otal 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.[50]

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.

Most mental traits are even more complex in appearance than
feeble-mindedness. None has yet been proved to be due to a single
germinal difference, and it is possible that none will ever be so
demonstrated.

[Illustration: FIG. 24.--The twins whose finger-prints are
shown in Fig. 25.]

Intensive genetic research in lower animals and plants has shown that a
visible character may b