THE INHERITANCE OF DURATION 179 



centage of the infantile death rate as the duration of 

 life of the father and mother increases. Among the chil- 

 dren of short-lived fathers and mothers, at the left end 

 of each line, is found the highest infant mortality, while 

 among the offspring of long-lived parents the lowest 

 infant mortality occurs, as shown at the right-hand end of 

 the diagram. 



The results so far presented regarding a selective 

 death rate and inheritance of duration of life, have come 

 from selected classes : the aristocracy, royalty or Quakers. 

 None of these classes can be fairly said to represent the 

 general population. Can the conclusion be transferred 

 safely from the classes to the masses? To the determina- 

 tion of this point one of Pearson's students, Dr. E. C. Snow, 

 addressed himself. The method which he used was, from 

 the necessities of the case, a much more complicated and 

 indirect one than that of Pearson and Ploetz. Its essen- 

 tial idea was to see whether infant deaths weeded out the 

 unfit and left as survivors the stronger and more resis- 

 tant. All the infants born in a single year were taken 

 as a cohort and the deaths occurring in this cohort in suc- 

 cessive years were followed through. Resort was had 

 to the method of partial or net correlation. The variables 

 correlated in the case of the Prussian data were these : 



1. a? = Births in year a given cohort started. 



2. Xi = Deaths in the first two years of life. 



3. a? 2 = Deaths in the next eight years of life. 



4. # 3 = Deaths in the ten years of all individuals not included in 



the particular cohort whose deaths are being followed. 



In the case of the English data the variables were : 



x = Births in specified year. 



Xi = Deaths in the first three years of life of those born in 



specified year. 

 x z = Deaths in fourth and fifth years of life of those born in 



specified year. 

 d? 3 = The "remaining" deaths under 5. 



