THE UNIQUENESS OF THE INDIVIDUAL 



evidence suggests that some sort of nuclear rehabilitation is 

 from time to time required. Ordinary asexual fission is, from 

 the mechanic'o of the process, a very exact division of the parent 

 organism into equal parts. The genetical sins of the parents — 

 the lethal or unwholesome mutant genes — are thus allotted to 

 their progeny with biblical justice and more than biblical pre- 

 cision. The nuclear reconstitution spoken of above is, in effect, 

 a device by which such genes may be eliminated from the 

 stock. The organisms which inherit them die soon, or fail to 

 reproduce; the others, often a minority, carry on.)^ 



With such new facts as these at his disposal, and others of 

 great value added by himself, Raymond PearP made the next 

 important attack on death in 1922. Pearl himself showed that 

 an animaPs span of life was governed by inherited factors and 

 was within certain limits subject to experimental modification. 

 The total span of life may be increased not by adding a few 

 extra years to its latter end nor, if it comes to that, by inter- 

 calating new life at any intermediate period, but rather by 

 stretching out the whole life span symmetrically, as if the seven 

 ages of man were marked out on a piece of rubber and then 

 stretched. The length of life may thus be treated as a function 

 of the rate of living. One simple way of lowering the rate of 

 living — an ingredient of many a centenarian''s recipe for long 

 life — is to withhold with known precision the sort of food that 

 is used for the supply of energy: a restriction of calories, as we 

 say, rather than a systematic maZnutrition. McCay and his 

 colleagues^ have shown that by such means the life span of rats 

 may be greatly lengthened. The same is true of flatworms, as 



1 Cf. B. F. Pierson, Biological Bulletin, 74, p. 235, 1938; T. M. Sonneborn, 

 ihid., p. 76. 1 am obliged to Professor J. B. S. Haldane for pointing out the 

 significance of their evidence. [For more recent evidence on senescence in 

 protozoa, see Alex Comfort, The Biology of Senescence, London, 1956.] 



^ The Biology of Death (Lowell Lectures), Philadelphia, 1922; The Rate 

 of Living, London, 1928. 



3 Cf. C. M. McCay, pp. 680-720, in Problems of Ageing (note 1, p. 27). 



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