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
time for the slow process of variation; hence the deposit will
generally have to be a very thick one; and the species undergoing
modification will have had to live on the same area throughout this
whole time.
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