THE BIOLOGY OF DEATH 453 



The Romano-African population diagram appears to start at nearly 

 the same point at birth as does the modern American and in general 

 the differences up to age 35 are not substantially more marked from 

 modern conditions than they are in the seventeenth century Breslau 

 table. The striking thing, however, is that at about age 40 the lines 

 cross, and from then on the expectation of life was definitely superior 

 in the early years of the Christian era to what it is now. 



It should be said that the curious zigzagging of the lines in all of 

 these Roman tables of Macdonell is due to the tendency, which ancient 

 Romans apparently had in common with present day American negroes, 

 towards heavy grouping on the even multiples of 5 in the statement 

 of their ages. 



Summarizing the whole matter we see that during a period of 

 approximately 2,000 years man's expectation of life at birth and sub- 

 sequent early ages has been steadily improving, while at the same 

 time his expectation of life at advanced ages has been steadily 

 worsening. The former phenomenon may be attributed essentially to 

 ever increasing knowledge of how best to cope with the lethal forces 

 of nature. Progressively better sanitation, in the broadest sense, down 

 through the centuries has saved for a time the lives of ever more and 

 more babies and young people who formerly could not withstand the 

 unfavorable conditions they met, and died in consequence rather 

 promptly. But just because this process tends to preserve the weak- 

 lings, who were speedily eliminated under the rigorous action of un- 

 mitigated natural 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 rigor- 

 ous regime of ancient Rome. 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 extraordinarily vigorous and resistant constitution. 

 Having come through successfully to 70 years of age it is no matter of 

 wonder that his prospects were for a longer old age than his descend- 

 ants 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 importance 

 in fixing the span of human longevity, namely that inherited constitu- 

 tion fundamentally and primarily determines how long an individual 

 will live. 



3. 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 d x line of a life table. 

 Figure 1 shows that this line, which gives the number of deaths occur- 



