Imatges de pàgina
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tion. Every separate feather resembles the bipinnate leaves of a plant; besides the obvious parts, the hollow quill, and solid stem bearded obliquely on both sides with an infinity of little plumes; each of these latter is also formed with a rachis or mid-rib set obliquely with plumelets, resembling hairs, and exactly incumbent on the preceding one, and adhering, by their means, closely to it, thus rendering the whole feather not only very light, but, as it were, air-tight. In the goose, the mid-rib of the plumelets of the primary feathers is dilated towards the base into a kind of keel, so that each plumelet at the summit looks like a feather, and at the base like a lamina or blade.

By the use of very fine microscopes of garnet and sapphire, Sir David Brewster succeeded in developing the structure of the plumelets; he discovered a singular spring, consisting of a number of slender fibres laid together, which resisted the division or separation of the minute parts of the feather, and closed themselves together when their separation had been forcibly effected.1

If we examine the whole wing, and the disposition and connection of the feathers that compose it, we shall find that one great object of its structure is to render it impervious to the air, so that it may take most effectual hold of it, and by pushing, as it were, against it with the wing, when the wingstroke is downwards, to force the body forwards. A person expert in swimming, or rowing, may easily get an idea how this is effected, by observing how the pressure of his arms and legs, or of his oars, against the denser medium, though not in the same direction, carries him, or his boat, forwards. In the case of the bird, the motion is not backwards and forwards, but upwards and downwards, which difference, perhaps, is rendered necessary by the rarer medium in which the motion takes place.

To facilitate the progress of the bird through the air, the head usually forms a trenchant point, that easily divides it, and overcomes its resistance; and often to this is added a long neck, which, in the case of many sea-birds, as wild geese and ducks, is stretched to its full length in flight; while in others, where centre of gravity requires it, as in the heron, bittern, &c., it is bent back.

3

The swiftness of the flight of some birds is wonderful, being four or five times greater than that of the swiftest quadruped. Directed by an astonishing acuteness of sight, the aquiline

1 Lit. Gazette, Oct. 11, 1824, 690.

2 Ardea cinerea.

3 A. stellaris.

tribes, when soaring in the air beyond human ken, can see a little bird or newt on the ground or on a rock, and dart upon it. in an instant, like a flash of lightning, giving it no time for escape. But though some birds are of such pernicious wing, there are others of the most gigantic size, for instance the ostrich, emu, &c., that have only rudiments of wings, and which never fly, and for their locomotions depend chiefly upon their legs, to which the muscles of power are given, instead of to the wings.

Amongst the terrestrial animals that give suck to their young, there is a single Family which the Creator has gifted with organs of excursive flight, and these afford the only example of the third kind of those organs mentioned above. These cannot, like insects and birds, traverse the earth upon legs, as well as flit through the air upon wings; for the analogues of the legs of quadrupeds, not solely of the anterior pair, as in birds, but of both pairs, form the bony structure by which the wing is extended and moved, and to which it is attached. It will be immediately seen that I am speaking of the bats and vampyres. These animals, which form the first Family of Cuvier's Order of Carnivorous Mammalians,3 are denominated Cheiroptera, or hand-winged, because in them the four fingers of the hand, the thumb being left free, are very much elongated so as to form the supports and extensors of the anterior portion of the membrane of which the wing is formed; while the hind leg and the tail, in most, perform the same office for the posterior portion of the wing: so that two wings appear to be united to form one ample organ of flight. The membrane itself, which forms the wing, is only a continuation of the skin of the flanks: as in the wings of insects, it is double, very fine, and so thin as to be semi-transparent; it is traversed by some blood-vessels, and muscular fibres-doubtless accompanied by nerveswhich, when the wings are folded form little cavities placed in rows, resembling the meshes of a net. As bats are not provided with air-cells, or air in their bones, like birds, and their flight is unassisted by feathers, these wants are compensated to them by wings four or five times the length of their body. Their flight is of a different character from that of birds, resembling rather the flitting of a butterfly; when we consider that the peculiar function of bats is to keep within due limits the numbers of crepuscular and nocturnal insects, especially moths, we see how necessary it was that they should be enabled to traverse every spot frequented by the objects their

1 Struthio.

2 Casuarius.

3 Les Carnassiers.

For this purpose

instinct urges them to pursue and devour. their wings are admirably adapted not only by their volume, but by their power of contracting them, and giving them various inflections in flight, so that their speed is regulated by the object they are pursuing.

When we further reflect that their eyes are small and deepseated, we may conjecture that it requires extraordinary tact and delicacy of sensation in some other organs to supply this defect in its sight. Spallanzani found that blind bats fly as well as those that have eyes; that they avoided most expertly threads of fine silk which he had so stretched as just to leave room for them to pass between them: that they contracted, at will, their wings, if the threads were near, so as to avoid touching them; as well as when they passed between the branches of trees; and also that they could suspend themselves in dark places, such as vaults, to the prominent angles. He deprived the same individuals of other organs of sensation, but they were equally adroit in their flight, so that he concluded they must have some sensiferous organs different from those of other animals to enable them to thread the labyrinths through which they ordinarily pass.

Dr. Grant observes on this subject-" Bats are nocturnal, but, contrary to what is generally the case with nocturnal animals, their eyes are minute and feeble, and indeed, comparatively speaking, of minor importance, for so exquisite is the sense of feeling diffused over the surface of their membranous wings, that they are able to feel any vibration of air however imperceptible by us; they can tell, by the slight rebound of the air, whether they are flying near any wall, or opposing body, or in free space though their eyes be sealed or removed." A similar observation was long ago made by Mr. Bingley."

We see in the circumstances here detailed a remarkable instance of the Power, Wisdom, and Goodness of the Creator, in compensating for the absence or imperfection of one or more senses, by adding to the intensity of another, and in establishing its principal seat in organs so nicely adapted to derive most profit by the information communicated.

An animal nearly related to the vampyres, the cat-ape, commonly called the flying cat, and by some the flying dog, though nearly related to the bats, and included by Cuvier in the same Family, differs essentially from them, in being furnished with organs formed by the skin of the flanks connected with the

1 Quoted in Lit. Gaz. Feb. 9, 1834. 3 Galeopithecus,

2 Mem. of Brit. Quad, 34.

legs of each extremity, which are calculated for suspension rather than flight, being used, as Cuvier remarks, merely as a parachute, and thus belong to the second kind of wings, mentioned above. This animal, which climbs like a cat, vaults from one tree to another, by the aid of the above skin, which supports it in the air. The petaurists,1 or flying squirrels, and the phalangists, or flying oppossums are similarly equipped, and for a similar purpose. The common squirrel, using its tail as a rudder, leaps with great agility from tree to tree, without the aid of this kind of parachute, the force of its spring being suflicient to counteract that of gravity. Providence has evidently added an organ of suspension, in the case of the three former animals, either because their vaults were necessarily longer, or because the greater weight of their bodies required it. The dreaded name of dragon, attached to the monsters of fable, has excited in our imagination ideas of beings clothed with unwonted terrors, from our carliest years, so that when we find the only animal that inherits their name is an insignificant lizard, not more than eight inches long, we are tempted to exclaim, Parturiunt montes. This little animal, under the name of wings, is furnished with two dorsal appendages independent of the legs, formed of the skin, and actually supported by the six first short ribs, which, instead of taking their usual curvature are extended in a right line. These organs are not used to fly with, but to support the animal in its leaps from branch to branch, and from tree to tree.

We see in this instance, how exactly the means are adapted to the end proposed. This animal walks with difficulty, and consequently seldom descends from the trees. It is therefore enabled to move from one part of a tree to another, not by its legs, but by an organ formed out of its ribs! How various and singular in this instance, as well as in that of serpents, before alluded to, are the means adopted by a Being, who is never at a loss to answer the foreseen call of circumstances by wise expedients.

Steering Organs.-But wings are not the only organs of flight with which the Creator has fitted those animals, to which he has assigned the air as the theatre of their most striking and interesting locomotions. They would be like a ship at sea without a rudder, and be altogether at the mercy of every wind of heaven, had they no means to enable them to steer their vessel through the fluctuations of the viewless element assigned

1 Petaurus.

4 See above, p. 258.

2 Phalangista.
5 Gubernacula,

3 Sciurus vulgaris.

to them. The eagle and the vulture would be gifted in vain with the faculty of seeing objects at a great distance, had they no other organ than their sail-broad vans to direct them in their flight. The same remark will apply as well to the insect as to the bird, which would in vain endeavour to discharge its functions, unless it could steer its course according to the direction of its will and the information furnished by its senses. But upon examination, we shall find that God hath not left himself without witness in this department, but hath furnished every bird and insect with such an organ of steerage as the case of each required; nay, even amongst the beasts and the reptiles we may discover similar means of directing their motions, especially when they leap, whether from the ground, or from tree to tree.

The caudal fin, or tail of fishes, may be regarded as belonging in some degree to this head; but as this is also their principal organ of locomotion, I thought it best to consider it with the other fins.

The abdomen of many insects seems to serve them as a rudder, being composed of several inosculating rings formed each of a dorsal and ventral segment; it is capable of considerable flexion in almost all directions; it can be elevated or depressed, and turned to either side, so that it seems, in a great degree calculated to enable insects to change the course of their flight according to their will. But besides this important organwhich by the air it is constantly inspiring adds force also to the internal impulse, and to the air-vessels in the wings-insects have other auxiliaries to keep them in their right course. Whoever has seen any grasshopper take flight, or leap from the ground, will find that they stretch out their hind legs, and, like certain birds, use them as a rudder. The tails also of the day-flies1 seem to be used by them as a kind of balancer in their choral dances up and down in the sun's declining beam.

But the most interesting and beautiful organ for steering animals in the air, is that formed by the tail feathers of birds, called by ornithologists, rectrices, or governing feathers, because they are used to direct their course; these are feathers planted in the rump, usually twelve in number-but in some amounting to nearly twenty-constituting two sets of feathers of six each, and forming together a kind of fork like the caudal fin of some fishes; the inside of each feather is set with much larger plumelets than the outside, so that there is a double series of corresponding feathers beginning one on the right side, and the

1 Ephemera.

2 Uropygium.

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