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cultivation of their vigour as ferments, cause Von Nageli's 

 hypothesis to have an inherent probabiHty. Not that the 

 idea that there is a struggle for existence between rival 

 microbes, enunciated by Pasteur in his "Etudes sur la Biere," 

 assumed by Klein to be the explanation of Buclmer's 

 supposed evolution of the anthrax bacillus from the hay 

 bacillus (both forms being supposed to have inadvertently 

 obtained admission to Buchner's culture solutions), and 

 referred to by Miquel as the explanation of the varying 

 proportions of germs in his cultures, may not also explain 

 the occurrence of epidemics. If we assume the absolute 

 inconvertibility of all species of microbes, epidemics might 

 still be explained as the consequence of the occurrence of 

 temporary conditions more favourable to the development 

 of noxious than of harmless species. But this would really 

 force us logically to the conclusion that all zymotic 

 diseases have specifically existed as long as life itself. 

 On the other hand we must be careful to distinguish be- 

 tween variability of species and what Pasteur means by 

 variability of vigour. Pasteur's idea is perhaps most 

 clearly conveyed by the use of the terms young and old 

 germs, or we may perhaps better grasp the idea by using 

 the terras tame and savage germs. Thus, let us take the 

 remarkable observation of Pasteur concerning the difi*erence 

 in the proportion of fermentation accomplished in brewing, 

 to weight of organism, in the presence or absence of free 

 oxygen. In shallow vessels in which the ferment easily 

 obtains free oxygen, 1 kilogramme of ferment will correspond 

 to 5 or 6 kilogrammes only of decomposed sugar; while in 

 deep vats, in which the free oxygen is speedily exhausted, 

 and the access of fresh supplies is prevented by the layer 

 of carbonic acid formed above the vats, a kilogramme 

 of ferment corresponds to 70, 80, 100, and even 150 

 kilogrammes of sugar decomposed. It is as though the 

 ferment in the presence of free oxygen lived the quiet and 



