PEEFACE 



npHIS essay is based upon notes and observations which I 

 -*- collected previous to the year 1913. It was only partly 

 written when, in August 1914, I proceeded on active service. 

 I was able, however, to complete it in the following summer 

 while serving with a Field Ambulance in France, and in the 

 autumn of the same year (1915) I submitted it in the form of 

 a Dissertation for the degree of M.D. at the University of 

 Cambridge. 



The difficulties in carrying out work of this character 

 while serving at the Front — remote from libraries and amidst 

 " alarms and excursions " which break up one's scanty leisure 

 — are sufficiently obvious and I trust may excuse some 

 of its defects. Some valuable materials which I had hoped to 

 utilise, including chapters on Viability and Agglutination 

 Reactions, were buried by a shell explosion and could not 

 be replaced. 



The claims of Army work have also precluded any attempt 

 on ray part to bring the work up to date by reference to 

 papers published since the beginning of the war. I particu- 

 larly regret having learnt too late to include any mention 

 of it in the following pages of the valuable research carried 

 out by Dr Thiele and Dr Embleton on the part played by 

 the body ferments in the pathogenicity of bacteria. 



Though I have endeavoured to suppress all irrelevant 

 matter, I am only too conscious of the discursiveness of this 

 essay. The topic is one of absorbing interest and at every step 

 one is tempted to digress. In the words of Dante, which I 

 have quoted on the title page, "The novelty must be my 

 excuse if my pen has wandered at all." 



