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*.

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 up 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 Spall. Dis. iii. sect. cxl.

*

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 animals are entirely without feet; yet loco-motive; 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 rapid 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-worm, as it crawls; the undulatory 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, affords, when com

pared 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 organization 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.

CHAP. XVII.

THE RELATION OF ANIMATED BODIES TO INANIMATE NATURE.

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 animal, 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 performed; which properties are different.— Was not this difference contemplated when the instruments were differently constituted?

II. The structure of the animal car 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 successive 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 the sense; which must be done before the internal machinery of the ear, subtile as it is, can act at all.

III. The organs of voice and respiration are, no less than the ear, indebted, for the success of their operation, to the peculiar qualities of the fluid in which the animal is immersed. They, therefore, as well as the ear, are constituted upon the supposition of such a fluid, i. e. of a fluid with such particular properties being always present. Change the properties of the fluid, and the organ cannot act; change the organ, and the properties of the fluid would be lost. The structure therefore of our organs, and the properties of our atmosphere, are made for one another. Nor does it alter the relation, whether you allege the organ to be made for the element (which seems the most natural way of considering it), or the element as prepared for the organ.

IV. But there is another fluid with which we have to do; with properties of its own; with laws of act

ing, and of being acted upon, totally different from those of air and water; and that is light. To this new, this singular element; to qualities perfectly peculiar, perfectly distinct and remote from the qualities of any other substance with which we are acquainted, an organ is adapted, an instrument is correctly adjusted, not less peculiar amongst the parts of the body, not less singular in its form, and in the substance of which it is composed, not less remote from the materials, the model, and the analogy of any other part of the animal frame than the element to which it relates is specific amidst the substances with which we converse. If this does not prove appropriation, I desire to know what would prove it.

Yet the element of light and the organ of vision, however related in their office and use, have no connexion whatever in their original. The action of rays of light upon the surfaces of animals has no tendency to breed eyes in their heads. The sun

might shine for ever upon living bodies, without the

smallest approach towards producing the sense of sight. On the other hand, also, the animal eye does not generate or emit light.

V. Throughout the universe there is a wonderful proportioning of one thing to another. The size of animals, of the human animal especially, when considered with respect to other animals, or to the plants which grow around him, is such as a regard to his conveniency would have pointed out. A giant or a pigmy could not have milked goats, reaped corn, or mowed grass; we may add, could not have rode a horse, trained a vine, shorn a sheep, with the same bodily ease as we do, if at all. A pigmy would have