From the journal Archives of Ophthalmology (Sept. 2010):
Charles Darwin was curious about the workings of the eye and corresponded with William Bowman and Cornelis Donders about its structure and function. In his On the Origin of Species in 1859, he took care to make the case that even such a complex organ as the eye could arise by natural selection. In other contexts, Darwin also gave particular weight to observations that revealed imperfections in development of organic features. It was exactly this line of inquiry that was so telling against the creationist credo of supernatural design. Darwin made a point of this in the second edition of his The Descent of Man, and Selection in Relation to Sex (1874) in which he made special reference to the views of Hermann Helmholtz about the imperfections of the eye.