No. 615] 



BACTERIAL PII Y LOG EX Y 



211 



means of polar flagella. It is not at all improbable that 

 ammonia may have been abundant enough on the primi- 

 tive earth to have constituted an adequate energy source 

 for the primitive bacteria. 



Which of these modern types most closely resemble- 

 the primitive organism living on autotrophic existence? 

 It is perhaps impossible to say. The modern representa- 

 tives of the nitrifiers and the methane and carbon 

 monoxid oxidizer- are apparently rather isolated group- 

 without numerous species and apparently not closely 

 related to other forms. The sulfur oxidizers, on the 

 other hand, are abundant, of many types, and show many 

 intergradations with other bacteria and the blue-green 

 algae. Possibly a somewhat better case can be made out 

 for them. However, it should be noted that all of these 

 forms have certain characters in common, they are all 

 autotrophic, all are aerobic, and when motile are elongate 

 cells with polar flagella. It is perhaps a fair inference 

 that the aerobiosis and the polar flagellation are primitive 

 characters. We may well conclude with Jensen that all 

 of these organisms discussed are related and may be 



placed in a single group. Expressed in terms of modern 

 representatives of the primitive types, the following dia- 

 gram might express the idea. 



We may next concern ourselves with possible and 

 probable relationship of these various forms to other 

 members of the Eubacteriales, disregarding the Thio- 

 bacteriales. The remainder of the Eubacteriales differ 

 from the autotrophic forms thus far discussed in that in 



