[19] That the movements of the extremities primarily emanate from the spine is rendered probable by the remarkable powers possessed by serpents. “It is true,” writes Professor Owen (_op. cit._ p. 261), “that the serpent has no limbs, yet it can outclimb the monkey, outswim the fish, outleap the jerboa, and, suddenly loosing the close coils of its crouching spiral, it can spring into the air and seize the bird upon the wing.” ... “The serpent has neither hands nor talons, yet it can outwrestle the athlete, and crush the tiger in the embrace of its ponderous overlapping folds.” The peculiar endowments, which accompany the possession of extremities, it appears to me, present themselves in an undeveloped or latent form in the trunk of the reptile. The extremities, when present, are provided with their own muscular cycles of extensor and flexor, abductor and adductor, pronator and supinator muscles,--these running longitudinally and at various degrees of obliquity, and enveloping the hard parts according to their direction--the bones being twisted upon themselves and furnished with articular surfaces which reflect the movements of the muscular cycles, whether these occur in straight lines anteriorly, posteriorly, or laterally, or in oblique lines in intermediate situations. The straight and oblique muscles are principally brought into play in the movements of the extremities of quadrupeds, bipeds, etc. in walking; in the movements of the tails and fins of fishes, whales, etc. in swimming; and in the movements of the wings of insects, bats, and birds in flying. The straight and oblique muscles are usually found together, and co-operate in producing the movements in question; the amount of rotation in a part always increasing as the oblique muscles preponderate. The combination of ball-and-socket and hinge-joints, with their concomitant oblique and longitudinal muscular cycles (the former occurring in their most perfect forms where the extremities are united to the trunk, the latter in the extremities themselves), enable the animal to present, when necessary, an extensive resisting surface the one instant, and a greatly diminished and a comparatively non-resisting one the next. This arrangement secures the subtlety and nicety of motion demanded by the several media at different stages of progression. [Illustration: FIG. 10. FIG. 11. FIG. 12. FIG. 13. FIG. 14. FIG. 10.--Extreme form of compressed foot, as seen in the deer, ox, etc., adapted specially for land transit.--_Original._ FIG. 11.--Extreme form of expanded foot, as seen in the _Ornithorhynchus_, etc., adapted more particularly for swimming.--_Original._ FIGS. 12 and 13.--Intermediate form of foot, as seen in the otter (fig. 12), frog (fig. 13), etc. Here the foot is equally serviceable in and out of the water.--_Original._