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II. The mechanical principles on which strength is combined with lightness,—or the proportion of strength greatly augmented beyond the quantum of matter employed, -or the power of resistance secured without a needless waste of matter, —are constantly observed in the system of nature. Thus the reeds of corn, and other tall-growing herbaceous plants, which have a weighty head to support, are hollow, and by this means while no matter is unnecessarily wasted, the strength of the stalk, like that of an iron tube or hollow pillar, is increased ; it is capable of supporting the ear, and bringing it to perfection by exposing it duly to the action of the sun and the air. The young shoots of trees, also, while in their soft and herbaceous state, especially of those that shoot up quickly, consist of a thin rind of firm woody fibres on the outside only, while the inside is filled with a spungy matter called pith; thus the diameter of the shoot is greatly augmented in proportion to its solid contents, and the shoot is capable of making a much greater resistance than it could otherwise have done. But when by age the woody fibres have acquired a firmer consistence, the pith is gradually diminished in size till at length it is wholly obliterated. This beautiful economy will be found to apply, not only to vegetable, but to animal productions. Had the quills of the wing and tail feathers in fowls been each a mass of solid matter, they would have formed an insupportable weight, and been obviously unsuitable to the purpose of flying ; on the other hand, had the whole of the matter proper for constituting any single quill been reduced into one solid bar instead of a tube, it would not have possessed, perhaps, one hundredth part of the strength which is equally requisite to fit it for its use.
In animals the bones, too, which have the principal weight to support, are all hollow.- Similar remarks might be made on the combination of lightness, pliability and strength, in the texture of the muscles, and the adaptation of the ligaments in the animal frame.
III. Referring to the work already commended, for a sufficiently full, and almost to every capacity intelligible view of the mechanical structure and arrangement of the bones, muscles, and vessels of animal bodies,*-it shall be enough to mention, that the muscular fibre which is transversely laid in other cases, has a spiral texture in the heart, a sign of original adaptation, no other form being suited to the peculiar contractions on which the proper function of that most important vit l organ depends. IV. Besides the relations of interior structure in living beings to inanimate nature, evident marks of contrivance appear, in the adaptation of their exterior form to the elements with which they are surrounded. If the world of waters was to be replenished with inhabitants, an organization different from that of those who walk the earth or wing the air must be requisite. The whole structure of fishes is, accordingly, the best adapted on mechanical principles to the element in which they live. Some are admirably fitted by their shape for cleaving the flood with rapidity ; none have any lateral projecting members calculated to retard their course. Expiration could not well be effected in the bosom of the deep ; but we find in the gills, an apparatus for passing the water regularly over the organ fitted for imbibing the vital principle of the air which the water contains. The fins on the back of the fish, with a slope or elevation proportioned to the quickness of movement for which it is destined, preserve its poise, while those under the belly serve the various purposes of oars-in advancing the body, turning it, changing its velocity, or even arresting its course. The tail, strong, pliant and active, is a helm properly situated, and at the same time contributes by its playing to advance the head, like an oar at the stern of a boat. And then, upon the principle that a body will swim only when lighter than that quantity of the fluid whose place it fills, the air-bag is calculated to increase or diminish the specific gravity, and in some the volume, of the fish, and thus enable him to rise to the top, sink to the bottom, or continue suspended at any determinate depth.-Birds who cannot by any process of this kind be either rendered lighter, or brought to an equal poise with the quantity of air they displace, are furnished with wings sufficiently strong for elevating their bodies, and at the same time so pliant as to give place in a certain degree to the current which is struck, that the apparatus be not injured. What an admirable piece of mechanism is a feather, all its filaments sloped in the proper direction, and hooked together with the due degree of adhesion, which when occasionally disturbed, is instantly resumed ! As lightness, however, is requisite, the bones of birds, though solid and substantial enough, to keep their bodies together, are notwithstanding so small and hollow, that they make no considerable addition to the weight of their flesh. The body of a bird is neither very massive, nor equally thick in all its parts; but well disposed for flight, sharp before, and gradually increasing to its proper bulk or dimensions.
* Paley, Nat. Theol. ch. viii. ix. x.
V.-The argument might be carried with great force into the region of comparative anatomy.
Consult the Natural Thcology, ch. xii.-Not to pass this department entirely, and yet to avoid as much as possible retailing what has been ably stated by the author of that work, the few remarks following are offered. In men and quadrupeds the anterior extremities, the arms or fore-legs, have, at the points of their insertion, but a middling degree of strength, sufficient, however, for the uses to which they are applied. The sternum is cartilaginous and the pectoral muscle small. In birds the structure is different, admirably fitting them for moving through a very thin and light element, in which the anterior extremities must perform the office of oars. In order to agitate a greater mass of air, these extremities require and are seen to possess a greater degree of extension in the form of wings, with a capacity of expanding. For moving force, the pectoral muscle is exceedingly thick and ample. To afford such a muscle a proper receptacle, the sternum is broad and ossified. There is a rising on the middle called the brisket, and the muscle fills up the projection which this forms on the sternum. A bone to which there is no analogy among quadrupeds, called the fork, is placed before the sternum, evidently to prevent the wings when Towered from approaching one another, and thus compressing the breast. The lungs too, instead of being freely suspended in a bag as in man and the quadrupeds, adhere to the sides in birds, without any envelope, pierced with holes each opening into a vescicle; and the whole abdomen is full of these vescicles, which being inflated, considerably augment the volume of the bird, and render it specifically lighter, answering in the manner best adapted to the density of the body compared with the element to be traversed, the same purpose with the bladder already noticed in fishes. This structure of lungs is at the same time that which fits the creature for sublime flight, and prevents them from being torn in rapid descents.-Let us search now for an exception. We find it in the Ostrich, that singular creature which, while it retains its place among birds, is yet, by certain alterations in the structure common to them, adapted to other habits and a different mode of life. The anterior extremities are short, scarcely deserving the name of wings. The sternum is small, but a sixth part the length of the trunk, and it wants the brisket. It is ossified indeed and very thick, but this is to support one of two callosities on which the animal leans when in a state of repose, and which the great weight of
the body rendered necessary. The other is inserted behind at the os pubis. The pectoral muscle is much smaller than the sternum, scarcely covering a third part of it, and covered with fat which forms a cushion for the anterior callosity. The air vescicles are reduced both with regard to number and capacity. There was no need of the fork; it exists, however, as—in connexion with outward form-a mark of the interior construction of a fowl; but being divided at the base, it is unfitted for answering the same purpose as in other fowls. The feathers, though of exquisite fabrie, entirely precluding the idea of mere rudiments in a yet unfinished being, are all detached filaments without adhesion, clearly not suited to the purpose of flying.
So much do the wonders of what is commonly denominated Instinct press themselves on the observation of mankind, that no detail of instances can be requisite to prove its existence. Curious as the subjeet is, however, it has not yet been investigated with the care it deserves. We are unable in some cases to distinguish between instinct and appetite, in others to mark the precise point where instinct terminates and the influence of instruction begins, or to discriminate it from certain degrees of intelligence influencing choice. But it is not necessary to be so minutely accurate ; enough may be found for our purpose, in a general definition, as the basis of two propositions, and the reader is referred to the Natural Theology, ch. xviii. xix., for illustrations which he can easily arrange under the one or the other.
I. Instincts are certain propensities undirected by reason, prior to experienee, and independent of instruction.-- Perhaps we might be allowed, for the sake of distinction, to say that some instincts are oceasional and others are permanent. The first term we would apply to those cases in which the propensity vaguely denominated instinct is called into action but seldom, and rather by artificial than by natural circumstances. It is in these cases, and chiefly because of the deviation from what is usual, that the actions or expedients imputed to instinct have been thought to indicate a certain degree of intelligence influencing choice. In all other cases the animal may be conceived to go on somewhat like a machine, but here is selection, such as would have been the result of the most correct judgment or reason in like circumstances; and as the
spectator or inquirer is a rational being accustomed to act from motives, he feels strongly impelled to interpret the actions and expedients of the most insignificant animals upon the same principle, in all these cases of deviation.* We are apt to err, however, though without intention, in judging analogically of other animals by our own habits, when there is nothing to support the analogy; and therefore, if there be no other facts sufficient to prove that the degree of intelligence supposed to be displayed in the cases referred to, really and permanently belongs to the animal, we ought still even in these cases to abide by the idea of instinct as a propensity undirected by reason, however inexplicable the subject may be ; and in its own sphere, it is not perhaps more inexplicable than the operations of reason, which we think we understand, because they are so familiar to us. The truth of the definition will be readily admitted with regard to permanent instincts, or those which are called into action by natural circumstances. The hen, who shews a pensity to incubation, gives no evidence of intelligence even in that very matter, since she will sit upon unimpregnated eggs, or upon pieces of chalk, and easily receives the eggs of the duck or the partridge instead of her own. Neither can we believe that the bee understands the mathematical principles upon which its cells are constructed, or that the lion-ant has reasoned about the form and site of its pit. Instinct operates with the most undeviating power among those animals whose faculties are of the lowest order. It is clearly a propensity prior to all experience and instruction. 6 The chick of the common dunghill-fowl, no sooner breaks the shell than it understands the cluck of its mother, and obeys her with the most prompt alacrity. The very day it is hatched, if a kite appear in the air, it discovers the most evident symptoms of terror, while other creatures of larger size are suffered to move about without occasioning any sort of alarm. The duckling, in like manner, the
first time it sees the water, runs towards it, and plunges at once into the pool, not only without the aid of
• See a paper on the apparent intelligence shown by Ants, Guardian, Nos. 156, 157 ; and the interesting observations and experiments in HUBER on Ants, with the concluding remarks in the Edinburgh Review of that work. -A bee, superintending the structure of cells by those who furnished the plates of wax, was remarked to have been somehow or other apprized of a foundation being inaccurately laid by a worker, which it immediately altered. Other instances of approach to reason and reflection, apt to strike with considerable surprise, repeatedly occur in KIRBY and SPENCE's Entomology, and the details in the Entertaining Library, on Insects.