vi INTRODUCTORY NOTE 



one or two statements of fact have been slightly 

 modified, in form, though not in substance, in the 

 belief that these changes would have been approved 

 by the author himself. In other respects the book 

 stands as it was written. 



My thanks are due to those of Dr. Herter's 

 scientific friends who had discussed with him some 

 of the subjects treated and were familiar with his 

 general views. Without their generous help and 

 encouragement I should have found it difficult to 

 prepare the manuscript for the press. 



SUSAN DOWS HERTER. 



