On The Gait Of Animals
WE have now to consider the parts which are useful to animals for
movement in place (locomotion); first, why each part is such as it
is and to what end they possess them; and second, the differences
between these parts both in one and the same creature, and again by
comparison of the parts of creatures of different species with one
another. First then let us lay down how many questions we have to
The first is what are the fewest points of motion necessary to
animal progression, the second why sanguineous animals have four
points and not more, but bloodless animals more than four, and
generally why some animals are footless, others bipeds, others
quadrupeds, others polypods, and why all have an even number of
feet, if they have feet at all; why in fine the points on which
progression depends are even in number.
Next, why are man and bird bipeds, but fish footless; and why do man
and bird, though both bipeds, have an opposite curvature of the
legs. For man bends his legs convexly, a bird has his bent
concavely; again, man bends his arms and legs in opposite
directions, for he has his arms bent convexly, but his legs concavely.
And a viviparous quadruped bends his limbs in opposite directions to a
man's, and in opposite directions to one another; for he has his
forelegs bent convexly, his hind legs concavely. Again, quadrupeds
which are not viviparous but oviparous have a peculiar curvature of
the limbs laterally away from the body. Again, why do quadrupeds
move their legs criss-cross?
We have to examine the reasons for all these facts, and others
cognate to them; that the facts are such is clear from our Natural
History, we have now to ask reasons for the facts.