80 ORGANIC EVOLUTION CONSIDERED 



only one species at a time in the same locality, for it 

 assumes that the individuals with the favorable vari- 

 ation survive at the expense of the parent form; nor 

 does it account for cross-sterility between closely- 

 related species. 



The later evolutionists, feeling the insufficiency of 

 Darwin's assumption, have bridged the difficulty by 

 introducing a still greater qne — that of cross-ster- 

 ility by birth. 



Darwin says that he cannot agree "that migration 

 and isolation are necessary for the formation of new 

 species."* 



Again: "Although isolation is of great importance 

 in the production of new species, on the whole I am 

 inclined to believe that largeness of area is still more 

 important, especially for the production of species 

 which shall prove capable of enduring for a long 

 period and spreading widely, "f 



Suppose, according to this, that we find two species 

 that have been evolved from a common stock occupy- 

 ing contiguous territories and mingling with each 

 other on the common border. How can we account 

 for the evolution of these two species in these locali- 

 ties? It is evident that the difficulties that arise from 

 the merging of new forms and of producing cross- 

 sterility remain in full force. These difficulties are 

 always present in a state of nature, whether the terri- 

 tory is large or small, for the new species and its 

 parent must occupy the same territory. The great 

 difficulty in all cases is to isolate new forms from the 

 parent form so that they will not be lost by mingling. 

 Nature has no method, so far as facts show, of doing 

 this. 



Again, Darwin says, that "isolation is of great 



* Origin of Species, 5th Edition, p. 105. t Ibid. p. 106. 



