VARIETY, RACE, MODIFICATION. 33 



non-plasmic substances, and hence do not give rise 

 to inheritable characters in the organism. Modi- 

 fications persist only so long as their causes, and 

 under other environments immediately pass over 

 into the modifications corresponding to them. The 

 transition is completed in the lowest plants during 

 a limited number of cell generations ; in an individ- 

 ual of the higher plants on the same stem during the 

 growth of a single year. Each variety and each 

 race appears clothed in a definite modification, and 

 can change it within a range peculiar to itself.* 



l8. SOCIAL AND INDIVIDUAL ORIGIN OF SPECIES. 



The species arises neither from the nutrition 

 variety nor from the race; it is always a more 

 advanced variety, and hence species formation is 

 identical with variety formation. Cause for varia- 

 tion and consequently for variety formation is always 

 shown, either when, environment remaining the 

 same, the automatic variation of the idioplasm has 

 advanced so far that the ontogeny is raised to a 

 higher grade of organization and division of labor, 

 or when external stimuli act for a sufficiently long 

 time in a manner not in harmony with the previous 

 adaptation. Hence various varieties arise easily 

 from a uniform kinship, when these are thrown 

 among unlike external influences by local separa- 

 tion, because in the separated places on the one 



*The distinctions which Nageli here erects are, of course, purely arbi- 

 trary, and hi* definitions are suitable for use only in his own thesis. Trans. 



