2 INTRODUCTION 



The development of bacteriology has been promoted by the 

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

 mists, 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 medidne 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 technic and because 

 of the peculiarly critical thought necessary in the interpretation 

 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 become con- 

 vinced only when overwhelming evidence compels conviction; 

 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 inany of these live as parasites and some 

 are important in the causation of disease, they are commonly 

 considered along with the pathogenic bacteria. The terms mi- 

 crobe and micro-organism properly include these as well as the 

 bacteria. There is thus an -evident tendency to extend the field 



