MENTAL DEFECTIVENESS 87 



THE OEIGIN AND CONTEOL OF MENTAL DEFECTIVENESS 



By Dr. CHAS. B. DAVENPORT 



STATION FOR EXPERIMENTAL EVOLUTION, CARNEGIE INSTITUTION OF WASHINGTON, 



■^VTOT long ago I spoke to a company of physicians and lawyers on 

 -^^ inheritance of certain types of imbecility, and exhibited some 

 charts that showed that imbecility in a child is due to defects in the 

 germ-plasm of both his parents. At the end of my remarks the chair- 

 man pointed out that the facts presented merely deferred the origin 

 of feeble-mindedness a generation or two and did not touch on its true 

 cause. I find this idea wide-spread; the point raised consequently 

 deserves further consideration: How did feeble-mindedness originate 

 in the first instance? 



Before we can answer the question as to the " cause " of feeble- 

 mindedness it is desirable to get a clear definition of the term. As a 

 matter of fact, very diverse definitions have been offered. An old legal 

 formula is as follows : " He that shall be said to be a sot and idiot from 

 his birth is such a person who can not coimt or number twenty pence, 

 not tell who was his father or mother, nor how old he is, so it may 

 appear that he hath no understanding or reason what shall be for his 

 profit or what for his loss; but, if he have sufficient understanding to 

 know and understand his letters, and to read by teaching or informa- 

 tion, then it seems he is not an idiot." While this definition lacks in 

 completeness and scope, it has a more philosophical basis than many 

 that are more recent. Of late the Binet- Simon tests of mental grade 

 have aroused new enthusiasm and have been thought to give an exact, 

 quantitative measurement and definition of the different classes of 

 mental backwardness. The method is simply that of establishing a 

 series of mental standards (questions, exercises, mental feats and 

 so on) for each year of school life, grading a given subject by these 

 standards and finding the difference between the actual age of the 

 subject and the standard age of the highest test passed by him. This 

 method of defining feeble-mindedness seems to assume that there is a 

 greater mental resemblance between two persons deficient three years 

 than there is between one who is deficient three years and one who is 

 deficient four years. And that, it seems to me, is fundamentally erro- 

 neous. For the modern biologist is coming to rely less on the idea of 

 races or groups and to realize that, in nature, we have only individuals, 

 made up of collections of traits that are, for the most part, separately 

 inheritable. Not individuals, but their transmittable characters, are 



