line of vision, Nature has made its eyes able to conform to its limbs,
for its eyes can move themselves obliquely, and therefore after a
fashion crabs are no exception but in this sense move forwards.
Birds bend their legs in the same way as quadrupeds. For their
natural construction is broadly speaking nearly the same. That is,
in birds the wings are a substitute for the forelegs; and so they
are bent in the same way as the forelegs of a quadruped, since when
they move to progress the natural beginning of change is from the
wings (as in quadrupeds from the forelegs). Flight in fact is their
appropriate movement. And so if the wings be cut off a bird can
neither stand still nor go forwards.
Again, the bird though a biped is not erect, and has the forward
parts of the body lighter than the hind, and so it is necessary (or at
least preferable for the standing posture) to have the thigh so placed
below the body as it actually is, I mean growing towards the back.
If then it must have this situation the flexion of the leg must be
backwards, as in the hind legs of quadrupeds. The reasons are the same
as those given in the case of viviparous quadrupeds.
If now we survey generally birds and winged insects, and animals
which swim in a watery medium, all I mean that make their progress
in water by dint of organs of movement, it is not difficult to see
that it is better to have the attachment of the parts in question
oblique to the frame, exactly as in fact we see it to be both in birds
and insects. And this same arrangement obtains also among fishes.
Among birds the wings are attached obliquely; so are the fins in water
animals, and the feather-like wings of insects. In this way they
divide the air or water most quickly and with most force and so effect
their movement. For the hinder parts in this way would follow forwards
as they are carried along in the yielding medium, fish in the water,
birds in the air.