Up to this point the non-endemic ferns and lycopods have been chiefly discussed. We will now briefly deal with the probable cause of the relative preponderance of peculiar or endemic species in Hawaii as contrasted with Fiji and Tahiti. In this respect the Hawaiian islands, as remarked at the commencement of this chapter, come into sharp contrast with the other two groups; but it would seem that the differentiation has rarely acquired a generic value (see [Note 66]). In this respect the age of ferns is markedly distinguished from the succeeding era, the age of the arborescent Compositæ and of Tree-Lobelias, to which a large number of peculiar genera belong. This, according to my view, is to be attributed to the circumstance that whilst the dispersion of spores by the wind is probably as active in our own time as it was in the earliest stage of the floral history of Hawaii, the dispersion of seeds by birds, to which the flowering plants in the main originally owe their presence in this group, has been greatly influenced by the various changes that have affected the migration of birds over the Pacific, a subject discussed in later pages.
Respecting the origin of the species of ferns and lycopods peculiar to Hawaii, it is first of importance to quote the remarks of Dr. Hillebrand on the subject. Speaking of the whole flora (p. xxv), but evidently with the ferns more especially in his mind, he says:—“Nature here luxuriates in formative energy. Is it because the islands offer a great range of conditions of life? Or is it because the leading genera are in their age of manhood, of greatest vigour? Or is it because the number of types which here come into play is limited, and, therefore, the area offered to their development comparatively great and varied?” It is deeply to be regretted that sickness and death intervened before the author was able to give to the world his matured views on the very important points here raised. Yet they are much the same questions that man is ever putting to the life around him. There is the same querulous note that we find in all, the question that begins, the question that ends, and the reply that never comes.
“The evolution theory (writes Dr. Hillebrand, p. xxix) could hardly find a more favourable field for observation than an isolated island-group in mid-ocean, large enough to have produced a number of original forms, and at the same time so diversified in conditions of temperature, humidity, and atmospheric currents as to admit an extraordinary development in nearly every direction of vegetable morphology, uninfluenced by intercrossing with foreign elements.” Isolation thus admittedly offers the preliminary determining or favouring conditions. This is directly indicated by the fact that Hawaii possesses fewer genera of ferns and lycopods than either Fiji or Tahiti, notwithstanding that it has the same area as Fiji, and is in extent three or four times the size of the whole Tahitian area. One effect of isolation in Hawaii has, therefore, been greater room for the development of new forms. It has, however, already been remarked that the islands of the Fijian area are much less isolated than those of the Hawaiian group, and that in consequence the free immigration possible in the one group has been checked in the other. Fiji possesses in respect to vascular cryptogams at least half as many species again as Hawaii, but Hawaii has three or four times the number of peculiar species. Yet before this great contrast can be ascribed to different degrees of isolation, it is necessary to exclude another possible cause presented by the greater range of life-conditions in Hawaii. It is possible that all the Hawaiian peculiar species may belong to the higher levels, elevations, as before shown, not represented in the Fijian islands, which correspond only to the lowlands of Hawaii, that is, to levels below 4,000 feet. If this is the case, the contrast between Fiji and Hawaii would be connected mainly with a difference in life-conditions, and, however potent the isolating influences might have been in Hawaii, they could hardly have been concerned with this striking difference.
In order to determine this point, I went carefully through the account given by Hillebrand of the Hawaiian ferns and lycopods, noting the altitudes there given, and making use of the maps and of my own local knowledge of the islands of Oahu and Hawaii, where the elevation is neither directly nor indirectly implied. As a result, I found that out of sixty-six endemic species available for my purpose, forty-seven had their stations at levels below 4,000 feet, that is in the region corresponding to Fiji, and nineteen at elevations exceeding this height. This, however, did not finally decide the question, since the proportion of endemic species may be much smaller in the region below 4,000 feet than in that above it. I, therefore, went over the ground again, and found, as shown in the table below, that the percentages of peculiar species amongst the total available for my use were not very far apart, 58 per cent. for the upper region and 43 per cent. for the lower region.
Distribution of the Hawaiian ferns and lycopods above and below 4,000 feet.
| Number. | Endemic. | Percentage of endemic species. | |
|---|---|---|---|
| Species below 4,000 feet | 110 | 47 | 43 |
| Species above 4,000 feet | 33 | 19 | 58 |
From the above it would appear that although the process of species-production in the Hawaiian islands has seemingly been rather more active above than below 4,000 feet, if we were to compare the entire vascular cryptogamic flora of Fiji with that of the corresponding lower levels of the Hawaiian group we should obtain much the same contrast in the proportion of peculiar species that we obtained when comparing all the ferns and lycopods of both groups. In other words, if we were to restrict our comparison with Fiji, and I may add Tahiti, to that lower portion of Hawaii that corresponds in elevation, we should not get results very different from those to be obtained by including the Hawaiian upland regions as well.
We are, I think, on these grounds justified in assuming that the relatively great development of new species of ferns and lycopods in Hawaii as contrasted with Fiji is not to be connected with the greater elevation of those islands. The only thing that we have been able to associate with the greater altitude of the Hawaiian Islands, and the consequent greater range of climatic conditions, when contrasting the Fijian and Hawaiian vascular cryptogams, is the occurrence of a number of peculiar mountain species and of wide-ranging temperate species that are found in the uplands of Hawaii, but not in the less elevated islands of Fiji.
On the whole, therefore, it is to be inferred that the greater display of formative power among the ferns and lycopods of the Hawaiian Islands is in great part to be associated with the isolation of this group as compared with those of Fiji and Tahiti. The indications supplied by the vascular cryptogams resemble in kind those we shall obtain from the study of the flowering plants, but there is this important distinction. In formative power, as shown in the development of new specific and generic types, the Hawaiian vascular cryptogams are far exceeded by the flowering plants where the proportion of endemic species amounts to 80 per cent. We have no reason to believe that the winds, to which the ferns and lycopods chiefly owe their dispersal, are less effective now in carrying their spores than they were in the earliest era of the floral history of Hawaii or in the intervening periods. In the course of ages the winds have been more uniform in their action as plant-dispersers even than the currents, and certainly far more than birds.
On the other hand, in the case of the Hawaiian flowering plants that depend on the varying influence of the migrant bird, the agency of dispersal has often been suspended altogether, and far greater differentiation or departure from the original type has resulted, the amount of change often reaching to the value of a generic distinction. It is a question, however, whether the isolation of the Hawaiian Islands is to be entirely connected with their mid-oceanic position. It will be shown in [Chapter XXXIII]. that effects almost as great have been produced in continental regions and in continental islands, and that the isolated situation of Hawaii has not induced but has intensified these results. In the later eras of plant-life a process of segregation has been ever active throughout the tropical world whether in the case of an elevated oceanic island or of a mid-continental mountain.