Nice little piece at Scientific American about the evolution of odd-ball structures like the human eyeball:
Charles Darwin was not yet 30 when he got the basic idea for the theory of evolution. But it wasn’t until he turned 50 that he presented his argument to the world. He spent those two decades methodically compiling evidence for his theory and coming up with responses to every skeptical counterargument he could think of. And the counterargument he anticipated most of all was that the gradual evolutionary process he envisioned could not produce certain complex structures.
Consider the human eye. It is made up of many parts—a retina, a lens, muscles, jelly, and so on—all of which must interact for sight to occur. Damage one part—detach the retina, for instance—and blindness can follow. In fact, the eye functions only if the parts are of the right size and shape to work with one another. If Darwin was right, then the complex eye had evolved from simple precursors. In On the Origin of Species, Darwin wrote that this idea “seems, I freely confess, absurd in the highest possible degree.”