They share this pathological condition with the Malayo-Polynesian peoples of the Pacific island-world. Among them both we find numerous examples of that outbreak of homicidal mania called “running amuck” (properly _amok_), where the maniac rushes into a crowd, killing whom he can; a crowd, not of enemies, as in the “Berserkerwuth” of the Northmen, but of friends and relatives. The abandonment of both races to alcoholism and narcotics is an evidence of the same morbid nervous excitability. This is an inherited racial pathological tendency and is not to be measured by the mere prevalence of nervous diseases. These may arise from the increased strain on the neurons when the struggle for existence is intensified. The enfranchised blacks since they have been obliged to support themselves present a much larger percentage of brain and nerve disease; such maladies among the Jews of Europe are six times more frequent than among the Aryans; and certain forms, such as progressive paralysis, are unknown in any but the most civilised communities. The immunity of races to disease, or its reverse, reacts powerfully on their mental life, leading in the latter case to discouragement and apathy, in the former to confidence and conquest. Two of the most striking examples are measles and smallpox. In the white race, the former has become merely one of the “diseases of children,” exciting little alarm, and, against the latter, vaccination provides an efficient protection. Among native Polynesians and Americans the ravages of both have been so dreadful as not merely to decimate a population but to leave the survivors mentally prostrate and indifferent to life. To such an extent has this mental depression sometimes progressed that some tribes, as the Lenguas of La Plata, have decided on the self-destruction of their race, and destroyed all their children at birth. The immunity of the white race to malignant measles is not due to any special power of resistance, but to well-known laws of natural selection in disease, and does not extend to many diseases. The Japanese are practically immune to scarlet fever, the black race to yellow fever, etc., and that all such exemptions react favourably on the ethnic mind cannot be doubted. Such immunity is strictly _cognatic_, a legacy of blood in the true physiological sense, the human cells having undergone changes by the repeated attacks of the disease-germs resulting in practical indifference to their assaults. Indirectly, the march of epidemics has often not only decided the fate of nations but worked remarkable changes in national character. A familiar and striking example is the result of the Black Death (bubonic typhus) in England in the reign of Edward III. CHAPTER III _THE INFLUENCE OF THE SOCIAL ENVIRONMENT_