The official returns of the Registrar-General record every year a proportion of deaths of infants which are entered as due to “congenital defects.” These defects are of many and various kinds. A frequent “congenital defect” is one which does not necessarily cause death, namely, the imperfection of the wonderfully elaborate organ of sight, rendering it useless. Children often are born blind, and so are a certain proportion of each species of animal normally provided with eyes—dogs, horses, cattle, birds, fish. Even insects, lobsters, and crabs are in quite considerable and definite number born blind. The inborn or congenital defects of the eyes which result in blindness are of several kinds. The whole of the eyeball or eye-structure may be atrophied, that is to say, dwindled and incomplete, or one essential part only may be defective, and the rest quite well-formed; or, again, the nerves connecting the eye with the brain or the several parts of the brain concerned in the function of sight may, one or more of them, be defective. Wild animals born in this condition must perish, except when they happen to be born in caverns or in the deep sea. Then they are no worse off than animals with “good” eyes. But the animals with good eyes, or even with only somewhat defective eyes, will follow up the gleams of light which in moving about they have the fortune to encounter, and so will escape from the cavern or dark depths of the sea, leaving behind in the dark only those with defective eyes. These will breed together, and perpetuate their defective eyes in a more and more marked degree in successive generations. Always those which can see, or see only a little, will leave the dark place, and so at last there will be a race of animals established in such places with defective eyes (as in the ocean, at a depth of two thousand fathoms, and in the great caves of Europe, and notably in the Mammoth Cave of Kentucky, U.S.A.), and often the eyes will altogether disappear. This, however, is a digression.