142 BACTERIA 



species feed directly on the proteins of living animals 

 or plants, some on various solid or liquid organic sub- 

 stances in the living body or outside it. Some can 

 get their nitrogen from comparatively simple organic 

 salts like ammonium tartrate, just as yeast can ; 

 others get it from even simpler salts ; others, again, can 

 use the free nitrogen of the air. A few can form 

 their protoplasm from inorganic substances alone, as 

 green plants can, though they do it in quite a different 

 way, using the energy liberated in the oxidation process 

 instead of the energy of sunlight. The great majority, 

 however, feed on more or less complex organic sub- 

 . stances, from which they obtain energy for growth and 

 multiplication. 



The actual form in which the food is absorbed by 

 the bacterial cell must of course be liquid or gaseous 

 — ^in the great majority of cases it is liquid. That is 

 why we class bacteria as plants, and, so far as their 

 nutrition is concerned, with the great group of colour- 

 less plants known as the fungi. But both in structure 

 and afifinities (by which we mean descent) the bacteria 

 are very different from and most probably have no 

 connexion with the fungi proper. Bacteria are most 

 probably more or less directly derived from the most 

 primitive forms of life, of which we have no direct 

 knowledge. We must, however, conceive of primitive 

 organisms as consisting of very minute forms living in 

 water, and obtaining their energy in different ways — 

 from light, from chemical energy such as that liberated 

 in the oxidation process, or from ingesting other minute 

 organisms. From this undifferentiated group the green 

 plants developed along one line ; the animals, with their 

 habit of feeding on other organisms, along another ; 

 while the bacteria as we know them to-day are a highly 



