506 HUMAN INHERITANCE 



more than his own generation. He may, to be sure, 

 ordinarily observe his own parents and his own children, 

 and more rarely his grandparents and grandchildren. At 

 most he may hope to see a part of four or five generations, 

 commonly totaling fewer than a hundred individuals. 

 Moreover he will have practically no control over the 

 various marriages, and this will render his meager ob- 

 servations of still less value. However, notwithstanding 

 all this, there are some human traits which are sufficiently 

 striking and simple that it has been possible to determine 

 with certainty how they are inherited, and to show that 

 they behave in the same way as similar characters in the 

 animals commonly used in genetic experiments. 



Heritable Characters in Man. — In plants and 

 the lower animals, practically all our knowledge of hered- 

 ity necessarily concerns physical traits. In man, however, 

 we are concerned with mental traits also. It is indeed 

 probable that knowledge of the way in which grades of 

 intelligence may be accurately measured and of the man- 

 ner in which they are inherited is of greater value than 

 any other sort of knowledge which we may ever hope 

 to attain, for on it depends the sole possibility of any 

 extensive improvement of the human race. Physical 

 traits appear, however, to be simpler. It will be desirable, 

 therefore, to treat briefly in this chapter of some physi- 

 cal traits which are either known to obey the ordinary 

 laws of heredity or may reasonably be supposed to do so 

 on the basis of the incomplete data available, and then 

 to consider the inheritance of mental traits in the next 

 chapter. 



Physical Characters. — It is an interesting fact that 

 man possesses very few distinctly marked physical traits. 

 Almost all of them vary through a wide range without 

 any sharp line of separation. Thus there are blondes 

 and brunettes, fat and lean people, straight-haired and 

 curly-haired, tall and short, strong and feeble, and all 

 gradations between. Certain more or less abnormal 



