More difficult for Darwin were highly evolved and complicated features that conveyed apparently no adaptive advantage to the organism. Writing to colleague Asa Gray in 1860, Darwin commented that he remembered well a "time when the thought of the eye made me cold all over, but I have got over this stage of the complaint, & now small trifling particulars of structure often make me very uncomfortable. The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!" Why should a bird like the peacock develop such an elaborate tail, which seemed at best to be a hindrance in its "struggle for existence"? To answer the question, Darwin had introduced in the "Origin" the theory of sexual selection, which outlined how different characteristics could be selected for if they conveyed a "reproductive" advantage to the individual. In this theory, male animals in particular showed heritable features acquired by sexual selection, such as "weapons" with which to fight over females with other males, or beautiful plumage with which to woo the female animals. Much of "Descent" is devoted to providing evidence for sexual selection in nature, which he also ties into the development of aesthetic instincts in human beings, as well as the differences in coloration between the human races.