For most nonhuman mammals the ocular refraction (the lens power required to form a clear retinal image of an object at infinity) for optimal distance vision tends strongly towards emmetropia, or perfect vision. Similarly, the variation of distance refraction and the presence of astigmatism is also lower than for humans. Primates brought up in captivity do tend to become myopic. Domestic animals may show similar defects when they get beyond the life expectancy in the wild, and old dogs or cats often go blind from cataracts. Replacing a pet dog’s lens is practically a routine operation nowadays. Our technology and ability to cooperate in societies allows us to overcome many of the things that kill other mammals. So relatively minor genetic defects such as poor eyesight tend to accumulate in our gene pool because natural selection cannot act against those individuals that have them. In recent times, anthropologists have noted that Inuit had almost universally perfect eyesight un til significant numbers of them became literate. It appears that literacy causes myopia; the constant switching between long and short distances appears to create myopia.

From On This Day


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