MENTAL INHERITANCE 465 



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 ex- 

 tensive 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 ques- 

 tionary, each giving the results of his intimate acquaintance with a 

 single family. The questions asked pertained to the ardor, impulsive- 

 ness, resolution, persistence, generosity, temperance, wit, patience, in- 

 dustry, 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 re- 

 semblance 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, more- 

 over, to stand in close agreement with Galton's law of ancestral in- 

 heritance, 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 re- 

 sults. 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. 

 JSTow without presuming to decide whether inheritance takes place in 

 all cases, or even as a rule, through the recombination of " uirit char- 

 acters," 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. 



VOL. xxlv. — 31. 



