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 be shifted to it again, it is evidently necessary for the leg from being bent to become straight again, while the point of movement of the leg thrust forward and its lower part remain still.