odour at the breeding season; many other animals do the same, though
in their case it does not seem so pleasant to our sense of smell.
It is true that the scent of the musk-deer and that of the beaver
(_Castoreum_), when much diluted, are agreeable to man, but others,
like the odours exhaled by stags or by beasts of prey, are very
disagreeable to us, though they have for the species that produce them
the same significance as the others, and are therefore to be referred
to sexual selection.
Darwin referred all the different _mechanisms for the production of
sounds_, up to the song of birds, to sexual selection, but it is
probable that natural selection has also to do with this in many ways.
It is certainly only the males which produce the well-known song of the
Cicadas, crickets, grasshoppers and birds, and I do not see any reason
to doubt that this 'music' affects the females by arousing sexual
excitement. To some extent, then, the rivalry among the males for the
possession of the females--that is to say, sexual selection--must have
produced these mechanisms of song; and how long-continued and gradual
the accumulations must have been which produced the song of the thrush
or of the nightingale from the chirping of the sparrow we may learn
from the innumerable species which, as regards beauty of song, may be
ranged between these two extremes.
My assumption that natural selection has also been operative in the
case of the song of insects and birds is based on the fact that many
of our songsters live widely scattered, and that the characteristic
note must be a means by which the two sexes find each other. That
they should find each other is an indispensable condition for the
maintenance of the species. Thus it is well known that each species has
a characteristic 'note' or love-call, which the male utters during the
breeding season, and which is answered by the female. From this simple
love-call the modern song of many species must have developed by means
of sexual selection.
It is remarkable that here again the various distinguishing characters
of the male seem to be often mutually restrictive or mutually
exclusive. The best singers among our birds are inconspicuously
coloured, grey or brown-grey, and this can hardly be regarded as due
to chance, but as the outcome of a greater sensitiveness on the part
of the females either to the song or to the beauty of their mates. And
since, according to the theory, only those characters of the males
could be increased which decided the choice, it therefore seems to
me that this mutual exclusiveness of the two kinds of distinguishing
characters is another indication of the reality of sexual selection.
It proves--so at least I am inclined to believe--that the excitement
of the female has been essentially affected by _only one_ of the
characters of the male, that in the bird of Paradise it was mainly
the brilliance of the plumage which roused excitement, while in the
nightingale it was mainly the song.
It might be objected to this that there are brilliant butterflies which
also possess scent-scales. This is really the case; thus a magnificent
blue iridescent _Apatura_ from Brazil has on the posterior wings a
large yellow brush of scent-hairs, and even the beautiful blue males of
our Lycænids have scent-scales in addition to their beautiful colour.
But this can hardly be considered as a contradiction, but is rather
an exception, which is the easier to explain since the odoriferous
apparatus is a relatively simple arrangement, which did not require
such a long series of generations for its evolution as the complicated
song-box and brain-mechanism of the singing-birds.
Moreover, it may also be that the scent-scales have arisen later
than the decorative colouring, and they would do so the more easily
since the brilliant blue, when once it was perfectly developed, and
was common to all the males of the species in an equal degree, was
no longer distinctive, and would have no specially exciting effect,
while a novel preferential character in the male would have a much
stronger effect. In the same way, the different parts of the body would
be furnished in succession with decorative and, therefore, exciting
distinctive characters. To understand this effect on the opposite
sex we need only think of analogous phenomena in human kind, and of
the strongly exciting effect that the sight of the secondary sexual
characters of the woman has upon the man.
By the successive additions of new decorative characters after the
older ones became general and reached a climax, the origin of the
extraordinary diversity of the decorative plumage in one and the same
species of bird, can be readily understood, and the same is true of
the complicated decorative coloration of the butterflies in so far as
it depends on sexual selection, and not on other factors. The details
did not arise all at once, but one after the other, and every character
went on increasing till it had reached its limit of increase, but
whenever it was common in its highest development to all the males
it was no longer an object of preference or the cause of specially
violent excitement, so that a new process of selection would begin in
reference to some other part of the body. We thus understand how, among
male birds of Paradise and humming-birds, such a marvellous diversity
of colours and of decorative feathers is found combined in one and the
same species.
Whoever has seen the Gould Collection of humming-birds in London must
have observed with amazement that among the 130 or so species of these
beautiful little birds nearly every group of feathers in the body has
been affected by the decorative colouring. In one species the little
feathers on the region of the throat are emerald green, metallic blue,
or rose; in another the feathers of the neck have been transformed into
an erectile collar of rose-coloured feathers with a metallic sheen; or,
again, it is the little feathers round the ear that stand erect and are
brilliantly coloured. Sometimes we find that the feathers of the tail
are lengthened, it may be only two of them, or the various lengths may
be graduated like steps; sometimes the tail has assumed the form of a
wedge, or is fan-like, or is shaped like the tail of a swallow, and all
this in combination with the most diverse colours and patterns, black
and white, ultramarine blue, and so forth. Or it may be the outermost
tail-feathers which are the longest, the inner ones the shortest, or
the four outer feathers are broad, pointed, directed outwards, and only
half as long as the other two, which are very long and straight. Some
species exhibit a sort of fine swan's down on the legs, others have a
gorgeous metallic red cap on the head--in short, the variety is beyond
description, just as we should expect it to be if now this and now that
chance variation attracted the favourable regard of the selecting sex,
and thus attained to its highest pitch of development.
The decorative colouring of male birds may be replaced, not only by
the power of song, but in other ways also. Not all the male birds
of Paradise possess the familiar feather ornaments. The Italian
traveller Beccari has called attention to a species, the males of
which are simply coloured brown, like the females of other species.
This _Amblyornis inornata_ entices its mate to itself in the pairing
time in a very peculiar manner, for it arranges in the midst of the
primitive forests of New Guinea a little 'love garden' or bower, a spot
several feet in extent, strewn with white sand, on which it places
shining stones and shells, and brightly coloured berries. In this case
a special instinct has developed, which has replaced the personal
charm of the bird in the eyes of the female. For this very reason the
case seems to me to have some theoretical importance, for it serves
indirectly to show that the personal excellences do actually function
as a means of exciting and attracting, if any one should still doubt it.
All the distinguishing characters of the male which we have hitherto
considered have had reference to gaining the favour of the female, but
there are many other secondary sexual characters which are employed in
quite a different manner to secure possession of the female. I have
already mentioned that in many butterflies the males possess a much
larger organ of smell. The antennæ of the males of numerous beetles,
such as the cockchafer and its relatives, are also much larger, and
furnished with much broader accessory branches, than those of the
female, and the same is the case in many of the lower crustaceans, like
the large transparent Daphnid of our lakes, _Leptodora hyalina_. Here
the anterior antenna bears (Fig. 56, _A_ and _B_, _at´_) olfactory
filaments; in the female this appendage is small and stump-like, while
in the male (_A_) it grows to a long, somewhat curved rod, which
is extended obliquely into the water, and in addition to the nine
olfactory filaments of the female (_ri_) bears from sixty to ninety
more (_ri´_).
[Illustration: FIG. 56. _Leptodora hyalina._ _A_, head of the male.
_B_, head of the female. _Au_, eye. _g. opt_, optic ganglion. _gh_,
brain. _at´_, first antenna with olfactory filaments _ri_ and _ri´_.
_sr_, œsophageal nerve-ring. _n_, nerve. _m_, muscles.]
In this and many other such cases it is not the struggle of the
species for existence which has so markedly augmented this distinctive
characteristic of the male; it is undoubtedly the struggle of the males
among themselves, their competition for the possession of the females.
In regard to decorative distinctions, the reality of a rivalry in
wooing and the ultimate victory of the most decorative may perhaps be
still doubted; but it is quite certain that, on an average, the male
which can smell and track best will also gain possession of the females
more easily than one less well equipped. Exactly the same is also true
of those cases in which the male distinguishing character does not
refer merely to finding the female, but to holding her fast, or, as we
may say, to capturing her.
Thus the males of the Copepods possess on their anterior antennæ an
arrangement which enables them to throw a long whiplike structure
like a lasso round the head of the female as she rapidly swims away.
The antennæ of the male Daphnids, too, are in one genus (_Moina_)
developed into a grasping apparatus, instead of into smelling organs as
in _Leptodora_. Fig. 57 shows the male, Fig. 58 the female of _Moina
paradoxa_; the first antennæ of the male are not only much longer and
stronger than those of the female (_at_^1), but they are also armed
with claws at the end, so that the males can catch their mates as with
a fork, and hold them fast. And even that was not enough, for, in
addition, the males of most Daphnids possess a large sickle-shaped but
blunt claw on the first pair of legs (Fig. 57, _fkr_), which enables
them to cling to the smooth shell of the female, and to clamber up on
it to get into the proper position for copulation.
[Illustration: FIG. 57. _Moina paradoxa_, male. _at^1_, first antennæ,
with claws at the tip for capturing the female. _at^2_, second antennæ.
_fkr_, claws on the first pair of legs for clambering. _gh_, brain.
_lbr_, upper lip. _md_, mandible. _md_, mid-gut, with the liver lobes
(_lh_). _h_, heart. _sp_, testis. _aft_, anus. _sb_, caudal setæ.
_skr_, caudal claws. _sch_, shell. _schr_, cavity of the shell. _kie_,
gill-plates. Magnified 100 times.]
If we inquire into the manner of the origin of secondary sexual
characters of this kind, we shall find that both may have been
increased by sexual selection, for a male with a better sickle will
succeed more quickly in getting into the proper position for copulation
than one with a less perfect mechanism. This assumption does not rest
on mere theory, for I was once able, by a happy chance, to observe for
a considerable time, under the microscope, a female to whose shell two
males were clinging, each trying to push the other off. Nevertheless it
seems to me very questionable whether the origin of this sickle-claw
can be referred to sexual selection, for without this clamping-organ
copulation in most Daphnids would not be possible. It was thus not as
an advantage which one male had over another that the clamping-sickle
evolved, but rather as a necessary acquisition of the whole family,
which must have developed in all the species at the same time as the
other peculiarities, and notably those of the shell. The competition of
the males among themselves is thus in this case simply an expression
of the struggle for existence on the part of the species as such, and
it is not a question merely of a character which makes it easier for
the males to gain possession of the females, but of one which had
necessarily to arise lest the species should become extinct. In other
words, in this case natural selection and sexual selection coincide.
[Illustration: FIG. 58. _Moina paradoxa_, female. The letters of Fig.
57 apply _mutatis mutandis_. _brr_, brood-pouch. _ov_, ovary. _sr_,
margin of shell.]
The case of the antennæ of _Moina_, which have been modified into
grasping organs, is quite different; these owe their origin not to
natural selection, but to sexual selection, for antennæ of that kind
are not indispensable to the existence of the species, as we can
see from the closely related genera, _Daphnia_ and _Simocephalus_,
where the males have quite short stump-like antennæ, furnished with
olfactory filaments not much more numerous than the females possess.
Just as these supernumerary olfactory filaments were produced by
sexual selection, and not by the ordinary natural selection, because
those males with the more acute sense of smell had an advantage over
those in which it was blunter, so the males of the genus _Moina_
which could grasp most securely had an advantage over those that
gripped less firmly, and thus arose these two different kinds of male
characteristics. Neither of them is of advantage to the species as
such, but only to the males in their competition for the possession of
the females.
But, where the production of a novel character in the male is
concerned, natural selection cannot proceed in a different manner
from sexual selection; the process of selection is exactly the same:
the better equipped males survive, the less well-equipped die without
begetting offspring; the difference lies only in the fact that in the
one case the improvement is in the species as such, in the other case
only in one sex without the existence of the species being thereby
made more secure. Such cases are instructive, because they make a
denial of the process of sexual selection quite impossible if that of
species-selection is admitted. If processes of selection are operative
at all as factors in transformation, they must act even where the
advantage is not to the species but only 'intra-sexual,' and the one
process must often run into the other, so that it is often quite
impossible to draw an exact line of demarcation between them.
Numerous secondary sexual differences probably depend purely on species
selection, that is to say, they include an improvement of the species
in relation to the struggle for existence. We may find a case in point
in the dwarf-like smallness of the males in many parasitic crustaceans,
in some worms, in many Rotifers, and in the Cirripedes. It can hardly
have been of advantage for the individual male to be smaller than
his fellows, but it was of advantage for the species to produce as
many males as possible in order to ensure a meeting with the females,
and, as the enormous production of males made it advantageous for the
species that as little material as possible should be used in their
individual production, we can readily understand the minuteness of the
males, and in some cases, as in the Rotifers and _Bonellia_, their
poor equipment, lack of nutritive organs, and ephemeral existence. The
marine worm, _Bonellia viridis_, whose female may be a foot in length,
is not the only case in which a microscopically small male lives like
a parasite inside the female. Among the round-worms, too, there is a
species called _Trichosomum crassicauda_, discovered by Leuckart in
the rat, the dwarf males of which live in the reproductive organs of
the female. All these are arrangements for securing the propagation of
the species, which might have been endangered if the males had had to
seek out the females, which, in the case of _Bonellia_, live in holes
in the rocks on the sea-floor, and, in the case of _Trichosomum_, are
concealed in the urinary bladder of the rat. Obviously, this is the
reason which, in addition to the one already mentioned, has conditioned
and produced, or helped to produce, the remarkable minuteness of
certain males.
From another category of sexual differences we see in how many ways
species-selection and sexual selection play into each other's hands.
In many species of animals the males are eager for combat, and they
are equipped with special weapons, or excel the females in general
strength of body. As these males struggle, in the literal sense of the
word, for the possession of the females, Darwin referred to sexual
selection those distinguishing characters which gave the stronger male
the victory over the weaker, and thus raised the victorious characters
to the rank of general characters of the species. And it certainly
cannot be doubted that, for instance, the strength and the antlers of
the stag must have been increased through the combats which recurred
every year at the breeding season, for the stronger always win in these
battles. The case is the same with the strength and the weapons of many
other male animals. The lion is effectively protected by his mane from
the bite of a rival, and the same protective arrangement occurs in
quite a different family of mammals--in an eared seal, which is called
the 'sea-lion' for this very reason. Among the seals the secondary
sexual characters are often very strongly developed, at least in all
the polygamous species, for in these the struggle for the females is
very keen. In the 'sea-lions' and 'sea-elephants' there are often fifty
females to one male, and the latter are 'enormously larger' than the
females, while in monogamous species of seal the two sexes are alike in
size.
Darwin has shown that actual combat for the females takes place among
most mammals, not only among stags, lions, and seals, but even among
the moles and the timid hares. Even among birds such combats occur, and
this is sometimes particularly noteworthy in those species in which the
males possess the most decorative colouring, like the humming-birds. In
some cases among birds there has also been a development of weapons.
Witness the spur of the cock, whose merciless combats with his rivals
Man has, as is well known, made positively atrocious for his own
amusement, by preventing the flight of the vanquished.
In Darwin's great work on sexual selection a considerable number of
cases are cited from among lower vertebrates, such as crocodiles
and fishes, and even from insects, in which the males fight for
the possession of the females, and exhibit distinctive masculine
characters adapted to such combats. But I do not propose to enter upon
a discussion of such cases, since my aim is rather to elucidate the
relation between sexual selection and species-selection than to discuss
all the phenomena of the former in detail. But the combats of males
illustrate with particular clearness the relation of sexual selection
and species-selection, since many of the weapons or protective
arrangements which may have arisen through sexual selection imply at
the same time an improvement to the species in relation to the struggle
for existence. Thus greater strength or sharper and larger teeth in the
males mean a gain to the species, and it is indifferent to the species
whether the weaker males succumb to a strange enemy (species-selection)
or to their stronger rivals (sexual selection), provided only that the
better equipped survive and leave descendants similarly endowed.
I have intentionally begun the consideration of sexual selection with
the cases most difficult to interpret on this theory, with those which
have called forth the greatest divergence of opinion--the decorative
colours and forms, the song of birds and of insects, the alluring
odours--in short, all the courtship-adaptations of the males; these are
the most difficult to deal with, because it is not easy to demonstrate
directly that the females _do_ choose. But if we revise them briefly
in reverse order, I believe that all doubt as to the reality of choice
on the part of the females will disappear. Thus the last-mentioned
sexual characters of greater strength and greater perfection of weapons
and defence in the males have been evolved by sexual selection in
close co-operation with species-selection. We should have to deny
species-selection altogether if we were to dispute this form of sexual
selection, which is closely connected with pure species-selection,
such, for instance, as is revealed in the production of dwarf males,
where there does not seem to be any aid from sexual selection at all.
Then came the cases in which the tracking and grasping organs of the
males were strengthened or were increased in number, and here too
species-selection may have had its share, for instance, in evolving
the sickle-claws of the Daphnids, which were inevitably advanced and
perfected through sexual selection, which must in this case have
operated independently of any choice on the part of the female. In
other cases the result may be referred to pure sexual selection,
as in the grasping antennæ of the male _Moina_, or in the highly
developed olfactory antennæ of the male _Leptodora_. That new organs,
too, can arise in this way is shown by the 'turban eyes'--to which
little attention has hitherto been paid--of some Ephemerids of the
genera _Cloë_ and _Potamanthus_, which were long ago described by
Pictet, the monographer of this family. These are large turban-shaped
compound eyes, occurring beside the ordinary eyes in the males alone,
which in these genera are in a majority of sixty to one. Whole swarms
of these males fly about over the water on the search for females,
and their highly developed organ of vision seems to decide matters
for them just as the organ of smell does for _Leptodora_. Neither of
these sense-organs can have any other advantage than that of making
their possessors aware of the female, for the whole activity of the
short-lived adult Ephemerides is limited to reproduction; they take no
food, and have nothing whatever to do except to reproduce.