The covering or horny part of the scale and the feather are alike
growths from the epidermis, and the initial stages of the growth
have certain analogies. But beyond this general conviction that
the feather is a development of the scale, we cannot proceed with
any confidence. Nor need we linger in attempting to trace the
gradual modification of the skeleton, owing to the material
change in habits. The horny beak and the reduction of the toes
are features we have already encountered in the reptile, and the
modification of the pelvis, breast-bone, and clavicle are a
natural outcome of flight.
In the Chalk period we find a large number of bird remains, of
about thirty different species, and in some respects they resume
the story of the evolution of the bird. They are widely removed
from our modern types of birds, and still have teeth in the jaws.
They are of two leading types, of which the Ichthyornis and
Hesperornis are the standard specimens. The Ichthyornis was a
small, tern-like bird with the power of flight strongly
developed, as we may gather from the frame of its wings and the
keel-shaped structure of its breast-bone. Its legs and feet were
small and slender, and its long, slender jaws had about twenty
teeth on each side at the bottom.
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