The features of the Archaeopteryx
The features of the Archaeopteryx ("primitive bird") have been
described so often, and such excellent pictorial restorations of
its appearance may now be seen, that we may deal with it briefly.
We have in it a most instructive combination of the characters of
the bird and the reptile. The feathers alone, the imprint of
which is excellently preserved in the fine limestone, would
indicate its bird nature, but other anatomical distinctions are
clearly seen in it. "There is," says Dr. Woodward, "a typical
bird's 'merrythought' between the wings, and the hind leg is
exactly that of a perching bird." In other words, it has the
shoulder-girdle and four-toed foot, as well as the feathers, of a
bird. On the other hand, it has a long tail (instead of a
terminal tuft of feathers as in the bird) consisting of
twenty-one vertebrae, with the feathers springing in pairs from
either side; it has biconcave vertebrae, like the fishes,
amphibia, and reptiles; it has teeth in its jaws; and it has
three complete fingers, free and clawed, on its front limbs.
As in the living Peripatus, therefore, we have here a very
valuable connecting link between two very different types of
organisms. It is clear that one of the smaller reptiles--the
Archaeopteryx is between a pigeon and a crow in size--of the
Triassic period was the ancestor of the birds. Its most
conspicuous distinction was that it developed a coat of feathers.
A more important difference between the bird and the reptile is
that the heart of the bird is completely divided into four
chambers, but, as we saw, this probably occurred also in the
other flying reptiles. It may be said to be almost a condition of
the greater energy of a flying animal. When the heart has four
complete chambers, the carbonised blood from the tissues of the
body can be conveyed direct to the lungs for purification, and
the aerated blood taken direct to the tissues, without any
mingling of the two. In the mud-fish and amphibian, we saw, the
heart has two chambers (auricles) above, but one (ventricle)
below, in which the pure and impure blood mingle. In the reptiles
a partition begins to form in the lower chamber. In the turtle it
is so nearly complete that the venous and the arterial blood are
fairly separated; in the crocodile it is quite complete, though
the arteries are imperfectly arranged. Thus the four-chambered
heart of the bird and mammal is not a sudden and inexplicable
development. Its advantage is enormous in a cold climate. The
purer supply of blood increases the combustion in the tissues,
and the animal maintains its temperature and vitality when the
surrounding air falls in temperature. It ceases to be
"cold-blooded."
But the bird secures a further advantage, and here it outstrips
the flying reptile. The naked skin of the Pterosaur would allow
the heat to escape so freely when the atmosphere cooled that a
great strain would be laid on its vitality. A man lessens the
demand on his vitality in cold regions by wearing clothing. The
bird somehow obtained clothing, in the shape of a coat of
feathers, and had more vitality to spare for life-purposes in a
falling temperature. The reptile is strictly limited to one
region, the bird can pass from region to region as food becomes
scarce.
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