There have been many stages in our evolution that contributed to our ability to fly. For example, we had to develop delicate wings, strong chest muscles, and several different kinds of feathers. Humans have long admired and envied our ability to fly. They have experimented and invented all kinds of means to duplicate what we do naturally in the sky. Since we birds are often far above the ground, we needed a number of adaptations to give us great eyesight. First, we have the largest eyes in the animal kingdom, relative to body size, which helps us to have very keen vision. From the outside, our eyes may appear small, but on the inside our eyes are so big that most of us cannot move them. Among warm-blooded animals, our ability to regenerate retinal neurons is outstanding. Scientists are trying to understand how it works, and believe there may be a way to unlock this ability in humans. If so, it would open the door to treatments for a wide variety of eye diseases.