1S90 ON PHYSIOLOGICAL SELECTION 203 



that the ' physiological isolation ' has to do with pre- 

 venting the species from fusing? But, if so, by 

 parity of reasoning, should we not expect to meet 

 ■with soi)ie degree of the same thing in the other cases, 

 which, although not here sufficiently pronounced to 

 block oflE frequent hybridisation, is nevertheless 

 sufficient to prevent the species from blending over 

 their common area ? 



And here, I may say, I should not at all object to 

 the charge of misunderstanding Darwin on any merely 

 trivial point such as the one you mention. But in this 

 instance it so happens that it is rather you who have 

 misunderstood me. I know that ' a hybrid is not an 

 intermediate form in his sense,' and this is just what 

 constitutes my difficulty against his paragraphs 

 quoted on p. 392 of my paper. For what I say is, 

 these intermediate forms ought to be hybrids, unless 

 pliysiological selection, (i.e. mutual sterility) lias been 

 at worJc. ' In his sense ' I cannot conceive how such 

 ' intermediate forms ' can exist in the circumstances 

 described, seeing that they are not hybrids, and yet 

 that (in the absence of any hypothesis of physiological 

 isolation for which I am contending) there is no 

 reason given why the two interlocking species should 

 not freely intercross. 



Regarding sexual selection I certainly am very 

 much in earnest about its parallel to p.s.^ If you in- 

 tend the meaning of n.s. so as to embrace s.s. it will 

 at the same time embrace also p.s. For s.s., like p.s., 

 has nothing to do with hfe-preserving characters ; 



' pA — physiological selection ; s.s. — sexual selection ; n.s. — natural 

 selection. 



