It is doubtless to birds of this description that we owe some of the specific connections of Coprosma between groups of the Western Pacific. That the dispersal of the species over distant regions was recently in active operation is shown by the close affinity, according to Dr. Stapf, of two species growing on the summit of Kinabalu, the Bornean mountain, with certain species from New Zealand and South-east Australia. Other Rubiaceous species, like Nertera depressa, possessing Coprosma-like fruits and fitted for the same mode of dispersal, link the heights of Kinabalu with the flora of high southern latitudes.
Being included in the Fijian area, the scanty mountain-flora of Samoa may be here referred to. As in Fiji, the endemic genera of Compositæ and Lobeliaceæ are not to be found, but we find in the central elevated district of Savaii, which rises to over 5,000 feet above the sea, a peculiar species of Vaccinium (4,900 feet), the Antarctic Nertera depressa (4,000 feet), and two species of Weinmannia, a genus hailing probably from high southern latitudes.
The Fijian Coniferæ.
It has been found most convenient to discuss here these interesting plants, which belong in a general sense to the mountain-flora of this archipelago. That which the Fijian flora loses in interest in the eyes of the student of plant-dispersal in not possessing the mysterious Composite and Lobeliaceous genera of Hawaii and Tahiti, it regains in the possession of its genera of Coniferæ. If he felt loth to apply his empirical principles to the above-named Hawaiian and Tahitian endemic genera, he feels more than uneasy when he comes to deal with the three Coniferous genera of Fiji, Dammara (Agathis), Podocarpus, and Dacrydium.
These three genera represent an order that has not found a home either in Tahiti or in East Polynesia generally, or in the more distant Hawaii; and they present at first sight in their existence in Fiji a powerful argument in favour of the previous continental condition of the islands of the Western Pacific. But in advocating this view we should remember that it involves the original continuity of the Fijian land-area, not only with the neighbouring islands of the New Hebrides and of New Caledonia where these genera alike occur, but also with New Zealand, Tasmania, and Australia, where they sometimes attain a great development.
In Fiji these trees often chiefly form the forests of the larger islands, extending in the moister regions from near the sea to the mountain-tops, and being often abundant on the great mountain-ridges of the interior. It may be at once remarked that, viewed merely from the standpoint of dispersal, there is no great difficulty in regarding it as probable that the seeds of Podocarpus and Dacrydium have been dispersed by frugivorous birds over tracts of ocean 500 or 600 miles across. Dammara, however, so far as my Fijian observations show, possesses none of the means of dispersal across oceans that we are at present acquainted with. The two first-named genera occur in South America as well as in the Australo-Polynesian region, some of the species in these two regions, though the Pacific Ocean divides them, being closely related. Dammara is, on the other hand, confined to a much more limited area, extending from New Zealand to Borneo. It is from the distribution of this genus that the continental theory derives its chief support.
Yet it may be remarked that something more than questions relating to the capacity for dispersal are involved here. This is at once indicated by the circumstance that although Podocarpus is known to be dispersed by frugivorous birds, it is not found in Polynesia east of Tonga, and the same may be said of Dacrydium, which does not occur east of Fiji. In this connection it is necessary to notice the intrusion of Araucaria into the tropical Pacific from Eastern Australia to New Caledonia and the New Hebrides. The fact of this genus not having been recorded from Fiji or any of the groups east of the New Hebrides is very remarkable, and scarcely in accordance with the continental hypothesis. There is a persistence in type of these genera of the Coniferæ during geological time that prevents us from dealing with them on the lines that are required by the mass of the flowering-plants. Other factors intervene, and we apply with hesitation the same canons of dispersal that we employ for the general bulk of the plants of the Pacific islands. If, as often happens, a specific distinction alone separates the Conifers of the same genus on either side of the Pacific Ocean, it must possess in point of time a very different value from that which we would usually attach to specific distinctions in the floras of the Pacific islands.
Dammara (Agathis).—The Dammara region includes Eastern Australia, New Zealand, New Caledonia, with the New Hebrides, Fijian, and Santa Cruz groups, and extends north-west to Java and Borneo. Only ten species are named in the Index Kewensis, and of these four are assigned to New Caledonia and two to Fiji, the focus of geographical distribution being, therefore, as Seemann long since pointed out, in the islands of the Western Pacific. The absence of the genus from the neighbouring Samoan and Tongan groups is very significant; and it is evident that the ordinary agencies of dispersal, whether birds, winds, or currents, have here failed to extend the genus over a few hundred miles of sea.
When by means of observation and experiment we turn to the fruits and look for a reply, we find in the first place that they are never to be noticed either whole or in part in the floating drift of sea or river, or amongst the stranded materials of the beaches. This is at once explained when we ascertain that the fresh cones sink in the river-water, and thus could never reach the coast in their entire condition. Nor could they do so in fragments, since the detached cone falls to pieces on the ground and the separate scales and seeds sink at once or float only for a few hours. In order to test the buoyancy of a cone after drying, it is necessary to bind it round with string to keep it from breaking down. One such fruit, after being kept for ten days, was placed in sea-water, where it floated heavily for eleven days and then sank. This is, of course, a most unnatural experiment, but it was well to have carried it out. That the entire fruit could never be transported by water is indirectly implied by Kirk respecting the fruit of Dammara australis, the Kauri Pine of New Zealand. In this case, when the fruit reaches maturity the scales, he remarks, fall away from the woody axis of the cone and the seeds are freed.
The fleshy, unprotected seeds, which, as above noted, possess little or no floating power, could scarcely withstand the injurious effect of sea-water; and they are absolutely unfitted for any known mode of dispersal by birds. It is observed by Kirk that the seeds of the New Zealand tree are widely spread by winds. But this could only avail them for local dispersion, and they appear ill-suited for being transported for more than a few paces. The seeds are winged, and are in form a little like the samara of the Maple (Acer); but they have not the same protective coverings, the wing being, however, only a little more than half the length of the entire seed. Those of both Dammara australis and D. vitiensis are about two-thirds of an inch in length, and are heavy-looking; and the agency of the wind could never be invoked except for local dispersion.