Pagina-afbeeldingen
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produces it if the fish desire to turn either way, a fingle 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 reft of the fins, therefore, fo far as refpects motion, seem to be merely fubfidiary to this. In their mechanical ufe, the anal fin may be reckoned the keel, the ventral fins, out-riggers; the pectoral mufcles, the oars: and if there be any fimilitude between these parts of a boat and a fish, observe, that it is not the refemblance of imitation, but the likeness which arises from applying fimilar mechanical means to the fame purpose.

We have seen that the tail in the fish is the great inftrument of motion. Now, in eetaceous or warm-blooded fish, which are obliged

Goldsmith's Hift. of An. Nat. vol. vi. p. 154.

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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 stroke, confequently, perpendicular to the horizon, which is the right direction for fending the fish to the top, or carrying it down to the bottom.

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REGARDING animals in their inftruments of motion, we have only followed the comparison through the first great divifion of ani-, mals into beafts, birds, and fish. If it were our intention to pursue the confideration further, I should take in that generic distinction amongst birds, the web foot 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 impoffible to notice the difference without acknowledging the defign. I am at a lofs 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 should 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,

fowl, and web footed; that fparrows, hawks, linnets, &c. which frequent the land, have, in process of time, and in the courfe of many generations, had this part worn away by treading upon hard ground. To fuch evasive assumptions must atheism always have recourse; and, after all, it confeffes 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, feals, otters, &c. fall under the fame obfervation.

IX. The five fenfes are common to most large animals: nor have we much difference to remark in their conftitution; or much however which is referable 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 ufe; but not at all known in the organization which produces it.

The external ears 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 ap

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proach of their enemy from behind, when he may fteal upon them unseen. This is a critical diftinction; 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. poffefs a faculty, not given to those of other species, namely, of clofing the pupil entirely. The final cause of which feems to be this. It was neceffary for such animals to be able to defcry objects with very small degrees of light. This capacity depended upon the fuperior fenfibility of the retina; that is, upon its being affected by the moft feeble impulfes. But that tenderness of ftructure, which rendered the membrane thus exquifitely fenfible, 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, so 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 fenfe.

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There appears to be alfo in the figure, and in fome properties of the pupil of the eye, an appropriate relation to the wants of different. animals. In horfes, 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 fide 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.

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