THE MOTIONS OF ANIMALS 65 



Among the movements made by animals, the moving 

 of the body from place to place, usually spoken of as lo- 

 comotion, generally requires the greatest energy or power. 

 The other motions are those of parts of the body, as the 

 arms, legs, head, etc. We shall here consider a few 

 examples of the motion of animals illustrating the 

 character of locomotion in different forms. 



There are three different ways in which locomotion 

 takes place, that is, by swimming in water, crawling or 

 walking or leaping on some solid object, as the ground 

 or the trunk of a tree, and by flying in the air. In 

 each of these three cases the body must first be sup- 

 ported, then either pushed or pulled along or perhaps 

 both pushed and pulled. 



In swimming the body is supported by the water. In 

 animals that swim it is either lighter than water, as in the 

 duck, or just as heavy or only a little heavier, as in fishes, 

 so that it is wholly or almost wholly held up by the 

 water, and the full power of the leg, fin, or tail used in 

 the motion can be devoted to pushing the animal along. 

 Animals crawling on the bottom in water also have very 

 little to do in holding up the body, the water supporting 

 them. But those that move on land or fly, with their 

 bodies immersed in air alone, have the body only very 

 slightly supported by the air. These animals must there- 

 fore devote energy to supporting the body as well as to 

 moving it along, and they have special means for this. 



As already said the body is moved by pushes or pulls. 

 In by far the most cases motion results from pushes given 

 by a part of the body against something outside. Now 

 it is plain that air is a very poor, thing to push against as 

 compared with water or a solid. Naturally since water 

 is a liquid it gives way readily to a push, but its heaviness 

 offers much greater resistance to motion than does the air. 

 The solid ground, of course, offers most of all. Currents 



