The Distribution of Senescence 



with a low rather than a very high standard of living throughout 

 life (Gumbel, 1938), an argument which fortunately has not so 

 far been advanced to justify starvation as a social policy. 

 Extreme records in man, occurring in excess of statistical prob- 

 ability, are chiefly of interest in suggesting that after a certain 

 age the rate of increase in the force of mortality is not main- 

 tained, either by reason of selection or from other causes. 



24 Distribution of Senescence in Vertebrates 



Actuarial senescence is known, or reasonably assumed, to 

 occur in all mammals, provided they live long enough. It is less 

 easily recognized, but apparently equally universal, in birds. 

 There are apparently no satisfactory life-table studies of birds 

 under domestic conditions apart from a single paper on fowls 

 (Gardner and Hurst, 1933), but individuals kept as pets cer- 

 tainly become increasingly enfeebled after an age which is 

 fairly constant for the species, and the reproductive senescence 

 of poultry, marked by a steep decline in egg production, is well- 

 known to farmers (Clark, 1940; Brody, 1945; Fig. 14). This 



180 



ISO 



120 



cL 

 5 90 



1X1 



a. 



60 



30 



YEAR OF EGG LAYING 



10 



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Fig. 14. — The decline in egg production in successive years of laying 

 (drawn from the data of Clark and of Hall and Marble) . 



63 



