The Nature and Criteria of Senescence 



metrical progression (Wiesner and Sheard, 1934). Real survival 

 graphs are commonly intermediate in form between the two 

 ideal contours. Pearl and Miner (1935) distinguished three 

 main types of observed death-curve, varying in skewness from 

 the nearly rectangular in organisms with a low standing death- 

 rate throughout life, but showing a tendency to die almost 

 simultaneously in old age, to the logarithmic decline char- 

 acteristic of populations which show no senescence, or which 

 die out before it can become evident (Fig. 6) . A fourth theoret- 

 ical type, in which the curve is rectangular but inverse to that 



TIME 

 Fig. 5. (a) — Survival curve at a 

 constant rate of mortality (50 per 

 cent per unit time) . 



TIME 

 Fig. 5 (b). — Survival curve of a 

 population which exhibits sene- 

 scence. 



found in the ideal senescent population, was recognized by 

 Pearl (1940) as a theoretical possibility; it seems to be realized 

 in nature among organisms which have a high infant mortality, 

 but whose expectation of life increases over a long period with 

 increasing age. This pattern of survival is characteristic of some 

 trees (Szabo, 1931) but probably also occurs in animals. 'There 

 may be animals in which the expectation of life increases con- 

 tinuously with age. This may be so for many fish under natural 

 conditions. It certainly goes on increasing for a considerable 

 time. Thus in a species where the expectation of life was equal 

 to the age, or better, to the age plus one week, no members 

 would live for ever, but a small fraction would live for a very 

 long time. A centenarian aware of the facts would pity a child, 

 with an expectation of life of only a few years, but would envy 

 a bicentenarian' (Haldane, 1953). 

 G 19 



