What good is half a wing?
The question, "What good is half a wing?" is often used by creationists who severely misunderstand evolutionary theory. The question implies that fossil birds should be discovered with literal "half-wings" — i.e., a wing missing half of itself. This is a misunderstanding of how wings likely evolved.
According to the modern theory, evolution is not oriented toward a final goal but merely involves small changes that aid in the survival of individuals to the age of reproduction. In particular, evolution in the ancestor of modern birds was not working toward full-blown modern wings as we know them today. The arms of, say, small dinosaurs might have developed small proto-feathers in order to help them stay in the air longer when they jumped to avoid predators or to pursue prey. This wasn't yet a wing, but it ostensibly had some small benefit to the animal. Over millennia the feathers may have grown longer, the arms leaner and the muscles stronger. Today their ancestors, modern birds, do have wings as we know them.
The "half a wing" argument is related to the idea that evolution is like a tornado moving through a junkyard and assembling a fully-functional 747.
What good is half an eye?
A closely-related question is, "what good is half an eye?" But we all know people who are nearsighted or farsighted, and who get along quite well. Even without glasses, it is better to be nearsighted than to have cataracts, which in turn is better than being blind. "Half an eye" or 50% of vision is 50% better than being blind (well, actually probably much more than 50% better).
Another way to look at it is to ask what constitutes 100% of an eye, the answer to which can shed light on what 50% of an eye is, and therefore what good it is.
- The human eye cannot be said to be a full eye in many respects:
- The nerves and blood vessels that carry visual signals and blood from the retina to the brain are wired so that they go inside the eye. Where they exit the eye, they crowd out light receptors, resulting in a blind spot that the brain must fill in essentially with educated guesswork. Squid and octopus eyes are not wired this way, and therefore do not have a blind spot.
- Humans see only three colors, not five as in some fish.
- Humans cannot see ultraviolet like bees.
- Humans cannot see infrared like pit vipers.
- Humans have poor underwater vision unlike penguins.
- Humans have poor night vision unlike owls.
- Humans have poor ranged vision unlike birds of prey.
- Humans cannot see magnetic fields unlike some migratory birds.
- Humans cannot easily detect polarized light unlike ants.
- Human eyes have a fovea, an area densely packed with light receptors, which we use for distinguishing colors and for resolving fine detail (this is why we can't read a book out of the corner of our eye: there aren't enough receptors to make out the shapes of the letters). But some birds of prey, such has hawks, have two foveas, which allows them to resolve detail in two areas at once.
- Chameleons have independently-targetable eyes: each eye can look at, and focus on, a separate object. They can estimate the distance to each object by the way the lens focuses on that object.
- Human retinas aren't properly attached to the eye and easily detach possibly causing blindness.
Eye evolution is well known and documented from a point of light sensitive cells, to a depression for the light sensitive cells, to a pin hole eye, to a lensed eye: each step conveys more and more use to an organism. So half an eye is useful, about half as useful.