1890 PHYSIOLOGICAL SELECTION 221 



ciated species as it is within each species, how is it 

 conceivable that they should continue to be distinct ? 

 In this connection it is well to consult Gulick's paper 

 already referred to (especially p. 259, paragraph 1st) 

 on the theoretical side, and Jordan's papers and 

 books on the practical side. I have repeated the 

 latter's observations on poppies, and find that where 

 any considerable number of individuals are concerned, 

 natural selection is not nearly so great a power in 

 this respect. (Even in cases where it happens that 

 in-breeding is necessarily confined to single herma- 

 phrodite individuals for numberless generations, the 

 handicapping is not fatal : witness flowers which 

 habitually fertilise themselves before opening es- 

 pecially some species of orchids, which never seem 

 to do otherwise, notwithstanding the elaborate pro- 

 visions for cross-fertilisation in other species.) Now, 

 I believe most of all in what I have called l collective 

 variation ' of the reproductive system in the way 

 of physiological selection, whereby, owing to some 

 common influence acting on a large number of indi- 

 viduals similarly and simultaneously, they all become 

 sexually co-adapted inter se while physiologically 

 isolated from the rest. This essential feature of the 

 theory seems to me entirely to remove the difficulty 

 about in-breeding, as well as that which Wallace 

 urged about the chances against a suitable meeting 

 of 'physiological complements.' 



As for my having attributed too much to the 

 swamping effects of intercrossing (Panmixia), this, I 

 am convinced, is the one and only particular wherein 

 I have at all departed from the judgments of Darwin ; 



