410 ME, G. J. ROMANES ON PHTSIOLO&ICAL SELECTION. 



selected to constitute a new species. Intercrossing is thus re- 

 garded as standing in the same kind of relation to the genesis of 

 species as the struggle for existence stands to that of adaptive 

 structures : it is the destroying tendency which furnishes the 

 needful condition to a selective process : it is the agency which 

 obliterates all other variations, save those of a particular kind. 

 Therefore, according to my theory of the origin of species, the 

 greater the swamping influence of intercrossing the better must 

 be the conditions for evolving species mutually sterile with one 

 another ; while, as we have seen, precisely the opposite conse- 

 quence follows from all previous theories upon this subject. 



Probably more than enough has now been said to dispose of the 

 criticism which I am considering, or to show that the theory of 

 physiological selection offers a real explanation of the origin of 

 species, and does so by going to work at the very root of the 

 problem. I will therefore only add that the real idea in the 

 minds of those who advanced this criticism must, it appears to 

 me, have been that my suggested explanation of the origin of 

 species opens up another and a more ultimate problem — namely, 

 granting that species have originated in the way supposed, what 

 have been the causes of the particular kind of variation in the 

 reproductive system which the theory requires ? This, of course, 

 is a perfectly intelligible question, and one that must immediately 

 suggest itself to the mind : my failure to meet it is therefore 

 apt to give rise to the impression that my theory is imperfect. 

 But, as briefly stated in the paper itself, this question is really 

 not one with which the theory of physiological selection can 

 properly be regarded as having anything to do. This theory has 

 only to take the facts of variation in general as granted, and then 

 to construct out of them its suggested explanation of the origin 

 of species. No doubt it would be most interesting to discover 

 the causes of every variation that constitutes the beginning of a 

 new specific character ; but our inability to do this does not in- 

 validate the theory of physiological selection, any more than 

 it does the theory of natural selection. Objections, indeed, have 

 been raised against the theory of natural selection on this very 

 ground — namely, that it does not explain the causes of those 

 variations on the occurrence of which it depends. But these 

 objections are clearly illogical. It constitutes no part of the 

 theory of natural selection to explain these variations ; this is a 

 problem which belongs to the future of physiology, and no doubt 



