I INTRODUCTION 9 



peculiar species of plants were adapted for dispersal by frugivorous 

 birds. The lesson to be learned from this island concerns the 

 Compositae, often arboreous, that constitute the principal feature of 

 its flora. St. Helena retains almost more than any other island 

 evidence of the age of Compositse which has left its impress on 

 many insular floras ;~ and when we discuss the original modes 

 of dispersal of the endemic Hawaiian genera of the same order we 

 shall look to the flora of this Atlantic island for assistance in the 

 matter. To the age of Compositae belong the beginnings of 

 several insular floras. 



To return to the main line of our argument, it would seem 

 that in a Pacific island there is a constant relation between free 

 means of dispersal and the preservation of specific characters. The 

 ocean-current and the aquatic bird are in our own time actively 

 engaged in dispersing the seeds of shore-plants and water-plants^ 

 and we see the same species ranging over the world. On the 

 other hand on the mountain-top the agencies of dispersal are 

 beginning to fail, and as a result many a mountain has some of its 

 species restricted to its higher regions. In the forest zone there 

 has been a more or less complete suspension of the activity of the 

 dispersing agencies, and new genera are formed w^hilst peculiar 

 species abound. Free means of communication with other regions 

 restrains but does not arrest the differentiating process that is ever 

 in progress throughout the organic world. Isolation within certain 

 limits gives it play. 



It is in this connection interesting to reflect that during the 

 differentiation of the inland flora the littoral plants have lagged 

 behind or have remained relatively unchanged. The currents have 

 been working without a break throughout the ages ; and the 

 cosmopolitan Ipomea, that now creeps over the sand of the beach, 

 or the wide-ranging Rhizophora, that forms the mangroves of the 

 coast-swamp, must have witnessed the arrival of the ancestors of 

 several of the endemic inland genera. The swamp-plants of the 

 littoral flora are probably older, however, than the beach-plants 

 which have been recruited from time to time in one region or 

 another of the tropics from the inland flora. Yet as a body the 

 littoral plants have lagged far behind the inland flora. We might 

 thus expect that in a Pacific island, excluding the wind-distributed 

 plants, such as the ferns and the lycopods, the most ancient types 

 of the plants would be found at the coast, the most modern in the 

 forests, whilst the plants of the mountain-summit would represent 

 an intermediate age. 



