CHAPTER III 



TAXONOMY 



The Question of ' Species ' among Bacteria ; Variability ; Involution 

 and Attenuation ; the Classification of Bacteria. 



DESPITE the morphological sameness of the bacteria, the functions they 

 perform in the economy of nature are numberless. When their extra- 

 ordinary versatility first became known, the idea arose that they were beyond 

 the influence of many of the laws of life which govern all other organisms. 

 Any opinion regarding bacteria, however absurd it might be, was permitted, 

 and even the existence of definite species among the bacteria was denied. 

 The controversy on the species question has been long and heated, and only 

 in the last few years has it been decided once for all that the conceptions of 

 genera and species are as justifiable among the bacteria as among other 

 organisms. The subject-matter of the whole discussion may be summed up 

 in two words : pleogeny or mutability of function, and pUomorphism or 

 mutability of shape (14). The pleomorphists maintained that a coccus did 

 not necessarily remain a coccus all its life long, that it could, under certain 

 conditions, stretch itself and assume the shape of a bacillus, that this again 

 could become curved and change into a vibrio, to return again later on to 

 the coccus form that it commenced with. Words like Micrococcus, Bacillus, 

 Vibrio, Spirillum, which we now know to have a definite taxonomic value, 

 were in the eyes of the pleomorphists worthless designations of transient 

 changes of shape. 



As an example of almost inexhaustible versatility the branched aquatic 

 bacterium Cladothrix dichotoma was advanced. But it has now been proved 

 to be not truly pleomorphic (15). Only at the season of reproduction, for 

 the purpose of securing new fields of growth, does the cell undergo change. 

 It loosens itself from its fellows in a filament, develops a tuft of cilia, and 

 emerges from the sheath (Fig. 12). The bacillus-like gonidia or swarm- 

 spores thus produced settle down sooner or later upon some solid body, to 

 which they adhere, and then grow out into new filaments. Neither coccus, 



