1. Bennett, Charles H., and Robert B. Brough. 1860. Shadow and substance. London: W. Kent.
2. Carlyle, Thomas. 1849. Occasional discourse on the negro question. Fraser’s Magazine for Town and Country 40: 670–679.
3. Darwin, Charles. 1871. The descent of man, and selection in relation to sex. New York: Appleton and Company.
4. Edgeworth, F.Y. 1881. Mathematical psychics. London: C. Kegan Paul & Co.
5. Greg, W.R. 1866. The Jamaica problem. Fraser’s Magazine for Town and Country 73: 277–305.