18 ESSAYS ON BACTERIOLOGY. 



This question, then, of the relation of animals to the 

 infectious diseases is evidently one of importance in 

 bacteriological investigations, and the uiicertaintie& 

 connected with it furnish an obstacle which is often 

 deplored. Obviously also, as the work progresses, 

 here must open fields for painstaking and interesting 

 research. Incidentally these researches may throw a 

 valuable light upon veterinary medicine. 



Is each of the infectious diseases due to a single and 

 constant, a specific cause? From the evident uni- 

 formity in their phenomena, it is generally believed 

 that this is true; and, for most of them at least, the 

 supposition is in all probability correct. Yet there 

 are some facts which have seemed to throw doubt upon 

 this belief, and in its present condition bacteriology 

 cannot answer the question with absolute certainty. 

 Two organisms, for instance, have been identified in 

 connection with croupous pneumonia; that of Fried- 

 lander, and the Sternberg-Pasteur-FrEenkel micrococ- 

 cus. It is claimed that by inoculation with each of 

 these a more or less typical exudative inflammation of 

 the lungs has been produced. But they do not satisfy 

 the postulates of Koch. Neither has been found con- 

 stantly in pneumonia, and both have been found un- 

 der other conditions. So true is this that Frankel has 

 called one of them the microbe of sputum septicemia 

 rather than of pileumonia. A similar condition of 

 uncertainty exists in the bacteriology of diphtheria. 



Such experimental results as these, even though 



