1. C. Darwin “Letter 2743 of Darwin correspondence project” (Cambridge University 1860). http://www.darwinproject.ac.uk/entry-2743.
2. C. Darwin, The Descent of Man, and Selection in Relation to Sex (John Murray, London, 1871).
3. Fossil evidence sheds light on sexual selection during the early evolution of birds
4. Lessons from nature, as manifested in mind and matter;Wallace A. R.;The Academy,1876
5. Darwin, sexual selection, and the brain