Tahitian Genera found in Fiji to the Exclusion of Hawaii.
Excluding the orchids, sedges, and grasses, as well as the few endemic genera, between sixty and seventy genera, or rather less than half of the genera of the flowering-plants of Tahiti, are found in Fiji to the exclusion of Hawaii. Of these, rather over a half are Old World genera; about a third occur in both the Old and the New World; four are confined to Polynesia, and not one is exclusively American. One-third are genera now possessing in the Tahitian region endemic species either entirely or in part, and in such cases we may consider that the agencies of dispersal are now inactive or partially suspended; the others belong entirely to the present era of dispersal. About half have more or less fleshy fruits fitted for dispersal by frugivorous birds. About a fourth have capsular or other dry fruits that must have been also dispersed by birds preferring a drier diet. Three only possess hairy seeds or fruits suitable for being carried in a bird’s plumage, namely, Commersonia, Weinmannia, and Alstonia. There remain about a fourth of the total that are shore-plants dispersed by the currents, being in two cases (Ximenia and Kleinhovia) assisted by birds; whilst Triumfetta, another littoral genus, is probably distributed by birds alone.
There are no cases of special difficulty from the standpoint of dispersal in these sixty and odd non-endemic genera that Tahiti possesses in common with Fiji to the exclusion of Hawaii. The lack of difficulties connected with the dispersal of all these Tahitian genera is worthy of note, because there are very few difficult genera amongst the rest of the Tahitian flora. Excluding Lepinia tahitensis, which has been already referred to, there are scarcely any “impossible” plants in the Tahitian region; and even in this case, when the modes of dispersal of Lepinia come to be investigated, it is likely that much of the difficulty will disappear. Hawaii, as we have before seen, abounds with perplexing questions of this nature. When dealing with the absentee Tahitian genera, later on in this chapter, it will be shown that “size” has played a prominent determining part in the exclusion of genera from Tahiti, genera with seeds or “stones” exceeding half an inch or twelve millimetres in dimension being, as a rule, unrepresented amongst the truly indigenous plants.
My further remarks on these Tahitian genera found in Fiji but not in Hawaii will be limited to some general observations from the standpoint of dispersal. I will first discuss some of those genera that possess only peculiar species. They belong to an era of dispersal that, as far as Tahiti is concerned, is passing or has passed away. Here we have the species of each genus more or less localised in the various South Pacific archipelagoes; but, as with Meryta, Alstonia, and Loranthus, it is often apparent that, although the Tahitian region is mainly outside the zone of present dispersal, the different groups of the Western Pacific are kept in touch by the possession of species in common. This testifies to the activity of dispersal in that region after it had become suspended in Eastern Polynesia. The connection between the isolated endemic species of Eastern Polynesia and a species ranging over the Western Pacific can sometimes be shown, as in the case of Loranthus, where a species confined to the Society Islands and to the Marquesas is very closely related to L. insularum, a widely-ranging West Polynesian species that reaches eastward as far as Rarotonga.
We next have those genera like Grewia, Nelitris, Melastoma, Randia, Geniostoma, Tabernæmontana, Fagræa, Bischoffia, Macaranga, and Ficus, that possess in Polynesia one or more widely-ranging species. The agency of the polymorphous species, which I have described in the preceding chapter in connection with the general dispersal of Malayan plants over the whole of Polynesia, is evidently also active when the work of dispersal is restricted to the South Pacific. Its operation is to be distinctly traced in all the genera above named except in Fagræa and Ficus. Thus, in the genera Grewia, Melastoma, Randia, Geniostoma, and Macaranga we find a single variable species ranging over the South Pacific from Fiji to Tahiti, keeping all the groups in touch, but associated in each, as a rule, with one or more peculiar species. A yet earlier stage in the process is to be seen in those genera which, like Nelitris, Tabernæmontana, and Bischoffia, possess only a solitary species ranging over the South Pacific, varying in each group, but not usually associated with endemic species. As with Melastoma, Macaranga, and others, we can often trace the widely-ranging species of Polynesia back to its home in Malaya, and with these and other genera the connection between a species confined to a group and a variable species ranging through all the archipelagoes of the South Pacific can sometimes be detected in the affinity of their characters.
It is thus seen that one of the principal determining causes of the differentiation of species in Polynesia lies in the failure of the dispersing agencies, a widely-ranging species becoming in consequence gradually isolated in the various groups. With some genera, as with Ophiorrhiza, it is possible to show that the resulting endemic species pass into each other by intermediate forms.
My further remarks on the Tahitian genera occurring in Fiji but not in Hawaii will be devoted mainly to those with which I was most familiar from the standpoint of dispersal.
The Tiliaceous genus Grewia offers a good example of those Polynesian genera which possess in the South Pacific a single widely-ranging species associated often with endemic species in the individual groups. It is likely that a polymorphous form, including most of the Polynesian species, could be here constituted. The fruits are dryish drupes, becoming black and moist when over-ripe, and containing three or four pyrenes suitable for distribution by birds and five or six millimetres in size.
The berries of Nelitris, a genus of the Myrtaceæ, contain a few hard seeds that are well fitted for dispersal by frugivorous birds. I am inclined to follow Drake del Castillo, who considers that there is only one varying species, N. vitiensis (Gray), which is distributed over the whole of the South Pacific from the Solomon Islands to Tahiti. The tendency of this widely-ranging species to vary in different groups is indicated in the fact that some botanists have distinguished other species within these limits. It is noteworthy that N. paniculata in Indo-Malaya and N. vitiensis in the Pacific cover the whole range of the genus. It would be interesting to establish a connection between them.
Melastoma, an Old World genus of forty and more species, has one very variable species, M. denticulatum, which, as defined by Bentham, has the range of the genus from tropical Asia across the Pacific to Tahiti. This plant is associated in some groups, as in Fiji, Tonga, and Samoa, with other more or less localised species, and it affords a good example of the principle of polymorphism in species-making. The berry-like fruits contain an abundance of minute seeds, half a millimetre in size, which, when rendered adhesive by adherent pulp, might readily stick to feathers, or they might pass unharmed through the alimentary canal of a bird. It is noteworthy that amongst the plants regarded by Prof. Penzig as introduced by frugivorous birds into Krakatoa since the eruption is a species of Melastoma.
Few genera in these islands would better repay a careful study of their species with regard both to the influence of station on specific characters and to the question of “mutations” than Ophiorrhiza. I found the three Fijian species of this Rubiaceous genus so often in close association, that I cannot doubt there is some connection between them. Seemann and Gray, indeed, characterise two of them as confluent species. The island of Tahiti alone possesses five peculiar species, and it is evident that this island has been a centre of development for species of Ophiorrhiza, just as Samoa has become the birthplace of many species of the Urticaceous genus Elatostema. The minute angular seeds of Ophiorrhiza would probably be transported in a bird’s feathers or in adherent soil. As my experiments showed, they do not become adhesive when wet.
The genus Loranthus as distributed in the South Pacific has already been briefly noticed. There is a species confined to the Tahitian region, and there is another peculiar to Samoa, whilst one widely-ranging species, L. insularum, that connects these regions together, reaching east to Rarotonga, is closely related with the Tahitian species. There was no doubt originally a single polymorphous plant that ranged over the tropical South Pacific. With regard to the mode of dispersal of the seeds of this genus of parasites, I should at once refer to the systematic and careful observations made by Mr. F. W. Keeble in Ceylon (Trans. Linn. Soc., v. 1895-1901). He formed the opinion that the seeds of Loranthus usually reach their host without passing through the alimentary canal of a bird, being merely wiped off its bill. This method would never carry the seeds to Tahiti or even to Fiji; and since this observer remarks that, although most of the seeds in the droppings were completely rotten, some of them “possibly pass through the gut uninjured,” we may accept this possibility as sufficient for the purpose of dispersal in the Pacific Ocean. Mr. Keeble notes the observation in Teil 3 of Engler’s Die Natürlichen Pflanzenfamilien that the seeds may germinate after passing through a bird’s intestine; and we may therefore infer that whilst the method he describes is typical of local dispersal, the other method is required in the instance of oceanic dispersal.
Alstonia, an Apocynaceous genus of tropical Asia and Australia, yields the caoutchouc of Fiji. Besides possessing in Fiji and Samoa peculiar species, the islands of Western Polynesia have in A. plumosa a species common to Fiji, Samoa, and New Caledonia. Another species, A. costata, is restricted to Eastern Polynesia, occurring in the different islands of the Tahitian Group as well as in Rarotonga. It is possible that the Pacific species may be connected with A. scholaris, a species possessing the range of the genus with the exception of Polynesia. The long ciliated or hairy seeds, six to nine millimetres in length, are fitted for transport by the winds and in birds’ plumage. The follicles dehisce on the tree, and, according to Horne, the light seeds are distributed locally by the wind. It is probable that the thick white juice oozing from a broken branch would at times aid the adhesion of the seeds to a bird’s feathers.
Geniostoma, a genus of the Loganiaceæ, is found in Malaya, Australia, and New Zealand. It possesses in G. rupestre a species that ranges across the South Pacific from New Caledonia to Tahiti, being associated with one or more endemic species in most of the groups. The fruit is a dehiscent capsule containing numerous small seeds imbedded in a yellowish pulp; and from the standpoint of dispersal it may be placed in the same category with Pittosporum and Gardenia (see pages [310], [313]).
The same principle involved in the occurrence of a species ranging the South Pacific from New Caledonia to Tahiti, and associated with one or more endemic species in most of the principal groups, is illustrated in the Euphorbiaceous genus Macaranga. It is specially noteworthy that M. tanarius, which ranges from India to East Australia and the New Hebrides, comes in touch in the group just named with M. harveyana, the widely-ranging plant of the South Pacific above alluded to, and itself an Asiatic species (see Burkill; Bot. Chall. Exped., iii. 191; Index Kewensis). The connection between M. harveyana, the widely-ranging species of the South Pacific, and the endemic species in the various groups is indicated by its affinity with M. reineckei, a Samoan species. The Macarangas in Fiji grow in a variety of situations, on the borders of estuaries, in the mountain forests, and on the isolated mountain peaks. It is to birds that we must look for the dispersal of the genus. In the case of a species, apparently M. seemanni, common in the Rewa delta, the seeds, which soon fall out of the cocci, are not infrequently found in the drift of the estuary, but they sink in a week or two. Other species examined showed no capacity for dispersal by currents. The fruit of M. harveyana is provided with a few prickles, but since it breaks up into the cocci, from which the seeds soon fall out, these appendages could scarcely aid its dispersal.
Like many other genera, Tabernæmontana, an Apocynaceous genus distributed through the tropics, is represented in Polynesia by a widely-ranging species, T. orientalis, which extends from Malaya and Eastern Australia through all the large groups of the South Pacific from the New Hebrides to Tahiti, and is associated in Fiji with one or two peculiar species, one of which, according to Mr. Burkill, is nearly related to it. This genus therefore seems to illustrate the earliest stage in the Pacific of that process by which a widely-ranging species takes on a polymorphous habit and through its variations gives rise to different species in various groups. Prof. Schimper ranks T. orientalis amongst the Malayan strand-flora; but in Fiji the Tabernæmontanas are only littoral where the soil is rich as in alluvial regions; and they have no capacity for dispersal by currents that is worth speaking of, the seeds in the case of T. orientalis and another species sinking after drying for years, whilst the follicles soon open in water and go to the bottom in a few days. The observations of Gaudichaud and Moseley indicate that some Malayan species are dispersed locally by the currents (Bot. Chall. Exped., iii, 279, 293); but the fruits of the genus are evidently quite unfit for oceanic dispersal by this agency. We find in the bird the agent that has carried the genus to the distant island-groups of the Pacific; and from the standpoint of dispersal the fruits may be placed with those of Pittosporum and Gardenia, being follicular, and in the Fijian plants possessing seeds, 5 to 10 millimetres in size, embedded in a pulp.
Fagræa, an Asiatic and Malayan genus of the Loganiaceæ, is represented in the Pacific by F. berteriana ranging through all the groups and islands of the South Pacific from the Solomon Islands and New Caledonia to Tahiti and the Marquesas, and by one or two other species in Fiji. It is with Fagræa berteriana that we are entirely concerned. The tree is often planted by the Pacific islanders near their villages; and since they value its timber and use its large fragrant flowers for personal decoration and for other purposes, it is probable that they have aided in its dispersal. But, as shown below, it behaves in most localities as an indigenous plant; and its berries are well fitted for promoting its dispersal by frugivorous birds.
I was familiar with Fagræa berteriana both in the Solomon Islands and in Fiji; and in the last-named locality I especially studied it from the standpoint of dispersal. All over the South Pacific, whether in the Solomon Islands, in Fiji, in Rarotonga, or in Tahiti, this tree, though thriving also in the lower levels, especially frequents rocky scantily vegetated or open-wooded hill-tops and crests up to 2,000 or 2,500 feet above the sea. In the rich alluvial soil of the Rewa delta in Fiji it attains a height of 25 or 30 feet or more, whilst in the poor, dry soil of the “talasinga” plains in this group it is much dwarfed, and often does not exceed 10 feet, and may be only 6 feet high. It is in these “talasinga,” or “sun-burnt,” plains of Fiji, especially in the Mbua province of Vanua Levu, that the tree, although dwarfed, seems most at home. Here it flowers and fruits abundantly whilst associated with Acacia, Casuarina, and Pandanus trees, and it is in such dry localities that this tree reflects in its choice of station the behaviour of different species of the genus in the Malay Peninsula, where they grow in open heath-country and sometimes on sandy heaths (Ridley in Trans. Linn. Soc. Bot., iii, 1888-94). The fruits and seeds of F. berteriana have little or no capacity for dispersal by currents. On the Fijian plains the berries partially wither and rot on the tree. In the western part of its area this tree almost comes in touch with the Asiatic species, F. obovata, that ranges from India and Ceylon to the Malayan region, a species that must be indebted to frugivorous birds for its wide distribution.
The Euphorbiaceous genus Bischoffia seems to offer another example of polymorphism in a wide-ranging species. Following Drake del Castillo, I take the genus as including only a single species, B. Javanica, a tree distributed over tropical Asia, Malaya, and Polynesia as far east as Tahiti. The variable character of the species is indicated by the different views held by the several botanists who have discussed the South Pacific species. Whilst it is a common forest-tree in Indo-Malaya, it affects in the Pacific islands the open-wooded districts of the lower levels, and it is not uncommon on the dry “talasinga” plains of Fiji. The fruits and seeds displayed in my experiments little or no capacity for dispersal by currents; nor do these dryish berries, with seeds four or five millimetres long, seem to be especially attractive for fruit-eating birds; and it is likely that the same birds that distribute Macaranga seeds also disperse those of this genus. The tree bears the same name over the South Pacific, “koka” in Fiji and Rarotonga, and “oa” in Samoa. Like many other Polynesian trees, it has its uses, but there is no reason to believe that the natives have aided materially in its dispersal.
Ficus, a large genus comprising several hundred species, attains its greatest development in tropical Asia and in Malaya. It is well represented in the Western Pacific from the Solomon Islands to Fiji and Samoa; but in Eastern Polynesia the species are very few, and the genus is altogether absent from Hawaii, although a species has been found in the North Pacific in Fanning Island, about 900 miles south of the Hawaiian group (see page [377]).
The Polynesian species are for the most part restricted to the Pacific islands, but there are only two species that range over the South Pacific as far east as Tahiti, namely, Ficus prolixa, the Tahitian banyan, and F. tinctoria. Some species are confined to Western Polynesia, such as F. obliqua, the Fijian banyan, F. scabra, and F. aspera, the last occurring in East Australia. Among the individual groups Fiji possesses probably fourteen or fifteen species, of which, perhaps, a third would be peculiar. According to Dr. Warburg, as cited in Dr. Reinecke’s paper, Samoa owns eight species, of which six may be endemic. In Rarotonga and Tahiti we find only F. prolixa and F. tinctoria. The species in the groups where they are best represented belong to three or four sections of the genus.
The banyans of the South Pacific are represented by three or four species, namely, Ficus prolixa, the Tahitian banyan, found all over the tropical groups of the South Pacific from the New Hebrides and New Caledonia to Tahiti, the Marquesas and Pitcairn Island (Maiden); F. obliqua, the Fijian banyan, confined to the islands of the Western Pacific from the New Hebrides to Tonga; and two new banyans in Samoa, as described by Dr. Warburg in Dr. Reinecke’s paper. In my paper on Polynesian plant-names it is shown that the banyans possess two names in the Pacific, one being “aoa,” the Polynesian name, found in all the groups from Samoa eastward, and connected linguistically with the Malayan and Malagasy banyan-words; the other, the Melanesian name typified in the Fijian “mbaka,” and represented in a variety of forms in the New Hebrides and neighbouring groups.
It is probable that the Pacific islanders have assisted in the dispersal of one or two of the species of Ficus, such as F. tinctoria, which they employ for different purposes, but, generally speaking, birds are active agents in distributing the genus. I need scarcely say that the agency of the currents is quite insufficient to explain the distribution of Ficus. When in Fiji I experimented on three or four different species of Ficus belonging to the sections of the genus there represented. The fruits may float at first, but within a week or ten days they break down, and the seeds escape and sink. Beneath a tree of F. scabra growing on the banks of the Wai Tonga in Viti Levu, I noticed a number of its fruits floating in a sodden condition among the reeds at the river-side.
It is with the banyans that the dispersal of the seeds by frugivorous birds becomes most evident. This is at once indicated by the frequent occurrence of these trees in the interior of coral islets in the Western Pacific, as in Fiji and in the Solomon Islands. Fruit-pigeons roost in their branches, and birds shot on these islets often contain the fruits in their crops (Bot. Chall. Exped., iv, 310). The process may also be seen in operation in Krakatoa. Professor Penzig found in 1897 that three species of Ficus had established themselves there since the eruption of 1883 through the agency of frugivorous birds. Besides pigeons, we find that parrots, hornbills, honey-eaters, &c., feed on these fruits, and I possess a large number of references to this subject. The Messrs. Layard in New Caledonia, Dr. Meyer in Celebes, Mr. Everett in Borneo, Dr. Forbes in Sumatra, and several other contributors to Ibis might be here mentioned. Dr. Beccari, in his Wanderings in the Great Forests of Borneo, speaks of “the facile dissemination of the various species of Ficus through the agency of birds,” and he arrives at certain important conclusions which are discussed in [Chapter XXXIII.]
I have before alluded to the absence of Ficus from Hawaii. This group possesses the Honey-Eaters (Meliphagidæ), birds well suited for dispersing species of Ficus over Polynesia; but this family of birds is only represented by peculiar genera in Hawaii, and therein lies the explanation. At the time when the Honey-Eaters roamed over Polynesia, the genus Ficus had not arrived from Malaya. The connection between the bird and the plant is well shown on Fernando Noronha, which possesses a peculiar species of Ficus and a peculiar species of dove, the only fruit-eating bird in the island (Ridley).