While, therefore, this Galton-Pearson law gives no advice in regard to individual marriages, it is yet of great value to applied eugenics. In the first place, it crystallizes the vague realization that remote ancestry is of much less importance than immediate ancestry, to an individual, while showing that every generation has a part in making a man what he is. In the second place, it is found, by mathematical reasoning which need not here be repeated, that the type of a population may be quickly changed by the mating of like with like; and that this newly established type may be maintained when not capable of further progress. Regression is not inevitable, for it may be overcome by selection.
To put the matter in a more concrete form, there is reason to think that if for a few generations superior people would marry only people on the average superior in like degree (superior in ancestry as well as individuality), a point would be reached where all the offspring would tend to be superior, mediocrities of the former type being eliminated; and this superiority could be maintained as long as care was taken to avoid mating with inferior. In other words, the Galton-Pearson Law gives statistical support for a belief that eugenic marriages will create an improved breed of men. And this, it seems to us, is the most important implication of that law for eugenics, although it is an implication that is generally ignored.
We do not propose to discuss further the laws of heredity; but it is likely that the reader who has made no other study of the subject may by this time find himself somewhat bewildered. "Can we talk only in generalities?" he may well ask; "Does eugenics know no laws of heredity that will guide me in the choice of a wife? I thought that was the purpose of eugenics!"
We reply: (1) The laws of heredity are vastly complicated in man by the complex nature of most of his characters. The definite way in which some abnormalities are inherited is known; but it has not been thought necessary to include an account of such facts in this work. They are set forth in other books, especially Davenport's _Heredity in Relation to Eugenics_. The knowledge of how such a trait as color-blindness is inherited may be of importance to one man out of a thousand in choosing a wife; but we are taking a broader view of eugenics than this. As far as the great mass of human characters go, they are, in our opinion, due to so many separately inheritable factors that it is not safe to dogmatize about exactly how they will behave in heredity. Such knowledge, desirable as it may be, is not necessary for race progress.