Vanity Fair (Sept 30, 1871)
The colorful panoply of feathers in a peacock’s tail are a challenge to the fundamental principles of the Darwinian theory of natural selection. These feathers are expensive to grow, requiring large amounts of energy which the peacock can deploy more fruitfully elsewhere. Encumbered with the weight of the feathers, the peacock moves at a much slower speed, and the dazzling design on the feathers makes it visible to a range of predators, thus vastly reducing its chances of survival in the Asian habitats where it has evolved.
The process of natural selection ought to have freed the peacock from the colorful tail feathers; in fact, the peacock should never have had such a tail. Later on Darwin came up with a theory of sexual selection by which he tried to explain what his theory of natural selection could not. While natural selection is the “struggle for existence,” sexual selection is the “struggle for mates”. He asserted that the tail is there in a peacock because the peahen admires it.
In a letter to Alfred R. Wallace, (dated March 24, 1968), Darwin wrote: “In regard to sexual selection. A girl sees a handsome man and without observing whether his nose or his whiskers are the tenth of an inch longer or shorter than in some other man, admires his appearance and says she will marry him. So I suppose with the peahen; and the tail has been increased in length merely by on the whole presenting a more gorgeous appearance.”
However, this means that, according to Darwin, man is not the only creature with an aesthetic sense—other creatures too have a sense of beauty.