VIII LITTORAL PLANTS AND CURRENTS OF THE PACIFIC 67 



in his memoir entitled Aves PolynesicE he remarks that certain 

 indications tend to show that the Pelew Islands have served as a 

 sort of bridge for the spread of species from I ndo-Austro- Malaya 

 right across the Pacific. Though I still think that the beach trees, 

 most of which would find a home on the numerous coral atolls of 

 the Marshall, Gilbert, and Ellice Groups, often followed that 

 track, yet I am now inclined to consider that the mangroves and 

 their associates, plants which find their most suitable home in the 

 estuaries of large elevated islands, like those of the Solomon 

 Group, in all probability reached Fiji in the mass by the 

 Melanesian route. 



Although the Old World has supplied to the Pacific islands 

 most of their littoral plants that are dispersed by the currents, 

 that is to say, the plants with buoyant seeds or seed vessels, yet 

 there is an appreciable American element, and it is with the plants 

 occurring in the New World that we are now concerned. The 

 total number of the littoral plants of these islands that possess 

 buoyant seeds or fruits is, according to the lists given under 

 Note 35, about seventy. Of these about forty-five are exclusively 

 Old World species, sixteen occur in both the Old and New 

 Worlds, three are exclusively American, and six are Polynesian. 



The question we have now to ask ourselves is whether the 

 shore plants common to both the Old World and America have 

 their homes in America, or whether they have been derived from 

 the other hemisphere. With one or two exceptions, as in the cases 

 of the Australian genera Dodonaea, Scaevola, and Cassytha, which, 

 as shown in a later page in this chapter, present no great difficulty, 

 there does not seem to be any serious objection, as far as the 

 numerical distribution of the species is concerned, in regarding 

 America as a possible home of the genus. It is not often we shall 

 come upon such a striking instance of the principle that where 

 the species are most numerous there is the home of the genus, as 

 in the instance of Cocos. The Coco-nut palm has been carried 

 around the world through the agencies of man and the currents, 

 whilst the home of the genus is in America. 



Now assuming that in having to choose between the Old World 

 and the New World as the home of most of the genera in the list 

 we selected the latter, we have to ask ourselves in what degree this 

 would be consistent with the place America holds with regard to 

 the distribution of tropical shore-plants dispersed by the currents 

 and with reference to the arrangement of the currents. If we 

 except the African continent, there is no part of the world that 



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