On The Parts Of Animals   

hairy animals. In these animals, then, the skin on the head is hard,
and so does not allow of the formation of an upper eyelid, whereas
lower down the integument is of a flesh-like character, so that the
lower lid can be thin and extensible.
The act of blinking is performed by the heavy-bodied birds by means of
the membrane already mentioned, and not by this lower lid. For in
blinking rapid motion is required, and such is the motion of this
membrane, whereas that of the lower lid is slow. It is from the
canthus that is nearest to the nostrils that the membrane comes. For
it is better to have one starting-point for nictitation than two; and
in these birds this starting-point is the junction of eye and
nostrils, an anterior starting-point being preferable to a lateral
one. Oviparous quadrupeds do not blink in like manner as the birds;
for, living as they do on the ground, they are free from the necessity
of having eyes of fluid consistency and of keen sight, whereas these
are essential requisites for birds, inasmuch as they have to use their
eyes at long distances. 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. 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.
Part 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. But in man, owing
to his upright attitude, the anterior and posterior surfaces of the
body are on an equality as regards need of protection. Nature
therefore has assigned the protective covering to the nobler of the
two surfaces; for invariably she brings about the best arrangement of
such as are possible. This then is the reason that there is no lower
eyelash in any quadruped; though in some a few scattered hairs sprout
out under the lower lid. This also is the reason that they never have