PREFACE 



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. 



The first three chapters of this book appeared 

 in Popular Science Monthly, June to Novem- 

 ber 1914, by previous arrangement with the 

 Princeton University Press, and a portion of 

 the last chapter was first published in Science 

 in January 1913. The illustrations are from 

 many sources: Figures 9, 10, 19, 20, 22, 23, 26- 

 29, 35, 40-43, 51-54, 58, 72-75, 77, 83 are origi- 

 nal; Figures 1-8, 11-18, 21, 24, 25, 30-34, 36- 

 39, 44-47, 49, 55-57, 59-62, 70 were redrawn 

 with more or less modification from original 

 sources which are indicated in every case; the 



