that their colours are dependent on the scales which thickly cover the
wings, and the significance of which, in part at least, is just to give
this or that colour to the wing. They are degenerate or colourless
among the transparent-winged butterflies, and their colour depends
partly on pigment, partly on fluorescence and interference conditioned
by the fine microscopical structure of a system of intercrossing lines
on faintly coloured scales. The scales of our male 'blue' butterflies
(_Lycæna_) only appear blue because of their structure, while the brown
scales of their mates are due to a brown pigment. If the pigment be
removed from the scales of the female by boiling with caustic potash,
and they be then dried, they do not look blue like those of the male;
the scales of the male, therefore, must possess something which those
of the female do not.
Still less will any one be disposed to regard the marvellous splendour
of the plumage of the male bird of Paradise, with its erectile
collars--glistening like burnished metal--on the neck, breast or
shoulders, with its tufts, with its specially decorative feathers
standing singly out from the rest of the plumage, on head, wings, or
tail, with its mane-like bunch of loose, pendulous feathers on the
belly and on the sides, in short, with its extraordinary, diverse, and
unique equipment of feathers, as a mere unintentional accessory effect
of a feather dress designed for flight and protective warmth. Such
conspicuous, diverse, and unusual specializations of plumage must have
some other significance than that just indicated.
Alfred Russel Wallace regards these distinctive features of the male
as an expression of the greater vigour, and the more active metabolism
of the males, but it is unproved that the vigour of the male birds
is greater than that of the females, and it is not easy to see why
a more active metabolism should be necessary for the production of
strikingly bright colours than for that of a dark or protective colour.
Moreover, there are brilliantly coloured females, both among birds
and butterflies, and in nearly allied species the males may be either
gorgeous or quite plain like the females.
Darwin refers the origin of these secondary sexual characters to
processes of selection quite analogous to those of ordinary natural
selection, only that in this case it is not the maintenance of the
species which is aimed at, but the attainment of reproduction by the
single individual. The males are to some extent obliged to struggle
for the possession of the females, and every little variation which
enables a male to gain possession of a female more readily than his
neighbour has for this reason a greater likelihood of being transmitted
to descendants. Thus, attractive variations which once crop up will be
transmitted to more and more numerous males of the species, and among
these it will always be those possessing the character in question
in the highest degree which will have the best chance of securing a
mate, and so the character will continue to be augmented as long as
variations in this direction appear.
Two kinds of preliminary conditions, however, must be assumed. As the
ordinary natural selection could never have operated but for the fact
that in every generation a great many individuals, indeed the majority
of them, perish before they have had time to reproduce, so the process
of sexual selection could never have come into operation if every
male were able ultimately to secure a mate, no matter what degree of
attractiveness to the latter he possessed. If the numbers of males and
females were equal, so that there was always one female to one male,
there could be no choice exercised either by male or female, for there
would always remain individuals enough of both sexes, so that no male
need remain unmated.
But this is not the case: the proportions of the sexes are very rarely
as 1 : 1; there is usually a preponderating number of males, more
rarely of females. Among birds the males are usually in the majority,
still more so among fishes; and among diurnal butterflies there are
often a hundred males to one female (Bates), although there seem to
be a few tropical Papilionidæ among which the females have rather the
preponderance. Darwin called attention to the fact that one could
infer the greater rarity of the females even from the pricelists of
butterflies issued by the late Dr. Staudinger in connexion with his
business, for the females in most species, except the very common ones,
are priced much higher than the males, often twice as high. In the
whole list of many thousands of species there are only eleven species
of nocturnal Lepidoptera in which the males are dearer than the females.
Among the Mayflies or Ephemerides, too, the males are in the majority;
in many of them there are sixty males to one female: but there are
other kinds of insects, such as the dragon-flies (Libellulidæ), in
which the females are three or four times as numerous. There are also,
it may be remembered, some kinds of insects, such as Aphides, which
have become capable of parthenogenetic reproduction, and in which the
males are becoming extinct, e.g. in the case of _Cerataphis_ in British
orchid-houses.
The first postulate implied in 'sexual selection,' namely, that there
be an unequal number of individuals in the two sexes, is therefore
fulfilled in Nature; we have now to inquire whether the second
condition postulated--the power of choice--may also be regarded as a
reality.
This point has been disputed from many sides, and even by one of the
founders of the whole selection theory, Alfred Russel Wallace. This
naturalist doubts whether a choice is exercised among birds by either
sex in regard to pairing, and maintains that, even if there could be
a choice, this could not have produced such differences in colour and
character of the plumage, since that would presuppose the existence of
similar taste in the females through many generations. In a similar way
it has been doubted whether butterflies can be said to exercise any
real power of sexual choice, whether a more beautiful male is as such
preferred to a less beautiful suitor.
It must be admitted that direct observation of choosing is difficult,
and that as yet there is very little that can be said with certainty
on this point. But there are, after all, some precise observations on
mammals and birds which prove that the female shows active inclination
to, or disinclination for, a particular male. If we hold fast to
this fact, and add to it that the distinctive markings of the males
are wonderfully developed during the period of courtship, and are
displayed before the females, and that they only appear in mammals,
birds, amphibians, and fishes at the time of sexual maturity, it seems
to me that there can be no doubt that they are intended to fascinate
the females, and to induce them to yield themselves to the males. The
opponents of the theory of sexual selection attach too much importance
to isolated cases; they imagine that each female must make a choice
between several males. But the theory of sexual selection does not
demand this, any more than the theory of natural selection requires the
assumption that every individual of a species which is better equipped
for the struggle for existence must necessarily survive and attain
to reproduction, or, conversely, that the less well equipped must
necessarily perish.
All that the theory requires is, that the selective and eliminative
processes do, _on an average_, secure their ends, and in the same way
the theory of sexual selection does not need the assumption that every
female is in a position to exercise a scrupulous choice from among a
troop of males, but only that, on an average, the males more agreeable
to the females are selected, and those less agreeable rejected. If this
is the case, it must result in the male characters most attractive to
the females gaining preponderance, and becoming more and more firmly
established in the species, increasing in intensity, and finally
becoming a stable possession of all the males.
When we go more into details we shall see that the _particular
qualities_ of the distinctive masculine characters are exactly such as
they would be if they owed their existence to processes of selection;
in other words, from this point of view the phenomena of the decorative
sexual characters can be understood up to a certain point. It seems
to me that we are bound to accept the process of sexual selection as
really operative, and instead of throwing doubt upon it, because the
choice of the females can rarely be directly established, we should
rather deduce from the numerous sexual characters of the males, which
have a significance only in relation to courtship, that the females of
the species are sensitive to these distinguishing characters, and are
really capable of exercising a choice.
In my mind at least there remains no doubt that the 'sexual selection'
of Darwin is an important factor in the transformation of species, even
if I only take into consideration those secondary sexual characters
which are related to wooing. We shall see, however, that there are
others in regard to whose origin through processes of selection doubt
is still less legitimate, and from which, on this account, we can argue
back to the courtship characters.
The first beginning of transformation is not, even in ordinary natural
selection, to be understood as due to selection, but is to be regarded
as _a given variation_ (the causes of which we shall discuss later on);
it is only the increase of such incipient variations in a definite
direction that can depend on natural selection, and they _must_ depend
on it in so far as the transformations are purposeful. Now, all
secondary sexual characters can be recognized as useful, save only the
decorative distinctions, although these also undoubtedly represent
intensifications of originally unimportant variations. Are we then
to regard these alone as the mere outcome of the internal impulsive
forces of the organism, while in the case of the analogous sexual
characters for tracking, catching, and holding the female, and so
forth, the augmentation and the directing must be referred to processes
of selection? But if there be any utility at all in the decorative
sexual characters it can only lie in their greater attractiveness to
the females, and it can only be of any account if the females have,
in a certain sense, the power of choice. Independently, therefore, of
direct observations as to the actual occurrence of choosing, we should
be compelled by our chain of reasoning to assume that there was such a
power of choice--and I shall immediately discuss it more precisely.
If we consider the decorative, distinctive characters of the males more
closely, we find that they are of very diverse kinds. The males of many
animals are distinguished from the females chiefly by greater beauty
of form, and especially of colour. This is the case in many birds,
some amphibians, like the water-salamander, many fishes, many insects,
and above all, in diurnal Lepidoptera. Especially among birds the
dimorphism between the sexes is in obvious relation to the excess in
the number of male individuals, or--what practically comes to the same
thing--to polygamy. For when a male attaches to himself four or ten
females the result is the same as if the number of female individuals
were divided by four or by ten. Thus the fowls and pheasants, which
are polygamous, are adorned by magnificent colours in the male sex,
while the monogamous partridges and quails exhibit the same colouring
in both sexes. Of course 'beautiful' is a relative term, and we must
not simply assume that what seems beautiful to us appears so to all
animals; yet when we see that all the male birds which are beautifully
decorative according to our taste--whether humming-birds, pheasants,
birds of Paradise, or rock-cocks (_Rupicola crocea_)--unfold their
'feather-wheels, 'fans,' 'collars,' and so forth, before the eyes of
the females in the breeding season, and display them in all their
brilliance, we must conclude that, in these instances at least, human
taste accords with that of the animals. That birds have sharp vision
and distinguish colours is well known; it is not for nothing that the
service berries and many other berries suitable for birds are red, the
mistletoe berries white, in contrast to the evergreen foliage of this
plant, the juniper berries black so that they stand out amid the snows
of winter; in this direction, then, there is no difficulty in the way
of sexual selection.
Even among much lower animals, like the butterflies, there seems to me
no reason for the assumption that they do not see the gorgeous colours
and often very complicated markings, the bars and eye-spots, on the
wings of their fellows of the same species. Of course if each facet of
the insect eye contributed only a single visual impression, as Johannes
Müller supposed, then even an eye with 12,000 facets would give but a
rough and ill-defined picture of objects more than a few feet away, and
I confess that for a long time I regarded this as an obstacle in the
way of referring the sexual dimorphism of butterflies to processes of
selection. But we now know, through Exner, that this is not the case;
we know that each facet gives a little picture, and not an 'inverted'
but an 'upright' one, and experiment with the excised insect eye has
directly shown that it throws on a photographic plate a tolerably clear
image of even distant objects, such as the frame of a window, a large
letter painted on the window, or even a church tower visible through it.
Furthermore, the structure of the eye allows of incomparably clearer
vision of near objects, for in that case the eyes act like lenses, and
reveal much more minute details than we ourselves are able to make out.
Here again, therefore, there is no obstacle to the Darwinian hypothesis
of a choice on the part of the females, for although it cannot be
demonstrated from the structure of the eye itself that insects see
colour, and that colours have a specially exciting influence on them,
yet we can deduce this with certainty from the phenomena of their life.
The butterflies fly to gaily coloured flowers, and as they find in them
their food, the nectar of the flowers, we may take for granted that the
sight of the colour of their food-providing plants is associated with
an agreeable sensation, and this is an indication that similar colours
in their fellows may awaken similar agreeable sensations.
[Illustration: FIG. 53. Scent-scales of diurnal butterflies. _a_, of
Pieris. _b_, of Argynnis paphia. _c_, of a Satyrid. _d_, of Lycæna. All
highly magnified.]
This conclusion is furthermore confirmed by the fact that, in the male
sex, numerous species of butterfly possess another means of exciting
the females, namely, by pleasant odours. Volatile ethereal oils are
secreted by certain cells of the skin, and exhale into the air through
specially constructed scales. Usually the apparatus for dispersing
fragrance occurs on the wing in the form of the so-called scent-scales
(_Duftschuppen_), peculiar modifications of the ordinary colour-scales
of the wing, but sometimes they take the form of brush-like hair-tufts
on the abdomen, and they are in all cases so arranged that the volatile
perfume from the cells of the skin penetrates into them, and then
evaporates through very thin spots on the surface of the scale, or
through brush-like, expanded fringes on their tips. Many of these have
long been known to entomologists, because their divergence in form
from the ordinary scales attracted attention; and it was also observed
that they never occurred on the females, but only on the males. Their
significance, however, remained obscure until, by a happy chance, Fritz
Müller, in his Brazilian garden, discovered the fact that there are
butterflies which give off fragrance like a flower, and then close
investigation revealed to him the connexion between this delicate
odour and the so-called 'male scales.' One can convince oneself of the
correctness of the observation even in some of our own butterflies by
brushing the finger over the wing of a newly caught male Garden White
(_Pieris napi_). The finger will be found covered with a white dust,
the rubbed-off wing-scales, and it will have a delicate perfume of
lemon or balsam, thus proving that the fragrance adheres to the scales.
[Illustration: FIG. 54. A portion of the upper surface of the wing of
a male 'blue' (_Lycæna menalcas_); after Dr. F. Köhler. _bl_, ordinary
blue scales. _d_, scent-scales. Highly magnified.]
[Illustration: FIG. 55. _Zeuxidia wallacei_, male, showing four tufts
of long, bristle-like, bright yellow scent-scales (_d_) on the upper
surface of the posterior wing.]
In the last case, that is, among the Whites (Pieridæ) (Fig. 53, _a_),
the scent-scales are distributed fairly regularly over the upper
surface of the wing, and the same is true of our blue butterflies, the
Lycænnidæ whose minute lute-shaped scales are shown singly in Fig.
53, _d_, but in their natural position among the ordinary scales in
Fig. 54. In many other diurnal, and also in nocturnal Lepidoptera, the
fragrant scales are united into tufts and localized in definite areas.
They then often form fairly large spots, stripes, or brushes, which
are easily visible to the naked eye. Thus the males of our various
species of grass-butterflies (Satyridæ) have velvet-like black spots
on the anterior wings, while the fritillary, _Argynnis paphia_, has
coal-black stripes on four longitudinal ribs of the anterior wing
which are absent in the females, and which are composed of hundreds of
odoriferous scales. Certain large forest butterflies of South America,
resembling our _Apatura_, bear in the middle of the gorgeous green
shimmering posterior wing a thick expansible brush of long, bright
yellow scent-scales, and a similar arrangement obtains in the beautiful
violet butterfly of the Malay Islands, the _Zeuxidia wallacei_ depicted
in Fig. 55. In many of the Danaides, which we have already considered
in relation to mimicry, the scent apparatus is even more perfect, for
it is sunk in a fairly deep pocket on the posterior wings, and in this
the scent-producing, hair-like scales lie concealed until the butterfly
wishes to allow the fragrance to stream forth. In many South American
and Indian species of _Papilio_ the fragrant hairs are disposed in a
sort of mane on a fold of the edge of the posterior wing, and so on.
The diversity of these arrangements is extreme, and they are widely
distributed among both diurnal and nocturnal Lepidoptera, in the
latter sometimes in the form of a thick, glistening, white felt which
fills a folded-over portion of the edge of the posterior wing. In many
cases the perfume can be retained, and then, by a sudden turning out
of the wing-fold, be allowed to stream forth. But there are a great
many species of butterfly which do not possess odoriferous scales, and
they are often wanting in near relatives of fragrant species; they are
obviously of very late origin, and arose only after the majority of
our modern species were already differentiated. It often seems as if
they bore a compensatory relation to beauty of colour, somewhat in the
same way as many modestly coloured flowers develop a strong perfume,
while, conversely, many magnificently coloured flowers have no scent at
all. Although among butterflies, as among flowers, there are species
which possess both beauty and fragrance, yet our most beautiful diurnal
butterflies, the Vanessas, the Apaturas, and Limenitis, possess no
scent-scales; and many inconspicuous, that is, protectively coloured
nocturnal Lepidoptera, are strongly fragrant, like most night-flowers:
I need only mention the convolvulus hawk-moth (_Sphinx convolvuli_),
whose musk-like odour was known to entomologists long before the
discovery of scent-scales.
It is, however, always only in the males that this odoriferous
apparatus is present. It must not be believed on this account that this
fragrance has the significance of a means of attraction comparable to
the perfume of the flowers which induces butterflies to visit them;
indeed, we cannot assume that the odour carries to a distance, for,
as far as we can make out, it is perceptible only within a very short
radius, and this is indicated also by the manifold arrangements of the
odoriferous organs, which are all calculated to retain the fragrance,
and then--in the immediate neighbourhood of the female--to let it
suddenly stream forth. Obviously, this arrangement can have no other
significance than that of a sexual excitant; its use is to incline the
female to the male, to fascinate her, just as do the beautiful colours,
in regard to which we must draw the same inference. It is in this
direction that the already mentioned relation of compensation between
beautiful colours and pleasant odours is particularly interesting, for
it confirms our interpretation of the decorative colours as a means of
sexual excitement. The most delicately fragrant or the most beautifully
coloured males were those which most excited the females, and thus
most easily attained to reproduction. The expression used by Darwin,
that the females 'choose,' must be taken metaphorically; they do not
exercise a conscious choice, but they follow the male which excites
them most strongly. Thus there arises a process of selection among
these distinctively male characteristics.
If the odoriferous organs we have been discussing had merely been a
means of attraction, serving to announce the proximity of a member of
the species, then they should have occurred, not in the males but in
the females, for these are sought out by the males, not conversely.
The males are able to track their desired mates from great distances,
and many remarkable examples of this are known, some of them indeed
sounding almost fabulous. The females must therefore also exhale a
fragrance, and perhaps continually, but it is much more delicate,
carries extraordinarily far, and is quite imperceptible to our weak
sense of smell. It is possible that it streams out from all the scales
covering the wings and body, for, as I long ago pointed out, all the
scales retain a connexion with the living cells of the skin, however
minute these may be, and it is therefore quite possible that the cells
produce scent imperceptible by us, and let it exhale through the
ordinary scales, since the male scent-scales owe their ethereal oil to
the large gland-like cells of the hypodermis on which they are placed.
Here we see very clearly the difference between ordinary natural
selection and sexual selection. The male odoriferous organs depend on
the latter, for they do not serve for the maintenance of the species,
but are of advantage in the courting competitions among the males for
the possession of the females, while the assumed fragrant cells of the
females must depend on natural selection, since they are of general
importance for the mutual discovery of the sexes, which would otherwise
be in most cases impossible. This hypothetical 'species scent,' as
we may call it, is first of all useful in securing the existence of
the species, and must therefore be referred to natural selection. The
other, the 'male scent,' might be, and actually is, wanting in many
species, although it may be necessary to reproduction in cases where it
has become a male specific character, and could not be absent from any
male without dooming him to sterility.
That the 'species scent' really exists admits of no doubt, although
we may be unable to perceive it. Entomologists have long been in the
habit of catching the males of the rarer Lepidoptera, especially of the
nocturnal forms, by freely exposing a captive female. Some years ago
I kept for some time in my study, with a view to certain experiments,
females of the eyed hawk-moth (_Smerinthus ocellatus_), and placed them
at first, without any special intention, in a gauze-covered vessel near
the open window. The very next morning several males had gathered and
were sitting on the window-sill, or on the wall of the room close to
the vessel, and by continuing the experiment I caught, in the course
of nine nights, no fewer than forty-two males of this species, which
I had never believed to be so numerous in the gardens of the town. The
males of the nocturnal Lepidoptera obviously possess an incredibly
delicate organ of smell, and its bearers, the antennæ, are usually
larger and more complex in structure in the male sex than in the female.