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an animal. The principle of the contrivance is clear; the application of the principle is also clear. The use of the organ to sustain, and, at will, also to elevate, the body of the fish in the water, is proved by observing, what has been tried, that, when the bladder is burst, the fish grovels at the bottoia; and also, that flounders, soles, skates, which are without the air-bladder, seldom rise in the water, and that with effort. The manner in which the purpose is attained, and the suitableness of the means to the end, are not dif
ficult to be apprehended. The rising and sinking of a fish in water, so far as it is indépendent of the stroke of the fins and tail, can only be regulated by the specific gravity of the body. When the bladder, contained in the body of the fish, is contracted, which the fish probably 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 inclosed and now com pressed o air, restores -the-dimensions of the bladder, * the tendent y downward s 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 chamelled 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 itself, and thus render it specifically heavier, or specifically lighter, than the water which surrounds it. Suppose this to be done, and the artist to solicit a patent for his invention. The inspectors of the model, whatever they might think of the use or value of the contrivance, could, by no possibility, entertain a question in their minds, whether it were a contrivance or not. No reason has ever been assigned,—no reason can be assigned, why the conclusion is not as certain in the fish, as it is in the machine; why the argument is not as firm in one case as the other. \ It would be very worthy of inquiry, if it were possible to discover, by what method an animal, which lives constantly in water, is able to supply a repository of air. The expedient, whatever it be, forms part, and perhaps the most curious part, of the provision. Nothing similar to the air-bladder is found in land-animals; and a life in the water has no natural tendency to produce a bag of air. Nothing can be further from an acquired organization than this is. 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. I. 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, lying down flat upon the jaw, but furnished with a muscle, which, with a jerk, and by the pluck, as it were, of a string, suddenly erects it. Under the tooth, close to its root, and communicating with the perforation, lies a small bag containing the venom. When the fang is raised, the closing of the jaw presses its root against the bag underneath; and the force of this compression sends out the fluid with a considerable impetus through the tube in the middle of the tooth. What more unequivocal, or effectual apparatus could be devised for the double purpose of at once inflicting the wound, and injecting the poison? Yet, though lodged in the mouth, it is so constituted, as, in its inoffensive and quiescent state, not to interfere with the animal's ordinary office of receiving its food. It has been observed also, that none of the harmless serpents, the black snake, the blind worm, &c. have these fangs, but teeth of an equal size; not moveable, as this is, but fixed into the jaw. II. In being the property of several different species, the preceding example is resembled by that which I shall next mention, which is the bag 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, furnished with
bones and muscles of its own. Two bones are placed before the os pubis, and joined to that bone as their base. These support, and give a fixture to, the muscles, which sérve to open the bag. To these muscles there are antagonists, which serve in the same manner to shut it; and this office they perform so exactly, that, in the living animal, the opening can scarcely be discerned, except when the sides are forcibly drawn asunder”. Is there any action in this part of the animal, any process arising from that action, by which these members could be formed? any account to be given of the formation, except design? III. As a particularity, yet appertaining to more species than one ; and also as strictly mechanical; we may notice a circumstance in the structure of the claws of certain birds. The middle claw of the heron and cormorant is toothed and notched like a saw. These birds are great fishers, and these notches assist them in holding their slippery prey. The . use is evident; but the structure such, as cannot at all be accounted for by the effort of the animal, or the exercise of the part. Some other fishing birds have these notches in their bills ; and for the same purpose. The gannet, or Soland goose, has the side of its bill
* Goldsmith, Nat. Hist, vol. iv. p. 244.