INTRODUCTION TO THE THIRD EDITION 



The kindly reception of Microbiology, which has been progressive, 

 makes a revision a pleasurable task. 



There has been little need of change in the basic facts presented, 

 but there is always room for a clarification of thought and improvement 

 in arrangement. As time has passed it has been found desirable, also, 

 to emphasize and extend some of the chapters. 



Teaching has demonstrated that, in most instances, the chapters 

 dealing with biological products follow more naturally and logically 

 the chapter on immunity. Since the chapters on diseases are more of a 

 reference character, they have been placed at the end. 



The war has made more prominent food contamination, preservation 

 and decomposition. For this reason all chapters considering food have 

 been brought together in a single division and greater attention has 

 been given the subject by rewriting, insertions and enlarging the scope. 

 Dairy microbiology has not been included in the division of food be- 

 cause it has such a distinctive field of its own. 



The editor has a deep feeling of indebtedness to the contributors who 

 have been so kindly disposed, ready and helpful in this revision, and 

 to Miss Marion F. Dondale, for her immeasurable assistance. 



CHARLES E. MARSHALL, EDITOR. 



.\\MIERST, MASSACHUSETTS. 



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