Sexual Selection

Abstract

IN the first volume of “The Descent of Man,” at page 396, Mr Darwin says, referring to butterflies, that “the lower surface (of the wings) generally affords to entomologists the most useful character for detecting the affinities of the various species.” I think, also, that this lower surface might afford another link in the chain of argument by which Mr. Darwin supports his theory of Sexual Selection. Thus, for example, to speak of British species only, in the cabbage butterflies, the under surface of the wings is alike in both sexes of Pieris Brassicæ. The black spots, however, which appear on both surfaces of the fore wing of the female vanish from the upper surface of that of the male, probably because the female has some dislike to them. There is no difference in food-plant, habit, or need of protection here; the only explanation seems to be a whim of the female or a whim of nature, and we have lately discarded all thought of nature being freakish. In P. Rapæ and P. Napi a similar difference prevails, though less constant and in a degree less marked. In the allied Anthocharis Cardamines the under surface of both sexes is alike, notwithstanding the vast difference of their upper surfaces. When these butterflies alight and close their wings, the under surfaces of the hind wings are alone visible, and these are, apparently, the parts of the insect modified for the sake of protection. The simple yellow in Brassicæ and Rapæ, the green-veined yellow in Napi, the green marbling in Cardamines, of the under sides of the hind wings, are well fitted to conceal those insects as they settle on the wild flowers which they prefer.