The Biology of Senescence 



depiction of the state of affairs in long-lived cold-blooded 

 vertebrates (at least there seems to be no good evidence to the 

 contrary), but it does not appear to obtain in mammals. 



We have already suggested that while we might have reason 

 to expect senescence, or one form of it, in the total absence of 

 cell division, either in the whole animal or in certain organs, 

 it is not self-evident why, in order to avoid senescence, an animal 

 should be obliged to increase constantly its total cell number or 

 its overall body size. If this were the case, it would suggest, 

 perhaps, not that growth prevents senescence, but that the 

 capacity for continued growth reflects a type of morphogenetic 

 physiology which does not produce senescence. 



The possibility exists, then, that vertebrate growth-cessation 

 might be of two kinds: that some cold-blooded vertebrates may 

 cease to grow visibly when a cell-population of a particular 

 size and composition is reached, and that this population there- 

 after remains substantially static, with replacement of all except 

 such mechanically irreplaceable cells as neurones, while mam- 

 malian growth is arrested by a more active process — probably 

 of differentiation rather than mere mitotic inhibition — affect- 

 ing a few key points. This would resemble in its effects the 

 difference between the behaviour of a society which voluntarily 

 limited its reproduction to replacement level, and one which, 

 when a predetermined figure was reached, summarily castrated 

 a vital and hereditary profession. Such a difference, if real, 

 would explain the apparently less catastrophic effects of growth 

 cessation upon those reptiles which exhibit virtual specific size, 

 compared with the rapid post-mature decline in most mammals. 

 Birds, significantly, occupy a midway position, since it is virtu- 

 ally certain that all species are subject to senescence in cap- 

 tivity, though at specific ages considerably higher than those of 

 mammals of comparable size and activity: their period of 

 growth, however, is proportionately much shorter. A serious 

 investigation of the phylogeny of senescence is badly needed. 

 The hypothesis put forward here would regard it, so far as 

 mammals are concerned, not as the consequence of general 

 growth cessation, but of a particular manner of growth cessation, 

 involving, perhaps, selective non-renewal of certain important 

 structures and changes in the specificity of the response in 



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