252 GEORGE JOHN ROMANES ibss- 



kind of variation ever takes place at all, we are not 

 concerned either with its causes or with its degrees. 

 Not with its causes, because in this respect the 

 theory of physiological selection is in just the same 

 position as that of natural selection ; it is enough for 

 both that the needful variations are provided, without 

 its being incumbent on either to explain the causes 

 which underlie the variations.' 



Nevertheless, just as it is of importance to any 

 one believing in natural selection to ascertain these 

 underlying causes of the variations on which it 

 depends, so &c. &c. Therefore I suggested some as 

 more or less probable — e.g. in plants slight differences 

 in the season of flowering. Now, your principle 

 seems to me a further suggestion on the same lines : 

 it seeks to explain the raison d'etre of cumulative 

 inter-sterility between two sections of a species. 



Lastly, I do not follow what you say about your 

 not requiring a ' segregation,' but only ' variations in 

 every possible direction, with no special factor brought 

 in besides to effect the beginning and progress of 

 separation.' It seems to me, on the contrary, 1st, 

 that you must assume the sexual variation to have 

 already been begun by some other cause, or causes 

 (else there could be no hybrids in the question) ; and 

 2nd, that when infertility — or other inferiority — of 

 the hybrids begins to tell on the parent forms in 

 the way supposed, that the only varirtion which 

 it is concerned in continuing and intensifying is 

 the variation on which physiological selection de- 

 pends — viz. infertility between A and B, with unim- 

 paired fertility between both A x A and B x B — where 



