AN UNSOLVED PROBLEM OF BIOLOGY 



From such a curve one may compute the death-rate at any 

 age of Kfe, for that is simply its slope, the rate of decline of the 

 number of survivors; the mean expectation of further life at birth 

 or at any other age; and the likelihood at any one age of living 



e 

 o 



> 



o 



c 



100,000 



75.000 - 



saooo 



2S.000 



age in years 

 Fig. 1 



brary 



r^i 



100 



to any other. The property that concerns us, however, is that 

 which is called the specific death rate or, less aridly, the 'force 

 of mortality\ the likelihood of dying within each interval of 

 age. In a first approximation, which is all that is necessary for 

 our purpose, the force of mortality is the quotient of this 

 fraction: 



Number of organisms that die within any chosen interval of age 

 Number of organisms alive at the beginning of the interval 



If, for example, 100 men reach age eighty-nine, but only 80 

 of them reach age ninety, then the force of mortality in the 

 ninetieth year of life is simply 0.2 (20 per cent., or 200 in every 

 1000). If there is no senescence in the population — if vitality 

 does not decline, so that there is no greater likelihood of dj'ing 

 at any one age than at any other — then the force of mortality 

 must necessarily be constant. Its members die, to be sure; but 



D 49 



