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. Ca-n the conclusion be transferred 

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

 tion of thispoint 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 bom 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. ir„ = Births in year a given cohort started. 



2. a;, = Deaths in the first two years of life. 



3. iBj ^ Deaths in the next eight years of life. 



4. CO, =: 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 : 



CDo = Births in specified year. 



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



specified year. 

 Xi = Deaths in fourth and fifth years of life of those born in 



specified year. 

 (Ts = The "remaining" deaths under 5. * 



