HOW THE EARTH WAS PEOPLED. 679
capricious in such an arrangement of lands and seas, and that there have been, if not always, at least from a very ancient period, emerged lands occupying a considerable part of the northern hemisphere, ad- vancing very far toward the pole, and describing around the Arctic Sea a belt of more or less contiguous countries and islands. This is, in effect, what geology teaches. The changes, immersions, and emersions have never been anything but partial and successive, while the skele- tons of the continents go back to the most remote ages. There have always been a Europe, an Asia, an America, and Arctic lands. We know certainly that there have always been around the Arctic pole ex- tensive territories, if not continents, long the home of the same plants as the rest of the globe, and that, beginning with an epoch that corre- sponds with the end of the Jurassic, the climate, at first as warm there as elsewhere, has tended gradually to become colder. The depression of temperature was at first manifested very slowly, and was far from having attained its present degree in the tertiary ; for the trees that then grew in Greenland the sequoias, magnolias, and plane-trees now attain their full development in Southern Europe, and are not suited with the climate of Central Europe. We are, then, assured of the ancient existence, near the Arctic pole, of a zone of lands covered with a rich vegetation. The permanent existence of a polar sea is none the less attested by fossils from all parts of the region. The neighborhood of the pole was long habitable, and inhabited by man in a time near that in which the vestiges of his industry begin to show themselves alike in Europe and America. In passing thus from the Arctic lands to those bordering on the polar circle, and through the latter into Asia, Europe, and America, man would only have taken the road which a host of plants and animals followed, either before him or at the same time, and under the stress of the same circumstances.
It is, in fact, by the aid of migrations from the neighborhood of the pole that we can generally explain the phenomenon of scattered or dis- joined species, a phenomenon identical with the one which man of the Old World and man of the New World present when they are compared. Combining present notions with the indications furnished by the fos- sils, we discover numerous examples of disjunction in which allied forms, often hardly distinguishable, have been distributed at the same time in scattered regions, at extremely remote points in the boreal hemisphere, without any apparent connection along the parallels, to ex- plain the common unit. Europe attests by undeniable fossils that it had formerly a host of vegetable types and forms that are now Ameri- can, which it could have received only from the extreme north. It had, for example, magnolias, tulip-trees, sassafras, maples, and pop- lars, comparable in all respects to those which grow in the United States. The two plane-trees, that of the West and that of Asia Minor, to which we may add an extinct fossil European plane-tree, illustrate the same phenomenon of dispersion. Europe in the Tertiary period