OLD AGE AND NATURAL DEATH 



chosen time divided by the size at that same time — in other 

 words, by the material theoretically available for further grow- 

 ing. It cannot be denied that the specific growth rate is a 

 measure of vitality, though not perhaps so complete a measure 

 as Minot in his time believed. Minot found that the power of 

 tissue to reproduce itself at the rate at which it was formed fell 

 off through life from earliest childhood onwards. He found that 

 the decline was faster in children than in their elders, and, 

 indeed, that it fell off more and more slowly as life went on. The 

 inferences he drew were these. There is no period of increasing 

 vitality leading to the mature state and thereafter to the senile; 

 the process of ageing goes on continuously throughout life. 

 And ageing is faster in young animals than their elders — ""a 

 strange, paradoxical statement\ ^Our notion that man passes 

 through a period of development and a period of decline is 

 misleading ... in reality we begin with a period of extremely 

 rapid decline, and then end life with a decline which is very 

 slow and very slight."* 



This is a good moment to ask what the life insurance com- 

 panies have to say about these problems. Their evidence is at 

 first sight very helpful. Look at the curve from which the 

 actuary computes the force of mortality at various ages — the 

 curve which defines, for each age of life, the numbers still living 

 of a certain initial number born alive. ^ From the twelfth or 

 fifteenth year onwards in human life, the curve is smooth; there 

 is no break or discontinuity, no hint at all that at such an age 

 the prime of life has ended and old age begins. Nor is this 

 generalization false for animals other than man. *'Life tables' 

 for them are pitifully meagre; but Leslie and Ranson made one 



^ For the terminology used in actuarial work, cf. L. Hogben: The Measure- 

 ment of Human Survival, in New Biology, ed. M. Abercrombie and M. L. 

 Johnson. Penguin Books, London, 1944. [See also L. I. Dublin, A. J. 

 Lotka and M. Spiegelman, Length of Life: a Study of the Life Table, New 

 York, 1949.] 



/>w J. 



