Though there are lepers in Iceland, in the Aleutian peninsula and in Kamschkatka, leprosy may be said to be a disease of tropical and subtropical countries. With the exception of a few insignificant islands, no country in the tropic zone seems to be entirely free from it. In India--the only large country in which accurate statistics have been taken--the proportion of lepers to the total population is estimated at 5 to 10,000, though errors of diagnosis and concealment have doubtless combined to make the estimate merely approximate. In China, judging from the numbers observed in the southern treaty ports, the proportion is probably higher, but both fall far short of the Fijian figure of one per cent., and the Hawaiian of one in thirty. Nothing was known of the specific cause of leprosy until 1874, when Armauer Hansen isolated the _Bacillus lepræ_, a discovery which has cleared the way for formulating precise ideas on the subjects of heredity and contagion, and the proper treatment of the leper as a public danger. It is, of course, impossible for any organism, however small, to create itself _de novo_. It must come from some pre-existing germ whose habitat may be earth, air, water, beast or man, and since leprosy has never been found in any animal except man, nor in any virgin country to which a human leper has not had access, and since the arrival of a leper in such a country is followed by an outbreak of leprosy among those who have associated with him, there is little room for doubt that man acquires the germ of the _Bacillus lepræ_ from man, and not from other animals, nor from local or climatic conditions. The most ancient, and, as it now turns out, the most correct belief, was that leprosy is contagious; the leper was unclean. Driven out from the society of men, he was compelled under heavy penalties to warn wayfarers of his approach by voice or bell. In comparatively recent times the belief arose that leprosy was hereditary, and even that it could be acquired from the soil of certain countries. The latter belief has been disproved absolutely by the behaviour of leprosy when introduced into virgin countries. The hereditary theory is also on the wane, although the Indian Commission on leprosy in the early nineties did not absolutely disprove it. If leprosy be hereditary, how explain the striking fact brought out by Hansen, the discoverer of the bacillus, that of the numerous offspring of 160 Norwegian lepers who emigrated to America none have developed the disease, or again the equally well-attested fact that children sometimes become lepers first, and their parents afterwards. Another strong argument against heredity is to be found in the fact that lepers become sterile at an early stage of the disease; unless, therefore, leprosy finds recruits in some other way than by heredity, the disease would inevitably die out in one or at the most in two generations. Moreover, leprosy is often developed quite late in life, and if the germ had been