as to allow the
same animals to flourish in it from end to end; and the example
chosen is only one of a long and ever-recurring series. It is
therefore much more reasonable to explain this, and all similar
cases, as owing to the _migration_ of the fauna, in whole or in
part, from one marine area to another. Thus, we may suppose an
ocean to cover what is now the European area, and to be peopled
by certain species of animals. Beds of sediment--clay, sands,
and limestones--will be deposited over the sea-bottom, and will
entomb the remains of the animals as fossils. After this has
lasted for a certain length of time, the European area may undergo
elevation, or may become otherwise unsuitable for the perpetuation
of its fauna; the result of which would be that some or all of the
marine animals of the area would migrate to some more suitable
region. Sediments would then be accumulated in the new area to
which they had betaken themselves, and they would then appear,
for the second time, as fossils in a set of beds widely separated
from Europe. The second set of beds would, however, obviously
not be strictly or literally contemporaneous with the first, but
would be separated from them by the period of time required for
the migration of the animals from the one area into the other.
It is only in a wide and comprehensive sense that such strata
can be said to be contemporaneous.

It is impossible to enter further into this subject here; but it
may be taken as certain that beds in widely remote geographical
areas can only come to contain the same fossils by reason of a
migration having taken place of the animals of the one area to
the other. That such migrations can and do take place is quite
certain, and this is a much more reasonable explanation of the
observed facts than the hypothesis that in former periods the
conditions of life were much more uniform than they are at present,
and that, consequently, the same organisms were able to range over
the entire globe at the same time. It need only be added, that
taking the evidence of the present as explaining the phenomena
of the past--the only safe method of reasoning in geological
matters--we have abundant proof that deposits which _are_ actually
contemporaneous, in the strict sense of the term, _do not contain
the same fossils, if far removed from one another in point of
distance_. Thus, deposits of various kinds are now in process of
formation in our existing seas, as, for ex