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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.
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