. Biology. Biology. 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 metaboli
. Biology. Biology. 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, FlG- 2g—Paramecium . caudatum in conjugation. for example, to Cirrhosis of the liver The micronuclei can be nr VirWv <;rlpro<;k of arteries etc seen in the Process of di~ Ul KlClHcy, hLlclUblh Ul dl LCilcb, ;j:_ J ' vicling. ending inevitably in death after a united 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. viding. The organisms are in the peristome region. From
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