with known pedigrees, and see how closely the characters of persons correspond with what we should expect were heredity the sole cause of mental and moral peculiarities—in other words, see if the results are as certain when applied to mental traits as to the more physical and tangible qualities like eye and hair, color, stature, etc. If it should be found that the human mind and moral character are subject to the law of Galton, and with an accuracy as constant as the coloration of animals, then we may conclude that the mind and character are very strongly inherited, since coloration in animals is due to what we at present at any rate consider heredity. Of course we do not expect to find the same accuracy in dealing with psychic aspects, since every one knows that moral traits, for instance, are much the result of environment—education, example, etc. Let us, by studying human characters and comparing them with their close blood relations, see how strong inheritance appears to be.
It is often impossible to say in any individual, how much is due to one and how much to another cause, but by taking a large number we may estimate in a rough way the proportionate reliance that is to be placed on each factor on the average. Galton's law, based on stature and color in animals, in human hair and eyes, etc., is this: Each child inherits one half of his make-up from his parents, one half of the remaining half from his grandparents, one half of the remaining one fourth from his great grandparents, and so on to infinity. Thus each parent contributes one fourth of the entire influence, each grandparent one fourth of one fourth, each great grandparent one eighth of one eighth, or one sixty-fourth, and so on. So we see how little is the influence to be expected from heredity from one distinguished great grandfather.
In order to get material for such a study, one might take individuals at random and then their brothers and sisters and all their ancestors to a reasonable degree of remoteness, say all the great grandparents, which would give 871⁄2 per cent, of the entire influence. This would be extremely difficult, as it is almost impossible to verify even the names of all the great grandparents of most people, let alone their mental and moral traits. Or one might use a large number of uncles and aunts to determine the latent inheritance of the ancestry, not known in the parents. Unless one had some proper way of selecting the material he might take instances that illustrate some theory and neglect others that do not.
The method I have employed has been to take individuals merely by blood relationship, and include every person about whom anything could be found. By doing this, I have escaped any selection of cases which illustrate a theory and at the same time know the exact blood relationship of every person to every other person. Of all families