Various Creationist books have a quote from The Origin of Species(1859):
To suppose that the eye [...] could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.This implies that Darwin didn't have faith in his own theory. Or, perhaps it implies that Darwin saw a hole in his theory, and didn't know how to solve it. But let's look at the next two sentences:
When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory.So Darwin was not in fact despairing. He was asking a rhetorical question, and then he started into a convincing answer.
The Origin of Species, Chapter Six, "Organs of extreme Perfection and Complication"
Today, a century-odd later, there is a highly developed theory of how an eye would have evolved. Many popular-science books, such as those of Richard Dawkins, explore the subject.