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placed on each side of it, which lifts and depresses it a: pleasure.*
V. The spider's web is a compensating contrivance The spider lives upon flies, without wings to pursue thein; 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 speci. fically 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 lens 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. [Pl. XXX. fig. 8.) 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. (Pl. XXX. fig. 9.] 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 may 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 chameleon. [Pl. XXXI. fig. 1.) 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 chameleon is inflexible. To make up for this, the eye is so prominent, as that more than half 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
* Goldan ith's Nat. Hist. vol. v. p. 274.
linary protection of eyelid, as well as a more than one diary supply of moisture; yet the motion of an eyelid, formed according to the common construction, would be impeded, as it should seen, 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 substitvtion. 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. [Pl. XXXI. fig. 2, 3.] 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 route, 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 contri vance 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 plaster. A muscle, which might seern, 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 in its constitution a difficulty se
great, that one could hardly conjecture beforehand 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, consists 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 simpliciy 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 a be successively fitted with new ones? If a change of shi] became necessary, how was the lobster to extricate hims If from his present confinement? How was he to uncase is buckler, or draw his legs out of his boots? The process, which fishermen have observed to take place, is as foll sws: At certain seasons, the shell of a lobster grows soft, the animal swells its body, the seams open, and the claws burst at the joints. When the shell has 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 over-charged 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 nis organs, ard to his propensities with respect to
food; i. 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.
For 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
* Blumenbach states, in his Manual of Natural History, that he had conversed with many Hollanders who had lived in Guiana, and from them collected, that this apparently miserable animal is rather an enviable one. First, he nourishes himself entirely from leaves, and, therefore, when he has once climbed a tree, he can live on the sa quarter of a year. Secondly, he does not drink at all. Thirdly, on a tree he is exposed to but few enemies, and when the sloth marks that 9 tiger-cat is climbing up a branch, it goes softly to the end of the branch, and rocks it till the tiger-cat falls oil, so that seldom is there an instance that a tiger-cat surprises one: even upon the ground, so powerful are the claws of the sloth, and so fearful its cries, that its enemies generally get the worst. So idle is Buffon's declamation against the goodness and wisdom of Providence, drawn from this beast.
which is necessary, but which was before left imperfect. 1 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. *
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 granivorous and herbivorous birds; such as common fowls, turkeys, ducks, geese, pigeons, &c. 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 plates, 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 here 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 farther 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 stoinach, accompanies the hooked, notched beak; the short
*Spall. dis. III. Sect. 140.