24 DERIVATION OF THE FLORA OF HAWAII 



history of its flora, and we can only conjecture what may have been the 

 agencies that have caused the disappearance of these missing types, 

 assuming that they formerly existed. The most obvious explanation 

 would be the extensive volcanic activity to which all parts of the islands 

 have been subjected. Every region has at some time or other been cov- 

 ered with lava flows, and it is not unreasonable to suppose that many 

 species, especially those of local distribution, may have been destroyed 

 as the direct result of volcanic eruptions. 



An indirect result of the purely volcanic nature of the present 

 islands may have been a more gradual extinction of species which require 

 certain constituents for their growth which are not present in volcanic 

 soils. The almost complete absence of granitic or calcareous soils, for 

 example, would practically prohibit the growth of many species that 

 might very well have been present at an earlier period before the sub- 

 mergence of areas now completely covered by volcanic formations. 



Another factor may also be considered, viz., the ravages of disease. 

 Just as at present, in large areas in the eastern United States, the chest- 

 nut has been practically exterminated by the ravages of a fungus, so we 

 may imagine species may have succumbed to similar enemies, and owing 

 to the complete isolation of the islands could not reestablish themselves. 



Whether climatic changes in the course of ages may have played a 

 part in the disappearance of any species, we cannot say. It is conceiv- 

 able, however, that this factor may have played a part in the extinction 

 of some of the original members of the Hawaiian flora. 



We believe that a study of the relationships and distribution of the 

 existing Hawaiian flora leads to the following conclusions : 



1. The archipelago is the remnant of a once much larger land area 

 which was connected more or less directly with the Southern Pacific area 

 to the southwest now occupied by Australasia and the Malayan Archi- 

 pelago. As the result of extensive subsidence throughout the Polynesian 

 area, the Hawaiian Islands have been left in their present extremely iso- 

 lated condition. The volcanic masses which constitute the whole of the 

 exposed portions of the islands have been superimposed upon an exten- 

 sive non-volcanic area now completely submerged. The isolation of the 

 islands probably occurred early in the Tertiary. 



2. The most ancient members of the Hawaiian flora are those de- 

 rived from some continental area to the south, and comprise the more or 

 less modified descendants of forms still represented by many Australasian 

 and Indo-Malayan genera. 



3. The American elements in the Hawaiian flora are of two kinds : 

 First, those which have been introduced since the isolation of the islands, 

 mostly from the west coast of North America, through the agency of 



