324 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 13 



There is a definite causal relationship between wind velocity, the 

 mechanical composition of the soil, and the evaporation rate. That is, 

 where the wind velocity is high the soil is coarse and the evaporation 

 rate high, and where the wind velocity is low the soil is finely textured 

 and the evaporation rate low. The wind appears to be the master factor 

 in the distribution and delimitation of the major plant communities. 



The pre-Pleistocene flora of the insular area probably contained 

 many plants which migrated there from the mainland during the lower 

 Miocene, when mainland connections have been postulated for the 

 ancient land of Catalinia. Thus the major affinity of the indigenous 

 island plants is with cismontane southern California. There is also a 

 remarkable relationship between the plants of the Channel Islands and 

 those of Guadalupe Island. The pre-Pleistocene flora was greatly en- 

 riched, during Pleistocene time, by migrations from the north, at a 

 time when Catalinia was again connected with the mainland. Presum- 

 ably the islands have not been connected with the mainland since that 

 epoch. The direction of the prevailing winds and the ocean currents 

 render unlikely the migration of mainland plants to the island by those 

 agencies since the separation. 



The nature of the native vegetation has been greatly modified by 

 human activities during the last four centuries, but particularly in the 

 past century when the islands were used for grazing. The overgrazing, 

 which was practiced for nearly a century, has been followed by such 

 great erosion by wind and water that, in many areas, the original climax 

 has disappeared. Much of what is now barren sand dunes, wind-eroded 

 uplands, or exotic grasslands was probably once covered by chaparral, 

 coastal scrub, or a mixed community of coarse, native grasses, forbs, 

 and low suffrutescents. Some areas, free from overgrazing for several 

 years, are now dominated by exotic grasses and are being gradually 

 invaded by plants of the mixed community mentioned. 



The presence of grazing animals on the islands since the Pleistocene, 

 prior to the occupation of the island by the white man, is doubtful. Many 

 of the weak-stemmed, herbaceous perennials and semi-shi-ubs once abun- 

 dant on the islands have been driven out of the areas which can be 

 reached by grazing animals. Most of the introduced plants are from 

 parts of the world possessing maritime or Mediterranean climates, where 

 grazing has been carried on during historic time. 



The sea bluffs occupy a considerable part of the insular area and 

 present a variety of habitats. The edaphic conditions are subject to 

 continuous change owing to the unceasing wave erosion. The gradual rise 

 in the ocean level since glacial time has brought new areas under wave 



