This too explains why birds with talons,
that have to search for prey by eye from aloft, and therefore soar
to greater heights than other birds, are sharpsighted; while common
fowls and the like, that live on the ground and are not made for
flight, have no such keenness of vision. For there is nothing in their
mode of life which imperatively requires it.
Fishes and Insects and the hard-skinned Crustacea present certain
differences in their eyes, but so far resemble each other as that none
of them have eyelids. As for the hard-skinned Crustacea it is
utterly out of the question that they should have any; for an
eyelid, to be of use, requires the action of the skin to be rapid.
These animals then have no eyelids and, in default of this protection,
their eyes are hard, just as though the lid were attached to the
surface of the eye, and the animal saw through it. Inasmuch,
however, as such hardness must necessarily blunt the sharpness of
vision, nature has endowed the eyes of Insects, and still more those
of Crustacea, with mobility (just as she has given some quadrupeds
movable ears), in order that they may be able to turn to the light and
catch its rays, and so see more plainly. Fishes, however, have eyes of
a fluid consistency. For animals that move much about have to use
their vision at considerable distances.
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