Although disease-producing micro-organisms take their place naturally among the exciting causes of disease, their life-history has attracted so much attention during the last twenty years, and has been so exhaustively studied during the last ten years, that it is imperative that they should receive special consideration. The discovery of the microscojie, which, according to Professor Edgar Crookshank, was an event of two and a half centuries ago, was as a matter of course followed by the detection of organisms in animal fluids and elsewhere, the existence of which previously could only be suspected. Shortly after the microscope was first brought into use, it was found that small living things were abundantly present in all decomposing substances, and Kircher believed that similar organisms could be found in various diseases. His researches were directed to the discovery of such organisms; but the modern microscopist would conclude without hesitation that Kircher's chances of success were extremely remote with the very primitive form of optical appliances which were then at his command. The knowledge of the forms and functions of bacteria advanced along with the progress in the development of the microscope, and a considerable step was taken when Anthony Van Leeuwenhoek devoted his attention to the construction of lenses, and made such improvements in the microscope as earned for him the title of the father of microscopy. In 1675 he described, in a series of letters to the Royal Society, numerous minute organisms in rain water, well water, infusions of pepper and hay, and also in many vegetable and animal substances. In 1683 the discoveries were illustrated by means of wood-cuts, and Professor Crookshank, from whose historical account in his work on Bacteriology these facts are quoted, remarks that there can be little doubt that the drawings were intended to represent leptothrix filaments, vibrios, and spirilla. In another communication in 1692 Kircher gives some idea of the size of these small organisms by stating that they were a thousand times smaller than a grain of sand. Further observations were made by Nicolas Andre in 1701, Lancisi in 1710, and in 1721 the plague in Toulon and Marseilles was attributed to the presence of animalcules, and the theory began to be entertained, amidst considerable ridicule, that all diseases arose from vermicules. Scientific knowledge of the subject was considerably advanced by the writings of Muller, who criticised the previous researches, which, he contended, had been too much directed to the finding of new organisms. Muller devoted himself to a consideration of the forms, movements, and other biological characters of the microbes, and attempted a system of classification. At this time the question of the origin of micro-organisms became paramount, and the theory of spontaneous generation with its alternative, development from pre-existing germs, was widely discussed. Many supporters were found for the theory of spontaneous generation, but the balance of the evidence, obtained by numerous experiments, was in favour of the germ theory. Even as late as 1872 Bastian published an account of his experiments with the object of proving that spontaneous generation actually took place. He found that decoctions of turnip and cheese, which had been filtered and boiled for ten minutes and hermetically sealed during the boiling, contained micro-organisms after a time. This evidence was very soon met by the further discovery that in milk, infusions of hay, and other substances the spores of bacilli are present, and that they are not destroyed by boiling. Tyndall further demonstrated that if the method on which Bastian relied was repeated two or three times, all the spores of organisms were destroyed; and thus the last attempt to demonstrate the truth of the theory of spontaneous generation utterly and entirely failed. During the controversy on the subject of spontaneous generation, several investigators, Latour, Schwann, Bassi. Henley, Davaine, Pasteur, and others, were working steadily in reference to the functions of bacteria, the various processes of fermentation, and production of disease. In 1850 Davaine and Rayer discovered a rod-like body in the blood of a sheep that had died of splenic fever (anthrax); Pollender also discovered similar bodies in the same disease, in the blood of cattle. There is no doubt that this rod-like body was the organism which is now familiarly known as the bacillus anthracis. It was not, however, until further investigations had been carried on by Davaine, Pasteur, Burdon Sanderson, Duguid, and others, that the rod-like body, bacillus anthracis, was accepted as the true cause of anthrax; and the demonstration of this important fact may be looked upon as the foundation of the doctrine of contagium vivum, as the origin of contagious disease.