CHAPTER III. 



Physiological Selection. 



So far we have been concerned with the principle 

 of Isolation in general. We have now to consider 

 that form of isolation which arises in consequence of 

 mutual infertility between the members of any group 

 of organisms and those of all other similarly isolated 

 groups occupying simultaneously the same area. 



Against the view that natural selection is a sufficient 

 explanation of the origin of species, there are two 

 fatal difficulties: one, the contrast between natural 

 species and domesticated varieties in respect of cross- 

 sterility ; the other, the fact that natural selection 

 cannot possibly give rise to polytypic as distinguished 

 from monotypic evolution. Now it is my belief that 

 the theory of physiological selection fully meets both 

 these difficulties. Indeed I hold this to be undeniable 

 in a formal or logical sense : the only question is as 

 to the evidence which can be adduced for the theory 

 in a practical or biological sense. Therefore in this 

 chapter, where the theory has first of all to be stated, 

 I shall restrict the exposition as much as possible 

 to the former, leaving for subsequent consideration the 

 biological side. 



