Man, however, with his superior mentality has freed himself from many of the conditions which impose restraint upon the expansion of animals. In his case selection through disease and social and economic competition has largely replaced selection through adjustment to the limitations of food supply. Man is the most cosmopolitan of animals and in one form or another thrives in the tropics and in the arctics, at sea level and on high plateaux, in the desert and in the reeking forests of the equator. Nevertheless, the various races of Europe have each a certain natural habitat in which it achieves its highest development. THE NORDIC HABITAT The Nordics appear in their present centre of distribution, the basin of the Baltic, at the close of the Paleolithic, as soon as the retreating glaciers left habitable land. This race was probably at that time in possession of its fundamental characters, and its extension from the plains of Russia to Scandinavia was not in the nature of a radical change of environment. The race in consequence is now, always has been and probably always will be, adjusted to certain environmental conditions, chief of which is protection from a tropical sun. The actinic rays of the sun at the same latitude are uniform in strength the world over and continuous sunlight affects adversely the delicate nervous organization of the Nordics. The fogs and long winter nights of the North serve as a protection from too much sun and from its too direct rays. Scarcely less important is the presence of a large amount of moisture but above all a constant variety of temperature is needed. Sharp contrast between night and day temperature and between summer and winter are necessary to maintain the vigor of the Nordic race at a high pitch. Uniform weather, if long continued, lessens its energy. Too great extremes as in midwinter or midsummer in parts of New England are injurious. Limited but constant alternations of heat and cold, of moisture and dryness, of sun and clouds, of calm and cyclonic storms offer the ideal surroundings. Where the environment is too soft and luxurious and no strife is required for survival, not only are weak strains and individuals allowed to survive and encouraged to breed but the strong types also grow fat mentally and physically, like overfed Indians on reservations or wingless birds on oceanic islands, which have lost the power of flight as a result of prolonged protective conditions. Men of the Nordic race may not enjoy the fogs and snows of the North, the endless changes of weather and the violent fluctuations of the thermometer and they may seek the sunny southern isles, but under the former conditions they flourish, do their work and raise their families. In the south they grow listless and cease to breed.