122 DORSAL FIN. 



with the body throughout a great part of its length, 

 and the hroad area which they present laterally to 

 the water, must obviously oppose a much greater 

 resistance to any rotatory motion of the animal or 

 its own axis, than any which it experiences in its 

 motions either upwards or downwards, forwards or 

 backwards. They thus operate in the same manner 

 as the keel of a ship, and serve to keep the animal 

 steady in its course ; and, for the same reason that a 

 flat-bottomed boat rolls with every wave, and can keep 

 its course at all only in very quiet waters, so a fish, 

 from which these fins have been removed, reels 

 continually to the right and left, and is able to 

 preserve any thing like an equilibrium only by 

 keeping its other fins in constant motion, as a man 

 does his arms when balancing himself upon a tight- 

 rope. But the dorsal and anal fins of fishes have 

 an advantage which the best constructed keel can 

 never possess; and that is, that their area and 

 tension can be increased, within certain limits, in 

 exact proportion to the necessity for greater secu- 

 rity, the spines on which they are built being 

 raised by proper muscles, which are under the 

 controul of the animal, so that it has but to call 

 these muscles into a greater or less degree of action 

 to expand or relax the fins to the precise point that 

 is required. It is thus that we may imitate Nature 

 in our contrivances, but we can never approach 

 her except at one or two removes; and the 

 meanest and most insignificant of her works gives, 

 every hour, lessons of mechanism to the most expert 



