The Biology of Senescence 



and the limitation of the individual life-span, as a positively- 

 beneficial adaptation, and his argument is, as we have seen, of 

 a circular kind. 



The theoretical difficulties of devising a system in which short 

 life is selected as a character of fitness are considerable though 

 not insuperable. 1 In any circumstances where a high number of 

 generations in unit time has an adaptive value, the Weisman- 

 nian argument against individual longevity might hold. The 

 most conspicuous adaptive modifications of life-span in phylo- 

 geny seem, however, to be chiefly in the other direction. The 

 development of social insects probably depended upon the 

 evolution of long-lived sexual forms, and it is very likely that 

 a similar process may have operated in human phylogeny, in 

 connection with the development of social behaviour and the 

 family unit. Not only was the evolution of neurones having a 

 long potential life a condition for the development of elaborate 

 learned behaviour and long parental dependence, but, with the 

 development of rational power and social organization, the 

 advantages of possessing the experience of even a few long-lived 

 members was probably very high in any early hominid com- 

 munity. The social animals, especially man, provide one of the 

 best examples where longevity depending on factors outside the 

 reproductive period can theoretically be subject to positive 

 selection in terms of fitness. 



The chief objection to Weismann's idea of senescence as an 

 adaptive effect is the rarity of its demonstrable occurrence in 

 nature. In all but the few forms discussed on pp. 108-13, 

 senescence is a potentiality, not a benefit or a handicap; it is 

 realized only when we interfere artificially with the animal or 

 its environment, and it is arguable whether evolution can select 

 for such potentialities. Bidder, it will be recalled (p. 12), con- 

 sidered that senescence in mammals was an evolutionarily un- 

 important 'by-product' of an important positive adaptation, the 

 limitation of size. It would indeed be possible to attribute senile 

 change to the accumulation of such by-products outside the 

 reproductive period. More recently it has been suggested that 



1 Ribbands (1953) found an apparent example in worker bees, where 

 the summer brood could increase its working life by consuming pollen, but 

 uses it instead to rear additional larvae. 



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