PRODUCED BY ALCOHOLIC FERMENTS. 157 



the employment of systematically selected species or races of 

 yeast. 



It was the result obtained on attacking the first of these 

 questions which caused me to include in these practical 

 studies also the other two questions. If, for instance, it had 

 been found that the alcoholic ferments do not cause disease, 

 there would have been no cogent reason for introducing true 

 pure cultures into practice, and in consequence also none for 

 the selection of a certain single species or race. Thus the 

 question of the diseases of beer and other fermented liquors 

 is one of great importance. Its solution was not arrived at 

 all at once, many investigators having worked at this problem 

 for a long time past. The investigations in this field were 

 closely connected with those relating to spontaneous genera- 

 tion, which, as is well known, led to the development of a new 

 experimental science, namely, the science of micro-organisms. 

 Amongst these, the doctrine relating to the diseases of fer- 

 mented liquids occupies a conspicuous place. An account of 

 the manner in which this doctrine gradually developed will 

 therefore be of interest both to the biologist and to the 

 practical zymotechnologist. This, however, will naturally 

 only be the case when the description is based upon a 

 thorough study from the original sources, and when it presents 

 to us in a clear manner the variable standpoints, not detached, 

 but in their consecutive order. In the following part of this 

 chapter I have attempted this. It is the first time that the 

 history of this question has been written. 



2. How THE DOCTRINE OF "DISEASES" IN FERMENTED 

 LIQUIDS WAS GRADUALLY DEVELOPED. 



It may here be pointed out that in speaking of " diseases," 

 this word is intended to apply to those objectionable changes 

 which fermented liquids, especially beer and wine, are liable 

 to undergo in consequence of the attack of micro-organisms. 



