46 



GROWTH 



Figure 26. The course of senescence in man 

 as measured by the increase in percentage 

 mortality with increasing age. The distri- 

 bution of the data points on a straight line 

 indicates that the percentage mortality in- 

 creases at a constant percentage rate. The 

 values of k when multiplied by 100 repre- 

 sent percentage mortality. 



60 90 



Figure 2J. Percentage mortality of the fruit 

 fly (Drosophila) at various ages. Plotted 

 from data by Pearl and coworkers. 



one in which each term bears 

 a constant ratio (= relative 

 rate) to the preceding term. It 

 is evident from these figures 

 that the law governing the 

 course of mortality of man 

 after the age of about fifteen 

 years is the same as the law 

 governing the course of mor- 

 tality of the fruit fly Droso- 

 phila after the age of ten days. 

 Indeed, by proportioning the 

 axes in an appropriate manner, 

 the two sets of curves can be 

 made to coincide as shown in 

 Figure 28. 



By way of summary of this 

 section, we may say that from 

 the available data it seems that 

 the descriptive law of the 

 course of declining vitality is 

 that the degree of senescence 

 increases, or the degree of 

 vitality decreases with age at a 

 constant ratio, indicated by k 

 on the charts. 



This statement may be put 

 in the form of a formula and, 

 in our judgment, it promises 

 to become a law of senescence. 

 The constancy of rate is the 

 fundamental feature charac- 

 terizing the process. In man 

 the constancy of the rate of 

 senescence begins to apply at 

 the age of about twenty years ; 



