PROGRESSION ON THE LAND.

Walking of the Quadruped, Biped, etc.-As the earth, because of its solidity, will bear any amount of pressure to which it may be subjected, the size, shape, and weight of animals destined to traverse its surface are matters of little or no consequence. As, moreover, the surface trod upon is rigid or unyielding, the extremities of quadrupeds are, as a rule, terminated by small feet. Fig. 18 (contrast with fig. 17).

FIG. 18.-Chillingham Bull (Bos Scoticus). Shows powerful heavy body, and the small extremities adapted for land transit. Also the figure-of-8 movements made by the feet and limbs in walking and running. u, t Curves made by right and left anterior extremities. r, s Curves made by right and left posterior extremities. The right fore and the left hind foot move together to form the waved line (s, u); the left fore and the right hind foot move together to form the waved line (r, t). The curves formed by the anterior (t. u) and posterior (r, s) extremities form ellipses. Compare with fig. 19, p. 39.-Original.

In this there is a double purpose-the limited area presented to the ground affording the animal sufficient support and leverage, and enabling it to disentangle its feet with the

utmost facility, it being a condition in rapid terrestrial progression that the points presented to the earth be few in number and limited in extent, as this approximates the feet of animals most closely to the wheel in mechanics, where the surface in contact with the plane of progression is reduced to a minimum. When the surface presented to a dense resisting medium is increased, speed is diminished, as shown in the tardy movements of the mollusc, caterpillar, and slowworm, and also, though not to the same extent, in the serpents, some of which move with considerable celerity. In the gecko and common house-fly, as is well known, the travelling surfaces are furnished with suctorial discs, which enable those creatures to walk, if need be, in an inverted position; and "the tree-frogs (Hyla) have a concave disc at the end of each toe, for climbing and adhering to the bark and leaves of trees, Some toads, on the other hand, are enabled, by peculiar tubercles or projections from the palm or sole, to clamber up old walls." A similar, but more complicated arrangement, is met with in the arms of the cuttle-fish.

The movements of the extremities in land animals vary considerably.

In the kangaroo and jerboa, the posterior extremities only are used, the animals advancing per saltum, i.e. by a series of leaps.3

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,* agouti, and guinea

1 Comp. Anat. and Phys. of Vertebrates, by Professor Owen, vol. i. pp. 262, 263. Lond. 1866.

2 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.

3 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.

4. The rabbit occasionally takes several short steps with the fore legs and

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.

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).

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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.

This twisting begets the figure-of-8 movements observed in walking, swimming, and flying. (Compare figs. 6, 7, and 26 x, pp. 28 and 55; figs. 18 and 19, pp. 37 and 39; figs. 32 and 50, pp. 68 and 97; figs. 71 and 73, p. 144; and fig. 81, p. 157.)

Locomotion of the Horse.-As the limits of the present volume forbid my entering upon a consideration of the movements of all the animals with terrestrial habits, I will describe briefly, and by way of illustration, those of the horse, ostrich, one long one with the hind legs; so that it walks with the fore legs, and leaps with the hind ones.

and man. In the horse, as in all quadrupeds endowed with great speed, the bones of the extremities are inclined obliquely towards each other to form angles; the angles diminishing as the speed increases. Thus the angles formed by the bones of the extremities with each other and with the scapulæ and iliac bones, are less in the horse than in the elephant. For the same reason they are less in the deer than in the horse. In the elephant, where no great speed is required, the limbs are nearly straight, this being the best arrangement for supporting superincumbent weight. The angles formed by the different bones of the wing of the bird are less than in the fleetest quadruped, the movements of wings being more rapid than those of the extremities of quadrupeds and bipeds. These are so many mechanical adaptations to neutralize shock, to increase elasticity, and secure velocity. The paces of the horse are conveniently divided into the walk, the trot, the amble, and the gallop. If the horse begins his walk by raising his near fore foot, the order in which the feet are lifted is as follows:-first the left fore foot, then the right or diagonal hind foot, then the right fore foot, and lastly the left or diagonal hind foot. There is therefore a twisting of the body and spiral overlapping of the extremities of the horse in the act of walking, in all respects analogous to what occurs in other quadrupeds1 and in bipeds (figs. 18 and 19, pp. 37 and 39). In the slowest walk Mr. Gamgee observes "that three feet are in constant action on the ground, whereas in the free walk in which the hind foot passes the position from which the parallel fore foot moves, there is a fraction of time when only two feet are upon the ground, but the interval is too short for the eye to measure it. The proportion of time, therefore, during which the feet act upon the ground, to that occupied in their removal to new positions, is as three to one in the slow, and a fraction less in the fast walk. In the fast gallop these proportions are as five to three. In all the paces the power of the horse is being exerted mainly upon a fore

1 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.

and hind limb, with the feet implanted in diagonal positions. There is also a constant parallel line of positions kept up by a fore and hind foot, alternating sides in each successive move. These relative positions are renewed and maintained. Thus each fore limb assumes, as it alights, the advanced position parallel with the hind, just released and moving; 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

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, rt). 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.

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."

In the trot, according to Bishop, the legs move in pairs