Pagina-afbeeldingen
PDF
ePub

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 fame 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 sparrows 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 clearness and precision of a dissector's knise. (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, i. o. both the upper and lower bill hooked, and their tips crossing. The spoon bill, enables ables 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 sut"liont deserves to be described in its particular 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, sharp-pointed prickles. These, though they should be called teeth, are not for the purpose of mastication, like the teeth of quadrupeds; nor yet, as in sish, 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, Curacy, the puddle, the brake, every mixture which is likely to contain her food. The operation is thus carried on. The liquid or semiliquid substances, in which the animal has plunged her bill, me 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 and 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

R the 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

But to return to the train of our observations. The similitude between the bills of birds and the mouths of quadrupeds, is exactly such, as, for the fake of the argument, might be wished for. It is near enough to shew the continuation of the fame plan: it is remote enough to exclude the supposition of the difference being produced by action or use. 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 shaft we so completely change, not onry 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 purpqscs of a mouth, furnished with lips, and armed with teeth, could be answered by an instrument which had none of these; could be supplied, and that with many additional advantages, by the hardness, and sharpness, and figure, of the bills of birds.

Every thing about the animal mouth is mechanical. The teeth of fish, have their points turned backwards, like the teeth of a wool- or cotton-card. The teeth of lobsters, work one against another, like the sides of a pair of shears. In many insects, the mouth is converted into a pump or sucker, fitted at the end sometimes with a wimble, sometimes with a forceps; by which double provision, viz. of the tube and the penetrating form of the point, the insect first bores through the integuments of its prey, and then extracts the juices. And, what is most extraordinary of all, one sort of mouth, as the occasion requires, shall be changed into another sort. The caterpillar could not live without teeth; in several species, the butterfly formed from it, could not use them. The old teeth therefore are cast off with the exuviæ of the grub; a new and totally different apparatus assumes their place in the fly. Amidst these novelties of form, we sometimes forget that it is, all the while, the animal's mouth; that, whether it be lips, or teeth, or bill,

R a or

« VorigeDoorgaan »