Liebig felt to the full the weight of Pasteur's criticisms; his reply [p013] was long delayed [1870], and, according to his biographer, Volhard [1909], caused him much anxiety. In it he admits the vegetable nature of yeast, but does not regard Pasteur's conclusion as in any way a solution of the problem of the nature of alcoholic fermentation. Pasteur's "physiological act" is for Liebig the very phenomenon which requires explanation, and which he still maintains can be explained by his original theory of communicated instability. On some of Pasteur's results, notably the very important one of the cultivation of yeast in a synthetic medium, he casts grave doubt, whilst he explains the production of glycerol and succinic acid as due to independent reactions. The phenomenon of fermentation is still for him one which accompanies the decomposition of the constituents of the cell, rather than their building up by vegetative growth. "When the fungus ceases to grow, the bond which holds together the constituents of the cell contents is relaxed, and it is the motion which is thus set up in them which is the means by which the yeast cells are enabled to bring about a displacement or decomposition of the elements of sugar or other organic molecules." Pasteur replied in a brief and unanswerable note [1872]. All his attention was concentrated on the one question of the production of yeast in a synthetic medium, which he recognised as fundamental. The validity of this experiment he emphatically reaffirmed, and finally undertook, from materials supplied by Liebig himself, to produce as much yeast as could be reasonably desired. This challenge was never taken up, and this communication formed the last word of the controversy. Pasteur had at this time firmly established his thesis, /no fermentation without life/, both for alcoholic fermentation and for those other fermentations which are produced by bacteria, and had put upon a sound and permanent basis the conclusions drawn by Schulze, Cagniard-Latour, Schwann, and Kützing from their early experiments. It became generally recognised that putrefaction and other fermentative changes were due to specific organisms, which produced them in the exercise of their vital functions. Pasteur subsequently [1875] came to the conclusion that fermentation was the result of life without oxygen, the cells being able, in the absence of free oxygen, to avail themselves of the energy liberated by the decomposition of substances containing combined oxygen. This view, which did not involve any alteration of Pasteur's original thesis but was an attempt to explain the physiological origin and function of fermentation, gave rise to a prolonged controversy, which cannot be further discussed in these pages. [p014]