Physiological Selection. 57 



Only two further points need be mentioned to 

 make this statement of physiological selection as 

 complete as the present rhumi of its main principles 

 requires. 



The first is, that, as Mr. Wallace remarks, " every 

 species has come into existence coincident both in 

 space and time with a pre-existing and closely allied 

 species." I regard this as important evidence that 

 physiological selection is one of the natural causes 

 concerned. For the general fact implied is that every 

 species has come into existence on an area occupied 

 by its parent type, and therefore under circumstances 

 which render it imperative that intercrossing with that 

 type should be prevented. In the case of monotypic 

 evolution by natural selection alone, intercrossing 

 with the parent type is prevented through the gradual 

 extinction of that type by successive generations of 

 the developing type. But in the case of polytypic 

 evolution^ intercrossing with the parent type can 

 only be prevented by some form of isolation other 

 than natural selection ; and here it is evident that 

 cross-infertility with the parent type must be as 

 efficient to that end as any other form of isolation 

 that can be imagined. Consequently we might 

 almost have expected beforehand, that in a large 

 proportional number of cases cross-infertility should 

 have been the means employed. And the fact that 

 this is actually the case so far corroborates the only 

 theory which is able to explain it. 



The second point is this. 



It appears to be comparatively rare for any cause 

 of specific divergence to prove effectual on common 

 areas, unless it sooner or later becomes associated with 



