12 GENERAL MORPHOLOGY AND BIOLOGY 



The Classification of Bacteria. In what we have to say 

 under this heading we shall chiefly confine ourselves to the 

 characters of the pathogenic bacteria. There have been 

 numerous schemes set forth for the classification of bacteria, the 

 fundamental 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 classifica- 

 tion 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. Some recent 

 attempts to carry out such a plan will be referred to in con- 

 nection with the principles of general bacteriological diagnosis 

 (p. 140). 



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, transitional forms have to be accounted for. In 

 subdividing the bacteria further, the forms they assume con- 

 stitute 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 subdivision are the planes in which fission 

 takes place and the presence or absence of spores. The recogni- 

 tion of actual species is often a matter of great difficulty. The 

 points to be observed in this will be discussed later (p. 137). 



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 /t to 2 /x, in diameter, but most measure about 1 /x. 

 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 and the succeeding systematic paragraphs, 

 vide Fig. 1. 



