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_.