Not even a man of the scientific attainments of Virchow was able to judge what constitutes tuberculosis, although during fifty years he had endeavored to formulate data for the differential diagnosis of post-mortem findings. Since we have learned that Koch’s bacillus is the bond of union between all the various tubercular diseases, we see that many inflammatory processes which Virchow described as entirely distinct and different are nothing else than special stages in the tubercular infection. The anatomical unit of the tubercular process is the tiny, translucent gray nodule of millet-seed size described by Laennec and Virchow, the miliary tubercle. But what may develop from this may take the form of most widely divergent pathological lesions, and it was not granted to Virchow to puzzle out the endless variety of things etiologically the same. Yet it is _etiologically_ that the vital processes must be analyzed if we wish to have a firm basis for our struggle against preventable diseases. This has more and more become the opinion of modern physicians. The progress that has been made, first in surgery and then in internal medicine, progress that only fifty years ago was declared by the most eminent representatives of medicine in those days to be impossible, has been possible only through the etiological investigations developed by Pasteur and by Koch. The first principle in the etiological investigation of all vital processes, and therefore of those concerned in infectious diseases caused by micro-organisms, is thus formulated by Darwin: “That which is of the same origin belongs to the same species.” Two infectious agents may resemble each other ever so closely, but if they have not the same genealogy, i.e., if they are phylogenetically widely separated, then, biologically, they belong to different species. And conversely, size, form, and other properties of certain micro-organisms may be ever so different. If, however, the organisms are of the same origin, then, biologically, they belong to the same species. The virus of anthrax occurs in two forms: as bacilli, and as oval spores. The quotidian malaria parasite has an extraordinarily complicated cycle of development. Nevertheless we have no hesitancy in speaking of either an anthrax virus or of a malaria parasite.[2]