194 INTRODUCTION TO EVOLUTION 



the present. The intervening time was divided approximately as follows 

 (Kulp, 1961): 



Pleistocene and Recent 1 million 



Pliocene 12 million 



Miocene 12 million 



Oligocene 1 1 million 



Eocene 22 million 



Paleocene 5 million 



While these estimates are based on the most accurate determinations 

 available, later research and further refinements in methods of dating rocks 

 will undoubtedly necessitate their revision. But they probably are of the 

 correct "order of magnitude." 



As indicated in Table 7.1 (p. 137), the first five periods are frequently 

 grouped together into a unit of the time scale called the "Tertiary," the 

 Pleistocene and Recent being relegated to a comparable unit called the 

 "Quaternary." This practice is being gradually abandoned, but the terms 

 are still widely used, especially "Tertiary," a convenient collective title for 

 designating all of the Cenozoic preceding the ice ages (Pleistocene). 



Climatic Changes 



During the first two periods of the Cenozoic the climate of much of 

 North America was mild. Much of the interior of the continent was a flat 

 lowland enjoying a subtropical climate resembling that of Florida. Palm 

 trees grew as far north as Minnesota and the Dakotas; crocodiles throve 

 in these same regions. Figs and magnolias grew in Alaska. Temperate 

 climates extended as far north as Greenland, where such trees as giant 

 redwoods, beeches, and elms were found. 



With the beginning of the Oligocene the climate slowly became cooler, 

 particularly in the interior of the continent. Palms and large crocodiles 

 disappeared from northern regions, though small alligators hved in Ne- 

 braska as late as Miocene times, along with plants similar to those found 

 at present in our Gulf states. Local arid regions began to appear on 

 the leeward side of newly elevated mountains. The moisture carried by 

 westerly winds was condensed and precipitated as rain on the windward, 

 western slopes of the mountains, as is the case today. The process culmi- 

 nated with the great regional uplift of the Cordilleran ranges in Pliocene 

 and Pleistocene times. The widespread aridity of western North America 

 followed that geologic occurrence. 



The glaciation occurring in the Pleistocene has already been mentioned. 



