FORM AND HABIT: THE WING. 21 In antarctic seas we lind the arctic Auks replaced bj the Penguins, a group in which all the members are flightless. They are possessed of remarkable aquatic 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. 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
