494 THE POPULAR SCIENCE MONTHLY. 



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 Lepi- 

 doptera, of either the membrane of the wing or the scaly cover- 

 ing ; 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 selec- 

 tion 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 differ- 

 entiation of sex in itself has been an important element in varia- 

 tion. 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 pro- 

 gressive, 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 pre- 

 ponderate, 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, particu- 

 larly, 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 StylopidcB j in very many 

 Lepidoptera and Coleoptera ; in the females of the Coccidcp, 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 



