A paragraph from Edgar Schuster[40] may appropriately be added. "After considering the published evidence a word must be said of facts which most people may collect for themselves. They are difficult to record, but are perhaps more convincing than any quantity of statistics. If one knows well several members of a family, one is bound to see in them likenesses with regard to mental traits, both large and small, which may sometimes be accounted for by example on the one hand or unconscious imitation on the other, but are often quite inexplicable on any other theory than heredity. It is difficult to understand how the inheritance of mental capacity can be denied by those whose eyes are open and whose minds are open too."
Strictly speaking, it is of course true that man inherits nothing more than the capacity of making mental acquirements. But this general capacity is made up of many separate capacities, all of these capacities are variable, and the variations are inherited. Such seems to us to be the unmistakable verdict of the evidence.
Our conclusions as to the inheritance of all sorts of mental capacity are not based on the mere presence of the same trait in parent and child. As the psychological analysis of individual traits proceeds, it will be possible to proceed further with the study of the inheritance of these traits. Some work has been done on spelling, which is particularly interesting because most people, without reflection, would take it for granted that a child's spelling ability depends almost wholly on his training. Professor Thorndike's exposition[41] of the investigation is as follows:
"E. L. Earle ('03) measured the spelling abilities of some 800 children in the St. Xavier school in New York by careful tests. As the children in this school commonly enter at a very early age, and as the staff and methods of teaching remain very constant, we have in the case of the 180 pairs of brothers and sisters included in the 600 children closely similar school training. Mr. Earle measured the ability of any individual by his deviation from the average for his grade and sex, and found the coefficient of correlation between children of the same family to be .50. That is, any individual is on the average 50% as much above or below the average for his age and sex as his brother or sister.
"Similarities of home training might account for this, but any one experienced in teaching will hesitate to attribute much efficacy to such similarities. Bad spellers remain bad spellers though their teachers change. Moreover, Dr. J. M. Rice in his exhaustive study of spelling ability ('97) found little or no relationship between good spelling and any one of the popular methods, and little or none between poor spelling and foreign parentage. Cornman's more careful study of spelling ('07) supports the view that ability to spell is little influenced by such differences in school or home training as commonly exist."
This is a very clear-cut case of a definite intellectual ability, differences in which might be supposed to be due almost wholly to the child's training, but which seem, on investigation, to be largely due to heredity.
The problem may be examined in still greater detail. Does a man merely inherit manual skill, let us say, or does he inherit the precise kind of manual skill needed to make a surgeon but not the kind that would be useful to a watchmaker? Is a man born merely with a generalized "artistic" ability, or is it one adapted solely for, let us say, music; or further, is it adapted solely for violin playing, not for the piano?
Galton, in his pioneer studies, sought for data on this question. In regard to English judges, he wrote: "Do the judges often have sons who succeed in the same career, where success would have been impossible if they had not been gifted with the special qualities of their fathers? Out of the 286 judges, more than one in every nine of them have been either father, son or brother to another judge, and the other high legal relationships have been even more numerous. There can not, then, remain a doubt but that the peculiar type of ability that is necessary to a judge is often transmitted by descent."
Unfortunately, we can not feel quite as free from doubt on the point as Galton did. The judicial mind, if that be the main qualification for a judge, might be inherited, or it might be the result of training. Such a case, standing alone, is inconclusive.
Galton similarly showed that the sons of statesmen tended to be statesmen, and that the same was true in families of great commanders, literary men, poets and divines. In his list of eminent painters, all the relatives mentioned are painters save four, two of whom were gifted in sculpture, one in music and one in embroidery. As to musicians, Mendelssohn and Meyerbeer are the only ones in his list whose eminent kinsmen achieved their success in other careers than music.