ME. G. J. EOMAKES OK PHTSTOLOGHCAL SELECTION. 393 



respect of being able freely to intercross with one another. 

 Therefore, how is it possible " to adapt two varieties to two large 

 areas, and a third (transitional) variety to a narrow intermediate 

 zone," in the face of free intercrossing on a continuous area? 

 Let A, B, and C represent the three areas in question. According 



B 



C 



to the argument, variety A passes first into variety B, and then 

 into variety C, while variety B eventually becomes exterminated 

 by the inroads both from A and C. But how can all this have 

 taken place with nothing to prevent intercrossing throughout the 

 entire area ABC? I confess that to me it seems this argument 

 can only hold on the supposition that the analogy between 

 varieties and species extends to the 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 showed some degree of 

 sterility towards one another. And, if so, we have of course to 

 do with the principles of physiological 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 transi- 

 tion on the area occupied by contiguous species, as well as from 

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

 all these facts combine to testify that the transition was origi- 

 nally due to that particular change in the reproductive systems 

 of the forms concerned, which still enables those forms to "inter- 

 lock " without intercrossing. 



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. Of 

 course this fact speaks in favour of evolution ; but where the 

 question is as to method, I confess that the theory of natural selec- 

 tion appears to me wholly irrevelant. For in all the numberless 

 eases to which I allude, the points of minute detail wherein the 

 allied species differ in respect of secondary distinctions are points 



