374 ME. G. J. EOMAIOIS ON PHTSIOLOGICAL SELECTION. 



on tte primary one, in the sense that, even if they he the causes, 

 they depend for their existence on the fact that they happen to 

 have been capable of producing this particular effect — a general 

 view of the case which appears to me abundantly justified by the 

 fact of their general association. Hence, if there are any cases — 

 and I do not doubt that there are many — where the secondary 

 distinctions have been the cause of the primary distinction, still 

 even here the former are, as I have phrased it, dependent on the 

 latter, inasmuch as the latter is a necessary condition to their 

 existence. Or, otherwise expressed, unless the secondary distinc- 

 tions had happened to be of a kind which induced the primary 

 distinction, they could not in themselves have survived, but would 

 have been reabsorbed by free intercossing. Thus, according to 

 my view, even in the minority of eases where the causes of the 

 primary distinction have been such changes in the organism as I 

 have called secondary distinctions, even in this minority of cases 

 the principles of physiological selection have been at work. Eor 

 these principles have in all those cases selected ^q particular kinds 

 of secondary distinctions which have proved themselves capable 

 of so reacting on the reproductive system as to bring about the 

 primary distinction. 



Suppose, for instance, that all our horticulturists and breeders 

 were suddenly to allow all domesticated varieties freely to 

 intercross, and suppose that some of these varieties had been 

 previously acted upon by artificial selection to an extent of 

 inducing sterility in a degree comparable with what evolutionists 

 imagine that natural selection may have been able to accomplish 

 in incipient species. Under these circumstances, physiological 

 selection would at once set to work to pick out all these sexually 

 protected forms, and band them on as permanent varieties (or, if 

 the sterility were sufficiently pronounced, as true species) ; while 

 all the other forms, no matter how much they might difier from 

 one another in respect of secondary distinctions, would be doomed 

 to extinction — or, as we should then say, to reversion, which merely 

 means reabsorption of secondary distinctions into parent forms. 

 Now, if so soon as the artificial barriers to intercrossing were re- 

 moved this is what would inevitably take place even with secon- 

 dary distinctions already formed, is it not evident that, in the 

 original absence of any kind of barrier otherwise given, none of 

 these secondary distinctions could ever have arisen, except those 



