VI PREFACE 



The author is much indebted to Mr. E. W. Nelson, Chief of the 

 Biological Survey of the United States Department of Agriculture, 

 and to some of his specialists, for reading the proofs of this book 

 and for various changes suggested. 



ALBERT M. REESE. 

 WEST VIRGINIA UNIVERSITY, 

 MORGANTOWN, W. V"A. 



