44 MANUAL OF DETERMINATIVE BACTERIOLOGY 



Streptococcus lactis var. maltigenes (the Streptococcus of milk producing 

 malt flavor). 



Some principles of taxonomy. It is important further that the student 

 of bacteriology recognize the meaning of certain terms used regularly in 

 classifications. 



(1) Species (plural species). A species of plant (or animal) is assumed 

 above to be one kind of plant. But how much difference must exist be- 

 tween two cultures of bacteria before one is justified in regarding the organ- 

 isms in them as being of distinct kinds or species? No rule can be laid 

 down. It depends largely upon convenience and a more or less arbitrary 

 decision. As stated by Hitchcock (Descriptive Systematic Botany, New 

 York, 1925, p. 8): "The unit of classification is a coherent group of like 

 individuals, called a species. The term is difficult to define with precision 

 because a species is not a definite entity, but a taxonomic concept." Huck- 

 er and Pederson (New York Agric. Exper. Sta. Tech. Bull. 167, 1930, p. 39) 

 state: "The difficulty met with among these lower forms in dividing them 

 into well-defined groups has led man}'' to question whether these small 

 groups or 'species' are natural groups and whether such groups can be 

 considered to be similar to 'species' among higher forms. However this 

 may be, it is necessary to arrange bacteria as well as possible into groups or 

 so-called 'species' for convenience in classification," and again (Hucker, 

 New York Agric. Exper. Sta. Tech. Bull. 100, 1924, 29), "characters 

 applicable to the differentiation of species must evidence a certain amount 

 of constancy when studied over a large series of tests. Furthermore, 

 characters adapted to the differentiation of larger natural groups or genera 

 should, in addition to constancy, show some correlation with other constant 

 characteristics. The presence of this relationship or correlation between 

 characters for the division of genera indicates that the groupings are being 

 made along natural rather than artificial lines." 



Type culture. It is quite evident that when a new species of bacterium 

 is described, it must include the particular culture from which the species 

 description was made. This original culture is termed the type culture. 

 We may develop a definition as follows : — A species of bacterium is the type 

 culture or specimen together with all other cultures or specimens regarded 

 b}'' an investigator as sufficiently like the type (or sufficiently closely related 

 to it) to be grouped with it. It is self-evident that different investigators 

 may not draw the same boundaries for a given species. This leads to some 

 practical difficulties, but no better definition has been evolved. 



There are certain special cases which require brief discussion. 



(a) How should one designate the different stages in an organism that 

 exhibits a growth cycle? There seems to be increasing evidence that certain 

 bacteria show cycles in morphology which parallel to some degree those well 



