PREFACE TO SEVENTH EDITION. 



DURING the interval that has elapsed since the appear- 

 ance of the Sixth Edition of this book the problem that 

 has chiefly engaged the attention of the foremost workers 

 in the field of Bacteriology is the old one of Infection 

 and Immunity. 



In its modern development this has become a question 

 of the broadest biological bearing, and, as knowledge 

 accumulates, we find it to be no longer of exclusive 

 bacteriological interest. 



Ramifying, as it does, in many diverse fields it grows 

 correspondingly complex, and a complete presentation 

 of the conceptions and methods would be inappropriate 

 to a work of this character. However, an effort has 

 been made to present in this edition sufficient to acquaint 

 the reader with the manifold directions taken by these 

 studies. 



Furthermore, the practical results, often of the utmost 

 importance to Preventive Medicine, have been included. 

 This, we believe, adds materially to the value of the book. 



The section on Methods has been brought fully up to 

 date, and, while not covering all the new technique, 



