e facts
displayed. The comparatively small size of the human family sometimes
makes it difficult to get data sufficiently extensive to be really
significant. And the long period that elapses between successive human
generations adds to the difficulty of getting precise information, for
in dealing with the heredity of some traits comparisons must be made
with individuals of the same ages, and the period of observation of a
single observer seldom exceeds the duration of a single generation.
Yet in spite of all these difficulties we have a fairly broad and
exact knowledge of human heredity in respect to some characteristics.

Human heredity involves both physical and psychical characters--both
the body and the mind are concerned. Among other animals little if
anything is known regarding psychic inheritance, but the physical
traits of men are inherited in just the same ways and to the same
degrees as in animals. This degree or intensity of inheritance may be
expressed in coefficients of heredity between the groups of relatives
being compared. To mention a few examples of coefficients for physical
traits we have the following:

CHARACTER OBSERVED PARENTAL FRATERNAL
COEFFICIENT COEFFICIENT
Stature .49-.51 } .51-.55 }
Span .45 } .55 }
Fore Arm .42 } .47 .49 } .53
Eye Color .55 } .52 }
Hair Color .57 - Average
Hair Curliness .52
Head Measurements-three .55 - "
Cephalic Index (Ratio between breadth and
length of cranium) .49

We might give many others, but it is unnecessary. Notice that these
parental and fraternal coefficients group about an average value of
about .50 or slightly less. Similar coefficients have been worked out
for other degrees of relationship; thus grandparental coefficients are
about .25.

Stated briefly, in less exact terms, these coefficients mean that,
with respect to such traits as deviate from the group average, the
resemblance of brothers and sisters to each other or of children to
their parents is, on the whole, approximately mid-way between being
complete in its deviation from the average and in not deviating at all
from the average in th