PREFACE TO FIRST EDITION 



The origin of species was probably the greatest biological 

 problem of the past century ; the origin of individuals is the great- 

 est biological subject of the present one. The many inconclusive 

 attempts to determine just how species arose led naturally to a 

 renewed study of the processes by which individuals came into 

 existence, for it seems probable that the principles and causes of 

 the development of individuals will be found to apply also to the 

 evolution of races. As the doctrine of evolution wrought great 

 change in prevalent beliefs regarding the origin and past history 

 of man, so present studies of development are changing opinions 

 as to the personality of man and the possibilities of improving the 

 race. The doctrine of evolution was largely of theoretical signifi- 

 cance, the phenomena of development are of the greatest practical 

 importance; indeed there is probably no other subject of such vast 

 importance to mankind as the knowledge of and the control over 

 heredity and development. Within recent years the experimental 

 study of heredity and development has led to a new epoch in our 

 knowledge of these subjects, and it does not seem unreasonable to 

 suppose that in time it will produce a better breed of men. 



The lectures which comprise this volume were given at North- 

 western University in February, 1914, on the Norman W. Harris 

 Foundation and were afterward repeated at Princeton University. 

 I gladly take this opportunity of expressing to the faculties, stu- 

 dents and friends of both institutions my deep appreciation of 

 their interest and courtesy. In attempting to present to a general 

 audience the results of recent studies on heredity and develop- 

 ment, with special reference to their application to man, the au- 

 thor has had to choose between simplicity and sufficiency of state- 

 ment, between apparent dogmatism and scientific caution, between 

 a popular and a scientific presentation. These are hard alterna- 



