MODES OF PROGRESSION. 93 



raf id efforts to compensate for this disadvantage. Kence it 

 requires a much greater expenditure of strength to fly than 

 to walk ; and, therefore, we find the great mass of muscles 

 in birds concentrated about the breast, (Fig. 30.) To facili- 

 tate its prDgress, the bird, after each flap of the wings, brings 

 them against the body, so as to present as little surface as 

 possible to the air ; for a still further diminution of resistance, 

 all birds have the anterior part of the body very slender, 

 Their flight would be much more difficult if they had large 

 heads and short necks. 



193. Some quadrupeds, such as the flying-squirrel and 

 Galeopithecus, have a fold of the skin at the sides, which 

 may be extended by the legs, and which enables them to 

 leap from branch to branch with more security. But this 

 is not flight, properly speaking, since none of the peculiar 

 operations of flight are performed. There are also some 

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

 to dart from the water, and sustain themselves for a consider- 

 able time in the air ; and hence they are called flying-fish. 

 But this is not truly flight. 



194. SWIMMING is the mode of locomotion employed by 

 the greater part of the aquatic animals. Most animals which 

 live in the water swim with more or less facility. Swimming 

 has this in common with flight, that the medium in which it 

 is performed, the water, becomes also the support, and read 

 ily yields also to the impulse of the fins. Only, as water is 

 much more dense than air, and as the body of most aquatic 

 animals is of very nearly the same specific gravity as water, 

 it follows that, in swimming, very little effort is requisite to 

 keep the body from sinking. The whole power of the mus- 

 cles is consequently employed in progression, and hence 

 swimming requires vastly less muscular force than flying. 



195. Swimming is accomplished by means of various or- 

 gans designated under the general term, fins, although in an 



