206 GEOEGE JOHN EOMANES I88I- 



18 Cornwall Terrace, Begent's Park, N.W. : December 27, 1888. 



I am most glad that in your last letter you deal 

 with what I consider the real ' question ' — viz. not 

 whether degrees of sterility obtain among a large 

 proportional number of species, but whether there is 

 any such correlation between them and absence of 

 isolation of other kinds as my theory would expect. 

 And, in dealing with this question you hit upon 

 precisely the two greatest difficulties which I have 

 myself concluded lie against the theory. The first 

 is about areas now discontinuous having been once 

 continuous, and our being so often unable to say 

 whether or not such has been the case. But this 

 difficulty is one that lies against verification of the 

 theory, not against the theory itself. It was in view 

 of this difficulty that I mentioned oceanic islands as 

 furnishing the best flora for trying experiments upon ; 

 but since I published the paper, I have not been able to 

 hear of any botanists visiting islands. Should you 

 ever hear of any you might let me know. 



The second difficulty is one that lies against the 

 theory itself, and has always seemed to me most 

 formidable. But as nobody else has ever mentioned 

 it, I have not hitherto done so, as I want to work it 

 out quietly. I allude to your remark about the ex- 

 traordinary difierences that obtain among different 

 genera with regard to the capability of intercrossing 

 exhibited by their constituent species. This, I 

 confess, has from the first appeared a tremendous 

 objection to my theory. On the other hand, I have 

 taken comfort from the consideration that besides 



