If now they live on land,
the air in which they move is transparent enough. But the water in
which fishes live is a hindrance to sharp sight, though it has this
advantage over the air, that it does not contain so many objects to
knock against the eyes. The risk of collision being thus small,
nature, who makes nothing in vain, has given no eyelids to fishes,
while to counterbalance the opacity of the water she has made their
eyes of fluid consistency.
14
All animals that have hairs on the body have lashes on the
eyelids; but birds and animals with scale-like plates, being hairless,
have none. The Libyan ostrich, indeed, forms an exception; for, though
a bird, it is furnished with eyelashes. This exception, however,
will be explained hereafter. Of hairy animals, man alone has lashes on
both lids. For in quadrupeds there is a greater abundance of hair on
the back than on the under side of the body; whereas in man the
contrary is the case, and the hair is more abundant on the front
surface than on the back. The reason for this is that hair is intended
to serve as a protection to its possessor. Now, in quadrupeds, owing
to their inclined attitude, the under or anterior surface does not
require so much protection as the back, and is therefore left
comparatively bald, in spite of its being the nobler of the two sides.
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