THE MODES OF PROGRESSION. 157 



fishes, whose pectoral tins are so extended as to enable them to 

 dart from the water, and sustain themselves for a short time in 

 the air ; and hence they are called flying fishes. But this is 

 not truly flight. 



302. SWIMMING is the mode of locomotion employed by 

 the greater number of aquatic animals. Swimming has this in 

 common with flight, that the medium in which it is performed 

 being also the support of the body, readily yields to the impulse 

 of the fins. But water being much more dense than air, and 

 the body of most aquatic animals being nearly the same weight 

 as the water it displaces, it follows, that in swimming, very little 

 effort is requisite to keep the body from sinking. The whole 

 power of the muscles is consequently employed in progression, 

 and hence swimming requires much less muscular force than 

 flying. 



303. Swimming is accomplished by means of various 

 organs, designated under the general term fins, although, in 

 an anatomical point of view, these represent very different 

 parts. In whales, it is the anterior extremities, and the tail, 

 which are transformed into fins. In fishes, the pectoral fins, 

 which represent the arms, and the ventral fins, which repre- 

 sent the legs, are employed for swimming, but they are not 

 the principal organs ; for it is by the tail, or caudal fin, that 

 progression is principally effected. Hence the swimming of 

 a fish is precisely that of a boat under the sole guidance of 

 the sculling-oar. In the same manner as a succession of 

 strokes, alternately right and left, propels the boat straight 

 forwards, so the fish advances by striking alternately right and 

 left with its tail. To advance obliquely, it has only to strike 

 in the opposite direction. Whales, on the contrary, swim 

 by a vertical movement of the tail ; and it is the same with a 

 few fishes also, such as the rays and the soles. The air-blad- 

 der facilitates the rising and sinking of the fish, by enabling it 

 to vaiy the specific weight of its body. 



304. Most land animals swim with more or less ease, by 

 simply employing the ordinary motions of walking or leaping. 

 Those which frequent the water, like the beaver, or which feed 

 on marine animals, as the otter, the duck, and other palmi- 

 pedes, have webbed feet, the toes being united by membranes, 

 which, when expanded, act as paddles. 



30.5. There is also a large number of invertebrate animals, 



