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ties 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.

The travelling surfaces of Animals modified and adapted to the medium on 07 in which they move.--In those land animals which take to the water occasionally, the feet, as a

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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.

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.
FIG. 14.-Foot of the seal, which opens and closes in the act of natation.

the organ being folded upon itself during the non-effective or return stroke,
and expanded during the effective or forward stroke. Due advantage is
taken of this arrangement by the seal when swimming, the animal rotating
on its long axis, so as to present the lower portion of the body and the
feet obliquely to the water during the return stroke, and the flat, or the
greatest available surface of both, during the effective or forward stroke. -
Original.

rule, are furnished with membranous expansions extending between the toes. Of such the Otter (fig. 12), Ornithorhynchus (fig. 11), Seal (fig. 14), Crocodile, Sea-Bear (fig. 37, p. 76), Walrus, Frog (fig. 13), and Triton, may be cited. The crocodile and triton, in addition to the membranous

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expansion occurring between the toes, are supplied with a powerful swimming-tail, which adds very materially to the surface engaged in natation. Those animals, one and all, walk awkwardly, it always happening that when the extremities are modified to operate upon two essentially different media (as, for instance, the land and water), the maximum of speed is attained in neither. For this reason those animals which swim the best, walk, as a rule, with the greatest difficulty, and vice versa, as the movements of the auk and seal in and out of the water amply testify.

In addition to those land animals which run and swim, there are some which precipitate themselves, parachutefashion, from immense heights, and others which even fly. In these the membranous expansions are greatly increased, the ribs affording the necessary support in the Dragon or Flying Lizard (fig. 15), the anterior and posterior extremities and tail, in the Flying Lemur (fig. 16) and Bat (fig. 17, p. 36).

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FIG. 15.

Fig. 16,

Fig. 15.---The Red-throated Dragon (Draco haematopogon, Gray) shows a large

membranous expansion (bb) situated between the anterior (d d) and posterior extremities, and supported by the ribs. The dragon by this arrangement can take extensive leaps with perfect safety.- Original. FIG. 16.-The Flying Lemur (Galeopithecus volans, Shaw). In the flying

lemur the membranous expansion (a b) is more extensive than in the Flying Dragon (fig. 15). It is supported by the neck, back, and tail, and by the anterior and posterior extremities. The flying lemur takes enormous leaps ; its membranous tunic all but enabling it to fly. The Bat, Phyllorhina gracilis (fig. 17), flies with a very slight increase of surface. The surface exposed by the bat exceeds that displayed by many insects

and birds. The wings of the bat are deeply concave, and so resemble the wings of beetles and heavy-bodied short-winged birds. The bones of the arm (r), forearm (d), and hand in, n, n) of the bat (fig. 17) support the anterior or thick margin and the extremity of the wing, and may not inaptly be compared to the nervures in corresponding positions in the wing of the beetle.- Original.

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FIG. 17.-The Bat (Phyllorhina gracilis, Peters). Here the travelling-surfaces

(rdes, a n n n) are enormously increased as compared with that of the land and water animals generally. Compare with figures from 10 to 14,

P. 34. r Arm of bat; d forearm of bat; e f, n n n hand of bat. - Original. Although no lizard is at present known to fly, there can be little doubt that the extinct Pterodactyles (which, according to Professor Huxley, are intermediate between the lizards and crocodiles) were possessed of this power. The bat is interesting as being the only mammal at present endowed with wings sufficiently large to enable it to fly. It affords an extreme example of modification for a special purpose, --its attenuated body, dwarfed posterior, and greatly elongated anterior extremities, with their enormous fingers and outspreading membranes, completely unfitting it for terrestrial progression. It is instructive as showing that flight may be attained, without the aid of hollow bones and air-sacs, by purely muscular efforts, and by the mere diminution and increase of a continuous membrane.

As the flying lizard, flying lemur, and bat (figs. 15, 16, and 17, pp. 35 and 36), connect terrestrial progression with aërial progression, so the auk, penguin (fig. 46, p. 91), and flyingfish (fig. 51, p. 98), connect progression in the water with progression in the air. The travelling surfaces of these anomalous creatures run the movements peculiar to the three highways of nature into each other, and bridge over, as it were, the gaps which naturally exist between locomotion on the land, in the water, and in the air.

1 The Vampire Bat of the Island of Bonin, according to Dr. Buckland, can also swim ; and this authority was of opinion that the Pterodactyle enjoyed similar advantages.-Eng. Cycl. vol. iv. p. 495.

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).

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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."1 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,4 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

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