FORM AXD HABIT: THE WING. 21 



In antarctic seas we find the arctic Auks replaced 

 l)y the Penguins, a group in whicli all the members are 



liightless. They are possessed of rcniarkahle a(|uatic 



Fig. 8. — Great Auk, showing relatively small wing. (Length of bird, 30 

 inches ; of wing, 5-75 inches.) 



powers, and can, it is said, outswim even fish. Thej 

 nest only on isolated islands, where they are not exposed 

 to the attack of predaceous mammals. 



Amons: Grebes and Ducks we have illustrations of 

 the way in which swimming birds may become tempo- 

 rarily flightless. With most land-inhabiting birds flight 

 is so important a faculty that any injury to the wings is 

 apt to result fatally. It is necessary, therefore, that 

 the power of flight shall not be impaired. Conse- 

 quently, when molting, the wing-feathers are shed 

 slowly and symmetrically, from the middle of the wing 

 both inwardly and outwardly ; the new feathers ap- 

 pear so quickly that at no time are there more than 

 two or three quills missing from either wing. But the 



