6 BACTERIA 



such plants can absorb them. Animals, on the other hand, 

 are only able to utilise compound food products which have 

 been, so to speak, prepared for them ; for example, albu- 

 minoids and proteids. They cannot directly feed upon the 

 elementary substances forming the diet of vegetables. This 

 distinction, however, did not at once clear up the difficult 

 matter of the classification of bacteria. It is true, they 

 possess motion, are free from chlorophyll, and even feed 

 occasionally upon products of decomposition — three physio- 

 logical characters which 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 un- 

 mistakably proclaim themselves to be of the vegetable 

 kingdom. In 1853 Cohn arrived at a conclusion to this 

 effect, and since that date they have become more and more 

 limited in classification and restricted in definition. 



Even yet, however, we are far from a scientific classifi- 

 cation for 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 subtilis, or 

 Bacterium termo, or Bacillus coli^ are now representative, 

 not of individuals, but of families and groups of species. 

 Again, isolated characteristics 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 micro- 

 scopic 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 



