CHAPTER XXIII
THE ERA OF THE NON-ENDEMIC GENERA OF FLOWERING PLANTS
The Mountain-Floras of the Pacific Islands as illustrated by the Non-endemic Genera
The mountain-flora of Hawaii.—A third of it derived from high southern latitudes.—An American element.—Compared with Tahiti and Fiji.—Capacities for dispersal of the genera possessing only endemic species.—Acæna, Lagenophora, Plantago, Artemisia, Silene, Vaccinium, &c.—Capacities for dispersal of the genera possessing non-endemic species.—Cyathodes, Santalum, Carex, Rhynchospora.—Fragaria chilensis, Drosera longifolia, Nertera depressa, Luzula campestris.—Summary.
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 The agencies of dispersal are often no longer active; but the period of inactivity has not been sufficiently prolonged to produce generic distinction, and the differentiating energy has been restricted to the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.
The Mountain-Floras of the Pacific Islands.
In the Hawaiian Islands there are at least 37 or 38 genera, making up about 19 or 20 per cent. of those belonging to this era, that may be designated mountain genera, nearly all of them being characterised as appertaining exclusively or in the main to temperate regions, or as frequenting mountain-tops in tropical latitudes. In Tahiti there are only 7 or 8 of such genera, about 4 per cent. of the total for the era. In Fiji, excluding the Conifers, there are only 4 or 5, or not 2 per cent. of the whole. In Samoa, which may be included in the Fijian area, there are 3, or about 2 per cent. of the total. These are results which we might have expected from the varying altitudes of these groups, as described in [Chapter XIX.]
Few things give more pleasure to the botanist than his recognition in some remote locality of plants long familiar to him in other regions. This will often be his lot on the mountain summits of Hawaii. If he has been a mountain-climber in many countries, he will there notice again the genera Artemisia, Geranium, Plantago, Ranunculus, Rubus, Sanicula, Vaccinium, and others that he has met perhaps either in the Rocky Mountains or in the Andes or in Equatorial Africa or in the Himalayas. If fresh from Chile he will find on these heights the familiar Gunnera and the Chilian Strawberry (Fragaria chilensis). If he has been in New Zealand and in the islands of the Southern Ocean he will find old friends in the genera Acæna and Coprosma. He may handle once again plants like Nertera depressa, that he gathered on Tristan da Cunha; and on the boggy summits of some of the mountains he will find the ubiquitous Sun-dew (Drosera longifolia).
Within the limited area occupied by the peaks of Tahiti he will find genera like Astelia and Coprosma that are at home in New Zealand or in Antarctic America, and may even find, as in the cases of Coriaria ruscifolia and Nertera depressa, the identical species that are at home in those distant regions. Even on the summit of Rarotonga he will gather a species of Vaccinium. In Fiji, here and there on some isolated mountain-top he may come upon a remnant of this Antarctic flora, such as a solitary species of Coprosma or Lagenophora, that will carry him back for a moment to high southern latitudes; and in the highlands of Savaii, in the neighbouring Samoan Group, he will find again Nertera depressa and a species of Vaccinium. But that which will interest him most in Fiji will be the tall conifers of the genera Dammara, Podocarpus, and Dacrydium, which will bring to him memories perhaps of New Zealand and southern Chile, of South Africa, and of the mountain-woods of Java and of Southern Japan.