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 540 THE AMERICAN JOURNAL OF SOCIOLOGY

Western Hemisphere falling ill of any new disease except malaria, we have quite definite accounts of the first introduction of this or that Old World malady to this and that region of the western world. The microbes of certain diseases, such as malaria and the sleeping-sickness, are transferred from one human being to another by winged insects, and may therefore prevail jn regions where the population is scanty. But most other zymotic diseases pass directly from man to man, and therefore prevail most where the population is densest. Thus the mortality from consumption, and therefore the stringency of selection by consumption, is much greater in the slums of great cities than in the open country. Dur- ing uncounted centuries, therefore, with the advance of civiliza- tion and the increase of population, man in the Old World under- went evolution against many forms of disease. By means of this evolution he achieved the power of dwelling in towns and cities, of living a civilized existence in spite of the prevalence of disease. Then, when the germs of disease were rife in every home and thick on the garments of every man, Columbus discovered America. At once the vastest tragedy in human existence began. The New World was swept from end to end by recurrent pesti- lences of air- and water-borne diseases, such as small-pox, measles, and cholera. Whole tribes and nations were destroyed or deci- mated. But an equal part was played by consumption. This disease particularly affects such dark and ill-ventilated houses as are built by men of European race in cold climates. The natives of all the temperate parts of the New World melted away before it. They could not at once achieve, under the worst conditions, an evolution which the natives of the Old World achieved during the course of many centuries, at the cost of hundreds of millions of lives, under conditions that became worse only very slowly. Nowhere in all the temperate parts of the New World has a settle- ment of white men a native quarter such as every white settle- ment has in Asia and Africa. The destruction wrought among the inhabitants of the tropical forests was less. Malaria, to which they had become resistant, protected them from the inrush of Europeans, while the abundance of heat and light, and the absence of towns and cities, checked the prevalence of consumption.