And to think that we might achieve the same results in our own species if we would only treat ourselves as well as we do our monkey captives! To "make a monkey of one's self" might have its advantages from a sanitary point of view. "But this method," some one will remind us, "would silence only a part of the enemy's infection batteries." Even supposing that we could prevent the spread of the disease from human sources, what of the animal consumptives and their deadly bacilli? If the milk that we drink, and the beef, pork, and poultry that we eat, are liable to convey the infection, what hope have we of ever stopping the invasion? The question is a serious one. But here again a thorough and careful study of the enemy's position has shown the danger to be far less than it appeared at first sight. Even bacilli have what the French call "the defects of their virtues." Their astonishing and most disquieting powers of adjustment, of accommodation to the surroundings in which they find themselves, namely, the tissues and body-fluids of some particular host whom they attack, bring certain limitations with them. Just in so far as they have adjusted themselves to live in and overcome the opposition of the body-tissues of a certain species of animals, _just to that degree they have incapacitated themselves to live in the tissues of any other species_. Some of the most interesting and far-reachingly important work that has been done in the bacteriology of tuberculosis of late years has concerned itself with the changes that have taken place in different varieties and strains of tubercle bacilli as the result of adjusting themselves to particular environments. The subject is so enormous that only the crudest outlines can be given here, and so new that it is impossible to announce any positive conclusions. But these appear to be the dominant tendencies of thought in the field so far. Though nearly all domestic animals and birds, and a majority of wild animals under captivity, are subject to the attack of tuberculosis, practically all the infections hitherto studied are caused by one of three great varieties or species of the tubercle bacillus: the _human_, infesting our own species; the _bovine_, attacking cattle; and the _avian_, inhabiting the tissues of birds, especially the domestic fowl. These three varieties or species so closely resemble one another that they were at one time regarded as identical, and we can well remember the wave of dismay which swept over the medical world when Robert Koch announced that the "_perlsucht_" of cattle was a genuine and unquestioned tuberculosis due to an unmistakable tubercle bacillus. But as these varieties were thoroughly and carefully studied, it was soon found that they presented definite marks of differentiation, until now they are universally admitted to be distinct varieties, each with its own life peculiarities, and, according to some authorities, even distinct species.