7. EVOLUTION OF THE CALIFORNIA FLORA 



The natural vegetation of California represents a set of plant 

 communities of varied historical origins. We have already pointed 

 out that some plant communities such as the North Coastal 

 Forest (or at least the segment of this community dominated by 

 Coast Redwood) have had a continuity back into the geological 

 past that extends as far as twenty million years, although these 

 plant communities have not always occupied their present geo- 

 graphical range. Other plant communities, such as those that are 

 adapted to the characteristic Mediterranean or desert climate of 

 California, are of relatively recent origin because the climatic 

 regime under which they exist is one of recent origin. 



The following geological time scale will be of some help in 

 visualizing the past history of the rich flora that now occupies 

 the California Floristic Province: 



EPOCH STARTED 



Recent post-glacial 



Pleistocene 1 million years ago 



Pliocene 10 million years ago 



Miocene 25 million years ago 



Oligocene 40 million years ago 



The Oligocene, Miocene, and Pliocene along with the older 

 Eocene and Paleocene epochs, constitute the Tertiary period. In 

 Oligocene and Miocene times the area occupied by California and 

 much of the rest of the western United States was a region of 

 rolling plains or low mountains. There was no major mountain 

 system in California until the close of the Tertiary period. 



The Arcto-Tertiary Geoflora 



As a consequence of topographical uniformity in the early 

 Tertiary, the climate of the western part of North America was 

 much less varied than it is at the present time. It is not surprising, 

 therefore, that much of North America, northern Asia, and 

 Europe was covered by a rather uniform type of vegetation, and 

 a rich one in terms of number of species present. In the west, 



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