174 DARWINISM chap. 



possibly have produced the difference that often occurs 

 between reciprocal crosses, one of these being sometimes 

 fertile, while the other is sterile. The extremely different 

 amounts of infertility or sterility between different species 

 of the same genus, the infertility often bearing no proportion 

 to the difference between the species crossed, is also an 

 important objection. But none of these objections would 

 have much weight if it could be clearly shown that natural 

 selection is able to increase the infertility variations of in- 

 cipient species, as it is certainly able to increase and develop 

 all useful variations of form, structure, instincts, or ha,bits. 

 Ample causes of infertility have been shown to exist, in the 

 nature of the organism and the laws of correlation; the 

 agency of natural selection is only needed to accumulate 

 the effects produced by these causes, and to render their final 

 results more uniform and more in accordance with the facts 

 that exist. 



About twenty years ago I had much correspondence and 

 discussion with Mr. Darwin on this question. I then believed 

 that I was able to demonstrate the action of natural selection in 

 accumulating infertility ; but I could not convince him, owing 

 to the extreme complexity of the process under the conditions 

 which he thought most probable. I have recently returned 

 to the question ; and, with the fuller knowledge of the facts of 

 variation we now possess, I think it may be shown that 

 natural selection is, in some probable cases at all events, able 

 to accumulate variations in infertility between incipient species. 



The simplest case to consider, will be that in which two 

 forms or varieties of a species, occupying an extensive area, are 

 in process of adaptation to somewhat different modes of life 

 within the same area. If these two forms freely intercross 

 with each other, and produce mongrel offspring which are 

 quite fertile inter se, then the further differentiation of the 

 forms into two distinct species will be retarded, or perhaps 

 entirely prevented ; for the offspring of the crossed unions 

 will be, perhaps, more vigorous on account of the cross, 

 although less perfectly adapted to the conditions of existence 

 than either of the pure breeds ; and this would certainly estab- 

 lish a powerful antagonistic influence to the further differentia 

 tion of the two forms. 



