Environmental Philosophies: Toward Unity Among Environmentalists . Bryan G. Norton. Oxford University Press, New York, 1991. xvi, 287 pp. $29.95.; After Earth Day . Continuing the Conservation Effort. Max Oelschlager, Ed. University of North Texas Press, Denton, 1992 (distributor, Texas A&M University Press, College Station). xxi, 241 pp. $24.50; paper, $15.95. Philosophy and Ecology