The distribution within the archipelago of the genera and species of the early Compositæ of Hawaii is worthy of notice from the light it throws, not only on the relative antiquity of the genera, but also on the subsequent conditions of isolation. Of the nine genera here referred to five are distributed over most of the islands of the group. These include all the genera possessing a number of species, namely, Tetramolopium with seven species, Lipochæta with eleven, Campylotheca with twelve, Dubautia with six, and Raillardia with twelve species. Of the four genera remaining all have only two species, and are restricted to two or three islands, Remya and Wilkesia being in both cases found in Kauai and Maui, whilst Argyroxiphium is confined to the adjacent islands of Maui and Hawaii, and Hesperomannia to those of Oahu, Lanai, and Maui. These four genera that are restricted to only two or three islands are the same before referred to as regarded by Hillebrand as the oldest, partly on account of their isolated generic position, and partly because in each case they only possess two species.
Although the early Hawaiian Compositæ were evidently originally transported to most of the islands of the group, it is noteworthy that their subsequent isolation from the rest of the world has in the later ages been repeated within the limits of the archipelago. Of the 56 species, all of which are now endemic, 28, or just half, as shown in the table on the following page, are confined to a single island. Of the remainder, almost all are restricted to two or three adjacent islands. Hillebrand gives only a solitary species, Lipochæta connata, as occurring in all the islands. This suspension, to a great extent, of the means of dispersal between the islands is also strikingly illustrated by the Lobeliaceæ.
We have only to mention the flora of Fiji and those of the adjacent groups of Samoa and Tonga to exclude them from any share in the early era of the Compositæ in the Pacific. The prevailing adventitious character of the Fijian Compositæ is indicated in the fact that the species of the majority of the genera are included by Seemann in his list of Fijian weeds. There are only one or two Fijian Compositæ, such as the mountain species of Lagenophora and the littoral species of Wedelia, that merit the special attention of the student of dispersal. So also with Samoa, Reinecke enumerates eight species, of which six are weeds either of aboriginal or of European introduction, the others being the littoral Wedelia above alluded to, and a species of Blumea found also in Fiji.
Distribution of the Endemic Genera of Compositæ in the Hawaiian Islands.
| Genus. | Distribution of the Species. | Total. | ||||
|---|---|---|---|---|---|---|
| One island. | Two islands. | Three islands. | Four islands. | General. | ||
| Remya | 2 | — | — | — | — | 2 |
| Tetramolopium | 1 | 4 | 2 | — | — | 7 |
| Lipochæta | 3 | 4 | 3 | — | 1 | 11 |
| Campylotheca | 5 | 4 | 3 | — | — | 12 |
| Argyroxiphium | 1 | 1 | — | — | — | 2 |
| Wilkesia | 2 | — | — | — | — | 2 |
| Dubautia | 4 | — | 2 | — | — | 6 |
| Raillardia | 9 | 1 | — | 2 | — | 12 |
| Hesperomannia | 1 | 1 | — | — | — | 2 |
| 28 | 15 | 10 | 2 | 1 | 56 | |
We have now, I venture to think, gone far to establish the existence of an early “Composite” flora with mainly American affinities in the Pacific islands, an ancient flora of which only the remnants now occur in the uplands of Hawaii, Tahiti, and Rarotonga. That the achenes were originally transported in birds’ plumage is, as we have seen, probable; but we are still quite in the dark as to the causes of the subsequent suspension of the means of dispersal and of the resulting period of isolation, during which the original immigrant plants acquired their endemic characters. In our uncertainty, therefore, we will look to Fiji in the hope that in the absence of the early Compositæ from that 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 Compositæ 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 not far exceed 2,000 feet in elevation. It is negatived also by the extensive development of shrubby and arborescent Compositæ 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 Compositæ 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 Compositæ 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 outside. We must in fact regard these genera from the standpoint of their position as members of the vast and ancient order of the Compositæ. It is now more than thirty years since Mr. Bentham completed his remarkable memoir on the classification, history, and geographical distribution of the Compositæ (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.
Accepting the antiquity of the order, and regarding it as probably dating far back in geological time, he observes that the evidence points to a very wide dispersion of its original stock at an early period. Africa, West America, and possibly Australia, possessed the order at the earliest recognisable stage. There must have existed, he contends, at this early period some means of reciprocal interchange of races between these regions. Then followed a stoppage of communication, or a suspension of the means of dispersal, between the tropical regions of the Old and New Worlds; but long after communication was broken off in the warmer regions, it still existed, as he holds, between the alpine heights in those regions and also between the high northern latitudes of both hemispheres. Referring particularly to the Hawaiian Group, he considers that the large endemic element among the Compositæ indicates that the ancient connection, whether with America or with Australasia, has been so long severed as not to have left a single unmodified common form. Fitchia, the Tahitian genus, as we have already remarked, is regarded as the only remnant of an ancient Composite flora in the tropical islands of the South Pacific.