FLIGHT OF PLANTS AXD ANIMALS. 437 



lived in North America before the Glacial period were found 

 subsequent to the Glacial period only in the Old World, 

 while the llamas, tapirs, and gigantic Edendata are South 

 American types. The progress of events seems to Lave been 

 about as follows : In the warm period preceding the Glacial 

 epoch, when the vegetation of the temperate zone flourished 

 about the north pole, there was land connection between 

 the continents, permitting the larger species of the Old World 

 to migrate to North America. At the same time the con- 

 ditions in North America were favorable to the tropical spe- 

 cies of animals which had developed and flourished in South 

 America. The refrigeration of the climate on the approach 

 of the Glacial period, and the advance of the ice from the 

 north, cut off retreat to the Old World species, and gradually 

 hemmed them in over the southern portion of the continent, 

 where all forms of life were compelled to readjust themselves 

 to new conditions. The struggle for existence probably re- 

 sulted, first, in the extinction of those South American spe- 

 cies which had invaded North America during the warmer 

 climate of later Tertiary times ; and the more hardy emi- 

 grants from the north would have the advantage from the 

 similarity in climate in the southern United States during 

 the Glacial period to that about the poles, where they had 

 flourished immediately before. With the withdrawal of ice 

 to the north, the struggle of these animals with the condition 

 of existence began anew, and the mammoth and some others 

 found themselves unable to cope with the changes to which 

 they were compelled to adjust themselves. From the abun- 

 dance of remains of these animals found in the peat-bogs of 

 kettle-holes and in the glacial terraces of gravel and loess, it 

 is evident that they followed close upon the retreating ice- 

 front, and some of them continued the retreat to the Arctic 

 Circle, where they still live and flourish ; while others, like 

 the elephant and mastodon, perished. 



Few things are better calculated to impress the scientific 

 imagination than this dispersion and final extinction in North 

 America of so many large animals native to the Old World ; 



