480 Proceedings of the Royal Physical Society. 



cause a more rapidly-produced actual variatiou, and vice 

 versa the increasing actual variation would give greater scope 

 for the Natural Selection to act on. The one would play 

 into the hands of the other. In course of time this process 

 would bring about a condition of things where the individuals 

 of a stock that was highly specialised along its own line 

 would become so alike in their potential variation that the 

 same cause of impaired fertility, and a weakness to the 

 environment that is so evident as the result of close in- 

 breeding, might come into play, with resulting extinction 

 of the group. 



