Introduction and Historical 



There are, however, a few which should be outlined in greater 

 detail here — either because they are still of importance, or 

 because, though untenable, they have a considerable surviving 

 influence. 



The most influential nineteenth-century contribution to this 

 second category was probably that of Weismann, whose theory 

 sprang directly from his distinction between germ plasm and 

 soma. Weismann regarded senescence as an inherent property 

 of metazoa, though not of living matter, since he failed to find 

 it in protozoans and other unicellular organisms. Its evolution 

 had gone hand in hand with the evolution of the soma as a 

 distinct entity, and it was the product of natural selection, aris- 

 ing like other mutants by chance, but perpetuated as a posi- 

 tively beneficial adaptation, because 'unlimited duration of life 

 of the individual would be a senseless luxury'. 'Death', accord-, 

 ing to this view, 'takes place because a worn-out tissue cannot 

 forever renew itself. . . . Worn-out individuals are not only 

 valueless to the species, but they are even harmful, for they take 

 the place of those which are sound' (1882). This argument both 

 assumes what it sets out to explain, that the survival value of 

 an individual decreases with increasing age, and denies its own 

 premise, by suggesting that worn-out individuals threaten the 

 existence of the young. It had the advantage, however, of being 

 an evolutionary theory, and we shall see later that this is the 

 only type of theory which today seems likely to offer a general 

 approach to the emergence of senescence in all the groups 

 which exhibit it. The idea that all somatic cells must necessarily 

 undergo irreversible senescence was challenged early in the 

 century by the studies of Child (1915) upon planarians, and of 

 Carrel (1912) upon tissue culture. The assumption that all 

 higher metazoa must ex hypothesi exhibit senescence, however, 

 dies hard, and the fallacious argument based on selection has 

 been repeated as recently as 1937 (Metalnikov, 1936, 1937). 



A considerable number of metabolic theories were based on the 

 fact that an inverse relationship exists between length of life 

 and 'rate of living'. On the basis of calorimetric experiments, 

 Rubner (1908, 1909) calculated that the amount of energy 

 required for the doubling of weight by body growth was 

 approximately equal in a number of mammals. The energy 



9 



