|March 22, 2012||Filled under All text of Paley Natural Theology, Chapter 13||
I believe that all the instances which I shall collect under this title, might, consistently enough with technical language, have been placed under the head of Comparative Anatomy. But there appears to me an impropriety in the use which that term hath obtained: it being, in some sort, absurd to call that a case of Comparative Anatomy, in which there is nothing to “compare;” in which a conformation is found in one animal, which hath nothing properly answering to it in another73. Of this kind are the examples which I have to propose in the present chapter: and the reader will see that, though some of them be the strongest, perhaps, he will meet with under any division of our subject, they must necessarily be of an unconnected and miscellaneous nature. To dispose them, however, into some sort of order, we will notice first, particularities of structure which belong to quadrupeds, birds, and fish, as such, or to many of the kinds included in these classes of animals; and then, such particularities as are confined to one or two species.
Along each side of the neck of large quadrupeds, runs a stiff, robust ligament, which butchers call the pax wax. No person can carve the upper end of a crop of beef without driving his knife against it. It is a tough, strong, tendinous substance, braced from the head to the middle of the back; its office is to assist in supporting the weight of the head. It is a mechanical provision, of which this is the undisputed use; and it is sufficient, an not more than sufficient, for the purpose which it has to execute. The head of an ox or a horse is a heavy weight, acting at the end of a long lever, (consequently with a great purchase,) and in a direction nearly perpendicular to the joints of the supporting neck. From such a force, so advantageously applied, the bones of the neck would be in constant danger of dislocation, if they were not fortified by this strong tape. No such organ is found in the human subject, because, from the erect position of the head, (the pressure of it acting nearly in the direction of the spine,) the junction of the vertebrae appears to be sufficiently secure without it. This cautionary expedient, therefore, is limited to quadrupeds: the care of the Creator is seen where it is wanted.
II. The oil with which birds prune their feathers, and the organ which supplies it, is a specific provision for the winged creation. On each side of the rump of birds is observed a small nipple, yielding upon pressure a butter-like substance, which the bird extracts by pinching the pap with its bill. With this oil, or ointment, thus procured, the bird dresses its coat; and repeats the action as often as its own sensations teach it that it is in any part wanted, or as the excretion may be sufficient for the expense. The gland, the pap, the nature and quality of the excreted substance, the manner of obtaining it from its lodgment in the body, the application of it when obtained, form, collectively, an evidence of intention which it is not easy to withstand. Nothing similar to it is found in unfeathered animals. What blind conatus of nature should produce it in birds? should not produce it in beasts?
III. The air-bladder also of a fish, [Pl. XXIII. fig. 3,] affords a plain and direct instance, not only of contrivance, but strictly of that species of contrivance which we denominate mechanical. It is a philosophical apparatus in the body of 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 bottom; 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 difficult to be apprehended. The rising and sinking of a fish in water, so far as it is independent 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 enclosed and now compressed air restores the dimensions of the bladder, the tendency downwards 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 enamelled 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 provision74. 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 farther 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. [Pl. XXIII. fig. 4, 5.] 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 movable, 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. [Pl. XXIV. fig. 1, 2, 3.] 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 conveyance, and their asylum. 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 serve 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 asunder75. 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 design76?
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. [Pl. XXV. fig. 1.] 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 edges of its bill irregularly jagged, that it may hold its prey the faster. [Pl. XXV. fig. 2.] 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 feed.
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. The stomach of the camel is well known to retain large quantities of water, and to retain it unchanged for a considerable length of time. [Pl. XXVI.] This property qualifies it for living in the desert. Let us see, therefore, what is the internal organization, upon which a faculty so rare, and so beneficial, depends. A number of distinct sacks 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.
II. The tongue of the woodpecker, is one of those singularities, which nature presents us with when a singular purpose is to be answered. [Pl. XXVII. fig. 1 and 2.] It is a particular instrument for a particular use: and what else but design, ever produces such? The woodpecker lives chiefly upon insect, lodged in the bodies of decayed or decaying trees. For the purpose of boring into the wood, it is furnished with a bill, straight, hard, angular, and sharp. When, by means of this piercer, it has reached the cells of the insects, then comes the office of its tongue; which tongue is, first, of such a length that the bird can dart it out three or four inches from the bill,—in this respect differing greatly from every other species of bird; in the second place, it is tipped with a stiff sharp bony thorn; and in the third place, (which appears to me the most remarkable property of all,) this tip is dentated on both sides, like the beard of an arrow or the barb of a hook. The description of the part declares its uses. The bird having exposed the retreats of the insects by the assistance of its bill, with a motion inconceivably quick, launches out at them this long tongue, transfixes them upon the barbed needle at the end of it, and thus draws its prey within its mouth. If this be not mechanism, what it? Should it be said, that, by continual endeavours to shoot out the tongue to the stretch, the woodpecker species may by degrees have lengthened the organ itself beyond that of other birds, what account can be given of its form, of its tip? How, in particular, did it get its barb, its dentation? These barbs, in my opinion, wherever they occur, are decisive proofs of mechanical contrivance.
I shall add one more example, for the sake of its novelty. It is always an agreeable discovery, when, having remarked in an animal an extraordinary structure, we come at length to find out an unexpected use for it. The following narrative, which Goldsmith has taken from Buffon, furnishes an instance of this kind. The babyrouessa, or Indian hog, a species of wild boar, found in the East Indies, has two bentteeth, more than half a yard long, growing upwards, and (which is the singularity) from the upper jaw. [Pl. XXVII. fig. 4.] These instruments are not wanted for offence; that service being provided for by two tusks issuing from the upper jaw, and resembling those of the common boar; nor does the animal use them for defence. They might seem therefore to be both a superfluity and an encumbrance. But observe the event: the animal hitches one of these bent upper teeth upon the branch of a tree, and then suffers its whole body to swing from it. This is its manner of taking repose, and of consulting for its safety. It continues the whole night suspended by its tooth, both easy in its posture, and secure; being out of the reach of animals which hunt it for prey7778.
73The objection here made to the use of the term, Comparative Anatomy, does not seem well founded. As commonly employed, it is intended to designate the anatomy of animals compared with that of men and of one another. It is only by comparison that the use of parts can be discovered. Generally, conformations found in one animal have something corresponding to them in other animals; but even where this is not the case, a comparison is not the less necessary to discover the use of the conformation. Thus, particularly, in the first instance mentioned by the author, he points out the function of the pax wax by the very process which he affirms cannot have place. It is by comparing the neck of large quadrupeds in which this provision is found, with that of man in which it is not found, and by comparing the position maintained by man with that maintained by quadrupeds, that he illustrates the object for which this provision is made.—Ed.
74Much obscurity still exists concerning the exact purpose which the air-bag is intended to perform. But with regard to the manner in which it is supplied with air, there seems no reason to doubt that it is effected by a secretion from the blood. It is an established fact in physiology, that many of the internal surfaces of the body have the power of producing gases in this way. In the air-bag of many fishes a very vascular organ is found which has been called the air-gland; and in some species vessels have been discovered conveying the air from this gland into the cavity of the bag. Even where this gland does not exist, it is probable that the internal surface of the bag may perform the [e.pk] office.—Ed.
75Goldsmith’s Nat. Hist. vol. iv. p. 244.
76There is a very considerable number of animals possessed of the same structure which is here described as existing in the opossum, to which the attention of naturalists has been more particularly called since the first publication of this work. The animals of this kind are called marsupial, from the pouch or marsupium which distinguishes them. This provision also has a relation to circumstances in the reproduction of these animals to which Dr. Paley has not referred. He appears merely to regard it as a place of refuge and deposit for the young; somewhat in the same way as the wings of a hen are for its brood. The fact is that the young of these animals are born prematurely, and in a very imperfect and unformed state; and the pouch of the parent seems properly intended for a residence during the completion of the process of developement [sic.]. The kangaroo is an instance of this kind. When full grown it is six feet in extreme length, and weighs but twenty grains. The fore legs are scarcely distinguishable, and the hind ones, which in the adult state form half the length of the body, are marked only by slight projections at the parts where they are afterwards to grow. In fact the kangaroo at birth is as imperfectly formed as the young of any other animal would be when but a quarter part of the proper period of its growth within its parent had elapsed.
It is remarkable that it has never yet been ascertained whether these little embryos are conveyed by the parent animal, or whether they find their own way, into the pouch. Having scarce the exercise of any of the senses, and being without limbs, it seems almost impossible they should make their way there by their own exertions. However this may be, they are found in the pouch closely attached, and as it were glued to the nipples, by the mouth or rather by that aperture which afterwards becomes a mouth. Here they remain, never quitting their hold, until a sufficient period has elapsed for their growth to be completed, and they have thus arrived in regard to form and structure upon an equality with other animals at the usual period of birth. When this is accomplished, they undergo, as it were, a second birth, and emerge from the pouch: but return occasionally for the purpose of feeding, and for that of protection from danger.
No marsupial animal was known before the discovery of America, of which the opossum is a native; and this animal was at first almost regarded as a sort of exception to the laws of nature; since the discovery of New Holland, however, and the investigation of its natural history, it has been found that the marsupial animals, so far from forming an exception to the general construction of animals on that continent, constitute the prevailing model. With a very few exceptions, all the native animals of New Holland are of the marsupial tribe.—Ed.
77Goldsmith’s Natural History, vol. iii. p. 195
78There does not seem to be any sufficient authority for ascribing this use to the tusks of this animal. Indeed one does not readily see how it could in the way described swing itself clear of its enemies, except by first climbing the tree; which is not pretended. The fact is doubted, it is believed, by many naturalists, and the opinion probably was in the first place founded upon mere conjecture. A modern and distinguished traveller has these remarks upon the subject. “Philosophers had long puzzled themselves in conjectures what the design of nature could be, as she does nothing without design, in giving to this animal a pair of large, curved tusks, pointing inwards to the face in such a manner as made it sufficiently clear they could not be used either for attack or defence, for procuring food, or for assisting the mastication of it when procured. At length it occurred, or was discovered, by whom I do not recollect, that the animal is fond of sleeping in a standing posture, and, that having a large, ponderous head, it finds a conveniency in hanging it upon the branch of a tree or shrub within the reach of its tusks, which serve on such occasions for hooks. This is at least an ingenious discovery, and may be true; but if so the habits of the animal, or one so like it that the difference is not distinguishable by any description or drawing that I have seen, is common among the rocks on the deserts of Southern Africa, where, within the distance of a hundred miles, there is neither tree nor shrub, except a few stunted heaths or shrivelled everlastings, thinly scattered over the barren surface. In such situations, where I have hunted and taken them, it would certainly be no easy matter for the babyrouessa to find a peg to hang its head upon.”—Barrow’s Voyage to Cochin-China.—Ed