The Biology of Senescence 



in later life. The prolongation of that part of the life cycle 

 which lies normally between the eighteenth and fiftieth years 

 of age, even by a small percentage, would come nearest to 

 fulfilling our objectives. The prolongation of childhood pre- 

 sents special interest, since, on existing mammalian evidence, 

 it is quite probably feasible by relatively simple, if heroic, 

 means such as calorie-restriction. Such a prolongation would have 

 interesting and far-reaching social consequences, both upon the 

 family and upon the acquisition of skills: its effects on character- 

 structure would be even more interesting. It seems very prob- 

 able that the time-scale of prepubertal development in the 

 majority of mammals is relatively labile. There is so far no 

 direct evidence that the same applies to later stages in the life 

 cycle. Dietary restriction, although it may favour longevity, 

 does not greatly delay senescence in adult mammals, and it may- 

 be that the time scale of the adult period, after somatic growth 

 has ceased, is not susceptible to any major interference without 

 at the same time destroying normal function. To assess the 

 possibilities of such interference we require to know how far 

 'marking time' at each stage of the mammalian developmental 

 programme is possible, and, if possible, is compatible with 

 functional health. It is also a matter of practical import whether 

 the rate of child growth influences the length of the period of 

 adult vigour in man (Sinclair, 1955). The degree of linkage 

 between growth, development, and metabolism may vary con- 

 siderably at different periods of the life cycle, and the bulk of 

 the work upon their separation has been carried out only in 

 non-mammalian embryos and larvae. We have to reckon with 

 the possibility that the post-pubertal mammal behaves like an 

 imago — that its life-span is closely linked to metabolism, which, 

 in homoeotherms, is virtually invariable by the methods which 

 can affect it in invertebrates, and that the fundamental change 

 which leads to eventual senescence has already taken place at 

 puberty. In this case, interference with the length of the adult 

 phase could only be prosthetic. 



There remains the possibility that a substantial change in the 

 specific age, and in the duration of healthy life, might result 

 from one particular adjustment. This was the hope which led to 

 the use of sex hormones for purposes of 'rejuvenation', and 



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