Morinda is another critical genus in this discussion. Besides the widespread littoral species (M. citrifolia) that is distributed by the currents and is also dispersed by man, there are in the Pacific islands a number of inland species, mostly climbers and denizens of the forests. In the Index Kewensis six are accredited to Fiji and five to New Caledonia. Hillebrand gives a peculiar Hawaiian species, and there is a widespread species (M. Forsteri) that ranges over the South Pacific from New Caledonia to the Marquesas and the Paumotu Islands. Since, as indicated in [Chapter II.] and in [Note 8], the pyrenes of the fruits of the inland species are not dispersed by the currents and could readily be transported by frugivorous birds, we are not called upon to connect them in their origin with M. citrifolia, the wide-ranging species of tropical beaches.
The fact of the dispersal of certain inland species of the genus over large areas of the tropics, such as in the case of Morinda umbellata through tropical Asia and Malaya, and M. Forsteri in the Pacific, is indeed sufficient proof that these inland plants are independent of any littoral species in the Pacific and possess their own means of distribution. Though the genus, comprising at least forty species, is mainly confined to the Old World, there are a few species in America; but M. citrifolia, the familiar beach species of the Old World and the Pacific, is not indigenous there, and, as far as I can gather, all the American species belong inland. Facts of distribution of this nature negative the possibility that the Pacific islands have received their inland species of Morinda through the intervention of the far-ranging littoral plant.
As respecting Calophyllum, which is represented all over the tropical South Pacific by the wide-ranging C. inophyllum and by a tree of the inland forests found also in Malaya and in Ceylon (C. spectabile), there are, apart from questions of affinity, grave objections against the derivation of the same inland species from the coast species all over this area. The fruits of the two inland species of Fiji, C. spectabile and C. burmanni, have sappy outer coverings and are quite suited for dispersal by fruit-pigeons. As observed in [Chapter II.] and [Note 9], they have limited floating capacities and their dispersal by birds is necessary to explain their distribution. Since the timber is greatly valued by the Polynesians, it is not unlikely, however, that those islanders have assisted in the distribution of the inland species. It is not possible to do more than touch on this subject here; but it may be inferred that the history of Calophyllum in the Pacific has not been one that would warrant our regarding the inland trees as derivatives of a coast species.
There are other genera of this section where, for reasons of a different character, there is no cause for assuming that the inland species are derived from the coast species, or vice versâ. Thus, in Fiji, Casuarina equisetifolia, a widely distributed species of the Old World, occurs at the coast and in the scantily wooded plains behind; while C. nodiflora, a New Caledonian species, finds its home in the lower forests. There are many endemic species in Australia and New Caledonia; and we are not called on to connect together these two species in Fiji. In the same way we are not under any obligation in the case of the numerous inland species of Ipomœa of the Pacific islands to connect them with the coast species. They are all widely ranging species, and their seeds have been carried to the islands, each in its own fashion. So again with the inland species of Hibiscus found in the Polynesian islands and often cultivated, we cannot either from the point of view of dispersal or of affinity connect them with the far-ranging littoral species, H. tiliaceus, which belongs to a section of the genus distinct from those sections to which the inland species belong.
In a similar way there is no ground for supposing that Cordia aspera, an inland species confined to Fiji, Tonga, and Samoa, is derived from C. subcordata, the widely distributed littoral species of the Pacific and of the Old World, since they belong to different sections of the genus. But, apart from any question of affinity, the drupes of inland species of Cordia are known to be well suited for dispersal by frugivorous birds, though, unlike the littoral species above named, not adapted for transportal by the currents. The genus Vitex, which is represented by a wide-ranging littoral species in the Pacific (V. trifolia), appears to be associated with inland species only in Fiji, where one or two, seemingly endemic, occur. But there is nothing in Dr. Seemann’s description of V. vitiensis, one of these species, that at all suggests its derivation from the strand species, a very variable plant that often extends far inland into the plains, adopting a different habit of growth in those localities. It is known that Vitex fruits can be dispersed both by birds and by currents. This genus is more fully discussed in a later chapter.
Of the genus Colubrina there seem to be only two Pacific species known—one the widely distributed shore-plant, C. asiatica, a straggling shrub with alternate leaves found in all the Pacific groups and on the beaches of much of the tropics of the Old World; the other a tree, C. oppositifolia, with opposite leaves, that is peculiar to the Hawaiian islands, where it frequents the open-wooded and scrubby inland districts. The seeds of the shore-plant float unharmed for many months, whilst the fruits of the inland plant, which differ in some important respects (see [Note 52]), would float only for a week or two. The strand species is also quite at home inland in many parts of the world; and there is nothing from the standpoint of affinity to indicate that in Hawaii it has given birth to an inland species so divergent in habit and in character. There is of course the difficulty of explaining how a plant like C. oppositifolia, with such a dry, unattractive fruit, could be indebted to birds for its original introduction into the group; but the same difficulty arises with a host of Hawaiian plants. It is, however, evident from its distribution over the islands of this archipelago that it possesses or has possessed some means of inter-island dispersal, and since it is not of much service to the aborigines we must look therefore to the bird.
In the instance of the genus Tacca there is in Fiji an inland species, T. maculata, associated with a wide-ranging beach species, T. pinnatifida, which also grows inland. The first-named is recorded from the north coast of Australia and from Samoa, and though, unlike the beach plant, its seeds are unfitted for dispersal by currents (see [Chapter II.]), they might be distributed by birds. Dr. Reinecke describes another inland species from Samoa, T. samoensis. The beach plant, T. pinnatifida, grows so typically (sometimes side by side with T. maculata) in the inland plains of Fiji that one would not be justified, apart from questions of affinity, in regarding it as the parent form of inland species in the Pacific islands.
For food and other purposes Tacca pinnatifida is or was much valued by the Pacific islanders, and it grows so abundantly that cultivation is rarely practised. That the Polynesians have aided the currents in the distribution of the plant there can be no doubt, and this is particularly indicated by its occurrence in Hawaii. The genus contains ten or a dozen species, of which at least three are peculiar to America; but T. pinnatifida, the characteristic shore-plant of the Old World, and according to Schimper the only one that can be so designated, is not found in America, where, as far as I can gather, there is no widely-spread beach species dispersed by the currents from which the peculiar species could have been derived. In the case of the Pacific species, however, it should be noted that I am not endeavouring to prove the improbability of the inland species having been derived from the coast species in other regions, as in Australia, but that my point is to show there is no reason to suppose that this has taken place in the Pacific. There is no difficulty in attributing the dispersal of inland species to birds; and we are therefore not called on to connect them with the beach plants.
Section II
This division includes those genera where the littoral species has apparently given rise to one or more inland species and both still exist in the same group of islands. Two genera alone, Vigna and Premna, come into this category. The first-named seems to present a good case for the derivation of an inland from a coast species in Hawaii. Besides Vigna lutea, the beach species, which is found not only all over the Pacific islands but on the tropical beaches of the Old World, there are in Hawaii two endemic species (V. sandwicensis and V. oahuensis) that occur in the mountains, usually at elevations of from 1,500 to 5,000 feet; but I do not find any more inland species recorded from the other Polynesian archipelagoes. It may at first be noted that Vigna lutea, which in some parts of the world strays inland, displays considerable variety in its littoral station in the Pacific. Thus, in Hawaii, I found it sometimes on the sandy beach, sometimes on a rocky shore, and sometimes on the edge of old lava-cliffs overlooking the sea. In Fiji, though usually a trailer on the beach, it may become a climber hanging from the trees bordering the creeks in the mangrove-swamps. Though Hillebrand makes no mention of forms intermediate between coast and inland species in Hawaii, I found in one locality at the coast some specimens of Vigna lutea displaying the twisted pods and two callosities on the standard that are characteristic of V. sandwicensis, one of the inland species. The seeds of Vigna lutea float in sea-water unharmed for months, and they are to be found in the stranded drift of the Hawaiian and Fijian beaches, and floating in the drift of the Fijian rivers. I was unable to obtain the mature seeds of the inland species, and it has therefore yet to be determined whether they follow the rule in the loss of buoyancy. It may be added that a plant of Vigna lutea raised in Hawaii from seed displayed some small tubers of the size of a pea on its roots.