66 HEREDITY IN RELATION TO EUGENICS 



tions. Such tests show that there are all grades of mental 

 ability. At one extreme is the idiot, without language and 

 incapable of attending to his bodily needs. He may retain 

 to maturity the mentality of a child of a few months. In a 

 higher grade mentality of a child of 3 to 5 years is retained 

 throughout life; such are the imbeciles; then come the 

 merely backward children who make dull adults of all 



Fig. 30. — Family group from a long-settled valley where much consan- 

 guineous marriage has taken place. 



grades to the normal condition (Fig. 30). Finally, there are 

 the exceptionally bright, quick children some of whom at 

 least, become superior adults. It is hard to recognize a 

 unit character in such a series any more than in human hair 

 color. Nevertheless there are laws of inheritance of general 

 mental ability that can be sharply expressed. Low men- 

 tality is due to the absence of some factor, and if this factor 

 that determines normal development is lacking in both 

 parents it will be lacking in all of their offspring. 



Two mentally defective parents will produce only mentally 

 defective offspring. This is the first law of inheritance of 



