ASPECTS OF KINETIC EVOLUTION 249 



effects of fertilization or cell-conjugation in endowing the new 

 organism with the power of vigorous growth. With this inter- 

 pretation of sexual differences in mind we are the more ready 

 to entertain the idea that specializations of heterism would be 

 beneficial, even apart from the sexual diversification of the 

 species, and are thus able to recognize and appreciate a group 

 of phenomena which has hitherto remained meaningless and 

 neglected. 



Since the time of Sprengel and especially since Darwin, it 

 has been known that many plants, even those which are 

 bisexual, or provided with both pollen and egg-cells, have many 

 specialized habits and devices which serve to secure cross-fer- 

 tilization. Although possessed of pollen of their own the flowers 

 are often so formed that the pistils receive pollen only from 

 abroad, and in many species foreign pollen is a necessity, pollen 

 from the same plant being entirely ineffective. The advantage 

 of cross-fertilization being admitted, the value of these adapta- 

 tions for securing it becomes obvious, but the benefits lie, as 

 Darwin discovered, not in the " crossing by itself" which " does 

 no good," but in the diversity of parentage which may in this 

 way be brought about. These specializations have, in other 

 words, a double function ; they assist in the crossing and 

 also minister to the diversity of descent which is the object of 

 the crossing. They have, in other words, the same function 

 as sexuality, and have been interpreted by naturalists as a simple 

 or incipient form of sexuality. 



Still simpler specializations of heterism have only one of these 

 two functions, that of maintaining the diversity, but without 

 assisting in the bringing of the diverse parents together. The 

 crossing is left, apparently, to chance, but when it takes place 

 the diversity renders it the more effective. As instances of this 

 simple type of specialized heterism may be cited such species 

 as Verbascum blattaria, the flowers of which are pink on some 

 plants and yellow on others. The two types grow freely inter- 

 mingled over wide ranges of country but no intermediates are 

 found. 



