a world-wide sheet of ice, to be as slowly driven 

 back by waves of tropic heat. 



In this double migration, owing to the configura- 

 tion of continents and mountain ranges, most of 

 the plants were destroyed, only a few vestiges of 

 the post-glacial families being extant, to-day, gath- 

 ered here and there upon the plains or stranded 

 upon the mountains. 



The means and manner of this destruction are 

 most interesting. The continents of both the old 

 and new world are greatly expanded at the north, 

 while the southern portions are attenuated to nar- 

 row peninsulas. 



These configurations give to the North Temper- 

 ate Zone its greatly dominant character, having 

 most of the existing families, while the peninsulas 

 are sparsely furnished. 



The Eurasian mountain ranges are mostly trans- 

 verse, like the Alps, Himalayas, and Thian-Chan 

 Mountains, forming barriers to the progress of 

 plants; while North- American ranges are nearly 

 longitudinal, permitting the plants to escape 

 southward, during a Glacial Age, and return 

 during a Thermal one. 



There is much evidence found as fossils in the 

 rock strata, that an abundant flora of monster trees 

 once occupied the Arctic regions, similar on the 

 two continents, owing to connections then existing 

 or to nearness of extremities. 



MANNER OF THIS DESTRUCTION. 



The formation of an ice-cap at the pole and of 

 snow and ice deposits on the summits of mountains 

 lower down in latitude, drove the plants down 



(7) 



