Nothing, however, that we see at the present day would justify us in imagining an ocean of such enormous extent, and at the same time so uniform in its depth, temperature, and other conditions of marine life, 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.