PREFACE. 



IN the following pages an attempt is made to give some 

 account of the main facts in Bacteriology, and of the 

 life-history of Bacteria and closely allied organisms, and also 

 to discuss the more important theories as to the part played 

 by them in Nature's Economy ; especially in their relation 

 to the commoner fermentative, putrefactive, and disease pro- 

 cesses. 



It may be held by some off my readers that too much pro- 

 minence is given to certain questions, whilst others have 

 been relegated to a comparatively obscure position, and that 

 some points have been accentuated, perhaps at the expense 

 of others ; but to such criticism it may be answered that 

 whilst some facts carry conviction along with them, others 

 can only be properly appreciated when seen by stage lights. 

 On the other hand, many points have of set purpose been 

 lightly touched upon because experience is constantly 

 bringing home the fact that what is new to-day may be 

 out of date to-morrow. I have, therefore, thought it better, 

 in most instances, to confine myself to an exposition of well- 

 accredited facts and to a discussion of some of the more 

 stable theories. 



Being privileged to hold a Sanitary Research Scholarship 

 of the Honourable Grocers' Company for some years, I 

 was enabled to devote considerable time to the study of the 

 relations of Bacteria to Disease, especially in the case of 

 tuberculosis, and many of the interpretations of facts here 

 mentioned are based on observations then made ; the re- 



