52 Darwin, and after Darwin. 



But, as previously remarked, this view is expressly 

 opposed to that of Darwin, who held that Wallace's 

 contention is erroneous. 



It will be seen, then, that both Darwin, and Wallace, 

 fully recognize the necessity of rinding some explana- 

 tion of the infertility of allied species, over and above 

 the mere reaction of morphological differentiation on 

 the physiology of the reproductive system, and they 

 both agree in suggesting additional causes, though 

 they entirely disagree as to what these causes are. 

 Now, the theory of physiological selection likewise 

 suggests an additional cause or, rather, a new ex- 

 planation and one which is surely the most probable. 

 For what is to be explained ? The very general 

 association of a certain physiological peculiarity witli 

 that amount of morphological change which dis- 

 tinguishes species from species, of whatever kind the 

 change may be, and in whatever family of the animal 

 or vegetable kingdom it may occur. Well, the theory 

 of physiological selection explains this very general 

 association by the simple supposition that, at least 

 in a large number of cases, it was the physiological 

 peculiarity which first of all led to the morphological 

 divergence, by interposing the bar of sterility between 

 two sections of a previously uniform species ; and by 

 thus isolating the two sections one from another, 

 started each upon a subsequently independent course 

 of divergent evolution. 



Or, to put it in another way, if the occurrence of 

 this physiological peculiarity has been often the only 

 possible means of isolating two sections of a species 

 occupying a common area, and thus giving rise to 

 a divergence of specific type (as obviously must have 



