[Why] do so many things come paired?--eyes, ears, nostrils, arms, legs, hands, feet, breasts, lungs, kidneys, lobes of the brain, wings on birds. Natural selection’s answer: “Why animals with two eys survived better than those with one.” But if it is just a survival matter, why stop at two? Why not three eyes, or eight?
These organs are not only paired, but are mirror images of each other. Symmetry is aesthetic; artists use it to convey beauty. It speaks for design more than chance and mutation.
James Perloff, Tornado in a Junkyard, Refuge Books 2002, p. 57