I shall commence with the question as to what relations exist between human and bovine tubercle bacilli. Two years ago, in London, Koch said that the excitants of bovine _perlsucht_ have nothing to do with human consumption. If this be true, then I must at once admit that my hopes regarding the utility of cattle immunization for human therapeutic purposes have very little foundation. It is well known that the protection afforded by immune bodies excited by the injection of any virus extends only to the same species of virus, not to that of different species. Nor, in our entire investigations in immunity, do we possess a single example showing the possibility of protecting animals or man by means of one infectious agent against the action of another of a different species. All this is now so well known that I need not discuss it. When, therefore, the researches at the Koch Institute show that it is possible to protect goats, donkeys, and cattle against _perlsucht_ by inoculating them with human tubercle bacilli, does it not show that in these diseases there is no difference of species? According to the recently expressed opinion of his co-workers, Schütz and Neufeld, Koch in his London address did not maintain that there is a difference in species. He merely maintained that the virus of human consumption is not identical with the virus of bovine _perlsucht_, and that, likewise, bovine tuberculosis is not identical with human tuberculosis. That, to be sure, would be something quite different from what the rest of the world understood. Human pulmonary consumption and bovine _perlsucht_ are anatomically so dissimilar that it required extraordinarily clever researches and keen reasoning to lead to the discovery of a common etiological factor in these diseases. But one can easily go further than this. One can maintain without exaggeration that tubercular lupus, tubercular scrofula, and other human tuberculosis localized in joints, serous surfaces, etc., are just as little identical with pulmonary consumption as are the cases of bovine _perlsucht_. The term _identical_ does not even fit all cases of consumption. Strictly speaking, a process or thing is identical only with itself. Certainly not all tubercle bacilli are identical, nor yet all kinds of tubercle bacilli. Koch’s assertion that there is an essential difference between human and bovine tubercle bacilli, and that these differences are not bridged by any connecting links, provoked the strongest opposition. This assertion has since called forth observations from all over the world which positively demonstrate the existence of intermediary stages in the virulence of tubercle bacilli derived from mammals. Generally, tubercle bacilli derived from cattle are more virulent for all animal species, thus far examined, than are human tubercle bacilli. And the opinion is constantly gaining ground that bovine tubercle bacilli are also more virulent for man.