vi PREFACE 



of whom facilitated the work by giving me introductions 

 throughout the districts with which they were best ac- 

 quainted. 



Miss M. B. Lee and Miss V. G. Jackson have 

 rendered valuable assistance with the proof sheets, and, 

 above all, I am specially indebted to Dr. E. J. Russell 

 for his constant advice and criticism throughout the pre- 

 paration of the book. 



W. E. B. 



ROTHAMSTED, 



June, 1920. 



