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 descendants, 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?" 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, probably much more than 50% better if you consider an animal trying to survive in the wild among sighted predators).
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.
When compared to those of other animals, the human eye cannot be said to be a "full eye" in many respects:
- Human vision is based on sensing only three colors (and sometimes less), not five as in some birds and fish.
- Humans cannot see ultraviolet, as can bees.
- Humans cannot see infrared, as can pit vipers.
- Humans have poor underwater vision, unlike penguins.
- Humans have poor night vision, unlike owls.
- Humans have relatively poor distance vision, unlike birds of prey.
- Humans cannot see magnetic fields, as migratory birds seem to be able to do.
- Humans cannot detect polarized light unlike bees and ants (the ability is most developed in the so-called mantis shrimp).
- 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.
- Human eyes (along with all vertebrates) have a built-in blind spot where the nerves and blood vessels that carry visual signals and blood from the retina to the brain exit the eye, crowding out light receptors. The brain must fill in the missing information using essentially educated guesswork. Squid and octopus eyes are not wired this way and therefore do not have a blind spot.
- Humans cannot point their eyes in two different directions and focus on two different objects (and judge the distance to each object independently), as can chameleons.
- Human retinas aren't securely attached to the eye and easily detach possibly causing blindness.
The sequence of steps necessary to develop a sophisticated eye is well known and each step has been extensively documented in nature, from a simple patch of light-sensitive cells, to a depression for the light-sensitive cells (to sense the direction of the incoming light), to a "pin hole" eye (to focus an image), to a lensed eye (to protect the eye contents and allow more light in): each step conveys additional benefits to an organism. So "half an eye" can be useful. (For more detail, see Wikipedia:Evolution of the eye.)