Tahiti, Hawaii, and Fiji. (See [note 63].)
| Group. | Number of genera. | Number of species. | Species to a genus. | Number of endemic species. | Percentage of endemic species. | Percentage of ferns and lycopods among the vascular plants. |
|---|---|---|---|---|---|---|
| Tahiti | 38 | 154 | 4·1 | 13 | 8 | 33 |
| Hawaii | 29 | 155 | 5·4 | 70 | 45 | 18 |
| Fiji | 40 | 237 | 5·9 | 20 | 8 | 21 |
The results, so far mentioned, are in the main consistent with the geographical position and the degree of isolation of these three areas. From their proximity to the large continental islands of the Western Pacific, the Fijian islands would have readily received a great number of immigrants from the west, since the intervening sea is not over 500 miles in breadth. They lie in the track of the main line of migration into and across the South Pacific, a track which has been followed by flowering plants and animals as well as by aboriginal man. Assuming that the migration of the vascular cryptogams extended from Fiji eastward to Tahiti, fewer of the immigrants would reach the last-named group. Fewer still would reach the Hawaiian islands, which excluding the groups of low coral islands to the southward are cut off on all sides, whether from the Fiji-Samoan and Tahitian areas, from the coasts of North America, or from the regions north and west, by a breadth of ocean that is never less than 1,500 miles.
That the main track of the ferns and lycopods across the South Pacific to Tahiti has been eastward there can be little doubt. This is indicated in the tables given by Drake del Castillo for Eastern Polynesia, and also by an analysis I have prepared of the distributions that he gives for the species of the Tahitian region (see [Note 64]). Out of the 154 species there are only two that belong exclusively to the American side of the Pacific; whilst 58 are derived exclusively from the Asiatic side, and mainly from Indo-Malaya. The drift of the ferns and lycopods eastward from Fiji is also brought out in the number of Tahitian species common to Hawaii and Fiji. Of these about 76 per cent. are common to Fiji or to the groups around, and only 30 per cent. occur in Hawaii. The Tahitian species found in Hawaii occur also in Fiji with the exception of two or three mountain species which have doubtless failed to find a suitable elevation in Fiji. These two or three mountain ferns and lycopods are probably the only vascular cryptogams possessed in common by Hawaii and Tahiti to the exclusion of other groups. (See [Note 64].)
The prevailing Indo-Malayan origin of the ferns and lycopods of the archipelagoes of the Fijian area (Fiji, Tonga, Samoa) is so well established in the writings of Seemann, Baker, Hemsley, Christ, and Burkill that there is no necessity to enter into details here. That the stream of vascular cryptogams to Hawaii has proceeded mainly from the Old World side of the Pacific is shown in the circumstance that of the eighty and odd species found outside the group nearly half are from the Asiatic side exclusively and only three from America alone, whilst about a fourth occur in both continents, and a fourth are confined to Polynesia. One point, says Dr. Hillebrand, comes out in strong relief, and that is “the great number of ferns scattered over the long track which leads from the Hawaiian islands through Polynesia and Malaysia to the east coast of tropical Africa.” But he adds significantly that “it cannot be inferred from this fact that all the species in question have travelled eastward to find the terminus of their long migration on this group, unless the principle be established, that the formative energy of a species or genus be greatest at the circumference or farthest extremity of its area” (p. 542).
Though evidently prepared to admit the general eastward trend of plants in the Pacific, Dr. Hillebrand (p. xxviii) puts forward in the case of the ferns the startling view that originally spores of a few simple species have been diffused over various countries and that they have there evolved on parallel lines “predetermined by the structure of the original immigrant” a series of higher forms, so that the same form might have been produced in two widely distant localities, as, for instance, in Ceylon and Hawaii. The editor, Mr. W. F. Hillebrand, gives good reasons for his belief that this does not represent the matured opinion of the author. It is, however, worth noting in this connection that Dr. Karl Mueller has advanced a similar view with respect to the lower orders of plants. (See a translation of his paper in Trans. and Proc. N. Z. Inst. Vol. 25.) Bearing in mind the known capacity of ferns for dispersal by the winds, an hypothesis of this kind, even if established, seems scarcely needed in the study of fern-dispersal.
It is probable that many of the ferns and lycopods reached Hawaii directly and not through South Polynesia. The mountain-ferns of this group could hardly have been received by that route, since, as is shown below, they do not as a rule occur in that region.
Some other interesting relations present themselves in connection with the Hawaiian ferns and lycopods when we consider the distribution of its non-endemic species in the other two groups of Fiji and Tahiti. Out of these species, some eighty in all, not more than half are common to all three groups, and about two dozen have not been found either in Fiji or in Tahiti. Of these last quite half are mountain species in Hawaii, having their station at elevations exceeding those of the highest districts of Fiji and of the several islands of the Tahitian area, excepting the limited region comprised in the uplands of Tahiti itself.
A glance at the list, given in [Note 65] of some of the mountain ferns of Hawaii not recorded from Fiji and Tahiti will show that these species are very widely distributed. Ferns and lycopods found in the Himalayas and in the Andes meet on the higher slopes of the lofty mountains of Hawaii and in no other of the less elevated island-groups of the open Pacific. This distribution of the vascular cryptogams thus foreshadows a principle that will come into prominence in the case of the flowering plants, namely, that difference in elevation has been an important factor in determining some of the contrasts between the Hawaiian, Fijian, and Tahitian floras. The contrasts here implied are those connected with the climatic conditions of station, since several plants of temperate regions, such as Aspidium filix mas, Asplenium trichomanes, Asplenium adiantum nigrum, &c., that are at home in the highlands of Hawaii, do not occur in either Fiji or Tahiti. We can infer that widely ranging ferns and lycopods have been dispersed over the oceanic groups of the tropical Pacific with a fair degree of uniformity, and that any marked contrasts in their distribution may be attributed to considerable differences in the altitude of the islands.
In appreciating such a conclusion, and in dealing with apparent exceptions to the rule, the relation between the vertical range of a species and its lateral distribution has to be considered. We find, for instance, that whilst the Common Bracken (Pteris aquilina) is a mountain plant in Hawaii, it occurs also in Fiji and Tahiti. Since, however, it is found all over the temperate and tropical regions, and has a vertical range in Hawaii of from 800 to 8,000 feet, any difficulty in this respect is thus explained. Aspidium aculeatum, a characteristic fern of temperate latitudes, seems at first to present a difficulty, which, however, proves to be more apparent than real. Whilst it has been recorded from Hawaii at heights of 6,000 to 9,000 feet, and from Tahiti at 4,000 feet, it has also been found in Fiji and Samoa; but since it was not collected by Seemann in Fiji, it can scarcely be common, and Horne seems only to have obtained it from the tops of mountains in Vanua Levu at an elevation of 1,800 feet.