTATION MUST HAVE A NATURAL BASIS, AND BECAUSE IT IS
THE ONLY POSSIBLE EXPLANATION OF THEM. (This has been discussed in many
of my earlier works. See for instance "The All-Sufficiency of Natural
Selection, a reply to Herbert Spencer", London, 1893.)
Many people are willing to admit that selection explains adaptations,
but they maintain that only a part of the phenomena are thus explained,
because everything does not depend upon adaptation. They regard
adaptation as, so to speak, a special effort on the part of Nature,
which she keeps in readiness to meet particularly difficult claims
of the external world on organisms. But if we look at the matter more
carefully we shall find that adaptations are by no means exceptional,
but that they are present everywhere in such enormous numbers, that it
would be difficult in regard to any structure whatever, to prove that
adaptation had NOT played a part in its evolution.
How often has the senseless objection been urged against selection that
it can create nothing, it can only reject. It is true that it cannot
create either the living substance or the variations of it; both must be
given. But in rejecting one thing it preserves another, intensifies
it, combines it, and in this way CREATES what is new. EVERYTHING in
organisms depends on adaptation; that is to say, everything must be
admitted through the narrow door of selection, otherwise it can take
no part in the building up of the whole. But, it is asked, what of the
direct effect of external conditions, temperature, nutrition, climate
and the like? Undoubtedly these can give rise to variations, but they
too must pass through the door of selection, and if they cannot do this
they are rejected, eliminated from the constitution of the species.
It may, perhaps, be objected that such external influences are often of
a compelling power, and that every animal MUST submit to them, and that
thus selection has no choice and can neither select nor reject. There
may be such cases; let us assume for instance that the effect of the
cold of the Arctic regions was to make all the mammals become black; the
result would be that they would all be eliminated by selection, and
that no mammals would be able to live there at all. But in most cases a
certain percentage of animals resists these strong influences, and
thus selection secures a foothold on which to work, eliminating the
unfavourable variation, and establishing a useful colouring, consistent
with what is required for the maintenance of the species.
Everything depends upon adaptation! We have spoken much of adaptation
in colouring, in connection with the examples brought into prominence by
Darwin, because these are conspicuous, easily verified, and at the same
time convincing for the theory of selection. But is it only desert and
polar animals whose colouring is determined through adaptation? Or the
leaf-butterflies, and the mimetic species, or the terrifying markings,
and "warning-colours" and a thousand other kinds of sympathetic
colouring? It is, indeed, never the colouring alone which makes up
the adaptation; the structure of the animal plays a part, often a very
essential part, in the protective disguise, and thus MANY variations may
cooperate towards ONE common end. And it is to be noted that it is by
no means only external parts that are changed; internal parts are ALWAYS
modified at the same time--for instance, the delicate elements of the
nervous system on which depend the INSTINCT of the insect to hold its
wings, when at rest, in a perfectly definite position, which, in the
leaf-butterfly, has the effect of bringing the two pieces on which
the marking occurs on the anterior and posterior wing into the same
direction, and thus displaying as a whole the fine curve of the midrib
on the seeming leaf. But the wing-holding instinct is not regulated in
the same way in all leaf-butterflies; even our indigenous species
of Vanessa, with their protective ground-colouring, have quite a
distinctive way of holding their wings so that the greater part of the
anterior wing is covered by the posterior when the butterfly is at rest.
But the protective colouring appears on the posterior wing and on the
tip of the anterior, TO PRECISELY THE DISTANCE TO WHICH IT IS LEFT
UNCOVERED. This occurs, as Standfuss has shown, in different degree in
our two most nearly allied species, the uncovered portion being
smaller in V. urticae than in V. polychloros. In this case, as in most
leaf-butterflies, the holding of the wing was probably the primary
character; only after that was thoroughly established did the protective
marking develop. In any case, the instinctive manner of holding the
wings is associated with the protective colouring, and must remain as it
is if the latter is to be effective. How greatly instincts may change,
that is to say, may be adapted, is shown by the case of the Noctuid
"shark" moth, Xylina vetusta. This form bears a most deceptive
resemblance to a piece of rotten wood, and the appearance is greatly
increased by the modification of the innate impulse to flight common to
so many animals, which has here been transformed into an almost contrary
instinct. This moth does not fly away from danger, but "feigns death,"
that is, it draws antennae, legs and wings close to the body, and
remains perfectly motionless. It may be touched, picked up, and thrown
down again, and still it does not move. This remarkable instinct must
surely have developed simultaneously with the wood-colouring; at all
events, both cooperating variations are now present, and prove that both
the external and the most minute internal structure have undergone a
process of adaptation.
The case is the same with all structural variations of animal parts,
which are not absolutely insignificant. When the insects acquired wings
they must also have acquired the mechanism with which to move them--the
musculature, and the nervous apparatus necessary for its automatic
regulation. All instincts depend upon compound reflex mechanisms and are
just as indispensable as the parts they have to set in motion, and all
may have arisen through processes of selection if the reasons which I
have elsewhere given for this view are correct. ("The Evolution Theory",
London, 1904, page 144.)
Thus there is no lack of adaptations within the organism, and
particularly in its most important and complicated parts, so that we may
say that there is no actively functional organ that has not undergone a
process of adaptation relative to its function and the requirements of
the organism. Not only is every gland structurally adapted, down to the
very minutest histological details, to its function, but the function
is equally minutely adapted to the needs of the body. Every cell in the
mucous lining of the intestine is exactly regulated in its relation to
the different nutritive substances, and behaves in quite a different way
towards the fats, and towards nitrogenous substances, or peptones.
I have elsewhere called attention to the many adaptations of the whale
to the surrounding medium, and have pointed out--what has long been
known, but is not universally admitted, even now--that in it a great
number of important organs have been transformed in adaptation to the
peculiar conditions of aquatic life, although the ancestors of the whale
must have lived, like other hair-covered mammals, on land. I cited a
number of these transformations--the fish-like form of the body, the
hairlessness of the skin, the transformation of the fore-limbs to fins,
the disappearance of the hind-limbs and the development of a tail fin,
the layer of blubber under the skin, which affords the protection
from cold necessary to a warm-blooded animal, the disappearance of the
ear-muscles and the auditory passages, the displacement of the external
nares to the forehead for the greater security of the breathing-hole
during the brief appearance at the surface, and certain remarkable
changes in the respiratory and circulatory organs which enable the
animal to remain for a long time under water. I might have added many
more, for the list of adaptations in the whale to aquatic life is by no
means exhausted; they are found in the histological structure and in the
minutest combinations in the nervous system. For it is obvious that a
tail-fin must be used in quite a different way from a tail, which serves
as a fly-brush in hoofed animals, or as an aid to springing in the
kangaroo or as a climbing organ; it will require quite different
reflex-mechanisms and nerve-combinations in the motor centres.
I used this example in order to show how unnecessary it is to assume a
special internal evolutionary power for the phylogenesis of species, for
this whole order of whales is, so to speak, MADE UP OF ADAPTATIONS; it
deviates in many essential respects from the usual mammalian type, and
all the deviations are adaptations to aquatic life. But if precisely the
most essential features of the organisation thus depend upon adaptation,
what is left for a phyletic force to do, since it is these essential
features of the structure it would have to determine? There are few
people now who believe in a phyletic evolutionary power, which is not
made up of the forces known to us--adaptation and heredity--but the
conviction that EVERY part of an organism depends upon adaptation has
not yet gained a firm footing. Nevertheless, I must continue to regard
this conception as the correct one, as I have long done.
I may be permitted one more example. The feather of a bird is a
marvellous structure, and no one will deny that as a whole it depends
upon adaptation. But what part of it DOES NOT depend upon adaptation?
The hollow quill, the shaft with its hard, thin, light cortex, and the
spongy substance within it, its square section compared with the round
section of the quill, the flat barbs, their short, hooked barbules
which, in the flight-feathers, hook into one another with just
sufficient firmness to resist the pressure of the air at each wing-beat,
the lightness and firmness of the whole apparatus, the elasticity of
the vane, and so on. And yet all this belongs to an organ which is only
passively functional, and therefore can have nothing to do with the
LAMARCKIAN PRINCIPLE. Nor can the feather have arisen through some
magical effect of temperature, moisture, electricity, or specific
nutrition, and thus selection is again our only anchor of safety.
But--it will be objected--the substance of which the feather consists,
this peculiar kind of horny substance, did not first arise through
selection in the course of the evolution of the birds, for it formed the
covering of the scales of their reptilian ancestors. It is quite true
that a similar substance covered the scales of the Reptiles, but why
should it not have arisen among them through selection? Or in what other
way could it have arisen, since scales are also passively useful parts?
It is true that if we are only to call adaptation what has been acquired
by the species we happen to be considering, there would remain a great
deal that could not be referred to selection; but we are postulating an
evolution which has stretched back through aeons, and in the course of
which innumerable adaptations took place, which had not merely ephemeral
persistence in a genus, a family or a class, but which was continued
into whole Phyla of animals, with continual fresh adaptations to
the special conditions of each species, family, or class, yet with
persistence of the fundamental elements. Thus the feather, once
acquired, persisted in all birds, and the vertebral column, once gained
by adaptation in the lowest forms, has persisted in all the Vertebrates,
from Amphioxus upwards, although with constant readaptation to the
conditions of each particular group. Thus everything we can see in
animals is adaptation, whether of to-day, or of yesterday, or of ages
long gone by; every kind of cell, whether glandular, muscular, nervous,
epidermic, or skeletal, is adapted to absolutely definite and specific
functions, and every organ which is composed of these different kinds
of cells contains them in the proper proportions, and in the particular
arrangement which best serves the function of the organ; it is thus
adapted to its function.
All parts of the organism are tuned to one another, that is, THEY ARE
ADAPTED TO ONE ANOTHER, and in the same way THE ORGANISM AS A WHOLE IS
ADAPTED TO THE CONDITIONS OF ITS LIFE, AND IT IS SO AT EVERY STAGE OF
ITS EVOLUTION.
But all adaptations CAN be referred to selection; the only point that
remains doubtful is whether they all MUST be referred to it.
However that may be, whether the LAMARCKIAN PRINCIPLE is a factor that
has cooperated with selection in evolution, or whether it is altogether
fallacious, the fact remains, that selection is the cause of a great
part of the phyletic evolution of organisms on our earth. Those
who agree with me in rejecting the LAMARCKIAN PRINCIPLE will regard
selection as the only GUIDING factor in evolution, which creates what
is new out of the transmissible variations, by ordering and arranging
these, selecting them in relation to their number and size, as the
architect does his building-stones so that a particular style must
result. ("Variation under Domestication", 1875 II. pages 426, 427.) But
the building-stones themselves, the variations, have their basis in the
influences which cause variation in those vital units which are handed
on from one generation to another, whether, taken together they form the
WHOLE organism, as in Bacteria and other low forms of life, or only
a germ-substance, as in unicellular and multicellular organisms. (The
Author and Editor are indebted to Professor Poulton for kindly assisting
in the revision of the proof of this Essay.)