PHYSIOLOGY OF MICRO-ORGANISMS 163 



and it has been the most important single method in establishing 

 our modern ideas of the relation of micro-organisms to infectious 

 diseases, and is the method of greatest promise for the immediate 

 futrue. 



Conditions of Physiological Study. The physiology of many 

 organisms is subject to only very limited experimental investi- 

 gation. Those organisms of very narrow biological adaptation, 

 such as many of the parasitic protozoa, can be studied only in 

 very close relation to their natural environment, the various 

 important elements of which are not readily subject to experi- 

 mental alteration and are largely unrecognizable. Our knowl- 

 edge of these forms must therefore be derived almost exclusively 

 from observations of form and structure, physical and chemical, 

 as they exist and change under the natural conditions of environ- 

 ment, and from changes which take place in the tissues surround- 

 ing the parasite, which we may ascribe with more or less justifica- 

 tion to their activity. Practically all that we know about the 

 physiological activity of the very numerous microbes not yet 

 brought into the group of artificially cultivable forms, has been 

 deduced from morphological observations. Even observations 

 of this kind, however, can be more successfully pursued in those 

 forms capable of artificial culture, and artificial culture is a prime 

 necessity for the study of cause and effect by the methods of ex- 

 perimental physiology. For this reason accurate knowledge of 

 what micro-organisms do is much richer in regard to the culti- 

 vable forms such as bacteria, yeasts and molds. In fact the mi- 

 crobic pure culture presents the most favorable object known for 

 the study of cellular physiology and bio-chemistry. Further- 

 more, the physiological activities of many microbes are of the 

 greatest practical importance. It is not surprising, therefore, 

 that, among the bacteria, many of which grow in artificial media 

 under a great variety of environmental conditions, the relative 

 ease of physiological experimentation, as compared with the dif- 

 ficulty of observation of the minute morphological details, and 

 the great practical importance of its results has lead to an enor- 



