Page:Popular Science Monthly Volume 75.djvu/468

464 about the central theme of human ability and its dependence upon the stock. In a study of three hundred eminent English families, Galton found the descent of great mental capacity to be far more intensive than in less eminent families; and found, further, that the closer the blood-relationship, the greater was the number of eminent individuals. Later studies, for example, Galton's own recent inquiry into the family history of Fellows of the Royal Society, have likewise shown that the person of superior mind is much more likely than the average to possess superior ancestors and descendants. These facts are justly interpreted as an indication that mental endowment depends in large measure upon direct inheritance.

This kind of inquiry then—the kind that takes human beings in the mass and applies a rough unit of measurement—reveals in a striking way the importance of the hereditary factor in mental ability. However, the method does not constitute a science of heredity. No evolutionist would be satisfied to know that large horses beget large horses, and small horses small horses. It is the degree of likeness of some particular organ, or quality, or function, or peculiarity that the student of heredity now attempts to state, and to state often in numerical terms.

So the present psychological problem of heredity comes back to the question of mental characters and of the best methods for their description and measurement. The psychologist may answer this question in either one of two ways. First, he may fall back upon the distinctions of every-day or popular psychology and say that "a good memory," "sound judgment," "conscientiousness," "affability," "sentimentality" and "industry" are mental characters, and that the way to calculate their heritability is to take a large number of persons, related and not related, estimate the eminence of these qualities in each, note their distribution and derive laws of resemblance. If you find that a good memory, or affability, or industry "runs in families," and is not to be attributed to a common environment, you may conclude that the characteristic in question is heritable. As a matter of fact, this is the method that, for the most part, has been employed within the last ten years; and it has been used either by biometricians themselves, or by the psychologist who has followed their initiative.

Let me cite two or three instances. Professor Karl Pearson, of the University of London, the leader of the biometrical school, collected from teachers data regarding some four thousand children. Color of hair and eyes and cephalic index were among the physical characters graded, and conscientiousness, temper and assertiveness among the mental traits. The degree of likeness between brothers and sisters was found to be substantially the same for physical and mental qualities; in Pearson's terms, each showed a correlation of about 0.5. Again,