ubtraction, but not in addition,
multiplication or division, it is to be expected that one at least of
his parents shows a like trait; or if he falls below the average in
subtraction and multiplication, and exceeds it in addition and division,
again the same will hold true of at least one of his parents." These
various kinds of arithmetic appear to be due to different functions of
the brain, and are therefore probably inherited independently, if they
are inherited at all.

To assume that the resemblance between parent and offspring in
arithmetical ability is due to association, training and imitation is
not plausible. If this were the case, a class of children ought to come
to resemble their teacher, but they do not. Moreover, the child
sometimes resembles more closely the parent with whom he has been less
associated in daily life.

From such data as these, we conclude that mental inheritance is
considerably specialized. This conclusion is in accord with Burris'
finding (cited by Thorndike) that the ability to do well in some one
high school study is nearly or quite as much due to ancestry as is the
ability to do well in the course as a whole.

To sum up, we have reason to believe not only that one's mental
character is due largely to heredity, but that the details of it may be
equally due to heredity, in the sense that for any particular trait or
complex in the child there is likely to be found a similar trait or
complex in the ancestry. Such a conclusion should not be pushed to the
point of assuming inheritance of all sorts of dispositions that might be
due to early training; on the other hand, a survey of the whole field
would probably justify us in concluding that any given trait is _more
likely than not_ to be inherited. The effect of training in the
formation of the child's mental character is certainly much less than is
popularly supposed; and even for the traits that are most due to
training, it must never be forgotten that there are inherited mental
bases.

If the reader has accepted the facts presented in this chapter, and our
inferences from the facts, he will admit that mental differences between
men are at bottom due to heredity, just as physical differences are;
that they are apparently inherited in the same manner and in
approximately the same degree.

CHAPTER V

THE LAWS OF HEREDITY

We have now established the bases for a practicable eugenics program.
Men differ; these differences are