ORIGIN OF ANAEROBIOSIS 275 



quire no oxygen; this is well illustrated in the glycogen 

 cycle in muscular contraction. Clark summarized the re- 

 lationships of the two i^rocesses in these words: ''What 

 we find in general is the reducing tendency of the isolated 

 cell; what we find in particular are special mechanisms 

 for the use of molecular oxygen." It is then obviously 

 very tempting to assume that the anaerobic processes are 

 the primitive ones and that those connected with the 

 uptake of molecular oxygen are secondarily acquired. 

 If one does not do so, one would have to assume two fun- 

 damentally different kinds of life, an anaerobic and an 

 aerobic one. Snyder already pointed out "that the 

 adaptation of anaerobic organisms to an oxygen habit 

 is conceivable and may be understood, whilst in the light 

 of our present knowledge the reverse is not."^ 



To point 5: That the atmosphere of the primitive 

 earth was devoid of oxygen is a hypothesis for which 

 Snyder cites the authority of such eminent scientists as 

 Lord Kelvin (1899) and Arrhenius (1907). The chief 

 argument seems to be that in the earth's crust vast 

 amounts of oxidizable but actually unoxidized materials 

 occur which at one time or another must have been ex- 

 posed to the atmosphere; if oxygen had been present, 

 these substances should have been oxidized. It is then 

 assumed that all the oxygen of the present-day atmos- 

 phere has been liberated by the photosynthetic activity 

 of plants; Snyder himself is inclined to ascribe the main 

 role in this process to marine algae rather than to a ter- 

 restrial flora. 



1. Snyder has in mind a hypothetical aerobic organism which does 

 not possess intermediate anaerobic processes. His statement should 

 not be understood as intimating that an animal endowed with the in- 

 termediate reactions cannot become adapted to anaerobic life. Most 

 "anaerobic" invertebrates living today are probably derived from 

 oxygen-consuming ancestors. This is illustrated in the case of intes- 

 tinal worms, the derivation of which will be discussed in greater 

 detail in Chapter 4. 



