As with mariners shipwrecked near a coast, it
would have been better for the good swimmers if they had been able to swim
still further, whereas it would have been better for the bad swimmers if
they had not been able to swim at all and had stuck to the wreck.
The eyes of moles and of some burrowing rodents are rudimentary in size,
and in some cases are quite covered by skin and fur. 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.
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