Charles Darwin (1861). “On the Origin of Species by Means of Natural Selection; Or, The Preservation of Favoured Races in the Struggle for Life”, p.167
St. Augustine, Dr. Howard Taylor, Mrs. Howard Taylor, Apostolic Fathers, J. Oswald Sanders (2010). “Moody Classics Complete Set: Includes 19 Classics of the Faith in a Single Volume”, p.1326, Moody Publishers