OBJECTIONS TO THE GERM THEORY. 87 



foreign cause, a ferment, is necessary, not only for the 

 commencement, but also for the continuance of the 

 movement. 



Liebig's view was, however, evidently a purely hypo- 

 thetical one ; the decomposing proteid substance which 

 was supposed to be the cause of the fermentation was by 

 no means demonstrated to be really present ; the only 

 experimental support of this supposition was the fact 

 that in the so-called self-fermentation of yeast which 

 takes place without the addition of sugar, and entirely 

 at the cost of the yeast substance, much more alcohol is 

 formed than could be derived from the amount of 

 cellulose in the yeast cells, and that thus some other 

 complex substance contained in the cells must furnish the 

 materials for the formation of the alcohol. This analy- 

 tical proof was, however, shown to be erroneous by Nageli 

 (Theorie der Gcihrung, p. 3), but at a much earlier period 

 Liebig had been compelled to modify his theory con- 

 siderably by the numerous experiments which absolutely 

 proved the direct dependence of the fermentative process 

 on the life of the yeast cells. 



In 1870 he declared that the living yeast cells con- Later 

 tained and produced the supposed ferment-like substance, S 

 and that therefore the formation of the ferment depends view 

 on the life of the cell ; the fermentative act, however, de- 

 pends on an unorganised ferment, and the yeast cells in 

 producing the ferment do nothing more than numerous 

 other cells do. Just as man produces diastatic ferment, 

 pepsin and trypsin, so all other animals and plants have 

 their ferments ; but the organisms are not identical with 

 these ferments, and the fermentative action cannot be 

 looked on as the direct work of the cells. If it were 

 possible to separate the ferments from the cells the 

 latter would then be no longer necessary for the com- 

 mencement and continuance of the fermentative process. 

 Similar views were taught by Traube in 1858 ; and in 

 1876 they were defended by Hoppe-Seyler. They rested 

 in part on the analogy between the fermentative and putre- 

 factive process, and the splitting up and decomposition 

 caused by unorganised ferments. The micro-organisms 



