_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). [Illustration: 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.”[21] A similar, but more complicated arrangement, is met with in the arms of the cuttle-fish. [21] Comp. Anat. and Phys. of Vertebrates, by Professor Owen, vol. i. pp. 262, 263. Lond. 1866. The movements of the extremities in land animals vary considerably. In the kangaroo and jerboa,[22] the posterior extremities only are used, the animals advancing _per saltum_, _i.e._ by a series of leaps.[23]