It is the summer migration of a heated belt from the south northward across the temperate zone, and the equally conspicuous winter advances of cold from the north southward across the same broad region, which gives to the United States and the southern portion of Canada a conspicuously changeable climate. The temperate zone, so far at least as North America is concerned, deserves its name only when the mean of the yearly changes in temperature is considered, as much of it is hotter in summer than equal areas between the tropics, and in winter over all of its northern half the cold is, at times, nearly or quite as intense as during the same season in the far north. As a whole, the portion of the continent embraced in the temperate zone is characterized by its pronounced seasonal changes, including wide extremes of heat and cold over large areas, and by its frequently sudden and strongly marked weather changes during short periods of time. It is a highly suggestive fact that of all the great climatic zones the one having the most changeable climate, the greatest extremes of heat and cold, and the most frequent storms should be the one in which man has reached the highest development both of body and mind. Evidently it is the _struggle_ for existence, when not too severe, which insures advancement. The part of North America most densely inhabited by descendants of Europeans, and the portion of the continent where intellectual development has made the greatest advance, is the east-central portion, where not only the variation of climate from season to season, but the weather changes from week to week and day to day are the most conspicuous.