PREFACE 



his subject plain and interesting, if he can, 

 rather than to talk to the scientific gallery over 

 the heads of the audience. In preparing the 

 lectures for publication it has not been possible 

 to avoid the technical treatment of certain sub- 

 jects, but in the main the lectures are still 

 addressed to the audience rather than to the 

 scientific gallery. Unfortunately biology is 

 still a strange subject to many intelligent 

 people and its terminology is rather terrifying 

 to the uninitiated ; but it is hoped that the glos- 

 sary at the end of the volume may rob these 

 unfamiliar terms of many of their terrors. 



I take this opportunity of thanking Dr. W. 

 E. Castle and Dr. J. H. McGregor for the 

 use of photographs which are reproduced in 

 Figures 81, 82 and 99; and I wish especially to 

 thank my assistant, Marguerite Ruddiman, 

 for her aid in preparing figures and manu- 

 script for publication. 



Princeton, December, 1914 



vii 



