FORM AND HABIT: THE WING. 21 
In antarctic seas we find the arctic Auks replaced 
by the Penguins, a group in which all the members are 
flightless. They are possessed of remarkable aquatic 
Fie. §.—Great Auk, showing relatively small wing. (Length of bird, 30 
inches; of wing, 575 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. 
Among 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 
