The Theriodonts seem to have been generally confined to
the southern continent, Gondwana Land (Brazil to Australia), of
which an area survives in South Africa. It is there also that we
find the early disputed remains of mammals. Now we saw that,
during the Permian, Gondwana Land was heavily coated with ice,
and it seems natural to suppose that the severe cold which the
glacial fields would give to the whole southern continent was the
great agency in the evolution of the highest type of the animal
world. From this southern land the new-born mammals spread
northward and eastward with great rapidity. Fitted as they were
to withstand the rigorous conditions which held the reptiles and
amphibia in check, they seemed destined to attain at once the
domination of the earth. Then, as we saw, the land was revelled
once more until its surface broke into a fresh semi-tropical
luxuriance, and the Deinosaurs advanced to their triumph. The
mammals shrank into a meagre and insignificant population, a
scattered tribe of small insect-eating animals, awaiting a fresh
refrigeration of the globe.
The remains of these interesting early mammals, restricted, as
they generally are, to jaws and teeth and a few other bones that
cannot in themselves be too confidently distinguished from those
of certain reptiles, may seem insufficient to enable us to form a
picture of their living forms.
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