xxi THE AGE OF COMPOSITE 245 



group we may find a clue that will enable us to divest this problem 

 of some of its difficulties. 



It might be at first considered that since these peculiar genera 

 of Compositae occur in the higher levels of Hawaii and Tahiti 

 their absence from Fiji might be connected with the relatively low 

 altitude of those islands, a character that is concerned with the 

 exclusion from the Fijian flora of many Hawaiian and Tahitian 

 mountain plants (see Chapters XXIII. and XXIV.). But this view 

 is at once negatived by the fact that Fitchia thrives in Rarotonga, 

 an island which does riot far exceed 2,000 feet in elevation. It is 

 negatived also by the extensive development of shrubby and 

 arborescent Compositae in the Galapagos Islands, on the equator, 

 in St. Helena in 16 South latitude, and in other tropical islands, 

 which are less than, or do not exceed, the Fijian Islands in their 

 altitude. 



During the age of the Compositae it is reasonable to suppose 

 that the dispersal was general over the Pacific. The absence of 

 genera indicating this era from the islands of the Fijian region, 

 that is, from Fiji, Tonga, and Samoa, would become intelligible if 

 these groups were submerged during this age of the general 

 dispersal of the order over this ocean. In my volume on the 

 geology of Vanua Levu in Fiji, I have shown that these island- 

 groups of the Western Pacific emerged from the sea towards the 

 close of the Tertiary period, a conclusion that would enable us to 

 assign the age of the general dispersal of the Compositae over the 

 tropical Pacific to an earlier portion of the same period. 



In order, however, to make further progress in the discussion of 

 this difficult problem we are obliged to approach it from the out- 

 side. We must in fact regard these genera from the standpoint of 

 their position as members of the vast and ancient order of the 

 Compositae. It is now more than thirty years since Mr. Bentham 

 completed his remarkable memoir on the classification, history, 

 and geographical distribution of the Compositae (Journal Linnean 

 Society, Botany, London, Vol. 13, 1873). Like De Candolle, when 

 dealing with the facts of distribution, he handled thousands of 

 species, and as a result he drew certain inferences which are of 

 prime importance to students of plant-dispersal. In his time the 

 order included nearly 10,000 known species, and although this 

 number has since no doubt been considerably increased, it is not 

 likely that his main conclusions, in so far as they are free from 

 purely hypothetical considerations, will be materially affected by 

 the later discoveries. 



