INTRODUCTION vii 



change, but these ideas are becoming more generally 

 diffused every day, and the unfamiliar reader may to 

 some extent gather them as he goes along. I must 

 express my indebtedness to Dr E. J. Russell for his 

 advice on certain sections of the book, and to Miss 

 W. E. Brenchley for the botanical drawings therein con- 

 tained. Miss L. M. Underwood also has greatly helped 

 me by compiling the index. As on other occasions 

 also, I have to thank Mr G. T. Dunkley for the care he 

 has taken in looking through the proofs, and verifying 

 the results quoted 



A. D. HALL. 



The Rothamsted Experimental Station, 

 June 1910. 



