GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 539 



of herbaceous plants, like the little Linnsea, the Iceland-poppy, 

 and other arctic and sub-arctic flowers. 



Some of these northern plants occur also on the summits of widely 

 separated mountains, where they were probably left stranded at the 

 close of the glacial epoch. 



With the great differences in climate which now prevail in the 

 different climatic zones, the uniform flora of the Tertiary has become 

 extraordinarily varied, and the Angiosperms have shown themselves 

 especially well fitted to modify their structure in accordance with 

 the demands of their new environment. The Fteridophytes and 

 Gymnosperms of the earlier geological formations have given way 

 more and more to the aggressive and adaptable Angiosperms ; and 

 it is only in exceptionally favored localities, such as the mountains of 

 the Pacific slope and the moist mountains of the Tropics, that they 

 can now compete successfully with their angiospermous rivals. 



Aquatics. The conditions in water have remained more uniform 

 than those on land, and, as might be expected, the aquatic flora of 

 the earth is much more uniform than that of the land. Most 

 genera and many species of fresh-water Algae are cosmopolitan, and 

 many genera of aquatic Spermatophytes, like Nymphaea, Potamo- 

 geton, Naias, etc., are very widely distributed. 



Terrestrial Plants. With the terrestrial plants it is different, and 

 the remote regions of the earth are characterized by their own types, 

 which often have little affinity with those of other regions, this 

 divergence becoming most marked as the equatorial regions are 

 approached. 



The Subpolar Zone As we have already seen, the flora of the 



high northern latitudes is much the same everywhere. The climatic 

 conditions are very similar throughout, and the survivors of the 

 preglacial flora which occupy it are much the same throughout its 

 whole extent, the differences being most marked along the southern 

 limits of the zone, where there is a mingling of southern forms. 



The few trees which characterize this northern zone, Birches, 

 Willows, Poplars, Firs, etc., are either identical throughout the 

 whole subpolar zone or are closely related species. At the extreme 

 limits of vegetation toward the pole all trees disappear. In North- 

 ern Europe, in Alaska, and in Canada, the peat bogs show many of 

 the same plants Sundews, Cranberries, Crowberries, and other 

 Heathlike plants, Cotton-grass and Orchids. In the woods we meet 

 the exquisite little Twin-flower (Linnsea) and the fragrant Pyrola, 

 and on the rocks the same Harebell in Scotland and in Canada. 



North Temperate Zone. Going southward the uniformity of the 

 vegetation becomes less marked in the Old World and the New. 

 Thus while the Northern United States and Southern Canada have 

 many plants similar to those of Europe and northern A sia, the num- 



