76 Darwin, and after Darwin. 



other hand, whether they occur rather on different 

 sides of physical barriers. If they occur intimately 

 associated, the form of homogamy to which their 

 differentiation was due must have presumably been 

 the physiological form ; whereas, if they are proved 

 to be correlated with physical barriers, the form of 

 homogamy which was concerned in their differen- 

 tiation must presumably have been the geographical 

 form. 



Now, at first this consideration was a trouble to 

 me, because Moritz Wagner had strenuously argued 

 — and supported his argument by a considerable 

 wealth of illustration — that allied species are always 

 found correlated with physical barriers or discon- 

 tinuous areas. Weismann's answer, indeed, had 

 shown that Wagner's statement was much too general : 

 nevertheless, I was disappointed to find that so 

 much could be said in favour of the geographical 

 (or topographical) form of isolation where closely 

 allied species are concerned. Subsequently, however. 

 I read the writings of Nageli on this subject, and 

 in them I find a very different state of matters 

 represented. 



Seeing as clearly as Wagner that it is impossible 

 under any circumstances for natural selection to 

 cause specific differentiation unless assisted by some 

 other forms of homogamy, but committing the same 

 oversight as Wagner and Weismann in supposing 

 that the only other form of homogamy in nature is 

 geographical isolation, Nageli, with great force of 

 reasoning, and by many examples, founded his argu- 

 ment against the theory of natural selection on the 

 ground that in the vegetable kingdom closely allied 



