And
this is why horses and such-like animals stand still with their legs
put forward criss-cross, not with the right or the left put forward
together at once. In the same fashion animals with more than four legs
make their movements; if you take two consecutive pairs of legs the
hind move criss-cross with the forelegs; you can see this if you watch
them moving slowly. Even crabs move in this way, and they are
polypods. They, too, always move criss-cross in whichever direction
they are making progress. For in direction this animal has a
movement all its own; it is the only animal that moves not forwards,
but obliquely. Yet since forwards is a distinction relative to the
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.
15
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.
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