VII ON THE INFERTILITY OF CROSSES 173 



a fact of great importance in considering the origin of species 

 by natural selection, since it shows us that, so soon as a slight 

 differentiation of form or colour has been effected, isolation 

 will at once arise by the selective association of the animals 

 themselves; and thus the great stumbling-block of "the 

 swamping eff'ccts of interprossing," Avhich has been so pvo- 

 minently brought forward by many naturalists, will be com- 

 pletely obviated. 



If now we combine with this fact the correlation of colour 

 with important constitutional peculiarities, and, in some cases, 

 with infertility ; and consider, further, the curious parallelism 

 that has been sho\vn to exist between the eiTects of changed 

 conditions and the intercrossing of varieties in producing 

 either an increase or a decrease of fertility, we shall have 

 obtained, at all events, a starting-point for the production of 

 that infertility which is so characteristic a feature of distinct 

 species when intercrossed. All we need, now, is some means 

 of increasing or accumulating this initial tendency ; and to a 

 discussion of this problem Ave Avill therefore address ourselves. 



The Influence of Natural Selection upon Sterility and Fertility. 



It Avill occur to many persons that, as the infertility or 

 sterility of incipient species Avould be useful to them Avhen 

 occupying the same or adjacent areas, by neutralising the 

 effects of intercrossing, this infertility might have been in- 

 creased by the action of natural selection ; and this Avill be 

 thought the more probable if Ave admit, as we have seen 

 reason to do, that variations in fertility occur, perhaps as 

 frequently as other variations. Mr. DarAvin tells us that, at 

 one time, this appeared to him probable, but he found the 

 problem to be one of extreme complexity ; and he Avas also 

 influenced against the view by many considerations Avhich 

 seemed to render such an origin of the sterility or infertility 

 of species Avhen intercrossed very improbable. The fact that 

 species Avhich occupy distinct areas, and Avhich nowhere come 

 in contact Avith each othei', are often sterile Avhen crossed, is one 

 of the difficulties ; but this may perhaps be overcome by the 

 consideration that, though now isolated, they may, and often 

 must, have been in contact at their origination. More 

 important is the objection that natural selection could not 



