THE BIOLOGY OF DEATH 



445 



equally distributed as to sex, and all born at the same instant of time. 

 How many of these individuals would die in each succeeding year, and 

 what would be the general picture of the changes in this cohort with 

 the passage of time? The facts on this point for the Registration 

 Area of the United States in 1910 are exhibited in Figure 1, which is 

 based on Glover's United States Life Tables. 



UNIT-CD STATES LIFE TABLE. - 1910 



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w#3 or Lire 



FIG. 1. LIFE TABLE DIAGRAM. FOR EXPLANATION SEE TEXT 



In this table are seen two curved lines, one marked l x and the 

 other d x . The l x line indicates the number of individuals, out of the 

 original 100,000 starting together at birth, who survived at the be- 

 ginning of each year of the life span, indicated along the bottom of 

 the diagram. The d x line shows the number dying within each year 

 of the life span. In other words, if we subtract the number dying 

 within each year from the number surviving at the beginning of that 

 year we shall get the series of figures plotted as the l x line. We note 

 that in the very first year of life the original hundred thousand lose 

 over one-tenth of their number, there being only 88,538 surviving at 

 the beginning of the second year of life. In the next year 2,446 drop 

 out, and in the year following that 1,062. Then the line of survivors 

 drops off more slowly between the period of youth and early adult life. 

 At 40 years of age, almost exactly 30,000 of the original 100,000 have 

 passed away, and from that point on the l x line descends with ever 

 increasing rapidity, until about age 80, when it once more begins to 

 drop more slowly, and the last few survivors pass out gradually, a few 

 each year until something over the century mark is reached, when the 

 last of the 100,000 who started so blithely across the bridge of life 

 together will have ended his journey. 



This diagram is a graphic representation of that important type of 

 document known as a life or mortality table. It puts the facts of mor- 

 tality and longevity in their best form for comparative purposes. The 



