Evidences of Physiological Selection. 77 



species are most frequently found in intimate asso- 

 ciation with one another, not, that is to say. in any 

 way isolated by means of physical barriers. This 

 argument is everywhere logically intact ; and. as he 

 sustains it by a large knowledge of topographical 

 botany, his indictment against natural selection as 

 a cause of specific differentiation appeared to be 

 insurmountable. And, in point of fact, it %vas in- 

 surmountable ; so that the whole problem of the 

 origin of species by differentiation on common areas 

 has hitherto been left in utter obscurity. Nor is there 

 now any escape from this obscurity, unless we enter- 

 tain the "supplementary factor" of selective fertility. 

 And, apparently, the only reason why this has not 

 been universally recognized, is because Darwinians 

 have hitherto failed to perceive the greatness of the 

 distinction between the differentiation and the trans- 

 mutation of species ; and hence have habitually met 

 such overwhelming difficulties as Nageli presented by 

 an illogical confounding of these two totally distinct 

 things. 



But if the idea of selective fertility had ever 

 occurred to Nageli as a form of segregation which 

 gives rise to specific differentiation, I can have no 

 doubt that so astute and logical a thinker would 

 have perceived that his whole indictment against 

 natural selection was answered. For it is incredible 

 that he should not have perceived how this physio- 

 logical form of homogamy (supposing it to arise before 

 or dnrijig. and not after the specific differentiation) 

 would perform exactly the same function on a con- 

 tinuous area, as he allowed that " isolation " does on 

 a discontinuous one. 



