204 Miscellaneous. Chapt. xxiii. 



This is doubtless the manner in which the tail is used when 

 the fish swims rapidly forwards in a straight line. "When it 

 wishes to turn abruptly to any side, successive strokes of the tail 

 are made on one side only, and the body curled round as much as 

 possible, and the pectoral fin on the inside of the curve is, I think, 

 thrust out, and the one on the outside of the curve worked. The 

 pectoral fins are certainly used for turning slowly, but the tail seems 

 to be the great motive power when turning rapidly, as, for instance, 

 when passing a fleeing fish and turning round so as to take it in head 

 foremost. Every ray in the tail, and in any other fin, is under as 

 much separate command as each toe in a duck's foot, and in drawing 

 up the tail for a blow, the fish can contract the tail as a duck does 

 its webbed foot, expanding it again for the blow ; or it can shape 

 and use it like the cross-fanned screw of a steam vessel. As the 

 ventral fins enable fish to maintain a horizontal position in the 

 water, so does the dorsal fin. I have noticed a gold-fish which had 

 no dorsal fin, and though it had been hatched thus it could not 

 well command its position in the water, but rolled slightly. Gold- 

 fish are unusually subject to deformity, not unfrequently having 

 two heads or two tails. The pectoral fins are used to swim slowly 

 backwards. The anal fin also seems to be used in maintaining the 

 horizontal. It and the dorsal fin are much prolonged in the 

 Murral, which, in accordance with its bottom-seeking habits, has not 

 so much of the ordinary compressed shape of a fish, but being 

 more than ordinarily depressed seems to need this additional pro- 

 vision. It is the same with the extensive anal fin of the bottom 

 feeding Wallago attu (Freshwater Shark). 



We may stay pleasurably for a moment to note a peculiarity in 

 the locomotion of fishes. It is based on principles markedly dif- 

 ferent from those which govern the locomotion of birds and beasts. 

 The flight of birds is in the midst of a highly elastic element, the 

 progression of beasts on the outside of an inelastic, immobile 

 substance, the swimming of fishes is conducts! in the midst of an 

 inelastic but easily-displaced element. Again, the locomotion of 

 both birds and beasts is dependent on the law of gravitation, 

 whereas fish may be almost said to dispense with it, at any rate 

 relatively to the element in which they move. It is not so with a 

 bird ; it could not move in the air if its superior weight therein 

 did not supply it with a fulcrum whence to apply power, and with 



