Evidences of Physiological Selection. 97 



parent form, and would certainly have been re- 

 absorbed into it had intercrossing in that direction 

 been possible. With Professor Le Conte, therefore, 

 I conclude that there is only one conceivable answer 

 to this question. Each crop of varieties must have 

 been protected from intercrossing with their parent 

 form. 



They must have been the result of a variation, which 

 rendered the affected individuals sterile with their 

 parent form, whilst leaving them fertile amongst them- 

 selves. The progeny of these individuals would then 

 have dispersed through the lake, physiologically isolated 

 from the parent population, and especially prone to 

 develop secondary variations as a direct result of the 

 primary variation. Thus, as we might expect, two or 

 three variations arose simultaneously, as expressions 

 of so many different lines of family descent from the 

 original or physiological variety ; these were every- 

 where prevented from intercrossing with their parent 

 form, yet capable of blending whenever they or their 

 ever-increasing progeny happened to meet. Thus, 

 without going into further details, we are able by 

 the theory of physiological selection to give an ex- 

 planation of all these facts, which otherwise remain 

 inexplicable. 



In view of the evidence which has now been pre- 

 sented, I will now ask five questions which must be 

 suitably answered by critics of the theory of physio- 

 logical selection. 



i. Can you doubt that the hitherto insoluble pro- 

 blem of inter-specific sterility would be solved, sup- 

 posing cross-infertility were proved to arise before or 



III II 



