With the wide prevalence of bovine and human tuberculosis it is little wonder that nearly every human being becomes more or less infected at some period of life. Autopsies on large numbers of individuals in some of our great hospitals have shown that as many as ninety-nine per cent. of the subjects show tubercular lesions of some kind. While it is true that the class of people who would come to autopsy in such public hospitals would perhaps be more likely to be tubercular than the average of the community, still it can not be denied that a very large degree of infection exists. Pearson, from statistics gathered in Europe, has shown that about eighty to ninety per cent. of the population have tubercular lesions before the age of eighteen. Hamburger found that in Vienna ninety-five per cent. of the children of the poor, between twelve and thirteen years of age, were infected with tubercular bacilli and he estimates that all would be before maturity. According to Doctor Mott, pathologist to the London County Asylums, the insane between the ages of fifteen and thirty-five are about fifteen times as likely to acquire tuberculosis as the sane are. Yet the mortality from tuberculosis, great though it be, is obviously not in proportion to the enormous degree of infection. The crux of the situation is mainly the matter of resistance. From the standpoint of heredity, therefore, the question largely resolves itself into one of the inheritance or non-inheritance of constitutional resistance. Some are predisposed to be non-resistant and hence succumb. The work of Karl Pearson[5] and other recent researches forcibly indicate that hereditary constitutional predisposition is one of the chief factors concerned in subjects who develop well defined attacks of the disease. Yet we must not forget that there are degrees of susceptibility and that therefore a constitutional predisposition which might be of little significance under good average conditions of nutrition and sanitation might be insufficient under unfavorable conditions. Before we can make any relatively accurate estimate of the exact degree to which the malady is based on inheritance we must have more data. Many difficulties beset the path of the investigator. In the first place, when one gets back a generation or two he finds that diagnosis was crude and uncertain; a given malady may or may not have been tuberculosis. The main error however was probably on the side of not recognizing it in mild or obscure cases. Then again the questions of virulence of the infection, of size and frequency of the dose, etc., are also complicating factors. Moreover, in very many cases the infection is a mixed one and hence we are dealing with other factors than straight tuberculosis.