THE UNIQUENESS OF THE INDIVIDUAL 



for only the second time, that senescence has had a very 

 orthodox evolutionary origin. But Weismann is arguing in what 

 a student of mine once called a vicious circle, or more exactly 

 a vicious figure-of-eight. He assumes that the elders of his race 

 are worn out and decrepit — the very state of affairs whose 

 origin he purports to be inferring — and then proceeds to argue 

 that because these dotard animals are taking the place of the 

 sound ones, so therefore the sound ones must by natural selec- 

 tion dispossess the old! This is all a great muddle, but there is 

 certainly some truth in it, and I shall spend the rest of my 

 lecture in an attempt to find out what that truth may be. 



My argument starts with a discussion of certain demo- 

 graphic properties of a population of potentially immortal 

 individuals, and it will be illustrated by an inorganic model 

 which I shall animate step by step. This choice makes it 

 possible to avoid two common traps. The first of these is to 

 argue that senescence in higher animals has come about because 

 they have a post-reproductive period; for ''unfavourable'' here, 

 ditary factors that reveal their action only in the post-repro- 

 ductive period are exempt from the direct effects of natural 

 selection and there is therefore little to stop them establishing 

 themselves and gaining ground. Any such argument is wholly 

 inadmissible. The existence of a post-reproductive period is one 

 of the consequences of senescence; it is not its cause. The 

 second trap, into which Weismann fell headlong, is to suppose 

 that a population of potentially immortal individuals subject 

 to real hazards of mortality consists in high proportion of very 

 aged animals with a relatively small number of no doubt brow- 

 beaten youngsters running round between their feet. It will 

 soon be clear that this idea is equally mistaken. 



I want you now to consider a population of objects, living or 

 not, which is at risk — in the sense that its members may be 

 killed or broken — but which is potentially immortal in the 

 sense that its members do not in any way deteriorate with 



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