Columbia subject headings:
Darwin, Charles, 1809-1882.
Marx, Karl, 1818-1883.
Wagner, Richard, 1813-1883.
Harvard subject headings:
Darwin, Charles, 1809-1882
Marx, Karl, 1818-1883
Wagner, Richard, 1813-1883
Princeton subject headings:
Darwin, Charles, 1809-1882.
Marx, Karl, 1818-1883.
Wagner, Richard, 1813-1883.