AN UNSOLVED PROBLEM OF BIOLOGY 



That one is obliged by the terms of my definition to admit 

 that there are two sorts of causes of senescence has, it will turn 

 out, no more than a minor nuisance value. I am of course chiefly 

 concerned with senescence of sort (a), and you will see that the 

 arguments put forward to account for its origin and evolution 

 are greatly strengthened by the fact that there may already 

 exist a senescence of sort (b). 



The time has now come for a formal definition of senescence, 

 and I shall adopt the usual practice of translating a statement 

 about the frequency of the occurrence of an event in a popula- 

 tion into a statement about the likelihood of its happening to 

 an individual. Senescence, then, may be defined as that change 

 of the bodily faculties and sensibilities and energies which 

 accompanies ageing, and which renders the individual pro- 

 gressively more likely to die from accidental causes of random 

 incidence. Strictly speaking, the Avord ""accidentar is redundant, 

 for all deaths are in some degree accidental. No death is wholly 

 'naturaP; no one dies tnerely of the burden of the years. 



By way of an interlude let me now, as a zoologist, apologize 

 for appealing so much to evidence from human beings. I do so 

 because we know so very much more about the death of human 

 beings than of other animals; and though I feel a professional 

 obligation to say something about the natural history of 

 senescence, there is no time to do so, and even if there were, 

 there would not be much to say. 



at each successive exposure to such a hazard; it will have 'remembered' 

 the earlier and accordingly learnt better. Two exposures to infection or 

 physical risk may therefore have a no more harmful consequence than one, 

 and the cumulative effects of some sorts of recurrent stress may therefore 

 be to some extent corrected by the benefactions of an immunological or 

 nervous memory. Memory, as Professor J. Z. Young has reminded me, is 

 also the outcome of some influence that has left a physical 'trace'. 



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