As frequent
inflammation of the eyes must be injurious to any animal, and as eyes
are certainly not indispensable to animals with 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 constantly aid the effects of disuse.
It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Styria and of 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. As it is difficult to
imagine that eyes, though useless, could be in any way injurious to
animals living in darkness, I attribute their loss wholly to disuse.
In one of the blind animals, namely, the cave-rat, the eyes are of
immense size; and Professor Silliman thought that it regained, after
living some days in the light, some slight power of vision. In the
same manner as in Madeira the wings of some of the insects have been
enlarged, and the wings of others have been reduced by natural
selection aided by use and disuse, so in the case of the cave-rat
natural selection seems to have struggled with the loss of light and
to have increased the size of the eyes; whereas with all the other
inhabitants of the caves, disuse by itself seems to have done its
work.
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