LOCOMOTIVE ORGANS. 321 



observe, an illustration of what natural philoso- 

 phers call the composition of motion, the two 

 forces which separately would move the animal to 

 the right or left, producing as their combined 

 result motion in an intermediate direction, that is 

 to say straight forwards. Human ingenuity has 

 in various ways applied the same combination of 

 force for a similar end. A boatman at the stern 

 of a boat, by means of a single oar, turns the 

 boat to the right or left, and by combining the 

 two motions of the oar which separately produce 

 that result, he imparts to the boat an onward 

 motion. The screw placed at the stern of the 

 steamship is an application of the same prin- 

 ciple. But thousands of years before man existed 

 the same natural laws, which he learnt to employ 

 only after centuries of slow and painful progress, 

 had already been taken advantage of by the All- 

 wise Artificer in the structure and application of 

 the fish's tail. The fins of fishes, which are 

 analogous to the legs of quadrupeds, appear to be 

 chiefly employed in balancing the animal's body, 

 and their structure is no less admirably adapted 

 to this purpose than the tail is.to its own proper 

 effect. 



Related to the locomotive powers of fishes there 

 is a peculiar organ possessed by many species, 

 which cannot be too much admired as the evi- 

 dence of an arrangement expressly adapted to a 

 certain purpose. It has been already remarked 

 that the weight of a fish's body is nearly the same 

 as that of an equal bulk of water. This equality, 



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