They had appeared, as we said, in the Permian period. Probably
they had been developed during the later Carboniferous, since we
find them already branched into three orders, with many
sub-orders, in the Permian. The stimulating and selecting
disturbances which culminated in the Permian revolution had begun
in the Carboniferous. Their origin is not clear, as the
intermediate forms between them and the amphibia are not found.
This is not surprising, if we may suppose that some of the
amphibia had, in the growing struggle, pushed inland, or that, as
the land rose and the waters were drained in certain regions,
they had gradually adopted a purely terrestrial life, as some of
the frogs have since done. In the absence of water their frames
would not be preserved and fossilised. We can, therefore,
understand the gap in the record between the amphibia and the
reptiles. From their structure we gather that they sprang from at
least two different branches of the amphibia. Their remains fall
into two great groups, which are known as the Diapsid and the
Synapsid reptiles. The former seem to be more closely related to
the Microsauria, or small salamander-like amphibia of the
Coal-forest; the latter are nearer to the Labyrinthodonts.
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