THE UNIQUENESS OF THE INDIVIDUAL 



owes more to his long neck than to the organ poised on top of 

 it; and the logic of brute fact tells us that the extinct reptile, 

 Diplodocus^ which had a brain in the pelvic region as well as up 

 in front, drew little advantage from his power to reason not 

 merely a priori. No: what kills the old animal is not in the first 

 place decrepitude, but something which has the dimensions of 

 the product of time by luck. 



Weismann had a theory not merely of the evolution of death 

 in animal populations but also of the mechanism of senescence 

 in the individual. He believed that a limit to life was set by an 

 inherent limitation in the power of germ cells to divide. ""We do 

 not know', he says, Svhy a cell must divide 10,000 or 100,000 

 times and then suddenly stop,''^ as he thought it did. As a 

 matter of fact, we now know that no such inherent limitation 

 exists; but Weismann'^s theory — if we disregard the fact that 

 the progeny of a cell which divided only 10,000 times would fill 

 the utter limits of known space — shows that he had not 

 appreciated the ""asymptotic'' character of the process of age- 

 decline. He had no grasp of the process of ageing. We don''t 

 grow old suddenly, and the cells within us do not suddenly stop 

 dividing. Those that do stop come to rest in a decent orderly 

 fashion. Charles Minot^ was the first to make this clear. He 

 took over Weismann''s idea that death had evolved by natural 

 selection, and turned his mind to ageing in the individual 

 alone. His views were original and still are theoretically import- 

 ant, so they deserve a fuller treatment than they commonly 

 get. 



Minot used growth as a measure of vitality; not the mere 

 rate of growth, but the specific rate, which gives us a measure 

 of the capacity of living tissue to reproduce itself at the rate 

 at which it was formed. It is simply the rate of growth at any 



1 Ibid., p. 22. 



2 The Problem of Age, Growth, and Death, Ldndon, 1908; a series of 

 lectures first published in Popular Science Monthly. 



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