oblique fibres are also present, and the tendons of the longitudinal muscles in some instances cross obliquely towards the tail, the fish has also the power of tilting or twisting its trunk (particularly the lower half) as well as the caudal fin. In a mackerel which I examined, the oblique muscles were represented by the four lateral masses occurring between the dorsal, ventral, and lateral longitudinal muscles-two of these being found on either side of the fish, and corresponding to the myocommas or grand muscle latéral" of Cuvier. The muscular system of the fish would therefore seem to be arranged on a fourfold plan,-there being four sets of longitudinal muscles, and a corresponding number of slightly oblique and oblique muscles, the oblique muscles being spiral in their nature and tending to cross or intersect at various angles, an arrest of the intersection, as it appears to me, giving rise to the myocommas and to that concentric arrangement of their constituent parts so evident on transverse section. This tendency of the muscular fibres to cross each other at various degrees of obliquity may also be traced in several parts of the human body, as, for instance, in the deltoid muscle of the arm and the deep muscles of the leg. Numerous other examples of penniform muscles might be adduced. Although the fibres of the myocommas have a more or less longitudinal direction, the myocommas themselves pursue an oblique spiral course from before backwards and from within outwards, i.e. from the spine towards the periphery, where they receive slightly oblique fibres from the longitudinal dorsal, ventral, and lateral muscles. As the spiral oblique myocommas and the oblique fibres from the longitudinal muscles act directly and indirectly upon the spines of the vertebræ, and the vertebræ themselves to which they are specially adapted, and as both sets of oblique fibres are geared by interdigitation to the fourfold set of longitudinal muscles, the lateral, sinuous, and rotatory movements of the body and tail of the fish are readily accounted for. The spinal column of the fish facilitates the lateral sinuous twisting movements of the tail and trunk, from the fact that the vertebræ composing it are united to each other by a series of modified universal joints-the vertebræ supplying the cup

shaped depressions or sockets, the intervertebral substance, the prominence or ball.

The same may be said of the general arrangement of the muscles in the trunk and tail of the Cetacea, the principal muscles in this case being distributed, not on the sides, but on the dorsal and ventral aspects. The lashing of the tail in the whales is consequently from above downwards or vertically, instead of from side to side. The spinal column is jointed as in the fish, with this difference, that the vertebræ (especially towards the tail) form the rounded prominences or ball, the meniscus or cup-shaped intervertebral plates the receptacles or socket.

When limbs are present, the spine may be regarded as being ideally divided, the spiral movements, under these circumstances, being thrown upon the extremities by typical ball-and-socket joints occurring at the shoulders and pelvis. This is peculiarly the case in the seal, where the spirally sinuous movements of the spine are transferred directly to the posterior extremities.1

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 extremi

1 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, presentthemselves in an undeveloped or latent form in the tr of the reptile.

C

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 or in which they move.-In those land animals which take to the water occasionally, the feet, as a

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.

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

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).

FIG. 15.-The Red-throated Dragon (Draco hæmatopogon, Gray) shows a large membranous expansion (b b) 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 (n, 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.

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