The Biology of Senescence 



morphogenesis (Baer, 1864; Cholodkowsky, 1882; Roux, 

 1881; Delage, 1903; Warthin, 1929) or the operation of an 

 Aristotelean entelechy (Driesch, 1941; Burger, 1954), metabolic 

 theories introducing the concept of a fixed-quantity reaction 

 or of a rate/quantity relationship in determining longevity 

 (Rubner, 1908; Loeb, 1908; Pearl, 1928; Robertson, 1923), 

 attainment of a critical volume-surface relationship (Muhl- 

 mann, 1910 etc.), depletive theories relating senescence to 

 reproduction (Orton, 1929) and finally an important group of 

 theories which relate senescence to the cessation of somatic 

 growth (Minot, 1908; Carrel and Ebeling, 1921; Brody, 1924; 

 Bidder, 1932; Lansing, 1947, 1951). Most of the older theories 

 have been reviewed, against a background of Drieschian neo- 

 vitalism, in the textbook of Burger (1954). 



The distribution of dates in this catalogue sufficiently indi- 

 cates the state of the subject. When Francis Bacon examined 

 the relationship between animal specific longevity, growth-rate, 

 size and gestation period, he concluded that the available facts 

 were unfortunately insufficient to support a general theory. 

 That conclusion remains valid in practically all the instances 

 quoted, but Bacon's self-denial failed to set a precedent for his 

 successors. Almost all these theories, judging from the literature, 

 continue at some point to influence biological thinking: some 

 can be partially, or even largely, justified by the suitable selec- 

 tion of instances. Others did not bear critical inspection at the 

 time they were first formulated, bearing in mind the known 

 behaviour of cells, and the known discrepancies in longevity 

 and in rate of ageing between animals of similar size, histo- 

 logical complexity, and physiological organization. Relatively 

 few are supported by any body of fundamental experiment. 

 The devising of general theories of senescence has employed 

 able men, chiefly in their spare time from laboratory research, 

 for many years. It seems reasonable to assume that almost all 

 the mechanisms which might theoretically be involved have 

 been considered, and if we are to understand what does in fact 

 occur in a given ageing organism, we now need a combination 

 of general observation with planned causal analysis in experi- 

 mental animals. 



The main theories of ageing will be discussed in the text. 



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