The case for Premna is stated in [Note 32]. In this genus, as with Vigna, the final test of experiment is needed; but the data at my disposal point to the probability that an inland species has here been derived from a littoral plant.
The summary of this chapter is given at the end of [Chapter XVI.]
CHAPTER XV
THE RELATION BETWEEN LITTORAL AND INLAND PLANTS
(continued)
Inland species of a genus developed from littoral species originally brought by the currents but no longer existing in the group.—Illustrated by the Leguminous genera, Erythrina, Canavalia, Mezoneuron, and Sophora, and by the Apocynaceous genus, Ochrosia.—The Hawaiian difficulty.
Section III
Here we have three genera of the Leguminosæ, namely, Erythrina, Canavalia, and Sophora, and one Apocynaceous genus, Ochrosia, in which it is considered that inland species have been probably developed from littoral species no longer found in the group. In this case the shore species, possessing buoyant seeds or fruits that are known to be dispersed by the currents, is absent from the particular group in which the inland species occurs; and since the last-named displays no capacity for distribution by currents, or seemingly by birds, we are driven to infer that it was originally derived from a coast species, brought by the currents, that has since disappeared.
Hawaii is the only region concerned here; and these four genera may be said to well illustrate the particular “Hawaiian difficulty.” If this explanation of the origin of the inland species is legitimate, then it offers us a mode of explaining still more perplexing cases in the Hawaiian flora, such as those relating to the endemic species of Mezoneuron (Leguminosæ) and to Hillebrand’s Vallesia (Apocynaceæ), where there is apparently no littoral species known from any region.
Dealing with the three Leguminous genera, it is at first to be remarked that the great floating powers of the seeds of the littoral species are in all three cases to be attributed to the buoyant kernel; whilst on account of the non-buoyancy of the kernel the seeds of all the inland species possess no floating power. Some very interesting points are raised in each of the three genera, and I will first deal with the genus Erythrina.
Erythrina.
If we look over the Pacific islands in search of a critical locality for the investigation of the genetic relation between the littoral and coast species of Erythrina, we discover it, as far as I can gather, only in one group. In Fiji, Tonga, and Samoa we find only the littoral species; in Hawaii there is only an inland species; whilst in Tahiti occur both the littoral and the inland species—E. indica, the wide-ranging shore-tree of the South Pacific, and E. monosperma, the inland tree of Hawaii—the last found nowhere else in Polynesia, and confined to the Pacific. In Tahiti there are no other species, and it is between these two species that the connection, if it exists, is to be sought. (Further details relating to the genus are given in [Note 53]. In this place only the facts bearing on the argument will be discussed.)