Columbia subject headings:
Nature--Social aspects--United States.
Nature.
Technology--Social aspects--United States.
United States--Civilization.
Harvard subject headings:
Emerson, Ralph Waldo, 1803-1882--Criticism and interpretation.
Melville, Herman, 1819-1891--Criticism and interpretation.
Nature.
Thoreau, Henry David, 1817-1862--Criticism and interpretation.
United States--Civilization.
Princeton subject headings:
Nature.
United States--Civilization.