CLASSIFICATION OF BACTERIA 5 



would ally them to the animal kingdom. Yet by their structure and 

 capsule of cellulose and by their life-history and mode of growth 

 they unmistakably proclaim themselves to be of the vegetable 

 kingdom. In 1853 Cohn arrived at a conclusion to this effect, and 

 since that date bacteria have become more and more limited in clas- 

 sification and restricted in definition. 



Even yet, however, we are far from a scientific classification of 

 bacteria. Nor is this matter for surprise. The development, in this 

 branch of biology has been so rapid that it has been impossible to 

 assimilate the facts collected. The facts themselves by their 

 remarkable variety have not aided classification. Names which a few 

 years ago were applied to individual species, like Bacillus suhtilis, or 

 Bacterium termo, or Bacillus coli, are now representative, not of 

 individuals, but of famflies and species. Again, isolated character- 

 istics of certain microbes such as motility, power of liquefying 

 gelatine, size, colour, and so forth, which at first sight might appear 

 as likely to form a basis for classification, are found to vary not only 

 between similar germs, but in the same germ. Different physical 

 conditions have so powerful an influence upon these microscopic cells 

 that their individual characters are constantly undergoing change. 

 For example, bacteria in old cultures assume a different size, and 

 often a different shape, from younger members of precisely the same 

 species; Bacillus pyocyaneus produces a green to olive colour on 

 gelatine, but a brown colour on potato ; the bacillus of Tetanus is 

 virulently pathogenic, and yet may not act thus unless in com- 

 pany with certain other micro-organisms. Hence it will at once 

 appear to the student of bacteriology that, though there is great 

 need for classification amongst the six or seven hundred named 

 "species" of microbes, our present knowledge of their life-history 

 is not yet advanced enough to form more than a provisional 

 arrangement. 



We know that bacteria are allied to Hyphomycetes on the one 

 hand and Saccharomycetes on the other, and that they have no 

 differentiation into root, stem, or leaf ; we know that they are fungi 

 (having no chlorophyll), in which no sexual reproduction occurs, and 

 that their mode of multiplication is by division. From such facts as 

 these we may build up a classification as follows : — 



[Vegetable Kingdom. 



