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If men were to colonize the moon or Mars—even with abundant supplies of oxygen, water, and food, as well as adequate protection against heat, cold, and radiation—they would not long retain their humanness, because they would be deprived of those stimuli which only Earth can provide. Similarly, we shall progressively lose our humanness even on Earth if we continue to pour filth into the atmosphere; to befoul soil, lakes, and rivers; to disfigure landscapes with junkpiles; to destroy wild plants and animals that do not contribute to monetary values; and thus transform the globe into an environment alien to our evolutionary past. The quality of human life is inextricably interwoven with the kinds and variety of stimuli man receives from the Earth and the life it harbors, because human nature is shaped biologically and mentally by external nature. (Rene Dubos qtd. in Kaltreider)
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