72 Darwin, and after Darwin. 



reproductive system ; or, in a sense more absolute 

 than the argument has in view, that "varieties do 

 not essentially differ from the species" which they 

 afterwards form, but from the first show some 

 degree of infertility towards one another. And, if so, 

 we have of course to do with the principles of physio- 

 logical selection. 



That in all such cases of species-distribution these 

 principles have played an important part in the 

 species-formation, appears to be rendered further 

 probable from the suddenness of transition on the 

 area occupied by contiguous species, as well as from 

 the completeness of it — i. e. the absence of connecting 

 forms. For these facts combine to testify that the 

 transition was originally due to that particular change 

 in the reproductive systems of the forms concerned, 

 which still enables those forms to '■ interlock " without 

 intercrossing. On the other hand, neither of these 

 facts appears to me compatible with the theory of 

 species-formation by natural selection alone. 



But this leads us to another general fact, also 

 mentioned by Darwin, and well recognized by all 

 naturalists, namely, that closely allied species, or 

 species differing from one another in trivial details, 

 usually occupy contiguous areas ; or, conversely stated, 

 that contiguity of geographical position is favourable 

 to the appearance of species closely allied to one 

 another. Now, the large body of facts to which 

 I here allude, but need not at present specify, appear 

 to me to constitute one of the strongest of all my 

 arguments in favour of physiological selection. Take, 

 for instance, a large continental area, and follow across 

 it a chain of species, each link of which differs from 



