i68 Darwin, and after Darwin. 



matical proof that such an association is always — and not 

 merely often — necessary, would not have materially affected 

 the theory of the origin of species by means of physiological 

 selection. We have now seen, however, that a competent 

 mathematical treatment proves the exact opposite ; and, there- 

 fore, that Mr. Wallace's criticism fails even as regards the 

 very subordinate point in question. 



