6 P. B. Medawar 



4,yrowing insufticiency of endocrine output, the loss of acuity 

 of the senses, the weakening of muscular power and coordina- 

 tion, the decline of the specific growth rate and rate of cellular 

 turnover, and so on. All these can be described and measured, 

 in fact or in principle; but as mere description cannot allot 

 causal priorities among their competing claims, no one of 

 them can be held to be the measure of senile decay. 



Considered one by one, each of these measurements gives 

 different estimates of the time of onset and rate of progress 

 of senescence. Take, for example, the specific growth rate of 

 the body, i.e. its rate of growth considered as a system increas- 

 ing by compound interest, in which that which is formed 

 by growth is itself capable of growing. Many years ago, 

 Charles Minot pointed out that the specific or percentage 

 rate of growth in man fell from birth onwards, rapidly at 

 first, and then progressively more slowly. Man grows like 

 money invested at a rate of compound interest which falls 

 progressively at a rate which itself progressively falls. Minot 

 argued that the specific growth rate was a particularly 

 searching and intimate measure of vitality; it follows that 

 senescence begins at birth and goes on faster in children than 

 in their elders. Estimates of the rate of healing of wounds 

 have been held to reveal a similar trend, but some of the data 

 will not stand up to ill-disposed criticism. 



Measurements by other standards give different answers. 

 Acuity of hearing is said to be sharpest at about the age of 

 ten and resistance to infectious disease to be at its maximum 

 at about fifteen. Muscular power and coordination are 

 presumably at their peak at about age twenty-five. If 

 therefore we think of the body as a multiplicative growth 

 system, there is a sense in which senescence could be said 

 to begin at birth; as an antibody forming system, at fifteen; 

 and as a muscular machine at twenty-five. These several 

 measures of vitality or senescence are therefore incongruent, 

 and it is an important empirical fact that they are so; but 

 for the time being we must concede that no one measure of 

 senescence based upon the properties of individual organisms 



