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 effects of intercrossing," which has been so pro- 

 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 shown to exist between the effects 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 we will therefore address ourselves. 



The Influence of Natural Selection upon Sterility and Fertility. 



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

 sterility of incipient species would be useful to them when 

 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 will be 

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

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

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

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

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

 influenced against the view by many considerations which 

 seemed to render such an origin of the sterility or infertility 

 of species when intercrossed very improbable. The fact that 

 species which occupy distinct areas, and which nowhere come 

 in contact with each other, are often sterile when 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 



