302 READINGS IN BIOLOGICAL SCIENCE 



The numerous groups of simple unicellular fungi associated with various 

 fermentative processes — of which ordinary yeast in alcoholic fermenta- 

 tion was only one — thus began to receive the attention of an exceedingly 

 astute observer. Pasteur's thought, from the very first, was that the yeast 

 globules and smaller, elongated bodies were the cells of living organisms, 

 and as they had life so they would require food. Not only were the cells 

 of yeast and other simple organisms taking their nourishment from the 

 solutions of sugar and other substances in which they lived — they were 

 also, of necessity, transforming it. They were using part of it to build up 

 their own substance and rejecting the rest. Hence the chemical changes 

 that took place in fermentation. 



This realization led Pasteur to a number of discoveries of great industrial 

 value. He discovered that the process of making vinegar from wine de- 

 pended upon the grow^th of a particular kind of fungus — "the vinegar plant" 

 — on the beach-wood chippings over which the wine was allowed to flow. 

 Where the wine would not "turn" to vinegar, he put in a little of the living 

 fungus and the vinegar-makers' troubles were at an end. He showed that a 

 souring of wine was due to the growth of an undesirable organism in it 

 after it had been bottled, and he showed the wine-makers how to over- 

 come the trouble, very simply, by heating the wine, to kill the cells of the 

 organism, before it was bottled. An early instance of "Pasteurization," al- 

 though, indeed, the Romans had been familiar with this dodge for preserv- 

 ing wine. The very idea that fermentation was brought about by any life- 

 process was rank heresy for the chemists at that time and Pasteur was 

 scornfully accused of attempting to put back the clock of up-to-date 

 nineteenth century progress. 



Pasteur's researches on "so-called spontaneous generation," which con- 

 tinued from 1859-1S65, arose directly out of his work on fermentation. 

 He had discovered that fermentation and putrefaction were dependent 

 upon the growth of Hving organisms. Where did those organisms come 

 from? Pasteur knew the answer before he began. The organisms came from 

 the air, in which their imponderable "spores" were always floating about, 

 as the grosser seeds of some of the flowering plants drifted in the wind. 

 But the apparently spontaneous appearance of minute organisms in in- 

 fusions of fermenting or decaying material was the very "fact" upon which 

 the last behef in spontaneous generation now depended. Pasteur set him- 

 self to prove experimentally what he knew beforehand must be true. As 

 the truth would be most unwelcome, and he would be challenged at every 

 step of the way, he had to contrive a series of experiments which would 

 give unambiguous results, and yet be of such simplicity that no one could 

 pretend to misunderstand them. 



He did not succeed in routing the materialist's belief in the possibility 

 of spontaneous generation. It was, in truth, impossible to prove that it never 

 occurred in nature. But Pasteur did show that the postulation of spontaneous 



