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 20 FORM AND HABIT: THE WING. of flight, and that as their wings, through disuse, became functionless, their ninning powers correspondingly in- creased. This, however, is theory, hut there are birds which have become flightless through some apparently known cause. They may be found among such widely separated families as Grebes, Auks, Ducks, Rails, Galli- nules. Pigeons, and Parrots. One of the characteristic water birds of our North Atlantic coasts is the Razor-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 cliff, 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, the wings 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 the case of the Razor-billed Auk's relative, the flightless, extinct Great Auk. The Razor-bill is sixteen inches long and its wing measures eight inches, while 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.