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
Manly P. Hall (2010). “The Secret Teachings of All Ages: An Encyclopedic Outline of Masonic, Hermetic, Qabbalistic and Rosicrucian Symbolical Philosophy”, p.92, Courier Corporation