[22] The jerboa when pursued can leap a distance of nine feet, and repeat the leaps so rapidly that it cannot be overtaken even by the aid of a swift horse. The bullfrog, a much smaller animal, can, when pressed, clear from six to eight feet at each bound, and project itself over a fence five feet high. [23] The long, powerful tail of the kangaroo assists in maintaining the equilibrium of the animal prior to the leaps; the posterior extremities and tail forming a tripod of support. The deer also bounds into the air in its slower movements; in its fastest paces it gallops like the horse, as explained at pp. 40–44. The posterior extremities of the kangaroo are enormously developed as compared with the anterior ones; they are also greatly elongated. The posterior extremities are in excess, likewise, in the horse, rabbit,[24] agouti, and guinea pig. As a consequence these animals descend declivities with difficulty. They are best adapted for slightly ascending ground. In the giraffe the anterior extremities are longer and more powerful, comparatively, than the posterior ones, which is just the opposite condition to that found in the kangaroo. [24] The rabbit occasionally takes several short steps with the fore legs and one long one with the hind legs; so that it walks with the fore legs, and leaps with the hind ones. In the giraffe the legs of opposite sides move together and alternate, whereas in most quadrupeds the extremities move diagonally--a remark which holds true also of ourselves in walking and skating, the right leg and left arm advancing together and alternating with the left leg and right arm (fig. 19). [Illustration: FIG. 19.--Diagram showing the figure-of-8 or double-waved track produced by the alternating of the extremities in man in walking and running; the right leg (_r_) and left arm (_s_) advancing simultaneously to form one step; and alternating with the left leg (_t_) and right arm (_u_), which likewise advance together to form a second step. The continuous line (_r_, _t_) gives the waved track made by the legs; the interrupted line (_s_, _u_) that made by the arms. The curves made by the right leg and left arm, and by the left leg and right arm, form ellipses. Compare with fig. 18, p. 37.--_Original._] In the hexapod insects, according to Müller, the fore and hind foot of the one side and the middle one of the opposite side move together to make one step, the three corresponding and opposite feet moving together to form the second step. Other and similar combinations are met with in the decapods. The alternating movements of the extremities are interesting as betokening a certain degree of flexuosity or twisting, either in the trunk or limbs, or partly in the one and partly in the other.