THE CLASSIFICATION OF BACTERIA 11 



various mineral salts, especially those of sodium, potassium, and 

 magnesium. The amount of different constituents varies ac- 

 cording to the age of the culture and the medium used for 

 growth, and certainly great variation takes place in the com- 

 position of different species. 



The Classification of Bacteria. There have been numerous 

 schemes set forth for the classification of bacteria, the funda- 

 mental principle running through all of which has been the 

 recognition of the two sub-groups and the type forms mentioned 

 in the opening paragraph above. In the attempts to still 

 further subdivide the group, scarcely two systematists are agreed 

 as to the characters on which sub-classes are to be based. Our 

 present knowledge of the essential morphology and relations of 

 bacteria is as yet too limited for a really natural classification to 

 be attempted. To prepare for the elaboration of the latter, 

 Marshall Ward suggested that in every species there should be 

 studied the habitat, best food supply, condition as to gaseous 

 environment, range of growth, temperature, morphology, life 

 history, special properties and pathogenicity. 



We must thus be content with a provisional and incomplete 

 classification. We have said that the division into lower and 

 higher bacteria is recognised by all, though, as in every other 

 classification, there occur transitional forms. In subdividing 

 the bacteria further, the forms they assume constitute at present 

 the only practicable basis of classification. The lower bacteria 

 thus naturally fall into the three groups mentioned, the cocci, 

 bacilli, and spirilla, though the higher are more difficult to deal 

 with. Subsidiary, though important, points in still further sub- 

 division are the planes in which fission takes place and the 

 presence or absence of spores. The recognition of actual species 

 is often a matter of great difficulty. The points to be observed 

 in this will be discussed later (p. 115). 



I. The Lower Bacteria. 1 These, as we have seen, are 

 minute unicellular masses of protoplasm surrounded by an 

 envelope, the total vital capacities of a species being represented 

 in every cell. They present three distinct type forms, the 

 coccus, the bacillus, and the spirillum ; endogenous sporulation 

 may occur. They may also be motile. 



1. The Cocci. In this group the cells range in different 

 species from '5 //, to 2 //, in diameter, but most measure about 1 p. 

 Before division they may increase in size in all directions. The 

 species are usually classified according to the method of division. 



1 For the illustration of this aud the succeeding systematic paragraphs, 

 vide Fig. 1. 



