The many branches of biology which have been developed independently in the course of the nineteenth century ought to remain in touch with one another, and co-operate with a clear apprehension of their task, if they are to attain their high purpose of framing a unified science embracing the whole field of organic life. Unfortunately, this common aim is often lost sight of in the specialization of study; the philosophical task is neglected in favor of the empirical. The confusion that has ensued makes it desirable to determine the mutual positions of the various biological disciplines. I went into this somewhat fully in my academic speech on the development and aim of zoology in 1869. But as this essay is little known, I will briefly resume the chief points of it. In correspondence with the long-established distinction between the plant and the animal, the two chief branches of biology, zoology and botany, have developed side by side, and are represented by two different chairs in the universities. Independently of these, there arose at the very beginning of scientific activity that field of inquiry which deals with human life in all its aspects--the anthropological disciplines and the so-called "mental sciences" (history, philology, psychology, etc.). Since the theory of descent has proved man's origin from vertebrate ancestors, and thus anthropology has been recognized as a part of zoology, we have begun to understand the inner historic connection between these various branches of anthropology, and to combine them in a comprehensive science of man. The immense extent and the great importance of this science have justified the creation of late years of special chairs of anthropology. It seems desirable to do the same for the science of the protists, or unicellular organisms. The cell theory, or cytology, as an elementary part of anatomy, has to be dealt with in both botany and zoology; but the lowest unicellular representatives of both kingdoms, the primitive plants (protophyta) and the primitive animals (protozoa), are so intimately connected, and throw so great a light, as independent rudimentary organisms, on the tissue cells in the _histon_, or multicellular organism, that we must regard as a sign of progress the recent proposal of Schaudinn to found a special institute and journal for the science of protists. One very important section of it is bacteriology.