PREFACE 



Recent great advances in our knowledge of heredity 

 lave revolutionized the methods of agriculturalists in im- 

 )roving domesticated plants and animals. V^It was early 

 ■ecognized that this new knowledge would have a far- 

 •eaching influence upon certain problems of human society 

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

 )opulation, of effectiveness, of health and vigorJ Now, 

 ;reat as are the potentialities of the new science of heredity 

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

 lot yet realized. A vast amount of investigation into the 

 aws of the inheritance of human traits will be required 

 )efore it will be possible to give definite instruction as to fit 

 narriage matings. Our social problems still remain prob- 

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

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

 n getting a better method of study. 



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

 in application of this method to some specific problems, 

 ispecially to the transmission of various human traits and 

 usceptibilities to disease, has been attempted. The sug- 

 ;estions made are by no means final but are made to illus- 

 rate the general method and give the most probable con- 

 tusions. Only with much more accurate data can the 

 aws of inheritance of family peculiarities be definitely de- 

 er mined. 



Some general consequences of the new point of view for 

 he American population have been set forth in Chap- 

 ers IV to VI. Their essential truth will, I trust, be generally 



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