"Beauty and nature often shows up in the vicinity of sex—think of the plumage of birds or mating rituals through out the animal kingdom. “Sexual selection”—that is, evolution’s favoring of features that increase a plant’s or animal’s attractiveness and therefore it's reproductive success—is the best explanation we have for the otherwise senseless extravagance of feathers and flowers, maybe also sports cars and bikinis. In nature, at least, the expense of beauty is usually paid for by sex. There may or may not be a correlation between the beautiful and the good, but there probably is one between beauty and health. (Which, I suppose, in Darwinian terms, is the good.) Evolutionary biologists believe that in many creatures beauty is a reliable indicator of health, and therefore a perfectly sensible way to choose one mate over another. Gorgeous plumage, lustrous hair, symmetrical features are “certificates of health,” as one scientist puts it, advertisements that a creature carries genes for resistance to parasites and is not otherwise under stress. A fabulous tail is a metabolic extravagance only the healthy can afford."