INTRODUCTION. 



11 



of tlie fish are never less than two, a caudal and a cephalic one. 

 They may and do exceed this number in the long-bodied fishes. 

 The tail of the fish is made to vibrate pendulum fashion on 

 either side of the spine, when it is lashed to and fro in the 

 act of swimming. It is made to rotate upon one or more of 

 the vertebrae of the spine, the vertebra or vertebrae forming 

 the centre of a lemniscate, which is described by the caudal 

 fin. There is, therefore, an obvious analogy between the tail 

 of the fish and the extremity of the biped. This is proved by 

 the conformation and swimming of the seal, — an animal in 

 which the posterior extremities are modified to resemble the 

 tail of the fish. In the swimming of the seal the hind legs are 

 applied to the water by a sculling figure-of-8 motion, in the 

 same manner as the tail of the fish. Similar remarks might 

 be made with regard to the swimming of the whale, dugong, 

 manatee, and porpoise, sea mammals, which still more closely 

 resemble the fish in shape. The double curve into which the 

 fish throws its body in swimming, and which gives continuity 

 of motion, also supplies the requisite degree of steadiness. 

 When the tail is lashed from side to side there is a tendency 

 to produce a corresponding movement in the head, which 

 is at once corrected by the complementary curve. Nor is 

 this all; the cephalic curve, in conjunction with the water 

 contained within it, forms the ^oint d'apimi for the caudal 

 curve, and vice versa. When a fish swims, the anterior and 

 posterior portions of its body (suj)posing it to be a short- 

 bodied fish) form curves, the convexities of which are 

 directed on opposite sides of a given line, as is the case 

 in the extremities of the biped when walking. The mass 

 of the fish, like the mass of the biped, when once set in 

 motion, contributes to progression by augmenting the rate 

 of speed. The velocity acquired by certain fishes is very 

 great. A shark can gambol around the bows of a ship in 

 full sail j and a sword-fish (such is the momentum acquired 

 by it) has been known to thrust its tusk through the copper 

 sheathing of a vessel, a layer of felt, four inches of deal, and 

 fourteen inches of oaken plank.^ 



The wing of the bird does not materially difi*er from the 

 1 A portion of the timbers, etc., of one of Her Majesty's ships, having the 



