He showed me also, in one of his books, the figures of the sun, moon, and stars, the zodiac, the tropics, and
polar circles, together with the denominations of many plains and solids.
Hence we may suppose that the organisms now living under the climate of latitude 60 deg, during the Pliocene period lived further north under the Polar Circle, in latitude 66 deg-67 deg; and that the strictly arctic productions then lived on the broken land still nearer to the pole.
During the slowly decreasing warmth of the Pliocene period, as soon as the species in common, which inhabited the New and Old Worlds, migrated south of the Polar Circle, they must have been completely cut off from each other.
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 areas completely sundered.