THE GENESIS AND MIGRATIONS OF PLANTS 22/ 



not indicate tropical heat in the far north, but only that of the 

 warm temperate zone ; but this in some portions of the period 

 certainly extends to the middle of Greenland, unless, without 

 any evidence, we suppose that the Cretaceous and lower Tertiary 

 plants differed in hardiness of constitution from their modern 

 representatives. They prove, however, considerable oscillations 

 of climate. Gardner, Nathorst and Reid have shown this in 

 Europe, and that it extends from the almost tropical flora of 

 the lower Eocene to the Arctic flora of the Pleistocene. In 

 America, owing, as Grey has suggested, to its great north and 

 south extension, the changes were more regular and gradual. 

 In the warmer periods of the Cretaceous, the flora as far north 

 as 55° was similar to that of Georgia and Northern Florida at 

 the present day, while in the cooler period of the Laramie 

 (Lower Eocene, or more probably Paleocene) it was not un- 

 like that of the Middle States. In the Pleistocene, the flora 

 indicates a boreal temperature in the Glacial age. Thus there 

 are no very extreme contrasts, but the evident fact of a warm 

 temperate or sub-tropical climate extending very far north at 

 the same times when Greenland had a temperate climate. As 

 I have elsewhere shown, ^ discoveries in various parts of North 

 America are beginning to indicate the precise geographical 

 conditions accompanying the warmer and colder climates. 



It would be wrong to leave this subject without noticing 

 that remarkable feature in the southward movement of the 

 later floras, to which I beUeve Prof. Gray was the first to 

 direct attention. In those periods when a warm climate pre- 

 vailed in the Arctic regions, the temperate flora must have been, 

 like the modern Arctic flora, circumpolar. When obliged to 

 migrate to the south, it had to follow the lines of the con- 

 tinents, and so to divide into separate belts. Three of these 

 at present are the floras of Western Europe, Eastern Asia, 

 and Eastern America, all of which have many representative 

 * Trans. Royal Society of Canada, iSgo-i. 



