12 GENERAL MORPHOLOGY AND BIOLOGY. 



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, tem- 

 perature, morphology, and 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, 

 bacilH, 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. 112). 



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

 minute unicellular masses of protoplasm surrounded by an 

 envelope, the total vital capacities of a species being repre- 

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

 the coccus, the bacillus, and the spirillum ; endogenous sporula- 

 tion may occur. They may also be motile. 



I. The Cocci. — In this group, the cells range in different 

 species from . 5 /a to 2 m in diameter, but most measure about i /i. 

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

 species are usually classified according to the method of division. 

 If the cells divide only in one axis, and through the consistency 

 of their envelopes remain attached, then a chain of cocci will be 

 formed. A species in which this occurs is known as a strepto- 

 coccus. If division takes place irregularly the resultant mass 



^ For the illustration of this and the succeeding systematic paragraphs, vide 

 Fig. I. 



