520 A NATURALIST IN THE PACIFIC chap. 



ing agencies. The area of active dispersion, as illustrated by the 

 non-endemic genera, at first comprised the whole of the tropical 

 Pacific. It was afterwards restricted to the South Pacific, and 

 finally to the Western Pacific only. The birds that carried seeds 

 all over this ocean became more and more restricted in their 

 ranges, probably on account of increasing diversity of climatic 

 conditions. The plants of necessity responded to the ever narrow- 

 ing conditions of bird-life in this ocean, and the differentiation of 

 the plant and the bird have taken place together. 



During the stages of decreasing activity in the dispersing 

 agencies, the widely-ranging highly variable species continued to 

 be an important factor in the development of new species in the 

 different groups. The role of the polymorphous species has always 

 been a conspicuous one in the Pacific. 



Yet, as in the case of the Cyrtandras, it is shown that the 

 display of great formative power within a genus is not a peculiarity 

 of an insular flora ; that the isolation of an oceanic archipelago 

 does not exclusively induce " endemism," but only intensifies it ; 

 that the development of new species may be nearly as active on a 

 mountain in a continent as on an island in mid-ocean ; and that 

 this is equally true of a land genus, like Embelia, exposed to an 

 infinite variety of conditions, and of an aquatic genus, like Naias, 

 where the conditions of existence are relatively uniform all the 

 world over. 



In framing a scheme by which the eras of the floral history of 

 the Pacific are brought into correlation with those of geological 

 time, the age of the Coniferae is placed in the Secondary period, 

 that of the Compositae and Lobeliaceas in the Tertiary period, 

 whilst the era of Malayan immigration is regarded as mainly post- 

 glacial. The age of the Coniferae is concerned only with the 

 Western Pacific, since the Hawaiian and Tahitian islands had not 

 then been formed. The age of the Compositae and Lobeliaceae is 

 concerned only with Hawaii and Tahiti, since the islands of the 

 Western Pacific were then more or less submerged. That of the 

 Malayan plants affects the whole Pacific as at present displayed 

 to us. 



In the chapter on the viviparous mangroves of Fiji it is shown 

 that both the Asiatic and the American species of Rhizophora 

 (R. mucronata and R. mangle) exist in that group, and that there 

 is in addition a seedless form, the Selala, which, although inter- 

 mediate in character between the two other species, comes nearest 

 to the Asiatic plant. Reasons are given for the belief that the 



