Europe been correlated with perfect accuracy.
With marine animals of all kinds, we may safely infer a large amount of
migration during climatal and other changes; and when we see a species
first appearing in any formation, the probability is that it
only then first immigrated into that area. It is well known, for
instance, that several species appeared somewhat earlier in the
pal ozoic beds of North America than in those of Europe; time having
apparently been required for their migration from the American to the
European seas. In examining the latest deposits of various quarters of
the world, it has everywhere been noted, that some few still existing
species are common in the deposit, but have become extinct in the
immediately surrounding sea; or, conversely, that some are now abundant
in the neighbouring sea, but are rare or absent in this particular
deposit. It is an excellent lesson to reflect on the ascertained amount
of migration of the inhabitants of Europe during the Glacial period,
which forms only a part of one whole geological period; and likewise to
reflect on the great changes of level, on the inordinately great change
of climate, on the prodigious lapse of time, all included within this
same glacial period. Yet it may be doubted whether in any quarter of
the world, sedimentary deposits, _including fossil remains_, have gone
on accumulating within the same area during the whole of this period.
It is not, for instance, probable that sediment was deposited during
the whole of the glacial period near the mouth of the Mississippi,
within that limit of depth at which marine animals can flourish; for we
know what vast geographical changes occurred in other parts of America
during this space of time. When such beds as were deposited in shallow
water near the mouth of the Mississippi during some part of the glacial
period shall have been upraised, organic remains will probably first
appear and disappear at different levels, owing to the migration of
species and to geographical changes. And in the distant future, a
geologist examining these beds, might be tempted to conclude that the
average duration of life
of the embedded fossils had been less than that of the glacial period,
instead of having been really far greater, that is extending from
before the glacial epoch to the present day.
In order to get a perfect gradation between two forms in the upper and
lower parts of the same formation, the deposit must have gone on
accumulating for a very long period, in order to have given sufficient
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