ACKNOWLEDGMENT 



I want to express to my colleagues and friends of the Department of 

 Agriculture and the National Museum my appreciation for helpful criticism 

 in the selection of the proper equivalents and in the reading and correcting 

 of the proofs. Grateful acknowledgment also is made on behalf of the 

 valuable services rendered by my wife, through the preparation of the 

 illustrations and such constant cooperation as alone assured the comple- 

 tion of the task. 



