246 NATURAL THEOLOGY. 



the resemblance of imitation,but the Hkeness which 

 arises from applying similar mechanical means to 

 the same 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 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, perpendicular 

 to the horizon, which is the right direction for 

 sending the fish to the top, or carrying it down to 

 the bottom.^ 



^ The poising and motion of fishes in the water has interested 

 some of our greatest philosophers, as Gahleo and Borelh. It is es- 

 timated that fishes make their way through a medium which resists 

 nine hundred times more than the atmosphere. But then, as it 

 offers a certain resistance to their progress, it resists also the mo- 

 tion of their tail and fins by which they have their power of pro- 

 gression. The breadth of the tail of fishes, compared v.ith that of 

 their fins, and its muscularity and power, declare what is affirmed 

 to us upon authority — that the tail is the great instrument of their 

 progression ; and we can see that when the trout darts away, the 

 force of his motion lays down the fins close upon his body. But 

 the fins direct him, as outriggers, and the pectoral fins especially, 

 by raising or depressing the head, give direction to the whole body 

 under the force of the tail. The lateral fins, and particularly the 

 pectoral fins, also sustain him in the right position in the water : 

 without the co-operation of these with the tail, the fish would move 

 like a boat sculled by one oar at the stern. As the digestion of 

 fishes, as well as that of other animals, is attended with the extri- 

 cation of air, and as the intestines are below the centre, the belly 

 would be turned up but for the action of these lateral fins : as we 

 see takes place in a dead fish. The tail and fins are the instru- 

 ments of motion ; but the incessant action of the muscles which 

 move these is a just matter of admiration. If a fish move with his 



