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the chafer； and to unsharded； like bees and wasps。 Further； birds that
are not made for flight have a tail that is of no use； for instance
the purple coot and the heron and all water…fowl。 These fly stretching
out their feet as a substitute for a tail； and use their legs
instead of a tail to direct their flight。 The flight of insects is
slow and frail because the character of their feathery wings is not
proportionate to the bulk of their body； this is heavy； their wings
small and frail； and so the flight they use is like a cargo boat
attempting to make its voyage with oars； now the frailty both of the
actual wings and of the outgrowths upon them contributes in a
measure to the flight described。 Among birds； the peacock's tail is at
one time useless because of its size； at another because it is shed。
But birds are in general at the opposite pole to flying insects as
regards their feathers； but especially the swiftest flyers among them。
（These are the birds with curved talons； for swiftness of wing is
useful to their mode of life。） The rest of their bodily structure is
in harmony with their peculiar movement； the small head； the slight
neck； the strong and acute breastbone （acute like the prow of a
clipper…built vessel； so as to be well…girt； and strong by dint of its
mass of flesh）； in order to be able to push away the air that beats
against it； and that easily and without exhaustion。 The hind…quarters；
too； are light and taper again； in order to conform to the movement of
the front and not by their breadth to suck the air。
11
So much then for these questions。 But why an animal that is to stand
erect must necessarily be not only a biped； but must also have the
superior parts of the body lighter； and those that lie under these
heavier； is plain。 Only if situated like this could it possibly
carry itself easily。 And so man； the only erect animal； has legs
longer and stouter relatively to the upper parts of his body than
any other animal with legs。 What we observe in children also is
evidence of this。 Children cannot walk erect because they are always
dwarf…like； the upper parts of their bodies being longer and stouter
than the lower。 With advancing years the lower increase
disproportionately； until the children get their appropriate size； and
then and not till then they succeed in walking erect。 Birds are
hunchbacked yet stand on two legs because their weight is set back；
after the principle of horses fashioned in bronze with their
forelegs prancing。 But their being bipeds and able to stand is above
all due to their having the hip…bone shaped like a thigh； and so large
that it looks as if they had two thighs； one in the leg before the
knee…joint； the other joining his part to the fundament。 Really this
is not a thigh but a hip； and if it were not so large the bird could
not be a biped。 As in a man or a quadruped； the thigh and the rest
of the leg would be attached immediately to quite a small hip；
consequently the whole body would be tilted forward。 As it is；
however； the hip is long and extends right along to the middle of
the belly； so that the legs are attached at that point and carry as
supports the whole frame。 It is also evident from these considerations
that a bird cannot possibly be erect in the sense in which man is。 For
as it holds its body now the wings are naturally useful to it； but
if it were erect they would be as useless as the wings of Cupids we
see in pictures。 It must have been clear as soon as we spoke that
the form of no human nor any similar being permits of wings； not
only because it would； though Sanguineous； be moved at more than
four points； but also because to have wings would be useless to it
when moving naturally。 And Nature makes nothing contrary to her own
nature。
12
We have stated above that without flexion in the legs or shoulders
and hips no Sanguineous animal with feet could progress； and that
flexion is impossible except some point be at rest； and that men and
birds； both bipeds； bend their legs in opposite directions； and
further that quadrupeds bend their in opposite directions； and each
pair in the opposite way to a man's limbs。 For men bend their arms
backwards； their legs forwards； quadrupeds their forelegs forwards；
their back legs backwards； and in like manner also birds bend
theirs。 The reason is that Nature's workmanship is never
purposeless； as we said above； but everything for the best possible in
the circumstances。 Inasmuch； therefore； as all creatures which
naturally have the power of changing position by the use of limbs；
must have one leg stationary with the weight of the body on it； and
when they move forward the leg which has the leading position must
be unencumbered； and the progression continuing the weight must
shift and be taken off on this leading leg； it is evidently
necessary for the back leg from being bent to become straight again；
while the point of movement of the leg thrust forward and its lower
part remain still。 And so the legs must be jointed。 And it is possible
for this to take place and at the same time for the animal to go
forward； if the leading leg has its articulation forwards；
impossible if it be backwards。 For； if it be forwards； the
stretching out of the leg will be while the body is going forwards；
but； if the other way； while it is going backwards。 And again； if
the flexion were backwards； the placing of the foot would be made by
two movements and those contrary to one another； one； that is；
backwards and one forwards； for in the bending together of the limb
the lower end of the thigh would go backwards； and the shin would move
the foot forwards away from the flexion； whereas； with the flexion
forwards； the progression described will be performed not with
contrary motions； but with one forward motion。
Now man； being a biped and making his change of position in the
natural way with his two legs； bends them forward for the reasons
set forth； but his arms bend backwards reasonably enough。 If they bent
the opposite way they would be useless for the work of the hands；
and for taking food。 But quadrupeds which are also viviparous
necessarily bend their front legs forwards。 For these lead off first
when they move； and are also in the forepart of their body。 The reason
that they bend forward is the same as in the case of man； for in
this respect they are like mankind。 And so quadrupeds as well as men
bend these legs forward in the manner described。 Moreover； if the
flexion is like this； they are enabled to lift their feet high； if
they bent them in the opposite way they would only lift them a
little way from the ground； because the whole thigh and the joint from
which the shin…bone springs would lie under the belly as the beast
moved forward。 If； however； the flexion of the hind legs were forwards
the lifting of these feet would be similar to that of the forefeet
（for the hind legs； too； would in this case have only a little room
for their lifting inasmuch as both the thigh and the knee…joint
would fall under the position of the belly）； but the flexion being
backwards； as in fact it is； nothing comes in the way of their
progression with this mode of moving the feet。 Moreover； it is
necessary or at least better for their legs to bend thus when they are
suckling their young； with a view to such ministrations。 If the
flexion were inwards it would be difficult to keep their young under
them and to shelter them。
13
Now there are four modes of flexion if we take the combinations in
pairs。 Fore and hind may bend either both backwards； as the figures
marked A； or in the opposite way both forwards； as in B； or in
converse ways and not in the same direction； as in C where the fore
bend forwards and the hind bend backwards； or as in D； the opposite
way to C； where the convexities are turned towards one another and the
concavities outwards。 Now no biped or quadruped bends his limbs like
the figures A or B； but the quadrupeds like C； and like D only the
elephant among quadrupeds and man if you consider his arms as well
as his legs。 For he bends his arms concavely and his legs convexly。
In man； too； the flexions of the limbs are always alternately
opposite； for example the elbow bends back； but the wrist of the
hand forwards； and again the shoulder forwards。 In like fashion；
too； in the case of the legs； the hip backwards； the knee forwards；
the ankle in the opposite way backwards。 And plainly the lower limbs
are opposed in this respect to the upper； because the first joints are
opposites； the shoulder bending forw