On The Gait Of Animals   

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