150 ESSAYS ON BACTERIOLOGY. 



guidance. The agriculturist would not misinterpret 

 his part of the analogy; neither need the physician 

 misinterpret his. 



We all recognize that there have been many fanci- 

 ful explanations of the infectious diseases, many 

 things fancifully invoked as factors in their causation. 

 But we all also recognize that even long before the 

 identification of disease germs various agencies, non- 

 bacterial in character, were shown to be closely asso- 

 ciated with and even to play a part in the production 

 of these maladies. The importance of these agencies 

 has been seen to vary in uifferent diseases and under 

 differing conditions, being sometimes but slight, some- 

 times seeming to stand in the foreground as exciting 

 or predisposing causes. But the fact of their impor- 

 tance and influence has been and is unquestioned and 

 unquestionable. 



l^'or example, no one asserts or believes that other 

 factors than the peculiar poison, except possibly age, 

 play an important role either as exciting or predis- 

 posing, causes of measles. But it is commonly and 

 strongly believed that the tubercle bacilli by no means 

 stand alone in the development of tuberculosis. 

 Every one recognizes, after the experience of recent 

 years, that the disease called la grippe, in its epidemic 

 form, attacks many without the aid of recognizable 

 predisposing causes. But every one also knows that 

 acute pneumonia seldom comes by infection alone. 



Granting that the infectious diseases are, in the last 



