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.
Of oviparous quadrupeds all those that live in holes, like
crocodiles, lizards, spotted lizards, freshwater tortoises, and
turtles, have their legs attached obliquely as their whole body
sprawls over the ground, and bend them obliquely.
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