RELATION TO ENVIRONMENT CLASSIFICATION 43 



constant, there are, nevertheless, many instances in which types 

 entirely similar in these respects must be differentiated. This can be 

 done only by careful study of staining reactions, finer structure, 

 cultural characteristics, and biological activities. 



As a matter of fact, while the botanical classification of the bac- 

 teria offers great difficulties, identification is not so complicated a 

 task as this would indicate. Identification, once roughly made on a 

 morphological basis, is further carried on by the aid of cultural char- 

 acteristics, by biochemical reactions and by pathogenic properties. 

 The bacteria occupy so important a place in agriculture, in medicine, 

 and in hygiene, that it rarely becomes necessary for a worker in any 

 particular field to survey the entire group. The habitat of a large 

 number of species is so well known that this consideration alone 

 often gives a clue to actual identification. 



Bacterial Mutation. The earlier views of bacteriologists con- 

 cerning mutation differed greatly, Naegeli holding that extensive 

 mutation was probably the rule; Cohn, on the other hand, holding 

 strictly to the constancy of form and species. The accumulated ex- 

 perience of many bacteriologists during the years since then seems 

 to point almost entirely in the direction indicated by Cohn, and, in 

 fact, most of our methods of classification are based upon the as- 

 sumption of such constancy. 



Form alone, of course, cannot be relied upon for classification 

 among organisms so simply constructed that the possibilities of 

 variation in form are very limited. In classifying bacteria, there- 

 fore, we are forced to take cognizance not only of morphology, but 

 also of staining characteristics, behavior on differential media, fer- 

 mentation reactions, pathogenicity, and, as a final appeal, reactions 

 with specific immune sera. The last especially, as utilized in agglu- 

 tination and complement-fixation, seems to indicate a fundamental 

 chemical difference in the constitution of bacteria often morpho- 

 logically very much alike. It is certainly a remarkable fact that 

 organisms such as those belonging to the colon-typhoid-dysentery 

 group, though morphologically not differentiable, may still retain 

 differences both in pathogenicity and in fermentation powers after 

 being kept for ten or more years in laboratory media, and we have 

 had the same experience with organisms belonging to the diphtheria 

 group. The virulence of plague and anthrax bacilli may be retained 

 for years in storage, and such evidence shows pretty definitely that 

 fundamental constant differences between organisms exist. 



