III.] LIFE AND DEATH. 1 47 



in the fact that there is only one generation of somatic cells in 

 the latter, and these are used up in the process of metaboHsm at 

 almost the same time that the reproductive cells are extruded, 

 while among the former there are successive generations of 

 somatic cells. I have elsewhere endeavoured to render the 

 duration of life in the animal kingdom intelHgible by the appli- 

 cation of this principle, and have attempted to show that its 

 varying duration is determined in different species by the 

 varying number of somatic cell-generations \ Of course, the 

 varying duration of each cell-generation materially influences 

 the total length of life, and experience teaches us that the dura- 

 tion of cell-generations varies, not only in the lowest Metazoa 

 as compared with the highest, but even in the various kinds of 

 cells in one and the same species of animal. 



We must, for the present, leave unanswered the question — 

 upon what changes in the physical constitution of protoplasm 

 does the variation in the capacity for cell-duration depend ; 

 and what are the causes which determine the greater or smaller 

 number of cell-generations. I mention this obvious difficulty 

 because it is the custom to meet every attempt to search 

 deeper into the common phenomena of life with the reproach 

 that so much is still left unexplained. If we must wait for the 

 explanation of these processes until we have ascertained the 

 molecular structure of cells, together with the changes that 

 occur in this structure and the consequences of the changes, we 

 shall probably never understand either the one or the other. 

 The complex processes of life can only be followed by degrees, 

 and we can only hope to solve the great problem by attacking 

 it from all sides. 



Therefore it is, in my opinion, an advance if we may assume 

 that length of life is dependent upon the number of generations 

 of somatic cells which can succeed one another in the course 

 of a single life ; and, furthermore, that, this number, as well as 

 the duration of each single cell-generation, is predestined in 

 the germ itself This view seems to me to derive support 

 from the obvious fact that the duration of each cell-generation, 

 and also the number of generations, undergo considerable in- 

 crease as we pass from the lowest to the highest Metazoa. 



* See the first essay on ' The Duration of Life.' 



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