Parental per cent 0-40 50 60 70 80 90 Ratio 75.3:24.7 76:23 79:21 82:18 78:22 85:15 There is a great deviation from 25 per cent in the "non-median" offspring of the 90 per cent parents, but in this particular case the total number of offspring is not large, and the deviation has a greater chance of being accidental. Altogether this explanation of the varying per cents of single comb on the ground of inheritance of varying potency in dominance seems best to fit the facts of the case. From the foregoing facts and considerations we may conclude that the Y comb represents imperfect dominance of median over no-median comb; that there is a fluctuation in the potency of the dominance, so that the proportion of the median element varies from 0 to over 90 per cent; that the more potent the dominance of median element is in any parents the more complete will be the dominance in the offspring and the smaller will be the percentage of imperfectly dominant, or Y-combed, offspring. _Dominance varies quantitatively and the degree of dominance is inheritable._ The index of heredity may be readily obtained in the familiar biometric fashion from table 9. This I have calculated and found to be 0.301± 0.002. This agrees with Pearson's theoretical coefficient of correlation between offspring and parent. The index is larger than it would otherwise be because it is measured with an _average_ of the parents and these parents assortatively mated. But this instance is, in any case, an interesting example of strong inheritance of a quantitative variation.