just been discussing, because it is obvious that it must depend on a large number of different factors, a man's size being due to the sum total of the sizes of a great number of bones, ligaments, tissues, etc. It is obvious that one can be long in the trunk and short in the legs, or vice versa, and so on through a great number of possible combinations. Here is a perfectly measurable character (no one has ever claimed that it is a genetic "unit character" in man although it behaves as such in some plants) as to the complex basis of which all will agree. And it is known, from common observation as well as from pedigree studies, that it is not inherited as a unit: children are never born in two discontinuous classes, "tall" and "short," as they are with color blindness or normal color vision, for example. Is it not a fair assumption that the difference between the apparent unit character of feeble-mindedness, and the obvious non-unit character of height, is a matter of difference in the number of factors involved, difference in the degree to which they hang together in transmission, variation in the factors, and certainly difference in the method of measurement? Add that the line between normal and feeble-minded individuals is wholly arbitrary, and it seems that there is little reason to talk about feeble-mindedness as a unit character. It may be true that there is some sort of an inhibiting factor inherited as a unit, but it seems more likely that feeble-mindedness may be due to numerous different causes; that its presence in one child is due to one factor or group of factors, and in another child to a different one.8
It does not fall wholly into the class of blending inheritance, for it does segregate to a considerable extent, yet some of the factors may show blending. Much more psychological analysis must be done before the question of the inheritance of feeble-mindedness can be considered solved. But at present one can say with confidence of this, as of other mental traits, that like tends to produce like; that low grades of mentality usually come from an ancestry of low mentality, and that bright children are usually produced in a stock that is marked by intelligence.