334 GEOEGE JOHN EOMANES 1893 



adequate reasons for doing so. You need not bother 

 with Dr. Vris, as he merely follows Focke, but I wish 

 you would read Focke ('Die Pflanzen-Mischlinge,' 

 p. 510, et sq.), and compare what he says with the 

 evidence which Darwin presents. 



As I do not know in what respects you have 

 found one part of my previous letter not to ' tally ' 

 with another, I cannot fully explain it ; but I fancy 

 that you will find they do, if, in reading the letter, 

 you carry in your mind the simple proposition that, 

 from the nature of the case, there can be no physio- 

 logical selection except where differentiating varieties 

 ( l incipient species ' ) occur upon common areas and 

 identical stations. I do not see any difficulty 

 about willows, roses, brambles, &c., since Naudin's 

 researches on Datura have shown how much vari- 

 ability, due to the hybridisation of any two species, 

 may give rise to the appearance of there being many 

 species. This, you will remember, is the view that 

 Naudin himself takes with regard to willows &c. - 

 although, of course, without any reference to phy. sel. 

 If you will refer to p. 405 of the paper on phy. sel. you 

 will find that from the first I have been aware of the 

 difficulty about discontinuous areas to which you 

 allude. But I think the converse line of evidence 

 (viz. that of cross-sterility between incipient species 

 on identical stations) will alone prove sufficient to 

 verify the theory. At the same time I look for more 

 corroboration from the cross-fertility of well-dif- 

 ferentiated species upon discontinuous areas where 

 these are, as you say, oceanic islands, or, still better, 

 mountainous districts where the allied species are 



