Oscar Wilde, Russell Jackson, Ian Small (2000). “The Complete Works of Oscar Wilde: Poems and poems in prose”, p.153, Oxford University Press on Demand
Charles Darwin, R. C. Stauffer (1987). “Charles Darwin's Natural Selection: Being the Second Part of His Big Species Book Written from 1856 to 1858”, p.279, Cambridge University Press
"The Descent of Man". Book by Charles Darwin, volume I, chapter II: "Comparison of the Mental Powers of Man and the Lower Animals", p. 40, darwin-online.org.uk. 1871.