calculated to counteract the effects of repletion.

flight. mal.

Or there may be cafes, in which a defect is artificial, and compenfated by the very cause which produces it. Thus the beep, in the domefticated state in which we fee it, is deftitute of the ordinary means of defence or escape; is incapable either of resistance or But this is not fo with the wild aniThe natural sheep is swift and active: and, if it lofe thefe qualities when it comes under the subjection of man, the lofs is compenfated by his protection. Perhaps there is no fpecies of quadruped whatever, which suffers fo little as this does, from the depredation of animals of prey.

FOR THE SAKE of making our meaning better understood, we have confidered this bufinefs of compenfation under certain particularities of conftitution, in which it appears to be most conspicuous. This view of the subject neceffarily limits the inftances to fingle fpecies of animals. But there are compenfations, perhaps, not lefs certain, which extend over large claffes, and to large portions, of living nature.

I. In quadrupeds, the deficiency of teeth

In

is ufually compenfated by the faculty of rumination. The sheep, deer, and ox tribe, are without fore teeth in the upper jaw. These ruminate. The horse and afs are furnished with teeth in the upper jaw, and do not ruminate. In the former clafs the grafs and hay defcend into the ftomach, nearly in the ftate in which they are cropped from the pasture, or gathered from the bundle. the ftomach they are foftened by the gastric juice, which in thefe animals is unusually copious. Thus foftened, and rendered tender, they are returned a second time to the action of the mouth, where the grinding teeth complete at their leifure the trituration which is neceffary, but which was before left imperfect. I fay the trituration which is necessary; for it appears from experiments that the gaftric fluid of fheep, for example, has no effect in digefing plants, unless they have been previously mafticated; that it only produces a flight maceration, nearly as common water would do in a like degree of heat: but that, when once vegetables are reduced to pieces by maftication, the fluid then exerts upon them its fpecific operation. Its first effect is to soften them, and to deftroy their natural consistency:

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 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 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 strong 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 thefe birds will not operate

Spal. Diff. III. fec. cxl.

X 4

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 corn. Yet why should a bill and á gizzard go together? Why should a gizzard never be found where there are teeth?

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 digestive organ within the body, bears a strict and mechanical relation to the external inftruments for procuring food. The foft membranous ftomach, accompanies the hooked, notched, beak; the fhort, mufcular legs; the strong, sharp, crooked talons: the cartilaginous ftomach, attends that conformation of bill and toes, which reftrains the bird to the picking of feeds or the cropping of plants.

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

derable

derable degree, fwift in their motion. How is the want of feet compenfated? It is done by the difpofition of the muscles and fibres of the trunk. In confequence of the just 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 ftretched out. The result of this action is a progreffive, and, in fome cafes, a rapid movement of the whole body, in any direction to which the will of the animal determines it. The meaneft 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 fhut 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 provisions for motion in other animals, proofs of new and appropriate mechanifm. Suppose that we had never seen an animal move upon the ground without feet, and that the problem

was,