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 0.73 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 15 to 17. 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 sane Fraternity, both before as well as after birth. Secondly, it depends upon the variety of ancestry and nurture in different Fraternities. 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 environments of a group of persons taken from the population