CHAP, ii.] BRITAIN CONNECTED WITH NORTH AMERICA. 21 



temperature or migration. If Tertiary floras of different 

 ages are met with in one area, great changes, on the con- 

 trary, 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 tempera- 

 ture 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 sec in the next 



1 Nature xx. 12. 



