can not be due to anything that
happens after they are born; and the facts presented in the second
chapter showed that these differences can not be due in an important
degree to any influences acting on the child prior to birth.

CHAPTER IV

THE INHERITANCE OF MENTAL CAPACITIES

We have come to the climax of the eugenist's preliminary argument; if
the main differences between human beings are not due to anything in the
environment or training, either of this or previous generations, there
can be but one explanation for them.

They must be due to the ancestry of the individual--that is, they must
be matters of heredity in the ordinary sense, coupled with the
fortuitous variations which accompany heredity throughout the organic
world.

We need not limit ourselves, however, to the argument by exclusion, for
it is not difficult to present direct evidence that the differences
between men are actually inherited by children from parents. The
problem, formally stated, is to measure the amount by which the likeness
of individuals of like ancestry surpasses the likeness of individuals of
different ancestry. After subtraction of the necessary amount for the
greater likeness in training, that the individuals of like ancestry will
have, whatever amount is left will necessarily, represent the actual
inheritance of the child from its ancestors--parents, grandparents, and
so on.

Obviously, the subtraction for environmental effects is the point at
which a mistake is most probable. We may safely start, therefore, with a
problem in which no subtraction whatever need be made for this cause.
Eye color is a stock example, and a good one, for it is not conceivable
that home environment or training would cause a change in the color of
brothers' eyes.

The correlation[30] between brothers, or sisters, or brothers and
sisters--briefly, the fraternal resemblance--for eye-color was found by
Karl Pearson, using the method described in Chapter I, to be .52. We are
in no danger of contradiction if we state with positiveness that this
figure represents the influence of ancestry, or direct inheritance, in
respect to this particular trait.

Suppose the resemblance between brothers be measured for stature--it
is .51; for cephalic index, that is, the ratio of width of skull to length
of skull--it is .49; for hair color--it is .59. In all of these points,
it will be admitted that no home training, or any other influence except
heredity