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.
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