It is difficult to imagine how only part of an adaptation could function, but Darwin explained this. How would he answer the question, “What good is 5% of an eye?”

a) Once an organism has the first 5% of an adaptation, the rest will quickly evolve.
b) Five percent of an eye is always better than a full eye since it is easier to grow. The difficulty is in explaining fully formed eyes.
c) Since variation is random, we don’t expect to see more than about 5% of an eye in any species.
d) Five percent of an eye, perhaps a simple light-sensitive spot, is often better than having no eye at all.

During the explanation of human eye formation, Darwin with his new theory faced several difficulties. As the human eye has several dependent parts like lens, pupil, retina, choroid etc and absence of any of these could impair vision, thus the natural selection or evolution of all of them as a whole is not explained by Darwin.

But Darwin explained the eye formation of humans as first the presence of light sensitive area on skin of some primitive animals which gave them advantages over the ones who lack them, and natural selection favored those species which had this spot.

Now some random changes led to the deepening of this spot which gave them sharper vision, then the aperture of this deepening decreased until a small hole, like a pinhole camera. Then the lens developed like a transparent fold of skin with fluid inside, which led to the formation of lens for accommodation. This is the 5% development rate which Darwin suggested.