ESSAYS ON BACTERIOLOGY. 151 



analysis, due to the growth of bacteria in the body, 

 the questions arise: What other agencies enter into 

 their production, and what role do such other factors 

 play? Can the facts of clinical experience and the 

 facts of laboratory work be harmonized? 



I believe that in large measure they may. And, 

 further, if one will but observe closely, it will be 

 found that laboratory experiment in this direction has 

 confirmed many of the facts of clinical experience, has 

 furnished a more correct interpretation of others, and 

 has thrown light upon almost all. When we come to 

 study the factors other than the mere presence of bac- 

 teria which enter into infection, we find that they fall 

 into two groups, one relating to the persons infected, 

 the other to the bacteria. What now are some of 

 these factors, and to what extent has bacteriology in- 

 creased our knowledge of their importance and 

 meaning? 



First, race. It has long been observed that certain 

 races of men seem less susceptible to certain infections 

 than others. For instance, the negro is believed to 

 be less susceptible to yellow fever than the white. 

 Can we find confirmation of such a principle in the 

 bacteriological laboratory? We do find many. Thus 

 the rat is comparatively unsusceptible to anthrax, 

 while the guinea pig readily succumbs to small dosage. 

 Algerian sheep resist anthrax strongly, while ordinary 

 sheep easily fall a prey to this serious disease. Hens 

 and frogs have a natural immunity against anthrax, 



