Introductory and Historical 



ally, and to show a steady progression from a large number of 

 speculative, to one or two highly probable, main hypotheses. 

 In the case of senescence this cannot profitably be done. The 

 general theories of its nature and cause which have been put 

 forward from the time of Aristotle to the present day have fallen 

 into a number of overall groups, and have been divided almost 

 equally between fundamentalist theories which explain all 

 senescence, or treat it as an inherent property of living matter 

 or of metazoan cells, and epiphenomenalist theories which 

 relate it to particular physiological systems or conditions. They 

 are also fairly evenly divided between the various categories of 

 Baconian idola. It is a striking feature of these theories that they 

 show little or no historical development; they can much more 

 readily be summarized as a catalogue than as a process of 

 developing scientific awareness. To the fundamentalist group 

 belong, in the first place, all theories which assume the exist- 

 ence of cellular 'wear and tear' (Abnutzungstheorie) without fur- 

 ther particularization (Weismann, 1882; Pearl, 1928; Warthin, 

 1929); the mechanochemical deterioration of cell colloids 

 (Bauer, Bergauer, 1924; Ruzicka, 1924; 1929; Dhar, 1932; 

 Lepeschkin, 1931; Szabo, 1931; Marinesco, 1934; Kopaczew- 

 ski, 1938; Georgiana, 1949); and pathological or histological 

 elaborations of these, which attribute senescence to inherent 

 changes in specified tissues, nervous (Muhlmann, 1900, 1910, 

 1914, 1927; Ribbert, 1908; Vogt and Vogt, 1946; Bab, 1948), 

 endocrine (Lorand, 1904; Gley, 1922; Dunn, 1946; Findley, 

 1949; to cite only a few from an enormous literature in which 

 the endocrine nature of mammalian senescence is discussed, 

 stated or assumed), vascular (Demange, 1886), or even con- 

 nective (Bogomolets, 1947). To the epiphenomenalist group 

 belong toxic theories based on products of intestinal bacteria 

 (Metchnikoff, 1904, 1907; Lorand, 1929; Metalnikov, 1937), 

 accumulation of 'metaplasm' or of metabolites (Kassowitz, 

 1899; Jickeli, 1902; Montgomery, 1906; Muhlmann, 1910; 

 Molisch, 1938; Heilbrunn, 1943; Lansing, 1942; etc.), the 

 action of gravity (Daranyi, 1930), the accumulation of heavy 

 water (Hakh and Westling, 1934) and the deleterious effect 

 of cosmic rays (Kunze, 1933). There are also general develop- 

 mental theories which stress the continuity of senescence with 



7 



