The origin of the feathers
These are the only remains of bird-life that we find in the
Mesozoic rocks. Admirably as they illustrate the evolution of the
bird from the reptile, they seem to represent a relatively poor
development and spread of one of the most advanced organisms of
the time. It must be understood that, as we shall see, the latter
part of the Chalk period does not belong to the depression, the
age of genial climate, which I call the Middle Ages of the earth,
but to the revolutionary period which closes it. We may say that
the bird, for all its advances in organisation, remains obscure
and unprosperous as long as the Age of Reptiles lasts. It awaits
the next massive uplift of the land and lowering of temperature.
In an earlier chapter I hinted that the bird and the mammal may
have been the supreme outcomes of the series of disturbances
which closed the Primary Epoch and devastated its primitive
population. 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. We cannot, of
course, from a few bones deduce that there already, in the
Triassic, existed an animal with a fully developed coat of fur
and an apparatus, however crude, in the breast for suckling the
young. But these bones so closely resemble the bones of the
lowest mammals of to-day that this seems highly probable. In the
latter part of the long period of cold it seems that some reptile
exchanged its scales for tufts of hair, developed a
four-chambered heart, and began the practice of nourishing the
young from its own blood which would give the mammals so great an
ascendancy in a colder world.
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