THE PERFECT EYE. “Despite a lack of molecular details available at the time, he offered a rational hypothesis to explain this seemingly impossible puzzle, ‘If numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist…then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.” (Darwin, 1859).
http://onlinelibrary.wiley.com/doi/10.1111/nph.12416/full