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Show 412 SEXUAL SELECTION. PART II. hy 1\ir. vVnllace 26 in the Malayan region, and by l\ir. Trimen in South Africa. As some writers 27 have felt much difficulty in understanding how the first steps in the process of mimickry could have been effected through natural selection, it may be well to remark that the process probably has never commenced with forms widely dissimilar in colour. But with t·wo species moderately like each other, the closest resemblance if beneficial to either form could readily be thus gained; and if the imitated form was subsequently and gradually modified through sexual selection or any other means, the imitating form would be led along the same track, and thus be modified to almost any extent, so that it might ultimately assume an appearance or colouring wholly unlike that of the other members of the group to which it belonged. As extremely slight variations in colour would not in many cases suffice to render a species so like another protected species as to lead to its preservation, it should be remembered that many species of Lepidoptera are liable to considerable and abrupt variations in colour. A few instances have been given in this chapter; but under this point of view Mr. Bates' original paper on mimickry, as well as 1\Ir. vVallace's papers, should be consulted. In the foregoing cases both sexes of the imitating species resemble the imitated; but occasionally the ~G 'Transact. Linn. Soc.' vol. xxv. 1865, p. 1; also 'Transact. Ent. Soc.' vol. iv. (3rd series), 1867, p. 301. 2i See an ingenious article entitled, "Difficulties of the Theory of Natural Selection," in the 'Month,' l8G9. The writer strangely supposes that I attribute the variations in colour of the Lepidoptera, by whieh certain species belonging to distinct families l1ave come to resemble others, to reversion to a common progenitor; but there is no more reason to attribute these variations to reversion than in the case <>f any ordinary variation. CHAP. XI. BUTTERFLIES AND MOTHS. 413 female alone mocks a brilliantly-coloured and protected species inhabiting the same district. Consequently the female differs in colour from her own male, and, which is a rare and anomalous circumstance, is the more brightly-coloured of the two. In all the few species of Pieridrn, in which the female is more conspicuously coloured than the male, she imitates, as I am informed by Mr. Wallace, some protected species inhabiting the· same region. The female of Diadema anomala is rich purple-brown with almost the whole surface glossed with satiny blue, and she closely imitates the Euplcea midamus, "one of the commonest butterflies of the East;" whilst the male is bronzy or olive-brown, with only a slight blue gloss on the outer parts of the wings.28 Both sexes of this Diadema and of IJ. bolina follow the same habits of life, so that the differences in colour between the sexes cannot be accounted for by exposure to different conditions ; 29 even if this explanation were admissible in other instances.30 The above cases of female butterflies which are more brightly-coloured than the males, shew us, firstly, that variations have arisen in a state of nature in the female sex, and have been transmitted exclusively, or almost exclusively, to the same sex; and, secondly, that this form of inheritance has not been determined through natural selection. For if we assume that the females, before they became brightly coloured in imitation of some protected kind, were exposed during each season for a longer period to danger than the males; or if we assume that 2s Wallace, "Notes on Eastern Butterflies," 'Transact. Ent. Sec.• 1869, p. 287. 29 Wallace, in 'Westminster Review,' July, 1867, p. 37; and in 'Journal of Travel and Nat. l-Iist.' vol. i. 1868, p. 88. 30 See remarks by Messrs. Bates and Wallace, in 'Proc. Ent. So< ... Nov. 19, 1866, p. xxxix. |
