The Descent of Man – Day 83 of 151

I have as yet only referred to the species in which the males are brighter coloured than the females, and I have attributed their beauty to the females for many generations having chosen and paired with the more attractive males. But converse cases occur, though rarely, in which the females are more brilliant than the males; and here, as I believe, the males have selected the more beautiful females, and have thus slowly added to their beauty. We do not know why in various classes of animals the males of some few species have selected the more beautiful females instead of having gladly accepted any female, as seems to be the general rule in the animal kingdom: but if, contrary to what generally occurs with the Lepidoptera, the females were much more numerous than the males, the latter would be likely to pick out the more beautiful females. Mr. Butler shewed me several species of Callidryas in the British Museum, in some of which the females equalled, and in others greatly surpassed the males in beauty; for the females alone have the borders of their wings suffused with crimson and orange, and spotted with black. The plainer males of these species closely resemble each other, shewing that here the females have been modified; whereas in those cases, where the males are the more ornate, it is these which have been modified, the females remaining closely alike.

In England we have some analogous cases, though not so marked. The females alone of two species of Thecla have a bright-purple or orange patch on their fore-wings. In Hipparchia the sexes do not differ much; but it is the female of H. janira which has a conspicuous light-brown patch on her wings; and the females of some of the other species are brighter coloured than their males. Again, the females of Colias edusa and hyale have “orange or yellow spots on the black marginal border, represented in the males only by thin streaks”; and in Pieris it is the females which “are ornamented with black spots on the fore-wings, and these are only partially present in the males.” Now the males of many butterflies are known to support the females during their marriage flight; but in the species just named it is the females which support the males; so that the part which the two sexes play is reversed, as is their relative beauty. Throughout the animal kingdom the males commonly take the more active share in wooing, and their beauty seems to have been increased by the females having accepted the more attractive individuals; but with these butterflies, the females take the more active part in the final marriage ceremony, so that we may suppose that they likewise do so in the wooing; and in this case we can understand how it is that they have been rendered the more beautiful. Mr. Meldola, from whom the foregoing statements have been taken, says in conclusion: “Though I am not convinced of the action of sexual selection in producing the colours of insects, it cannot be denied that these facts are strikingly corroborative of Mr. Darwin’s views.” (23. ‘Nature,’ April 27, 1871, p. 508. Mr. Meldola quotes Donzel, in ‘Soc. Ent. de France,’ 1837, p. 77, on the flight of butterflies whilst pairing. See also Mr. G. Fraser, in ‘Nature,’ April 20, 1871, p. 489, on the sexual differences of several British butterflies.)

As sexual selection primarily depends on variability, a few words must be added on this subject. In respect to colour there is no difficulty, for any number of highly variable Lepidoptera could be named. One good instance will suffice. Mr. Bates shewed me a whole series of specimens of Papilio sesostris and P. childrenae; in the latter the males varied much in the extent of the beautifully enamelled green patch on the fore-wings, and in the size of the white mark, and of the splendid crimson stripe on the hind-wings; so that there was a great contrast amongst the males between the most and the least gaudy. The male of Papilio sesostris is much less beautiful than of P. childrenae; and it likewise varies a little in the size of the green patch on the fore-wings, and in the occasional appearance of the small crimson stripe on the hind-wings, borrowed, as it would seem, from its own female; for the females of this and of many other species in the Aeneas group possess this crimson stripe. Hence between the brightest specimens of P. sesostris and the dullest of P. childrenae, there was but a small interval; and it was evident that as far as mere variability is concerned, there would be no difficulty in permanently increasing the beauty of either species by means of selection. The variability is here almost confined to the male sex; but Mr. Wallace and Mr. Bates have shewn (24. Wallace on the Papilionidae of the Malayan Region, in ‘Transact. Linn. Soc.’ vol. xxv. 1865, pp. 8, 36. A striking case of a rare variety, strictly intermediate between two other well-marked female varieties, is given by Mr. Wallace. See also Mr. Bates, in ‘Proc. Entomolog. Soc.’ Nov. 19, 1866, p. xl.) that the females of some species are extremely variable, the males being nearly constant. In a future chapter I shall have occasion to shew that the beautiful eye-like spots, or ocelli, found on the wings of many Lepidoptera, are eminently variable. I may here add that these ocelli offer a difficulty on the theory of sexual selection; for though appearing to us so ornamental, they are never present in one sex and absent in the other, nor do they ever differ much in the two sexes. (25. Mr. Bates was so kind as to lay this subject before the Entomological Society, and I have received answers to this effect from several entomologists.) This fact is at present inexplicable; but if it should hereafter be found that the formation of an ocellus is due to some change in the tissues of the wings, for instance, occurring at a very early period of development, we might expect, from what we know of the laws of inheritance, that it would be transmitted to both sexes, though arising and perfected in one sex alone.

On the whole, although many serious objections may be urged, it seems probable that most of the brilliantly-coloured species of Lepidoptera owe their colours to sexual selection, excepting in certain cases, presently to be mentioned, in which conspicuous colours have been gained through mimicry as a protection. From the ardour of the male throughout the animal kingdom, he is generally willing to accept any female; and it is the female which usually exerts a choice. Hence, if sexual selection has been efficient with the Lepidoptera, the male, when the sexes differ, ought to be the more brilliantly coloured, and this undoubtedly is the case. When both sexes are brilliantly coloured and resemble each other, the characters acquired by the males appear to have been transmitted to both. We are led to this conclusion by cases, even within the same genus, of gradation from an extraordinary amount of difference to identity in colour between the two sexes.

But it may be asked whether the difference in colour between the sexes may not be accounted for by other means besides sexual selection. Thus the males and females of the same species of butterfly are in several cases known (26. H.W. Bates, ‘The Naturalist on the Amazons,’ vol. ii. 1863, p. 228. A.R. Wallace, in ‘Transactions, Linnean Society,’ vol. xxv. 1865, p. 10.) to inhabit different stations, the former commonly basking in the sunshine, the latter haunting gloomy forests. It is therefore possible that different conditions of life may have acted directly on the two sexes; but this is not probable (27. On this whole subject see ‘The Variation of Animals and Plants under Domestication,’ 1868, vol. ii. chap. xxiii.) as in the adult state they are exposed to different conditions during a very short period; and the larvae of both are exposed to the same conditions. Mr. Wallace believes that the difference between the sexes is due not so much to the males having been modified, as to the females having in all or almost all cases acquired dull colours for the sake of protection. It seems to me, on the contrary, far more probable that it is the males which have been chiefly modified through sexual selection, the females having been comparatively little changed. We can thus understand how it is that the females of allied species generally resemble one another so much more closely than do the males. They thus shew us approximately the primordial colouring of the parent-species of the group to which they belong. They have, however, almost always been somewhat modified by the transfer to them of some of the successive variations, through the accumulation of which the males were rendered beautiful. But I do not wish to deny that the females alone of some species may have been specially modified for protection. In most cases the males and females of distinct species will have been exposed during their prolonged larval state to different conditions, and may have been thus affected; though with the males any slight change of colour thus caused will generally have been masked by the brilliant tints gained through sexual selection. When we treat of Birds, I shall have to discuss the whole question, as to how far the differences in colour between the sexes are due to the males having been modified through sexual selection for ornamental purposes, or to the females having been modified through natural selection for the sake of protection, so that I will here say but little on the subject.

In all the cases in which the more common form of equal inheritance by both sexes has prevailed, the selection of bright-coloured males would tend to make the females bright-coloured; and the selection of dull-coloured females would tend to make the males dull. If both processes were carried on simultaneously, they would tend to counteract each other; and the final result would depend on whether a greater number of females from being well protected by obscure colours, or a greater number of males by being brightly-coloured and thus finding partners, succeeded in leaving more numerous offspring.

In order to account for the frequent transmission of characters to one sex alone, Mr. Wallace expresses his belief that the more common form of equal inheritance by both sexes can be changed through natural selection into inheritance by one sex alone, but in favour of this view I can discover no evidence. We know from what occurs under domestication that new characters often appear, which from the first are transmitted to one sex alone; and by the selection of such variations there would not be the slightest difficulty in giving bright colours to the males alone, and at the same time or subsequently, dull colours to the females alone. In this manner the females of some butterflies and moths have, it is probable, been rendered inconspicuous for the sake of protection, and widely different from their males.

I am, however, unwilling without distinct evidence to admit that two complex processes of selection, each requiring the transference of new characters to one sex alone, have been carried on with a multitude of species,–that the males have been rendered more brilliant by beating their rivals, and the females more dull-coloured by having escaped from their enemies. The male, for instance, of the common brimstone butterfly (Gonepteryx), is of a far more intense yellow than the female, though she is equally conspicuous; and it does not seem probable that she specially acquired her pale tints as a protection, though it is probable that the male acquired his bright colours as a sexual attraction. The female of Anthocharis cardamines does not possess the beautiful orange wing-tips of the male; consequently she closely resembles the white butterflies (Pieris) so common in our gardens; but we have no evidence that this resemblance is beneficial to her. As, on the other hand, she resembles both sexes of several other species of the genus inhabiting various quarters of the world, it is probable that she has simply retained to a large extent her primordial colours.

Finally, as we have seen, various considerations lead to the conclusion that with the greater number of brilliantly-coloured Lepidoptera it is the male which has been chiefly modified through sexual selection; the amount of difference between the sexes mostly depending on the form of inheritance which has prevailed. Inheritance is governed by so many unknown laws or conditions, that it seems to us to act in a capricious manner (28. The ‘Variation of Animals and Plants under Domestication,’ vol. ii. chap. xii. p. 17.); and we can thus, to a certain extent, understand how it is that with closely allied species the sexes either differ to an astonishing degree, or are identical in colour. As all the successive steps in the process of variation are necessarily transmitted through the female, a greater or less number of such steps might readily become developed in her; and thus we can understand the frequent gradations from an extreme difference to none at all between the sexes of allied species. These cases of gradation, it may be added, are much too common to favour the supposition that we here see females actually undergoing the process of transition and losing their brightness for the sake of protection; for we have every reason to conclude that at any one time the greater number of species are in a fixed condition.

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