e due to

1. Independent multiple factors in the germ-plasm, as in the case of
wheat mentioned a few pages back.

2. Multiple allelomorphs, that is, a series of different grades of a
single factor.

3. One distinct Mendelian factor (or several such factors), with
modifying factors which may cause either (a) intensification, (b)
inhibition, or (c) dilution.

4. Variation of a factor.

5. Or several or all of the above explanations may apply to one case.

Moreover, the characters of which the origin has been most completely
worked out are mostly color characters, whose physiological development
seems to be relatively simple. It is probable that the development of a
mental character is much more complicated, and therefore there is more
likelihood of additional factors being involved.

To say, then, that any mental trait is a unit character, or that it is
due to a single germinal difference, is to go beyond both the evidence
and the probabilities.

And if mental traits are, in their germinal foundations, not simple but
highly complex, it follows that any advice given as to how human matings
should be arranged to produce any precise result in the progeny, should
be viewed with distrust. Such advice can be given only in the case of a
few pathological characters such as color-blindness, night-blindness, or
Huntington's Chorea. It is well that the man or woman interested in one
of these abnormalities can get definite information on the subject; and
Huntington's Chorea, in particular, is a dysgenic trait which can and
should be stamped out. But it can not be pretended that any of man's
traits, as to whose inheritance prediction can be made with confidence,
is of great importance to national eugenics.

In short, a knowledge of heredity shows that attempts to predict the
mode of inheritance of the important human traits (particularly mental
traits) are still uncertain in their results. The characters involved
are too complex to offer any simple sequences. If two parents have brown
eyes, it can not be said that all their children will have brown eyes;
still less can it be said that all the children of two musically gifted
parents are certain to be endowed with musical talent in any given
degree.

Prediction is possible only when uniform sequences are found. How are
such sequences to be found in heredity, if they do not appear when a
parent and his offspring are examined? Obviously it is necessary to
examine _a large nu