THE C L ASS 1 PICA TION OF BA C TERIA. 1 5 



has taken place, according to the age of the culture, and 

 also according to the medium used. Besides nitrogenous 

 material, salts of sodium, potassium, magnesium, and phos- 

 phorus may be present in the bacterial protoplasm. In 

 certain cases traces of cellulose or of substances resembling 

 cellulose, and also fatty bodies have been isolated. It is 

 probable that the composition of bacteria varies somewhat 

 in different species. 



The Classification of Bacteria. There have been 

 numerous schemes set forth for the classification of bac- 

 teria, 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 classification to be 

 attempted. To prepare for the elaboration of the latter, 

 Marshall Ward suggests that in every species there should 

 be studied the habitat, best food supply, condition as 

 to gaseous environment, range of growth, temperature, 

 morphology, and life history, special properties and patho- 

 genicity. 



We must thus be content with a provisional and in- 

 complete 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 classifica- 

 tion. 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 recognition of actual species is often a matter 

 of great difficulty. The points to be observed in this will 

 be discussed later (p. 125). 



