This state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection.

In South America, a burrowing rodent, the tuco-tuco, or Ctenomys, is even more subterranean in its habits than the mole; and I was assured by a Spaniard, who had often caught them, that they were frequently blind.

One which I kept alive was certainly in this condition, the cause, as appeared on dissection, having been inflammation of the nictitating membrane.

As frequent inflammation of the eyes must be injurious to any animal, and as eyes are certainly not necessary to animals having subterranean habits, a reduction in their size, with the adhesion of the eyelids and growth of fur over them, might in such case be an advantage; and if so, natural selection would aid the effects of disuse.
It is well known that several animals, belonging to the most different classes, which inhabit the caves of Carniola and Kentucky, are blind.

In some of the crabs the foot-stalk for the eye remains, though the eye is gone; the stand for the telescope is there, though the telescope with its glasses has been lost.