The
Dibranchiate Cephalopods appear with equal apparent suddenness in
the older Mesozoic deposits, and no known type of the Palaeozoic
period can be pointed to as a possible ancestor. The _Hippuritidoe_
of the Cretaceous burst into a varied life to all appearance
almost immediately after their first introduction into existence.
The wonderful Dicotyledonous flora of the Upper Cretaceous period
similarly surprises us without any prophetic annunciation from
the older Jurassic.
Many other instances could be given; but enough has been said
to show that there is a good deal to be said on both sides, and
that the problem is one environed with profound difficulties.
One point only seems now to be universally conceded, and that
is, that the record of life in past time is not interrupted by
gaps other than those due to the necessary imperfections of the
fossiliferous series, to the fact that many animals are incapable
of preservation in a fossil condition, or to other causes of a
like nature. All those who are entitled to speak on this head
are agreed that the introduction of new and the destruction of
old species have been slow and gradual processes, in no sense of
the term "catastrophistic." Most are also willing to admit that
"Evolution" has taken place in the past, to a greater or less
extent, and that a greater or less number of so-called species of
fossil animals are really the modified descendants of pre-existent
forms.
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