378 A NATURALIST IN THE PACIFIC CHAP. 



Fiji and Tahiti. In a still later period the dispersing agencies have 

 confined their operations mainly to Western Polynesia and the last 

 immigrant genera have not reached beyond the Fijian region. 



The whole story of plant-life in the tropical Pacific is bound up 

 with these successive stages of decreasing activity of the dispersing 

 agencies. The story of plant-distribution in this region is well 

 illustrated in its earlier phases of general dispersion in the floral 

 history of Hawaii, in its later phase by those Asiatic genera that 

 have only crossed the South Pacific to Tahiti, and in its last phase 

 by those genera that have never extended beyond the groups of 

 the Fijian area. The area of active dispersion, that first comprised 

 the whole of the tropical Pacific, was afterwards restricted to the 

 South Pacific, and finally to the western portion of that area. It 

 can scarcely be doubted that these successive stages in the con- 

 traction of the area of active dispersion of plants in the Pacific were 

 accompanied by a corresponding diminution in the general distribu- 

 tion of birds in the same ocean, to which it stood in the relation of 

 an effect to a cause. 



TAHITI. 



The peculiarities of the Tahitian flora as compared with Hawaii 

 and Fiji may be discussed by treating first those genera that are 

 alone represented in Tahiti, the " residual " genera ; then those that 

 it possesses in common first with Hawaii and then with Fiji ; and 

 lastly by pointing out the more noticeable gaps in the flora. By 

 Tahiti is typically signified the whole Tahitian region, which 

 includes the Austral and Cook Groups, the Society Islands, the 

 Paumotus, and the Marquesas. 



THE TAHITIAN RESIDUAL GENERA. 



The non-endemic genera occurring alone in the Tahitian region 

 and not found either in Hawaii or in one or other of the three 

 groups of the Fijian region (Fiji, Tonga, Samoa) are not more than 

 half a dozen. These six genera are exceedingly interesting ; but 

 since each tells a different story and gives its own independent 

 indication they cannot be treated in a collective sense. Nor are 

 they all to be regarded as anomalies in plant-distribution, since 

 with a single exception there is scarcely one concerning which it is 

 not in some way possible to give an explanation of its isolation 

 without coming into conflict with the principles of plant-dispersal. 

 The exception is Lepinia tahitensis, which, without presenting any 



