PLANTS 537 



probably contemporaneous with those of the Northern Hemisphere, 

 and Patagonia was extensively glaciated. 



The causes of the climatic changes which led up to the Glacial 

 epoch and to the later disappearance of the ice-sheets, are still 

 wrapped in mystery. Many attempts have been made to solve 

 this most difficult problem, but none is convincing or satisfactory. 



Pleistocene Life 



The frequent and extreme climatic changes, of which we find 

 such abundant evidence in the Pleistocene, caused extensive 

 migrations and dispersions of animals and plants, and the rapid 

 succession of Arctic and temperate forms. Many land bridges 

 between different continents, or between continents and what are 

 now islands, were not severed until the end of the Pleistocene, per- 

 mitting migrations which are no longer possible. The extension 

 of the ice-sheets brought with them Arctic floras and faunas, which 

 retreated in the Interglacial times, while temperate animals and 

 plants spread northward to replace them. These conditions pro- 

 duced a very severe struggle for existence and were fatal to a 

 great many large mammals, causing numerous extinctions over the 

 larger part of the world. 



Pleistocene plants are almost all of the same species as those 

 now living, but they are often very differently distributed. The 

 Glacial cold greatly impoverished the European forests, which in 

 the Pliocene had many kinds of trees now found only in North 

 America or in eastern Asia. Owing to the east and west trend of 

 the European mountains, the forests could not retire before the ice, 

 and return, as they did in, the United States, where no mountain 

 barriers shut them off from the warm latitudes of the south. 

 When the ice-sheets melted and the climate was ameliorated, the 

 Arctic flora and fauna were forced to retreat in their turn ; they 

 did so not only by following the retiring ice-front, but also by 

 ascending the mountains as the latter were cleared of ice. Thus, 

 high mountains in the Northern Hemisphere have on their upper 

 slopes an Arctic flora and fauna, separated, perhaps, by hundreds 

 of miles from the nearest similar colony. For example, the higher 



