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ERTAINLY, from a scientific point of view, no question in medicine is more important than that which relates to the causation of disease. An accurate knowledge of the specific etiological agents concerned in the production of specific infectious diseases forms the very basis of scientific medicine; and, as you all know, researches which have been made during the past thirty years have given us this accurate knowledge for a considerable number of these diseases. I say thirty years, in order to include the researches of Davaine upon anthrax; but, as a matter of fact, the principal portion of our knowledge relating to specific disease-germs has been acquired during the past decade.

As an introduction, a brief historical review of the progress of our knowledge will perhaps not be out of place. But first I must call your attention to the fact that this progress has been made possible by certain improvements in methods of research, and especially by the following: first, the use of a cotton filter to exclude atmospheric organisms from our culture media (Schröder and Von Dusch, 1854); second, the sterilization of culture media by heat (methods perfected by Pasteur, Koch, and others); third, the use of aniline dyes as staining agents (first recommended by Weigert in 1877); fourth, the introduction of solid culture media, and the "plate method" for obtaining pure cultures (by Koch in 1881); fifth, the perfection of methods for cultivating anaerobic bacteria. I have already referred to the researches of Davaine relating to the disease of cattle and sheep known as anthrax. Having ascertained that the blood of an infected animal constantly contained a rod-shaped micro-organism, and that the smallest quantity of this blood inoculated into a susceptible animal gave rise to the disease and caused its death, Davaine, in 1803, boldly announced his belief that the bacillus was the specific etiological agent in this disease. The experiments of Davaine were not, however, generally accepted as conclusive, because, in inoculating an animal with blood containing the bacillus, the living micro-organism was associated with material from the body of the diseased animal. This objection was subsequently removed by the experiments of Pasteur, Koch, and many others with pure cultures of the bacillus. These were shown to have the same pathogenic effects as had been obtained in inoculation experiments with the