12 P. B. Med A WAR 



There is therefore no difficulty in coming to terms with 

 this first objection of principle, if need be. The second is of 

 an entirely diff'erent kind. 



By the word "senescence" we clearly wish to describe a 

 change which is of innate origin, which is built into the 

 developmental structure of an animal, and which could take 

 place even under the most "favourable" conditions it is 

 possible to envisage. The force of mortality does indeed 

 measure this inborn decline, but the difficulty is that it must 

 also measure another sort of decline, of purely contingent 

 origin, as well. 



It cannot be strictly true to say, as I said earlier, that 

 plates and test-tubes are subject to a constant force of 

 mortality, even when given equality of use and of exposure 

 to the risk of being broken. They suffer indeed no innate 

 decline (if we disregard such refinements as the slow cry- 

 stallization of glass) but they may deteriorate in a diff'erent 

 way. Plates and test-tubes get cracked and chipped, but 

 may yet be usable, at least in the more frugal homes and 

 laboratories. A cracked test-tube or plate is more vulnerable 

 than one which is still intact; it is more likely to get broken 

 in the course of ordinary use. There is therefore, or may be, 

 an increase of vulnerability which has nothing at all to do 

 with innate decline, but which is due to the persistent, there- 

 fore cumulative, effect of recurrent injury or stress. 



There are innumerable examples in living organisms of an 

 increase of vulnerability caused by the persistence and, with 

 time, the summation of the effects of recurrent damage, 

 stress, and biological error (cf. Medawar, 1952). Any injury 

 which leaves any physical trace is an actuarial debt, and will 

 add to the likelihood of dying. A single example will iUus- 

 trate the distinction which may be drawn between deteriora- 

 tion of innate and of merely contingent origin: the formation 

 of ectopic flexure lines or creases in the skin, which (if the 

 efforts made to conceal them are anything to go by) are among 

 the most characteristic and reliable signs of the deterioration 

 associated with increasing age. 



