VARIATIONS IN THE SACCHAROMYCETES. 287 



top-fermentation yeasts yielded only a small number of cells 

 which displayed bottom-fermentation phenomena. We are 

 not dealing, therefore, as in the previous cases (asporogenesis) 

 with the action of definite factors producing a transformation, 

 but with unknown causes, and probably with sudden variations 

 of the same kind as the mutations studied by H. de Vries. 

 According to these researches the two physiological forms, 

 top- and bottom-fermentation yeasts, are not independent. 

 On the contrary, they may both occur in a growth derived 

 from an individual cell. They can exist together in the same 

 liquid, one or other securing the upper hand in their compe- 

 tition and thus determining the character of the growth. 



Since 1887 the author, who has long enjoyed the co-opera- 

 tion of his laboratory superintendent, H. Rafn, has treated as 

 one of his principal problems the study of the variations of 

 yeasts during their application in the different branches of the 

 fermentation industry. The number of his investigations has 

 now increased to many thousands. The difficulty in work 

 of this character, where large masses of yeast are under in- 

 vestigation, is to make sure that the growths observed by 

 the separation of a certain number of cells with abnormal 

 characteristics are real varieties of the parent cells, and have 

 not been derived from infection by foreign species. 



The botanical and biological investigation can never form 

 more than part of the examination, and must, moreover, be 

 carried out with the utmost care. We must take refuge to a 

 great extent in the different characteristics that are developed, 

 partly during large scale fermentations and partly during 

 parallel fermentations carried out in the laboratory with small 

 quantities. For such experiments it is obvious that only yeast 

 masses can be used which have been derived from a single cell. 



As a result of observations carried out during a series of 

 years, it has been definitely established that variations do 

 very frequently take place. They occur without any obvious 

 cause, and on occasion they may develop in such quantities 

 that the whole mass of yeast changes its character or " de- 

 generates." This expression, which is used in practice, only 

 indicates that the yeast mass in the special brewery or distillery 

 concerned no longer suffices for the particular requirements. 



