|
have concluded that the average duration of each formation is twice or thrice as long as the average duration of specific forms. But insuperable difficulties, as it seems to me, prevent us
to any just conclusion on this head. When we see a species first appearing in the middle of any formation, it would be rash in the extreme to infer that it had not elsewhere previously existed. So again when we find a species disappearing before the
layers have been deposited, it would be equally rash to suppose that it then became
extinct. We forget how small the area of Europe is compared with the rest of the world; nor have the several stages of the same formation throughout Europe been correlated with perfect accuracy. |
|
→We may safely infer that with
marine animals of all
→there has been
a large amount of migration
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
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,
→and on the great
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
|
