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, can conceivably play an important part. We could go on with a
long list of such measurements, which biometrists have made; and if they
were all summed up it would be found that the fraternal correlation in
these traits as to the heritability of which there can be no dispute, is
about .52. Here is a good measure, albeit a technical one, of the
influence of heredity from the near ancestry. It is possible, too, to
measure the direct correlation between a trait in parent and the same
trait in offspring; the average of many cases where only heredity can be
thought to have had any effect in producing the result, is .
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