THE UNIQUENESS OF THE INDIVIDUAL 



which the factor which governs some heritable endowment 

 appears in the population. 



I said, earlier on, that any theory of the origin of the ageing 

 process must take two things into account: the early onset of 

 what is in the technical sense senescence, and the continuity 

 of its expression through life. I would like now to suggest that 

 the 'force of mortality"* has been moulded by a physical oper- 

 ator that has the dimensions of time x luck. Let us examine 

 how natural selection will work upon a population that is 

 potentially immortal; of which the individuals remain, for all 

 the time that they are alive, in the fullness of physical maturity. 

 Such a population will contain old animals and young. The old 

 are old in years alone: we are so used to hearing the overtones 

 of senility in the word 'old"* that we must forcibly adjust our- 

 selves to accept this important qualification. The old animals 

 I shall speak about are '"in themselves"* (to use a category of lay 

 diagnosis) '■young\ They will no doubt have the advantage of 

 their juniors in reflex and immunological wisdom, but these 

 advantages will in the first approximation be disregarded. 



Upon this population exempt from age decline we shall now 

 superimpose a variety of causes of death that are wholly 

 random or haphazard in their manner of incidence. The causes 

 of death being random in nature, and susceptibility to it inde- 

 pendent of age, it follows that the probability that an animal 

 alive at the beginning of any span of time will die within its 

 compass is likewise constant. The one-year-old is just as likely 

 to see his second birthday as is the fifty-year-old to see his 

 fifty-first. But the chances at birth of living to age 1 and age 50 

 are very diff'erent indeed; for as Weismann pointed out, though 

 the significance of it escaped him, the older an animal becomes 

 the more frequently is it exposed to the hazard of random 

 extinction. Likewise a coin that has turned up heads ten times 

 running will turn up heads on the eleventh spin in just 50 per 

 cent, of trials; but the chances of turning up heads eleven times 



36 



