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become loose upon the body, the animal makes a second effort, and by a tremulous, spasmodic motion, casts it off. In this state, the liberated but defenceless fish retires into holes in the rock. The released body now suddenly pushes its growth. In about eight-and-forty hours, a fresh concretion of humour upon the surface, is é, a new shell, is formed, adapted in every part to the increased dimensions of the animal. This wonderful mutation is repeated every year.
If there be imputed defects without compensation, I should suspect that they were defects only in appearance. Thus, the body of the sloth has often been reproached for the slowness of its motions, which has been attributed to an imperfection in the formation of its limbs. But it ought to be observed, that it is this slowness which alone suspends the voracity of the animal. He fasts during his migration from one tree to another: and this fast may be necessary for the relief of his overeharged vessels, as well as to allow time for the concoction of the mass of coarse and hard food which he has taken into his stomach. The tardiness of his pace seems to have reference to the capacity of his organs, and to his propensities with respect to food; ; i. e. is calculated to counteract the effects of repletion. Or there
cases, in which a defect is artificial, and compensated by the very cause which produces it. Thus the sheep, in the domesticated state in which we see it, is destitute of the ordinary means of defence or escape ; is incapable either of resistance or flight. But this is not so with the wild animal. The natural sheep is swift and active : and, if it lose these qualities when it comes under the ection of man, the loss is compensated by his protection. - Perhaps there is no species of quadruped whatever, which suffers so little as this does from the depredation of animals of prey.
For the sake of making our meaning better understood, we have considered this business of compensation under certain particularities of constitution, in which it appears to be most conspicuous. This view of the subject necessarily limits the instances to single species of animals. But there are compensations, perhaps not less certain, which extend over large classes, and to large portions of living nature,
I. In quadrupeds, the deficiency of teeth is usually compensated by the faculty of rumination. The sheep, deer, and ox tribe, are without fore-teeth in the upper jaw. These ruminate. The horse and ass are furnished with teeth in the upper jaw, and do not ruminate. In the former class, the grass and hay descend into the stomach, nearly in the state in which they are cropped from the pasture, or gathered from the bundle. In the stomach, they are softened by the gastric juice, which in these animals is unusually copious. Thus softened and rendered tender, they are returned a second time to the action of the mouth, where the grinding teeth complete at their leisure the trituration which is necessary, but which was before left imperfect. I say, the trituration which is necessary; for it appears from experiments, that the gastric fluid of sheep, for example, has no effect in digesting plants, unless they have been previously masticated; that it only produces a slight maceration, nearly as common water would do in a likedegree of heat; but that when once vegetables are reduced to pieces by mastication, the fluid then exerts upon them its specific operation. Its first effect is to soften them, and to destroy their natural consistency; it then goes on to dissolve them ; not sparing even the toughest parts, such as the nerves of the leaves*.
* Spall. Dis. iii. sect. cxl.
I think it very probable, that the gratification also of the animal is renewed and prolonged by this faculty. Sheep, deer, and oxen, appear to be in a state of enjoyment whilst they are chewing the cud. It is then, perhaps, that they best relish their food.
II. In birds, the compensation is still more striking. They have no teeth at all. What have they then to make
for this severe want? I speak of granivorous and herbivorous birds; such as common fowls, turkeys, ducks, geese, pigeons, &c.; for it is concerning these alone that the question need be asked. All these are furnished with a peculiar and most powerful muscle, called the gizzard; the inner coat of which is fitted up with rough plaits, which, by a strong friction against one another, break and grind the hard aliment as effectually, and by the same mechanical action, as a coffee-mill would do. It has been proved by the most correct experiments, that the gastric juice of these birds will not operate upon the entire grain ; not even when softened by water or macerated in the crop. Therefore without a grinding machine within its body, without the trituration of the gizzard, a chicken would have starved upon a heap of corn. Yet why should a bill and a gizzard go together? Why should a gizzard never be found where there are teeth ?
Nor does the gizzard belong to birds as such. A gizzard is not found in birds of prey. Their food requires not to be ground down in a mill.
The compensatory contrivance goes no farther than the necessity. In both classes of birds, however, the Digestive organ within the body bears a strict and mechanical relation to the external instruments for procuring food. The soft membranous stomach accompanies' a hooked, notched beak: short, muscular legs; strong, sharp, crooked talons: the cartilaginous stomach attends that
conformation of bill and toes, which restrains the bird to the picking of seeds, or the cropping of plants.
III. But to proceed with our compensations.--A very numerous and comprehensive tribe of terrestrial ani. mals are entirely without feet; yet locomotive; and in a very considerable degree swift in their motion. How is the want of feet compensated? It is done by the disposition of the muscles and fibres of the trunk. In consequence of the just collocation, and by means of the joint action of longitudinal and annular fibres, that is to say, of strings and rings, the body and train of reptiles are capable of being reciprocally shortened and lengthened, drawn up and stretched out. The result of this action is a progressive, and, in some cases, a rápid movement of the whole body, in any direction to which the will of the animal determines it. The meanest creature is a collection of wonders. The play of the rings in an earth-norm, as it crawls; the undu. latory motion propagated along the body; the beards or prickles with which the annuli are armed, and which the animal can either shut up close to its body, or let out to lay hold of the roughness of the surface upon which it creeps; and the power arising from all these, of changing its place and position, afford, when compared with the provisions for motion in other animals, proofs of new and appropriate mechanism. Suppose that we had never seen an animal move upon the ground without feet, and that the problem was; Muscular action, i. e. reciprocal contraction and relaxation being given, to describe how such an animal might be constructed, capable of voluntarily changing place, Something, perhaps, like the organisation of reptiles, might have been hit upon by the ingenuity of an artist : or might have been exhibited in an automaton, by the combination of springs, spiral wires, and ringlets: but
to the solution of the problem would not be denied, surely, the praise of invention and of successful thought: least of all could it ever be questioned, whether intelligence had been employed about it, or not.
THE RELATION OF ANIMATED BODIES TO INANIMATE
We have already considered relation, and under different views; but it was the relation of parts to parts, of the parts of an animal to other parts of the same ani. mal, or of another individual of the same species.
But the bodies of animals hold, in their constitution and properties, a close and important relation to natures altogether external to their own: to inanimate substances, and to the specific qualities of these ; e. g. they hold a strict relation to the ELEMENTS by which they are surrounded.
I. Can it be doubted, whether the wings of birds bear a relation to air, and the fins of fish to water ? They are instruments of motion, severally suited to the properties of the medium in which the motion is to be per, formed: which properties are different. Was not this difference contemplated, when the instruments were differently constituted ?
II. The structure of the animal ear depends for its use not simply upon being surrounded by a fluid, but upon the specific nature of that fluid. Every fluid would not serve: its particles must repel one another ; it must form an elastic medium : for it is by the succes. sive pulses of such a medium, that the undulations excited by the surrounding body are carried to the organ; that a communication is formed between the object and