In the scientific controversy as to whether the virus of consumption and that of _perlsucht_ belong to the same species or not, various misunderstandings have prevented an agreement. Very often the terms “similar species” and “different species” have been used in a different sense. It is, of course, wholly a matter of individual judgment where to draw the line limiting the membership of what, in our minds, constitutes a species. Even if we apply Darwin’s definition and include under the term “species” only “organisms of similar origin,” the term is still very elastic. However, in the light of our present biological knowledge, based on Darwin’s theory of evolution, we are under no circumstances permitted to take as a criterion for like and unlike species such an easily influenced factor as the degree of virulence, i.e., the capacity for giving rise to pathological changes in another organism. _Nowadays all botanists and zoologists are agreed on this, that membership in a species is determined by the origin of the organism._ An anthrax culture, which has been made entirely avirulent by means of Pasteur’s procedure for reducing virulence, surely belongs to the same species as does the virulent variety from which it was derived. And in this case the difference in the degree of virulence is far greater than it ever is between human and bovine tubercle bacilli. If we keep in mind all the functional differences that have experimentally been shown to exist between tubercle bacilli which have long lived in a human body and those which have long lived in a bovine body, we shall surely not go wrong when we assume that with a little patience and expert knowledge we shall be able to make these two varieties absolutely similar again, even in respect to their virulence. Passing now to the discussion of the real subject of my lecture I shall try to give you an idea of the distribution of tuberculosis in the human species. Although the number of persons dying of consumption is frightfully large, the number of tubercular, and tubercularly infected, persons is much larger. Not until recent years have we secured tolerably reliable data to enable us to judge of the distribution of tuberculosis in civilized countries. If we classify as tuberculous every case in which there are pathological changes due to tubercle bacilli, then surely the oft-quoted saying of the noted Greifswald physician is true, “We are all a bit tuberculous.” It has not been easy to demonstrate this fact scientifically, and it would have been impossible of demonstration without the two epoch-making discoveries of Robert Koch, that of tubercle bacilli, and of tuberculin.