PHYTOGEOGRAPHY OF MANOA VALLEY, HAWAIIAN ISLANDS 603 
high humidity at relatively low temperatures places the vegetation 
of a montane rain-forest under conditions which are so unfavorable ^ 
as to be comparable with the conditions of many extremely arid regions. 
The collective physiological activities of the rain-forest are continuous 
but slow; those of arid regions are rapid, but confined to very brief 
periods. In the regions of the earth which present intermediate con- 
ditions between those of the desert and the reeking montane rain- 
forest may be sought the optimum conditions for the operation of all 
essential plant processes. It is indeed, in such intermediate regions — 
tropical lowlands and moist temperate regions — that the most luxuri- 
ant vegetation of the earth may be found, and it is also in such regions 
that the maximum origination of new plant structures and new species 
has taken place." 
From the standpoint of conditions in the Hawaiian Islands, the 
closing words of the above quotation are of particular significance. 
Evidence is accumulating which indicates the former elevation of these 
islands far above their present levels. There undoubtedly has been 
a period of prolonged subsidence, amounting perhaps to several 
thousands of feet. The very rich endemic flora that today occupies 
the Manoa rain-forest very likely did not originate there, but rather 
upon warm lowlands that are now submersed beneath the ocean. 
In other words, Hawaii's remarkable endemic flora evolved upon 
prehistoric lowlands, and through slow subsidence of the land has been 
slowly crowded up the mountain slopes, into zones distinctly unfavor- 
able for plant evolution. This hypothesis is also applicable to the 
various groups of animals — birds, snails, and insects, that today 
occupy the upper levels. 
College of Hawaii, Honolulu 
