260 



OUTLINE OF PLANT GEOGRAPHY 



Aa in all volcanic islands, ferns play an important role in the 

 vegetation of the mountain forests, and this is notably the casein 

 Hawaii. As has already been mentioned in regard to the restora- 

 tion of the vegetation of Krakatau, after the great eruption, so 

 in Hawaii, ferns are among the pioneers on the new lava dis- 

 charged from the active craters. 



Doubtless one of the important factors determining the pecu- 

 liarities of these volcanic is- 

 land floras, is the preponder- 

 ance of volcanic soils which 

 are not always suited to 

 plants from outside. Thus 

 in Hawaii it has been ob- 

 served that only the strictly 

 indigenous species seem able 

 to get a foot-hold on the new 

 lava. 



It has been very commonly 

 held that these isolated vol- 

 canic islands have always 

 been completely separated 

 from any larger body of land, 

 and that the vegetation has 

 been introduced from outside 

 since the first appearance of 

 the islands. There are very 

 serious objections to this view, 

 the greatest being the over- 

 whelming preponderance of 

 plants of Malayan and Aus- 

 tralasian affinities, although 

 very many of these are quite unfitted for natural transporta- 

 tion from these remote regions by any agency that is com- 

 prehensible. On the other hand, the number of strictly American 

 types is very small, and can mostly be explained as introduced 

 into the islands by currents, wind, or by birds, of which there are 

 many regularly migrating between the American mainland and 

 the islands. 



The palms, screw-pines, acacias, and most of the indigenous 



Fig. 74. — Tree-lobelia (Delissea longifolia) 

 Photo., Dr. J. F. Rock. 



