GROWTH, AGE, AND DEATH 299 



ing the physiological and structural characteristics o£ a young ani- 

 mal. Accompanying the reversion of growth is a corresponding 

 reversion of differentiation. In other forms that reproduce agamically 

 there is a similar reversal of tissue condition from an old to a young 

 state. But these methods of rejuvenescence are not to be applied to 

 complex vertebrates nor to mammals. The mammalian body repre- 

 sents a very complete differentiation of tissues and an extremely 

 closely correlated mechanism in which the continuation of life in 

 one tissue depends on the normal activities of all other tissues. Not 

 all tissues and organ systems have the same energy requirement; 

 hence starvation of a tissue may reach a degree fatal to its function 

 before others are extensively affected. The whole mechanism breaks 

 down when undergoing starvation as a result of the failure of one 

 or more parts and death results. There is no reversing of growth 

 nor of differentiation. 



Two other concepts of the nature of old age have attracted some 

 attention. According to one theory, the human body poisons itself 

 by reason of the absorption of toxic substances produced by bac- 

 teria from the contents of the digestive system; the long-continued 

 accumulation of effects of such absorption is said to be responsible 

 for senescence. There is sufficient fact to lend some weight to this 

 theory, for the blood stream does continually absorb substances 

 that result from the fermentation of foods in the intestine. However, 

 while this may be contributory to the onset of various infirmities 

 that characterize old age in Man, it can hardly be regarded as the 

 cause of age; senescence is a metabolic phenomenon, not a bac- 

 terial disease. According to another theory that has attained some 

 prominence with the growth of knowledge of the endocrines (p. 

 203), senescence is a consequence of decline of the normal activity o£ 

 certain endocrine glands, particularly of the gonads. Attempts have 

 been made to restore bodily vigor by implantations of glands from 

 various sources. The results have not in general afforded support for 



