The Biology of Senescence 



increasing longevity per se. Although one may guess that early 

 man occasionally reached the point at which his powers of 

 homoeostasis began to fail through age, he must have died 

 through environmental pressure, like Murie's sheep, very early 

 in the process. Out of 173 palaeolithic and mesolithic indivi- 

 duals whose age could be determined, only 3 (all males) 

 appeared to have been older than 50 years, and none much 

 older (Vallois, 1937). Palaeolithic man in the Chinese deposits 

 normally died from violence at a presenile age (Wiedenreich, 

 1939). In rather more civilized societies, the fall in mortality 

 with increasing age becomes more evident: according to Lack 

 (1943a, 1954) the curve of mortality based on the ages given in 

 Roman funerary inscriptions (Macdonnell, 1913) is much like 

 that for birds. Hufeland's (1798) and Silbergleit's (Vischer, 

 1947) figures (Fig. 7) illustrate further stages in the transition 

 to the rectangular survival curve of modern societies in privi- 

 leged countries: many other examples have been collected by 

 Dublin and his fellow actuaries (1949). 



2-6-2 INVERTEBRATES 



Senescence also occurs in the wild in some invertebrates, 

 though it is often probably of the type of the 'parental' deaths 

 of shotten eels. Senescence in one form or another has been 

 invoked to account for the fixity of size and life-span in some 

 fresh-water gastropods (Sewell, 1924; Van Cleave, 1934, 1935). 

 The figures of Fischer- Piette (1939), relating longevity inversely 

 to growth rate in Patella, also suggest the operation of senescence. 

 It very probably occurs in the long-lived sexual forms of social 

 insects, such as termite primaries, and has been found to con- 

 tribute to the mortality of worker bees (Ribbands, 1952). 

 Among other insects, Jackson (1940) observed a factor of sen- 

 escence in tsetse flies (Glossina) occurring only during the rainy 

 season, when the life-span of the flies is longer. Dowdeswell, 

 Fisher and Ford (1940) infer the possibility of a decline in the 

 viability of butterflies (Polyommatus icarus) throughout imaginal 

 life. The position in insects is considerably complicated by the 

 existence of specialized overwintering forms. Overwintering 

 Gerrids show changes in the muscles which appear to precede 

 natural death — mechanical wear of the rostrum, which occurs 



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