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dioxide but even from the deadly monoxide, 1 is shared by- 

 many absolute anaerobes, even though it is not by Bacterium 

 pasteurianum. Nostoc is apparently better able to propagate 

 rapidly under the atmospheric conditions which now prevail but 

 as we shall see it is at least extremely doubtful if these same 

 conditions obtained in the earlier epochs of the earth's history. 



The conclusion that the anaerobic nitrifiers constitute the 

 more primitive form is supported by the behaviour of these 

 and other low forms towards an oxygen supply. It is now 

 a well-established fact that many, if not all, of the incontestably 

 anaerobic bacteria may, under suitable conditions, be cultivated 

 and reproduce rapidly under ordinary atmospheric conditions. 2 

 On the other hand, some at least of the supposedly absolute 

 aerobes, as Rosenthal, Tarozzi and others have shown, if passed 

 through a series of cultures in which the supply of oxygen 

 is more and more attenuated, may at last be cultivated freely 

 under completely anaerobic conditions. 3 In the ancient phrase 

 they may be " adapted " to an anaerobic life. 



It has long been known that there are intermediate forms 

 which are indifferently oxygen users or abstainers, according 

 as this element is present or absent ; in other words, the oxygen 

 habit is apparently one which may be readily assumed and 

 readily put aside. On the other hand, there does not seem to 

 be any clear evidence that a corresponding nitrogen habit may 

 be acquired where it is non-existent. The ability to assimilate 

 uncombined nitrogen seems to be an extremely primitive faculty, 

 quickly put aside and finally lost as the complexity of the 

 organism increases. 



In the light of this conclusion and the fact that complete 

 and enduring anaerobiosis is to be found only among the more 

 primitive forms, it seems almost impossible to regard the 

 faculty of living without oxygen as an " acquirement." We 

 shall have decisive proof in the sequel that it is not. It is 

 the ability to use uncombined oxygen which is the "acquire- 

 ment " ; and we seem therefore driven to the conclusion that 

 the anaerobic was the more primitive and the earlier form. 



1 Koserer, Cent./. Bakt. II. Bd. 16, 1906. Further literature in O. Richter, 

 Die Badeutung der Reinkultur (Berlin, 1907). 



3 G. Tarozzi, Atti R. Accad. dei Fisiocratidi, Sienna, serie 4, v. 17, 1905 ; also 

 vol. 15, and 1906. 



3 G. Rosenthal, C. r. Soc. Biol. Paris, No. 25, 1906. 



