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Show 110 SEXUAL SELECTION. PART II. known laws or conditions, that they seem to us to be most capricious in their action ; 23 and we can so far understand how it is that with closely-allied species the sexes of some differ to an astonishing degree, whilst. the sexes of others are identical in colour. As the successive steps in the process of variation are necessarily all 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 no difference at all between the sexes of the species within the same group. These cases of gradation 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. With respect to the differences between the females of the species in the same genus or family, we can perceive that they depend, at least in part, on the females partaking of the colours of their respective males. This is well illustrated in those groups in which the males are ornamented to an extraordinary degree; for the females in these groups generally partake to a certain extent of the splendour of their male partners. Lastly, we continually find, as already remarked, that the females of almost all the species in the same genus,. or even family, resemble each other mueh more closely in colour than do the males; and this indicates that. the males have undergone a greater amount of modification than the females. 23 'The Variation of Animals anu PLmts under Domestication, .. vol. ii. chap. xii. p. 17. • CrrAP. XI. BUTTERFLIES AND MOTHS. 411 Mimiclcry.-This principle was first made clear in an admirable paper by Mr. Bates,24 who thus threw a flood. of light on many obscure problems. It had previously been observed that certain butterflies in S. America belonging to quite distinct families, resembled the Heli-· conidre so closely in every stripe and shade of colour that they could not be distinguished except by an experienced entomologist. As the Heliconidre are coloured in their usual manner, whilst the others depart from the usual colouring of the groups to which they belong, it is clear that the latter are the imitators, and the Heliconidre the imitated. Mr. Bates further observed that the imitating species are comparatively rare, whilst the imitated swarm in large numbers ; the two sets living mingled together. From the fact of the Helieonidre being conspicuous and beautiful insects, yet so numerous in individuals and species, he concluded that they must be protected from the attacks of birds by some secretion or odour ; and this hypothesis has now been confirmed by a considerable body of curious evidence.25 From these considerations Mr. Bates inferred that the butterflies which imitate the protected 8pecies had acquired their present marvellously deceptive appearance, through variation and natural selection, in order to be mistaken for the protected kinds and thus to escape being devoured. No explanation is here attempted of the brilliant colours of the imitated, but only of the imitating butterflies. We must account for the colours of the former in the same general manner, as in the cases previously discussed in this chapter. Since the publication of Mr. Bates' paper,. similar and equally striking facts have been observed ~4 ' Transact. Linn. Soc.' vol. xxiii. 1862, p. 495 . ~5 'Proc. Ent. Soc.' Dec. 3rd, 1866, p. xlv. |