10. Fr. Darwin og A.C.Seward (red), More letters of Ch. Darwin (New York, Appleton, 1903) Vol.I, s321.

Fr. Darwin, The life and letters of Ch. Darwin (New York, Appleton, 1887) Vol. I, s278-279, Vol.II, s105-106

J. Wells, "Ch. Darwin and the Teleology of Evolution," International Journal on the Unity of Sciences 4 (Summer 1991): s133-156