Chapter XII.
Comparative Anatomy

Whenever we find a general plan pursued, yet with such variations in it as are, in each case, required by the particular exigency of the subject to which it is applied, we possess, in such a plan and such adaptation, the strongest evidence that can be afforded of intelligence and design—an evidence which the most completely excludes every other hypothesis. If the general plan proceeded from any fixed necessity in the nature of things, how could it accommodate itself to the various wants and uses which it had to serve under different circumstances and on different occasions? Arkwright's mill was invented for the spinning of cotton. We see it employed for the spinning of wool, flax, and hemp, with such modifications of the original principle, such variety in the same plan, as the texture of those different materials rendered necessary. Of the machine's being put together with design, if it were possible to doubt while we saw it only under one mode, and in one form, when we came to observe it in its different applications, with such changes of structure, such additions and supplements, as the special and particular use in each case demanded, we could not refuse any longer our assent to the proposition, "that intelligence, properly and strictly so called—including, under that name, foresight, consideration, reference to utility—had been employed, as well in the primitive plan as in the several changes and accommodations which it is made to undergo."

Very much of this reasoning is applicable to what has been called comparative anatomy. In their general economy, in the outlines of the plan, in the construction as well as offices of their principal parts, there exists between all large terrestrial animals a close resemblance. In all, life is sustained, and the body nourished, by nearly the same apparatus. The heart, the lungs, the stomach, the liver, the kidneys, are much alike in all. The same fluid—for no distinction of blood has been observed—circulates through their vessels, and nearly in the same order. The same cause, therefore, whatever that cause was, has been concerned in the origin, has governed the production of these different animal forms.

When we pass on to smaller animals, or to the inhabitants of a different element, the resemblance becomes more distant and more obscure; but still the plan accompanies us.

And, what we can never enough commend, and which it is our business at present to exemplify, the plan is attended, through all its varieties and deflections, by subserviences to special occasions and utilities.

I. The covering of different animals—though whether I am correct in classing this under their anatomy, I do not know—is the first thing which presents itself to our observation; and is, in truth, both for its variety and its suitableness, to their several natures, as much to be admired as any part of their structure. We have bristles, hair, wool, furs, feathers, quills, prickles, scales; yet in this diversity both of material and form, we cannot change one animal's coat for another without evidently changing it for the worse; taking care, however, to remark, that these coverings are, in many cases, armor as well as clothing; intended for protection as well as warmth.

The human animal is the only one which is naked, and the only one which can clothe itself. This is one of the properties which renders him an animal of all climates, and of all seasons. He can adapt the warmth or lightness of his covering to the temperature of his habitation. Had he been born with a fleece upon his back, although he might have been comforted by its warmth in high latitudes, it would have oppressed him by its weight and heat, as the species spread towards the equator.

What art, however, does for man, nature has, in many instances, done for those animals which  are incapable of art. Their clothing, of its own accord, changes with their necessities. This is particularly the case with that large tribe of quadrupeds which are covered with furs. Every dealer in hare-skins and rabbit-skins knows how much the fur is thickened by the approach of winter. It seems to be a part of the same constitution and the same design, that wool, in hot countries, degenerates, as it is called, but in truth—most happily for the animal's ease—passes into hair; while, on the contrary, that hair, in the dogs of the polar regions, is turned into wool, or something very like it. To which may be referred, what naturalists have remarked, that bears, wolves, foxes, hares, which do not take the water, have the far much thicker on the back than the belly; whereas in the beaver it is the thickest upon the belly, as are the feathers in water-fowl. We know the final cause of all this, and we know no other.

The covering of birds cannot escape the most vulgar observation; its lightness, its smoothness, its warmth—the disposition of the feathers all inclined backward, the down about their stem, the overlapping of their tips, their different configuration in different parts, not to mention the variety of their colors, constitute a vestment for the body so beautiful, and so appropriate to the life which the animal is to lead, as that, I think, we should have had no conception of any thing equally perfect, if we had never seen it, or can now imagine any thing more so. Let us suppose—what is possible only in supposition—a person who had never seen a bird, to be presented with a plucked pheasant, and bid to set his wits to work how to contrive for it a covering which shall unite the qualities of warmth, levity, and least resistance to the air, and the highest degree of each; giving it also as much of beauty and ornament as he could afford. He is the person to behold the work of the Deity, in this part of his creation, with the sentiments which are due to it.

The commendation which the general aspect of the feathered world seldom fails of exciting, will be increased by further examination. It is one of those cases in which the philosopher has more to admire than the common observer. Every feather is a mechanical wonder. If we look at the quill, we find properties not easily brought together—strength and lightness. I know few things more remarkable than the strength and lightness of the very pen with which I am writing. If we cast our eye to the upper part of the stem, we see a material made for the purpose, used in no other class of animals, and in no other part of birds, tough, light, pliant, elastic. The pith also, which feeds the feathers, is, among animal substances, sui generis—neither bone, flesh, membrane, nor tendon.*

But the artificial part of a feather is the beard, or, as it is sometimes, I believe, called, the vane. By the beards are meant what are fastened on each side of the stem, and what constitute the breadth of the feather—what we usually strip off from one side or both, when we make a pen.  The separate pieces, or laminae, of which the beard is composed, are called threads, sometimes filaments or rays. Now, the first thing which an attentive observer will remark is, how much stronger the beard of the feather shows itself to be when pressed in a direction perpendicular to its plane, than when rubbed, either up or down, in the line of the stem; and he will soon discover the structure which occasions this difference, namely, that the laminae whereof these beards are composed are flat, and placed with their flat sides towards each other; by which means, while they easily bend for the approaching of each other, as any one may perceive by drawing his finger ever so lightly upwards, they are much harder to bend out of their plane, which is the direction in which they have to encounter the impulse and pressure of  the air, and in which their strength is wanted and put to the trial.

This is one particularity in the structure of a feather; a second is still more extraordinary. Whoever examines a feather cannot help taking notice, that the threads or laminae of which we have been speaking, in their natural state unite—that their union is something more than the mere apposition of loose surfaces—that they are not parted asunder without some degree of force—that nevertheless there is no glutinous cohesion between them—that therefore, by some mechanical means or other, they catch or clasp among themselves, thereby giving to the beard or vane its closeness and compactness of texture. Nor is this all: when two laminae which have been separated by accident or force are brought together again, they immediately reclasp; the connection, whatever it was, is perfectly recovered, and the beard of the feather becomes as smooth and firm as if nothing had happened to it. Draw your finger down the feather, which is against the grain, and you break probably the junction of some of the contiguous threads; draw your finger up the feather, and you restore all things to their former state. This is no common contrivance: and now for the mechanism by which it is effected. The threads or laminae above mentioned are interlaced with one another; and the interlacing is performed by means of a vast number of fibres or teeth, which the laminae shoot forth on each side, and which hook and grapple together. A friend of mine counted fifty of these fibres in one-twentieth of an inch. These fibres are crooked, but curved after a different manner: for those which proceed from the thread on the side towards the extremity of the feather, are longer, more flexible, and bent downwards; whereas those which proceed from the side towards the beginning or quill end of the feather, are shorter, firmer, and turn upwards. The process, then, which takes place is as follows: when two laminae are pressed together, so that these long fibres are forced far enough over the short ones, their crooked parts fall into the cavity made by the crooked parts of the others; just as the latch that is fastened to a door enters into the cavity of the catch fixed to the door-post, and there hooking itself, fastens the door; for it is properly in this manner that one thread of a feather is fastened to the other.

This admirable structure of the feather, which it is easy to see with the microscope, succeeds perfectly for the use to which nature has designed it; which use was, not only that the laminae might be united, but that when one thread or lamina has been separated from another by some external violence, it might be reclasped with sufficient facility and expedition.*

In the ostrich, this apparatus of crotchets and fibres, of hooks and teeth, is wanting; and we see the consequence of the want. The filaments hang loose and separate from one another, forming only a kind of down; which constitution of the feathers, however it may fit them for the flowing honors of a lady's headdress, may be reckoned an imperfection in the bird, inasmuch as wings composed of these feathers, although they may greatly assist it in running, do not serve for flight.

But under the present division of our subject, our business with feathers is as they are the covering of the bird. And herein a singular circumstance occurs. In the small order of birds which winter with us, from a snipe downwards, let the external color of the feathers be what it will, their Creator has universally given them a bed of black down next their bodies. Black, we know, is the warmest color; and the purpose here is, to keep in the heat arising from the heart and circulation of the blood. It is further likewise remarkable, that this is not found in larger birds; for which there is also a reason. Small birds are much more exposed to the cold than large ones, forasmuch as they present, in proportion to their bulk, a much larger surface to the air. If a turkey were divided into a number of wrens—supposing the shape of the turkey and the wren to be similar—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 for.

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 the swine works in the ground, after the protruding snout, like a prong or ploughshare, 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 while it picks its bones. In some birds of this class we have the cross-bill, that is, 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 penetrate 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, sharp­pointed 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 semiliquid 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 endeavor, 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 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 provision, namely, 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 exuviae 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.

III. To the mouth adjoins the gullet: in this part also, comparative anatomy discovers a difference of structure, adapted to the different necessities of the animal. In brutes, because the posture of their neck conduces little to the passage of the aliment, the fibres of the gullet which act in this business run in two close spiral lines, crossing each other; in men, these fibres run only a little obliquely from the upper end of the oesophagus to the stomach, into which, by a gentle contraction, they easily transmit the descending morsels: that is to say, for the more laborious deglutition of animals which thrust their food up instead of down, and also through a longer passage, a proportionably more power­ful apparatus of muscles is provided—more powerful, not merely by the strength of the fibres, which might be attributed to the greater exercise of their force, but in their collocation, which is a determinate circumstance, and must have been original.

IV. The gullet leads to the intestines: here, likewise, as before, comparing quadrupeds with man, under a general similitude we meet with appropriate differences. The valvulae 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 innermost 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 opposition which would arise from pressure is greater in the upright trunk than in any other. That theory, therefore, is pointedly contradicted 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 intestines of a sheep, unravelled, measured thirty times the length of the body. The intestines of a wild cat are 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 route.

V. In comparing the bones of different animals, we are struck, in the bones of birds, with a propriety which could only proceed from the wisdom of an intelligent and designing Creator. In the bones of an animal which is to fly, the two qualities required are strength and lightness. Wherein, therefore, do the bones of birds—I speak of the cylindrical bones—differ in these respects from the bones of quadrupeds? In three properties: first, their cavities are much larger, in proportion to the weight of the bone, than in those of quadrupeds; secondly, these cavities are empty; thirdly, the shell is of a firmer texture than is the substance of other bones. It is easy to observe these particulars even in picking the wing or leg of a chicken. Now, the weight being the same, the diameter, it is evident, will be greater in a hollow bone than in a solid one; and with the diameter, as every mathematician can prove, is increased, caeteris 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 or tubular by exercise. What appetency could excavate a bone?

VI. The lungs also of birds, as compared with the wings 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 air-vessels 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 oviparous. This likewise 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 while 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 clutch 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 in 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 wines 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 no obscure resemblance to the fore-leg 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.

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 winds 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. The sky­lark could never ascend by such an action as this; for, though the stroke upon the air by the underside of her wing would carry her up, the stroke from the upper side, when she raised her wing again, would bring her down. In order, therefore, to account for the advantage which the bird derives from her wing, it is necessary to suppose that the surface of the wing, measured upon the same plane, is contracted while the wing is drawn up, and let out to its full expansion when it descends upon the air for the purpose of moving the body by the reaction of that element. Now, the form and structure of the wing, its external convexity, the disposition and particularly the overlapping of its larger feathers, the action of the muscles and joints of the pinions, are all adapted to this alternate adjustment of its shape and dimensions. Such a twist, for instance, or semirotary motion, is given to the great feathers of the wing, that they strike the air with their flat side, but rise from the stroke slantwise. The turning of the oar in rowing, while the rower advances his hand for a new stroke, is a similar operation to that of the feather, and takes its name from the resemblance. I believe that this faculty is not found in the great feathers of the tail. 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, besides 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, that is, the pair which lies close behind the gills, the head falls prone to the bottom; if the right pectoral fin only be cut off, the fish leans to that side; if the ventral fin on the same side be cut away, then it loses its equilibrium entirely; if the dorsal and ventral fins be cut off, the fish reels to the right and left. When the fish dies, that is, when the fins cease to play, the belly turns upwards. The use of the same parts for motion is seen in the following observation upon them when put in action. The pectoral, and more particularly the ventral fins, serve to raise and depress the fish: when the fish desires to have a retrograde motion, a stroke forward with the pectoral fin effectually produces it; if the fish desires to turn either way, a single blow with the tail the opposite way sends it round at once; if the tail strike both ways, the motion produced by the double lash is progressive, and enables the fish to dart forward with an astonishing velocity.* The result is not only in some cases the most rapid, but in all cases the most gentle, pliant, easy animal motion with which we are acquainted. However, when the tail is cut off, the fish loses all motion, and gives itself up to where the water impels it. The rest of the fins, therefore, so far as respects motion, seem to be merely subsidiary to this. In their mechanical use, the anal fin may be reckoned the keel; the ventral fins, outriggers; 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.

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 the bottom.

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 among 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 web­footed; that sparrows, hawks, linnets, etc., 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, etc., 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 forward, to seize the sounds which are before them, namely, 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, etc., possess a faculty not given to those of other species, namely, of closing the pupil entirely. The final cause of which seems to be this: it was necessary for such animals 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.

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, and 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, or those which come from objects immediately in front of the animal's face.