LOCOMOTION OF FISHES. 97 



the right side be sufficiently long-continued, or the 

 sweep and force with which it moves from right to 

 left sufficiently exceed that with which it moves 

 from left to right, the animal will wheel completely 

 round, or may be even made to revolve upon the 

 same horizontal plane, as upon a pivot driven verti- 

 cally through its centre of gravity. Now, it is 

 exactly on the same principle that the flat osseous 

 fishes, which have no air-bladder, use their tail, 

 not only in swimming in a straight line through the 

 water, but also in rising and sinking in this fluid ; 

 for the same loss of balance in the motions of a tail 

 moving from side to side as would turn an animal 

 to the right or left, in those of a tail moving verti- 

 cally, will, of course, depress or raise it. And it is 

 thus also that, in the cetaceous tribes, the necessity 

 of an air-bladder is superseded ; since, when they 

 desire to rise in the water, all that they have to do 

 is to strike a few smart blows with their tail down- 

 wards, when their heads are necessarily carried in 

 an opposite direction ; and when they wish to sink, 

 a few similar blows with the tail in the upward di- 

 rection, at once serves to bury their heads beneath 

 the waters. 



But the tail of fishes is useful to them still in 

 another capacity, besides that of either a paddle or 

 a rudder, since it is chiefly by means of this organ 

 that they are enabled to leap out of the water; and 

 the height to which some of them are capable of 

 thu'S bounding into the air is astonishing. From 

 the enormous basking-shark to the minute stickle- 



G 



