14 HEREDITY AND SEX 



new character that appeared would be grafted, so to 

 speak, on to the body of the species in which it appeared. 

 Once introduced it would be brought into combination 

 with all the possible combinations, or races, already 

 existing within the species. Some of the hybrid com- 

 binations thus formed might be very vigorous and would 

 survive. This reasoning, while hypothetical, and, per- 

 haps not convincing, points at least to a way in 

 which new varieties may become incorporated into 

 the body of a species and assist in the process of 

 evolution. 



It might be argued against this view that the same 

 end would be gained, if a new advantageous variation 

 arose in a species that propagated by non-sexual 

 methods or in a species that propagated by self-fertili- 

 zation. The offspring of such individuals would trans- 

 mit their new character more directly to the offspring. 

 Evolution may, of course, at times have come about 

 in this way, and it is know^n that in many plants self- 

 fertilization is largely or exclusively followed. But in 

 a species in which cross-fertilization was the estab- 

 lished means of propagation, the new character would 

 be brought into relation with all the other variations 

 that are found in the component races and increase 

 thereby its chances of favorable combinations. We 

 have in recent years come to see that a new heritable 

 character is not lost by crossing, or even weakened by 

 ''blending," as was formerly supposed to be the case; 

 hence no loss to the character itself will result in the 

 union with other strains, or races, within the species. 



If then we cannot explain the origin of sexual re- 

 production by means of the theory of evolution, we 



