There is variation in the susceptibility to infection in the different races of man. If a race be confined to one habitat with close intercourse between the people, such a race may acquire a high degree of immunity to local diseases by a gradual weeding out of the individuals who are most susceptible. A degree of comparative harmony may be gradually established between host and parasite, as is the case in wild animals. These have few diseases, the weak die, the resistant breed; they harbor, it is true, large numbers of parasites, but there is mutual adjustment between parasite and host. Diseases in animals greatly increase under the artificial conditions of domestication. Certain highly specialized breeds of cattle, as the Alderneys, are much more susceptible to tuberculosis than the less specialized. The high development of the variation which consists in a marked ability to produce milk fat is probably combined with other qualities, shown in diminished resistance to disease, and under natural conditions the variation would not have persisted. The introduction of a new disease into an isolated people has often been attended with dire consequences. It is much the same thing with the introduction of disease of plants. In Europe the brown-tail moth and the gypsy moth produce continuously a certain amount of damage to the trees, but their parasitic enemies have developed with them and check their increase. These pests were brought to this country in which there were no conditions retarding their increase and have produced great damage. It is very difficult to estimate the degree of racial susceptibility. The negro race seems to be more susceptible to certain diseases, such as tuberculosis and smallpox, less so to others, as yellow fever, malaria and uncinariasis. What are apparently differences in susceptibility may be explained by racial customs. A statistical inquiry into death in India from poisonous snakes might be interpreted as showing a marked resistance on the part of the white to the action of the venom, but it is merely a question of the boots of the whites and the naked feet and legs of the natives. The relatively greater frequency of smallpox in the blacks is due to the greater difficulties in carrying out vaccination measures among them and the greater opportunity for infection which results from their less hygienic life. It has always been noted that when plague prevails in Oriental cities, the natives are more frequently attacked than are Europeans. This does not depend upon differences in susceptibility, but on the better hygienic conditions of the whites which prevent the close relation to rats and vermin by which infection is extended. There would be but little extension of the hookworm disease in a community where shoes were worn and the habits were cleanly.