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i 

 AN UNSOLVED PROBLEM OF BIOLOGY ' 



disclosed the changes of senescence. I hope that makes it 

 clear. 



Senescence means a decline of vitality. How is this to be 

 more precisely defined and measured? One may set about trying 

 to measure senescence in two entirely different sorts of ways. 



The first sort of measure is personal, in the sense that it is 

 carried out on individual animals. Quite a number of schemes j 



of measurement are at our disposal. For example, the rate at 

 which wounds heal provides some sort of measure of what we i 



vaguely mean by vitality, since it depends on the multiplication I 



i 



or migratory activity of cells. What sort of answer does it give? 

 So far as we know, the answer is that the rate of wound healing 

 is highest at birth and steadily declines thereafter. In terms of 

 this measurement, therefore, senescence begins at birth and 

 the *"prime of life"* is something of a fiction. Or we might reason- 

 ably choose a measure founded on the acuity of the senses. 

 The acoustical prime of life, for example, appears to be in the 

 neighbourhood of the age often, for we are said to hear sounds 

 of higher pitch at ten than earlier or thereafter. On the other 

 hand physical strength, endurance and the niceties of muscular 

 co-ordination reach their peak at about twenty- five. 



All these are very piecemeal measures. The best, perhaps, is 

 that originally devised by Minot — the multiplicative power of 

 the tissues of the body, that is, their capacity to increase by 

 further growth in the manner in which they themselves were 

 formed. Organisms tend to grow by compound interest, for that 

 which is formed by growth is itself usually capable of further 

 growing. But the rate of interest falls; the organism grows like 

 a sum of money which, invested at birth at (say) 10 per cent, 

 compound interest, gathers in a lower rate of interest year by 

 year. The rate of interest does indeed fall from birth, and it is 

 at birth, if Minot is to be believed, that senescence must be 

 said to begin. And so, in some perfectly respectable sense, it 

 does; but if we pursue this train of thought by asking in what 



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