Imagens das páginas
PDF
ePub

of his arms and legs, and the awkward manner in which they are applied to and withdrawn from the water.

Swimming of the Turtle, Triton, Crocodile, etc.—The swimming of the turtle differs in some respects from all the other forms of swimming. While the anterior extremities of this

[graphic]

FIG. 44. _The Turtle (Chelonia imbricata), adapted for swimming and diving.

the extremities being relatively larger than in the seal, sea-bear, and walrus. The anterior extremities have a thick anterior margin and a thin posterior one, and in this respect resemble wings. Compare with figs. 36 and 37, pp. 74 and 76.-Original.

quaint animal move alternately, and tilt or partially rotate during their action, as in the sea-bear and walrus, the posterior

[graphic]

FIG. 45.- The Crested Newt (Triton cristatus, Laur.) In the newt a tail is

superadded to the extremities, the tail and the extremities both acting in swimming.-Original.

extremities likewise move by turns. As, moreover, the right anterior and left posterior extremities move together, and reciprocate with the left anterior and right posterior ones, the creature has the appearance of walking in the water (fig. 44).

The same remarks apply to the movements of the extremities of the triton (fig. 45, p. 89) and crocodile, when swimming, and to the feebly developed corresponding members in the lepidosiren, proteus, and axolotl, specimens of all of which are to be seen in the Zoological Society's Gardens, London. In the latter, natation is effected principally, if not altogether, by the tail and lower half of the body, which is largely developed and flattened laterally for this purpose, as in the fish.

The muscular power exercised by the fishes, the cetaceans, and the seals in swimming, is conserved to a remarkable extent by the momentum which the body rapidly acquires— the velocity attained by the mass diminishing the degree of exertion required in the individual or integral parts. This holds true of all animals, whether they move on the land or on or in the water or air.

The animals which furnish the connecting link between the water and the air are the diving-birds on the one hand, and the flying-fishes on the other,—the former using their wings for flying above and through the water, as occasion demands; the latter sustaining themselves for considerable intervals in the air by means of their enormous pectoral fins.

Flight under water, etc.—Mr. Macgillivray thus describes a flock of red mergansers which he observed pursuing sand-eels in one of the shallow sandy bays of the Outer Hebrides : “ The birds seemed to move under the water with almost as much velocity as in the air, and often rose to breathe at a distance of 200 yards from the spot at which they had dived."1

In birds which fly indiscriminately above and beneath the water, the wing is provided with stiff feathers, and reduced to a minimum as regards size. In subaqueous flight the wings may act by themselves, as in the guillemots, or in conjunction with the feet, as in the grebes. To convert the

1 History of British Birds, vol. i. p. 48.

2 The guillemots in diving do not use their feet; so that they literally fly under the water. Their wings for this purpose are reduced to the smallest possible dimensions consistent with flight. The loons, on the other hand, while they employ their feet, rarely, if ever, use their wings. The subaqueous progression of the grebe resembles that of the rog.-Cuvier's Animal Kingdom, Lond. 1840, pp. 252, 253.

wing into a powerful oar for swimming, it is only necessary to extend and flex it in a slightly backward direction, the mere act of extension causing the feathers to roll down, and giving to the back of the wing, which in this case communicates the more effective stroke, the angle or obliquity necessary for sending the animal forward. This angle, I may observe, corresponds with that made by the foot during extension, so that, if the feet and wings are both employed, they act in harmony. If proof were wanting that it is the back or convex surface of the wing which gives the more effective stroke in subaquatic flight, it would be found in the fact that in the penguin and great auk, which are totally incapable of flying out of the water, the wing is actually twisted

[graphic][ocr errors]

FIG. 46.— The Little Penguin (Aptenodytes minor, Linn.), adapted exclusively

for swimming and diving. In this quaint bird the wing forms a perfect screw, and is employed as such in swimming and diving. Compare with fig. 37, p. 70, and fig. 44, p. 89.-Original.

round in order that the concave surface, which takes a better hold of the water, may be directed backwards (fig. 46). The thick margin of the wing when giving the effective stroke is turned downwards, as happens in the flippers of the sea-bear, walrus, and turtle. This, I need scarcely remark, is precisely the reverse of what occurs in the ordinary wing in aërial flight. In those extraordinary birds (great auk and penguin) the wing is covered with short, bristly-looking feathers, and is a mere rudiment and exceedingly rigid, the

1 In the swimming of the crocodile, turtle, triton, and frog, the concave surfaces of the feet of the anterior extremities are likewise turned backwards.

movement which wields it emanating, for the most part, from the shoulder, where the articulation partakes of the nature of a universal joint. The wing is beautifully twisted upon itself, and when it is elevated and advanced, it rolls up from the side of the bird at varying degrees of obliquity, till it makes a right angle with the body, when it presents a narrow or cutting edge to the water. The wing when fully extended, as in ordinary flight, makes, on the contrary, an angle of something like 30° with the horizon. When the wing is depressed and carried backwards, the angles which its under surface make with the surface of the water are gradually increased. The wing of the penguin and auk propels both when it is elevated and depressed. It acts very much after the manner of a screw; and this, as I shall endeavour to show, holds true likewise of the wing adapted for aërial flight.

Difference between Subaquatic and Aërial Flight.--The difference between subaquatic flight or diving, and flight proper, may be briefly stated. In aërial flight, the most effective stroke is delivered downwards and forwards by the under, concave, or biting surface of the wing which is turned in this direction ; the less effective stroke being delivered in an upward and forward direction by the upper, convex, or nonbiting surface of the wing. In subaquatic flight, on the contrary, the most effective stroke is delivered downwards and backwards, the least effective one upwards and forwards. In aërial flight the long axis of the body of the bird and the short axis of the wings are inclined slightly upwards, and make a forward angle with the horizon. In subaquatic flight the long axis of the body of the bird, and the short axis of the wings are inclined slightly downwards and make a backward angle with the surface of the water. The wing acts more or less efficiently in every direction, as the tail of the fish does. The difference noted in the direction of the down stroke in flying and diving, is rendered imperative by the fact that a bird which flies in the air is heavier than the medium it navigates, and must be supported by the wings; whereas a bird which flies under the water or dives, is lighter than the water, and must

1 The effective stroke is also delivered during flexion in the shrimp, prawn, and lobster.

For the same reason, she has furnished the diving bird with a certain degree of buoyancy, and the flying bird with a certain amount of weight-levity tending to bring the one to the surface of the water, weight the other to the surface of the earth, which is the normal position of rest for both. The action of the subaquatic or diving wing of the king penguin is well seen at p. 94, fig. 47. ,

From what has been stated it will be evident that the wing acts very differently in and out of the water; and this is a point deserving of attention, the more especially as it seems to have hitherto escaped observation. In the water the wing, when most effective, strikes downwards and backwards, and acts as an auxiliary of the foot; whereas in the air it strikes downwards and forwards. The oblique surfaces, spiral or otherwise, presented by animals to the water and air are therefore made to act in opposite directions, as far as the down strokes are concerned. This is owing to the greater density of the water as compared with the air,—the former supporting or nearly supporting the animal moving upon or in it; the latter permitting the creature to fall through it in a downward direction during the ascent of the wing. To counteract the tendency of the bird in motion to fall downwards and forwards, the down stroke is delivered in this direction ; the kite-like action of the wing, and the rapidity with which it is moved causing the mass of the bird to pursue a more or less horizontal course. I offer this explanation of the action of the wing in and out of the water after repeated and careful observation in tame and wild birds, and, as I am aware, in opposition to all previous writers on the subject.

The rudimentary wings or paddles of the penguin (the movements of which I had an opportunity of studying in a tame specimen) are principally employed in swimming and diving. The feet, which are of moderate size and strongly webbed, are occasionally used as auxiliaries. There is this difference between the movements of the wings and feet of this most curious bird, and it is worthy of attention. The wings act together, or synchronously, as in flying birds ; the feet, on the other hand, are moved alternately. The wings are wielded with great energy, and, because of their

« AnteriorContinuar »