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produces it: if the fisti desire to turn either* way, a single blow with the tail the opposite way, sends it round at once: if the tail strike both ways, the motion produced by the double lasli is progressive; and enables the fish to dart forwards with an astonishing velocity'. The result is, not only, in some cases, the most rapid, but, in all cafes, the most gentle, pliant, easy, animal motion, with which we are acquainted. However, when the tail is cut off, the fish loses all motion, and gives itself up to where the water impels it. The rest of the fins, therefore, so far as respects motion, seem to be merely subsidiary to this. In their mechanical use, the anal fin may be reckoned the keel, the ventral sins, out-rig* gers; the pectoral muscles, the oars: and if there be'any similitude between these parts of a boat and a fish, observe, that it is not the resemblance of imitation, but the likeness which arises from applying similar mechanical means to the fame purpose. ..
We have seen that the tail in the fish is the great instrument of motion. Now, in cetaceous or warm-blooded fish, which are obliged
q GoklsiniuYs Hist, of An. Nat. rol. vi. p. 154.
to to rise every two or three minutes to the surface to take breath, the tail, unlike what it is in other fish, is horizontal; its stroke, consequently, P rpendicular to the horizon, which is the right direction for sending the fish to the top, or carrying it down to the bottom.
Regarding animals in their instruments of motion, we have only followed the comparison through the first great division of animals into beasts, birds, and fish. If it were our intention to pursue the consideration further, I should take in that generic distinction amongst birds, the web soot of water fowl. It is an instance 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 seems impossible to notice the difference without acknowledging the 'design. I am at a loss to know, how those who deny the agency of an intelligent Creator, dispose of this example. There is.jiothing in the action of swimming, as carried on by a bird upon the surface of the water, that should generate a membrane between the toes. As to that membrane, it is an exercise of constant resistance. The only supposition I can think of is, that all birds have been originally water
fowl, fowl, and web footed; that sparrows, hawks, linnets, &c. which frequent the land, have4 in process of time, and in the course of many generations, had this part worn away by treading upon hard ground. To such evasive assumptions must atheism always have recourse; and, after all, it consesses that the structure of the feet of birds, in their original form, was critically adapted to their original destination. The web feet of amphibious quadrupeds, seals, otters, &c. fall under the fame observation.
IX. The five senses are common to most large animals: nor have we much difference to remark in their constitution; or much however which is referable to mechanism.
The superior fagacity of animals which hunt their prey, and which, consequently, depend for their livelihood upon their nofct is well known, in its use; but not at all known in the organization which produces it.
The external ears of beasts of prey, of lions, tigers, wolves, have their trumpet part or concavity standing forwards, to seize the sounds which are before them, viz. the sounds 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 steal upon them unseen. This is a critical distinction; and is mechanical: but it may be suggested, 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. possess a faculty, not given to those of other species, namely, of closing the pupil entirely. The final cause of which seems to be this. It was necessary for such animals to be able to descry objects with very small degrees of light. This capacity depended upon the superior sensibility of the retina; that is, upon its being affected by the most feeble impulses. But that tenderness of structure, which rendered the membrane thus exquisitely sensible, rendered it also liable to be offended by the access of stronger degrees of light. The contractile range therefore of the pupil is increased in these animals, so as to enable them to close the aperture entirely; which includes the power of diminishing it in every degree; whereby at all times such portions, and only such portions of light are admitted, as may be received without injury to the sense.
There appears to be also 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, sheep, the pupil of the eye is elliptical; the transverse axis being horizontal: by which structure, although the eye be placed on the side of the head, the anterior elongation of the pupil catches the forward rays, or those which come from objects immediately in front of the ani? mal's face, .;