1S90 ON PHYSIOLOGICAL SELECTION 201 



either in first crosses or in their hybrid progeny. 

 But now, the most important thing for me is mutual 

 sterihty that is not absolute (though, on m}- theory, 

 perhaps on its way to becoming so) but relative, i.e. 

 there being a lower degree of fertility between A xB 

 or B X A, than there is between A x A or B x B. 



Hitherto very few experiments have been made 

 on these comparative degrees of fertUity, yet it is 

 by such alone, it seems to me, that physiological 

 selections can be tested. Thus, e.g., my point about 

 the ' interlocking ' species (p. 392) is that in such 

 cases I should expect a higher degree of fertiHty in 

 A X A and B x B than crosswise. Indeed, my fear is 

 that when I shall have proved by experiment that 

 such is the general rule in such cases, naturaHsts 

 will tm'n round and say : ' Well, of course, on merely 

 a priori gTounds you might have known that such 

 must have been the case ; for otherwise the two 

 interlocking species could never have existed as 

 separate species, they would have hybridised freely 

 along the whole frontier Hne and eventually blended 

 over the whole area.' And stiU more may this be 

 said in the case of alhed species, not merely inter- 

 locking, but interm ix ed through common areas. 

 Therefore, as a beheving P.E.S. said to me the other 

 day, ' Your letters in " Xature '' will at least have the 

 effect of blunting the edge of such possible criticism 

 in the future.' Of course j'ou wiU laugh at the 

 robustness of my faith in thus forecasting the Hne of 

 future opposition, but I would hke to ask you this 

 much: Supposing, for the sake of argument, that 

 twenty years hence I pubhsh one hundred instances 



