the Lycaenidae, or the Hesperidae, there are forms almost or quite indistinguishable on external examination, but a glance at the genitalia suffices to distinguish numerous species, while on the contrary among Pieridae a great range of species show scarcely any difference in these respects: and again in occasional species the genitalia show very considerable variations. The proposition that animals and plants are on the whole divisible into definite and recognisable species is an approximation to the truth. Such a statement is readily defensible, whereas to assert the contrary would be palpably absurd. For example, a very competent authority lately wrote: "In the whole Lepidopterous fauna of England there is no species of really uncertain limits."[7] Others may be disposed to make certain reservations, but such exceptions would be so few as scarcely to impair the validity of the general statement. The declaration might be extended to other orders and other lands. We know, of course, that the phenomenon of specific diversity is complicated by local differentiation: that, in general, forms which cannot disperse themselves freely exhibit a multitude of local races, and that of these some are obviously adaptative, and that a few even owe their peculiarity to direct environmental effects. Every systematist also is perfectly aware that in dealing with collections from little explored countries the occurrence of polymorphism or even of sporadic variation may make the practical business of distinguishing the species difficult and perhaps for the time impossible; still, conceding that a great part of the diversity is due to geographical differentiation, and that some is sporadic variation, our experience of our own floras and faunas encourages the belief that if we were thoroughly familiar with these exotic productions it would usually be possible to assign their specific limitations with an approach to certainty. For apart from any question of the justice of these wider inferences, if we examine the phenomenon of specificity as it appears in those examples which are nearest to hand, surely we find signs in plenty that specific distinction is no mere consequence of Natural Selection. The strength of this proposition has lain mainly in the appeal to ignorance. Steadily with the growth of knowledge has its cogency diminished, and such a belief could only have been formulated at a time when the facts of variation were unknown.