234 THE GEOLOGICAL HISTORY OF PLANTS. 



from the soufh aud again occupied the land, though they 

 have not been able, in their decimated condition, to re- 

 store the exuberance of the flora of the earlier Tertiary. 

 In point of fact, as we shall see in the next chapter, it is 

 the floras originating within the polar circle and coming 

 down from the north that are rich and copious. Those 

 that, after periods of cold or submergence, return from 

 the south, are comparatively poor. Hence the modern 

 flora is far inferior to that of the Middle Kainozoic. In 

 America, however, and in eastern Asia, for reasons al- 

 ready stated, the return was more abundant than in 

 Europe. 



Simultaneously with the return of the old temperate 

 flora, the arctic plants that had overspread the land re- 

 treated to mountain-tops, now bared of ice and snow, and 

 back to the polar lands whence they came ; and so it hap- 

 pens that, on the White Mountains, the Alps, and the 

 Himalayas, we have insular patches of the same groups of 

 plants that exist around the pole. 



These changes need not have required a very long 

 time, for the multiplication and migration of plants are 

 very rapid, especially when aided by the agency of migra- 

 tory animals. Many parts of the land must, indeed, have 

 been stocked with plants from various sources, and by 

 agencies — as that of the sea — which might at first sight 

 seem adverse to their distribution. The British Islands, 

 for example, have no indigenous plants. Their flora 

 consists mainly of Germanic plants, which must have 

 migrated to Britain in that very late period of the Post- 

 glacial when the space now occupied by the North Sea 

 was mostly dry land. Other portions of it are Scandi- 

 navian plants, perhaps survivors of the Glacial age, or 

 carried by migratory birds ; and still another element 

 consists of Spanish plants, brought north by spring mi- 

 grants, and establishing themselves in warm and sheltered 

 spots, just as-the arctic plants do on the bleak hill-tops. 



