THE ORIGIN AND THE EXTINCTION OF SPECIES 349 



adaptatious. for we have seen that it depends upon the co-operation 

 of different grades of selective processes, that in many cases it is 

 produced solely by germinal selection, but that in very many more 

 personal selection plays the chief part, whether in bringing about 

 sexual adaptations, or adaptations to the conditions of existence. 



When we have thus recognized that the origin of a variation in 

 a definite direction results as inevitably when it is called forth by the 

 indirect influence of conditions, that is, through the need for a new 

 adaptation, as when it is induced in the germ-plasm by direct causes such 

 as those of climate, we shall not be disposed to estimate very highly the 

 part played by mutual sterility in the origin of species. We shall 

 rather be inclined to assign it a role at a later stage, after the 

 separation of the forms has taken place, and this view is supported by 

 the fact of the mutual sterility of most nearly related species, and by 

 the theoretical consideration that the frequency of hybrids, even if 

 these are always eliminated in the struggle for existence, must signify 

 a loss for both the parent species. But no certain conclusion can be 

 based upon either of these arguments — not upon the theoretical one, 

 because here again we are unable to estimate the extent of this loss ; 

 and not upon the argument from fact, because the results of experi- 

 ments in crossing animals have generally been overestimated, since 

 we are apt to regard the most nearly related animals that are at our 

 disposal as being very closely related. Thus, for instance, horse and 

 ass, horse and zebra are undoubtedly rightly included within the 

 same genus, but the fact that there are several species of zebra in 

 Africa gives us an idea of the number of transition stages that may 

 have existed between the horse and the zebra. Entomologists have 

 sometimes reared hybrids between the most nearly related indigenous 

 species of hawk-moth of the genus Siniervnthtis — hybrids of Smerinthus 

 ocellata, the eyed hawk-moth, and Smerinthus 2^opuli, the poplar 

 hawk-moth. I have myself made many experiments of this kind, and 

 have often succeeded in getting the two species to pair and even to 

 deposit eggs, but I have never seen a caterpillar emerge from them. 

 The hybrids do occur, however, and they have been repeatedly 

 obtained by Standfuss. In external appearance tliey are intermediate 

 between the parent forms, but with marked divergences, thus, for 

 instance, the beautiful blue eye on the posterior wing of S. ocellata 

 (Fig. 5, vol. i. p. 69) may have almost disappeared or be only indicated. 

 They are sterile. But we know three species of Smerinthus in North 

 America, which are all much nearer to S. ocellata than S. pojmli is, 

 for they all possess the eye-spot referred to, although it is less well 

 developed. The proof that the most nearly related species do not 



