THE INHERITANCE OF DURATION 177 



preferred to state the conclusion in terms of death, rates, 

 as it was originally stated by Pearson, because of the 

 bearing it has upon a great deal of the public health 

 propaganda so loosely flung about. It need only be re- 

 membered that there is a perfectly definite functional 

 relation between death rate and average duration of life 

 in an approximately stable population group, expres- 

 sible by an equation, in order to see that any conclusion 

 as to the relative influence of heredity and environment 

 upon the general death rate must apply with equal force 

 to the duration of life. 



THE SELECTrVB DEATH BATE IN MAN 



If the duration o^ Ufe were inherited it would logical- 

 ly be expected that some portion of the death rate must 

 be selective in character. For inheritance of duration 

 of life can only mean that when a person dies is in part 

 determined by that individual's biological constitution or 

 makeup. And equally it is obvious that individuals of 

 weak and unsound constitution must, on the average, 

 die earlier than those of strong, sound, and vigorous con- 

 stitution. Whence it follows that the chances of leaving 

 offspring will be greater for those of sound constitution 

 than for the weaklings. The mathematical discussion 

 which has just been given indicates that from one-half 

 to three-fourths of the death rate is selective in char- 

 acter, because that proportion is determined by hereditary 

 factors. Just in proportion as heredity determines 

 the death rate, so is the mortality selective. The reality of 

 the fact of a selective death rate in man can be easily 

 shown graphically. 



In Figure 44 are seen the graphs of some data from 

 European royal families, where no neglect of children, 



12 



