NO. 2 GENTRY : LAND PLANTS 85 



than an archipelago during a considerable portion of the Upper Ter- 

 tiary. The evolution of the biota, the distributions and speciations, are 

 linked with the physiography. 



It was inevitable that the evolution of the fauna and flora developed 

 synchronously with the radical physiographic stages, but even though we 

 have both spacial and temporal yardsticks, we have yet to measure these 

 events in terms of plant development and to make specific correlations 

 between organic and inorganic evolution. Plant populations were re- 

 peatedly restricted or provided with new areas and divergent habitats. 

 This effected lines of descent with swamping, with infrequent crossing 

 opportunities, with new placements for variants and chance natural selec- 

 tions, and also with entire eliminations. However, it appears that plant 

 evolution can be evaluated in accordance with rates of divergence in 

 many groups, in so far as isolation has fostered it, or as migration has 

 interrupted it, and as speciation has expressed it. 



The California Gulf Region was split by the sea invasion, creating 

 disjunct populations on the peninsula, the mainland, and the islands. 

 Specific divergence due to isolation should be greatest about the mouth 

 of the gulf because the sea invaded that end first and the distances sepa- 

 rating mainland and peninsula populations are greatest. The Cape Dis- 

 trict appears to have been isolated, except for its Quaternary union 

 with the peninsula, since early Tertiary times and its high ratio of en- 

 demism is to be expected and correlates nicely with a tempo-spacial 

 yardstick. Endemism in the disjunct segments of the upper part of the 

 gulf dates generally from the Upper Miocene or Lower Pliocene. Cor- 

 respondingly speciation is not so clearly developed and we encounter 

 difficulties in separating the entities to our taxonomic satisfaction. The 

 Pleistocene disjuncts are even less mature and here we often engage the 

 aggravating problem of choosing between species, subspecies, or varieties. 



Nevertheless we have to deal with them, since they represent stages 

 in a natural rate of evolution. Were it not for the irrepressible tendency 

 of life to vary, the units of life would be fixed and we would be denied 

 many fascinating problems. By a study of the events that demark the 

 periods of California Gulf history in relation to the evolving organisms, 

 the origins of our desert flora should be less obscure. 

 The Postinsular Localities 



The Cape District, consisting primarily of a granitic batholithic 

 block and volcanic intrusives, was not until recently a continuous part 

 of the peninsula, as has been stated above. Westward and southward 

 of La Paz are a series of fossiliferous beach deposits underlaid by marine 



