94 BIOLOGY OF DEATH 



ural selection, there appear now in the higher age groups 

 of the population many weaker individuals than formerly 

 ever got there. Consequently the average expectation 

 of life at ages beyond say 60 to 70 is not nearly so good 

 now as it was under the more rigorous regime of ancient 

 times. Then, any individual who attained age 70 was 

 the surviving resultant of a bitterly destructive process 

 of selection. To run successfully the gauntlet of early 

 and middle life, he necessarily had to have an extraor- 

 dinarily vigorous and resistant constitution. Having 

 come through successfully to 70 years of age it is no mat- 

 ter of wonder that his prospects were for a longer old 

 age than his descendants of the same age to-day can look 

 forward to. Biologically, these expectation of life curves 

 give us the first introduction to a principle which we 

 shall find as we go on to be of the very foremost impor- 

 tance in fixing the span of human longevity, namely that 

 inherited constitution fundamentally and primarily de- 

 termines how long an individual will live. 



ANALYSIS OF THE LIFE TABLE 



I shall not develop this point further now, but instead 

 will turn back to consider briefly certain features of the 

 dx line of a life table. Figure 18 shows that this line, 

 which gives the number of deaths occurring at each age, 

 has the form of a very much stretched letter S resting 

 on its back. Some years ago, Pearson undertook the 

 analysis of this complex curve, and drew certain inter- 

 esting conclusions as to the fundamental biological causes 

 lying behind its curious sinuosity. His results are shown 

 in Figure 25. 



He regarded the dx line of the life table as a compound 

 curve, and by suitable mathematical analysis broke it up 



