PREFACE 



Recent great advances in our knowledge of heredity 

 have revolutionized the methods of agriculturalists in im- 

 proving domesticated plants and animals. It was early 

 recognized that this new knowledge would have a far- 

 reaching influence upon certain problems of human society 

 — the problems of the unsocial classes, of immigration, of 

 population, of effectiveness, of health and vigor. Now, 

 great as are the potentialities of the new science of heredity 

 in its application to man it must be confessed that they are 

 not yet realized. A vast amount of investigation into the 

 laws of the inheritance of human traits will be required 

 before it will be possible to give definite instruction as to fit 

 marriage matings. Oiu- social problems still remain prob- 

 lems. For a long time yet our watchword must be investi- 

 gation. The advance that has been made so far is chiefly 

 in getting a better method of study. 



In this book I have sought to explain this new method. 

 An application of this method to some specific problems, 

 especially to the transmission of various human traits and 

 susceptibiUties to disease, has been attempted. The sug- 

 gestions made are by no means final but are made to illus- 

 trate the general method and give the most probable con- 

 clusions. Only with much more accurate data can the 

 laws of inheritance of family peculiarities be definitely de- 

 termined. 



Some general consequences of the new point of view for 

 the American population have been set forth in Chap- 

 ters IV to VI. Their essential truth will, I trust, be generaUy 



ill 



