20 FORM AND HABIT: THE WING. 



of flight, and that as their wings, throiigli disuse, became 

 functionless, their running powers correspondingly in- 

 creased. This, however, is theory, but tliere are birds 

 wliieh have become flightless through some apparently 

 known cause. They may be found among such widely 

 separated famihes as Grebes, Auks, Ducks, Rails, Galli- 

 nules. Pigeons, and Parrots. 



One of the characteristic water birds of our Korth 

 Atlantic coasts is the Kazor-billed Auk. It is a strictly 

 aquatic species, nearly helpless on land, which, as a rule, 

 it visits only when nesting. Its egg is laid in the crevice 

 of a rocky cliif, frequently at some height from the sea. 

 During the winter it migrates southward as far as Long 

 Island. Flight is therefore a necessary faculty, and we 

 find the bird with well-developed wings, which it uses 

 effectively. We can, however, imagine conditions under 

 which it would not be necessary for the Razor-bill to 

 fly. It might become a permanent resident of isolated 

 islands, laying its egg on accessible beaches. Already 

 an expert diver, obtaining its food in the water, it would 

 not be obliged to rise into the air, and, as a result of dis- 

 use, tlie ^vings would finally become too small to support it 

 in aerial flight, though fully answering the purpose of oars. 



Apparently this is what has happened in tlie case of 

 the Razor-billed Auk's relativ^e, the flightless, extinct 

 Great Auk. The Razor-bill is sixteen inches long and 

 its wing measures eight inches, wliile the Great Auk, 

 with a length of thirty inches, has a wing only five and 

 three fourths inches in length. Aside from this differ- 

 ence in measurements these birds closely resemble each 

 other. So far as we are familiar with the Great Auk's 

 habits, they agreed with those of the hypothetical case I 

 have just mentioned, and we are warranted, I think, in 

 assuming that the bird lost the power of flight through 

 disuse of its wings. 



