382 A NATURALIST IN THE PACIFIC chap. 



species ranging over the South Pacific, varying in each group, but 

 not usually associated with endemic species. As with Melastoma, 

 Macaranga, and others, we can often trace the widely-ranging 

 species of Polynesia back to its home in Malaya, and with these 

 and other genera the connection between a species confined to 

 a group and a variable species ranging through all the archipelagoes 

 of the South Pacific can sometimes be detected in the affinity of 

 their characters. 



It is thus seen that one of the principal determining causes of 

 the differentiation of species in Polynesia lies in the failure of the dis- 

 persing agencies, a widely-ranging species becoming in consequence 

 gradually isolated in the various groups. With some genera, as 

 with Ophiorrhiza, it is possible to show that the resulting endemic 

 species pass into each other by intermediate forms. 



My further remarks on the Tahitian genera occurring in Fiji 

 but not in Hawaii will be devoted mainly to those with which I 

 was most familiar from the standpoint of dispersal. 



The Tiliaceous genus Grewia offers a good example of those 

 Polynesian genera which possess in the South Pacific a single widely- 

 ranging species associated often with endemic species in the indi- 

 vidual groups. It is likely that a polymorphous form, including 

 most of the Polynesian species, could be here constituted. The 

 fruits are dryish drupes, becoming black and moist when over-ripe, 

 and containing three or four pyrenes suitable for distribution by 

 birds and five or six millimetres in size. 



The berries of Nelitris, a genus of the Myrtaceae, contain a 

 few hard seeds that are well fitted for dispersal by frugivorous 

 birds. I am inclined to follow Drake del Castillo, who considers 

 that there is only one varying species, N. vitiensis (Gray), which is 

 distributed over the whole of the South Pacific from the Solomon 

 Islands to Tahiti. The tendency of this widely-ranging species to 

 vary in different groups is indicated in the fact that some botanists 

 have distinguished other species within these limits. It is note- 

 worthy that N. paniculata in Indo-Malaya and N. vitiensis in the 

 Pacific cover the whole range of the genus. It would be interest- 

 ing to establish a connection between them. 



Melastoma, an Old World genus of forty and more species, 

 has one very variable species, M. denticulatum, which, as defined by 

 Bentham, has the range of the genus from tropical Asia across the 

 Pacific to Tahiti. This plant is associated in some groups, as in 

 Fiji, Tonga, and Samoa, with other more or less localised species, 

 and it affords a good example of the principle of polymorphism in 



