Evolution of eye

"Likewise, there was no need for an eye to pop out of an animal’s head all at once. Invertebrates such as flatworms have nothing more than nerves with endings coated in light-sensitive pigments. Some crustaceans have eyes that consist of little more than a layer of pigment coated by a membrane. Over time, this membrane could separate from the pigment and begin to act like a crude lens. With small alterations, such an eye could turn into the precise telescopes that birds and mammals use. Because a little eyesight is better than none at all , each new step along the way would be rewarded by natural selection."