Carl Zimmer writes in his blog, The Loom:
The eye has always had a special place in the study of evolution, and Darwin had a lot to do with that. He believed that natural selection could produce the complexity of nature, and to a nineteenth century naturalist, nothing seemed as complex as an eye, with its lens, cornea, retina, and other parts working together so exquisitely.The notion that natural selection could produce such an organ “seems, I freely confess, absurd in the highest possible degree,” Darwin wrote in the Origin of Species.
For Darwin, the key word in that line was seems. He realized that if you look at the different sort of eyes out in the natural world, and consider the ways in which they could have evolved, the absurdity disappears. The objection that the human eye couldn’t possibly have evolved, he wrote, “can hardly be considered real.”
The more scientists study the eye, the more they recognize that Darwin was right.
More here.