The expression _hereditary transmission of tuberculosis_, or rather, _of tubercle bacilli_, may be construed in several different ways. It may mean the hereditary transmission from father or mother, or from grandfather or grandmother, or from ancestors still further back. If we designate the parental transmission as _congenital heredity_, transmission from further back as _pregenital heredity_, then in my view of the origin of pulmonary consumption, generally neither congenital nor pregenital heredity comes into play. And, looking at the matter as I do, if one is still desirous of speaking of the hereditary influences of relatives, one ought to use the term _postgenital_. It is now almost everywhere conceded that human tuberculosis as a rule is actually of postgenital origin. Experience has taught me that if, in scientific investigation, one wishes to discover something new, one should study the exceptions to the rule. In actual practice, on the other hand, it is well to keep to the rule. We may therefore safely ignore the cases of congenital tuberculosis, but must all the more thoroughly study the circumstances which in extra-uterine life govern the tubercular infections which lead to consumption. And here I believe I have discovered a new principle which may be expressed thus: “_The milk fed to infants is the chief cause of consumption._” This assertion will at first sight be surprising, for it has long been maintained that the suckling infant receives milk free or almost free from germs. Mother’s or nurse’s milk is taken by the child in this condition, and the cow’s milk for artificially nourished children is usually first boiled or scalded. In later life, to be sure, much less attention is paid to securing a milk as sterile as possible. How, then, is this to be reconciled with the above statement, that it is especially the _milk fed to infants_ which constitutes the chief danger in causing tuberculosis? And yet this statement is true, not because the milk fed to infants is at all worse than other milk, but because the human infant, like the young of all other mammals, is destitute of the protective agencies in his alimentary system which at a later period of life prevent the entrance of disease germs into his tissues. It has taken many years of experimental work to demonstrate this fact conclusively. At present, however, the chain of evidence is so strong that I have not the least hesitancy in building on it my entire plan for the suppression of tuberculosis. In this lecture I can do no more than summarize the main proofs for my assertion regarding the ready penetrability of the infantile alimentary tract for all disease germs, but especially for tubercle bacilli.