146 THE BIOLOGY OF BIRDS 



in most cases it folds together neatly in being drawn forwards 

 for the next stroke, thus reducing the friction. In some 

 cases, as in divers, the tarso-metatarsus region before the 

 toes begin is laterally compressed like a knife blade. When 

 the leg strikes quickly backwards, all the force is concen- 

 trated in the webbed foot, the blade of the oar as it were ; 

 and when the leg is drawn forward for the next stroke the 

 sharp edge of the tarso-metatarsus and the collapse of the 

 webbed foot must reduce the friction greatly. In ordinary 

 cases the feet are used alternately, but many may have 

 noticed how an excited swan will kick violently backwards 

 in the water with both feet at once. 



(c) The water-ouzel or dipper uses its wings as organs 

 for swimming under water, and they do not show any 

 notable transformation. The Diving Petrel swims deeply 

 with its wings and emerges flying from the surface of the 

 sea. A similar use of the wings as organs for under-water 

 swimming is seen in various members of the auk family, 

 where the wing is long, narrow, and pointed. In the 

 flightless Great Auk the wing had no use except in the water. 

 A storm-petrel, diver, puffin, little auk, or any similar 

 bird, moving on the surface of the sea is obviously swimming 

 by means of its feet only. But the very reverse is to be seen 

 in the penguins, where the hind legs are stretched out 

 behind as a rudder, but the transformed wings are powerful 

 paddles. They are flipper- like and relatively short, covered 

 with flat reduced feathers, and move en bloc except at the 

 elbow joint. The bones are greatly compressed laterally 

 and their detailed shapes tend to restrict the possibility of 

 their moving on one another, except at the elbow where a 

 restricted mobility admits of a somewhat screw-like twisting. 

 The musculature of this strangely transformed wing is 

 reduced to a vanishing point, but there are strong sinews 

 which bind the bones tightly together. No bird that has a 

 functional fore-limb at all shows such a transformation as 

 the penguin, and yet it is important to observe how little 

 the essential architecture has been departed from. Hilz- 

 heimer notes that the fossil penguins of Seymour Island 



