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 ) 
