THE UNIQUENESS OF THE INDIVIDUAL 



obvious way.* To go back to colloquial speech, all these effects 

 are the effects of age but not necessarily the effects of ageing; 

 they may take their toll even if ageing is not accompanied by 

 an innate deterioration. Senescence, as it is measured by 

 increase of vulnerability or the likelihood of an individuaPs 

 dying, is therefore of at least twofold origin.^ There is (a) the 

 innate or ingrained senescence, which is, in a general sense, 

 developmental or the effect of '"nature^; and (b) the senescence 

 comprised of the accumulated sum of the effects of recurrent 

 stress or injury or infection. The latter is environmental in 

 origin and thus, paradoxically, the effect of '•nurture\ There is 

 always an empirical test for distinguishing between the two in 

 principle — one has only to find out whether a, first injury or 

 physiological abuse or stress is less well tolerated by older 

 animals than their juniors — but in the actual records of vulner- 

 ability the two are inextricably combined.^ 



* [Mammals which have what is optimistically described as a 'permanent 

 dentition' — i.e. a second and final set of teeth — obviously depend upon its 

 remaining in working order; but teeth are bound to get chipped or damaged 

 in the ordinary course of biting, and this is a good example of deterioration 

 of the kind classified below under heading (6).] 



1 Dr Whitear has pointed out that a third and quite distinct sort of 

 change with ageing which influences and will ultimately increase the 

 vulnerability of older animals is that entailed by the differential growth and 

 changing proportions of the several organs of the body or ingredients of a 

 complex tissue. As a general rule, it may be said that every fixed regimen 

 of differential growth will, if growth is indeterminate, inevitably lead to 

 mechanical or physiological ineptitude of one sort or another, although not 

 necessarily involving a loss of 'vitality' at the cellular or tissue level. The 

 problem is discussed more fully later. 



2 Higher organisms have means for counteracting the cumulative effect 

 of recurrent injuries. Two of the three principal reflex (i.e. response-to- 

 stimulus) systems of the body, the immunological and the nervous, have 

 the power of 'storing their information' for long periods. The hormone 

 system, apparently, has not. In general, an animal is less likely to contract 

 a particular infection on its second exposure than on its first, and this is 

 mainly due to the fact that what immunologists call the 'secondary' 

 response to an immunity-provoking agent is a good deal brisker than the 

 first. An animal is also less likely to get bitten, burnt, or otherwise abused 



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