In the course of the 20th century the human race, without intending anything of the sort, undertook a giant, uncontrolled experiment on the earth. In time, according to John McNeill in his new book, the environmental dimension of 20th century history will overshadow the importance of its world wars, the rise and fall of communism, and the spread of mass literacy. Contrary to the wisdom of Ecclesiastes that there is nothing new under the sun, McNeill sets out to show that the massive change we have wrought in our physical world has indeed created something new. To a degree unprecedented in human history, we have refashioned the earth's air, water and soil, and the biosphere of which we are a part.