THE RANGE OF DOMINANCE 151 



embryonic or slightly differentiated tissue of the grow- 

 ing tip itself. 



In the higher animals the extension of dominance is 

 evidently very much greater than in the lower forms. 

 In the medullated nerve fibers of the higher vertebrates 

 the transmission-decrement is so slight that some authors 

 have denied its existence. Various lines of experiment 

 have indicated, however, that a transmission-decrement 

 does exist even in vertebrate nerves (see pp. 173-75). 

 Tashiro has shown that a gradient in carbon-dioxide 

 production exists in nerve fibers, and I have observed a 

 distinct susceptibility gradient in certain nerves. The 

 nerve is essentially a specialized protoplasm which 

 conducts with less decrement and therefore to greater 

 distances than other kinds of protoplasm, and the 

 central nervous system arises in those regions of the 

 body where the transmitted changes primarily originate. 



The extension of dominance during the development 

 of the higher animals is so great that the range of domi- 

 nance is undoubtedly very much greater than the size 

 of the individual. In these forms individual size is 

 limited, not by the range of dominance, but by the 

 decrease in metabolic rate which accompanies the pro- 

 gressive differentiation, and so limits growth. Only 

 in early stages of development, or in the lower organ- 

 isms, where nerves are either absent or not very good 

 conductors, does the size of the individual equal the 

 range of dominance. 



EXPERIMENTAL PHYSIOLOGICAL ISOLATION AND 

 REPRODUCTION IN PLANTS 



The course of development in the single plant 

 individual suggests the dominance of the growing tip 



