The Age of the Endemic Genera of Flowering Plants.
We are now entering an era distinguished from the preceding age of the endemic genera, the age chiefly of the Compositæ and Lobeliaceæ, by the fact that the extreme isolation that followed that era no longer prevails. In a sense these island-floras are in touch again with the world around, though the main stream of plant-migration now comes from the south and from the west. Yet in a large number of cases, the amount varying greatly in the different groups, it is evident that this stream has not flowed continuously to the present day. The agencies of dispersal are often no longer active; but the period of inactivity has not been sufficiently prolonged to produce generic distinction, and the differentiating energy has been restricted to the development of new species.
Yet within these limits the development of new forms, as indicated in Table B on p. [233], has often been very great. Thus, nearly half the Hawaiian genera that are non-endemic are composed entirely of species not found outside the group; and in this sense they may be regarded as cut off from the regions around. In Fiji and Tahiti only about a fourth are in this manner isolated, the agencies of dispersal being still effective with the majority of the genera. It is apparent, therefore, that the same question concerning the cause of the failure of the means of dispersal presents itself in this era as in the last, and most markedly in the instance of Hawaii.
The simplest and quickest plan for bringing into relief the prominent features of this age is first to regard the genera from the standpoint of the elevation of their stations. We have before remarked that in the occurrence of extensive regions of great altitude the Hawaiian Islands differ conspicuously from the groups of Tahiti and Fiji (and I may add Samoa); and that they present conditions for the development of a temperate mountain-flora that are not found at all in Fiji and are barely represented in Tahiti. That the Hawaiian flora responds to this contrast between the elevations of the three groups is well established; and I will now proceed to refer more in detail to the subject.