ASPECTS OF KINETIC EVOLUTION 35 I 



diversification of the sexes has not taken place twice only 

 among the plants, but probably hundreds of times, independ- 

 ently, and in different and unrelated natural groups, the ances- 

 tors of which were bisexual. Separate sexes, though well-nigh 

 universal among the higher animals, both arthropods and verte- 

 brates, show, nevertheless, numberless independent specializa- 

 tions. In short, no tendency of evolution has been so definite 

 and so general as that leading toward the accentuation of sexual 

 differences. This can hardly mean anything less than that 

 diversity of descent, to which sexuality ministers, has a general 

 physiological importance and is not merely incidental to fortuitous 

 collocations of character-units. No doubt it will be found that 

 the details of sex-determination differ much in the different 

 groups of animals and plants, but this will not diminish the 

 general significance of the phenomenon. 



Sex-determination by purely mechanical means might still 

 serve the purposes of symbasic interbreeding, but the heredity 

 which might be due to the existence and operations of such 

 mechanisms would not afford the basis of a complete theory 

 of evolution. It would still be in need of an evolutionary 

 explanation. 



VEGETATIVE MODIFICATIONS OF HEREDITY. 



Further reasons for preferring this idea of polar or positional 

 relations of the ancestral hereditary elements to that of charac- 

 ter units or determinants, is to be found in the fact that the 

 hereditary attributes of form and structure are apparently ca- 

 pable of change at any time in the life-history of the organism, 

 and not merely at the time of conjugation when under the more 

 mechanical theory the nature of the individual should be deter- 

 mined, once for all. 



As a matter of fact, plants do make extensive and permanent 

 alterations of their characters during the vegetative period. 

 Such cases, though relatively rare, are numerous in the aggre- 

 gate. The best known instances are those of bud varia- 

 tions or "sports/' as the gardeners call them, where^a single 

 bud produces a branch as different from the others as seed- 

 grown individuals, or more so. A bud mutation of coffee found 



