Now, if all these beds were contemporaneous, in the literal sense of the term, we should have to suppose that the ocean at one time extended uninterruptedly between all these points, and was peopled throughout the vast area thus indicated by many of the same animals.

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.