George Washington, United States. President (1789-1797 : Washington) (1847). “Washington's Farewell Address to the People of the United States of America: Published in September, 1796”, p.11
W. E. B. Du Bois' statement of 1910, as quoted in Gregory Michael Dorr "Fighting Fire with Fire: African Americans and Hereditarian Thinking, 1900-1942" by Gregory Michael Dorr on Wake Forest University Homepage, March 13, 2006.