THE UNIQUENESS OF THE INDIVIDUAL 



differentiation is the cause of the progressive fall away of 

 growth potential. Cellular differentiation — the degree of muscli- 

 ness of a muscle fibre, for example — has never been measured, 

 but Minot guessed that if such a measurement were to be 

 made, the curve of increasing differentiation would be found 

 to be the exact complement of that which plots the declining 

 energies of growth. To put it in another way: that which we 

 call '"development'' when looked at from the birth end of life 

 becomes senescence when looked at from its close. It is an 

 attractive idea, but such little evidence as we have speaks 

 against it. The tissue cultivator, who grows cells in blood and 

 tissue media outside the body, finds that ""old"* cells have just as 

 high a capacity for growth as young ones. They simply take a 

 longer time to set about it.^ It is perfectly true that some very 

 highly differentiated cells, like those of nerve and muscle, lose 

 their power to multiply by fission. But that is more of a mech- 

 anical accident than a slur upon their vitality; after all, a nerve 

 cell may be some yards long. Neither adult nerve nor adult 

 muscle has lost the power to grow^ and if a muscle or nerve 

 fibre is cut into two, healing and replacement will start up from 

 one end or the other. But whatever the rights and wrongs of 

 Minofs special theory, he has left us with two ideas which 

 any future theory of the ageing process must analyse and 

 suitably explain: the first is the continuity of the ageing pro- 

 cess, the second its great span in time. 



Some mention must now be made of the celebrated and 

 widely misinterpreted views of Elie Metchnikoff on ageing.^ 

 Metchnikoff believed that much of what in ageing seems to us 

 to be very '"naturaP is in fact abnormal. How much of ageing 

 he held to be so is far from clear, though he seemed to think, 

 as Buffon did and later Flourens, that an animaPs total span 



^ Cf. the evidence summarized by P. B. Medawar, Proceedings of the 

 Royal Society of Medicine^ 35, p. 690, 1942. 



2 The Prolongation of Life (trans. P. Chalmers Mitchell), London, 1910. 



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