portion as the curves into which the body is thrown in swimming are diminished, the degree of rotation at the tail or in the fins is augmented, some fishes, as the mackerel, using the tail very much after the manner of a screw in a steam-ship. The fish may thus be said to drill the water in two directions, viz. from behind forwards by a twisting or screwing of the body on its long axis, and from side to side by causing its anterior and posterior portions to assume opposite curves. The pectoral and other fins are also thrown into curves when in action, the movement, as in the body itself, travelling in spiral waves; and it is worthy of remark that the wing of the insect, bat, and bird obeys similar impulses, the pinion, as I shall show presently, being essentially a spiral organ.

The twisting of the pectoral fins is well seen in the common perch (Perca fluviatilis), and still better in the 15-spined Stickleback (Gasterosteus spinosus), which latter frequently progresses by their aid alone.1 In the stickleback, the pectoral fins are so delicate, and are plied with such vigour, that the eye is apt to overlook them, particularly when in motion. The action of the fins can be reversed at pleasure, so that it is by no means an unusual thing to see the stickleback progressing tail first. The fins are rotated or twisted, and their free margins lashed about by spiral movements which closely resemble those by which the wings of insects are propelled.2 The rotating of the fish upon its long axis is seen to advantage in the shark and sturgeon, the former of which requires to turn on its side before it can seize its prey,-—and likewise

1 The Syngnathi, or Pipefishes, swim chiefly by the undulating movement of the dorsal fin.

2 If the pectoral fins are to be regarded as the homologues of the anterior extremities (which they unquestionably are), it is not surprising that in them the spiral rotatory movements which are traceable in the extremities of quadrupeds, and so fully developed in the wings of bats and birds, should be clearly foreshadowed. "The muscles of the pectoral fins," remarks Professor Owen, "though, when compared with those of the homologous members in higher vertebrates, they are very small, few, and simple, yet suffice for all the requisite movements of the fins-elevating, depressing, advancing, and again laying them prone and flat, by an oblique stroke, upon the sides of the body. The rays or digits of both pectorals and ventrals (the homologues of the posterior extremities) can be divaricated and approximated, and the intervening webs spread out or folded up."-Op. cit. vol..i. p. 252.

in the pipefish, whose motions are unwontedly sluggish. The twisting of the tail is occasionally well marked in the swimming of the salamander. In those remarkable mammals, the whale,1 porpoise, manatee, and dugong (figs. 33, 34, and 35), the movements are strictly analogous to those of the fish,

FIG. 33.-The Porpoise (Phocæna communis). Here the tail is principally engaged in swimming, the anterior extremities being rudimentary, and resembling the pectoral fins of fishes. Compare with fig. 30, p. 65.-Original.

FIG. 34.-The Manatee (Manatus Americanus). In this the anterior extremities are more developed than in the porpoise, but still the tail is the great organ of natation. Compare with fig. 33, p. 73, and with fig. 30, p. 65. The shape of the manatee and porpoise is essentially that of the fish.-Original.

the only difference being that the tail acts from above downwards or vertically, instead of from side to side or laterally. The anterior extremities, which in those animals are comparatively perfect, are rotated on their long axes, and applied obliquely and non-obliquely to the water, to assist in balancing and turning. Natation is performed almost exclusively by the tail and lower half of the trunk, the tail of the whale exerting prodigious power.

It is otherwise with the Rays, where the hands are princi

1 Vide "Remarks on the Swimming of the Cetaceans," by Dr. Murie, Proc. Zool. Soc., 1865, pp. 209, 210.

pally concerned in progression, these flapping about in the water very much as the wings of a bird flap about in the air. In the beaver, the tail is flattened from above downwards, as in the foregoing mammals, but in swimming it is made to

FIG. 35.-Skeleton of the Dugong. In this curious mammal the anterior extremities are more developed than in the manatee and porpoise, and resemble those found in the seal, sea-bear, and walrus. They are useful in balancing and turning, the tail being the effective instrument of propulsion. The vertebral column closely resembles that of the fish, and allows the tail to be lashed freely about in a vertical direction. Compare with fig. 29, p. 65.-(After Dallas.)

act upon the water laterally as in the fish. The tail of the bird, which is also compressed from above downwards, can be twisted obliquely, and when in this position may be made to perform the office of a rudder.

Swimming of the Seal, Sea-Bear, and Walrus.--In the seal, the anterior and posterior extremities are more perfect than in the whale, porpoise, dugong, and manatee; the general

FIG. 36. The Seal (Phoca foetida, Müll.), adapted principally for water. The extremities are larger than in the porpoise and manatee. Compare with figs. 33 and 34, p. 73.-Original.

form, however, and mode of progression (if the fact of its occasionally swimming on its back be taken into account), is essentially fish-like.

A peculiarity is met with in the swimming of the seal, to which I think it proper to direct attention. When the lower portion of the body and posterior extremities of these creatures are flexed and tilted, as happens during the back and least effective stroke, the naturally expanded feet are more or less completely closed or pressed together, in order to diminish the extent of surface presented to the water, and, as a consequence, to reduce the resistance produced. The feet are opened to the utmost during extension, when the more effective stroke is given, in which case they present their maximum of surface. They form powerful propellers, both during flexion and extension.

The swimming apparatus of the seal is therefore more highly differentiated than that of the whale, porpoise, dugong, and manatee; the natatory tail in these animals being, from its peculiar structure, incapable of lateral compression.1 It would appear that the swimming appliances of the seals (where the feet open and close as in swimming-birds) are to those of the sea-mammals generally, what the feathers of the bird's wing (these also open and close in flight) are to the continuous membrane forming the wing of the insect and bat.

The anterior extremities or flippers of the seal are not engaged in swimming, but only in balancing and in changing position. When so employed the fore feet open and close, though not to the same extent as the hind ones; the resistance and non-resistance necessary being secured by a partial rotation and tilting of the flippers. By this twisting and untwisting, the narrow edges and broader portions of the flippers are applied to the water alternately. The rotating and tilting of the anterior and posterior extremities, and the opening and closing of the hands and feet in the balancing and swimming of the seal, form a series of strictly progressive and very graceful movements. They are, however, performed so rapidly, and glide into each other so perfectly, as to render an analysis of them exceedingly difficult.

1 In a few instances the caudal fin of the fish, as has been already stated, is more or less pressed together during the back stroke, the compression and tilting or twisting of the tail taking place synchronously.

In the Sea-Bear (Otaria jubata) the anterior extremities attain sufficient magnitude and power to enable the animal to progress by their aid alone; the feet and the lower portions of the body being moved only sufficiently to maintain, correct, or alter the course pursued (fig. 73). The anterior extremities are flattened out, and greatly resemble wings, particularly those of the penguin and auk, which are rudimentary in character. Thus they have a thick and comparatively stiff anterior margin; and a thin, flexible, and more or less elastic posterior margin. They are screw structures, and when elevated and depressed in the water, twist and untwist, screw-fashion, precisely as wings do, or the tails of the fish, whale, dugong, and manatee.

FIG. 37. The Sea-Bear (Otaria jubata), adapted principally for swimming and diving. It also walks with tolerable facility. Its extremities are larger than those of the seal, and its movements, both in and out of the water, more varied.-Original.

1

This remarkable creature, which I have repeatedly watched at the Zoological Gardens 1 (London), appears to fly in the water, the universal joints by which the arms are attached to the shoulders enabling it, by partially rotating and twisting

1 The unusual opportunities afforded by this unrivalled collection have enabled me to determine with considerable accuracy the movements of the various land-animals, as well as the motions of the wings and feet of birds, both in and out of the water. I have also studied under the most favourable circumstances the movements of the otter, sea-bear, seal, walrus, porpoise, turtle, triton, crocodile, frog, lepidosiren, proteus, axolotl, and the several orders of fishes.