When it comes to an organ of such complexity as the eye, it is not difficult to understand why some people cannot accept that such perfection was arrived at by the trial and error, or gradual development, of natural selection. Yet people thought the Earth stood still until Copernicus told them otherwise. In the same way, it shouldn't be hard to believe that a complex eye could be formed by natural selection if it can be shown that there were numerous stages from a simple and imperfect eye to a complex and perfect one, with each development being useful to its possessor and the variations being inherited.
However, the search for the stages through which an organ in any one species has come to perfection, which ideally would mean looking exclusively at its past generations, is rarely possible. Therefore, researchers are forced to examine species and genera of the same group to discover what stages or gradual developments are possible. Even the state of development of the same organ in a different class of creature may throw light on the steps taken towards perfection.
Some people object that in order for the eye to modify and still remain a useful instrument to its owner, many changes would have had to take place simultaneously. However, it is not necessary to suppose this if the modifications were extremely slight and gradual.