Evidences of Physiological Selection. 63 



nothing to do either with its causes or its degrees. 

 Not with its causes, because in this respect the 

 theory of physiological selection is in just the same 

 position as that of natural selection : it is enough for 

 both if the needful variations are provided, without 

 its being incumbent on either to explain the causes 

 which produce them. Not with its degrees, because, 

 in the first place, it can only be those degrees of 

 variation which in particular cases are supposed 

 adequate to induce specific divergence, that fall 

 within the scope of the theory ; and because, in the 

 second place, degrees which are adequate only to 

 induce or to assist in inducing varietal divergence, 

 must always tend to increase, or pass into higher 

 degrees. 



Antecedent Standing of the Theory. 



The antecedent standing or logical basis of the 

 theory has already been in large measure displayed 

 in the preceding chapter ; for it was impossible to 

 state the theory without thereby showing in how 

 considerable a degree it is self-evident. A brief 

 recapitulation is therefore all that is here necessary. 



It has been shown that divergent or polytypic 

 evolution on common areas is inexplicable by natural 

 selection alone. Hence the question arises: What 

 form of isolation has, under such circumstances, 

 rendered possible divergent evolution? In answer 

 to this question the theory of physiological selection 

 suggests that variations in the reproductive function 

 occur in such a way as to isolate more or less 

 perfectly from each other different sections of a 

 species. While cross-fertility remains unimpaired 



