dental) which now occupy the general area from which this flora 

 is believed to have migrated northwestward. 



The Madro-Tertiary geoflora consisted largely of sclerophyl- 

 lous, small-leaved trees and shrubs similar to those that presently 

 occupy many areas of California. Even as early as the middle 

 Eocene some of the immediate ancestors of species present in 

 Oiaparral, Coastal Sage Scrub, and Valley and Foothill Woodland 

 were present in CaUfornia, although it is doubtful if these plant 

 communities would have been recognizable at that time. During 

 the middle PUocene times, the oak woodland dwindled and 

 disappeared over many areas of what later became the south- 

 western deserts, since the rainfall in these areas became sharply 

 reduced as the Sierra Nevada and the Peninsular ranges were up- 

 lifted. The increasing dryness that characterizes the climatologi- 

 cal history of the Pacific coast is one that has been developing 

 gradually, although erratically, over perhaps as many as a hund- 

 red million years. Therefore, during this time there have been 

 plants that have been gradually adapting to these altered moisture 

 conditions; those that did not adapt either became extinct or else 

 persist only in areas where the rainfall is sufficiently high to sup- 

 port them. For example, Catalina Ironwood {Lyonothamnus 

 floribundus , Rosaceae) was once present on the mainland of Cal- 

 ifornia but presumably became extinct there because of its in- 

 ability to tolerate the increasingly dry climatic conditions. As a 

 consequence, it now persists as a relictual endemic of the Chan- 

 nel Islands. Likewise, some of the pine species in the Closed-Cone 

 Pine Forest once were also more widely distributed, and these 

 are barely persisting in isolated pockets along the cool and rela- 

 tively moist Pacific coast. There is evidence that the increasing 

 aridity of the California climate has altered the composition of 

 some plant communities that we consider to be characteristic of 

 the contemporary climate. For example. Chaparral was probably 

 once more widespread than it now is and also contained a larger 

 number of species of shrubs than it now does. There were also 

 areas where the Chaparral contained some trees and also some of 

 the thorn-scrub plants that are now found only to the south of 

 the present range of Chaparral. Such genera 2iS Acacia and Fou- 

 quieria (Ocotillo) once were components of Chaparral, but now 



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