The Nature and Criteria of Senescence 



Much information about the behaviour of self-restoring and 

 self-regulating systems, and a number of important general 

 concepts, are now available from the study of mechanical 

 models. These analogies apply, strictly, only to the elucidation 

 of single components of the process of maintaining physiological 

 stability; the most important feature of 'cybernetics' and homoe- 

 ostasis in the organism has no precise mechanical analogy. 

 This is the fact that the homoeostatic process, the state of 

 quantitative invariance, or self-restoration, in various physio- 

 logical systems, is superimposed on qualitative and quantitative 

 change in the nature of the systems themselves, their specificity, 

 relative proportions, and function — in other words, upon de- 

 velopmental change. There is a strong inference that senescence 

 occurs when these long-term changes, which are probably con- 

 trolled or initiated largely by the same humoral mediators which 

 function in day-to-day homoeostasis, pass out of control, or 

 reach a point beyond which homoeostasis is no longer possible. 



This argument ultimately stands or falls by the result of our 

 study of the phylogeny of senescence. If mammalian senescence 

 results from morphogenetic processes which ultimately escape 

 from the homoeostatic mechanisms that operate during adult 

 vigour, and if, on the other hand, some other vertebrates reach 

 a state of growing, or self-replacing, equilibrium, even over 

 limited periods, the problem of understanding mammalian sen- 

 escence will be very greatly restricted in theoretical scope, 

 though probably not very greatly simplified in experimental 

 detail. Such an equilibrium would be most likely to be found 

 in those forms where differential growth is least evident. The 

 evidence on this point will be examined later. 



1 -3 Senescence in Evolution 



Senescence has frequently been regarded as an evolved 

 adaptation, rather than as an inherent property of somatic 

 organization. This view, which is reasonably well in accord 

 with the existing, and very incomplete, evidence of its distri- 

 bution in phylogeny, was held by Weismann in spite of his 

 insistence on the contrast between germinal immortality and 

 somatic mortality. Weismann, however, regarded senile change, 



37 



