For those which only jump and so move from place to place do not need feet for this movement at least, but those which not only jump but also need to walk, finding that movement not sufficient for their purposes, evidently either are better able to progress with even feet or cannot otherwise progress at all; for every animal which has limbs must have an even number for as this kind of movement is effected by part of the body at a time, and not by the whole at once as in the movement of leaping, some of the feet must in turn remain at rest, and others be moved, and the animal must act in each of these cases with opposite limbs, shifting the weight from the limbs that are being moved to those at rest.