148 BACTERIOLOGY. 



tions in 1878 could yet in 1880 confidently 

 advance his three requirements). The fact 

 soon became manifest also that not all those 

 bacteria which the microscope showed to be 

 associated with certain diseases could be culti- 

 vated outside of the body ; in such cases it 

 was not possible to obtain pure cultures and 

 infect animals with them. It became evident 

 further that the bacterial forms themselves 

 were not always distinct, typical and specific, 

 and that the simple form-characteristics relied 

 on in earlier times could not be made use of, 

 but recourse must be had to other differential 

 diagnostic characters. Many bacteria also 

 were found to be commonly harmless, although 

 able to cause disease under special conditions. 

 In the light of these discoveries, all our 

 classifications of bacteria are seen to be beset 

 with insuperable difficulties. If we classify 

 according to bacterial genera, the germs of 

 disease are widely separated from one another ; 

 if we classify according to diseases, the bac- 

 terial groups are broken up. Often one does 

 not know, in a given disease, whether to attach 

 more importance to the localization of the 

 germs in single organs or tissues or to the 

 general effect. The grounds for many of 

 these perplexities will be explained 'later very 

 simply when I take up scientifically the dis- 



