another hunting experience later in life that was formative for Leopold's later views. Here Leopold describes the death of a she-wolf killed by his party during a time when conservationists were operating under the assumption that elimination of top predators would make game plentiful. The essay provides a non-technical characterization of the trophic cascade where the removal of single species carries serious implications for the rest of the ecosystem. The book ends with a section of philosophical essays grouped together under the heading "The Upshot". Here Leopold explores ironies of conservation: in order to promote wider appreciation of wild nature and engender necessary political support, one encourages recreational usage of wilderness that ultimately destroys it. Musings on "trophies" contrasts the way that some need a physical specimen to prove their conquest into the wilderness, though photographs may be less damaging than a trophy head to be mounted on the wall. He suggests that the best trophy is the experience of wilderness itself, along with its character-building aspects. Leopold also rails against the way that policy makers need to find an economic motive for conservation. In the concluding essay, "The Land Ethic", Leopold delves into a more appropriate rationale for conservation. In "The Ecological Conscience" section, he wrote: "Conservation is a state of harmony between men and land." Leopold felt it was generally agreed that more conservation education was needed; however quantity and content were up for debate. He believed that land is not a commodity to be possessed; rather, humans must have mutual respect for Earth in order not to destroy it. He philosophizes that humans will cease to be free if they have no wild spaces in which to roam. Leopold's home, Aldo Leopold Shack and Farm, was listed on the U.S. National Register of Historic Places in 1978, and was uplisted to National Historic Landmark status in 2009. Importance and influence In a 1990 poll of the membership by the American Nature Study Society, A Sand County Almanac and Rachel Carson's Silent Spring stand alone as the two most venerated and significant environmental books of the 20th century. The book was little noticed when published but, during the environmental awakening of the 1970s, a paperback edition turned into a surprise bestseller. It still sells about 40,000 copies a year. The book has had immense popular influence and has been described as: "one of the benchmark titles of the