240 



THE AMERICAN NATURALIST 



[Vol. LII 



should examine carefully other possibilities. It is con- 

 ceivable (though scarcely probable) that hydrogen may 

 have constituted a larger percentage of the atmosphere 

 in times past than now. Several species of modern bac- 

 teria have been described which in the presence of hydro- 

 gen and oxygen may secure their growth energy by com- 

 bining these elements directly or indirectly. These 

 species are motile rods with polar nagella. These 

 modern members of the genus Hydrogenomonas, how- 

 ever, are very far from primitive because under ordinary 

 conditions they are pantotrophous growing well on ordi- 

 nary laboratory media. Thus far no organism strictly 

 prototrophic capable of utilizing hydrogen has been 

 found. This does not prove that such organism has not 

 existed, but throws the burden of proof upon any one 

 who would urge hydrogen oxidation as a primitive method 

 of securing growth energy. The results of Kaserer 

 (1906) seem to indicate that the organism catalytically 

 causes the transformation in the presence of hydrogen 

 of carbonic acid into formaldehyde, the cell then using the 

 formaldehyde as food. 



Methane and carbon monoxide are also oxidized by cer- 

 tain of our modern bacteria, the organisms securing their 

 growth energy in this manner. These organisms accord- 

 ing to the descriptions are autotrophic and do not thrive 

 in the presence of organic material. It is possible that 

 these represent primitive characters. The organisms are 

 rods, motile or non-motile, when motile with polar 

 nagella. If either methane or carbon monoxide were 

 common in the atmosphere of the early earth, forms of 

 this general type may have flourished. That these gases 

 were sufficintly abundant does not seem probable, but the 

 possibility must be admitted. 



Several types of modern bacteria are known which 

 oxidize ammonia to nitrites and nitrites to nitrates, utiliz- 

 ing the energy thus secured for chemosynthesis of food 

 from inorganic materials. At least one species of the 

 nitrifying bacteria is a coccus, others are rods, motile by 



