- Multiple allelomorphs, that is, a series of different grades of a single factor.
- One distinct Mendelian factor (or several such factors), with modifying factors which may cause either (a) intensification, (b) inhibition, or (c) dilution.
- Variation of a factor.
- 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