228 SENESCENCE AND REJUVENESCENCE 



substratum, the greater the possibiUties of speciaHzation and differ- 

 entiation along the axis in relation to the gradient and therefore the 

 more intimate and complex the correlation between parts and the 

 higher the degree of unity in the whole. In the lower forms, where 

 structures once formed may disappear in a few hours or a few 

 days under altered physiological conditions, there is no possibihty 

 of such minute and dehcate interrelation and adjustment of parts 

 to each other as in the higher forms, where regressive changes are 

 much less extensive. In fact, the advance in development of the 

 nervous system itself from the lower to the higher forms is in part 

 dependent upon the increase in stabihty of the structural sub- 

 stratum. 



The degree of individuation is dependent upon the rate of 

 metabolism. At any given stage of development the higher the 

 rate of metabohsm, the higher the degree of individuation. But 

 we cannot properly compare earher and later stages of development 

 in this way, for, although the rate of metabohsm decreases during 

 development, the degree of individuation increases in most cases 

 up to the adult stage, because of the increasing efficiency of conduc- 

 tion and the specialization and interrelation of parts. It is only 

 after the adult stage is attained that the further decrease in meta- 

 bohc rate with advancing senescence determines a gradual decrease 

 in the degree of individuation, a physiological disintegration. 



Many other incidental and external factors may alter the degree 

 of individuation in organisms. In general, depressing factors 

 decrease and stimulating factors, at least up to a certain Hmit, 

 increase it. The point of chief importance is, however, the possi- 

 bihty of distinguishing different degrees of individuation and of 

 interpreting them to some extent, however incompletely, in physico- 

 chemical terms. 



PHYSIOLOGICAL ISOLATION AND AGAMIC REPRODUCTION 



If the axiate individual consists of a dominant and of sub- 

 ordinate parts, the structure, differentiation, and special function 

 of the subordinate parts are dependent, at least to a considerable 

 degree, upon their relation to the dominant part. Isolation of such 

 parts from the influence of the dominant part must result, if the 



