In addition, examples can be found of human differences in tolerance to climatic conditions through prolonged acclimatization. Among them are the ability of persons native to Tierra del Fuego to sleep in freezing temperatures, and of persons native to the high Andean mountains to live with half the oxygen values found at sea level. Prolonged genetic selection appears to have developed functional differences, such as the greater number of sweat glands in peoples native to tropical regions, and the higher blood flow and surface temperature in those native to Arctic regions. Many common diseases also show a relationship to climatic variations, but a direct cause-and-effect correlation is difficult to establish. For one, it is hard to differentiate weather-induced illnesses from other illnesses; for two, great changes have taken place in sanitation and health education through time.
Climate and Human Culture
Writers such as Friedrich Ratzel and Ellen Semple attempted to draw up scientific laws to explain differences in human culture and development. Perhaps the best-known writer in the field of climatic determinism was Ellsworth Huntington, who sought to describe the role of climate in determining racial character, religion, and the rise and fall of civilization. While his theories have been severely criticized as ethnocentric overgeneralizations that were based on limited data and that ignored contradictory evidence, theories such as the one that climatic changes affect migration patterns and cultural developments remain as working principles.