Malaria is the curse of plains and lowlands, while mountainous regions have almost the monopoly of goitre and cretinism. These endemic maladies directly diminish the mental powers through disturbing the circulation of the brain. They contribute largely to the inferior intellectual status of mountaineers, already prepared by the isolation of their lives. The most important ethnic question in connection with climate is that of the possibility of a race adapting itself to climatic conditions widely different from those to which it has been accustomed. This is the question of Acclimatisation. Its bearings on ethnic psychology can be made at once evident by posing a few practical inquiries: Can the English people flourish in India? Will the French colonise successfully the Sudan? Have the Europeans lost or gained in power by their migration to the United States? Can the white or any other race ultimately become the sole residents of the globe? It will be seen that on the answers to such questions depends the destiny of races and the consequences to the species of the facilities of transportation offered by modern inventions. The subject has therefore received the careful study of medical geographers and statisticians. I can give but a brief statement of their conclusions. They are to the effect, first, that when the migration takes place along approximately the same isothermal lines, the changes in the system are slight; but as the mean annual temperature rises, the body becomes increasingly unable to resist its deleterious action until a difference of 18° F. is reached, at which continued existence of the more northern race becomes impossible. They suffer from a chemical change in the condition of the blood-cells, leading to anæmia in the individual and to extinction of the lineage in the third generation. This is the general law of the relation to race and climate. Like most laws, it has its exceptions, depending on special conditions. A stock which has long been accustomed to change of climate adapts itself to any with greater facility. This explains the singular readiness of the Jews to settle and flourish in all zones. For a similar reason a people who at home are accustomed to a climate of wide and sudden changes, like that of the eastern United States, supports others with less loss of power than the average. A locality may be extremely hot, but unusually free from other malefic influences, being dry, with regular and moderate winds, and well drained, such as certain areas between the Red Sea and the Nile, which are also quite salubrious. Finally, certain individuals and certain families, owing to some fortunate power of resistance which we cannot explain, acclimate successfully where their companions perish. Most of the instances of alleged successful acclimatisation of Europeans in the tropics are due to such exceptions, the far greater number of the victims being left out of the count.