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.
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