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.
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