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It would be very worthy of enquiry, 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 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 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, and 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 re
ceived 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 conveyance, and their asylum. Can the use of this structure be doubted of? Nor is it a mere doubling of the flan, 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 serve to open the bag. To these muscles there are antagonists, which serve in the fame 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 draw*n 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
* Goldsmith's Nat. Hist. vol. iv. p. 244. '*
more 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 faw. These birds are great fishers, and these notches assist: them in holding their flippery 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 lills; and for the fame purpose. The gannet, or Soland goose, has the tide of its bill irregularly jagged, that it may hold its prey the faster. Nor can the structure in this, more than in the former case, arise from the manner of employing the part. The smooth surfaces, and soft flesh of fish, were less likely to notch the bills of birds, than the hard bodies upon which many other species seed.
We now come to particularities strictly so called, as being limited to a single species of animal. Of these I shall take one from a quadruped, and one from a bird.
I. Tiie Jlomacb of the camel is well known to retain large quantities of water, and to retain it unchanged for a considerable length of time. This property qualifies it for living in the defart. Let us see therefore what is the internal organization, upon which a faculty, so rare and so beneficial, depends. A number of distinct facs -or bags (in a dromedary thirty of these have been counted) are observed to lie between the membranes of the second stomach, and to open into the stomach near the top by small square apertures. Through these orifices, after the stomach is full, the annexed bags are filled from it. And the water, so deposited, is, in the first place, not liable to pass into the intestines; in the second place, is kept separate from the solid aliment; and, in the third place, is out of the reach of the digestive action of the stomach, or of mixture with the gastric juice. It appears probable, or rather certain, that the animal, by the conformation of its muscles, possesses the power of squeezing back this water from the adjacent bags into the stomach, whenever thirst excites it to put this power in action,