1



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

consider.

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.

2