s commonly
exist.[1]

[Footnote 1: Thorndike: _loc. cit._, p. 78.]

In general the influence of heredity may be said far to outweigh
the influence of home training. In all the cases
reported, the resemblances were about the same in traits subject
to training, and in those not subject to training. Thus industry
and conscientiousness and public spirit, which are clearly
affected by environment, show no greater resemblance than
such practically unmodifiable traits as memory, original
sensitiveness to colors, sounds, and distances.

The influence of parentage, it must be added, consists in the
transmission of specific traits, not of a certain "nature" as a
whole. There are in the germ and the ovum which constitute
the inheritance of each individual, certain determinant elements.
The elements that determine the original traits with
which each individual will be born vary, of course, in the
germs produced by a single parent less than among individuals
chosen at random, but they vary none the less. In this
variation of the determining elements in the germs of the
same individual is to be found the cause of the variation in
the physical and mental traits among children of the same
parents.

Since the determining elements, the unit characters that
appear in the sperm or ovum of each individual, do not appear
uniformly even in children of the same parents, brother
and sister may resemble each other in certain mental traits,
and differ in others. "A pair of twins may be indistinguishable
in eye color and stature, but be notably different in hair
color and tests of intellect."

Mental inheritance, as well as physical, is, then, organized
in detail. It is not the inheritance of gross total natures, but
of particular "mental traits." If we had sufficient data, we
should be able to analyze out the unit characters of an
individual's mental equipment, so as to be able to predict with
some accuracy the mental inheritance of the children of any
two parents. In the case of physical inheritance, the laws of
the hereditary transmission of any given traits are known in
considerable detail. The detailed quantitative investigations
of inheritance, following the general lines set by Mendel, have
given striking results.

Physical traits have been found to be analyzable into unit-characters
(that is, traits hereditarily transmitted as units),
such as "curliness of hair," "blue eyes," and the like. Mental
traits, however, do not seem a