Part 25

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.

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.

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.

Butterflies are by no means the only creatures that produce a peculiar 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.