EVOLUTION OF BACTERIA 89 



ment: these colloidal substances have the food-value of protein 

 and form the primary food of many Protozoa, the most ele- 

 mentary forms of animal life. Chemical messengers in the 

 form of enzymes of three kinds exist, proteolytic, oxidizing, and 

 synthetic.^ The proteolytic enzymes are similar to the tryptic 

 enzymes of animals, being able to digest only the proteoses 

 and simple proteins (casein, albumin) but not the complex 

 proteins. Powerful oxidizing enzymes are present, but their 

 character is not known. Synthetic enzymes, bringing together 

 new living chemical compounds^ must also exist, though as yet 

 there is no positive information concerning them. 



Armed with these physicochemical powers, which may 

 have been acquired one by one, the primordial bacteria begin 

 to mimic the subsec|uent evolution of the higher plant and 

 animal world by an adaptive radiation into groups which 

 respectively seek new sources of energy, either directly from 

 the inorganic world or parasitically from the developing organic 

 bacterial and plant foods in protein and carbohydrate form, 

 the different groups living together in large communities and 

 interacting chemically upon one another by the changes pro- 

 duced in their environment. 



The parasitic life of bacteria, beginning with their symbiotic 

 relations with other bacteria, was extended into intimate rela- 

 tions with the plants and finally with the entire living world. 



Like other forms of life, bacteria need oxygen for combus- 

 tion in their intracellular actions and reactions; but free oxygen 

 is not only unnecessary but actually toxic to the anaerobic 

 bacteria, discovered by Pasteur in 1861, which derive their 

 oxygen from inorganic and organic compounds. There is, 

 however, a transitional group of bacteria, known as the faculta- 

 tive anaerobes, which can use either free or combined oxygen, 



1 1. J. Kligler. 



