also, so in insects, we find offensive organs highly developed in the male, and either lacking or but partially developed in the female, wherever the struggle for the possession of the female is by force or strength. It has evolved scent-organs in the various parts of the body, causing modification, especially in the Lepidoptera, of either the membrane of the wing or the scaly covering; it has induced profound modification in the structure of the legs, whether the anterior, middle, or posterior pair, and whether in the whole number or some part of it, or in its covering. The subject has been so fully treated by Darwin, however, that it is not necessary to elaborate it further in this connection. Strictly speaking, it may be said to act in two ways, viz., by conflict of the males for possession of the female, or by attractiveness, the former being most conspicuous among mammals, the latter among birds, and both coming conspicuously into play among insects. It is rather difficult to define the limit of sexual selection as a factor in evolution, but I would not confound it with another factor, not hitherto generally recognized, but which I think must be all-powerful, namely, sexual differentiation.
Sexual Differentiation.—It seems evident that the mere differentiation of sex in itself has been an important element in variation. The principle elaborated by Brooks as a modification of the theory of pangenesis is a good one, and in the main the male may be said to be the more complex and to represent the progressive, and the females the more simple and to represent the conservative element in nature. "When the conditions of life are favorable, the female preponderates, and exercises a conservative influence. When the conditions are unfavorable, the males preponderate, and with their greater tendency to vary induce greater plasticity in the species, and hence greater power of adaptation. Sexual differentiation may, I think, be used to include many other variations and differentiations not otherwise satisfactorily accounted for, and to express the law of the interaction of the sexes upon one another, inducing great differentiation entirely apart from the struggle of the males for the possession of the females, or the struggle for existence. Among insects, particularly, though the same is true among other classes, we find many illustrations of this that can hardly be explained by the other forms of selection.
A few of the more notable in Hexapods may be instanced, as the degraded form of the female in Stylopidæ; in very many Lepidoptera and Coleoptera; in the females of the Coccidæ, in Homoptera, etc. In most of these cases it is the female which has been modified, without any very special modification in the male, though it is a general rule that, in proportion as the female is degradational and stationary, the organs which permit him to