ORIGIN AND EVOLUTION OF LIFE 295
(d) the catalytic or enzyme action, both within and without the
organism. Thus the chemical composition of bacteria is analogous to that of the higher plant and animal cells, but no chlorophyll and no cellulose is found.
Bacterial suspensions manifest the characteristics of colloidal suspen- sions, namely, of fluids containing minute gelatinous particles which are kept in motion by molecular movement: these colloidal substances have the food value of protein and form the primary food supply of many Protozoa, the most elementary forms of animal life. Enzymes of three kinds exist, proteolytic, oxidizing and synthetic. ^^ The pro- teolytic enzymes are similar to the tryptic enzymes of animals, being able to digest only the proteoses and amino acids but not the complex proteins. Powerful oxidizing enz}Tnes are present but their character is not known. Synthetic enzymes must also exist though as yet there is no positiye information concerning them. Like other forms of life, bacteria need oxygen for combustion 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, how- ever, a transitional group of bacteria, known as the facultative anaerobes^ which can use either free, or combined oxygen, thus forming a link to all the higher forms of Uf e in which free oxygen is an absolute essential. There is a group of the higher spore-forming bacteria which must have free oxygen. These constitute probably the last stage in bacterial evo- lution and form the link to the higher forms.
Armed with these physico-chemical powers, which may have been acquired one by one, the primordial bacteria mimic the 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 produced in the environment. For ex- ample, the iron bacteria discovered by Ehrenberg in 1838 obtain their energy from the oxidation of iron compounds, tlie insoluble oxide re- maining stored in the cell and accumulating into iron as the bacteria die.** In general the beds of iron ore found in the pre-Cambrian strati-
12 I. J. Kligler.
13 It is claimed that iron bacteria play an important part in the formation of numerous small deposits of bog-iron ore and it seems possible that their activities may be responsible for extensive sedimentary deposits as well. Fur- ther, the fact of finding iron bacteria in underground mines opens the possi- bility that certain underground deposits of iron ore may have been formed by them. Harder, E. C, 1915, p. 311.
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