86 THE ORIGIN AND EVOLUTION OF LIFE 



bearing algal plant from the Newland limestone of the Algon- 

 kian of Montana, the age of which is estimated to be about 

 33,000,000 years. They point to a very long antecedent stage 

 of bacterial evolution. In this section (Fig. 11, A), at the 

 points indicated by the arrows, there is a little chain of cells 

 closely similar to those in the existing species of Azotohader, an 

 organism that fixes atmospheric nitrogen and converts it into 

 a form utilizable by the plant. The Algonkian form is related 

 to the other nitrifiers, Nitroso coccus, Nitroso monas, and to 

 Nitrohacter which lives on simple salts with carbon dioxide 

 (CO2) as a source of carbon. 



The gradual evolution of a cellular structure in these organ- 

 isms can be partly traced despite their excessively minute size. 

 The cell structure of the Algonkian and of the recent Nitroso 

 coccus bacteria (Fig. 11, A, B) is very primitive and uniform in 

 appearance, the protoplasm being naked or unprotected; this 

 primitive structure is also seen in C, another type of nitrogen- 

 fixer of the soil, which is chemically more complex because it 

 can obtain its nitrogen either from the inorganic nitrogen 

 compounds or from the organic nitrogen compounds (amino- 

 acids), which are fatal to the Nitroso monas and the Nitro- 

 hacter forms. The arrow points to a group of cells similar in 

 appearance to those in B. A higher stage of granular structure 

 appears in D, a nitrogen-fixer from the root nodules of legumes, 

 which like B and C lives on inorganic chemical compounds, 

 but draws upon the atmosphere for nitrogen and upon sugar 

 for its carbon; we observe an uneven granular structure in this 

 cell. This may be an illustration of an early type of parasitic 

 adaptation. The next type of bacterium {E) is a denitrifer, 

 which derives its oxygen from the nitrates, reducing them to 

 nitrites and free nitrogen and ammonia. A further stage of 

 structural and chemical evolution is seen (F) in four elongated 



