CHAP. IX SELF-FERTILE VARIETIES. 347 



sterile plants is the evidence which they afford of the 

 advantage, or rather of the necessity, of some degree or 

 kind of differentiation in the sexual elements, in order 

 that they should unite and give birth to a new being. 

 It was ascertained that the five plants of Reseda odorata 

 which were selected by chance, could be perfectly 

 fertilised by pollen taken from any one of them, but 

 not by their own pollen ; and a few additional trials 

 were made with some other individuals, which I have 

 not thought worth recording. So again, Hildebrand 

 and Fritz Muller frequently speak of self-sterile plants 

 being fertile with the pollen of any other individual ; 

 and if there had been any exceptions to the rule, these 

 could hardly have escaped their observation and my 

 own. We may therefore confidently assert that a 

 self-sterile plant can be fertilised by the pollen of any 

 one out of a thousand or ten thousand individuals of 

 the same species, but not by its own. Now it is 

 obviously impossible that the sexual organs and 

 elements of every individual can have been specialised 

 with respect to every other individual. But there 

 is no difficulty in believing that the sexual elements 

 of each differ slightly in the same diversified manner 

 as do their external characters ; and it has often been 

 remarked that no two individuals are absolutely 

 alike. Therefore we can hardly avoid the conclusion, 

 that differences of an analogous and indefinite nature 

 in the reproductive system are sufficient to excite the 

 mutual action of the sexual elements, and that unless 

 there be such differentiation fertility fails. 



The appearance of highly self-fertile Varieties. We 

 have just seen that the degree to which flowers are 

 capable of being fertilised with their own pollen differs 

 much, both with the species of the same genus, and 



