90 THE ORIGIN AND EVOLUTION OF LIFE 



thus forming a link to all the higher forms of life 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 a late stage in bacterial evolution and 

 form the link to the higher forms. 



The iron bacteria discovered by Ehrenberg in 1838 obtain 

 their energy from the oxidation of iron compounds, the insolu- 

 ble oxide remaining stored in the cell and accumulating into 

 iron as the bacteria die.^ In general the beds of iron ore found 

 in certain of the pre-Cambrian stratified rocks, which have an 

 estimated age of 60,000,000 years, are believed to be of bac- 

 terial origin. Sulphur bacteria similarly obtain their energy 

 from the oxidation of hydrogen sulphide. 



Bacteria in the Balance of Life 



Bacteria thus anticipate the plant world of alg£e, diatoms, 

 and carbon-formers, as well as the animal world of Protozoa 

 and Mollusca, by playing an important role in the formation 

 of the new crust of the earth. This is observed in the primor- 

 dial limestone depositions composed of calcium carbonate 

 formed by bacterial action on the various soluble salts of cal- 

 cium present in solution in sea-water, a process exemplified 

 to-day- in the Great Bahama Banks, where chalk mud is now 

 precipitated through accumulation by B. calcis. Doubtless in 

 the shallow continental seas of the primal earth such bacteria 

 swarmed, as in the shallow coastal seas of to-day, having both 

 the power of secreting and precipitating Hme and, at the same 



' 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 respon- 

 sible for extensive sedimentary deposits as well. Further, the fact of finding iron bac- 

 teria in underground mines opens the possibility that certain underground deposits of 

 iron ore may have been formed by them. — Harder, E. C, 1915, p. 311. 



- Drew, George H., 1914, p. 44. 



