ME. &. J. ROMANES ON PHYSIOLOGICAL SELECTION. 381 



whatever degree the sterility first occurs, in that degree it may 

 remain ; for, ex hypothesi, it must from the first have been suffi- 

 cient to cause at least so much of physiological separation of the 

 varietal type as to admit of the continuance of that type. If this 

 degree of sterility were from the first but small, a longer time 

 would be required to eflfect a complete separation between the 

 parent and the variety, than if this degree were from the first 

 considerable. But, as we have before seen, this is all the difference 

 that would arise ; and therefore, upon my theory, we may regard 

 degrees of sterility as matters o£ no significance — although I do 

 think it is extremely probable that when once sterility in any 

 degree has arisen it will afterwards become increased, not so 

 much for the reason assigned by Mr. Darwin (viz. prolonged ex- 

 posure to uniform conditions), as from the general tendency which 

 variations of all kinds present to continue in the lines of their 

 initial deviation. I cannot doubt that if the theory of physiolo- 

 gical selection had occurred to Mr. Darwin, he would have seen 

 in this latter consideration a much more cogent reason than the 

 one which he assigns for the general sterility that obtains between 

 species. But he was precluded from applying this consideration 

 because it did not occur to him that sterility might itself be ori- 

 ginally due to independent variation, and thus itself be subject 

 to the laws of variation in general. 



I trust, then, that these considerations will have shown that, al- 

 though natural selection cannot have been directly instrumental in 

 causing sterility between an incipient species audits parent form, 

 if the incipient species were such in virtue of a variation in its re- 

 productive system tending from the first to prevent intercrossing 

 with its parent form, then there would be a variation the further 

 development of which might be favoured by natural selection. 

 ¥ov if, as Mr. Darwin thought, " it would profit an incipient 

 species if it were rendered in some degree sterile with its parent 

 form," although this profit could not have been initially conferred 

 by natural selection, yet when it once arises from a spontaneous 

 variation in the reproductive system itself, I see no reason to 

 doubt that it should forthwith be favoured by natural selection, 

 just as is the case with favourable variations in general. That is 

 to say, natural selection would set a premium upon infertility 

 with the parent form, and would thus cooperate with physiological 

 selection in splitting up the specific type. Por, although natural 

 selection is powerless to induce sterility between allied forms, 



