FLORAS OF SECONDARY AND TERTIARY PERIODS. 475 



56', no less than ninety-five species of fossil plants have 

 been obtained, including Taxodium of two species, hazel, 

 poplar, alder, beech, plane-tree, and lime. Such a vigorous 

 growth of trees within 12° of the pole, where now a dwarf 

 willow and a few herbaceous plants form the only vegetation, 

 and where the ground is covered with almost perpetual snow 

 and ice, is truly remarkable." 



Taking the Miocene flora as a whole, Dr Heer concludes 

 from his study of about 3000 plants contained in the Euro- 

 pean Miocene alone, that the Miocene plants indicate tropi- 

 cal or sub-tropical conditions, but that there is a striking 

 intermixture of forms which are at present found in countries 

 widely removed from one another. It is impossible to state 

 with certainty liow many of the Miocene plants belong to 

 existing sj)ecies, 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 inhabiting the "warmer parts of tlie United States ; and 

 this has led to the suG;2;estion that in Miocene times the 

 jSTorth 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 tlie 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 determined that tlie Miocene plants 

 indicate a temperate climate in that country, with a mean 

 annual temperature at least 30° warmer than it is at present. 



Tlie 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 



