MODES OF PROGRESSION. 69 



advantage. Hence 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 facilitate its flight, the bird, after 

 each flap of the wings, brings them against the body, so as 

 to present as little surface to the air as possible ; 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 have a fold of the skin at the 

 sides, which may be extended by the legs, and which ena- 

 bles them to leap from branch to branch, with more facility, 

 such as the flying-squirrel and Galeopithecus. But this 

 is not flight, properly speaking, since none of the pecu- 

 liar operations of flight are performed. There are also 

 some fishes, whose pectoral fins are so extended as to ena- 

 ble them to dart from the water, and sustain themselves 

 for a considerable 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 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 sup- 

 port, and readily 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 

 weight as water, it follows that in swimming, very little 

 effort is requisite to keep the body from sinking. The 

 whole effort of its muscles is consequently employed in pro- 

 gression, and hence swimming requires vastly less muscular 

 force than flying. 



195. Swimming is accomplished by means of various or- 



