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only then first immigrated into that area. It is
for instance, that several species appeared
earlier in the palæozoic 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
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
which forms only a part of one whole geological
and likewise to reflect on the
changes of level, on the
change of climate,
→on the prodigious
lapse of time, all included within this same glacial period. Yet it may be doubted
in any quarter of the world, sedimentary deposits,
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
for we know
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
of species and to geographical changes. And in the distant future, a
examining these beds,
be tempted to conclude that the average duration of life
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