From the eighteenth century to the twentieth century, American literature has recorded America's perception of nature as it has shifted from nature an untamed wilderness to nature a conquered and disappearing entity. In the eighteenth century, writers like William Bartram wrote of the newly founded America, which existed on the edge of a vastly untamed frontier that needed cultivating to be useful for economic growth. As a result, by the end of the eighteenth century, the untamed frontier was pushed further west as settlers subdued and conquered it. In the nineteenth century, writers like Henry David Thoreau began to record the usefulness of nature beyond economic pursuits. Nature became a place of spiritual renewal and an escape from the toils of industrialization. Nature, however, also continued to be exploited for its natural resources as the capitalistic endeavors of the American economy cultivated more of the land. Twentieth century writers such as Aldo Leopold recorded how the wilderness was disappearing and America's natural habitat was in danger of being eradicated by poor stewardship and over-consumption by society. Individuals could no longer achieve spiritual renewal through nature due to the disappearance of the natural wilderness to economic pursuits. Even the so-called protected wildernesses of the national parks were being raped and conquered as motels and paved roads violated the wildness of the wilderness and made it a commodity instead for entrepreneurs to make capital gain while providing a "natural" experience for their clients. Nature writers have recorded this shift from nature as an untamed wilderness to a conquered land in danger of disappearing. The evolution of this shift can be traced from the eighteenth century's sincere awe of the untamed wilderness and its economic possibilities to the twentieth century's desperate attempts to protect what is left of the disappearing wilderness from the hands of human destruction. Three writers whose writings portray this shift are William Bartram from the eighteenth century, Henry David Thoreau from the nineteenth century, and Aldo Leopold from the twentieth century.