inheriting
some inferiority in common; and, therefore, as new and improved groups
spread throughout the world, old groups disappear from the world; and
the succession of forms everywhere tends to correspond both in their
first appearance and final disappearance.

There is one other remark connected with this subject worth making. I
have given my reasons for believing that most of our great formations,
rich in fossils, were deposited during periods of subsidence; and that
blank intervals of vast duration, as far as fossils are concerned,
occurred during the periods when the bed of the sea was either
stationary or rising, and likewise when sediment was not thrown down
quickly enough to embed and preserve organic remains. During these
long and blank intervals I suppose that the inhabitants of each region
underwent a considerable amount of modification and extinction, and
that there was much migration from other parts of the world. As we have
reason to believe that large areas are affected by the same movement,
it is probable that strictly contemporaneous formations have often been
accumulated over very wide spaces in the same quarter of the world;
but we are very far from having any right to conclude that this has
invariably been the case, and that large areas have invariably been
affected by the same movements. When two formations have been deposited
in two regions during nearly, but not exactly, the same period,
we should find in both, from the causes explained in the foregoing
paragraphs, the same general succession in the forms of life; but the
species would not exactly correspond; for there will have been a
little more time in the one region than in the other for modification,
extinction, and immigration.

I suspect that cases of this nature occur in Europe. Mr. Prestwich, in
his admirable Memoirs on the eocene deposits of England and France, is
able to draw a close general parallelism between the successive stages
in the two countries; but when he compares certain stages in England
with those in France, although he finds in both a curious accordance in
the numbers of the species belonging to the same genera, yet the species
themselves differ in a manner very difficult to account for considering
the proximity of the two areas, unless, indeed, it be assumed that an
isthmus separated two seas inhabited by distinct, but contemporaneous
faunas. Lyell has made similar observations on some of the later
tertiary formations