538 BOTANY 



The tropical types, such as the Palms, Laurels, and Figs, disap- 

 pear gradually from the northern regions, and their place is taken 

 by the Oaks, Birches, Willows, etc., which characterize the cooler 

 zones. 



Toward the end of the Tertiary, as shown by Pliocene deposits, the 

 flora of Europe was similar to that which prevails in North America 

 to-day, the more southern regions also including a number of such 

 subtropical types as Palms and Bamboos. Among the American 

 genera living in Europe in Tertiary times, but which are now extinct, 

 were Sequoia, Sassafras, Magnolia, Liriodendron, Oreodaphne, and 

 others. These also occur in more northern regions, and extend into 

 Asia, indicating a very similar flora for the whole of the northern 

 hemisphere during the later Tertiary. 



Our knowledge of the Tertiary flora of the southern hemisphere is 

 very incomplete, but it would appear that although there were a 

 number of forms allied to those of the northern hemisphere, the 

 peculiar southern types were already differentiated. 



The great factor in the changed distribution of the plants of the 

 northern hemisphere was the rapid cooling of the northern regions 

 with the gradual oncoming of the glacial epoch. With the lowering 

 of the temperature, and the advance of the great ice-sheet, vegeta- 

 tion was driven southward. The hardier forms, like some of the 

 Willows and Birches, and certain Alpine flowers, could maintain 

 themselves up to the edge of the glaciers as they do to-day in the 

 Arctic regions, and upon lofty mountains; but the tender forms were 

 obliged to retreat to a more genial climate. 



The fate of these plants was very different in America and in 

 Europe. This was the result of the very different configurations of 

 the two continents. In the former the great mountain systems run 

 north and south, and there is direct land communication with the 

 Tropics. In Europe, which lies farther north and was subjected to 

 much more extensive glaciation than America, the mountain chains 

 of the Alps and Pyrenees extend east and west, and moreover the 

 Mediterranean lies between its southern shores and Africa. Thus 

 overtaken by the advancing glaciers, nearly all the tropical and sub- 

 tropical types which still exist in North America and Eastern Asia 

 were completely exterminated. The extraordinary number of closely 

 related plants which occur in Eastern Asia and Atlantic North 

 America are therefore simply the survivors of the once continuous 

 Tertiary flora, which have disappeared from Europe. 



With the retreat of the ice-sheet the plants advanced north again, 

 and the hardier forms which kept near the ice-sheet still form a con- 

 tinuous subpolar flora. A large number of northern plants are met 

 with in the higher latitudes of both the old and new world. Among 

 these are the White-birch, Aspen, Willow, Fir, and many species 



