THE MEANING OF EVOLUTION 267 



and in the cavities of the animal body — the mouth, 

 ahmentary canal, nose, and vagina. Proteids are 

 decomposed by them into simple chemical compounds 

 such as amido-acids, and then these substances, along 

 with carbohydrates, are fermented so as ultimately to 

 form water, carbonic acid, and salts of nitric acid. 

 These bacteria obtain their energy from the conversion 

 of chemical compounds of high potential energy into 

 compounds of low potential energy. Prototrophic bac- 

 teria are never parasites, nor do they live in the cavities 

 of the bodies of animals : they always live in the open. 

 They carry on still further the action of the putrefactive 

 bacteria by converting ammonia into nitrous acid, and 

 nitrous acid into nitric acid. Others reverse this series 

 of changes by reducing nitric acid to nitrous acid, 

 nitrous acid to ammonia, and ammonia to free nitrogen. 

 Others again oxidise sulphuretted hydrogen to sulphuric 

 acid, others ferrous hj^drate to ferric hydrate, while it 

 has recently been shown that some bacteria are ap- 

 parently able to oxidise the carbon of coal to carbonic 

 acid. Some are able to oxidise the free nitrogen of the 

 atmosphere into nitrous and nitric acids. How pre- 

 cisely the energy necessary for these transformations is 

 obtained is not at all clearly understood, and it may be 

 possible that some of the prototrophic bacteria obtain 

 their energy by making use of the un-co-ordinated kinetic 

 energy of the medium in which they live. From our 

 point of view the net result of the activity of the pre- 

 dominant species of bacteria which inhabit the earth 

 is that they reverse the processes which are the mani- 

 festations of the metabolism of plants and animals. 

 The result of the metabolism of plants is the accumula- 

 tion of stores of high potential compounds such as 

 carbohydrates, and the depletion of the terrestrial 

 stores of carbon dioxide and other materials necessary 



