PREFACE 



The origin of species was probably the great- 

 est biological problem of the past century ; the 

 origin of individuals is the greatest biological 

 subject of the present one. The many incon- 

 clusive attempts to determine just how species 

 arose led naturally to a renewed study of the 

 processes by which individuals come into 

 existence, for it seems probable that the prin- 

 ciples and causes of the development of indi- 

 viduals will be found to apply also to the 

 evolution of races. As the doctrine of evolu- 

 tion wrought great change in prevalent be- 

 liefs 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 theo- 

 retical significance, the phenomena of develop- 

 ment are of the greatest practical importance ; 

 indeed there is probably no other subject of 



