1890 PHYSIOLOGICAL SELECTION 181 



Dear Darwin, — Criticism of an intelligent kind is 

 what I feel most in need of, and therefore it is no 

 merit on my part to like it when it comes. 



The point about the combined action of natural 

 and physiological selection is, after all, a very sub- 

 ordinate one, and, as I said in ■ Nature ' some weeks 

 ago, is the most highly speculative and least trust- 

 worthy part of the theory. Moreover, it is the only 

 part that is directly opposed to an expressed conclusion 

 in the ' Origin,' though, even here, the opposition 

 is not real. If natural selection can do anything 

 at all in the way of bringing about sterility with 

 parent forms, it can only do so by acting on the type 

 or whole community (for I quite agree with the 

 reasoning in the ' Origin,' that it cannot do so by 

 acting on individuals) ; and whether natural selection 

 could in any case act on a type is a question which 

 your father has told me he could never quite make 

 up his mind about, except in the case of social 

 hymenoptera and moral sense of man. 



You will see what I mean by ' secondary varia- 

 tions ' by looking at page 366 of my paper. It is 

 merely a short-hand expression for all other specific 

 differences save the sexual difference of sterility. My 

 view is that these secondary differences are always 

 sure to arise sooner or later in some direction or another 

 wherever a portion of a species is separated from the 

 rest, whether by geographical or physiological isolation, 

 which, indeed, as regards the former, is no more than 

 you (following Weismann, &c.) acknowledge. Now, to 

 me it seems obvious that Weismann's ' variations " 

 (i.e. slight changes in the form of shells) cannot 



