172 Darwin, and after Darwin. 



logical divergence, as must almost certainly have been the 

 case if such polytypic evolution had occurred on a common 

 area. Indeed, one of the two lines of experimental verifica- 

 tion of our theory consists in selecting cases where nearly 

 allied species are separated by geographical barriers, and 

 proving that, in such cases, there is no cross-sterility. 



Fertility of Domesticated Varieties. Some writers have 

 sought to explain the contrast between domesticated varieties 

 and natural species in respect of fertility when crossed, by 

 the consideration that it is only those natural species which 

 have proved themselves so far flexible as to continue fertile 

 under changed conditions of life that can have ever allowed 

 themselves to become domesticated. But although this 

 condition may well serve to explain the unimpaired fertility 

 under domestication of such species as for this very reason have 

 ever become domesticated, I fail to see how it explains the 

 further and altogether different fact, that this fertility continues 

 unimpaired between all the newly differentiated morphological 

 types which have been derived from the original specific type. 

 It is one thing that this type should continue fertile after 

 domestication : it is quite another thing that fertility should 

 continue as between all its modified descendants, even 

 although the amount of modification may extend much 

 further than that which usually obtains between different 

 natural species. 



Testing for Cross-infertility among varieties growing on 

 the same area is a much more crucial line of verification than 

 testing for unimpaired fertility between allied species which 

 occupy different areas, because while in the former case we 

 are dealing with " incipient species" with a view to ascertain- 

 ing whether the divergence which they have already undergone 

 is accompanied by physiological isolation, in the latter case 

 we can never be sure that two allied species, which are now 

 widely disconnected geographically, have always been so 



