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Wetmore, Alexander, 1886-, et al. / Warm-blooded vertebrates

Chapter II: adaptations for progression by flying ,   pp. 13-24 ff. PDF (4.7 MB)

Page 13

FLIGHT has been a powerful factor in controlling the bodily
form of birds, maintaining it within the boundaries of a
very definite pattern, and allowing only such variation as
does not interfere with a flying existence. It is true that a
number of species of living birds, like the ostriches,
penguins, and numerous rails, as well as various peculiar
fossil forms (see Plates I7 and i9), do not, or did not, fly.
Some authorities have gone so far as to argue that the
ostriches and penguins have always been flightless, and
that they have not descended from the same ancestor as
have the flying birds. I hold, however, with many others,
that the form of the wing in ostriches and penguins, with
the fused bones of the hand and other peculiarities, indi-
cates definitely that these groups have come from flying
ancestors and have lost the power of flight: as they became
specialized for their own peculiar habits of life. No
definite basis seems to exist for the suggestion that birds
have originated from two distinct and separate lines of
The adaptations of birds for flight, though presenting a
number of departures from those found in other flying
animals, are relatively simple. We find the most striking
modification in the forelimb, which has become a wing
with feathers projecting from its posterior margin. The
wings form a strong supporting plane which maintains the
bird in the air, provided it has a certain forward momen-
tum, and at the same time can be flexed easily. Thus as
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