INTRODUCTION vii 



animalculse, etc., in water, and so gave a basis for the 

 " infinitely little " view of the nature of disease germs. And 

 it was a study of the processes of fermentation that led 

 Pasteur to the sure ground on which we now stand. Starting 

 as a pure chemist, and becoming interested in the science of 

 crystallography, it was not until his life at Lille, a town with 

 important brewing industries, that Pasteur became interested 

 in the biological side of chemical problems. Many years before 

 it had been noted by Cagniard-Latour that yeast was composed 

 of cells capable of reproducing themselves by a sort of budding, 

 and he made the keen suggestion that it was possibly through 

 some effect of their vegetation that the sugar was transformed. 

 But Liebig's view everywhere prevailed that the ferment was 

 an alterable, organic substance which exercised a catalytic 

 force, transforming the sugar. It was in August, 1857, that 

 Pasteur sent his famous paper on Lactic Acid Fermentation 

 to the Lille Scientific Society ; and in December of the same 

 year he presented to the Academy of Sciences a paper on 

 Alcoholic Fermentation, in which he concluded that the 

 deduplication of sugar into alcohol and carbonic acid is cor- 

 relevant to a phenomena of life. These studies had the signal 

 effect of diverting the man from the course of his previous more 

 strictly chemical studies. It is interesting to note how slowly 

 these views dislocated the dominant theories of Liebig. More 

 than ten years after their announcement I remember that we 

 had in our chemical lectures the catalytic theory very fully 

 presented. 



Out of these researches arose a famous battle which kept 

 Pasteur hard at work for four or five years the struggle over 

 spontaneous generation. It was an old warfare, but the 

 microscope had revealed a new world, and the experiments on 

 fermentation had lent great weight to the omne vivum ex ovo 

 doctrine. The famous Italians, Redi and Spallanzani, had led 

 the way in their experiments, and the latter had reached the 

 conclusion that there is no vegetable and no animal that has 

 not its own germ. But heterogenesis became the burning 

 question, and Pouchet in France, and Bastian in England, 



