AN UNSOLVED PROBLEM OF BIOLOGY 



event in life — its end. It is a notorious fact that Maxwell's 

 Demon can reduce all such measures to absurdity, since he can 

 strike down perfectly vigorous, or indeed potentially immortal, 

 animals at just such ages as will exactly imitate any chosen 

 force-of-mortality curve. 



There are many other serious reservations. The use of the 

 force of mortality as a measure of senescence assumes that all 



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members of the population are equally at risk. This is not true, 

 because wage-earners are more exposed to risk than school- 

 children or those who have retired. A third difficultv is that if 

 a life table is constructed in the way I have suggested — that is 

 by following the life histories of a cohort of the newly born — 

 it is only too likely to be corrupted by secular changes in the 

 hazards of which human beings may be victims. Individuals 

 aged seventy to-day were born in 1881, when the causes of 

 the death of children, and their likelihood of surviving early 

 youth, were very different from what they are to-day. A fourth 



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