CONCLUSIONS FROM EXPERIMENTS 183 



of senescence as in some sense secondary or incidental, and not as 

 a necessary and inevitable consequence or a part of the cycle of 

 development. According to such theories senescence is due to 

 incomplete excretion of toxic products of metabolism of one kind 

 or another, or to a wearing out of certain organs for one reason or 

 another, to evolutionary adaptation, or to some other incidental 

 factor. The theories of the other group regard senescence as a 

 result of the same processes which determine growth, differentia- 

 tion, and what we call development in general. These theories 

 attempt to find the conditions and processes which determine 

 senescence in the conditions and processes which underlie develop- 

 ment. From this point of view senescence is a feature of develop- 

 ment. The experimental data presented in the preceding chapters 

 leave little room for doubt that both senescence and rejuvenescence 

 are necessary and inevitable features of the life cycle. Certainly 

 the worn-out organs of old animals cannot be repaired by an 

 extended period of starvation, nor is the elimination of toxic meta- 

 bolic products likely to be assisted by the structural degeneration 

 of parts which occurs in various cases of reconstitution. Senescence 

 and development are simply two aspects of the same complex 

 dynamic activities. 



Since our knowledge of the metabolic reactions, on the one hand, 

 and of the colloid substratum of the organism, on the other, is not 

 very far advanced, we cannot at present determine the exact nature 

 of the relation between growth, differentiation, and senescence, and 

 reduction, dedifferentiation, and rejuvenescence. Nevertheless we 

 can point with considerable confidence to certain features of growth 

 and development as affording a basis for the changes of the age 

 cycle. 



It was pointed out in Part I that during development the 

 general metabolic substratum of the organism, the unspecialized 

 or embryonic cell, undergoes a progressive change in the direction 

 of greater physiological stability in consequence of changes in the 

 substratum and additions to it in the course of growth and differ- 

 entiation. The general result of these changes is a decrease in the 

 metabolic activity of each unit of weight or volume of the organism 

 because the proportion of the relatively stable constituents in the 

 substratum increases. 



