40 INDIVIDUALITY IN ORGANISMS 



change upon it, and the gradient will be further intensi- 

 fied by the fact that slight transmitted changes do not 

 reach the more remote parts at all while they do affect 

 the parts nearer the dominant region. It is this general 

 average which determines the more conspicuous and 

 lasting effects at different levels. 



The continued existence of a metabolic gradient of 

 this kind undoubtedly determines an increase in the 

 conductivity of the protoplasm for the transmitted 

 excitation. Many facts indicate that within certain 

 limits the occurrence and repetition of transmission 

 increase the conductivity, and in all animals except the 

 simplest a nervous system which possesses a very high 

 degree of conductivity develops in relation to the primary 

 gradients. In most organisms there is therefore an 

 .extension of dominance during development, the trans- 

 mitted changes become effective through a greater dis- 

 tance, and their limit of effectiveness, w^hich of course 

 determines the range of dominance, becomes farther and 

 farther removed from the point of origin. This extension 

 of dominance, however, is itself limited by the changes 

 known as senescence, which become evident in a general 

 decrease in reaction rate. These relations of parts, 

 dependent in the final analysis on differences in meta- 

 bolic rate, constitute, as I believe, the foundation of 

 unity and order in the organic individual, the starting- 

 point of physiological individuation. 



If this conclusion is correct, the organic individual, 

 as a living entity possessing some degree of physiological 

 — not merely physical — unity and order, consists in its 

 simplest forms of one or more gradients in part of a cell, 

 a cell, or a cell mass of specific physico-chemical consti- 



