The Distribution of Senescence 



old (30 per cent), and there were signs of a decreasing force of 

 mortality with age. In some cases the decrease may be even 

 steeper. In some vertebrates the enormous infant mortality 

 would completely overshadow subsequent trends in any life- 

 table based upon a cohort at birth: in the mackerel, for 

 instance (Sette, 1943), survival to the 50 mm. stage is less 

 than 0-0004 per cent. 



There are a certain number of apparent instances where 

 senescence occurs as a regular phenomenon in wild populations 

 of animals, both vertebrate and invertebrate, quite apart from 

 occasional records of 'old' individuals. Murie (1944), from the 

 examination of the skulls of 608 mountain sheep (Ovis dalli), 

 constructed a life-table in which the death-rate was minimal 

 between 1 f and 5 years of age, and climbed thereafter. The 

 main deaths in old and young sheep appear to have been due 

 to predation by wolves. The Arctic fin whales studied by 

 Wheeler (1934) appeared to undergo an increase in mortality 

 after the fifteenth year of age (in females) ; the apparent increase 

 may however have been the result of the failure of the older 

 specimens to return from their winter quarters to the regions 

 where they can be caught and recorded. A good many larger 

 carnivores and herding animals probably survive occasionally 

 into old age in the wild state, though death must as a rule occur 

 very early in the process of declining resistance. It is evidently 

 impossible, in population studies, to assume either a constant 

 mortality with age or a mortality increasing with increasing age, 

 without some prior evidence of the behaviour of similar forms. 



The 'normal' or 'wild' pattern of mortality in man is, of 

 course, an abstraction, since even man in modern urban society 

 is, biologically speaking, living 'in the wild', albeit after much 

 social and behavioural adaptation. Early and primitive human 

 societies almost certainly resembled in their ageing behaviour 

 those populations of animals which occasionally reach old age, 

 and in which the force of mortality shows some decrease during 

 middle adult life. This is the pattern one would expect in social 

 animals, where the survival of certain experienced individuals 

 has probably a positive survival-value for the group, although 

 in man the adaptation has been expressed in increasing capacity 

 for abstract thought and social organization, rather than in 



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