Biological Approach in Study of Ageing 5 



justification, besides our original preoccupation with man, for 

 seeing a biological unity in these processes, and for accepting a 

 general definition of this kind, but it is in terms of the place 

 which senescence seems to occupy in evolution, not, in the 

 first place, of cytochemistry. 



In a population subject to the high mortality which affects 

 most wild animals, the evolutionary relevance of individuals 

 depends on the relative numbers of offspring they contribute, 

 and the selection pressure toward vigour and homeostasis 

 probably declines very rapidly with increasing age (Medawar, 

 1955a). It has therefore been suggested that the common 

 feature of ageing processes in animals is that they represent a 

 "running out of programme", and an escape from the selec- 

 tion pressures which maintain fitness early in life. Where 

 individuals rarely reach even mid-adult life unless protected, 

 the postponement of an adverse effect is as effective as its 

 removal, and this will apply both to selection pressure towards 

 further somatic-cell renewal and probably to the elimination 

 of late-acting lethal genes (Haldane, 1941). If this is so, we 

 might expect the processes which limit the lives of different 

 animals to be diverse. At the same time, since the cells of 

 animals are in general similar in many important respects, it 

 is quite probable that some deteriorative processes at the 

 cellular level are shared by all animals. It is probable, in 

 particular, that the lifespan of any non-dividing cell is fixed — 

 perhaps by processes similar to those which Hinshelwood 

 (1951) found in non-dividing bacteria. If so, any animal con- 

 taining essential and unrenewable cells would eventually 

 display a loss of vigour from that cause. But it does not follow 

 that this is in fact the cause of the senescence of mammals, 

 either through loss of neurones or through endocrine deteriora- 

 tion. Their senescence might equally depend on progressive 

 changes in the quality of cells produced in regenerating tissues. 

 It might conceivably depend on mechanochemical and non- 

 cellular changes in tissue materials which have a low turnover 

 rate, analogous to the changes which cause the senescence of 

 some trees. But these are hypotheses, not definitions, and the 



