We
shall find them developing with great richness in the following
period, but, imperfect as the record is, we may venture to say
that they were checked in the Cretaceous. There were good
conditions for preserving them, but few are preserved. And of the
other groups of invertebrates we need only say that they show a
steady advance toward modern types. The sea-lily fills the rocks
no longer; the sea-urchin is very abundant. The Molluscs gain on
the more lowly organised Brachiopods.
To complete the picture we must add that higher types probably
arose in the later Cretaceous which do not appear in the records.
This is particularly true of the birds and mammals. We find them
spreading so early in the Tertiary that we must put back the
beginning of the expansion to the Cretaceous. As yet, however,
the only mammal remains we find are such jaws and teeth of
primitive mammals as we have already described. The birds we
described (after the Archaeopteryx) also belong to the
Cretaceous, and they form another of the doomed races. Probably
the modern birds were already developing among the new vegetation
on the higher ground.
These are the facts of Cretaceous life, as far as the record has
yielded them, and it remains for us to understand them.
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