The former influence of the glacial climate on the distribution of the inhabitants of Europe, as explained by Edward Forbes, is substantially as follows. But we shall follow the changes more readily by supposing a new glacial period slowly to come on, and then pass away, as formerly occurred. As the cold came on, and as each more southern zone became fitted for the inhabitants of the north, these would take the places of the former inhabitants of the temperate regions. The latter, at the same time, would travel farther and farther southward, unless they were stopped by barriers, in which case they would perish. The mountains would become covered with snow and ice, and their former Alpine inhabitants would descend to the plains. By the time that the cold had reached its maximum, we should have an Arctic fauna and flora, covering the central parts of Europe, as far south as the Alps and Pyrenees, and even stretching into Spain. The now temperate regions of the United States would likewise be covered by Arctic plants and animals, and these would be nearly the same with those of Europe; for the present circumpolar inhabitants, which we suppose to have everywhere traveled southward, are remarkably uniform round the world. As the warmth returned, the Arctic forms would retreat northward, closely followed up in their retreat by the productions of the more temperate regions. And, as the snow melted from the bases of the mountains, the Arctic forms would seize on the cleared and thawed ground, always ascending, as the warmth increased and the snow still further disappeared, higher and higher, while their brethren were pursuing their northern journey. Hence, when the warmth had fully returned, the same species, which had lately lived together on the European and North American lowlands, would again be found in the Arctic regions of the Old and New Worlds, and on many isolated mountain-summits far distant from each other. Thus we can understand the identity of many plants at points so immensely remote as the mountains of the United States and those of Europe. THE THEORY OF CREATION INADEQUATE. [Page 334.] As on the land, so in the waters of the sea, a slow southern migration of a marine fauna, which, during the Pliocene or even a somewhat earlier period, was nearly uniform along the continuous shores of the Polar Circle, will account, on the theory of modification, for many closely allied forms now living in marine areas completely sundered. Thus, I think, we can understand the presence of some closely allied, still existing and extinct tertiary forms on the eastern and western shores of temperate North America; and the still more striking fact of many closely allied crustaceans (as described in Dana’s admirable work), some fish and other marine animals, inhabiting the Mediterranean and the seas of Japan--these two areas being now completely separated by the breadth of a whole continent and by wide spaces of ocean.