nection with the necessities of life. Neither touches the origin of the variation, but both express laws thereof or methods by which it is accumulated. The inherent tendency to vary, whether in external or adaptive structure, or internal or reproductive character, is simply an observed fact, the causes of which we are endeavoring to analyze.
Physiological selection is remarkably exemplified in insects, and probably in no other class are the modifications which may be attributed to it more easily studied; for in no other class are the genitalia of the male so variable or so complex. There has so far been no attempt to homologize the different parts in the different orders of insects, so that they have received different names according to individual authors. Ordinarily there are two pairs of claspers, themselves very variable, associated with sundry hooks and tufts of hair. There are families, as in the Cecidomyidæ, among the Diptera, in which many species are almost, and others absolutely, indistinguishable except by the differences in the male genitalia. In all other orders there are an immense number of forms which can only be distinguished by a careful study of those organs. Descriptive entomology to-day, which does not take account of these organs, is in fact almost valueless, and we must necessarily assume that, where there is differentiation of structure in these important parts, it implies a corresponding modification on the part of some associated female, even where no other differentiated characters are to be detected, and upon Romanes's law such must be looked upon as physiological varieties, and will be counted good species in proportion as the differentiation involves other observable characters or as their life-habits determine.
Sexual Selection.—The part of sexual selection in inducing variation may next be considered. While it is evidently at the bottom of the diversity in sex so common among many animals, it is difficult to see how it can play any very important part in the differentiation of species, except on the hypothesis that the greater the differentiation between the sexes the greater the tendency to vary in the offspring. In no class of organism is this factor more notable than in insects, and volumes might be written to record the interesting and curious facts in this class alone. As a general rule it may be said that with insects, as with other animals, it acts chiefly in inducing secondary sexual characteristics in the male, and in simplifying the characteristics of the female. Nowhere do we find greater contrasts between the sexes, involving almost every organ, both colorationally and structurally. Where color is affected, the greater brilliancy almost always belongs to the male sex, as in birds. So, where song or sound is employed to attract, the sound-organs are either peculiar to, or most highly developed in, the males. As in higher animals,