ESSAYS ON BACTERIOLOGY. 123 



diphtheria. Having done this, we are able to iden- 

 tify it at any time or place, and, by recognizing it, to 

 diagnose the disease to which it belongs. 



Were the attempt to identify the germ of each 

 specific infectious disease so easily carried to a suc- 

 cessful issue, little would now remain to be done in 

 this direction. Unfortunately, the task is not usually 

 so simple. Difficulties or temporary defeats are likely 

 to be met with at every step. To begin with, the ma- 

 terial taken from the throat, though containing vari- 

 ous bacteria, might not include the germ of diph- 

 theria. The final inoculation experiments would 

 therefore be negative. Or the culture soil might not 

 be suitable for this particular microbe, and again the 

 results would be negative. Unfavorable conditions, 

 such as the presence or absence of air, might defeat 

 the experiment; or the growth of one or more of the 

 other bacteria might obscure, retard or prevent the 

 growth of those sought for. Finally, though the de- 

 sired germ might be obtained in pure culture, the ani- 

 mals experimented upon might not be susceptible to 

 the disease in question, and again a negative or neu- 

 tral result would be reached. Here, therefore, as 

 in agriculture, though the general principles and 

 methods be simple, a reliable conclusion is to be 

 reached only by a successful management of details. 

 But having once identified a germ and demonstrated 

 its specific causative relation to a disease, its identifi- 

 cation and manipulation thereafter usually becomes 



