THE UNIQUENESS OF THE INDIVIDUAL 



senescence is a self-enhancing process. The theorem in the form 

 in which I have just put it does not depend upon the existence 

 of a post-reproductive period; it only requires that the repro- 

 ductive value of each age-group should diminish with increas- 

 ing age. I have argued that this must necessarily diminish even 

 with a population of potentially immortal and indeterminately 

 fertile individuals, provided only that they are subject to real 

 dangers of mortality. In such a population a younger age-group 

 must necessarily outnumber an older, for the older represents 

 the residue of those who have been longer exposed to mortal 

 hazards. If you should have, as I believe, unjustified qualms 

 about an argument based upon combining an innate potential 

 immortality with a contingent real mortality, I would recall to 

 you my earlier distinction between senescence of sorts (a) and 

 (b). Senescence of sort (b) is not innate or 'laid on'' develop- 

 mentally; it represents the outcome of the cumulative effects of 

 recurrent physical damage, physiological stress, or faulty 

 cellular replication. If you will admit that senescence of this 

 sort is a means by which, irrespective of any genetical back- 

 ground, the reproductive value of each individual in a popula- 

 tion is caused to diminish with increasing age, then my argument 

 is quantitatively strengthened, because the numerical pre- 

 ponderance of the younger age-groups will become so much the 

 more pronounced. And if, further, a post-reproductive period 

 of life is already established, then indeed it becomes, as it were, 

 a dustbin for the effects of deleterious genes. But these pro- 

 positions are mere glosses or refinements. The argument must 

 stand or fall on the case which I first proposed. 



I have now suggested three agencies which may have played 

 a part in the evolution of ''innate'' senescence: (1) the inability 

 of natural selection to counteract the feeble pressure of 



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