The Nature and Criteria of Senescence 



tional morphogenetic control, and would resemble an auto- 

 matic control device which has run out of 'programme'. In any 

 such system the equilibrium must be increasingly unstable. 

 These two views of senescence, as accumulation of delayed 

 lethal or sublethal genetic effects, and as a withdrawal of the 

 evolutionary pressure towards homoeostasis with increasing age, 

 are complementary, though probably only partial, pictures of 

 its evolutionary significance. The concept of senescence as 

 exhaustion of programme also restores a far greater unity to 

 our definition of ageing, which includes a great many effects 

 having little in common beyond their destructive effect on 

 homoeostasis. All such effects fall within the idea of deteriora- 

 tion lying outside the 'terms of reference' of each species, as laid 

 down by natural selection. The 'flying bomb' which failed to 

 dive on its objective would ultimately 'die' either of fuel 

 exhaustion, or through wear in its expendable engine. If its 

 design had been produced by evolution, and its evolutionary 

 relevance ceased at the moment of passing its objective, or 

 decreased as a function of the distance flown, both these events 

 would be outside the programme laid down by the selective 

 equilibrium, as they were outside the calculation of the design- 

 ing engineers. Death in such an expendable system may result 

 from one of many factors, and even, as Bidder recognized, from 

 the consequence of processes which contribute to fitness during 

 earlier life, such as systems of differential growth. We shall find 

 a good deal of gerontology is primarily the study of a living 

 system's behaviour after its biological programme is exhausted. 

 The various evolutionary explanations of ageing already com- 

 bine to offer us some idea of the reasons why this may be so. 



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