184 NATURAL INHERITANCE. [CHAP. 



Let the Q of the Co-Fraternity be called n. The Grade 

 at which this amount of deviation occurs should be 

 found in Table 8 opposite to the value of 073 divided 

 by n. 



Next as regards the value to be assigned to n, we 

 may be assured that the Q of a Co-Fraternity cannot 

 exceed that of the general population. Therefore n 

 cannot exceed 1. In the case of Stature the relation 

 between the Q of the Co-Fraternity and that of the 

 Population was found to be as 1 5 to 1 7. If the same 

 proportion held good here, its value would be 0'9. 

 This is I think too high an estimate for the following 

 reasons. The variability of the Co-Fraternity depends 

 on two groups of causes. First, on fraternal variability ; 

 which itself is due in part to mixed ancestry, and in 

 part to variety of nurture in the same Fraternity, both 

 before as well as after birth. Secondly, it depends upon 

 the variety of ancestry and nurture in different Frater- 

 nities. As to the first of the two groups of causes, 

 they seem to affect consumptive fraternities in the same 

 way as others, but not so with respect to the second 

 group. The household arrangements of vigorous, of 

 moderately vigorous, and of invalided parents are 

 not alike. I have already spoken of infection. There 

 is also a tradition in families that are not vigorous, of 

 the necessity of avoiding risks and of never entering 

 professions that involve physical hardship. There is 

 no such tradition in families who are vigorous. Thus 

 there must be much greater variability in the environ- 

 ments of a group of persons taken from the population 



