AGE AND NATURAL DEATH 



69 



term old age, ending in natural death. The length of life varies 

 within wide limits, animals on the whole having shorter lives 

 than plants; some of the giant trees of California, for example, 

 live for tens of centuries while some of the insects (May-flies) 

 are born and live out their adult life within a single day. On the 

 other hand, some animals like the tortoise may live for hundreds 

 of years. 



Senescence. In the higher animals or metazoa the cells are 

 differentiated for the performance of different functions, and the 

 various activities of metabolism are 

 relegated to different types of special- 

 ized cells. These are the links in 

 the chain of vital phenomena which 

 weaken, give out and lead to old age. 

 The secreting cells, for example, ulti- 

 mately cease functioning one by one, 

 and their places in the tissue are 

 taken by non-functioning connective- 

 tissue cells ; when enough of these are 

 thus worn out and replaced, activity 

 of the organ is impaired and the 

 general vitality of the entire organism 

 is correspondingly weakened. In the 

 human organism this process results 

 in hardening of the tissues, leading, 

 for example, to cirrhosis of the liver 

 or kidney, sclerosis of arteries, etc., 

 ending inevitably in death after a 

 longer or shorter time. 



The problem of old age therefore 



resolves itself into the question, why do the individual cells 

 give out? It would seem that these differentiated cells of the 

 body are endowed with a limited possibility of action, or with 

 a " potential of vitality," which is gradually exhausted by 

 continued use. Yet some of these epithelial cells, under cir- 

 cumstances abnormal to the organism, have the capacity to live 

 far beyond the limits of the natural life of the organism to 



W- 



FIG. 29. Paramecinm 

 caudatum in conjugation. 

 The micronuclei can be 

 seen in the process of di- 

 viding. The organisms are 

 united in the peristome 

 region. From a photograph 

 of a preparation. 



