temperature or migration. If Tertiary floras of different ages are met with in one area, great changes, on the contrary, are seen, and these are mainly due to progressive changes in climate. From middle Eocene to Meiocene the heat imperceptibly diminished. Very gradually the tropical members of the flora disappeared; that is to say, they migrated, for most of their types, I think, actually survive at the present day, many but very slightly altered. Then the sub-tropical members decreased, and the temperate forms, never quite absent even in the middle Eocenes, preponderated. As decreasing temperature drove the tropical forms south, the more northern must have pressed closely upon them. The northern Eocene, or the temperate floras of that period, must have pushed from their homes in the far north more and more south as climates chilled, and at last, in the Meiocene time, occupied our latitudes. The relative preponderance of these elements, I believe, will assist in determining the age of Tertiary deposits in Europe, more than any minute comparisons of species. Thus it is useless to seek in the Arctic regions for Eocene floras, as we know them in our latitudes; for during the Tertiary period the climatic conditions of the earth did not permit their growth there."[1]
Before such a migration of plants as this could take place there must have been land extending far north, so as to bring Europe into close relation with the Polar regions. The position of this ancient continent is indicated by the fossil floras of Iceland, Greenland, and Spitzbergen, which flourished in those latitudes in the Eocene, and subsequently established themselves in Europe in the Meiocene age, as we shall see in the next
- ↑ Nature xx. 12.
