The Biology of Senescence 



requirement for the maintenance of metabolism, per unit 

 adult body weight, was also approximately equal between 

 species. Rubner inferred that senescence might, from these 

 energy relationships, represent the completion of one particu- 

 lar system of chemical reactions, depending on a fixed total 

 energy expenditure. He was obliged to erect a special category 

 for man, whose energy requirement was found to be far higher 

 than in laboratory or domestic animals. Loeb (1902, 1908) 

 attempted to find out whether the temperature coefficient of 

 this hypothetical reaction was identical with that of general 

 rate of development. Working with echinoderm eggs at various 

 temperatures, and using a hatchability criterion to determine 

 'senescence', if the word can be used in such a highly- 

 specialized instance, he concluded that the two coefficients were 

 distinct. The importance of this work has been that its pre- 

 suppositions have recurred in later studies, where some authors 

 have based very similar inferences about the relationship of 

 growth and senescence to a 'monomolecular, autocatalytic 

 reaction' on the shape and supposed mathematical proportions 

 of the growth curve. As d'Arcy Thompson pointed out, this 

 might equally prove the 'autocatalytic' character of growth in 

 a human population. In fact, with suitable adjustment, curves 

 based on biological material can be made to provide support 

 for almost any hypothesis of this kind. 



Little need be said of the various toxic or pathological theories 

 of mammalian senescence. We are really left with five historic- 

 ally important theories, or groups of observations: the sugges- 

 tion of Weismann that senescence is evolved, not intrinsic in 

 all cellular matter; the work of Pearl (1928) which leads to the 

 conception of a 'rate of living', such that factors which retard 

 development or reduce metabolism tend in many organisms to 

 prevent or postpone senescence; the work of Minot (1913, 

 1908), of which the most important surviving parts are his 

 relation of senescence to the decline of growth, and his insist- 

 ence upon its continuous and gradual character and its con- 

 tinuity with morphogenesis; the experimental studies of Child 

 (1915), which showed that cellular differentiation and 'sen- 

 escence' in planarians is reversible, and of Carrel (1912), who 

 demonstrated that some tissue cells derived from adult animals 



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