ORIGIN OF AXAEROBIOSIS 277 



Whatever the origin of the bulk of the atmospheric 

 oxygen may have been, true animal life must have been 

 preceded by plant life since plants represent the ulti- 

 mate source of animal food. One can safely assume, 

 therefore, that the primitive animals had access to, at 

 least, some oxygen. Whether they were originally able 

 to utilize it, we obviously cannot say. 



From the point of view of economy of food — suppos- 

 ing that nature considers economy — it may be pointed 

 out that anaerobic processes involve more waste, the 

 more so since the reactions possible to animal proto- 

 plasm depend on organic materials as mother-substances. 

 Oxidative processes using molecular oxygen are much 

 more economical; i.e., with the same amount of food-sub- 

 stance available, a much richer aerobic than anaerobic 

 fauna can be supported. Could it not be, then, that the 

 type of life that we call animal life became possible only 

 after the development of aerobic processes! 



The fact that most animals which possess pronounced 

 anaerobic functions are nevertheless capable of utilizing 

 oxygen when they have access to it, suggests that they 

 are only invaders of anaerobic habitats, that they have 

 become adapted to them. The sapropelic ciliates may 

 represent an exception ; though, as explained previously, 

 we do not yet know whether or not they are capable of 

 utilizing molecular oxygen of low tension. In whatever 

 way future investigation may decide this question, it 

 should not be overlooked that a function once possessed 

 by an organism may be lost again. If it should be dem- 

 onstrated that these ciliates are incapable of utilizing 

 any oxygen, one would not be justified in assuming that 

 this is necessarily a primitive condition. One might as 

 well argue that the necessary enzyme mechanism was 

 '^ discarded" once they became established in their pe- 

 culiar habitat. One could compare such a hypothetical 



