FLORAS OF SECONDARY AND TERTIARY PERIODS. 503 



plants are found many trees, such as Conifers, Beeches, Oaks, 

 Maples, Plane-trees, Walnuts, Magnolias, &c., with numerous 

 shrubs, ferns, and other smaller plants. 



Taking the Miocene flora as a whole, Dr Heer concludes 

 from his study of about 3000 plants contained in the European 

 Miocene alone, that the Miocene plants indicate tropical or 

 sub-tropical conditions, but that there is a striking intermixture 

 of forms which are at present found in countries widely re- 

 moved from one another. It is impossible to state with cer- 

 tainty how many of the Miocene plants belong to existing 

 species, but it appears that the larger number are extinct. 

 According to Heer, the American types of plants are most 

 largely represented in the Miocene flora, next those of Europe 

 and Asia, next those of Africa, and lastly those of Australia. 

 Upon the whole, however, the Miocene flora of Europe is 

 mostly nearly allied to the plants which we now find inhabit- 

 ing the warmer parts of the United States ; and this has led 

 to the suggestion that in Miocene times the Atlantic Ocean 

 was dry land, and that a migration of American plants to 

 Europe was thus permitted. This view is borne out by the 

 fact that the Miocene plants of Europe are most nearly allied 

 to the living plants of the eastern or Atlantic seaboard of the 

 United States, and also by the occurrence of a rich Miocene 

 flora in Greenland. As regards Greenland, Dr Heer has de- 

 termined that the Miocene plants indicate a temperate climate 

 in that country, with a mean annual temperature at least 30 

 warmer than it is at present. 



The present limit of trees is the isothermal which gives the 

 mean temperature of 50 Fahr. in July, or about the parallel 

 of 67 N. latitude. In Miocene times, however, the Limes, 

 Cypresses, and Plane-trees reach the 79th degree of latitude, 

 and the Pines and Poplars must have ranged even further 

 north than this. 



PLIOCENE PLANTS. The vegetation of the Pliocene period 

 is, upon the whole, so closely allied to that now existing as to 

 call for no special mention. It is worthy of notice, however, 

 that the Pliocene flora of Europe was strikingly similar to that 

 now existing in North America. Thus, we find in the Plio- 

 cene of Europe genera such as the Locust-trees (Robinia}, 

 the Honey-locusts (Gleditschia], the Sumach (Rhus], the Bald 

 Cypress (Taxodium), the Tulip-tree (Liriodendrori), the Sweet- 

 gum Tree (Liquidambar\ the Sour-gum Tree (Nyssa), &c., 

 which do not now occur in Europe, but are at present charac- 

 teristic forms in the flora of temperate North America. 



