No. 615] 



BACTERIAL PIIYLOGEXY 



A study of the organisms which at the present time are 

 known to secure energy by the oxidation of 1LS, I'll ,. IF.,, 

 CO and NIL show that they possess certain character- 

 istics in common. For the most part the organisms are 

 cocci or rods, occasionally spiral, in some cases motile, 

 and then always with polar flagella. While there are 

 some exceptions to the rule, the organisms for the most 

 part do not thrive in a medium containing much organic 

 matter. It is not improbable that the primitive organism 

 had characters not greatly unlike these enumerated. 

 Just what type of oxidation is most primitive it is diffi- 

 cult if not impossible to determine, although certain con- 

 jectures may not be out of place. Probably one of the 

 most common of the easily oxidized substances of the 

 primitive earth was hydrogen sulfid. It undoubtedly 

 was a common constituent of thermal springs. The 

 modern representatives of the groups which thrive in 

 water containing hydrogen sulfid are abundant both in 

 numbers and in species. By means of the energy which 

 they secure from the oxidation of H 2 S and S they probably 

 take up CO.. and transform it into food and protoplasm. 

 Apparently all of the forms which have been investigated 

 are motionless cocci or rods or spirals motile by means 

 of polar flagella. No modern form is known which pro- 

 duces spores. Many of the species contain a pigment 

 bacteriopurpurin and swim or grow toward light, show- 

 ing positive chemotaxis or chemotropism. We may find 

 every gradation between the modern representatives of 

 these forms and the blue-green alga, on the one hand, and 

 the true bacteria, on the other. Many of the blue-green 

 alga? contain a purple coloring material in addition to the 

 blue and green pigments. From the standpoint of evolu- 

 tionary requirements, therefore, it is evident that some 

 primitive organism having much the same type of metab- 

 olism as the modern sulphur bacteria would be a satis- 

 factory starting form. 



Before additional stress is laid upon a sulfur bac- 

 terium as a possible progenitor of modern forms, we 



