THE UNIQUENESS OF THE INDIVIDUAL 



difficulty is that if the population is rather crudely subdivided 

 into the innately (that is, genetically) less tough and tougher, 

 then the population that reaches age sixty will be by no means 

 a genetically fair sample of the cohort with which the life table 

 began. Presumably each pattern of genetic constitution endows 

 its owners with a characteristic mode of increase of vulner- 

 ability; but in a cohort of mixed origins all such distinctions 

 must inevitably be confused. 



These are grave difficulties,"* but all of them can be over- 

 come in principle, and some in laboratory practice. I now turn 

 to a much more important difficulty in the use of vulnerability 

 as a measure of senescence; it is ingrained, and in practice 

 ineradicable, and it leads us to distinguish between two sorts 

 of causes of senescence. 



IV 



Consider wrinkles and lines on the skin, for these are familiar 

 outward signs of ageing in its colloquial sense. People who often 

 frown get lines between the eyebrows; the supercilious reveal 

 their temperament by furrows across the forehead; deep lines 

 down the corners of the mouth are allegedly the consequence 

 of having a ready smile. What is the history of wrinkles? Every 

 time one grins or frowns some physical trace is left in the 

 cellular or fibrous structure of the skin. These traces are cumu- 

 lative, and if only one folds or creases the skin sufficiently often, 

 they will add up to form a visible flexure line. One perfectly 

 good reason why elderly people should have more lines and 



* [There is another difficulty in accepting 'vulnerability' as a measure 

 of senescence: the decline and loss of reproductive power (e.g. in the meno- 

 pause) is beyond question a form of senescence, but it is not accompanied 

 by any increase of vulnerability in an actuarial sense. I consider this 

 problem, and deal more fully with the other difficulties mentioned in the 

 text, in The definition and measurement of senescence, Ciba Foundation 

 Colloquia on Ageing, Vol. 1, p. 4, 1955.] 



52 



