Fig. 24.—The twins whose finger-prints are shown in Fig. 25.
Intensive genetic research in lower animals and plants has shown that a visible character may be 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.