122 Darwin, and after Darwin. 



It is in relation to my presentment of the im- 

 possibility of natural selection alone causing poly- 

 typic evolution, that Mr. Wallace has been at the 

 pains to show how the permission of intercrossing 

 (panmixia) is necessary for natural selection in its 

 work of causing monotypic evolution. And not only 

 has he thus failed to perceive that the " difficulty " 

 which intercrossing raises against the view of natural 

 selection being of itself capable of causing polytypic 

 evolution in no way applies to the case of monotypic ; 

 but as regards this " difficulty," where it does apply, 

 he says:- 



Professor G. J. Romanes has adduced it as one of the 

 difficulties which can alone be overcome by his theory of physio- 

 logical selection '. 



This, however, is a misapprehension. I have by 

 no means represented that the difficulty in question 

 can alone be overcome by this theory. What I have 

 represented is, that it can be overcome by any of the 

 numerous forms of isolation which I named, and 

 of which physiological selection is but one. And 

 although, where common areas are concerned, I believe 

 that the physiological form of isolation is the most 

 important form, this is a very different thing from 

 entertaining the supposition which Mr. Wallace here 

 assigns to me. 



I may take this opportunity of correcting a some- 

 what similar misunderstanding which has been more 

 recently published by Professor W. A. Herdman, of 

 Liverpool ; and as the case which he gives is one of 



1 Danvinism, p. 143. 



