the odour which emanates from so many animals at the breeding season.
It is possible that this odour also served at first merely to give
notice of the presence of individuals of the other sex, but it soon
became an excitant, and as the individuals which caused the greatest
degree of excitement were preferred, it reached as high a pitch of
perfection as was possible to it. I shall confine myself here to the
comparatively recently discovered fragrance of butterflies. Since
Fritz Mueller found out that certain Brazilian butterflies gave off
fragrance "like a flower," we have become acquainted with many such
cases, and we now know that in all lands, not only many diurnal
Lepidoptera but nocturnal ones also give off a delicate odour, which
is agreeable even to man. The ethereal oil to which this fragrance is
due is secreted by the skin-cells, usually of the wing, as I showed
soon after the discovery of the _scent-scales_. This is the case in
the males; the females have no _special_ scent-scales recognisable as
such by their form, but they must, nevertheless, give off an extremely
delicate fragrance, although our imperfect organ of smell cannot
perceive it, for the males become aware of the presence of a female,
even at night, from a long distance off, and gather round her. We may
therefore conclude, that both sexes have long given forth a very
delicate perfume, which announced their presence to others of the same
species, and that in many species (_not in all_) these small
beginnings become, in the males, particularly strong scent-scales of
characteristic form (lute, brush, or lyre-shaped). At first these
scales were scattered over the surface of the wing, but gradually they
concentrated themselves, and formed broad, velvety bands, or strong,
prominent brushes, and they attained their highest pitch of evolution
when they became enclosed within pits or folds of the skin, which
could be opened to let the delicious fragrance stream forth suddenly
towards the female. Thus in this case also we see that characters, the
original use of which was to bring the sexes together, and so to
maintain the species, have been evolved in the males into means for
exciting the female. And we can hardly doubt, that the females are
most readily enticed to yield to the butterfly that sends out the
strongest fragrance,--that is to say, that excites them to the highest
degree. It is a pity that our organs of smell are not fine enough to
examine the fragrance of male Lepidoptera in general, and to compare
it with other perfumes which attract these insects.[43] As far as we
can perceive them they resemble the fragrance of flowers, but there
are Lepidoptera whose scent suggests musk. A smell of musk is also
given off by several plants: it is a sexual excitant in the
musk-deer, the musk-sheep, and the crocodile.
As far as we know, then, it is perfumes similar to those of flowers
that the male Lepidoptera give off in order to entice their mates and
this is a further indication that animals, like plants, can to a large
extent meet the claims made upon them by life, and produce the
adaptations which are most purposive,--a further proof, too, of my
proposition that the useful variations, so to speak, are _always
there_. The flowers developed the perfumes which entice their
visitors, and the male Lepidoptera developed the perfumes which entice
and excite their mates.
There are many pretty little problems to be solved in this connection,
for there are insects, such as some flies, that are attracted by
smells which are unpleasant to us, like those from decaying flesh and
carrion. But there are also certain flowers, some orchids for
instance, which give forth no very agreeable odour, but one which is
to us repulsive and disgusting; and we should therefore expect that
the males of such insects would give off a smell unpleasant to us, but
there is no case known to me in which this has been demonstrated.
In cases such as we have discussed, it is obvious that there is no
possible explanation except through selection. This brings us to the
last kind of secondary sexual characters, and the one in regard to
which doubt has been most frequently expressed,--decorative colours
and decorative forms, the brilliant plumage of the male pheasant, the
humming-birds, and the bird of Paradise, as well as the bright colours
of many species of butterfly, from the beautiful blue of our little
Lycaenidae to the magnificent azure of the large Morphinae of Brazil.
In a great many cases, though not by any means in all, the male
butterflies are "more beautiful" than the females, and in the Tropics
in particular they shine and glow in the most superb colours. I really
see no reason why we should doubt the power of sexual selection, and I
myself stand wholly on Darwin's side. Even though we certainly cannot
assume that the females exercise a conscious choice of the
"handsomest" mate, and deliberate like the judges in a court of
justice over the perfections of their wooers, we have no reason to
doubt that distinctive forms (decorative feathers), and colours have a
particularly exciting effect upon the female, just as certain odours
have among animals of so many different groups, including the
butterflies. The doubts which existed for a considerable time, as a
result of fallacious experiments, as to whether the colours of flowers
really had any influence in attracting butterflies have now been set
at rest through a series of more careful investigations; we now know
that the colours of flowers are there on account of the butterflies,
as Sprengel first showed, and that the blossoms of Phanerogams are
selected in relation to them, as Darwin pointed out.
Certainly it is not possible to bring forward any convincing proof of
the origin of decorative colours through sexual selection, but there
are many weighty arguments in favour of it, and these form a body of
presumptive evidence so strong that it almost amounts to certainty.
In the first place, there is the analogy with other secondary sexual
characters. If the song of birds and the chirping of the cricket have
been evolved through sexual selection, if the penetrating odours of
male animals,--the crocodile, the musk-deer, the beaver, the
carnivores, and, finally, the flower-like fragrances of the
butterflies have been evolved to their present pitch in this way, why
should decorative colours have arisen in some other way? Why should
the eye be less sensitive to _specifically male_ colours and other
_visible_ signs _enticing to the female_, than the olfactory sense to
specifically male odours, or the sense of hearing to specifically male
sounds? Moreover, the decorative feathers of birds are almost always
spread out and displayed before the female during courtship. I have
elsewhere[44] pointed out that decorative colouring and
sweet-scentedness may replace one another in Lepidoptera as well as in
flowers, for just as some modestly coloured flowers (mignonette and
violet) have often a strong perfume, while strikingly coloured ones
are sometimes quite devoid of fragrance, so we find that the most
beautiful and gaily-coloured of our native Lepidoptera, the species of
Vanessa, have no scent-scales, while these are often markedly
developed in grey nocturnal Lepidoptera. Both attractions may,
however, be combined in butterflies, just as in flowers. Of course, we
cannot explain why both means of attraction should exist in one genus,
and only one of them in another, since we do not know the minutest
details of the conditions of life of the genera concerned. But from
the sporadic distribution of scent-scales in Lepidoptera, and from
their occurrence or absence in nearly related species, we may conclude
that fragrance is a relatively _modern_ acquirement, more recent than
brilliant colouring.
One thing in particular that stamps decorative colouring as a product
of selection is _its gradual intensification_ by the addition of new
spots, which we can quite well observe, because in many cases the
colours have been first acquired by the males, and later transmitted
to the females by inheritance. The scent-scales are never thus
transmitted, probably for the same reason that the decorative colours
of many birds are often not transmitted to the females: because with
these they would be exposed to too great elimination by enemies.
Wallace was the first to point out that in species with concealed
nests the beautiful feathers of the male occurred in the female also,
as in the parrots, for instance, but this is not the case in species
which brood on an exposed nest. In the parrots one can often observe
that the general brilliant colouring of the male is found in the
female, but that certain spots of colour are absent, and these have
probably been acquired comparatively recently by the male and have not
yet been transmitted to the female.
Isolation of the group of individuals which is in process of varying
is undoubtedly of great value in sexual selection, for even a solitary
conspicuous variation will become dominant much sooner in a small
isolated colony, than among a large number of members of a species.
Any one who agrees with me in deriving variations from germinal
selection will regard that process as an essential aid towards
explaining the selection of distinctive courtship-characters, such as
coloured spots, decorative feathers, horny outgrowths in birds and
reptiles, combs, feather-tufts, and the like, since the beginnings of
these would be presented with relative frequency in the struggle
between the determinants within the germ-plasm. The process of
transmission of decorative feathers to the female results, as Darwin
pointed out and illustrated by interesting examples, in the
_colour-transformation of a whole species_, and this process, as the
phyletically older colouring of young birds shows, must, in the course
of thousands of years, have repeated itself several times in a line of
descent.
If we survey the wealth of phenomena presented to us by secondary
sexual characters, we can hardly fail to be convinced of the truth of
the principle of sexual selection. And certainly no one who has
accepted natural selection should reject sexual selection, for, not
only do the two processes rest upon the same basis, but they merge
into one another, so that it is often impossible to say how much of a
particular character depends on one and how much on the other form of
selection.
(_b_) _Natural Selection_
An actual proof of the theory of sexual selection is out of the
question, if only because we cannot tell when a variation attains to
selection-value. It is certain that a delicate sense of smell is of
value to the male moth in his search for the female, but whether the
possession of one additional olfactory hair, or of ten, or of twenty
additional hairs leads to the success of its possessor we are unable
to tell. And we are groping even more in the dark when we discuss the
excitement caused in the female by agreeable perfumes, or by striking
and beautiful colours. That these do make an impression is beyond
doubt; but we can only assume that slight intensifications of them
give any advantage, and we _must_ assume this _since otherwise
secondary sexual characters remain inexplicable_.
The same thing is true in regard to natural selection. It is not
possible to bring forward any actual proof of the selection-value of
the initial stages, and the stages in the increase of variations, as
has been already shown. But the selection-value of a finished
adaptation can in many cases be statistically determined. Cesnola and
Poulton have made valuable experiments in this direction. The former
attached forty-five individuals of the green, and sixty-five of the
brown variety of the praying mantis (_Mantis religiosa_), by a silk
thread to plants, and watched them for seven days. The insects which
were on a surface of a colour Similar to their own remained uneaten,
while twenty-five green insects on brown parts of plants had all
disappeared in eleven days.
The experiments of Poulton and Sanders[45] were made with 600 pupae of
_Vanessa urticae_, the "tortoise-shell butterfly." The pupae were
artificially attached to nettles, tree-trunks, fences, walls, and to
the ground, some at Oxford, some at St. Helens in the Isle of Wight.
In the course of a month 93% of the pupae at Oxford were killed,
chiefly by small birds, while at St. Helens 68% perished. The
experiments showed very clearly that the colour and character of the
surface on which the pupa rests--and thus its own conspicuousness--are
of the greatest importance. At Oxford only the four pupae which were
fastened to nettles emerged; all the rest--on bark, stones and the
like--perished. At St. Helens the elimination was as follows: on
fences where the pupae were conspicuous, 92%; on bark, 66%; on walls,
54%; and among nettles, 57%. These interesting experiments confirm our
views as to protective coloration, and show further, _that the ratio
of elimination in the species is a very high one, and that therefore
selection must be very keen_.
We may say that the process of selection follows as a logical
necessity from the fulfilment of the three preliminary postulates of
the theory: variability, heredity, and the struggle for existence,
with its enormous ratio of elimination in all species. To this we must
add a fourth factor, the _intensification_ of variations which Darwin
established as a fact, and which we are now able to account for
theoretically on the basis of germinal selection. It may be objected
that there is considerable uncertainty about this _logical_ proof,
because of our inability to demonstrate the selection-value of the
initial stages and the individual stages of increase. We have
therefore to fall back on _presumptive evidence_. This is to be found
in _the interpretative value of the theory_. Let us consider this
point in greater detail.
In the first place it is necessary to emphasize what is often
overlooked, namely, that the theory not only explains the
_transformations_ of species, it also explains _their remaining the
same_; in addition to the principle of varying, it contains within
itself that of _persisting_. It is part of the essence of selection,
that it not only causes a part to _vary_ till it has reached its
highest pitch of adaptation, but that it _maintains it at this pitch.
This conserving influence of natural selection_ is of great
importance, and was early recognised by Darwin; it follows naturally
from the principle of the survival of the fittest.
We understand from this how it is that a species which has become
fully adapted to certain conditions of life ceases to vary, but
remains "constant," as long as the conditions of life _for_ it remain
unchanged, whether this be for thousands of years, or for whole
geological epochs. But the most convincing proof of the power of the
principle of selection lies in the innumerable multitude of phenomena
which cannot be explained in any other way. To this category belong
all structures which are only _passively_ of advantage to the
organism, because none of these can have arisen by the alleged
_Lamarckian principle_. These have been so often discussed that we
need do no more than indicate them here. Until quite recently the
sympathetic coloration of animals--for instance, the whiteness of
Arctic animals--was referred, at least in part, to the _direct_
influence of external factors, but the facts can best be explained by
referring them to the processes of selection, for then it is
unnecessary to make the gratuitous assumption that many species are
sensitive to the stimulus of cold and that others are not. The great
majority of Arctic land-animals, mammals and birds, are white, and
this proves that they were all able to present the variation which was
most useful for them. The sable is brown, but it lives in trees, where
the brown colouring protects and conceals it more effectively. The
musk-sheep (_Ovibos moschatus_) is also brown, and contrasts sharply
with the ice and snow, but it is protected from beasts of prey by its
gregarious habit, and therefore it is of advantage to be visible from
as great a distance as possible. That so many species have been able
to give rise to white varieties does not depend on a special
sensitiveness of the skin to the influence of cold, but to the fact
that Mammals and Birds have a general tendency to vary towards white.
Even with us, many birds--starlings, blackbirds, swallows,
etc.--occasionally produce white individuals, but the white variety
does not persist, because it readily falls a victim to the carnivores.
This is true of white fawns, foxes, deer, etc. The whiteness,
therefore, arises from internal causes, and only persists when it is
useful. A great many animals living in a _green environment_ have
become clothed in green, especially insects, caterpillars, and
Mantidae, both persecuted and persecutors.
That it is not the direct effect of the environment which calls forth
the green colour is shown by the many kinds of caterpillar which rest
on leaves and feed on them, but are nevertheless brown. These feed by
night and betake themselves through the day to the trunk of the tree,
and hide in the furrows of the bark. We cannot, however, conclude
from this that they were _unable_ to vary towards green, for there are
Arctic animals which are white only in winter and brown in summer
(Alpine hare, and the ptarmigan of the Alps), and there are also green
leaf-insects which remain green only while they are young and
difficult to see on the leaf, but which become brown again in the last
stage of larval life, when they have outgrown the leaf. They then
conceal themselves by day, sometimes only among withered leaves on the
ground, sometimes in the earth itself. It is interesting that in one
genus, Chaerocampa, one species is brown in the last stage of larval
life, another becomes brown earlier, and in many species the last
stage is not wholly brown, a part remaining green. Whether this is a
case of a double adaptation, or whether the green is being gradually
crowded out by the brown, the fact remains that the same species, even
the same individual, can exhibit both variations. The case is the same
with many of the leaf-like Orthoptera, as, for instance, the praying
mantis (_Mantis religiosa_) which we have already mentioned.
But the best proofs are furnished by those of ten-cited cases in which
the insect bears a deceptive resemblance to another object. We now
know many such cases, such as the numerous imitations of green or
withered leaves, which are brought about in the most diverse ways,
sometimes by mere variations in the form of the insect and in its
colour, sometimes by an elaborate marking, like that which occurs in
the Indian leaf-butterflies, _Kallima inachis_. In the single
butterfly-genus Anaea, in the woods of South America, there are about
a hundred species which are all gaily coloured on the upper surface,
and on the reverse side exhibit the most delicate imitation of the
colouring and pattern of a leaf, generally without any indication of
the leaf-ribs, but extremely deceptive nevertheless. Anyone who has
seen only one such butterfly may doubt whether many of the
insignificant details of the marking can really be of advantage to the
insect. Such details are for instance the apparent holes and splits
in the apparently dry or half-rotten leaf, which are usually due to
the fact that the scales are absent on a circular or oval patch so
that the colourless wing-membrane lies bare, and one can look through
the spot as through a window. Whether the bird which is seeking or
pursuing the butterflies takes these holes for dewdrops, or for the
work of a devouring insect, does not affect the question; the
mirror-like spot undoubtedly increases the general deceptiveness, for
the same thing occurs in many leaf-butterflies, though not in all, and
in some cases it is replaced in quite a peculiar manner. In one
species of Anaea (_A. divina_), the resting butterfly looks exactly
like a leaf out of the outer edge of which a large semi-circular piece
has been eaten, possibly by a caterpillar; but if we look more closely
it is obvious that there is no part of the wing absent, and that the
semi-circular piece is of a clear, pale yellow colour, while the rest
of the wing is of a strongly contrasted dark brown.
But the deceptive resemblance may be caused in quite a different
manner. I have often speculated as to what advantage the brilliant
white C could give to the otherwise dusky-coloured "Comma butterfly"
(_Grapta C. album_). Poulton's recent observations[46] have shown that
this represents the imitation of a crack such as is often seen in dry
leaves, and is very conspicuous because the light shines through it.
The utility obviously lies in presenting to the bird the very familiar
picture of a broken leaf with a clear shining slit, and we may
conclude, from the imitation of such small details, that the birds are
very sharp observers and that the smallest deviation from the usual
arrests their attention and incites them to closer investigation. It
is obvious that such detailed--we might almost say such
subtle--deceptive resemblances could only have come about in the
course of long ages through the acquirement from time to time of
something new which heightened the already existing resemblance.
In face of facts like these there can be no question of chance and no
one has succeeded so far in finding any other explanation to replace
that by selection. For the rest, the apparent leaves are by no means
perfect copies of a leaf; many of them only represent the torn or
broken piece, or the half or two-thirds of a leaf, but then the leaves
themselves frequently do not present themselves to the eye as a whole,
but partially concealed among other leaves. Even those butterflies
which, like the species of Kallima and Anaea, represent the whole of a
leaf with stalk, ribs, apex, and the whole breadth, are not actual
copies which would satisfy a botanist; there is often much wanting. In
Kallima the lateral ribs of the leaf are never all included in the
markings; there are only two or three on the left side and at more
four or five on the right, and in many individuals these are rather
obscure, while in others they are comparatively distinct. This
furnishes us with fresh evidence in favour of their origin through
processes of selection, for a botanically perfect picture could not
arise in this way; there could only be a fixing of such details as
heightened the deceptive resemblance.
Our postulate of origin through selection also enables us to
understand why the leaf-imitation is on the lower surface of the wing
in the diurnal Lepidoptera, and on the upper surface in the nocturnal
forms, corresponding to the attitude of the wings in the resting
position of the two groups.
The strongest of all proofs of the theory, however, is afforded by
cases of true "mimicry," those adaptations discovered by Bates in
1861, consisting in the imitation of one species by another, which
becomes more and more like its model. The model is always a species
that enjoys some special protection from enemies, whether because it
is unpleasant to taste, or because it is in some way dangerous.