THE UNIQUENESS OF THE INDIVIDUAL 



manner the rate of interest falls, we shall be led by Minot into 

 an attractive paradox. The answer is that from birth onwards 

 the rate of interest falls steadily at a rate which itself steadily 

 falls. Not only does senescence begin at birth, but it is going 

 on much faster in the early years of life than latterly. The child 

 is hurrying precipitately towards his grave; his elders, appro- 

 priately enough, proceed there in a more decent and orderly 

 fashion. 



None of these personal measures is of more than limited 

 value. They are together incomplete, and severally give differ- 

 ent answers; nor can they be made to add up to give a single 

 figure that represents a measure of senescence in the round. 

 Let us therefore turn to a scheme of measurement founded on 

 wholly different principles. 



Ill 



The second sort of measure is not personal, but statistical. 

 We have agreed that senescence is a decline of what may be 

 vaguely called vitality, and must now ask what property it is 

 that changes as a direct outcome of that decline. The property 

 is, in a word, vuhierability to all the mortal hazards of life; and 

 it is measured by the likelihood of dying within any chosen 

 interval of age. 



The measurement of vulnerability is in principle very easy. 

 Imagine 100,000 animals, each of which is labelled or otherwise 

 identified at birth and followed throughout its life; and suppose 

 one keeps a record of the age at which each dies, keeping the 

 record open until the death of the most long-lived. Such a 

 record might well be called a Death Table, but, by an agreeable 

 euphemism, it is in fact called a Life Table. If we plot the 

 number of survivors against age, the curve so defined starts 

 with age at 100,000 and falls to zero at the age of about 100 

 years. Fig. 1 illustrates the shape of the life-table curve for 

 human beings. 



48 



