Aric2000: Yo, PH, where is that great explanatory graphic, I've seen it around here someplace. Time for a repost!!
Is this what your refering to?
Source: http://www.abarnett.demon.co.uk/atheism/evolution.html
The green represents the creature that is evolving, the pink represents light-sensitive cells. Imagine these changes happening over a period of two million years (which is a microscopic fraction of the history of the Earth), each generation of creatures only changing by a fraction of a percent).
A: The creature is blind. This has obvious disadvantages as it cannot see predators approaching, and has to rely on sound, smell etc.
B: A random mutation has given this creature a patch of light-sensitive cells (not a problem - see below). It can detect light and dark. A sudden change of light to dark could indicate a predator approaching, allowing the creature to defend itself (by fleeing, fighting etc.) and dramatically increase it's chances of surviving and reproducing. Which is more likely to survive long enough to reproduce - a creature that runs when a shadow passes over it, or one that stands still? Your skin is covered in cells/nerves that detect heat, pressure, taste, smell and so on. Light is not that much different (see below).
C: Two patches, one either side of the head have developed (no surprise, as symettrical mutations are very common). The creatures can now determine which side the shadow is approaching from, and run in the opposite direction (or it may distinguish open spaces from dark shelter, for instance). Again, a huge improvement in their chances of survival from a fairly small change in their body. Slugs and snails see like this (admittedly, they can't run very well, but who wants to eat a slug anyway?).
D: If the patch of cells becomes hollow, cup-shaped, it gives the ability to better determine the direction of light (a dome would work just as well, but would be easier to damage. Also, a hollow would help create greater contrast with well-defined shadows). One side of the cup will be better-lit than the other. This creature can therefore better determine the direction that a shadow is approaching from (or, again, find a dark shelter more easily).
E: The hollow deepens over time and starts to close in on itself. A photographer would recognise this a pin-hole camera. It will form a reasonable image on the cells (retina), allowing the creature to see shapes, not just differentiate between light and dark. The Nautilus (a marine mollusc) has eyes exactly like this.
F: A transparent membrane covers the pin-hole, forming a crude lens (alternatively, the eye may be filled with a transparent jelly). This will make the images formed on the retina much sharper as well as protecting the delicate surface from dirt and infections. The creature can see predators/prey much more clearly now.
G: Muscles around the lens develop, allowing the creature to alter the shape of the lens and change focus. Now it can clearly see objects close by or far away. This is how the eyes of most mammals (such as humans) function. The Chameleon's eyes are quite different - instead of changing the shape of the lens, muscles move the lens backwards and forwards to focus the image, in the same way that an auto-focus camera works.
Further incremental refinements include the iris (to restrict the amount of light), eyelids (to protect and clean the surface of the eye) and muscles to rotate the eyes.
This is not the way all eyes develop - there are thousands of different styles of eye in the world, all doing a similar job in different ways. Eyes do not develop quickly, but over thousands of generations (on the geological timescale, this is still just the blink of an eye. Ho ho!). To say that "half an eye is no use" is wrong. Half an eye (eg. example D) is much better than no eye at all.
Many creationists would say "But how did those first light-sensitive cells appear?". Well, your entire body is covered with light-sensitive cells. Your skin can detect heat radiation, can it not? What is this radiation? Infra-red light! It is easy to see how small mutations could lead infra-red sensitive cells to become more sensitive to shorter wavelengths of light, ie. "visible" light. Also, photons of certain wavelengths are absorbed by certain pigments/chemicals, affecting the chemistry of the cell in a manner that the brain may detect.
Remember, the creatures do not consciously attempt to grow eyes, and evolution does not drive them towards having any particular type of eye. Each new, tiny, random change will either increase or decrease that individual's chances of surviving to reproduce. Obviously, the ones that do manage to reproduce will pass on those characteristics that enabled them to do so.
Our eyes are allright, and serve us well, but there are creatures with far more complex optical systems than the human eye. Ours may evolve a bit more, but they do their job well enough for survival, which is what it's all about.
As an aside, it should be noted that the human eye is still quite badly "designed":
Your eye has a blind spot caused by the blood vessels that cover the surface of the retina converging on one point to exit the eye. If they were behind the retina it would be much better.
The optic cells (called rods and cones) are the wrong way round (the blood vessels come out into the eye rather than go out the back.
The number of people wearing glasses gives an idea of how imperfect human eyes are.
(Personally, I'd like a good zoom-lens and the ability to choose the wavelengths I can perceive. A God could have done a better job of it, but we're stuck with crappy old natural evolution. Oh well...)