What good is half a wing?

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What good is half an eye?

A closely-related question is, "what good is half an eye?" But 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, actually probably much more than 50% better).


  • The human eye cannot be said to be a full eye in many respects:
    • The nerves and blood vessels that carry visual signals and blood from the retina to the brain are wired so that they go inside the eye. Where they exit the eye, they crowd out light receptors, resulting in a blind spot that the brain must fill in essentially with educated guesswork. Squid and octopus eyes are not wired this way, and therefore do not have a blind spot.
    • Humans see only three colors, not five as in some fish.
    • Humans cannot see ultraviolet like bees.
    • Humans cannot see infrared like pit vipers.
    • Humans have poor underwater vision unlike penguins.
    • Humans have poor night vision unlike owls.
    • Humans have poor ranged vision unlike birds of prey.
    • Humans cannot see magnetic fields unlike some migratory birds.
    • Humans cannot easily detect polarized light unlike ants.
    • Chameleons have independently-targetable eyes: each eye can look at, and focus on, a separate object. They can estimate the distance to each object by the way the lens focuses on that object.
    • Human retinas aren't properly attached to the eye and easily detach possibly causing blindness.

Eye evolution is well known and documented from a point of light sensitive cells, to a depression for the light sensitive cells, to a pin hole eye, to a lensed eye: each step conveys more and more use to an organism. So half an eye is useful, about half as useful.

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