vi Preface - 



lives, but the first duty of a lecturer is to address his audience and 

 to make 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 subjects, 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 glossary 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 Sci- 

 ence Monthly, June to November 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 illus- 

 trations are from many sources: *Figures 9, 10, 19, 20, 22, 23, 

 26-29, 35, 40-43, 51-54, 58, 72-75, 77, 83 are original; 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 remaining figures, 

 namely Figures 48, 50, 63-69, 71, 76, 78-80, 84-96 were copied 

 with little or no modification from various papers, photographs 

 and books, the original source being given in each case. The 

 writer wishes to express his obligation to all authors and publish- 

 ers upon whose works he has drawn, and he thanks especially the 

 Columbia University Press for permission to use Figures 48 and 

 50, and the Open Court Publishing Company for Figures 84-87. 



I take this opportunity of thanking Dr. W. E. Castle ond Dr. 

 J. H. McGregor for the use of photographs which are reproduced 

 in Figures 81, 82 and 96; and I wish especially to thank my as- 

 sistant, Marguerite Ruddiman, for her aid in preparing figures 

 and manuscript for publication. 

 Princeton, December, 1914 



* These figure numbers apply to the first and second editions but not to 

 the third. 



