I believe that this faculty is not found in the great feathers of the tail. This is the place also for obferv. ing, that the pinions are fo fet on upon the body, as to bring down the wings, not vertically, but in a direction obliquely tending towards the tail: which motion, by virtue of the common refolution of forces, does two things at the fame time; fupports the body in the air, and carries it forward.

The steerage of a bird in its flight is effected partly by the wings, but, in a principal degree, by the tail. And herein we meet with a circumftance not a little remarkable. Birds with long legs have short tails; and, in their flight, place their legs close to their bo dies, at the fame time fretching them out backwards as far as they can. In this polition the legs extend beyond the rump, and become the rudder; fupplying that fteerage which the tail could not.

From the wings of birds, the tranfition is easy to the fins of fish. They are both, to their respective tribes, the inftruments of their motion; but, in the work which they have to do, there is a confiderable difference, founded in this circumftance. Fish, unlike birds, have very nearly the fame fpecific gravity with the element in which they move. In the cafe of fish, therefore, there is little or no weight to bear up what is wanted, is only an impulfe fufficient to carry the body through a refifting medium, or to maintain the pofture, or to fupport, or reftore the balance of the body, which is always the most unfteady where there is no weight to fink it. For these offices the fins are as large as neceffary, though much fmaller than wings, their action mechanical, their pofition, and the mufcles by which they are moved, in the highest degree, convenient. The following fhort account of fome experiments upon filh, made for the purpose of afcertaining the ufe of their fins, will be the beft confirmation of what we affert. In most fish,

befide the great fin the tail, we find two pair of fins upon the fides, two fingle fins upon the back, and one upon the belly, or rather between the belly and the tail. The balancing ufe of thefe organs is proved in this manner. Of the large headed fifh, if you cut off the pectoral fins, i. e. the pair which lies close behind the gills, the head falls prone to the bottom if the right pectoral fin only be cut off, the fifh leans to that fide; if the ventral fin on the fame fide be cut away, then it lofes its equilibrium entirely if the dorfal and ventral fins be cut off, the fish reels to the right and left. When the fish dies, that is, when the fins cease to play, the belly turns upwards. The ufe of the fame parts for motion is feen in the following obfervation upon them when put in action. The pectoral, and more particularly the ventral fins, ferve to raife and deprefs the fish: when the fish defires to have a retrograde motion, a ftroke forward with the pectoral fin effectually produces it if the fifh defire to turn either way, a single blow with the tail the oppofite way, fends it round at once: if the tail ftrike both ways, the motion produced by the double lafh is progreffive; and enables the fish to dart forwards with an aftonishing velocity. The refult is, not only, in fome cafes, the most rapid, but, in all cafes, the most gentle, pliant, eafy, animal motion, with which we are acquainted. However, when the tail is cut off, the fish lofes all motion, and gives itself up to where the water impels it. The rest of the fins, therefore, fo far as refpects motion, seem to be merely fubfidiary to this. In their mechanical use, the anal fin may be reckoned the keel, the ventral fins, out-riggers; the pectoral fins, the oars and if there be any fimilitude between these parts of a boat and a fish, observe, that it is not the resemblance of imitation, but

*Goldfmith's Hift, of An. Nat. vol. vi. p. 154.

P

the likenefs which arifes from applying similar méchanical means to the fame purpose.

We have seen that the tail in the fish is the great inftrument of motion. Now, in cetaceous or warmblooded fifh, which are obliged to rife every two or three minutes to the furface to take breath, the tail, unlike what it is in other fish, is horizontal; its ftroke, confequently, perpendicular to the horizon, which is the right direction for fending the fish to the top, ór carrying it down to the bottom.

Regarding animals in their inftruments of motion, we have only followed the comparison through the First great 'divifion of animals into beafts, birds, and fifh. If it were our intention to purfue the confider. ation further, I fhould take in that generic diftinction among birds, the web foot of water fowl. It is an inftance which may be pointed out to a child. The utility of the web to water fowl, the inutility to land fowl, are so obvious, that it feems impoffible to notice the difference without acknowledging the defign. I am at a loss to know, how those who deny the agency of an intelligent Creator, difpofe of this example. There is nothing in the action of fwimming, as carried on by a bird upon the surface of the water, that Thould generate a membrane between the toes: As to that membrane, it is an exercife of conftant refiftance. The only fuppofition I can think of is, that all birds have been originally water fowl, and web footed; that sparrows, hawks, linnets, &c. which frequent the land, have, in procefs of time, and in the courfe of many generations, had this part worn away by treading upon hard ground. To fuch evafive affumptions must atheism always have recourfe; and, after all, it confefles that the ftructure of the feet of birds, in their original form, was critically adapted to their original deftination. The web feet of amphibious quadrupeds, feals, otters, &c. fall under the fame observation.

IX. The five fenfes are common to most large ani. mals nor have we much difference to remark in their conftitution; or much however which is refer able to mechanism.

The fuperior fagacity of animals which hunt their prey, and which, confequently, depend for their livelihood upon their nofe, is well known, in its use; but not at all known in the organization which produces it. The external cars of beafts of prey, of lions, tigers, wolves, have their trumpet part or concavity ftanding. forwards, to feize the founds which are before them, viz. the founds of the animals, which they pursue or watch. The ears of animals of flight are turned backward, to give notice of the approach of their enemy from behind, when he may fteal upon them unfeen. This is a critical diftinction; and is mechanical; but it may be fuggefted, and, I think, not without probability, that it is the effect of continued habit.

The eyes of animals which follow their prey by night, as cats, owls, &c. poffefs a faculty, not given to thofe of other fpecies, namely, of clofing the pupil entirely. The final caufe of which feems to be this: It was neceffary for fuch animals to be able to defcry objects with very fmall degrees of light. This ca pacity depended upon the fuperior fenfibility of the retina; that is, upon its being affected by the most feeble impulfes. But that tenderness of ftructure, which rendered the membrane thus exquifitely fenfi ble, rendered it also liable to be offended by the accefs of ftronger degrees of light. The contractile range therefore of the pupil is increased in these animals, fo as to enable them to close the aperture entirely; which includes the power of diminishing it in every degree; whereby at all times fuch portions, and only fuch portions of light are admitted, as may be received without injury to the sense.

There appears to be allo in the figure, and in some

properties of the pupil of the eye, an appropriate relation to the wants of different animals. In horses, oxen, goats, fheep, the pupil of the eye is elliptical; the transverse axis being horizontal: by which structure, although the eye be placed on the fide of the head, the anterior elongation of the pupil catches the forward rays, or those which come from objects im. mediately in front of the animal's face.

I

CHAPTER XIII.

PECULIAR ORGANIZATIONS.

BELIEVE that all the inftances which I fhall colfect under this title, might, confiftently enough with technical language, have been placed under the head of Comparative Anatomy. But there appears to me an impropriety in the ufe which that term hath obtained it being, in fome fort, abfurd, to call that a cafe of comparative anatomy, in which there is nothing to "compare;" in which a conformation is found in one animal, which hath nothing properly anfwering to it in another. Of this kind are the examples which I have to propofe in the prefent chapter; and the reader will fee, that though fome of them be the ftrongeft, perhaps, he will meet with under any divifion of our fubject, they must neceffarily be of an unconnected and mifcellaneous nature. To dispose them however into fome fort of order, we will notice, firft, particularities of ftructure which belong to quadrupeds, birds, and fifh, as fuch, or to many of the kinds included in thefe claffes of animals; and then, fuch particularities as are confined to one or two pecies.

I. Along each fide of the neck of large quadrupeds, runs a stiff robust cartilage, which butchers call the