Lack of Evidence
We cannot complain of any lack of evidence connecting the mammal
with a reptile ancestor. The earliest remains we find are of such
a nature that the highest authorities are still at variance as to
whether they should be classed as reptilian or mammalian. A skull
and a fore limb from the Triassic of South Africa (Tritylodon and
Theriodesmus) are in this predicament. It will be remembered that
we divided the primitive reptiles of the Permian period into two
great groups, the Diapsids and Synapsids (or Theromorphs).
The
former group have spread into the great reptiles of the Jurassic;
the latter have remained in comparative obscurity. One branch of
these Theromorph reptiles approach the mammals so closely in the
formation of the teeth that they have received the name "of the
Theriodonts", or "beast-toothed" reptiles. Their teeth are, like
those of the mammals, divided into incisors, canines (sometimes
several inches long), and molars; and the molars have in some
cases developed cusps or tubercles. As the earlier remains of
mammals which we find are generally teeth and jaws, the
resemblance of the two groups leads to some confusion in
classifying them, but from our point of view it is not unwelcome.
It narrows the supposed gulf between the reptile and the mammal,
and suggests very forcibly the particular branch of the reptiles
to which we may look for the ancestry of the mammals. We cannot
say that these Theriodont reptiles were the ancestors of the
mammals. But we may conclude with some confidence that they bring
us near to the point of origin, and probably had at least a
common ancestor with the mammals.
The distribution of the Theriodonts suggests a further idea of
interest in regard to the origin of the mammals. It would be
improper to press this view in the present state of our
knowledge, yet it offers a plausible theory of the origin of the
mammals. 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
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