The moving animal lifts up one foot while resting the other one on the ground.

The moving animal lifts up one foot while resting the other one on the ground. Bipeds lift one and get supported on the other. Quadrupeds lift one pair and rest on the other, on the opposite sides. If quadrupeds had lifted the pair of legs on the same side, grope on the other would have been difficult, just as a board cannot stand on two legs. The front leg of the right side and the hind leg of the left side are lifted together, and vice versa, for steady locomotion.