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GROWTH AND SENESCENCE 



5-1 'Rate of Living' 



The idea of the life-span as a fixed quantity is an old one. In 

 a great many organisms it has long been recognized that the 

 contrast, perhaps originally moralistic, between a long life and 

 a high 'rate of living' had valid biological applications. The 

 phrase 'rate of living' we owe to Pearl, and it conveys the con- 

 cept very satisfactorily without making too many assumptions. 

 In many organisms the life-span, like the rate of development, 

 is a function of the temperature over a considerable range. In 

 such forms it appears that a fixed quantity of something, which, 

 for want of a better term, we have called 'programme', must 

 run out and be succeeded by senescence. The organism must 

 pass through a fixed sequence of operations, metabolic or 

 developmental, the rate of its passage determining the observed 

 life-span. 



The period in which the kinetics of metabolism were being 

 discovered expressed this 'programme' in directly chemical 

 terms. Life had an observable temperature-coefficient. Growth, 

 in the classical conception of Robertson (1923), followed the 

 same course as a monomolecular autocatalytic reaction. Loeb 

 (1908) attempted to answer by the determination of tempera- 

 ture coefficients a fundamental question about the 'rate of liv- 

 ing' in relation to ageing — what is the nature of the 'pro- 

 gramme' which has to be fulfilled before senescence begins? 

 Is it a programme of differentiation, or growth, or maintenance 

 metabolism, or of all three? Loeb's experiments showed that 

 the temperature coefficient of the rate of 'ageing' in echinoderm 

 ova differed greatly from that of their respiration. Later work 

 has shown the relationship between development and tempera- 



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