PREFACE 



Schaeffer, Mr. E. L. Shaffer, Dr. C. W. Stiles, Dr. F. B. Sumner, Prof. 

 A. L. Treadwell, Dr. T. W. Vaughan, Prof. D. D. Whitney, Prof. H. H. 

 Wilder, Prof. S. R. Williams, Prof. H. V. Wilson, and Prof. R. H. Wolcott. 

 The author wishes to thank Prof. J. P. Givler of Southwestern College 

 for assistance in revising the proof sheets, he having read a large part of 

 the text and made many corrections and useful suggestions. He also wishes 

 to acknowledge his indebtedness to Prof. Ludwig von Graff of the University 

 of Graz, Austria, who extended to him the use of his laboratories and 

 extensive library during a residence of two semesters in that city, and whose 

 unfailing courtesy and kindness will ever be gratefully remembered. His 

 greatest obligation, however, in preparing this work is to Dr. Charles B. 

 Davenport of Cold Spring Harbor, Long Island, at whose suggestion it 

 was undertaken in the first place, and without whose constant cooperation 

 from year to year during its progress it would not have been completed. 



H. S. P. 

 Haverford, Penn. 



