204 PASTEUR I THE HISTORY OF A MIND 



least gives very little; it is a complete combustion which 

 has followed the incomplete combustion of the preced- 

 ing experiment in the same way as an incomplete com- 

 bustion followed a complete one in the case of the 

 mucor when deprived of air. Finally, a last analogy: 

 the yeast, like the mucor, multiplies and increases mark- 

 edly in weight in contact with air, so that, this time, 

 for 100 grams of sugar used, we find 20 to 25 grams of 

 yeast produced. 



Some modifications of form accompany here also these 

 changes of function. The yeast in contact with air is 

 less full of cavities, has a much finer content, and is 

 younger in aspect, as the comparison of the two halves 

 of Fig. 17 shows. 



In this very aerobic existence, the yeast, therefore, 

 approaches the Mucedinese. It differs from them 

 in that it can lead the anaerobic life, to which it is adapted 

 for a very much longer time, than even the Mucor. 

 But here also, the appearance of the ferment-character 

 accompanies life without air. 



So that in the presence of the general character of 

 these facts, Pasteur had been led to ask himself the 

 following question: Can we admit that the yeast and the 

 other plants capable of becoming alcoholic ferments 

 which absorb and consume oxygen so actively when 

 they have it at their disposal, cease to have need of it 

 when it is refused them, and in this case change com- 

 pletely their mode of existence? If we reply, "Yes," 

 to this question, then there is no relation between 

 aerobic and anaerobic life. These are two different 

 living organisms which succeed each other in the same 

 protoplasm and within the same cell-wall. If we admit, 

 on the contrary, as it is evidently more natural to do, 

 that the needs of the cell in its two modes of existence 

 remain the same, and that only the means of satisfying 



