20 FORM AND HABIT: THE WING. 
of flight, and that as their wings, through disuse, became 
functionless, their running powers correspondingly in- 
creased. This, however, is theory, but 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 on.y 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 aérial 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 flighc through 
disuse of its wings. 
