How the Eye Evolved (The New York Academy of Sciences)
"If you look at the different sort of eyes out in the natural world and consider the ways in which they could have evolved, Darwin realized, the absurdity disappears. The objection that the human eye couldn't possibly have evolved, he wrote, 'can hardly be considered real.'"
Diverse eyes (Berkeley)
"The human eye, on the other hand, relies on more machinery. We have a lens and cornea, which help focus light, and a mobile iris, which controls the amount of light entering the eye much like a shutter on a camera. The many light-sensing cells at the back of our eyeballs form a structure called the retina, which collects information that the brain can assemble into an image. Many different animals have this sort of single-lens, image-forming eye."