Thus it is
proved that the invertebrate animals were succeeded in time by the higher
vertebrates, which is exactly what the evidences of the previous
categories have shown. When we remember that the lower animals are devoid
as a rule of skeletal structures that might be fossilized, and when we
recall the fact that the strata of the palaeozoic provided the materials
out of which the upper layers were formed afterwards, we can understand
why the ancient members of the invertebrate groups are not known as well
as the later and higher forms like vertebrates. Yet all the fossils of
these relatively unfamiliar creatures clearly prove that no complex animal
appears upon a geological horizon until after some simple type belonging
to a class from which it may have taken its origin; in brief, there are no
anachronisms in the record, which always corresponds with the record
written by comparative anatomy, wherever the facts enable a comparison to
be made.
But the extinct animals of the third and fourth ages are more interesting
to us, because there are more of them and because they are more like the
well-known organisms of our present era. These two ages are called the
Mesozoic or Secondary, and the Cenozoic or Tertiary.
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