DIODONS AND TETRODONS. 387 



this may partly be accounted for by the size of 

 their muscles, aad the advantageous mode of their 

 insertion, yet these advantages would avail but 

 little, were it not for the sudden manner in which 

 their power is exerted. Where the great length 

 and flexibility of the spine tend to impair the force 

 with which the tail strikes the water, the resulting 

 motion is slow and desultory, as is the case with 

 Eels, and other fishes of the same elongated con- 

 struction.* Most fishes, however, move with the 

 utmost rapidity, and with scarcely any visible 

 effort; and perform long journeys without apparent 

 fatigue. Sharks often follow ships across the 

 Atlantic, not only outstripping them in their swift- 

 est sailing, but playing round them on every side, 

 just as if the vessels were at rest. 



Chapter VIII. 



REPTILIA. 



§ 1 . Terrestrial Vertebrata in general. 



The numerous tribes of vertebrated animals which 

 are strictly terrestrial, or destined to move on land, 

 differ widely in their modes of progression, and in 

 the mechanical advantages of their formation. The 

 greater number are quadrupeds ; some formed for 

 climbing trees ; others for burrowing in the earth ; 

 some for treading on sandy plains; some for scaling 

 precipices. A few seem scarcely capable of ad- 



* Carlisle, Phil. Trans, for 1806, p. 9. 



