As far as the bird is concerned, this may be doubted
on the ground that it first appears in the upper or later
Jurassic, and is even then still largely reptilian in character.
We must remember, however, that the elevation of the land and the
cold climate lasted until the second part of the Triassic, and it
is generally agreed that the bird may have been evolved in the
Triassic. Its slow progress after that date is not difficult to
understand. The advantage of a four-chambered heart and warm coat
would be greatly reduced when the climate became warmer. The
stimulus to advance would relax. The change from a coat of scales
to a coat of feathers obviously means adaptation to a low
temperature, and there is nothing to prevent us from locating it
in the Triassic, and indeed no later known period of cold in
which to place it.
It is much clearer that the mammals were a product of the Permian
revolution. They not only abound throughout the Jurassic, in
which they are distributed in more than thirty genera, but they
may be traced into the Triassic itself. Both in North America and
Europe we find the teeth and fragments of the jaws of small
animals which are generally recognised as mammals.
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