Evidences of Physiological Selection. 91 



remaining fact that is required to complete the case 

 in favour of the present theory. We are here brought 

 back to the very earliest stages of physiological differ- 

 entiation or to the stages which lie behind Jordan's 

 " Physiological Species " ; and therefore, when taken 

 in conjunction with his results, the phenomena d 

 prepotency may be said to give us the complete and 

 final demonstration of one continuous development, 

 which, beginning in an almost imperceptible amount 

 of cross-infertility, ends in absolute cross-sterility. 

 The " elective capacity " to which Darwin alludes as 

 having been " acquired " by all the species of Com- 

 positae since they "branched off from a common 

 progenitor," is thus seen among innumerable other 

 species actually in process of acquisition ; and so 

 we can perfectly well understand, what is otherwise 

 unintelligible, that closely allied species of plants 

 occur, in ninety-five per cent, of cases, intimately asso- 

 ciated on common areas, while exhibiting towards one 

 another the character of mutual sterility. 



But more than this. The importance of the wide- 

 spread phenomena of prepotency to the theory of 

 physiological selection does not consist merely in 

 thus supplying the last link in the chain of evidence 

 touching the origin of species by selective fertility, 

 or "elective capacity." These phenomena are of 

 further importance as showing how in plants, at all 

 events, physiological selection appears to be frequently 

 capable of differentiating specific types without the 

 necessary assistance of any other form of homogamy. 

 In my original statement of the theory, I was careful 

 to insist upon the great value, as differentiating agents, 

 of even small degrees of other forms of homogamy 



