it then goes on to diffolve them; not sparing even the toughest parts, fuch as the nerves of the leaves *.

I think it very probable that the gratification alfo of the animal is renewed and prolonged by this faculty. Sheep, deer, and oxen, appear to be in a state of enjoyment whilft they are chewing the cud. It is then, perhaps, that they beft relish their food.

II. In birds, the compenfation is ftill more ftriking. They have no teeth at all. What have they then to make up for this fevere want? I speak of graminivorous and herbivorous birds; fuch as common fowls, turkeys, ducks, geefe, pigeons, &c. for it is concerning these alone that the question need be asked. All these are furnished with a peculiar and moft powerful muscle, called the gizzard; the inner coat of which is fitted up with rough plaits, which, by a ftrong friction against one another, break and grind the hard aliment, as effectually, and by the fame mechanical action, as a coffee-mill would do. It has been proved by the most correct experiments, that the gaftric juice of these birds will not operate

* Spal. Diff. III. fec. cxl.

X 4

2

upon

upon the entire grain; not even when foftened 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 ftarved upon a heap of Yet why should a bill and a gizzard go together? Why should a gizzard never be found where there are teeth?

corn.

Nor does the gizzard belong to birds as fuch. A gizzard is not found in birds of prey. Their food requires not to be ground down in a mill. The compenfatory contrivance goes no further than the neceffity. In both claffes of birds however, the digeftive organ within the body, bears a ftrict and mechanical relation to the external inftruments for procuring food. The foft membranous ftomach, accompanies the hooked, notched, beak; the short, mufcular legs; the ftrong, fharp, crooked talons: the cartilaginous ftomach, attends that conformation of bill and toes, which restrains the bird to the picking of feeds or the cropping of plants.

III. But to proceed with our compenfations. A very numerous and comprehensive tribe of terrestrial animals are entirely without feet; yet locomotive; and, in a very confi

4

derable

derable degree, swift in their motion. How is the want of feet compensated? It is done by the difpofition of the muscles and fibres of the trunk. In confequence of the juft collocation, and by means of the joint action of longitudinal and annular fibres, that is to fay, of ftrings and rings, the body and train of reptiles are capable of being reciprocally shortened and lengthened, drawn up and stretched out. The refult of this action is a progreffive, 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 roughneffes of the furface upon which it creeps; and, the power arising from all these, of changing its place and position, affords, when compared with the provifions. for motion in other animals, proofs of new and appropriate mechanifin. Suppose that we had never seen an animal move upon the ground without fect, and that the problem.

was,

was, muscular action, i. e. reciprocal contraction and relaxation being given, to describe how fuch 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 artift; or might have been exhibited in an automaton, by the combination of fprings, fpiral wires, and ringlets: but to the folution of the problem would not be denied, furely, the praise of invention and of fuccefsful thought; leaft of all could it ever be queftioned, whether intelligence had been employed about it, or not.

CHAP

CHAPTER XVII.

THE RELATION OF ANIMATED BODIES TO INANIMATE NATURE.

We have already confidered 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 fame animal, or of another individual of the fame fpecies.

But the bodies of animals hold, in their conftitution and properties, a clofe and important relation to natures altogether external to their own; to inanimate fubftances, and to the specific qualities of these, e. g. they hold a ftrict 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 fif to water? They are inftruments of motion, severally fuited 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 inftruments were differently constituted?

II. The