fringe of North America. The Neotropical Tertiary geoflora was 

 composed of diverse tropical or subtropical trees such as figs, 

 avocados, cinnamon, palms, and others. This geoflora has now 

 disappeared from most of North America, although it is rep- 

 resented in the region of southern Mexico southward to northern 

 South America. This geoflora is now restricted to tropical areas 

 with a high rainfall. Its disappearance from much of its former 

 area was due to the cooler, drier climates that have prevailed 

 over much of North America since Eocene times. There are few 

 Uving remnants of this rich flora left in California, although some 

 genera managed to survive here into the Pliocene. 



The description given above of the former and present ranges 

 of the Arcto-Tertiary and Neotropical Tertiary geofloras and 

 their response to climatic changes suggests that there is a large 

 area of the southwestern portion of North America that is gen- 

 erally unfavorable for occupancy by the modern descendants of 

 either one of these geofloras. Yet this southwestern area is now 

 occupied by a rich flora. What is the origin of these plants? 



The Madro-Tertiary Geoflora 



According to D. I. Axelrod of the Department of Botany, 

 University of California, Davis, in middle Miocene times the 

 countryside south of San Francisco was occupied largely by an 

 oak woodland flora that had migrated into this area from the 

 Sierra Madre Occidental region in northwestern Mexico. The 

 plants of this oak woodland were adapted to a year-round rain- 

 fall but to a warmer, somewhat drier climate than were the plants 

 of the Arcto-Tertiary forest to the north of it. Presumably this 

 oak woodland occupied some of the area that formerly had been 

 occupied by the Neotropical Tertiary geoflora, which in turn had 

 become gradually eliminated from the American southwest due 

 to climatic changes. It is probable that some species of this oak 

 woodland evolved directly from subtropical precursors in the 

 Neotropical Tertiary geoflora. This oak woodland has been termed 

 by Axelrod the Madro-Tertiary geoflora, the "Madro-" coming 

 from the name of the Mexican mountains (Sierra Madre Occi- 



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