2 INTRODUCTION 



colleagues. The commonly used meat-water-pepton-gelatin was 

 introduced by Loffler; agar by Frau Hesse. 



The development of bacteriology has been promoted by the 

 work of biologists, botanists, chemists, pathologists and agronom- 

 ists, many of whom have been willing to include bacteriology 

 as a subdivision of their own field. The practical importance of 

 Bacteriology to these various fields is becoming progressively 

 more evident. The relation to pathology and medicine is per- 

 haps most clearly recognized, although the importance of bac- 

 teria in chemical technology and in agriculture is no longer 

 questioned. The relationships to general biology have not been 

 so completely developed as yet, partly because these have seemed 

 to offer less promise of immediate practical application, and partly 

 because few well-trained zoologists or botanists have devoted 

 serious attention to bacteriology. 



As a matter of fact, bacteriology must be ranked as a distinct 

 science, especially because of its peculiar special technic and be- 

 cause of the peculiarly critical thought necessary in the inter- 

 pretation of bacteriological observations and experiments. The 

 importance of these can be fully appreciated only after actual 

 experience in handling microbes. Here is a science in which 

 skepticism is a necessary safeguard, a skepticism which will be- 

 come convinced only when overwhelming evidence compels con- 

 viction; and, while regarding other conclusions with interest or 

 even with enthusiasm, still carefully reserves final judgment as 

 long as the observed phenomena are open to more than one 

 interpretation. 



These methods of thinking and of working have been applied 

 to organisms other than the bacteria, on the one hand to the 

 unicellular animals, the protozoa, on the other to more complex 

 plant-forms such as the yeasts and molds, and more especially to 

 the study of the still undefined types of living things known as 

 filterable viruses or more vulgarly as the ultramicroscopic mi- 

 crobes. Inasmuch as many of these live as parasites and some 

 are important in the causation of disease, they are commonly 



