« VorigeDoorgaan »
is sometimes, I believe, called, the vane. By the beards are meant, what are fastened on each side of the stem, and what constitute the breadth of the feather; what we usually strip off, from one side or both, when we make a pen. The separate pieces of laminæ, of which the beard is composed, are called threads, sometimes filaments, or rays. Now the first thing which an attentive observer will remark is, hcw much stronger the beard of the feather shows itself to be, when pressed in a direction perpendicular to its plane, than when rubbed, either up or down, in the line of the stem; and he will soon discover the structure which occasions this difference, viz. that the laminæ, whereof these beards are composed, are flat, and placed with their flat sides towards each other; by which means, whilst they easily bend for the approaching of each other, as any one may perceive by drawing his finger ever so lightly upwards, they are much harder to bend out of their plane, which is the direction in which they have to encounter the impulse and pressure of the air, and in which their strength is wanted and put to the trial.
This is one particularity in the structure of a feather; a second is still more extraordinary.
Whoever examines a feather, cannot help taking notice, that the threads or laminæ, of which we have been speaking, in their natural state unite; that their union is something more than the mere apposition of loose surfaces; that they are not parted asunder without some degree of force; that nevertheless there is no glutinous cohesion between them; that, therefore, by some mechanical means or other, they catch or clasp among themselves, thereby giving to the beard or vane its closeness and compactness of texture. Nor is this all: when two laminæ, which have been separated by accident or force, are brought together again, they immediately reclasp; the connexion, whatever it was, is perfectly recovered, and the beard of the feather becomes as smooth and firm as if nothing had happened to it. Draw your finger down the feather, which is against the grain, and you break probably the junction of some of the contiguous threads; draw your finger up the feather, and you restore all things to their former state. This is no common contrivance: and now for the mechanism by which it is effected.* The threads or laminæ above mentioned, are in
* By the aid of the microscope it appears, that the laminæ are not fat, as they appear to the unassisted eye, but are semi-tubular, having on their outward edge a series of bristles, termed in the text fibres, set in pairs opposite one another, which clasp with the bristles of the approximate
terlaced with one another; and the interlacing is perforined by means of a vast number of fibres, or teeth, which the laminæ shoot forth on each side, and which hook and grapple together. A friend of mine counted fifty of these fibres in one twentieth of an inch. These fibres are crooked; but curved after a different manner: for those which yroceed from the thread on the side towards the extremity of the feather, are longer, more flexible, and bent downward; whereas those which proceed from the side towards the beginning, or quill-end of the feather, are shorter, firmer, and turn upwards. The process then which takes place is as follows: When two laminæ are pressed together, so that these long fibres are forced far enough over the short ones, their crooked parts fall into the cavity made by the crooked parts of the others; just as the latch that is fastened to a door enters into the cavity of the catch fixed to the door-post, and there hooking itself, fastens the door; for it is properly in this manner, that one thread of a feather is fastened to the other.
This admirable structure of the feather, which it is easy to see with the microscope, succeeds perfectly for the use to which nature has designed it; which use was, not only that the laminæ might be united, but that when one thread or lamina has been separated from another by some external violence, it might be reclasped with sufficient facility and expedition.*
In the ostrich, this apparatus of crotchets and fibres, of hooks and teeth, is wanting: and we see the consequence of the want. The filaments hang loose and separate from one another, forming only a kind of down; which constitution of the feathers, however it may fit them for the flowing honors of a lady's head-dress, may be reckoned an imperfection in the bird, inasmuch as wings, composed of these feathers, although they may greatly assist it in running, do not serve for flight.
But under the present division of our subject, our business with feathers is, as they are the covering of the bird. And herein a singular circumstance occurs. In the small order of birds which winter with us, from a snipe downlaminæ, and cause that adhesiveness observable between the several .aminæ of the vane.
The bristles are not of the same form on each side of one lamina ; the ower tier, Tab. XXIII. fig. 6. form a simple and slight curve, while the upper, fig. 7. terminate with three or four little hooks, which serve to catch the simple corresponding bristle, fig. 6. of the next lamina.
* The above account is taken from Memoirs for a Natural Histry of Animals, by the Royal Academy of Paris, published 1701, p. 209.
wards, let he external color of the feathers be what it will, their Creator has universally given them a bed of black down next their bodies. Black, we know, is the warmest color; and the purpose here is, to keep in the heat, arising from the heart and circulation of the blood. It is farther likewise remarkable, that this is not found in larger birds; for which there is also a reason:-small birds are much more exposed to the cold than large ones; forasmuch as they present, in proportion to their bulk, a much larger surface to the air. If a turkey were divided into a number of wrens, (supposing the shape of the turkey and the wren to be similar,) the surface of all the wrens would exceed the surface of the turkey, in the proportion of the length, breadth, (or, of any homologous line,) of a turkey to that of a wren; which would be, perhaps, a proportion of ten to one.
It was necessary, therefore, that small birds should be more warmly clad than large ones: and this seems to be the expedient by which that exigency is provided for.
II. In comparing different animals, I know no part of their structure which exhibits greater variety, or, in that variety, a nicer accommodation to their respective conveniency, than that which is seen in the different formations of their mouths. Whether the purpose be the reception of aliment merely, or the catching of prey, the picking up of seeds, the cropping of herbage, the extraction of juices, the suction of liquids, the breaking and grinding of food, the taste of that food, together with the respiration of air, and, in conjunction with it, the utterance of sound; these various offices are assigned to this one part, and, in different species, provided for, as they are wanted, by its different constitution. In the human species, forasmuch as there are hands to convey the food to the mouth, the mouth is flat, and by reason of its flatness, fitted only for reception; whereas the projecting jaws, the wide rictus, the pointed teeth of the dog and his affinities, enable them to apply their mouths to snatch and seize the objects of their pursuit. The full lips, the rough tongue, the corrugated cartilaginous palate, the broad cutting teeth of the ox, the deer, the horse, and the sheep, quality this tribe for browsing upon their pasture; either gathering large mouthfuls at once, where the grass is long, which is the case with the ox in particular; or biting close, where it is short, which the horse and the sheep are able to do, in a degree that one could hardly expect. The retired under jaw of a swine works in the ground, after the protruding snout, lika
a prong or ploughshare, has made its way to the roots upon which it feeds. A conformation so happy was not the gift of chance.
In birds, this organ assumes a new character; new both in substance and in form; but in both, wonderfully adapted to the wants and uses of a distinct mode of existence. We have no longer the fleshy lips, the teeth of enamelled bone; but we have, in the place of these two parts, and to perform the office of both, a hard substance (of the same nature with that which composes the nails, claws, and hoofs of quadrupeds) cut out into proper shapes, and mechanically suited to the actions which are wanted. The sharp edge and tempered point of the sparrow's bill picks almost every kind of seed from its concealment in the plant; and not only so, but hulls the grain, breaks and shatters the coats of the seed, in order to get at the kernel. The hooked beak of the hawk tribe separates the flesh from the bones of the animals which it feeds upon, almost with the cleanness and precision of a dissector's knife. The butcher-bird transfixes its prey upon the spike of a thorn, whilst it picks its bones. In some birds of this class, we have the cross bill,
e. both the upper and lower bill hooked, and their tips crossing. The spoon bill, enables the goose to graze, to collect its food from the bottom of pools, or to seek it amidst the soft or liquid substances with which it is mixed. The long tapering bill of the snipe and woodcock, penetrates still deeper into moist earth, which is the bed in which the food of that species is lodged. This is exactly the instrument which the animal wanted. It did not want strength in its bill, which was inconsistent with the slender form of the animal's neck, as well as unnecessary for the kind of aliment upon which it subsists; but it wanted length to reach its object.
But the species of bill which belongs to birds that live by suction, deserves to be described in its relation to that office. They are what naturalists call serrated or dentated bills; the inside of them, towards the edge, being thickly set with parallel or concentric rows of short, strong, sharppointed prickles. These, though they should be called teeth, are not for the purpose of mastication, like the teeth of quadrupeds: nor yet, as in fish, for the seizing and retaining of their prey; but for a quite different use. They form a filter The duck by means of them discusses the mud; examining with great accuracy the puddle, the brake, every mixture which is likely to contain her food. The -peratior is thus carried on :-The liquid or semi-liquid sub
stances, in which the animal has plunged her bill, she draws, by the action of her lungs, through the narrow interstices which lie between these teeth; catching, as the stream passes across her beak, whatever it may happen to bring along with it, that proves agreeable to her choice, and easily dismissing all the rest. Now, suppose the purpose to have been, out of a mass of confused heterogeneous substances, to separate for the use of the animal, or rather to enable the animal to separate for its own, those few particles which suited its taste and digestion; what more artificial, or more commodious instrument of selection, could have been given to it, than this natural filter?* It has been observed, also, (what must enable the bird to choose and distinguish with greater acuteness, as well, probably, as what increases its gratification and its luxury,) that the bills of this species are furnished with large nerves, that they are covered with a skin,—and that the nerves run down to the very extremity. In the curlew, woodcock, and snipe, there are three pairs of nerves, equal almost to the optic nerve in thickness, which pass first along the roof of the mouth, and then along the upper chap, down to the point of the bill, long as the bill is. (Pl. XXIII. fig. 1.]
But to return to the train of our observations. The simnitude between the bills of birds and the mouths of quadrupeds, is exactly such as, for the sake of the argument, might be wished for. It is near enough to show the continuation of the same plan; it is remote enough to exclude the supposition of the difference being produced by action
A more prominent contour, or a wider gape might be resolved into the effect of continued efforts, on the part of the species, to thrust out the mouth, or open it to the stretch. But by what course of action, or exercise, or endeavour, shall we get rid of the lips, the gums, the teeth; and acquire, in the place of them, pincers of horn? By what habit shall we so completely change, not only the shape of the part, but the substance of which it is composed? The truth is, if we had seen no other than the mouths of quadrupeds, we should have thought no other could have been formed: little could we have supposed, that all the purposes of a mouth furnished with lips, and armed with
* There is a remarkable contrivance of this kind the genus balæna, or proper whale. Numerous parallel plates of the substance called whalebone, cover the palatine surface of the uper jaw, and descend vertically into the mouth; the lower edges are fringed by long fibres, which serve the animal, when taking in the water, to retain the molluscr, with whi h the water abounds, and which constitute its food.-Paxton