FORM AND HABIT: THE WING. 



21 



In antarctic seas we find the arctic Auks replaced 

 by the Penguins, a group in which all the memhers are 

 flightless. They are possessed of I'eniarkahle aquatic 



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

 inches ; of wing, 5-7.5 inches.) 



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

 nest only on isolated islands, where they are not exposed 

 to the attack of predaceous mammals. 



AmoTig 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 fliglit shall not be impaired. Conse- 

 quently, when molting, the wing-feathers ai-e shed 

 slowly and synimetrically, 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 



