310 SENESCENCE AND REJUVENESCENCE 



changes. This characteristic of the nervous system suggests the 

 probability that the natural or physiological length of Hfe in these 

 forms is determined primarily by the length of Ufe of the nervous 

 system and that physiological death is primarily the death, as the 

 final stage of senescence, of the nervous system. This view is 

 supported by various facts of observation. 



Physiological or natural death is not something which has 

 originated in the course of evolution from the lower to the higher 

 forms. All organisms, from the lowest to the highest, from the 

 simplest to the most complex, undoubtedly die of old age, unless 

 senescence is compensated by rejuvenescence. In the lower forms 

 the death point may never be attained under the usual conditions 

 because the low stability of the substratum and the consequent 

 low degree of individuation permit the frequent occurrence of a high 

 degree of rejuvenescence. In the higher forms death becomes 

 inevitable and necessary because the capacity for rejuvenescence 

 is limited by the greater stabihty of the substratum. For his high 

 degree of individuation man pays the penalty of individual death, 

 and the conditions and processes in the human organism which 

 lead to death in the end are the conditions and processes which 

 make man what he is. The advance of knowledge and of experi- 

 mental technique may make it possible at some future time to 

 bring about a greater degree of rejuvenescence and retardation of 

 senescence in man and the higher animals than is now possible, but 

 when we remember that the present condition of the protoplasmic 

 substratum of these organisms is the result of milUons of years of 

 evolutionary equilibration, we cannot but admit that this task may 

 prove to be one of considerable difficulty. 



REFERENCES 

 Benedict, F. G. > 



1907. "The Influence of Inanition on Metabolism," Carnegie Inst. Publ., 



No. 77. 

 1915. "A Study of Prolonged Fasting," Carnegie Inst. Publ., No. 203. 

 Champy, C. 



1913. "La diff6renciation des tissus cultives en dehors de I'organisme," 

 BiUiogr. Anat., XXIII. 



1914. "Notes de biologie cytologique. Quelques resultats de la methode 

 de culture de tissus: III, Le rein," Arch, de zool. exp., LIV. 



