Evidences of Physiological Selection. 75 



section, that the greater the number of individuals 

 of the same species on a given area, the less must 

 be the power of natural selection to split that species 

 into two or more allied types ; because, the more 

 crowded the population, the greater must be the 

 uniformitarian effect of free intercrossing. This ob- 

 vious fact has been insisted upon by several previous 

 writers on Darwinism ; and the only reason why it 

 has not been recognized by all naturalists is, that so 

 few of them have observed the all-important dis- 

 tinction between monotypic and polytypic evolution. 

 The denser the population, and therefore the greater 

 the intercrossing and the severer the struggle for 

 existence within the species, the better will it be 

 for transmutation of the species by natural selection ; 

 but the worse it will be for differentiation of the 

 species by this form of homogamy. On the other 

 hand, if physiological selection be entertained as 

 a form of homogamy, the denser the population, the 

 better opportunity it will have of differentiating the 

 species, first, because a greater number of individuals 

 will be present in which the physiological change 

 may arise, and. secondly, because, if it does arise, the 

 severity of the struggle for existence will then give 

 natural selection a better chance of acting rapidly 

 and effectually on each of the isolated sections. 



Hence, where the question is whether selective 

 fertility has played any large or general part in the 

 differentiation of specific types, the best criterion we 

 can apply is to ascertain whether it is a general 

 rule that closely allied species occur in intimate 

 association, so that their individual members con- 

 stitute, as it were, a single population, or, on the 



