72 STRUCTURE AND COMPARISON. 



bird reaches his foot forward and pulls until he has drawn his 

 body ahead of his feet, and then (continuing the stroke but 

 changing the kind of action) pushes against the water until his 

 feet trail behind. 



If we think of the water as less easily moved than the bird's 

 body, we can easily understand the paddling motion. In order 

 to paddle faster a man must take either more strokes or longer 

 strokes in a minute, or else use a longer or a wider paddle ; that 

 is, he must displace more water in a given time. The bird, in 

 order to swim fast, must do the same, quicken his stroke, 

 or lengthen it, or oppose a greater surface to the water. The 

 bird that can do all three without exhausting his strength is an 

 expert swimmer. 



D 



FIG. 13. LEG -BONES OF A LOON. 

 A Thigh-bone. B Knee. G Tarsus. D Heel. 



If you will compare your chicken's leg-bones with this 

 picture of the leg-bones of a loon, you will observe some 

 marked differences in the relative length of bones, the arrange- 

 ment of toes, and in that little bony spur that stands up in 

 front of the knee joint, which the chicken does not have. 



