PREFACE TO THE SEVENTH EDITION. 



To his scientific friends, whose continued appreciation 

 and patronage have made necessary the preparation of this 

 seventh edition of the PATHOGENIC BACTERIA, the author 

 desires to extend his sincere thanks. 



In so far as his pages have been found a useful and reli- 

 able guide, he is elated; in so far as they may have failed, 

 he feels humiliated, but 'is stimulated to renewed and more 

 earnest endeavors on their behalf. 



The flight of time has brought with it many changes, but 

 perhaps in no department of learning have they come in 

 greater number or with more startling rapidity than in 

 Microbiology. 



When, some eighteen years ago, the author was appointed 

 to give the first systematic course of lectures upon Bacteri- 

 ology, in the Medical Department of the University of Penn- 

 sylvania, there were few text-books suitable for the use of 

 students, and the preparation of the "Pathogenic Bacteria" 

 seemed to be a justified, though a doubtful venture. To-day 

 our shelves groan beneath the weight of many excellent vol- 

 umes. 



When the " Pathogenic Bacteria" appeared, all of the 

 existing books were general in character. The title adopted 

 by the author seemed to be of doubtful expediency, lest it 

 should limit the success of the work by contracting the 

 sphere of its usefulness. But it was fortunate in meeting 

 with a cordial reception, and, in spite of its title, soon 

 came to be looked upon and used as a general text-book. 

 All of the early revisions were directed toward increasing 

 its general usefulness and making it serviceable in all the 

 fields in which Bacteriology was taught or practised. 



But now, times have changed, and it can no longer be said 

 that anything short of a many volume encyclopedia can be 

 regarded as an adequate "general" work upon Bacteriology. 

 There are now excellent books devoted to microbiology; to 



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