50 THE ANIMALS AND MAN 



power of motion and are sensitive to light, heat, and other 

 influences as are animals, but to a far less degree. 



Among the movements made by animals, the moving of 

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

 tion, generally requires the greatest energy or power. The 

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

 legs, head, etc. 



There are three different ways in which locomotion takes 

 place, namely, 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 supported, 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 therefore 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 com- 

 pared 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 in wa- 

 ter and air are of peculiar help in this matter. Water cur- 



