38 . . BIOLOGY AND TECHNIQUE 



Genus V. Thiothrix. Unbranched, non-motile threads, inclosed in fine 

 sheaths. Division of cells in one direction only. Cells contain sulphur 

 granules. 



FAMILY V. BEGGIATOACE.E. Cells united in sheathless threads. Division 

 in one direction of space only. Motility by undulating membrane as in 

 Oscillaria. 

 Genus Beggiatoa. Cells with sulphur granules. 



It will be seen in reviewing the classification just given that the sub- 

 divisions are based upon questions of form, motility, and situation of 

 flagella. While these characteristics, so far as we know, are constant, 

 there are, nevertheless, many instances in which types entirely similar 

 in these respects must be differentiated. This can be done only by care- 

 ful study of staining reactions, finer structure, cultural characteristics, 

 and biological activities. 



As a matter of fact, while the botanical classification of the bacteria 

 offers great difficulties, identification is not so complicated a task as this 

 would indicate. Identification, once roughly made on a morphological 

 basis, is further carried on by the aid of cultural characteristics, by bio- 

 chemical reactions and by pathogenic properties. The bacteria occupy 

 so important a place in agriculture, in medicine, and in hygiene, that it 

 rarely becomes necessary for a worker in any particular field to survey 

 the entire group. The habitat of a large number of species is so well 

 known that this consideration alone often gives a clew to actual identi- 

 fication. 



Bacterial Mutation. The earlier views of bacteriologists concerning 

 mutation differed greatly, Naegeli holding that extensive mutation was 

 probably the rule; Cohn, on the other hand, holding strictly to the 

 constancy of form and species. The accumulated experience of many 

 bacteriologists during the years since then seems to point almost entirely 

 in the direction indicated by Cohn, and, in fact, most of our methods of 

 classification are based upon the assumption of such constancy. 



Form alone, of course, cannot be relied upon for classification among 

 organisms so simply constructed that the possibilities of variation in 

 form are very limited. In classifying bacteria, therefore, we are forced 

 to take cognizance not only of morphology, but also of staining character- 

 istics, behavior on differential media, fermentation reactions, patho- 

 genicity, - and, as a final appeal, reactions with specific immune sera. 

 The last especially, as utilized in agglutination and complement-fixation, 



