Paul has the rights of it! There is no organ which cannot be conceived of as a series of tiny changes, each improving the chances. The only constraint on outcome is that if it doesn't work, it tends to get un-selected through reduced survival or less successful reproduction.
Onepoint that often gets overlooked is that many things are not binary variables but continuous -- a tiny bit of light sensitivity is a tiny bit better than none, and each tiny bit of increase also better. The real world is full of gradient values and continuous variables which our slower minds try to stuff into black or white judgments.