Descent of Man, and Selection in Relation to Sex, v.1

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414 SEXUAL SELECTIOX. PART II. they could not escape so swiftly from their enemies, we can understand how they alone might originally have acquired through natural selection and sexually­limited inheritance their present protective colours. But except on the principle of these variations having been transmitted exclusively to the female offspring, we cannot understand why the males should have re­mained dull-coloured; for it would surely not have been in any way injurious to each individual male to have partaken by inheritance of the protective colours of the female, and thus to have had a better chance of escaping destruction. In a group in which brilliant colours are so common as with butterflies, it cannot be supposed that the males have been kept dull-coloured through sexual selection by the females rejecting the individuals which were rendered as beautiful as them­selves. We may, therefore, conclude that in these cases inheritance by one sex is not due to the modification through natural selection of a tendency to equal inherit­ance by both sexes. It may be well here to give an analogous case in another Order, of characters acquired only by the female, though not in the least injurious, as far as we can judge, to the male. Amongst the Phasmidre, or spectre-insects, nfr. wallace states that "it is often the females alone "that so strikingly resemble leaves, while the males show "only a rude approximation." Now, whatever may be the habits of these insects, it is highly improbable that it c?uld be disadvantageous to the males to escape de­tectwn by resembling leaves.31 Hence we may conclude 31 See 1\lr. Wallace in 'Westminster Review,' July, 1867, p. 11 and 3!. ~he male of no butterfly, as Mr. Wallace informs me, is known to d1ffer m col?ur, ~s a protection, from the female ; and he asks me how I can explam th1s fact on the principle that one sex alone bas varied and has transmitted its variations exclusively to the same sex, without Crr.AP. XI. BUTTERFLIES AND MOTHS. 415 that the females alone in this latter as in the previous cases originally varied in certain characters; these cha­racters having been preserved and augmented through ordinary selection for the sake of protection and from the first transmitted to the female off:;pring alone. Bright Colours of Caterpillars.-'Vhilst reflecting on the beauty of many butterflies, it occurred to me that some caterpillars were splendidly coloured, and as sexual selection could not possibly have here acted, it appeared rash to attribute the beauty of the mature insect to this agency, unless the bright colours of their larvre could be in some manner explained. In the first place it may be observed that the colours of caterpillars do not stand in any close correlation with those of the mature insect. Secondly, their bright colours do not the aid of selection to check the variations being inherited by the other sex .. No doubt if it could be shewn that the females of very many species bad been rendered beautiful through protective mimickry, but that this has never occuned with the males, it would be a serious difficulty. But the number of cases as yet known hardly suffices for a fair judgment. We can see that the males, from having the power of flying more swiftly, and thus escaping danger, would not be so likely as the females to have had their colours modified for the sake of protec­tion; but this would not iu the least have interfered with their receivin"' protective colours through inheritance from the females. In the second place, it is probable that sexual selection would actually tend to prevent a beautiful male from becoming obscure, for tl1e less brilliant individuals would be less attractive to the females. Supposing that the beauty of the male of any species had been mainly acquired through sexual s?lection, yet if this beauty likewise served as a protection, the acquisi­twn would have been aided by natural selection. But it would be quite beyond our power to distinguish between the two processes of sexual and ordinary selection. Hence it is not likely that we should be abl~ to adduce cases of the males having been rendered brilliant exclus1~ely through protective mimickry, though this is comparatively easy With the females, which have rarely or never been rendered beau­tiful, as far as we can judge, for the sake of sexual attraction, although they have often received beauty through inheritance from their male parents.