It has long been known that fruit-pigeons are fond of the fruits of wild species of Eugenia, and I found the Solomon Islanders and the Fijians well acquainted with the fact. The fruits of a tall Eugenia tree, near E. richii, common in the interior of the coral islets of Bougainville Straits in the Solomon Group, were found by me in quantities in the crops of fruit-pigeons shot by Lieut. Heming and Lieut. Leeper on the islets (Solomon Islands, pp. 293, 297; Bot. Chall. Exped., Introd. 46, iv. 312). Dr. Seemann remarks that in Fiji the red fruits of E. brackenridgei are eaten by pigeons. The somewhat thin coverings of the seeds of this genus would seem to offer but a slight protection in a bird’s stomach, though in one species the test was almost crustaceous.
Most species possessed only one or two large seeds in each fruit, though this number may vary in the same individual. Thus, out of ten fruits of Eugenia rariflora in Fiji, six had one seed, three had two seeds, and one had three seeds. In the fruit of E. neurocalyx, however, the seeds range from three to five.
It is the question of size that is of importance in considering the possibility of birds transporting the seeds over a broad tract of ocean. Eugenia rariflora, the species found all over the Pacific, has seeds that measure in the Fijian plant one-fourth to one-third of an inch (6 to 8 mm.) across; and in Hawaii, according to Hillebrand, they would perhaps be rather smaller. In point of size there is less difficulty with regard to the transport by birds across the ocean to Hawaii of the seeds of Eugenia rariflora than with the “stones” and seeds of some other genera, like Elæocarpus, Osmanthus, and Sideroxylon, that must have been conveyed there by the same agency. The fruits of several of the Fijian species are of the size of a large cherry; but it is noteworthy that in those species like E. corynocarpa and E. neurocalyx, where the fruits are large and the seeds about an inch in size, the plants are confined to the Western Pacific only, namely, to the Fiji-Samoa region.
There is therefore no difficulty, from the standpoint of size, in accounting for the distribution by birds of the widely-ranging Eugenia rariflora over Polynesia; but at first sight there seems to be a real difficulty with regard to the protective coverings of the seed. Yet Nature speaks with no hesitating voice in the matter. The West Indian and Florida species, E. monticola, regarded as indigenous in the Bermudas, must have reached that group through the agency of birds that carried its seeds over quite 800 or 900 miles of sea; and it may here be noted that South Trinidad, lying some 600 miles off the coast of Brazil, and Rodriguez, distant about 330 miles from Mauritius, each possess species (Bot. Chall. Exped., Introd., 12, i. 32, ii. 128). If fruit-pigeons can transport Eugenia seeds across 600 or 800 miles of ocean, there would be no difficulty in accounting for the stocking of the Fijian, Tongan, and Samoan Islands with the genus from regions to the west. But the occurrence of the genus in Hawaii seems to compel us to assume that the seeds have been carried in a bird’s stomach over 1,500 to 2,000 miles of ocean. This difficulty, however, does not really exist. Eugenia rariflora, the Polynesian species found in Hawaii, frequents, as before observed, coast districts and coral islets in Fiji, and if we suppose that the low islands of the Fanning and Phœnix Groups, lying between Hawaii and Samoa, have served as stepping-stones, a capacity of crossing 1,000 miles of ocean would be alone required. This is not much in excess of the distance that must have been traversed by the bird that first brought the seeds of Eugenia monticola to the Bermudas.
Other genera like Morinda and Scævola, possessing fleshy fruits dispersed by frugivorous birds, have been mentioned in different connections in other parts of this work, and will not be further dealt with here. But before concluding this chapter I will refer briefly to one of the disquieting mysteries in the flora of the Pacific which is presented to us in the genus Gossypium. Three species are, or were, truly indigenous in this region. One is Gossypium drynarioides, a small endemic tree found by Nelson, the companion of Captain Cook, in Hawaii, which was very rare in Hillebrand’s time, and is perhaps now extinct. The second is G. tomentosum (Nuttall), which is also peculiar to Hawaii, where it is found on the beaches. I am following here the Index Kewensis; but it should be remarked that this species occurs also in Fiji, though Seemann regards it as introduced. The third is G. religiosum (L.), found by Captain Cook’s botanists growing wild in Tahiti, and hailing from the tropics of the Old World. The seeds of the first species are covered with a short brownish tomentum, and could never have been of any value. The tawny wool of the seeds of the second species has a staple too short for cultivation; whilst the Tahitians do not seem to have made any use of the third species. It is difficult to draw any conclusion concerning the presence of these plants in the Pacific islands at the time of their discovery; nor can Dr. Seemann, who was especially well informed in these matters, aid us much in our endeavours to solve the mystery. From the aboriginal names we get no clue. The Hawaiian name of “huluhulu” seemingly refers to the hairy covering of the seed; whilst the Tahitian “vavai” and “ovari” simulate the Fijian “vauvau,” which is merely the reduplicated form of “vau” (the word in many shapes for Hibiscus tiliaceus in Malaya and Polynesia), and is applied by the Fijians to Hibiscus esculentus and to the introduced species of Gossypium.
When in Hawaii I ascertained that neither the seeds of the littoral plant, Gossypium tomentosum, nor those of two cultivated species possessed any fitness for dispersal by the currents, the scraped seeds sinking at once, whilst when covered with the wool they floated only for a few days. Further references to G. tomentosum in Hawaii are given in the index of this volume.
The Last Stage of the General Dispersal of Plants of the Malayan Era.
We arrive now at the close of the era of the general dispersal of tropical plants, mainly Malayan, over the Pacific, and this brings us down to our own age. The few genera that are still dispersed have no peculiar species in particular groups. The species which often range over all the groups, and retain as a rule their characters in most of them, do not therefore display, except in a few cases, that extreme variation which would give them a place in the ranks of the polymorphous species. The dispersing agencies, in fact, are sufficiently active to check marked variations, and the process of isolation has scarcely begun.
We perceive the reason of this when we look at the nine genera which are taken as samples of this period, viz., Rhus, Osteomeles, Viscum, Plectronia, Boerhaavia, Polygonum, Pipturus, Boehmeria, and Dianella, most of them being known to be dispersed by birds at the present day. Six of the genera possess fruits likely to attract frugivorous birds; whilst one of them, Boerhaavia, has sticky fruits that would be apt to adhere to plumage. Actual observations in the cases of Rhus, Viscum, and Plectronia establish the fact of their dispersal by fruit-eating birds; and there is no difficulty in postulating the same agency for Osteomeles, Pipturus, and Dianella. A method by which Boerhaavia fruits would be transported in the plumage of birds has been observed by Mr. Lister; whilst the nutlets of Polygonum are known to afford food to a variety of birds and to be thus distributed.
In this period the plants all hail from the Asiatic side of the Pacific. Three of the genera, Plectronia, Pipturus, and Dianella, belong almost exclusively to the Old World. Five occur in both the Old and New Worlds, but, as with Rhus, Viscum, Boerhaavia, and Boehmeria, are represented by Old World species in the Pacific, or, as with Polygonum, possess a cosmopolitan species (P. glabrum) ranging over the warm regions of the globe. Even Osteomeles presents no exception to the rule, since the Pacific plant is the only one of its species that is not American.