THEORIES OF INDIVIDUALITY 47 



isolation of parts probably does not occur except occa- 

 sionally in embryonic stages, for with the evolution and 

 development of the nervous system in the individual the 

 transmission-decrement decreases and the effective range 

 of transmission therefore increases until in the nerves 

 of mammals the transmission-decrement is inappreciable 

 under natural conditions in the lengths of nerve fiber 

 available for experiment. In these forms the physio- 

 logical limit of size of the individual determined by the 

 range of dominance is very great and is never attained 

 by the individual because growth is limited by the 

 progress of differentiation in the course of development. 

 In such organisms, then, physiological isolation does not 

 occur except occasionally in embryonic stages before the 

 nervous system has developed or under special condi- 

 tions which limit the range of dominance or decrease 

 the receptivity of subordinate parts. 



Moreover, in the higher animals the degree and 

 stability of specialization of parts of the body is so great 

 that in most cases they do not respond to physiological 

 or physical isolation by reproduction, but either die or 

 remain largely unchanged. For these reasons asexual 

 reproduction among the higher animals is rare and is 

 limited to early developmental stages. Sexual or 

 gametic reproduction which results from the union of the 

 two gametes or sex cells, which are usually speciaKzed 

 and differentiated as egg and sperm, is somewhat more 

 complex than asexual reproduction, but I have already 

 endeavored to show that there is a fundamental physio- 

 logical similarity in the two processes,^ and I shall con- 

 sider the question briefly in a later chapter. 



^ Child, Senescence and Rejuvenescence, 1915, chaps, vi, x, xiii, xiv, xv. 



