x.] DISEASE. 183

buted, and that the law of hereditary regression from a deviation of three units on the part of either parent to an average of one unit in the child, may be supposed to apply here, just as it did to Stature and to the other subjects of the preceding chapters.

Let the scale by which consumptivity is measured be such that the Q of the general population = 1. Let its M = N, when measured on the same scale ; the value of N is and will remain unknown. Let N + C be the number of units of consumptivity that just amount to actual consumption. Our data tell us that 16 per cent, of the -population have an amount of consumptivity that exceeds N + C. On referring to Table 8, we find the value of C that corresponds to the

Grade of (100°-16°), or of 84°, to be 1.47. There

fore whenever the consumptivity of a person exceeds N + 1.47, he has actual consumption.

Adding together the tabular values in Table 8 at all the odd grades above 84°, we shall find their average value to be 2.23. We may therefore assume (see p. 160) that a group of persons each of whom has a consumptivity of N + 2.23 will approximately represent all the grades above 84°. The Co-Fraternity descended from such a group will have an M whose value according to the law of Regression ought to be [N + 3 (2.23)] or [N + 0.74 units.]

Those members of the Co-Fraternity are consumptive whose consumptivity exceeds N + 1-47; these are the same as those whose deviation from [N+0-74] which is the M of the Co-Fraternity, exceeds + 0.7.3 unit.