2 ] The Classification of Lower Organisms 



turbed? One proposes changes in order to express what one supposes to be improved 

 knowledge of the kinds of organisms which belong together as facts of nature. If here 

 I place bacteria in a different kingdom from plants, and Infusoria in a different king- 

 dom from animals, it is because I believe that everyone will have a better understand- 

 ing of each of these four groups if he does not think of any two of them as belonging 

 to the same kingdom. 



The course of evolution believed to have produced those features of the natural 

 system to which the present work gives taxonomic expression is next to be described. 



Life originated on this earth, by natural processes, under conditions other than 

 those of the present, once only. These are the opinions of Oparin ( 1938) 1, and appear 

 sound, although some of the details which he suggested may not be. When the crust 

 of the earth first became cool, it was covered by an atmosphere of ammonia, water 

 vapor, and methane, and by an ocean containing the gases in the atmosphere above 

 it and minerals dissolved from the crust. This is to state the hypotheses that organic 

 matter in the form of methane is older than life; and that whereas conditions on the 

 face of the earth tend now to cause oxidation, they tended originally to cause reduc- 

 tion. In a medium of the nature of the supposed primitive ocean, spontanous chemical 

 changes will occur and produce organic compounds of considerable complexity: this 

 has repeatedly been demonstrated by experiment. To convert a solution of ammonia, 

 methane, and minerals into protoplasm, Oparin postulates a very long series of 

 changes, producing successively more complicated compounds and mixtures, and re- 

 quiring perhaps hundreds of millions of years. The changes are conceived as acci- 

 dents; they are supposed to have been probable accidents, like throwing a seven at dice, 

 not events which could only very rarely occur by accident, like throwing twenty sevens 

 in succession. By supposing that some of these processes used up the m.aterials neces- 

 sary for them, Oparin provides an explanation of the single origin of life: we are 

 confident that all life is of one origin, because all protoplasm is of the same general 

 nature, and all life consists of essentially the same processes. The course of events 

 described would have yielded, as the original form of life, anaerobic saprophytes; this 

 is in harmony with the fact that anaerobic energesis is in a sense the basic metabolic 

 process. The original organisms would scarcely have possessed nuclei: Oparin's 

 theories indicate, as the most primitive form of life which has been able to survive, 

 the anaerobic bacteria. The anaerobic bacteria are indeed very far removed from any 

 lifeless things; their protoplasm and their metabolism are fundamentally the same 

 as ours. 



Life requires energy. Under anaerobic conditions, an organism can obtain energy 

 by converting sugars to alcohol, but it can not use alcohol as a source of energy. This 

 example means that anaerobic energesis yields energy in strictly limited quantity and 

 produces incompletely oxidized compounds. So long as all life was anaerobic, it was 

 engaged in converting the organic matter upon which it depended into forms which 

 it could not use; life under these conditions, at least if they persisted for any great 

 period of time, was surely very sluggish. A further scries of changes in the metabolic 

 system, occurring accidentally in certain organisms and preserved by natural selec- 

 tion, brought photosynthesis into existence. The purple bacteria are believed to rep- 

 resent stages in the evolution of photosynthesis, which exists in its fully developed 

 form, involving the release of elemental oxygen, in the blue-green algae. Once photo- 



^ Dates in parentheses are references to works which have been consulted and listed in 

 the bibliography. 



