Aldo Leopold, a forest ranger, hunter, and ecologist, argued for protecting wild lands and defined wilderness as an area still wild enough and large enough to travel on horseback for two weeks without crossing roads or seeing other signs of human development. He concluded that the Gila Forest met that standard, and in 1924 he persuaded the Forest Service to set aside 755,000 acres of the Gila so future generations could have that experience. This became the country's first designated wilderness. Leopold worked for the Forest Service in Arizona and New Mexico between 1909 and 1924, and he was also a philosopher, writer, and teacher whose 1949 book A Sand County Almanac became one of the most widely read and respected books about the natural world. The Wilderness Act of 1964 later defined wilderness as a place where the land is affected primarily by the forces of nature and where humans are only visitors, entered on foot or by horse or mule.