F

tric juice, which in these animals is unusually copious. Thus foftened, and rendered tender, they are returned a fecond time to the action of the mouth, where the grinding teeth complete at their leisure the trituration which is neceffary, but which was before =left imperfect. I fay the trituration which is neceffa ry; for it appears from experiments that the gastric fluid of fheep, for example, has no effect in digeftiing 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 specific operation. Its firft effect is to foften them, and to destroy their natural confiftency : it then goes on to diffolve them; not fparing 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, dear, and oxen, appear to be in a state of enjoyment whilft they are chewing the cud. It is then, perhaps, that they best relifh 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 fpeak of graminivorous and herbivorous birds; fuch as common fowls, turkeys, ducks, geese, pigeons, &c. for it is concerning these alone that the queftion 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 coffe-mill would do. It has been proved by the most correct experiments, that the gastric juice

*Spal. Diff, III. fec. cxl.

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 ftarved upon a heap of corn. Yet why fhould a bill and a gizzard go together? Why fhould 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 com. penfatory contrivance goes no further than the neceffity. In both claffes of birds, however, the digestive 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 fhort, mufcular legs; the ftrong. fharp, 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 terrestrial animals are entirely without feet; yet locomotive; and, in a very confiderable 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 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 fhortened and lengthened, drawn up and ftretched out. The refult 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 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 arifing from all thefe, of changing its place and pofition, affords, when compared with the provifions for motion in other animals, proofs of new and appropriate mechanifm. Suppose that we had never feen an animal move upon the ground without feet, and that the problem was, mufcular action, i. e. reciprocal contraction and relaxation being given, to describe how fuch an animal might be conftructed, 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 praife of invention and of fuccefsful thought; leaft of all could it ever be queftioned, whether intelligence had been employed about it, or not.

CHAPTER XVII.

THE RELATION OF ANIMATED BODIES TO IN ANIMATE 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 constitution and properties, a clofe and important relation to

natures altogether external to their own; to inanimate fubftances, and to the fpecific qualities of thefe, e. g. they hold a strict relation to the elements by which they are furrounded.

I. Can it be doubted, whether the wings of birds bear a relation to air, and the fins of fish to water? They are inftruments of motion, feverally 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 ftructure of the animal ear depends for its ufe not fimply upon being furrounded by a fluid, but upon the specific nature of that fluid. Every fluid would not ferve: its particles must repel one another; it must form an elastic medium: for it is by the fucceffive pulfes of fuch a medium, that the undulations excited by the founding body are carried to the organ; that a communication is formed between the abject and the sense; which must be done, before the internal machinery of the ear, fubtile as it is, can act at all.

III. The organs of fpeech, and voice, and respiration, are, no lefs than the ear, indebted, for the fuccefs of their operation, to the peculiar qualities of the fluid, in which the animal is immerfed. They, therefore, as well as the ear, are conflituted upon the fuppofition of fuch a fluid, i. e. of a fluid with fuch particular properties, being always prefent. Change the properties of the fluid, and the organ cannot act : change the organ, and the properties of the fluid would be loft. The ftructure therefore of our organs, and the properties of our atmosphere, are made for one anothNor does it alter the relation, whether you alledge the organ to be made for the element, (which feems the most natural way of confidering it,) or the element as prepared for the organ..

er.

IV. But there is another fluid with which we have to do; with properties of its own; with laws of aćting, and of being acted upon, totally different from thofe of air or water:-and that is light. To this new, this fingular element; to qualities perfectly peculiar, perfectly diftin&t and remote from the qualities of any other fubftance with which we are acquainted, an organ is adapted, an inftrument is correctly adjusted, not lefs peculiar amongst the parts of the body, not lefs fingular in its form, and, in the fubftance of which it is compofed, not lefs 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 fpecific amidst the substances with which we converfe. If this does not prove appropriation, I defire to know what would prove it.

Yet the element of light and the organ of vifion, however related in their office and use, have no con. nexion whatever in their original. The action of rays of light upon the furfaces of animals has no tendency to breed eyes in their heads. The fun might shine for ever upon living bodies without the fmalleft approach towards producing the fenfe of fight. On the other hand alfo, the animal eye does not generate or emit light.

V. Throughout the universe there is a wonderful proportioning of one thing to another. The fize of animals, of the human animal especially, when confidered with respect to other animals, or to the plants which grow around him, around him, is fuch, as a regard to his conveniency would have pointed out. A giant or a pigmy could not have milked goats, reaped corn, or mowed grafs; we may add, could not have rode a horfe, trained a vine, fhorn a fheep, with the fame. bodily ease as we do, if at all. A pigmy would have been loft amongft rufhes, or carried off by birds of prey.