SENESCENCE IN HIGHER ANIMALS AND MAN 267 



and the factors concerned in it is of course the problem of the evo- 

 lution of the individual, but only certain aspects of this problem 

 need consideration here. 



The evolution of the individual is evidently closely associated 

 with an increasing functional and structural stability of protoplasm. 

 In the higher forms a cell or a group of cells, once started along a 

 certain course of development, reacts less readily than in the lower 

 organisms to altered conditions by regression and change in the 

 course of development. In the adult vertebrates the capacity for 

 regression is in most cases so narrowly limited that the cells of one 

 tissue are under any known conditions incapable of giving rise to 

 other tissues. In other words, the ability of the cells, so conspicu- 

 ous in the lower organisms, to react to altered conditions by a change 

 in activity which brings about the breakdown and elimination of 

 previously accumulated structural substance is very slight in the 

 iiigher animals. From this point of view evolution appears as a 

 change from less stable to more stable dynamic equilibrium, in the 

 course of which the morphogenetic and functional behavior of the 

 organism has become less directly dependent on external and more 

 dependent on internal conditions. This increase in structural and 

 functional stability results in a greater degree of continuity in 

 progressive development and so in a greater specialization of parts 

 and a greater differentiation of structural mechanisms with definite 

 functions, which in turn provide a basis for a more varied and 

 intimate correlation of parts and so for a wider range and greater 

 delicacy of functional adjustment. 



Among these changes the most important for the integration of 

 the individual are the functional and structural evolution of the 

 nervous system. The high metabolic rate in the cells of the cen- 

 tral nervous system undoubtedly determines that the accumulation 

 and transformation of substance in the structural substratum 

 which bring about senescence occur less rapidly here than in other 

 tissues; because of its high rate of metabolic flow, the nerve cell 

 deposits structural sediment relatively slowly. This is particularly 

 true after the stage of specialized functional activity is attained, 

 for then stimulation through the sense-organs and other parts of 

 the body plays a very important part in maintaining the nerve 



