510 



THE NATIONAL GEOGRAPHIC MAGAZINE 



evidence that a tendency to longevity is 

 an inheritable characteristic. For ex- 

 ample, divide the 1,594 cases into three 

 groups : 



t. Those whose parents, neither of 

 them, lived to be eighty. 



2. Those having one parent who lived 

 to be eighty or older, and 



3. Those having parents both of whom 

 lived to be eighty or older. 



Now note the proportion of long-lived 

 persons in each group. Only about 5 per 

 cent of the persons in group one lived to 

 be eighty; about 10 per cent of the per- 

 sons in group two, and 20 per cent in 

 group three (exact percentages 5.3, 9.8, 

 and 20.6). Few of the persons who did 

 not have long-lived parents behind them 

 lived to be old. The long-lived propor- 

 tion was practically doubled where one 

 parent lived to be old and quadrupled 

 where both parents lived to be old. The 

 evidence indicates that heredity is deeply 

 involved in the production of longevity. 



If we divide the 1,594 cases into groups 

 arranged according to the ages reached 

 by the parents, and then calculate the 

 average duration of life of all the persons 

 in each group, not simply the proportion 

 who lived to be old, we find that the per- 

 sons constituting - the longest-lived group 

 were the offspring of the longest-lived 

 narents, the members of the shortest- 

 lived group came from the shortest-lived 

 parents, with intermediates intermediate. 



age periods the duration of life was inter- 

 mediate. 



The figures indicate very clearly that 

 there is a correlation between the dura- 

 tion of life of the individual and the 

 duration of life of his parents ; and, con- 

 versely, we may conclude that the longest- 

 lived parents, on the average, had the 

 longest-lived children ; the shortest-lived 

 parents the shortest-lived children ; with 

 intermediates intermediate. 



We have only to glance around us at 

 the different forms of animal life to find 

 plentiful indications that the duration of 

 life is influenced, and indeed controlled, 

 by hereditv. Each species has its own 

 limit of life, and man is no exception. 



The contrasts are often very great: 

 For example, a horse born the same day 

 as a child dies of old age before the child 

 has reached full maturity. Just think of 

 the differences. The horse may become a 

 parent when the child is only a toddling 

 three-year-old, a grandparent by the time 

 the child is six, and several generations 

 of horses may appear before the child has 

 even reached marriageable age. 



The duration of life of each species is 

 controlled and limited by heredity, and 

 heredity even establishes different limits 

 for groups of animals within the same 

 species. The long-lived tend to produce 

 long-lived offspring, the short-lived, short- 

 lived offspring, etc. 



A million people may be born on the 



Number of Cases 

 Mother's age at death 

 -60 60-80 80 + 



151 



206 



184 



251 



328 



172 



128 



120 



74 



Average Duration of Life 



Mother's age at death 



- 60 60-80 80 ■ 



42.3 



45.5 



52.7 



35.8 



38.0 



45.0 



32.8 



33.4 



36.3 



s 8 



8^ 

 I 



The persons whose parents both died 

 before sixty lived, on the average, 32.8 

 years. Those whose parents both lived 

 beyond eighty averaged 52.7 years ; and 

 where the parents died at the intermediate 



same day, and we know that multitudes 

 of them will die during the very first year 

 of life. So great is the mortality during 

 infancy and childhood that we may be 

 perfectly certain that the majority of the 



