12 OUTLINE OF PLANT GEOGRAPHY 



where they formerly lived — as their retreat was cut off by the 

 mountain ranges to the south. 



It is clear that the climate of the middle Tertiary was decidedly 

 warmer than at present in the northern hemisphere, as most of 

 the characteristic trees flourished far north of their present range, 

 and there was a practically uniform circumpolar vegetation, as 

 there is at present, but made up of elements requiring a much 

 warmer climate than suffices for present arctic and sub-arctic 

 vegetation. 



Within the United States there seems to have been a sorting 

 out of the Tertiary types at the time of the glacial advance. It 

 is likely that at this time the climatic conditions in the northern 

 states were not very different from those now prevailing, i. e., the 

 Pacific Slope was a region of dry summers and mild wet winters; 

 the Atlantic of hot humid summers and severe winters. Between 

 the two regions lay the semi-arid plains between the Mississippi 

 and the Rocky Mountains, a region unsuited, for the most part, 

 for tree growth. As the vegetation retreated southward, before 

 the advancing glaciers, this central area acted as a barrier to 

 further advance, and formed a wedge on either side of which there 

 was a migration toward the more favorable coastal regions. In 

 this migration the species adapted to the dryer summer conditions 

 of the West Coast have survived in the Pacific region, while 

 those requiring more humidity now constitute the bulk of the 

 Atlantic forest. Thus at present we find a preponderance of ever- 

 green trees, especially conifers in the Pacific forests, while the 

 Atlantic forest is largely composed of deciduous species. 



As the glaciers retreated northward the arctic and sub-arctic 

 plants followed closely and established themselves in their present 

 circumpolar area. This area, once occupied by the warm temperate 

 Tertiary flora, is now much too cold to support most of these, many 

 of which, however, survive in the warm temperate regions of 

 eastern Asia and North America. 



In some cases, arctic species, instead of following the receding 

 ice northward, attained the same climatic conditions by ascending 

 the mountains until they reached an elevation where the tem- 

 perature was favorable to their growth, and as the lowland climate 

 became warmer, they moved to still higher elevations, so that 

 these arctic refugees are now found stranded on the summits of 



