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ization supplements them. Identification at the species level and often 

 even at the genus level is sometimes almost entirely biochemical. 



Among the common physiological tests are those for determining 

 the ability to liquefy gelatin, to reduce potassium nitrate to nitrite or 

 even to ammonia or gaseous nitrogen, to change the reaction of milk, 

 and to digest the curd produced in milk by some types of micro-organ- 

 isms. Many other examples are given in the Manual of Microbio- 

 logical Methods (6) and in textbooks of bacteriology. 



It must not be supposed that an unknown must be completely charac- 

 terized before identification by classification keys is attempted. In 

 fact, keys will suggest which tests are necessary at each step in the 

 identification process. 



Nomenclature and taxonomy are complex subjects. Only the fun- 

 damentals have been presented here. Two more principles might be 

 emphasized however. The first is that in taxonomic work only charac- 

 teristics that are constantly reproducible can be relied upon, and the 

 second is that to be really of value a characteristic should correlate 

 with one or more other properties. Obviously every characteristic will 

 not be of taxonomic value. Merely because two otherwise similar 

 micro-organisms differ, for example, in ability to reduce nitrates, it 

 does not follow that they are different species. The proper conclusion 

 might rather be that the ability to reduce nitrates is inconstant and 

 not suitable for use in the classification of these micro-organisms. 



SYSTEMS OF CLASSIFICATION 



Most groups of micro-organisms have been studied taxonomically 

 by several investigators. It must be realized that no classification or 

 treatment, no matter how complete and thorough, can ever be consid- 

 ered perfect. Generic and specific concepts are subjective and vary 

 with the monographer. In some instances a species is clearly distinct 

 from all its relatives and therefore its limits do not in any way overlap 

 those of other species in the same genus. For example, Absidia 

 glauca Hagem is always found in the soil and has globose spores and 

 a green aerial mycelium. Never has a culture been found to have 

 other colors or other shaped spores, or to come from any place but soil. 

 This organism is clearly a distinct taxon. However, one expert might 

 consider it a species and the next a color variety of another soil-inhabit- 

 ing species with globose spores and colored mycelium. 



A species is actually a filing system, in which individuals in nature 

 may be conveniently grouped. Several related groups of micro-or- 

 ganisms then may be considered as distinct entities ; but since they are 

 closely related, they are placed together in a genus. The best classifi- 

 cation of a particular group will be determined by the usefulness of the 

 system ; the one most usable on the natural population will be the best. 

 Often a taxonomic work quite adequate for a given geographical loca- 

 tion will be completely useless in another region. 



A second factor is that increased study of more and more individuals 

 will cause the expert to alter his species concept : Species A and B were 

 created on the basis of two isolates, one of A and one of B. Later 

 when a hundred or more forms have been found, A and B cannot be 

 distinguished because of all the degrees of intergradation between the 



