THIRD PART 



SPONTANEOUS GENERATIONS 



I 

 SPONTANEOUS GENERATION AND FERMENTATION 



Pasteur's contributions to the study of fermentations, 

 which we have just seen going on under our eyes, may 

 be summed up in a few words. Fermentation is no 

 longer a vague transformation, indeterminate in its cause 

 and in its origins, capable of taking place under the 

 influence of any organic substance whatsoever: it is a 

 specific phenomenon, due also to the existence and 

 development of a specific organism, the study of which 

 under the microscope is facilitated in proportion as 

 we remove from the liquid undergoing fermentation 

 those insoluble organic substances which it was formerly 

 believed necessary to add to it. Since, by working 

 with a clear bouillon, it is possible to follow closely the 

 organism sown, and to be sure that the bouillon con- 

 tains this and this alone, the study of its nutrition be- 

 comes easy. Now, by acting on the nutrition of an 

 organism, we become the master of it; we can sow it and 

 cultivate it with as much certainty, and with the same 

 absence of weeds, as we can lettuce in a garden. We 

 can also banish it from liquids where there is no occasion 

 for its presence. In short, this infinitely small organism 

 becomes tangible and open to experiment: a capital 

 idea, which the whole life of Pasteur will henceforth be 

 spent in developing. 



Nevertheless, the logic of his studies placed before him 



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