viii PREFACE TO THE REVISED EDITION 



manuscript critically read by Dr. J. W. Wilson of 

 Brown University. 



There are thirty-four new figures and diagrams 

 which are either original or copied from acknowledged 

 sources. Mr. C. J. Fish made the drawings for figures 

 22 and 32. The proof was read by my wife and by my 

 niece, Miss Dorothy Walter. 



I wish to acknowledge the help I have received from 

 all of these sources as well as from many unnamed 

 friends who have given valuable suggestions. 



H. E. W. 



LA JOLUA, CALIF., 

 March 18, 



