FOREWORD 19 



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of bacteriologists is in its application to public 

 health doctrines. My views, while in no way ad- 

 verse to doctrines of contagion, favour the notion 

 that infectious or communicable diseases may, at 

 times, and under special conditions, still arise de 

 novo as they must certainly once have done, unless 

 some special creative acts are postulated. But 

 to such doctrines bacteriologists are, as a rule, 

 strongly opposed. They are ultra-contagionists, 

 and for the most part seem disposed to ignore the 

 possibility of the de novo origin of such diseases. 



There are, however, some notable exceptions, and 

 I would especially call attention to an authoritative 

 statement made by Lehmann and Neumann in their 

 Principles of Bacteriology (Translation 1901, pp. 

 118-119), in which they say: "The division of 

 bacteria into pathogenic and non-pathogenic, etc., 

 as is still always done in text-books, has failed 

 absolutely. We can understand and know the 

 pathogenic varieties only if we study simulta- 

 neously the non-pathogenic, from which the 

 former have once originated and still always 

 originate." They say also : We certainly be- 

 lieve it belongs to the future to convert varieties 

 of bacteria into others in a manner scarcely to be 



