204 GEOEGE JOHN EOMANES i88i- 



yet, also like p.s., it has to do with the differentiation 

 of specific forms. (There is no distinction to be 

 drawn between ' the species of a cock ' and ' the 

 plumage of a cock ' : plumage is the most favourite 

 part of a bird with ornithologists on which to found 

 specific diagnoses.) Therefore, if p.s. is true at all — 

 which, of course, is another question — even my cele- 

 brated powers of ' dialectical subtlety ' are completely 

 unable to perceive any difference between p.s. and 

 s.s. in respect of their relation to n.s. 



Lastly, as regards NageH, no doubt he is an out- 

 and-out Lamarckian, but I did not see that this 

 should make any difference touching his opinion on 

 a matter of fact not more connected with Lism. than 

 Dism. I will look up ' Nature ' for 1870. 



With best Christmas wishes and many thanks for 

 botanical hints. 



December 26, 1888. 



It has occurred to me that if you know ChurchiU's 

 address, I might save time by writing to him before 

 seeing him when he comes in spring. 



It has also occurred to me that I might perhaps 

 put the argument on pp. 801-4 better before you 

 thus : 



If phys. sel. is true, it would follow that as between 

 alhed species, mutual sterility ought to occur in all 

 degrees (from zero to absolute), and that there ought 

 to be a correlation between these degrees of steriHty 

 and degrees of non-separation, topographically. 



Now, you cannot possibly doubt that the first 

 expectation is realised in nature ; as between alKed 



