CHAPTER V. 



THE PRINCIPLES OF CLASSIFICATION AND THEIR 

 APPLICATION TO 1TIE SULPHUR BACTERIA. 



The Principles of a Natural Classification, and their 

 Application to the Grouping of the Sulphur Bacteria. 



All bacteria are usually assigned to the plant kingdom, 

 mainly because the genus Bacillus, which includes most 

 bacteria, is, in its structure and life-history, of a distinctive 

 plant nature. This cannot, however, be said of some of the 

 other genera in this group. Some, as for example. Spirillum, 

 are more allied to animals that to plants. In the classification 

 of more highly evolved organisms the ditihculty would be a 

 serious one, but bacteria are at a stage of development where 

 the distinction between a plant and an animal is not so well 

 defined as in higher organisms. They are in fact not sufiiciently 

 far removed from ultramicroscopic organisms that cannot be 

 assigned exclusively to either the plant or to the animal 

 kingdom. 



All those bacteria which contain sulphur inclusions are 

 grouped under the name sulphur bacteria. These organisms 

 are morphologically very varied but physiologically similar. 

 Physiological similarity does not, however, necessarily imply 

 genetic affinity, and the systematist is therefore confronted 

 with a difficulty. Fie must effect the grouping of a large 

 number of species on the basis of a common physiological 

 attribute, when some of the constituent units can be assigned 

 to the plant kingdom, whilst others readily find a place in 

 the animal kingdom. A grouping on physiological lines is 

 unsatisfactory, for, with increasing knowledge, it is becoming 

 more evident that the mode of metabolism of an individual 

 bacterium is not fixed and unalterable. As a class bacteria 



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