124 CLASSIFICATION OF BACTERIA 



nations. Yet new perfections in technique constantly cast new organisms on the shores 

 of the bacterial sea, where they become accessible. Workers with the anaerobic group 

 tell us that it is c^uite possible to describe totally new organisms at the rate of several 

 a week, and it is only when someone gets interested in a special problem that we get 

 a list of the organisms concerned. It is clearly impossible to foresee new discoveries 

 beyond what may be safely prophesied from analogy, and the best we can do is to 

 make and preserve a scheme which we believe will act as a framework on which to 

 hang not only the groups we have, but those which may turn up from time to time. 



It seems clear, then, that in common with students of other biological groups we 

 have reached only a small fraction of the varieties which make up the mass, and that 

 it is logical to believe that the changes in our knowledge and beliefs which have fol- 

 lowed the lights of informative changes in technique are only an earnest of what is to 

 follow. Bacteriology, as we now understand it, is less than fifty years old, and inas- 

 much as bacteria are either plants or animals, it fell heir to all the taxonomic litera- 

 ture relating to both. The discovery of the principles of pure-culture study resulted 

 in such a sudden burst of investigation that it was a lost month in which a new organ- 

 ism was not described, catalogued, and laid away, very frequently in the wrong grave. 



With this introduction we can enter upon a discussion of the criteria used in past 

 and present times to establish the various groups from order to variety with an at- 

 tempt at a critical summary of modern opinion, 



BASES OF CLASSIFICATION 



In the minds of many, perhaps most persons, the actual definition of the words 

 used in taxonomy are vague, and some definitions may be helpful. Inasmuch as the 

 "genus" is the center, so to speak, the grouping which brings together a collection of 

 species, and itself forms the base for larger groupings, let us begin with it. Agassiz de- 

 fined genera as "most closely allied groups of [animals] differing .... simply in the 

 ultimate structural peculiarities of some of their parts." The Century Dictionary 

 (iQoi) speaks of it as a 



classificatory group ranking next above the species, containing a group of species (sometimes 

 a single species), possessing certain structural characters differing from those of any others. 

 The value assigned to a genus is wholly arbitrary, that is, it is entirely a matter of opinion 

 or current usage what characters shall be considered generic .... a genus has no natural, 

 much less necessary, definition, its meaning being at best a matter of expert opinion, and the 

 same is true of the species, family, order, class, etc. A genus of the animal kingdom in the 

 time of Linnaeus was a group of species approximately equivalent to a modern family, some- 

 times even to an order. 



Stiles {loc. cit.) defines it as "a taxonomic complex of specimens grouped for the mo- 

 ment (according to our subjective and never absolutely perfect knowledge) around a 

 genotype." 



These definitions emphasize that it is impossible to lay down rules which will hold 

 future experts, at least beyond a certain general plan, and also show why it is that in 

 so many cases the desire for revision starts with the study of a genus, and later in- 

 volves its position in connection with other genera. 



Perhaps one of the most important points in the description of genera is the selec- 



