Only after the work of Danielssen and Boeck can one say that Leprosy entered the ranks of the scientifically investigated diseases. At that time, in 1840, when they commenced their investigations, Humoral Pathology held the field. Most diseases were ascribed to changes in the blood, and they therefore endeavoured to establish that there was in lepers a change in the blood which they regarded as the cause of the disease symptoms, especially the node formation. These changes in the blood they believed were caused by unfavourable conditions of living, and as they were not able to find any convincing evidence of the power of infection of the disease but several of its limitations to certain families, they drew the conclusion that Lepra, as they called it, might appear spontaneously, that is to say, that the sanguineous dyscrasia which led to leprosy could be developed under unfavourable conditions of life, but that it was in most cases hereditary. It must, however, be noted that Danielssen always regarded Leprosy as a specific disease, described it as such, and sought for a specific cause, and the fact that he did not find it must be ascribed to the circumstance that microscopical technique and microscopical aids, especially the immersion lens, were at that time either insufficiently developed, or not yet discovered. The teaching of Danielssen and Boeck was everywhere adopted, especially their view of the heredity of the disease. The fish diet and damp cold theories are only attempts to explain the so-called spontaneous development of the disease, and they are founded on the fact that Leprosy is chiefly present in littoral districts and on islands. Of other Norwegian investigators, the late Dr. Hjorth held the view that Leprosy could not be ascribed to a specific cause, and that it was certainly not hereditary. Dr. Holmsen regarded it as a specific miasmatic and non-hereditary disease, and finally Prof. Lochmann stated that it was specific, contagious, and hereditary. While Danielssen and Boeck always required a leprous ancestor in order to recognise a case as hereditary, and when this was not forthcoming, found in the presence of the disease in other branches of the family, proof of its heredity, Biedenkap, as he was unable in many of his cases to determine the existence of leprous ancestors, widened the definition of heredity by assuming that unfavourable conditions of life might produce in the organism conditions which became hereditary, and showed themselves in later generations as leprosy. In the year 1869, Dr. Drognat-Landré published a book with the title, _De la contagion seule cause de la propagation de la lépre_, in which he sought to prove that heredity had nothing to do with the spread of Leprosy. That is, according to our view, the right standpoint, as we shall endeavour to demonstrate.