The theme of Darwin's struggle between reason and imagination is perhaps most clearly shown in his efforts in the Origin to conceive of how the eye of the body might have developed through natural selection. Thus he wrote:
To arrive, however, at a just conclusion regarding the formation of the eye, with all its marvellous yet not absolutely perfect characters, it is indispensable that the reason * should conquer the imagination; but I have felt the difficulty far too keenly to be surprised at others hesitating to extend the principle of natural selection to so startling a length.