era
there has habitually existed a recognizable similarity between the
groups of organic forms inhabiting all the different parts of the Earth;
and that the causes which have in one part of the Earth changed the
organic forms into those which characterize the next era, have
simultaneously acted in all other parts of the Earth, in such ways as to
produce parallel changes of their organic forms. Now this is not only a
large assumption to make; but it is an assumption contrary to
probability. The probability is, that the causes which have changed
Faunas have been local rather than universal; that hence while the
Faunas of some regions have been rapidly changing, those of others have
been almost quiescent; and that when those of others have been changed,
it has been, not in such ways as to maintain parallelism, but in such
ways as to produce divergence.

Even supposing, however, that districts some hundreds of miles apart,
furnished groups of strata which completely agreed in their order of
superposition, their mineral characters, and their fossils, we should
still have inadequate proof of contemporaneity. For there are
conditions, very likely to occur, under which such groups might differ
widely in age. If there be a continent of which the strata crop out on
the surface obliquely to the line of coast--running, say,
west-north-west, while the coast runs east and west--it is clear that
each group of strata will crop out on the beach at a particular part of
the coast; that further west the next group of strata will crop out on
the beach; and so continuously. As the localization of marine plants and
animals, is in a considerable degree determined by the natures of the
rocks and their detritus, it follows that each part of this coast will
have its more or less distinct Flora and Fauna. What now must result
from the action of the waves in the course of a geologic epoch? As the
sea makes slow inroads on the land, the place at which each group of
strata crops out on the beach will gradually move towards the west: its
distinctive fish, mollusks, crustaceans, and sea-weeds, migrating with
it. Further, the detritus of each of these groups of strata will, as the
point of outcrop moves westwards, be deposited over the detritus of the
group in advance of it. And the consequence of these actions, carried on
for one of those enormous periods which a geologic change takes, will be
that, corresponding to each eastern stratum, there will ar