Professor Thorndike, of Columbia University, in a study of fifty pairs of twins, by the use of tests and measurements, derived nearly the same degree of correlation (just exceeding.75) for mental and physical characters—a much higher degree of resemblance, by the way, than he found in brothers and sisters not twins. The result is noteworthy, even though we may doubt the full validity of the method. The most extensive and painstaking investigation of this order was recently made by two Dutch psychologists, Heymans and Wiersma, of the University of Groningen. Some four hundred physicians responded to a questionary, each giving the results of his intimate acquaintance with a single family. The questions asked pertained to the ardor, impulsiveness, resolution, persistence, generosity, temperance, wit, patience, industry, etc.—about ninety topics in all—of each member of the family selected. The results when thrown into tabular form indicate a high degree of resemblance between parent and child—a higher resemblance between father and son, mother and daughter, than between father and daughter and mother and son. Even after allowance had been made for cultural influences, the degree of likeness was about the same as the inheritance of bodily stature, and the result seems, moreover, to stand in close agreement with Galton's law of ancestral inheritance, which accords to the average parent one quarter the heritage of the offspring.
It is, now, a matter of interest that these studies and others that might be brought under survey suggest that our mental traits and capabilities are derived, very much as are our bodily characteristics, from hereditary endowment. You must, however, have been struck by the grossness of the method of collecting facts. What is the scientific value, you may have asked yourselves, of a teacher's or physician's opinion that A is more vivacious or less generous than B? Well, the outcome does show, I think, that careful mathematical treatment of extensive data thus collected will yield noteworthy and valuable results. But the more important the results, the greater the demand for refinement of method. Can the method be improved? I think that it can. The biometrician having shown that the problem is capable of solution, let us see if his arch-enemy, the follower of Mendel, can not suggest the improvement in procedure. The improvement that I find suggested is this: the exclusive inheritance of Mendel lays emphasis upon the analysis and separate treatment of individual characters. Now without presuming to decide whether inheritance takes place in all cases, or even as a rule, through the recombination of "unit characters," mental or physical, psychology may profit by the Mendelian* principles so far as to insist that inheritance be studied, not in the; gross, but in terms of definite and measurable mental structures and functions.
