The origin of the feathers
The question of the origin of the feathers can be discussed only
from the speculative point of view, as they are fully developed
in the Archaeopteryx, and there is no approach toward them in any
other living or fossil organism. But a long discussion of the
problem has convinced scientific men that the feathers are
evolved from the scales of the reptile ancestor. The analogy
between the shedding of the coat in a snake and the moulting of a
bird is not uninstructive. In both cases the outer skin or
epidermis is shedding an old growth, to be replaced by a new one.
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. No modern bird has teeth;
though the fact that in some modern species we find the teeth
appearing in a rudimentary form is another illustration of the
law that animals tend to reproduce ancestral features in their
development. A more reptilian character in the Ichthyornis group
is the fact that, unlike any modern bird, but like their reptile
ancestors, they had biconcave vertebrae. The brain was relatively
poor. We are still dealing with a type intermediate in some
respects between the reptile and the modern bird. The gannets,
cormorants, and pelicans are believed to descend from some branch
of this group.
The other group of Cretaceous birds, of the Hesperornis type,
show an actual degeneration of the power of flight through
adaptation to an environment in which it was not needed, as
happened, later, in the kiwi of New Zealand, and is happening in
the case of the barn-yard fowl. These birds had become divers.
Their wings had shrunk into an abortive bone, while their
powerful legs had been peculiarly fitted for diving. They stood
out at right angles to the body, and seem to have developed
paddles. The whole frame suggests that the bird could neither
walk nor fly, but was an excellent diver and swimmer. Not
infrequently as large as an ostrich (five to six feet high), with
teeth set in grooves in its jaws, and the jaws themselves joined
as in the snake, with a great capacity of bolting its prey, the
Hesperornis would become an important element in the life of the
fishes. The wing-fingers have gone, and the tail is much
shortened, but the grooved teeth and loosely jointed jaws still
point back to a reptilian ancestry.
|
