This chapter concerns factors of causation which are all more or less ascertained and defined. The influence of heredity is considered as acting in two ways: by direct transmission before birth from parent to infant of the germs of disease; or by the transmission from parent to offspring of a special weakness or openness rendering certain persons more liable to infection than others. Baumgarten believes that either germinal or intra-uterine transmission of infection is the most common cause of tuberculosis, and that long latency of the infection is the rule rather than the exception. Phthisis is regarded as a typically hereditary disease, in the causation of which family predisposition plays a large part. Phthisis, like scarlet fever, is a common and an infectious disease, and the futility of depending on statistics as evidence of hereditary predisposition. The chapter discusses the practical bearing of the facts and considerations on the marriage of, and particularly on the marriage between, consumptives.