Darwin said:
“If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely breakdown"
Darwin thought about the eye a lot. He came up with a scenario illustrating how the eye socket could have formed. The diagram shows on the left side a light sensitive spot (purple) on the skin of an animal. You can see that as oblique light rays shine on the spot, all areas of the spot are struck equally consequently the spot has no way to determine the direction of the light. But suppose the animal's light sensitive spot began to mutate into a cavity. Then the angled light would not strike all parts of the light sensitive spot. One side would be in shadow. This way the spot could deliver enough information to the animal's brain such that it could interpolate which direction the light is coming from. While the light sensitive spot capable of sensing light and darkness is beneficial, this directional capability would be even more so. Darwin then imagined the cavity getting larger, forming into a socket, eventually capturing a gel like substance which eventually becomes an eyeball complete with a lens. Now I know I went fast on that last one but that was left up to our imagination to figure how that happened.
Also notice how Darwin started with a light sensitive spot. Easy to say but would it be easy to do with random mutations and natural selection? Take a look at the five minute video of how a photon of light is captured by a rod in the retina of the eye and turned into a nerve signal to the brain.
Some light sensitive spot, huh? Have we sufficiently demonstrated such a complex organ for Darwin's challenge? Well of course Darwin did know know what we know today but he did say:
“To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection seems, I freely confess, absurd in the highest degree.”
If we stopped there, you might think that Darwin was stumped. But he is really just setting up a strawman to knock down so that he can praise his own theory. So Darwin does not admit he is stumped; he goes on to say:
“In living bodies, variations will cause the slight modifications, generation will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. Let this process go on for millions of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass as the works of the Creator are to those of man?"
"May we not believe...", that's pretty bold. If only he could see how the eye really works. Oh yeah, that's right, now he knows the truth.