ASPECTS OF KINETIC EVOLUTION 329 



The character-unit hypothesis of heredity is one of the corol- 

 laries of the environmental causation hypothesis of evolution. 

 It seemed necessary to predicate 'something in addition to the 

 observed methods and sequences of organic existence, in order to 

 explain the evolutionary progress of species. How could the 

 environment change the characters of organisms, and how could 

 the changes of the characters be inherited and bring about the 

 transformation of the characters of the species? These are the 

 questions which Darwin sought to answer by his hypothesis of 

 pangenesis, a migration of determinant particles from all parts 

 of the body of the parent to the reproductive cells, so as to 

 repeat in the offspring the modifications which the parent organ- 

 ism had experienced. The doctrine of pangenesis never found 

 any support or justification in fact, since it could not be as- 

 certained that characters caused by the environment are in- 

 herited by pangenesis or otherwise. Nevertheless, the doctrine 

 of determinant character-unit particles has been kept alive by 

 the speculations of Nageli, Weismann, and many other mathe- 

 matically inclined students of evolutionary problems. 



The kinetic theory does not approach the problem from this 

 standpoint, for it finds causes of evolution in the facts of sym- 

 basic interbreeding and normal intraspeciric diversity. The 

 first significant fact in the direction of an explanation of evolu- 

 tion is the method of interweaving of the network of descent in 

 which evolutionary progress is carried forward. In place of 

 the assumption by static theories of a hypothetical mechanism 

 of character-determination, with an equally hypothetical result of 

 ideal uniformity, the kinetic theory presents for our study con- 

 junctions of lines of diverse descent and results of continued 

 diversity of offspring. 



HEREDITY IN CELL SPECIALIZATION. 



The fact that the germ-cells of the higher plants and animals 

 are so different from those of which the various tissues and 

 organs of the adult body are composed, has been taken to mean 

 that they have some special function of heredity. A long series 

 of exceedingly difficult and detailed investigations have been 

 made in the hope of discovering these causes of development 



