Page:The Zoologist, 4th series, vol 4 (1900).djvu/573

Rh For instance, in those wonderful cases which are found so frequently among insects, the habits of each species are so intimately correlated with its abnormal structure and colouring,(n4) that it is unreasonable to believe that these characters have been developed independently by different factors; the latter by natural selection, and the former by the "evolution of active mimicry," whatever that may mean. These special structures cannot be accounted for by "active mimicry," neither can they be explained by any general theory of internal or external causes, for, as the late Mr. Romanes has well remarked, "Were it not that some of Darwin's critics have overlooked the very point wherein the great value of protective colouring as evidence of natural selection consists, it would be needless to observe that it does so in the minuteness of the protective resemblance which in so many cases is presented. Of course, where the resemblance is only very general, the phenomena might be ascribed to mere coincidence, of which the instincts of the animal have taken advantage. But in the measure that the resemblance becomes minutely detailed, the supposition of mere coincidence is excluded, and the agency of some specially adaptive cause demonstrated" ('Darwin and after Darwin,' p. 318, note).

Thus a strong objection may be lodged against the whole suggestion of active mimicry, as opposed to that of natural selection, in that the former suggestion is essentially incomplete and cannot explain all the facts of the case. Let us take the instance of the leaf-butterflies of the genus Kallima, of which Mr. Distant says: "The partiality of this insect for settling on dry and withered leaves appears a true instance of active mimicry " (l.c., 1899, p. 531).(n6) Upon the theory of natural selection (granted the undisputed facts of variation and the struggle for existence), it is easy to understand that any marked variations in the direction of leaf-like shapes or markings, which would afford better concealment, would tend to be preserved and further augmented, both by heredity and by the increased keenness of enemies, until the present admirable resemblance had been arrived at. "But, as Mr. Badenoch has well enquired, 'Of what avail would be the disguise were the insect prone to settle upon a flower, or green leaf, or other inappropriate surface?'"(l.c.). Quite true; and the fact that the insect is not so inclined is readily explainable by the Darwinian theory; for it is clear that a much greater proportion of those individuals which were prone to render themselves conspicuous by settling on inappropriate surfaces would be picked off by their enemies than of those which selected suitable resting places; and thus, by a gradual process of elimination, the progeny of those individuals, which possessed a well-defined instinct to settle upon withered leaves, &c,