Physiological Selection. 59 



absence of cross-infertility, such divergence seems, as 

 a general rule, to have been incapable of attaining to a 

 specific value. 



5. Therefore, in the vast majority of such cases, it 

 must have been those varietal changes of structure, 

 size, colour, &c., which happened to have afterwards 

 been assisted by the reproductive change that were 

 on this account selected as successful candidates for 

 specific differentiation. 



6. It follows, that it makes no difference to the 

 general theory of physiological selection in what pro- 

 portion of cases the physiological change has been 

 the initial change ; for, whether prior or subsequent 



'to the varietal changes with which it becomes associ- 

 ated, its presence has been equally important as a 

 condition to specific divergence. 



7. When physiological isolation becomes associated 

 with natural selection, or any other form of homogamy, 

 the segregative power of both is augmented. More- 

 over, so great is the augmentation that even very 

 moderate degrees of physiological isolation — them- 

 selves capable of effecting little or nothing — become 

 very powerful when associated with moderate degrees 

 of any other kind of homogamy, and vice versa. 



8. The theory of physiological selection effec- 

 tually explains the divergent evolution of specific 

 types and the cross-infertility of such types when 

 evolved. 



To prevent, if possible, the continuance of certain 

 misunderstandings with regard to my original state- 

 ment of the new theory, let me here disclaim some 

 views which have been assigned to me. They are : 



