the hind feet move by turns, in sequence to their diagonal fore, and in priority to their parallel fellows, which following they maintain for nearly half their course, when the fore in its turn is raised and carried to its destined place, the hind alighting midway. All the feet passing over equal distances and keeping the same time, no interference of the one with the other occurs, and each successive hind foot as it is implanted forms a new diagonal with the opposite fore, the latter forming the front of the parallel in one instant, and one of the diagonal positions in the next: while in the case of the hind, they assume the diagonal on alighting and become the terminators of the parallel in the last part of their action.” [25] If a cat when walking is seen from above, a continuous wave of movement is observed travelling along its spine from before backwards. This movement closely resembles the crawling of the serpent and the swimming of the eel. [Illustration: FIG. 20.--Horse in the act of trotting. In this, as in all the other paces, the body of the horse is levered forward by a diagonal twisting of the trunk and extremities, the extremities describing a figure-of-8 track (_s u_, _r t_). The figure-of-8 is produced by the alternate play of the extremities and feet, two of which are always on the ground (_a_, _b_). Thus the right fore foot describes the curve marked _t_, the left hind foot that marked _r_, the left fore foot that marked _u_, and the right hind foot that marked _s_. The feet on the ground in the present instance are the left fore and the right hind. Compare with figs. 18 and 19, pp. 37 and 39.--_Original._] In the trot, according to Bishop, the legs move in pairs diagonally. The same leg moves rather oftener during the same period in trotting than in walking, or as six to five. The velocity acquired by moving the legs in pairs, instead of consecutively, depends on the circumstance that in the trot each leg rests on the ground during a short interval, and swings during a long one; whilst in walking each leg swings a short, and rests a long period. The undulations arising from the projection of the trunk in the trot are chiefly in the vertical plane; in the walk they are more in the horizontal.