PREFACE TO THE FOURTH EDITION. 



When the revision of the present edition was begun it 

 became obvious that some fundamental change would 

 be necessary in the treatment. Since the first edition 

 was published in June, 19 12, the subject has grown 

 enormously : it has now completely run away from the 

 166 pages that proved a sufficient allowance in those 

 days. Several courses were open ; the one chosen was 

 to retain the general characteristics of the older editions, 

 dealing broadly with the whole subject, emphasising the 

 outlines and endeavouring to maintain a dispassionate 

 balance between the various parts. Detailed treatment 

 has been avoided, partly because it is now hopeless for 

 any one individual to attempt it, and partly because I 

 did not wish to overload the book and make it into a 

 huge " Handbuch " where the outlines are all obscured 

 and the perspective lost. 



At the same time, however, the student must have 

 access to the more detailed treatment, and it would have 

 helped him little had I simply published a card index 

 and left him to find out the rest for himself. 



The problem was solved by enlisting the sympathy 

 of the Heads of the Departments at Rothamsted, who 

 each undertook to write a monograph dealing with his 

 or her special branch, and which Messrs. Longmans will 

 publish uniformly with this volume. Each one has 

 discussed with me the corresponding sections of this 



