The Biology of Senescence 



increasing early output of eggs or offspring. Greenwood (1932) 

 found that the fertility and hatchability of hens' eggs decline 

 with age of the parent to such an extent that attempts to 

 improve the stock by breeding from long-lived birds were 

 economically impracticable. Apart from the obvious difficulty 



Fig. 32. — Drosophila subobscura. Strain K. Survival curves of flies raised in 



each generation from eggs laid by adults which had passed the thirtieth day 



of imaginal life. Compare Fig. 33. 



of breeding for long life in any animal with a substantial 

 post-reproductive period, which involves rearing all the progeny 

 of large numbers of animals throughout life, the consequence 

 of inbreeding per se, and the tendency of inbred laboratory 

 stocks to reach a very stable equilibrium life-span (Pearl 

 and Parker, 1922) militate against any such experiment. In 

 Pearl's own experiments (Pearl, 1928) the long- and short-lived 



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