FORM AND IIAIUT: TIIK WING. 



21 



III antarctic seas we tiiid tlie arctic Auks replaced 

 l)v tlie Penguins, a tj^ronp in w liicli all tlie members are 

 They are possessed of i-eniarkable aquatic 



lliglitless 



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

 inches; of wing, 5-75 inches.) 



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

 nest only on isolated islands, where they are not exposed 

 to tlie attack of predaceous mammals. 



Amono^ Grebes and Ducks we have illustrations of 

 the way in which swinnning birds may become tempo- 

 rarily flightless. AVith 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 an<l 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 



