The Biology of Senescence 



development. Senescence is a change in the behaviour of the 

 organism with age, which leads to a decreased power of survival 

 and adjustment. It is not a single overall process, except in the 

 evolutionary sense which we have outlined. Various factors in 

 varying proportions contribute to the senile change in different 

 species. Among these are the deterioration of irreplaceable 

 structures; the sum of previous injuries which are imperfectly 

 repaired; and progressive morphogenetic changes in the nature 

 and specificity of cell response and organ function. Any or all 

 of these factors may contribute to senescence in a given species. 

 Experimental removal of the factor which operates earliest in 

 the life-span may reveal another subsequent to it, and so on. 

 There is no conclusive evidence to incriminate cessation of 

 growth as a 'cause' of senescence, except in cases where cell 

 division ceases altogether. Senescence is not an 'inherent' pro- 

 perty of the metazoa, but one which they have on several occa- 

 sions acquired as a potentiality, probably through the opera- 

 tion of evolutionary forces directed to other biological ends. 

 In this respect the senescence of insects and of man is probably 

 a comparable process only to the same extent that the eyes of 

 these organisms are comparable structures. It is obvious that 

 such a conception, while it does not prevent us from ascertain- 

 ing what factors produce the age-deterioration in a given 

 species, excludes general physiopathological theories of the 

 'causation' of ageing as a whole. 



Unlike the functional evolution of the eye, senescence is 

 typically an undirected process — not a part of the programme, 

 but a weakening of the directive force of the programme, an 

 escape from co-ordination, combined with the arrears of pro- 

 cesses which once contributed to fitness but are now running 

 free. Attempts to invest the programme of morphogenesis with 

 metaphysical or supra-natural properties (Driesch, 1941; Bur- 

 ger, 1954) have already been adequately answered by J. Need- 

 ham (1942), and need not be dealt with here. The idea of sen- 

 escence as the 'fated' or 'destined' end of the organism, i.e. a 

 positively-subsistent and ordered process of life-curtailment, 

 though it is not always the fruit of an avowed vitalism, has 

 much in common with it. Such treatment of senescence as an 

 evolved entity, and the idea that it must have developed as a 



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