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.

Nor can we complain of any lack of evidence connecting the mammal with a
reptile ancestor. The earliest remains we find are of suc