Evidences of Physiological Selection. 71 



intermediate region, having intermediate conditions of life, why 

 do we not now find closely-linking intermediate varieties ? This 

 difficulty for a long time quite confounded me. But I think it 

 can in large part be explained 1 . 



His explanation is that, " as the neutral territory 

 between two representative species is generally narrow 

 in comparison with the territory proper to each, 

 . . . and as varieties do not essentially differ from 

 species, the same rule will probably apply to both ; and, 

 therefore, if we take a varying species inhabiting 

 a very large area, we shall have to adapt two varieties 

 to two large areas, and a third variety to a narrow 

 intermediate zone." It is hence argued that this 

 third or intermediate variety, on account of its existing 

 in lesser numbers, will probably be soon overrun and 

 exterminated by the larger populations on either side 

 of it. But how is it possible "to adapt two varieties 

 to two large areas, and a third [transitional] variety 

 to a narrow intermediate zone," in the face of free 

 intercrossing on a continuous area ? Let A, B, and 

 C represent the three areas in question. According to 



the argument, variety A passes first into variety B, 

 and then into variety C, while variety B eventually 

 becomes exterminated by the inroads both from 

 A and C. But how can all this have taken place 

 with nothing to prevent intercrossing throughout the 

 entire area A, B, Cf I confess that to me it seems this 

 argument can only hold on the supposition that the 

 analogy between varieties and species extends to the 



1 Origin of Species, ed. 6, pp. 134, 135. 



