h 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