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.
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