THE DEFINITION AND MEASUREMENT OF 

 SENESCENCE 



P. B. MeDAWAK, D.Sc, f.r.s. 

 Deportment of Zoology, University College, London. 



Introduction 



Nothing is clearer evidence of the immaturity of geronto- 

 logical science than the tentative and probationary character 

 of its system of definitions and measurements. "What is 

 ageing?" is a question to which there is no agreed answer. 

 It is therefore very natural that the first contribution to this 

 colloquium should be an attempt to suggest the ways by 

 which an answer might be arrived at. 



"Ageing", in the literal sense of merely growing older, 

 might well be a beneficial process; i.e. there are certain reasons 

 why an animal's expectation of further life should increase, 

 instead of becoming shorter, with advancing years. Animals 

 grow wiser as they grow older: so far as memory can influence 

 their actions, the survivors of earlier hazards are less likely 

 to be their victims when exposed to them for a second time. 

 For example, Lack (1951) has shown that beyond the first 

 few months of life the mortality of wild birds is independent 

 of their age; but game birds (whose census is partly carried 

 out by gunfire) may suffer a disproportionately high mortality 

 during the first two years of life, after which the survivors 

 presumably know better. Then again, animals become wiser 

 in an immunological sense, for antibody formation is a process 

 with a long memory, and a second response to an antigenic 

 stimulus is much more effective than the first. (Immuno- 

 logists were speaking of the anamnestic response, a not- 

 forgetting of past behaviour, long before biologists came to 

 regard the storage of information as a conceptual novelty.) 



Yet ageing in man is accompanied by that general deteriora- 

 tion which the word itself colloquially implies, and the same 



