136 COMPARATIVE ANATOMY. 



instruments of their motion ; but, in the work which they 

 have to do, there is a considerable difference, founded on 

 this circumstance. Fish, unlike birds, have very nearly 

 the same specific gravity with the element in which they 

 move. In the case of fish, therefore, there is little or no 

 weight to bear up ; what is wanted is only an impulse, suffi- 

 cient to carry the body through a resisting medium, or to 

 maintain the posture, or to support or restore the balance 

 of the body, which is always the most unsteady where 

 there is no weight to sink it. For these offices the fins are 

 as large as necessary, though much smaller than wings, 

 their action mechanical, their position, and the muscles by 

 which they are moved, in the highest degree convenient. 

 The following short account of some experiments upon fish, 

 made for the purpose of ascertaining the use of their fins, will 

 be the best confirm.ation of what we assert. In most fish, 

 beside the great fin, the tail, we find two pairs of fins upon 

 the sides, two single fins upon the back, and one upon the 

 belly, or rather between the belly and the tail. The hal- 

 ancing use of these organs is proved in this manner. Of 

 the large-headed fish, if you cut off" the pectoral fins, i. e. 

 the pair which lie close behind the gills, the head falls 

 prone to the bottom : if the right pectoral fin only be cut 

 off, the fish leans to that side ; if the ventral fin on the 

 same side be cut away, then it loses its equilibrium entire- 

 ly ; if the dorsal and ventral fins be cut off, the fish reels 

 to the right and left. When the fish dies, that is, when 

 the fins cease to play, the belly turns upwards. The use 

 of the same parts for motion is seen in the following obser- 

 vations upon them when put in action. The pectoral, and 

 more particularly the ventral fins, serve to raise and depress 

 the fi.sh : when the fish desires to have a retrograde motion, 

 a stroke forward with the pectoral fin effectually produces 

 it : if the fish desire to turn either way, a single blow with 

 the tail the opposite way, sends it round at once : if the 

 tail strike both ways, the motion produced by the double 

 lash is progressive ; and enables the fish to dart forward 

 with an astonishing velocity.* The result is, not only, in 

 some cases, the most rapid, but, in all cases, the most gen- 

 tle, pliant, easy animal motion, with which we are acquaint- 

 ed. However, when the tail is cut off, tlie fish loses all 



* Goldsmith's History of Animated Nature, vol. iv. p. 154. The 

 velocity with which fish swim from one part of the globe to another 

 is astonishing ; when a ship is sailing at the rate of fourteen miles an 

 hour, the porpoises will pass it with as much ease as when at anchor. 



Pttxton. 



