, 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 .49. By the
two methods of measurement, therefore, quite comparable results are
obtained.

So much work has been done in this subject that we have no hesitation in
affirming .5 to represent approximately the average intensity of
heredity for physical characters in man. If any well-marked physical
character be measured, in which training and environment can not be
assumed to have had any part, it will be found, in a large enough number
of subjects, that the resemblance, measured on a scale from 0 to 1, is
just about one-half of unity. Of course, perfect identity with the
parents is not to be expected, because the child must inherit from both
parents, who in turn each inherited from two parents, and so on.

So far, it may be said, we have had plain sailing because we have
carefully chosen traits in which we were not obliged to make any
subtraction whatever for the influence of training. But it is evident
that not all traits fall in that class.

This is the point at which the inheritance of mental traits has been
most often questioned. Probably no one will care to dispute the
inheritance of such physical traits as eye-color. But in considering the
mind, a certain school of popular pseudo-psychological writers question
the reality of mental inheritance, and allege that the proofs which the
geneticist offers are worthless because they do not make account of the
similarity in environment or training. Of course, it is admitted that
some sort of a mental groundwork must be inherited, but extremists
allege that this is little more than a clean slate on which the
environment, particularly during the early years of childhood, writes
its autograph.

We must grant that the analysis of the inheritance of mental traits is
proceeding slowly. This is not the fault of the geneticist, but rather
of the psychologist, who has not