THE FLORA OF THE COAST ISLANDS. 517 



According to Wallace, it is one of the primary divisions of 

 the Nearctic Region. 



During the late Pliocene and early Quaternary, as already 

 seen, the islands were still a part of the mainland, and the 

 whole was occupied by the same flora, viz: the distinctively 

 Californian (with some differences doubtless), now found in 

 both, together with the peculiar island-species. 



During the oscillations of the Quaternary the then west- 

 ernmost coast range was separated by subsidence, and has 

 remained ever since as islands. Simultaneously with, or 

 after, this separation, came the invasion of northern species , 

 driven southward by glacial cold. Then followed the 

 mingling of invaders with the natives, the struggle for 

 mastery, the extermination of many (viz : the peculiar island 

 species), and perhaps the slight modification of all, and the 

 final result is the California flora of to-day. But the island 

 flora was saved from this invasion by isolation, and there- 

 fore far less changed than the flora of the mainland, i. e., 

 the" invading species are mostly wanting, and many species 

 survived there which were destroyed, or else modified into 

 other species, on the mainland, and the remainder prob- 

 ably less modified than on the mainland. The flora of 

 these islands, therefore, represents somewhat nearly the 

 character of the flora of the whole country during the 

 Pliocene times. Some modification they have doubtless 

 suffered, but the time has been too short for any great change 

 in the absence of severe competition. 



The question naturally arises, " How is it that with a 

 separation of only 20 — 30 miles the two floras — insular and 

 mainland — have not become entirely similar by mutual 

 colonization ?" The prevailing winds being landward would, 

 I suppose, largely prevent the colonization of common 

 American forms on the islands, although some such coloni- 

 zation has in fact taken place. But with the prevailing- 

 winds in this direction, why have not all the peculiar island 

 species been long ago colonized on the mainland? Accord- 



