340 MR. 6. J. ROMANES ON PHYSIOLOGICAL SELECTION. 



by this author, and afterwards in a more elaborate form by 

 Professor Mivart, as to the improbability of a variation being 

 from the first of sufficient utility to come under the influence 

 of natural selection, I feel it impossible to doubt that a most 

 formidable opposition is presented. Eor even if, for the sake of 

 argument, we waive Professor Mivart's objection as to the pro- 

 bable inutility of many incipient variations which afterwards, or 

 in a higher degree of perfection, begin to become useful, even if 

 we waive this objection and assume that all useful variations are 

 useful from the first moment of variation, still we have to meet 

 the difficulty from the swamping efi'ects of free intercrossing on 

 the incipient variation, however useful. 



Here then we have three great obstructions in the road of 

 natural selection, considered as an explanation of the origin of 

 species. Por the sake of brevity I will hereafter allude to these 

 difficulties as those relating to sterility, to inutility, and to inter- 

 crossing. Let us now consider how these difficulties have been 

 dealt with in the later editions of Mr. Darwin's works. 



Sterility between Species. 



Pounding his argument for natural selection upon the basis 

 furnished by the known effects of artificial selection, Mr. Darvsdn 

 had to meet the question why it is that the supposed products of 

 the former differ from the known products of the latter in being 

 so much more sterile infer se ; or, in other words, why it is that 

 natural species differ so conspicuously from artificial varieties in 

 respect of mutual fertility. In order to meet this question, Mr. 

 Darwin adduced a variety of consideratians, each of which he 

 substantiated by so large an accumulation of facts, that, as I have 

 already observed, his discussion of the question as a whole is one 

 of the most laboured portions of all his laborious work. Prom 

 which we may perceive how fully Mr. Darwin recognized the 

 formidable nature of this difficulty. I will now summarize the 

 considerations whereby he sought to overcome it. And this I 

 can do most briefly by arranging them in an order of my own. 



ancestors, and begot so many children ; those quaHties, in fact, which the 

 struggle for existence would select, if it could select anything ? 



" Here is a case in which a variety was introduced with far greater advantages 

 than any sport ever heard of, advantages tending to its preservation, and yet 

 powerless to perpetuate the new variety." 



