Darwin, troubled even with his 19th century level of knowledge and technology, made the following admissions:
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. ii
The eye to this day gives me a cold shudder, but when I think of the fine known gradations, my reason tells me I ought to conquer the cold shudder. iii
The recur to the eye. I really think it would have been dishonest, not to have faced the difficulty. iv
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 break down. v
But it must be gradual when it is being used to explain the coming into existence of complicated, APPARENTLY DESIGNED OBJECTS, like eyes. For if it is not gradual in these cases, IT CEASES TO HAVE ANY EXPLANATORY POWER AT ALL. Without gradualness in these cases, we are back to miracle, which is simply a synonym for the total absence of explanation.iv