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lar,) the surface of all the wrens would exceed the surface of the turkey, in the proportion of the length, breadth (or, of any homologous line,) of a turkey to that of a wren; which would be, perhaps, a proportion of ten to one. It was necessary therefore that small birds should be more warmly clad than large ones: and this seems to be the expedient by which that exigency is provided
II. In comparing different animals, I know no part of their structure which exhibits greater variety, or in that variety, a nicer accommodation to their respective conveniency, than that which is seen in the different formations of their mouths. Whether the purpose be the reception of aliment merely, or the catching of prey, the picking up of seeds, the cropping of herbage, the extraction of juices, the suction of liquids, the breaking and grinding of food, the taste of that food, together with the respiration of air, and, in conjunction with it, the utterance of sound; these various offices are assigned to this one part, and in different species, provided for, as they are wanted, by its different constitution. In the human species, forasmuch as there are hands to convey the food to the mouth, the mouth is flat, and by reason of its flatness, fitted only for reception; whereas the projecting jaws, the wide rictus, the pointed teeth of the dog and his affinities, enable them to apply their mouths to snatch and seize the objects of their pursuit. The full lips, the rough tongue, the corrugated cartilaginous palate, the broad cutting teeth of the ox, the deer, the horse, and the sheep, qualify this tribe for browsing upon their pasture; either gathering large mouthfuls at once, where the grass is long, which is the case with the ox in particular; or biting close, where it is short, which the horse and the sheep are able to do, in a degree that one could hardly expect. The retired under-jaw of a swine works in the ground, after the protruding snout, like a prong or plough-share, has made its way to the roots upon which it feeds. A conformation so happy, was not the gift of chance.
In birds, this organ assumes a new character; new both in substance and in form: but in both, wonderfully adapted to the wants and uses of a distinct mode of existence. We have no longer the fleshy lips, the teeth of enamelled bone; but we have, in the place of these two parts, and to perform the office of both, a hard substance (of the same nature with that which composes the nails, claws, and hoofs, of quadrupeds,) cut out into proper shapes, and mechanically suited to the actions which are wanted. The sharp edge and tempered point of the sparrow's bill picks almost every kind of seed from its concealment in the plant; and not only so, but hulls the grain, breaks and shatters the coats of the seed, in order to get at the kernel. The hooked beak of the hawk tribe separates the flesh from the bones of the animals which it feeds upon, almost with the cleanness and precision of a dissector's knife. The butcher-bird transfixes its prey upon the spike of a thorn, whilst it picks its bones. In some birds of this class, we have the cross-bill, i. e. both the upper and lower bill hooked, and their tips crossing. The spoon-bill enables the goose to graze, to collect its food from the bottom of pools, or to seek it amidst the soft or liquid substances with which it is mixed. The long tapering bill of the snipe and woodcock, penetrates still deeper into
moist earth, which is the bed in which the food of that species is lodged. This is exactly the instrument which the animal wanted. It did not want strength in its bill, which was inconsistent with the slender form of the animal's neck, as well as unnecessary for the kind of aliment upon which it subsists; but it wanted length to reach its object.
But the species of bill which belongs to the birds that live by suction, deserves to be described in its relation to that office. They are what naturalists call serrated or dentated bills; the inside of them towards the edge, being thickly set with parallel or concentric rows of short, strong, sharp-pointed prickles. These, though they should be called teeth, are not for the purpose of mastication, like the teeth of quadrupeds; nor yet, as in fish, for the seizing and retaining of their prey; but for a quite different use. They form a filter. The duck by means of them discusses the mud; examining with great accuracy the puddle, the brake, every mixture which is likely to contain her food. The operation is thus carried on:-The liquid or semi-liquid substances, in which the animal has plunged her bill, she draws, by the action of her lungs, through the narrow interstices which lie between these teeth; catching, as the stream passes across her beak, whatever it may happen to bring along with it, that proves agreeable to her choice, and easily dismissing all the rest. Now, suppose the purpose to have been out of a mass of confused and heterogeneous substances, to separate for the use of the animal, or rather to enable the animal to separate for its own, those few particles which suited its taste and digestion; what more artificial, or more commodious, instrument of selection, could have been given to it, than this natural filter? It has been observed also (what must enable the bird to choose and distinguish with greater acuteness, as well, probably, as what greatly increases its luxury,) that the bills of this species are furnished with large nerves,--that they are covered with a skin,-and that the nerves run down to the very extremity. In the curlew, woodcock, and snipe, there are three pairs of nerves, equal almost to the optic nerve in thickness, which pass first along the roof of the mouth, and then along the upper chap down to the point of the bill, long as the bill is.
But to return to the train of our observations.The similitude between the bills of birds and the mouths of quadrupeds, is exactly such, as, for the sake of the argument, might be wished for. It is near enough to show the continuation of the same plan: it is remote enough to exclude the supposition of the difference being produced by action or use. A more prominent contour, or a wider gap, might be resolved into the effect of continued efforts, on the part of the species, to thrust out the mouth, or open it to the stretch. But by what course of action, or exercise, or endeavour, shall we get rid of the lips, the gums, the teeth; and acquire, in the place of them, pincers of horn? By what habit shall we so completely change, not only the shape of the part, but the substance of which it is composed? The truth is, if we had seen no other than the mouths of quadrupeds, we should have thought no other could have been formed: little could we have supposed, that all the purposes of a mouth, furnished with lips, and armed with teeth, could be answered by an instrument which had none of these; could be supplied,
and that with many additional advantages, by the hardness, and sharpness, and figure, of the bills of birds. Every thing about the animal's mouth is mechanical. The teeth of fish have their points turned backward, like the teeth of a wool or cotton card. The teeth of lobsters work one against another, like the sides of a pair of shears. In many insects, the mouth is converted into a pump or sucker, fitted at the end sometimes with a wimble, sometimes with a forceps; by which double provisions, viz, of the tube and the penetrating form of the point, the insect first bores through the integuments of its prey, and then extracts the juices. And, what is most extraordinary of all, one sort of mouth as the occasion requires, shall be changed into another sort. The caterpillar could not live without teeth; in several species, the butterfly formed from it, could not use them. The old teeth therefore are cast off with the exuvia of the grub; a new and totally different apparatus assumes their place in the fly. Amid these novelties of form, we sometimes forget that it is, all the while, the animal's mouth; that whether it be lips, or teeth, or bill, or beak, or shears, or pump, it is the same part diversified and it is also remarkable, that, under all the varieties of configuration with which we are acquainted, and which are very great, the organs of taste and smelling are situated near each other.
position which would arise from pressure, is greater in the upright trunk than in any other. That theory therefore is pointedly contracted by the example before us. The structure is found where its generation, according to the method by which the theorist would have it generated, is the most difficult; but (observe) it is found where its effect is most useful.
The different length of the intestines in carnivorous and herbivorous animals, has been noticed on a former occasion. The shortest, I believe, is that of some birds of prey, in which the intestinal canal is little more than a straight passage from the mouth to the vent. The longest is in the deer kind. The intestines of a Canadian stag, four feet high, measured ninety-six feet.* The intestine of a sheep, unravelled, measured thirty times the length of the body. The intestine of a wild cat is only three times the length of the body. Universally, where the substance upon which the animal feeds is of slow concoction, or yields its chyle with more difficulty, there the passage is circuitous and dilatory, that time and space may be allowed for the change and the absorption which are necessary. Where the food is soon dissolved, or already half assimilated, an unnecessary or, perhaps, hurtful detention is avoided, by giving to it a shorter and a readier
III. To the mouth adjoins the gullet: in this V. In comparing the bones of different animals, part also, comparative anatomy discovers a differ- we are struck, in the bones of birds, with a proence of structure, adapted to the different necessi-priety, which could only proceed from the wisdom ties of the animal. In brutes, because the posture of an intelligent and designing Creator. In the of their neck conduces little to the passage of the bones of an animal which is to fly, the two qualialiment, the fibres of the gullet, which act in this ties required are strength and lightness. Wherein, business, run in two close spiral lines, crossing therefore, do the bones of birds (I speak of the each other: in men, these fibres run only a little cylindrical bones) differ, in these respects, from obliquely from the upper end of the esophagus to the bones of quadrupeds? In these properties: the stomach, into which, by a gentle contraction, first, their cavities are much larger in proportion they easily transmit the descending morsels; that to the weight of the bone, than in those of quais to say, for the more laborious deglutition of ani- drupeds; secondly, these cavities are empty; mals, which thrust their food up instead of down, thirdly, the shell is of a firmer texture, than is and also through a longer passage, a proportionably the substance of other bones. It is easy to obmore powerful apparatus of muscles is provided; serve these particulars, even in picking the wing more powerful, not merely by the strength of the or leg of a chicken. Now, the weight being the fibres, which might be attributed to the greater same, the diameter, it is evident, will be greater exercise of their force, but in their collocation, in a hollow bone than in a solid one, and with the which is a determinate circumstance, and must diameter, as every mathematician can prove, is have been original. increased, cæteris paribus, the strength of the cylinder, or its resistance to breaking. In a word, a bone of the same weight would not have been so strong in any other form; and to have made it heavier, would have incommoded the animal's flight. Yet this form could not be acquired by use, or the bone become hollow and tubular by exercise. What appetency could excavate a bone?
VI. The lungs also of birds, as compared with the lungs of quadrupeds, contain in them a provision, distinguishingly calculated for this same purpose of levitation; namely, a communication (not found in other kinds of animals) between the airvessels of the lungs and the cavities of the body: so that by the intromission of air from one to the other (at the will, as it should seem, of the animal,) its body can be occasionally puffed out, and its tendency to descend in the air, or its specific gravity, made less. The bodies of birds are blown up from their lungs, (which no other animal bodies are,) and thus rendered buoyant. VII. All birds are oriparous. This likewise
Mem. Acad. Paris, 1701, p. 170.
IV. T gullet leads to the intestines: here, likewise, as before, comparing quadrupeds with man, under a general similitude we meet with appropriate differences. The ralvulæ conniventes, or, as they are by some called, the semilunar valves, found in the human intestine, are wanting in that of brutes. These are wrinkles or plates of the irmermost coat of the guts, the effect of which is to retard the progress of the food through the alimentary canal. It is easy to understand how much more necessary such a provision may be to the body of an animal of an erect posture, and in which, consequently, the weight of the food is added to the action of the intestine, than in that of a quadruped, in which the course of the food, from its entrance to its exit, is nearly horizontal: but it is impossible to assign any cause, except the final cause, for this distinction actually taking place. So far as depends upon the action of the part, this structure was more to be expected in a quadruped than in a man. In truth, it must in both have been formed, not by action, but in direct opposition to action and to pressure; but the op
carries on the work of gestation with as little increase as possible of the weight of the body. A gravid uterus would have been a troublesome burden to a bird in its flight. The advantage, in this respect, of an oviparous procreation, is, that, whilst the whole brood are hatched together, the eggs are excluded singly, and at considerable intervals. Ten, fifteen, or twenty young birds may be produced in one cletch or covey, yet the parent bird have never been encumbered by the load of more than one full-grown egg at one time. VIII. A principal topic of comparison between animals, is their instruments of motion. These come before us under three divisions; feet, wings, and fins. I desire any man to say, which of the three is best fitted for its use; or whether the same consummate art be not conspicuous in them all. The constitution of the elements, in which the motion is to be performed is very different. The animal action must necessarily follow that constitution. The Creator, therefore, if we might so speak, had to prepare for different situations, for different difficulties: yet the purpose is accomplished not less successfully in one case than in the other. And, as between wings and the corresponding limbs of quadrupeds, it is accomplished without deserting the general idea. The idea is modified, not deserted. Strip a wing of its feathers, and it bears an obscure resemblance to the foreleg of a quadruped. The articulations at the shoulder, and the cubitus are much alike; and, what is a closer circumstance, in both cases the upper part of the limb consists of a single bone, the lower part of two.
feathers of the tall. This is the place also for observing, that the pinions are so set upon the body, as to bring down the wings not vertically, but in a direction obliquely tending towards the tail; which motion, by virtue of the common resolution of forces, does two things at the same time; supports the body in the air, and carries it forward. The steerage of a bird in its flight is effected partly by the wings, but in a principal degree by the tail. And herein we meet with a circumstance not a little remarkable. Birds with long legs have short tails; and in their flight, place their legs close to their bodies, at the same time stretching them out backwards as far as they can. In this position, the legs extend beyond the rump, and become the rudder supplying that steerage which the tail could not.
From the wings of birds, the transition is easy to the fins of fish. They are both, to their respective tribes, the instruments of their motion; but, in the work which they have to do, there is a considerable difference, founded in this circumstance. Fish, unlike birds, have very nearly the same specific gravity with the element in which they move. In the case of fish, therefore, there is little or no weight to bear up; what is wanted, is only an impulse sufficient to carry the body through a resisting medium, or to maintain the posture, or to support or restore the balance of the body, which is always the most unsteady where there is no weight to sink it. For these offices, the fins are as large as necessary, though much smaller than wings, their action mechanical, their position, and the muscles by which they are moved, in the highest degree convenient. The following short account of some experiments upon fish, made for the purpose of ascertaining the use of their fins, will be the best confirmation of what we assert. In most fish, beside the great fin the tail, we find two pairs of fins upon the sides, two single fins upon the back, and one upon the belly, or rather between the belly and the tail. The balancing use of these organs is proved in this manner. Of the large-headed fish, if you cut off the pectoral fins, i. e. the pair which lies
But, fitted up with its furniture of feathers and quills, it becomes a wonderful instrument, more artificial than its first appearance indicates, though that be very striking at least, the use which the bird makes of its wings in flying, is more complicated, and more curious, than is generally known. One thing is certain, that if the flapping of the wings in flight were no more than the reciprocal motion of the same surface in opposite directions, either upwards and downwards, or estimated in any oblique line, the bird would lose as much by one motion as she gained by another.close behind the gills, the head falls prone to the The skylark could never ascend by such an ac-bottom: if the right pectoral fin only be cut off, tion as this; for, though the stroke upon the air the fish leans to that side; if the ventral fin on by the under side of her wing would carry her the same side be cut away, then it loses its equiliup, the stroke from the upper side, when she brium entirely; if the dorsal and ventral fins be raised her wing again, would bring her down. In cut off, the fish reels to the right and left. When order, therefore, to account for the advantage the fish dies, that is, when the fins cease to play, which the bird derives from her wing, it is neces- the belly turns upwards. The use of the same sary to suppose, that the surface of the wing, parts for motion is seen in the following observameasured upon the same plane, is contracted, tion upon them when put in action. The pectowhilst the wing is drawn up; and let out to its ral, and more particularly the ventral fins, serve full expansion, when it descends upon the air for to raise and depress the fish: when the fish the purpose of moving the body by the reaction desires to have a retrograde motion, a stroke of that element. Now, the form and structure of forward with the pectoral fin effectually produces the wing, its external convexity, the disposition, it; if the fish desire to turn either way, a single and particularly the overlapping, of its larger fea- blow with the tail the opposite way, sends it round thers, the action of the muscles, and joints of the at once: if the tail strike both ways, the motion pinions, are all adapted to this alternate adjust-produced by the double lash is progressive, and ment of its shape and dimensions. Such a twist, enables the fish to dart forwards with an astonishfor instance, or semirotatory motion, is given to ing velocity. The result is, not only in some the great feathers of the wing, that they strike the cases, the most rapid, but in all cases, the most air with their flat side, but rise from the stroke gentle, pliant, easy, animal motion, with which slantwise. The turning of the oar in rowing, we are acquainted. However, when the tail is whilst the rower advances his hand for a new cut off, the fish loses all motion, and gives itself stroke, is a similar operation to that of the feather, up to where the water impels it. The rest of the and takes its name from the resemblance. I believe that this faculty is not found in the great
*Goldsmith, Hist. of An. Nat. vol. vi. p. 154.
cal use, the anal fin may be reckoned the keel; the ventral fins, out-riggers; the pectoral muscles, the oars; and if there be any similitude between these parts of a boat and a fish, observe, that it is not the resemblance of imitation, but the likeness which arises from applying similar mechanical means to the same purpose.
fins, therefore, so far as respects motion, seem to seems to be this:-It was necessary for such anibe merely subsidiary to this. In their mechani-mals to be able to descry objects with very small degrees of light. This capacity depended upon the superior sensibility of the retina; that is, upon its being affected by the most feeble impulses. But that tenderness of structure, which rendered the membrane thus exquisitely sensible, rendered it also liable to be offended by the access of stronger degrees of light. The contractile range therefore of the pupil is increased in these animals, so as to enable them to close the aperture entirely: which includes the power of diminishing it in every degree; whereby at all times such portions, and only such portions, of light are admitted, as may be received without injury to the sense.
We have seen that the tail in the fish is the great instrument of motion. Now, in cetaceous or warm-blooded fish, which are obliged to rise every two or three minutes to the surface to take breath, the tail, unlike what it is in other fish, is horizontal; its stroke consequently, perpendicular to the horizon, which is the right direction for sending the fish to the top, or carrying it down to
There appears to be also in the figure, and in some properties of the pupil of the eye, an appropriate relation to the wants of different animals. In horses, oxen, goats, sheep, the pupil of the eye is elliptical; the transverse axis being horizontal; by which structure, although the eye be placed on the side of the head, the anterior elongation of the pupil catches the forward rays, those which come from objects immediately in front of the animal's face.
Regarding animals in their instruments of motion, we have only followed the comparison through the first great division of animals into beasts, birds, and fish. If it were our intention to pursue the consideration farther, I should take in that generic distinction amongst birds, the web foot of waterfowl. It is an instance which may be pointed out to a child. The utility of the web to water-fowl, the inutility to land-fowl, are so obvious, that it seems impossible to notice the difference without acknowledging the design. I am at a loss to know, how those who deny the agency of an intelligent Creator, dispose of this example. There is nothing in the action of swimming, as carried on by a bird upon the surface of the water, that should generate a membrane between the toes. As to that membrane, it is an exercise of constant resistance. The only supposition I can think of is, that all birds have been originally water-fowl, and web-footed; that sparrows, hawks, linnets, &c. which frequent the land, have in process of time, and in the course of many generations, had this part worn away by treading upon hard ground. To such evasive assumptions must atheism always have recourse! and, after all, it confesses that the structure of the feet of birds, in their original form, was critically adapted to their original destination! The web-feet of amphibious quadrupeds, seals, otters, &c. fall under the same observation.
IX. The five senses are common to most large animals: nor have we much difference to remark in their constitution; or much, however, which is referable to mechanism.
The superior sagacity of animals which hunt their prey, and which, consequently, depend for their livelihood upon their nose, is well known, in its use; but not at all known in the organization which produces it.
The external ears of beasts of prey, of lions, tigers, wolves, have their trumpet-part, or concavity, standing forwards, to seize the sounds which are before them, viz. the sounds of the animals which they pursue or watch. The ears of animals of flight are turned backward, to give notice of the approach of their enemy from behind, whence he may steal upon them unseen. This is a critical distinction; and is mechanical: but it may be suggested, and, I think, not without probability, that it is the effect of continual habit.
The eyes of animals which follow their prey by night, as cats, owls, &c. possess a faculty not given to those of other species, namely, of closing The pupil entirely. The final cause of which
I BELIEVE that all the instances which I shall collect under this title, might, consistently enough with technical language, have been placed under the head of Comparative Anatomy. But there appears to me an impropriety in the use which that term hath obtained; it being, in some sort, absurd to call that a case of comparative anatomy, in which there is nothing to "compare;" in which a conformation is found in one animal, which hath nothing properly answering to it in another. Of this kind are the examples which I have to propose in the present chapter; and the reader will see that, though some of them be the strongest, perhaps, he will meet with under any division of our subject, they must necessarily be of an unconnected and miscellaneous nature. To dispose them, however, into some sort of order, we will notice, first, particularities of structure which belong to quadrupeds, birds, and fish, as such, or to many of the kinds included in these classes of animals; and then, such particularities as are confined to one or two species.
I. Along each side of the neck of large quadrupeds, runs a stiff, robust cartilage, which butchers call the paxwax. No person can carve the upper end of a crop of beef without driving his knife against it. It is a tough, strong, tendinous substance, braced from the head to the middle of the back: its office is to assist in supporting the weight of the head. It is a mechanical provision, of which this is the undisputed use; and it is sufficient, and not more than sufficient, for the purpose which it has to execute. The head of an ox or a horse is a heavy weight, acting at the end of a long lever (consequently with a great purchase) and in a direction nearly perpendicular to the joints of the supporting neck. From such a force, so advantageously applied, the bones of the neck would be in constant danger of dislocation, if they were not fortified by this strong tape. No such
organ is found in the human subject, because, | no possibility, entertain a question in their minds, from the erect position of the head (the pressure whether it were a contrivance or not. No reason of it acting nearly in the direction of the spine,) has ever been assigned-no reason can be assignthe junction of the vertebræ appears to be suffi-ed, why the conclusion is not as certain in the ciently secure without it. This cautionary expe- fish, as it is in the machine; why the argument dient, therefore, is limited to quadrupeds: the care is not as firm in one case as the other. of the Creator is seen where it is wanted.
It would be very worthy of inquiry, if it were
II. The oil with which birds prune their fea-possible to discover by what method an animal thers, and the organ which supplies it, is a specific which lives constantly in water, is able to supply provision for the winged creation. On each side a repository of air. The expedient, whatever it of the rump of birds is observed a small nipple, be, forms part, and perhaps the most curious part, yielding upon pressure a butter-like substance, of the provision. Nothing similar to the air-bladwhich the bird extracts by pinching the pap with der is found in land-animals; and a life in the its bill. With this oil, or ointment, thus procured, water has no natural tendency to produce a bag the bird dresses its coat; and repeats the action of air. Nothing can be farther from an acquired as often as its own sensations teach it that it is in organization than this is. any part wanted, or as the excretion may be sufficient for the expense. The gland, the pap, the nature and quality of the excreted substance, the manner of obtaining it from its lodgment in the body, the application of it when obtained, form, collectively, an evidence of intention which it is not easy to withstand. Nothing similar to it is found in unfeathered animals. What blind conatus of nature should produce it in birds; should not produce it in beasts?
These examples mark the attention of the Creator to the three great kingdoms of his animal creation, and to their constitution as such.-The example which stands next in point of generality, belonging to a large tribe of animals, or rather to various species of that tribe, is the poisonous tooth of serpents.
1. The fang of a viper is a clear and curious example of mechanical contrivance. It is a perforated tooth, loose at the root; in its quiet state, III. The air-bladder also of a fish affords a lying down flat upon the jaw, but furnished with plain and direct instance, not only of contri- a muscle, which, with a jerk, and by the pluck, as vance, but strictly of that species of contrivance it were, of a string, suddenly erects it. Under which we denominate mechanical. It is a philo- the tooth, close to its root, and communicating sophical apparatus in the body of an animal. The with the perforation, lies a small bag containing principle of the contrivance is clear: the applica- the venom. When the fang is raised, the closing tion of the principle is also clear. The use of the of the jaw presses its root against the bag underorgan to sustain, and, at will, also to elevate, the neath; and the force of this compression sends body of the fish in the water, is proved by observ- out the fluid with a considerable impetus through ing, what has been tried, that, when the bladder the tube in the middle of the tooth. What more is burst, the fish grovels at the bottom; and also, unequivocal or effectual apparatus could be dethat flounders, soles, skates, which are without the vised, for the double purpose of at once inflicting air-bladder, seldom rise in the water, and that with the wound and injecting the poison? Yet, though effort. The manner in which the purpose is at-lodged in the mouth, it is so constituted, as, in its tained, and the suitableness of the means to the inoffensive and quiescent state, not to interfere end, are not difficult to be apprehended. The with the animal's ordinary office of receiving its rising and sinking of a fish in water, so far as it food. It has been observed also, that none of the is independent of the stroke of the fins and tail, harmless serpents, the black snake, the blind can only be regulated by the specific gravity of worm, &c. have these fangs, but teeth of an equal the body. When the bladder, contained in the size; not moveable, as this is, but fixed into the body of the fish, is contracted, which the fish pro- jaw. bably possesses a muscular power of doing, the bulk of the fish is contracted along with it; whereby, since the absolute weight remains the same, the specific gravity, which is the sinking force, is increased, and the fish descends: on the contrary, when, in consequence of the relaxation of the muscles, the elasticity of the enclosed and now compressed air restores the dimensions of the bladder, the tendency downwards becomes proportionably less than it was before, or is turned into a contrary tendency. These are known properties of bodies immersed in a fluid. The enamelled figures, or little glass bubbles, in a jar of water, are made to rise and fall by the same artifice. A diving-machine might be made to ascend and descend, upon the like principle; namely, by introducing into the inside of it an air-vessel, which, by its contraction, would diminish, and by its distension enlarge, the bulk of the machine it-nished with bones and muscles of its own. Two self, and thus render it specifically heavier, or bones are placed before the os pubis, and joined to specifically lighter, than the water which sur-that bone as their base. These support, and give rounds it. Suppose this to be done, and the ar- a fixture to, the muscles which serve to open the tist to solicit a patent for his invention. The bag. To these muscles there are antagonists, inspectors of the model, whatever they might think which serve in the same manner to shut it; and of the use or value of the contrivance, could, by this office they perform so exactly, that, in the
11. In being the property of several different species, the preceding example is resembled by that which I shall next mention, which is the beg of the opossum. This is a mechanical contrivance, most properly so called. The simplicity of the expedient renders the contrivance more obvious than many others, and by no means less certain. A false skin under the belly of the animal, forms a pouch, into which the young litter are received at their birth; where they have an easy and constant access to the teats; in which they are transported by the dam from place to place; where they are at liberty to run in and out; and where they find a refuge from surprise and danger. It is their cradle, their asylum, and the machine for their conveyance. Can the use of this structure be doubted of? Nor is it a mere doubling of the skin; but it is a new organ, fur