PREFACE 9 



reading the text of the chapters covering their respective fields and for their 

 constructive criticism; to the late Professor Thaxter of Harvard University 

 and to Dr. Charles Thorn of the Bureau of Soils for the kind interest and 

 advice they have so generously given; to Mr. James F. Ballard and Miss 

 Lotta McCrea of the Boston Medical Library and to Miss Ella B. Lawrence 

 of the Library of the Washington University School of Medicine for their 

 sympathetic help in finding incorrectly cited references ; to many of my former 

 students for helpful suggestions ; to the Chancellor of Washington University 

 and to the Director of the Missouri Botanical Garden for a leave of absence 

 to complete reading in the files of rarer periodicals in Boston before begin- 

 ning my duties in their respective institutions ; and to my wife who has helped 

 throughout in the dreary tasks of preparing the manuscript for the printer, 

 correcting the proofs and preparing the index. 



— C. W. D. 



St. Louis. 

 August 1, 1935. 



