If these alleged successful cases, or that of the Jews or Arabs, be closely examined, it will almost surely be discovered that another physiological element has been active in bringing about acclimatisation, and that is the mingling of blood with the native race. In the American tropics the Spaniards have survived for four centuries; but how many of the _Ladinos_ can truthfully claim an unmixed descent? In Guatemala, for example, says a close observer, _not any_. The Jews of the Malabar coast have actually become black, and so has also in Africa many an Arab claiming direct descent from the Prophet himself. But along with this process of adaptation by amalgamation comes unquestionably a lowering of the mental vitality of the higher race. That is the price it has to pay for the privilege of survival under the new conditions. But, in conformity to the principles already laid down as accepted by all anthropologists, such a lowering must correspond to a degeneration in the highest grades of structure, the brain-cells. We are forced, therefore, to reach the decision that the human species attains its highest development only under moderate conditions of heat, such as prevail in the temperate zones (an annual mean of 8°–12° C.); and the more startling conclusion that the races now native to the polar and tropical areas are distinctly _pathological_, are types of degeneracy, having forfeited their highest physiological elements in order to purchase immunity from the unfavourable climatic conditions to which they are subject. We must agree with a French writer, that “man is not cosmopolitan,” and if he insists on becoming a “citizen of the world” he is taxed heavily in his best estate for his presumption. The inferences in racial psychology which follow this opinion are too evident to require detailed mention. Natural selection has fitted the Eskimo and the Sudanese for their respective abodes, but it has been by the process of regressive evolution; progressive evolution in man has confined itself to less extreme climatic areas. The facts of acclimatisation stand in close connection with another doctrine in anthropology which is interesting for my theme, that of “ethno-geographic provinces.” Alexander von Humboldt seems to have been the first to give expression to this system of human grouping, and it has been diligently cultivated by his disciple, Professor Bastian. It rests upon the application to the human species of two general principles recognised as true in zoölogy and botany. The one is, that every organism is directly dependent on its environment (the _milieu_), action and reaction going on constantly between them; the other is, that no two faunal or floral regions are of equal rank in their capacity for the development of a given type of organism.