[Chap. LI I PLANTS OF THE PAST 733 



Central and South America, the tip of Florida, and the islands of the 

 Caribbean Sea. 



From the Miocene through the Pliocene and the glaciations of the 

 Pleistocene the outstanding changes in the vegetation have been the 

 extinction of manv species and numerous genera, and the reduction of 

 some genera to single species which survived in but one or only a few 

 comparatively small regions. Much of this extinction of species and 

 restriction in distribution of others had apparently taken place before 



Fig. 366. Animals of the Mississippi \'alley at the close of the Glacial epoch. 

 Restoration by C. R. Knight. Copyright by American Museum of Natural History, 

 New York. 



the end of the Pliocene ( Fig. 365 ) . The grasslands of the Central States 

 began their extension from the southwest in Eocene times as the Rockies 

 were elevated. Many other species became extinct after the glacial 

 period. This is strikingly illustrated bv the large mammals (Fig. 366). 

 The four great ice invasions from Canada into the Northern States 

 were separated by long time intervals during a period of at least a mil- 

 lion years (Fig. 367). At the beginning of the Pleistocene period the 

 continent as a whole seems to have been considerably higher and more 

 extensive than at present. The elevation of the land may have been a 

 factor in the decrease in temperature associated with the development 

 of continental ice sheets. The destruction of vegetation outside the ice- 

 covered areas was brought about partly by the small reduction (5° to 

 10° F. ) in average temperature and partly by the reversal of northward- 

 flowing streams, floods, and pondine^ where ice lobes pushed across 

 valleys. The apparent survival of preglacial species in protected coves 

 and in rock gorges just south of the ice indicates that the maximum 

 lobes were being pushed into territory where the summer temperatures 

 were not greatly below those of the present time. 



