I've never seen a gorilla with glasses, a chameleon with contacts, or a moose（[动] 驼鹿） with a monocle, although I did see a spectacle（景象） bear once. But, like seven out of ten Americans, my eyes don't bend light correctly, so I've worn glasses since fifth grade. It's a confusing question. How did our species survive the trials（试验） of natural selection（选择） if so many of us can't see? We can't take otters（水獭） or ostriches（鸵鸟） to the ophthalmologist, so it's hard to know exactly how common bad eyesight is in the animal kingdom（王国）. But if you can smell, hear, or feel your way through life, good eyes are expendable（消耗品）.

  Many species that rely（依赖） on seeing sacrifice（牺牲） clear vision（视） all over to see extra sharp where natural selection（选择） demands. For most visual（视觉的） species, you're more likely to become a meal before you pass on your blurry genes（[遗] 基因）, so they vanish（突然不见） from the population. But modern humans live a pretty comfortable life, so natural selection has let us slip（滑动） through with bad eyesight. The lens（透镜） inside your eye isn't rigid, more like really firm（ 坚固的） jello. It's made from flexible（柔韧性） proteins（蛋白质） called crystalline（ 水晶的）. They let visible（可见的） wavelengths（[物] 波长） pass through, but the lens's rounded shape bends the light.

  Refraction, just like what happens where air meets water. We can't focus by moving our lens closer or farther from the retina（[解剖] 视网膜）, the way a camera would. We actually squish and pull our lens to change how it bends light. The objects reflect light in all directions. The farther away an object is, the angles（角度） between the light rays（射线） entering our eyes are smaller, and our lens doesn't need to bend them as much, so tiny muscles relax and flatten（击败） it.