mplete 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.

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,