An important part of the scientific knowledge of living organisms is dependent on a study of their forms and relationships. As has been stated, Leeuwenhoek considered bacteria to be “animalcules” because they showed independent movement. But little attention was paid to the natural history of these animalcules for nearly a hundred years after Leeuwenhoek. During the last quarter of the eighteenth century, however, workers busied themselves chiefly with the discovery and description of new forms. Among these students were Baron Gleichen, Jablot, Lesser, Reaumur, Hill and others. Müller, of Copenhagen, in 1786 published the first attempt at classification, a most important step in the study of these organisms. Müller introduced the terms Monas, Proteus and Vibrio, which are still in use. Ehrenberg, in his work on _Infusoria_, or the organisms found in infusions, published in 1838, introduced many generic names in use at present, but still classed the bacteria with protozoa. Joseph Leidy, the American naturalist, considered that the “vibrios” of previous writers were plants and not “animalcules.” He seems to have been the first to have made this distinction (1849). Perty (1852) recognized the presence of spores in some of his organisms. Ferdinand Cohn (1854) classed the bacteria among plants. Nägeli (1857) proposed the name “Schizomycetes” or “fission fungi,” which is still retained for the entire class of bacteria. Cohn in the years 1872-1875 established classification on a modern basis and added greatly to the knowledge of morphology and natural history of bacteria. He described spore formation and the development of spores into active bacteria, and showed the close relationships as well as differences between the bacteria and the lower algæ. Robert Koch was a pupil of Cohn. An examination of the accompanying chronological table will show how the investigations and discoveries in connection with “spontaneous generation,” the “contagium vivum” theory and putrefaction and fermentation must have been mutually suggestive: 1546. Fracastorius, disease germs theory and direct and indirect contagion. 1671. Kircher, “contagium vivum” theory. 1675. Leeuwenhoek, first saw bacteria, “animalcules.” 1701. Andry, “animalcules” cause of diseases. 1718. Lancisi, “animalcules” cause of malaria. 1749. Needham, described development of organisms in water around barley grains. 1762. Plenciz, arguments for “living cause” theory and that “animalcules” cause putrefaction. 1768. Bonnet, suggested that probably Needham’s organisms came from germs in the liquid. 1776. Spallanzani, boiled and sealed infusions. 1786. Müller, first classified “animalcules.” 1787. Wollstein, glanders pus infectious. 1795-1798. Jenner, vaccination against smallpox. 1797. Viborg, transmitted glanders repeatedly.