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known of the higher classes of organised beings. The observation that each of these creatures was distinctly differentiated was no more than the differences of other species would lead us to anticipate. The several varieties must of necessity have their own habitats, and their own surroundings, if they were subject to the same laws of life as other animals and vegetables; and each must fulfil its own purpose in the economy of Nature. In the progress of this investigation, it was found that some of these minute organisms were traceable as parasites in other living beings; and the strong presumption was that they were associated with certain morbid states. It further became evident that this development was not restricted by the action of chemical laws, but that the processes of putrefaction and fermentation might be superseded by morbid processes which have hitherto, at all events, refused to be classed under chemical or physical laws. One additional step led to the conclusion that, in each instance, the special organism was of a different kind. It would be somewhat rash to assert that all of these facts have yet been brought into the domain of laws of Nature. But if we may trust some of the most careful observers, we must admit that certain morbid states (e.g., splenic fever) are accompanied of necessity by the development of a specific bacillus (e.g., the bacillus anthracis), and that this
