Compensation is a species of relation. It is relation, when the defects of one part, or of one organ, are supplied by the structure of another part, or of another organ. Thus,
I. The short, unbending neck of the elephant, is compensated by the length and flexibility of his proboscis.* He could not have reached the ground without it: or, if it be supposed that he might have fed upon the fruit, leaves, or branches of trees, how was he to drink? Should it be asked, Why is the elephant's neck so short? it may be answered that the weight of a head so heavy could not have been supported at the end of a longer lever. To a form therefore, in some respects necessary, but in some respects also inadequate to the occasions of the animal, a supplement is added, which exactly makes up the deficiency under which he laboured.
If it be suggested, that this proboscis may have been produced in a long course of generations, by the constant endeavour of the elephant to thrust out his nose, (which is the general hypothesis by which it has lately been attempted to account for the forms of animated nature,)* I would ask, how was the animal to subsist in the mean time; during the process; until this prolongation of snout were completed? What was to become of the individual, whilst the species was perfecting?
Our business at present is, simply to point out the relation, which this organ bears to the peculiar figure of the animal, to which it belongs. And, herein, all things correspond. The necessity of the elephant's proboscis arises from the shortness of his neck; the shortness of the neck is rendered necessary by the weight of the head. Were we to enter into an examination of the structure and anatomy of the proboscis itself, we should see in it one of the most curious of all examples of animal mechanism. The disposition of the ringlets and fibres, for the purpose, first, of forming a long cartilaginous pipe; secondly, of contracting and lengthening that pipe; thirdly, of turning it in every direction at the will of the animal; with the superaddition, at the end, of a fleshy production, of about the length and thickness of a finger, and performing the office of a finger, so as to pick up a straw from the ground; these properties of the same organ, taken together, exhibit a specimen, not only of design, (which is attested by the advantage,) but of consummate art, and, as I may say, of elaborate preparation, in accomplishing that design.
II. The hook in the wing of a bat, is strictly a mechanical, and, also, a compensating contrivance. At the angle of its wing there is a bent claw, exactly in the form of a hook, by which the bat attaches itself to the sides of rocks, caves, and buildings, laying hold of crevices, joinings, chinks, and roughnesses. It hooks itself by this claw; remains suspended by this hold; takes its flight from this position: which operations compensate for the decrepitude of its legs and feet. Without her hook, the bat would be the most helpless of all animals. She can neither run upon her feet, nor raise herself from the ground. These inabilities are made up to her by the contrivance in her wing: and in placing a claw on that part, the Creator has deviated from the analogy observed in winged animals. A singular defect required a singular substitute.
III. The crane kind are to live and seek their food amongst the waters; yet, having no web feet, are incapable of swimming. To make up for this deficiency, they are furnished with long legs for wading, or long bills for groping; or usually with both. This is compensation. But I think the true reflection upon the present instance is, how every part of nature is tenanted by appropriate inhabitants. Not only is the surface of deep waters peopled by numerous tribes* of birds that swim, but marshes and shallow pools are furnished with hardly less numerous tribes of birds that wade.
IV. The common parrot has, in the structure of its beak, both an inconveniency, and a compensation for it. When I speak of an incon-veniency, I have a view to a dilemma which frequently occurs in the works of nature, viz. that the peculiarity of structure by which an organ is made to answer one purpose, necessarily unfits it for some other purpose. This is the case before us. The upper bill of the parrot is so much hooked, and so much overlaps the lower, that, if, as in other birds, the lower chap alone had motion, the bird could scarcely gape wide enough to receive its food: yet this hook and overlapping of the bill could not be spared, for it forms the very instrument by which the bird climbs: to say nothing of the use which it makes of it in breaking nuts, and the hard substances upon which it feeds. How therefore has nature provided for the opening of this occluded mouth? By making the upper chap moveable, as well as the lower. In most birds the upper chap is connected, and makes but one piece, with the skull; but, in the parrot, the upper chap is joined to the bone of the head by a strong membrane, placed on each side of it, which lifts and depresses it at pleasure.1
V. The spider's web is a compensating contrivance. The spider lives upon flies, without wings to pursue them; a case, one would have thought, of great difficulty, yet provided for; and provided for by a resource, which no stratagem, no effort of the animal, could have produced, had not both its external and internal structure been specifically adapted to the operation.
VI. In many species of insects the eye is fixed; and consequently without the power of turning the pupil to the object. This great defect is however perfectly compensated; and by a mechanism which we should not suspect. The eye is a multiplying glass; with a lense looking in every direction, and catching every object. By which means, although the orb of the eye be stationary, the field of vision is as ample as that of other animals; and is commanded on every side. When this lattice work was first observed,* the multiplicity and minuteness of the surfaces must have added to the surprise of the discovery. Adams tells us,* that fourteen hundred of these reticulations have been counted in the two eyes of a drone bee.
In other cases, the compensation is effected, by the number and position of the eyes themselves. The spider has eight eyes, mounted upon different parts of the head, two in front, two in the top of the head, two on each side. These eyes are without motion; but, by their situation, suited to comprehend every view, which the wants or safety of the animal render it necessary for it to take.
VII. The Memoirs for the Natural History of Animals, published by the French Academy, A. D. 1687, furnish us with some curious particulars in the eye of a camelion. Instead of two eyelids it is covered by an eyelid with a hole in it. This singular structure appears to be compensatory, and to answer to some other singularities in the shape of the animal. The neck of the camelion is inflexible. To make up for this, the eye is so prominent, as that more than half of the ball stands out of the head. By means of which extraordinary projection, the pupil of the eye can be carried by the muscles in every direction,
and is capable of being pointed towards every object. But then so unusual an exposure of the globe of the eye, requires for its lubricity and defence, a more than ordinary protection of eyelid, as well as more than ordinary supply of moisture; yet the motion of an eyelid, formed according to the common construction, would be impeded, as it should seem, by the convexity of the organ. The aperture in the lid meets this difficulty. It enables the animal to keep the principal part of the surface of the eye under cover, and to preserve it in a due state of humidity, without shutting out the light; or without performing every moment a nictitation,* which, it is probable, would be more laborious to this animal than to others.
VIII. In another animal, and in another part of the animal economy, the same Memoirs describe a most remarkable substitution. The reader will remember what we have already observed concerning the intestinal canal; that its length, so many times exceeding that of the body, promotes the extraction of the chyle from the aliment, by giving room for the lacteal vessels to act upon it through a greater space. This long intestine, wherever it occurs, is, in other animals, disposed in the abdomen from side to side in returning folds. But, in the animal now under our notice, the matter is managed otherwise. The same intention is mechanically effectuated; but by a mechanism of a different kind. The animal of which I speak, is an amphibious quadruped, which our authors call the alopecias, or sea fox. The intestine is straight from one end to the other: but in this straight, and consequently short intestine, is a winding, corkscrew, spiral passage, through which, the food, not without several circumvolutions, and in fact by a long rout, is conducted to its exit. Here the shortness of the gut is compensated by the obliquity of the perforation.
IX. But the works of the Deity are known by expedients. Where we should look for absolute destitution; where we can reckon up nothing but wants; some contrivance always comes in to supply the privation. A snail, without wings, feet, or thread, climbs up the stalks of plants, by the sole aid of a viscid humour discharged from her skin. She adheres to the stems, leaves, and fruits of plants, by means of a sticking plaister. A muscle, which might seem, by its helplessness, to lie at the mercy of every wave that went over it, has the singular power of spinning, strong, tendinous threads, by which she moors her shell to rocks and timbers. A cockle, on the contrary, by means of its stiff tongue, works for itself a shelter in the sand. The provisions of nature extend to cases the most desperate. A lobster has a difficulty in its constitution so great, that one could hardly conjecture before hand how nature would dispose of it. In most animals, the skin grows with their growth. If, instead of a soft skin, there be a shell, still it admits of a gradual enlargement. If the shell, as in the tortoise, consist of several pieces, the accession* of substance is made at the sutures. Bivalve shells grow bigger by receiving an accretion at their edge: it is the same with spiral shells at their mouth. The simplicity of their form admits of this. But the lobster's shell being applied to the limbs of the body, as well as to the body itself, allows not of either of the modes of growth which are observed to take place in other shells. Its hardness resists expansion; and its complexity renders it incapable of increasing its size by addition of substance to its edge. How then was the growth of the lobster to be provided for? Was room to be made for it in the old shell, or was it to be successively fitted with new ones? If a change of shell became necessary, how was the lobster to extricate himself from his present confinement? How was he to uncase his buckler, or draw his legs out of his boots? The process, which fishermen have observed to take place, is as follows. At certain seasons, the shell of the lobster grows soft; the animal swells its body; the seams open, and the claws burst at the joints. When the shell is thus become loose upon the body, the animal makes a second effort, and by a tremulous, spasmodic motion, casts it off. In this state the liberated, but defenceless, fish, retires into holes in the rock. The released body now suddenly pushes its growth. In about eight-and-forty hours, a fresh concretion of humour upon the surface, i. e. a new shell, is formed, adapted in every part to the increased dimensions of the animal. This wonderful mutation* is repeated every year.
If there be imputed defects without compensation, I should suspect that they were defects only in appearance. Thus, the body of the sloth has often been reproached for the slowness of its motions, which has been attributed to an imperfection in the formation of its limbs. But it ought to be observed, that it is this slowness, which alone suspends the voracity of the animal. He fasts during his migration from one tree to another; and this fast may be necessary for the relief of his overcharged vessels, as well as to allow time for the concoction of the mass of coarse and hard food which he has taken into his stomach. The tardiness of his pace seems to have reference to the capacity of his organs, and to his propensities with respect to food; h. e.* is calculated to counteract the effects of repletion.
Or there may be cases, in which a defect is artificial, and compensated by the very cause which produces it. Thus the sheep, in the domesticated state in which we see it, is destitute of the ordinary means of defence or escape; is incapable either of resistance or flight. But this is not so with the wild animal. The natural sheep is swift and active: and, if it lose these qualities when it comes under the subjection of man, the loss is compensated by his protection. Perhaps there is no species of quadruped whatever, which suffers so little as this does, from the depredation of animals of prey.
F or the sake of making our meaning better understood, we have considered this business of compensation under certain particularities of constitution, in which it appears to be most conspicuous. This view of the subject necessarily limits the instances to single species of animals. But there are compensations, perhaps, not less certain, which extend over large classes, and to large portions, of living nature.
I. In quadrupeds, the deficiency of teeth is usually compensated by the faculty of rumination. The sheep, deer, and ox tribe, are without fore teeth in the upper jaw. These ruminate. The horse and ass are furnished with teeth in the upper jaw, and do not ruminate. In the former class the grass and hay descend into the stomach, nearly in the state in which they are cropped from the pasture, or gathered from the bundle. In the stomach they are softened by the gastric juice, which in these animals is unusually copious. Thus softened, and rendered tender, they are returned a second time to the action of the mouth, where the grinding teeth complete at their leisure the trituration which is necessary, but which was before left imperfect. I say the trituration which is necessary; for it appears from experiments that the gastric fluid of sheep, for example, has no effect in digesting plants, unless they have been previously masticated; that it only produces a slight maceration, nearly as common water would do in a like degree of heat: but that, when once vegetables are reduced to pieces by mastication, the fluid then exerts upon them its specific operation. Its first effect is to soften them, and to destroy their natural consistency: it then goes on to dissolve them; not sparing even the toughest parts, such as the nerves of the leaves.1
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 compensation is still more striking. They have no teeth at all. What have they then to make up for this severe want? I speak of graminivorous and herbivorous* birds; such as common fowls, turkeys, ducks, geese, pigeons, etc. for it is concerning these alone that the question need be asked. All these are furnished with a peculiar and most 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 same mechanical action, as a coffee-mill would do. It has been proved by the most correct experiments, that the gastric juice 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 starved upon a heap of corn. Yet why should a bill and a gizzard go together? Why should a gizzard never be found where there are teeth?
Nor does the gizzard belong to birds as such. A gizzard is not found in birds of prey. Their food requires not to be ground down in a mill. The compensatory contrivance goes no further than the necessity. In both classes of birds however, the digestive organ within the body, bears a strict and mechanical relation to the external instruments for procuring food. The soft membranous stomach, accompanies the hooked, notched, beak; the short, muscular legs; the strong, sharp, crooked talons: the cartilaginous stomach, attends that conformation of bill and toes, which restrains the bird to the picking of seeds or the cropping of plants.
III. But to proceed with our compensations. A very numerous and comprehensive tribe of terrestrial animals are entirely without feet; yet locomotive; and, in a very considerable degree, swift in their motion. How is the want of feet compensated? It is done by the disposition of the muscles and fibres of the trunk. In consequence of
the just collocation, and by means of the joint action of longitudinal and annular fibres, that is to say, of strings and rings, the body and train of reptiles are capable of being reciprocally shortened and lengthened, drawn up and stretched out. The result of this action is a progressive, 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 roughnesses of the surface 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 mechanism. Suppose that we had never seen an animal move upon the ground without feet, and that the problem was, muscular action, i. e. reciprocal contraction and relaxation being given, to describe how such 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 artist; or might have been exhibited in an automaton, by the combination of springs, spiral wires, and ringlets: but to the solution of the problem would not be denied, surely, the praise of invention and of successful thought; least of all could it ever be questioned, whether intelligence had been employed about it, or not.
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