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. 



