Any account of the scientific progress of this century which omits the name of Louis Pasteur would be lamentably incomplete.  In that part of science which relates strictly to human life and the means of preserving it, the work of this great man must be placed in the first rank.  Indeed, we believe that no other stride in biological investigation from the beginning of time has been so great in its immediate and prospective results as has been the increment contributed by Pasteur and his contemporary Koch.  The success of these two experimental philosophers grew out of the substitution of a new theory for one that had hitherto prevailed respecting some of the fundamental processes in living matter.

Up to about the close of the third quarter of this century, the belief continued to prevail in the possibility of the propagation and production of germ life without other germ life to precede it.  It was held that fermentation is not dependent upon living organisms, and that fermentation may be excited in substances from which all living germs have been excluded.  This belief led to the theory of abiogenesis so-called—­a term signifying the production of life without life to begin with.

The question involved in this theory was hotly debated by philosophers and scientists in the Sixties and Seventies.  The first great work of Pasteur in biological investigation was his successful demonstration of the impossibility of spontaneous generation.  About 1870, he became a careful experimenter with the phenomena of fermentation.  As his work proceeded, he was more convinced that fermentation can never occur in the absence and exclusion of living germs; and this view of the deep-down processes in living matter has now been accepted as correct.

The next stage in the work of Pasteur was the discovery that certain substances, such as glycerine, are products of fermentation.  From this foundation firmly established he passed on to consider the phenomena of disease.  He had been, in the first place, a teacher in a normal school at Paris.  In 1863, when he was thirty-nine years of age, he was a professor of geology.  Afterward he had a chair of chemistry at the Sorbonne.  In 1856 we find him experimenting with light, and after that he turned to biological investigations.  This led him to the results mentioned above, and presently to the discovery that the contagious and infectious diseases with which men and the lower animals are affected are in general the results of processes in the system that are nearly analagous to fermentation, and that such diseases are therefore traceable ultimately to the existence of living germs.

This view of the case brought Pasteur to a large and general investigation of bacteria.  The bacterium may be defined as a microscopic vegetable organism; or it may be called an animal organism; for in the deep-down life of germs there is not much difference between vegetable and animal—­perhaps no difference at all.  The bacterium is generally a jointed rod-like filament of living matter, and its native world seems to be any putrefying organic substance.