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 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.
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