Definition and Measurement of Senescence 5 



is true of all mammals and birds which have been artificially 

 kept alive long- enough to suffer the inroads of senile decay. 

 The deterioration that goes with ageing could be described 

 as a decline of "vitalitv" or a lowerino- of biolooical efficiencv 

 — of an organism's power to maintain itself as a going concern; 

 but this is more of an innuendo than a formal definition, and 

 the question remains, how is senescence to be measured and 

 defined with acceptable precision? (It will be convenient to 

 use the seventeenth century word senescence to stand for the 

 deterioration that accompanies ageing, and to leave ageing 

 itself to stand for merely growing old.) 



The process of senescence and the state of senile deteriora- 

 tion can be measured or assessed in two entirely different 

 ways (Medawar, 1952). First, there is the kind of measure 

 that may be called personal, because it purports to measure 

 a process that takes place in the life history of an individual 

 animal; second, there is a statistical or actuarial measure, 

 which is founded upon the mortality of a population of 

 individuals and which bears only indirectly upon the changes 

 that occur within the lifetime of anyone. The former is used 

 by the pathologist, physiologist, and (by an extension of his 

 terms of reference) the embryologist. The latter is used by 

 biologists generally, and in particular by ecologists and by 

 students of the genetical theory of evolution. 



The Personal Measure of Senescence 



If we knew of some master factor or prime mover in the 

 process of senescence — if, for example, every ingredient of 

 decay was causally dependent upon changes occurring in the 

 ])ineal gland — then a personal measure of senescence would 

 be in principle entirely adequate. The one master process 

 could be measured, and the rest should follow. But we know 

 of no such master factor; the processes of senescence are in 

 gear, but we do not know which is the driving wheel. All that 

 can be done, therefore, is to record separately the several 

 manifestations of decay — the wrinkling of the skin and grey- 

 ing of hair, the anatomical involution of the organs, the 



