MATERIALS OF EVOLUTION 47 



pies, especially in plants, in which cross fertilization 

 between individuals belonging to groups, unmis- 

 takably species according to ordinary standards, 

 takes place. Darwin was also impressed by the fact 

 that even self-sterility often occurs in hermaphro- 

 ditic sjDecies, not only because self-fertilization is 

 often made difficult in one way or another, but also 

 because it does not actuallv occur even when the 

 spenn has access to the egg, as has been sho\^^l by 

 suitable tests. 



Darw^in was also familiar with the sterility of 

 hybrids from species-crosses, and here again he em- 

 jjhasized the lack of any sharp distinction; for in 

 some cases the sterility is complete, in others partial, 

 and in still others it is not present. All this is in har- 

 mony with his conception of the gradual breaking 

 vip of a species into new groups or varieties which as 

 they become more and more different may show, 

 when crossed, various degrees of fertility, and of 

 sterility in their offspring. 



Since Darwin, the subject has not advanced much 

 further, although genetics has contributed a little 

 more information and holds out promise of furnish- 

 ing more. It has been shown in one or two hermaph- 

 roditic species that genetic factors are present that 

 are concerned with self-sterilitv, and in a few other 

 cases it has been shown that similar conditions are 

 explicable on the assumption of more than one such 



