AUTHOR'S INTRODUCTION 3 



the subjects, a summary has, on the suggestion of the Committee, 

 been prefixed to each chapter. 



These summaries are as far as possible free from technical 

 terms, and contain in many cases explanatory notes upon the 

 more technical side of the subject-matter of the chapter which 

 follows. It is hoped, nevertheless, that the chapters have for 

 the most part been so presented as to be intelligible after perusal 

 of the summary to those with slight knowledge of the subject. 



A few of the papers referred to in the bibliography have been 

 inaccessible, but nearly all have been read by the writer. 



On some questions it is believed that the bibliography relating 

 to original work is complete, but this would have been impossible 

 to accomplish in regard to those parts of the subject in which 

 opinions and discussions form the main literature. Moreover, on 

 some questions the literature is too voluminous to permit of a com- 

 plete set of references. In such cases the main research papers have 

 been referred to, and any research worker can, it is believed, obtain 

 full references from an investigation of the works quoted. It may 

 perhaps not be out of place to mention the magnificent library 

 belonging to the Patent Office, which is especially valuable for 

 periodicals dealing with agricultural and ' borderland ' subjects. 



The appendices which have been prepared deal with special 

 extensions of the work discussed in some of the chapters It was 

 felt that although the subject-matter might be of interest, it was 

 hardly of sufficient general importance to include in the chapter 

 whose contents corresponded most closely to the appendix in 

 question. 



I have pleasure also in acknowledging my indebtedness to the 

 Controller of H.M. Stationery Office, by whose kind permission the 

 diagrams relating to the work of Prof. Delepine, Dr. Savage, and of 

 myself are reproduced ; also to Prof. Delepine, Dr. Savage, Dr. David 

 Forsyth, Mr. Wilfred Buckley, and the Liverpool Corporation, for 

 permission to reproduce diagrams and plates relating to different 

 subjects. 



I am also deeply indebted to Prof. Hewlett and Mr. Reiss, for 

 being so kind as to supply me with microscopical slides, from which 

 the drawings on the plates of cells and streptococci in milk are taken. 



B 2 



