Abstract

Most geologists are now prepared to admit that the occurrence of the same species of marine animals in strata very widely removed from one another in point of distance may be accepted as a proof of “migration” having taken place. It does not matter, from this point of view, whether the deposits of the two areas are approximately of the same age, or whether they succeed one another in point of time—except that in the former instance we can hardly ever be sure which of the areas has been peopled from the other. It follows also from this view that strata containing exactly the same species of fossils, if very widely separated in point of distance, can never be exactly “contemporaneous.” They may have been formed nearly at the same time, and they will belong to the same geological epoch; but the one must really succeed the other in point of time. It will be understood, of course, that this statement is only intended to apply to cases in which identical fossils are found at points so far removed from one another over the earth's surface that we can never suppose them to have been connected by a single ocean the conditions of which were so uniform as to have allowed the spread of the same animals over its entire extent.

If we were thoroughly acquainted with the range of any given fossil vertically, and, at the same time, had all the necessary details as to what may be called