202 PASTEUR I THE HISTORY OF A MIND 



their individuality. It is a question of a functional 

 elasticity of the cell, permitting it to adapt itself without 

 changing its nature, to be and to become according to 

 varied conditions of existence. We see to what heights 

 he had raised the debate: by changing the mode of 

 interpretation of facts already known, he caused them to 

 give birth to a new theory. 



IV 

 AEROBIC LIFE OF ANAEROBIC SPECIES 



The preceding facts had in reality a bold counter- 

 part. We have just seen that some species, aerobic 

 under ordinary conditions, can lead an anaerobic life 

 for a greater or less length of time. In like manner 

 we ought to be able to acclimate a species which is 

 ordinarily anaerobic to an aerobic life. 



Such is the yeast of beer. Let us try first to ascertain 

 just how far it can go in its anaerobic life by sowing it 

 in a nutrient solution which we have completely de- 

 prived of oxygen, it matters not how. In this dis- 

 aerated medium the yeast lives, but feebly; its develop- 

 ment is slow, and the fermentation takes a long time. 

 But it comes to an end, and if, when it has terminated, 

 we investigate the relation between the weight of the 

 sugar transformed and the weight of yeast present we 

 find a very high figure, of 150 to 200. If, for example, 

 we work with 100 grams of sugar, we shall see that from 

 5 to 7 decigrams of yeast have been sufficient to trans- 

 form it into alcohol and carbonic acid. 



Let us now give to the yeast a little more air. Let us 

 sow it in a fermentable aerated liquid, contained in a 

 flask Which we shall not fill completely, so that the yeast 



