WHAT IS THE PURE YEAST OF PASTEUR? 133 



tion of pure cultures. If this were so, I should have to 

 acknowledge having committed a great error, and having 

 wasted my time with useless investigations. 



Against Duclaux's reasoning, Miquel (' Annales de 

 Micrographie,' 1889, p. 140), Alfred Jorgensen (' Botan. 

 Centralbl.,' xl. Bd., 1889, p. 316), and Denamur ('La Gazette 

 du Brasseur,' 1889, p. 887) immediately raised the same 

 weighty objection, namely, that it is absolutely impossible, 

 after so long a period, to ascertain with certainty what growths 

 had been introduced into the flasks by Pasteur. Flasks 

 which contained only one species when examined may very 

 well have contained several species originally ; in short, we are 

 not able to discover those species which may have perished 

 in the course of years. If Duclaux wished above all to 

 revive and defend the old methods of Pasteur, it would have 

 been more correct to have shown by means of theoretical 

 investigations and practical tests what these methods are" 

 really able to effect. As stated, the examination of the old 

 flasks really throws no light on the subject. In the following 

 pages I intend, in the first place,, to discuss the scope of 

 Pasteur's method from a theoretical point of view, and 

 then to show by experiments what can be attained by its 

 means. 



Duclaux's investigations, mentioned above, teach us that 

 several species can live together in one and the same flask for 

 even fifteen to seventeen years. It is evident that under these 

 circumstances there can be no question of a pure culture, and 

 thus we here have cases before us in which the method does 

 not stand the test. I am ready to acknowledge that there 

 are species of yeast which, under the conditions mentioned, 

 possess different degrees of vital activity, and when a mixture 

 of such species is present in one flask, the time will naturally 

 come when all the weaker species will be dead and only the 

 strongest will have survived ; the question now is : how are 

 we to know when this point has arrived and how to determine 



