Male peacock, Pavo cristatus, displaying his ocellated tail feathers. The peacock's tail, cumbersome and expensive to produce, required an explanation under Darwin's theory of natural selection. He once wrote to Asa Gray in 1860 \the sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!\". Darwin addressed the peacock in his 1871 \"Descent of Man and Selection in Relation to Sex\". The theory of sexual selection proposes that, when females have a choice of mates, the males compete between each other to be the most attractive. Females may choose tails as a way to tell the health of a male, and in later evolution, simply as an indicator of a male who is likely to be attractive to other females (and sire young with the same qualitities). Wallace disagreed. The degree of runaway selection possible in such secondary sexual characters is a matter of debate."