In order not to be misunderstood, I wish to emphasize here that I do not at all deny that infection can be caused in adults by inoculation with tubercular virus. In fact, I assume that few of us in advanced life escape such infection. But that this infection leads to cavity formation in the lungs, is, I believe, fully as unproved as the assertion that bovine tubercular virus has caused human pulmonary consumption in even a single instance. Koch has very properly pointed out the entire absence of proof for this last statement. I can go still further in my concessions to the prevalent view that consumption results from the inhalation of particles of dust or moisture laden with tubercle bacilli. I concede not only the possibility but the actual occurrence of pulmonary tuberculosis going on to consumption, as a result of infection of an adult person. I concede this in the sense that on the basis of an infantile infection a pulmonary tuberculosis has developed which becomes manifest only through the agency of the additional infection. However, the opportunity for infection with tubercle bacilli cannot by itself be a deciding factor in the development of pulmonary consumption. I can here cite the experience of Dr. Moritz Schmidt of Frankfurt a. M., who has examined a great number of cases of tubercular laryngitis. In his experience of over forty years he has certainly been exposed, more than others, to tubercular infection. But neither he nor any of his numerous assistants has ever become consumptive. My experiments on animals have shown me that the lesions characteristic of human pulmonary consumption are developed only after there have been extensive and long-continued disturbances of the vital functions of the organism. Our ancestors introduced the term “dyscrasia” and “diseased constitution” to express this idea. I have succeeded, especially in goats, but also in other animals, in producing a clinical picture exactly similar to that of human pulmonary consumption. In these animals I first produced a moderate degree of immunity against tuberculosis by a lengthy course of treatment, and then I injected a strong tubercular virus into the circulation. I regard the lesions in pulmonary consumption as being produced in similar fashion. They are the expression of an infection in an individual who, owing to a very early previous infection with tubercle bacilli, is less susceptible to the new infection. These late infections may in isolated cases be referable to the inhalation of tubercle bacilli. They may, however, be due to already existing tubercular lesions, and so be regarded as auto-infections or metastases. _Were we to inject into the tissue juices of a person not yet partially immunized against tuberculosis an amount of tubercle bacilli equal to that usually found in the lungs of consumptives, the person would die of an acute miliary tuberculosis, but he would never develop pulmonary consumption._