ORIGIN OF THE SPECIFIC TYPE 339 



diminished fertility of the crossings could be established simultaneously 

 with the other variations. This would be useful, since pure and 

 well-defined variations would be better adapted to their life-conditions 

 than hybrids, and would become increasingly so in the course of 

 generations. But as soon as it is useful it must actually come about, 

 if that is possible at all. It may be, however, as we have said before, 

 that the two divergent forms depend merely upon quantitative 

 variations of the already existing characters ; sexual attraction, 

 whether it depends upon very delicate chemical substances, or on 

 odours, or on mutual complementary tensions unknown to us, will 

 always fluctuate upwards and downwards, and plus or minus deter- 

 minants, which lie at the root of these unknown characters in 

 the germ-plasm, must continually present themselves and form the 

 starting-point for selection-processes of a germinal and personal 

 kind, which may bring about sexual antipathy and mutual sterility 

 between the varieties. I therefore consider Romanes's idea correct 

 in so far that separation between species is in many cases accompanied 

 by increasing sexual antipathy and mutual sterility. While Romanes 

 supposed that ' natural selection could in no case have been the 

 cause ' of the sterility, I believe, on the contrary, that it could only 

 have been produced by natural selection ; it arises simply, as all 

 adaptations do, through personal selection on a basis of germinal 

 selection, and it is not a preliminary condition of the separation 

 of species, but an adaptation for the purpose of making as pure 

 and clean a separation as possible. It is obviously an advantage 

 for both the divergent tendencies of variation that they should 

 intermingle as little as possible. This is corroborated by the fact 

 that by no means all the marked divergences of species are accom- 

 panied by sexual alienation, and that the mutual sterility so frequently 

 seen is not an inevitable accompaniment of differences in the rest 

 of the organism. 



That this is not the case is very clearly proved by our domesti- 

 cated animals. The differences in structure between the various breeds 

 of pigeon and poultry are very great, and breeds of dog also diverge 

 from one another very markedly, especially in shape and size of 

 body. Yet all these are fertile with one another, and they yield 

 fertile offspring. But they are products of artificial selection by 

 man, and he has no interest in making them mutually sterile, so 

 that they have not been selected with a view to sexual alienation, 

 but in reference to the other characters. The segregation of animal 

 species into several sub-species on the same area is probably usually 

 accompanied by sexual antipathy, since in this case it would be 



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