ASPECTS OF KINETIC EVOLUTION 383 



ration, while the other is approaching that of complete sterility. 

 Horticulturists have not uncommonly believed that the longer 

 the succession of " grafted generations " of tree fruits the greater 

 the likelihood of deviations from the type of the original seedling, 

 but this idea seems not to have received scientific consideration 

 or support, perhaps because it appeared to contradict the opinion 

 of Darwin 1 and many other evolutionary writers who have held 

 that characters can be permanently " fixed " by inbreeding, or 

 close selective segregation, of which propagation by cuttings 

 may be taken to be the extreme form. The kinetic theory of 

 evolution permits us to understand, however, that the "fixity" 

 to be secured either by inbreeding or by asexual propagation is 

 only relative, and that its result in both cases is to predispose 

 the organism to abrupt variations and reproductive debility. 



ORIGIN OF LINIC AND CLONIC CONDITIONS. 



The occurrence of self-fertilization , parthenogenesis, and vege- 

 tative propagation in nature has undoubtedly caused many 

 writers to suppose that these methods of descent represent 

 truly normal evolutionary conditions. Indeed, no abnormality 

 need be charged in the many cases where the species maintains at 

 the same time the normal network of descent by sexual repro- 

 duction with free interbreeding. The abnormal condition super- 

 venes when the species loses its network of symbasic descent 

 and is resolved into disconnected lines. Such a condition may 

 result whenever the normally sexual and symbasic reproduction 

 becomes less effective than autogamous or purely vegetative 

 methods of propagation. Thus, in such little plants as Draba 

 and Viola, which have to avoid the competition of larger neigh- 

 bors by blossoming early in the spring, the non-symbasic 

 methods of propagation take on great importance, for insects 

 are scarce and the weather often so inclement as to completely 

 prevent the transfer of pollen. 



Similarly, in alpine and arctic conditions, vegetative propaga- 

 tion is much safer, and usually much more successful than sex- 

 ual reproduction. The short and treacherous seasons often pre- 

 vent the ripening of seed. The formation of apogamic bulblets 



'The Effects of Cross and Self-Fertilization in the Vegetable Kingdom, p. 27. 



