In the light of these reflections it will be interesting to glance at the general distribution of the shrubby and arborescent or woody Compositæ. Mr. Hemsley, having generally discussed the subject, arrived at the conclusion that, “although they form so large a proportion of the floras of St. Helena, Juan Fernandez, the Sandwich Islands, and some other islands, they are not specially insular.” There are scores of them, he goes on to say, in South America, Africa, Madagascar, India, Australia, and New Zealand from twenty to forty feet high, and more truly arboreous than the insular ones; whilst nearly every sub-order has its arboreous representatives. He was, however, unable to form any definite opinion of the method of distribution of the woody Compositæ. Taking those of St. Helena and Juan Fernandez, he observes that they are not more closely allied to the Compositæ of the nearest continents than they are to those of more distant regions. The occurrence of arboreous Compositæ, belonging in each case to different tribes, in so many remote oceanic islands, coupled with the distribution of the genera to which they bear the greatest affinity, seems, he observes, to indicate that they are the remains of very ancient types (Introd. Bot. Chall. Exped., pp. 19-24, 66, 68; also Parts ii. p. 61, and iii. p. 23).
The further discussion of this subject would lead us into a wide field of inquiry, quite beyond the scope of this work. There is, however, an inference that I think we may legitimately draw from geological evidence in this region. With respect to the antiquity of the woody Compositæ of the Pacific as illustrated by the endemic genera, both Mr. Bentham and Mr. Hemsley view them as belonging to ancient types. Mr. Wallace, in his Island Life, a book that becomes more and more indispensable for the student of dispersal as years progress, dwells on the importance of these ancient Compositæ in the floral history of the Pacific islands. We may look upon the Hawaiian Compositæ, he remarks, as representing the most ancient portion of the existing flora, carrying us back to a very remote period when the facilities for communication with America were greater than they are now. The date of this period of oceanic dispersal of the Compositæ we can now approximately determine, since these plants are absent from the Fijian region, an area of submergence during the Tertiary era. Before the island-groups of the Fijian region had emerged towards the close of the Tertiary period the achenes of the early Compositæ had been dispersed far and wide over the tropical Pacific.
But this is not all that we can infer from the convergence of these independent lines of botanical and geological investigation. Mr. Bentham observes that the tribes of the Compositæ had acquired the essential characters now employed in classification before the dispersion of the order over the Pacific. Since this general dispersion took place, as we hold, during the Tertiary submergence of the island-groups of West Polynesia (Fiji, Tonga, Samoa), it follows that the birth of the tribes of the Compositæ antedates that period. If this interesting order could supply us with a “datum-mark” in the history of the Pacific floras, it would be stated in terms of the development of specific and generic characters, but not of those of a tribe.
Summary of Chapter.
(1) The Hawaiian Islands present the same contrast with the Fijian and Tahitian groups as regards the development of new species in the case of the flowering plants that they offer in the case of the vascular cryptogams (ferns and lycopods). But the contrast is intensified, and it is further emphasised as respecting the flowering plants by the evolution of a large number of endemic genera.
(2) This great preponderance of peculiar species and genera in Hawaii is not to be connected with the relative antiquity of the group but with its degree of isolation.
(3) The earliest stage of the flowering plants of the islands of Hawaii and of Eastern Polynesia (the Tahitian region) is indicated by the endemic genera, particularly those of the Compositæ and Lobeliaceæ. Such genera are numerous in Hawaii, and occur also in the Tahitian region, as in Tahiti and Rarotonga; but do not exist in the groups of the Fijian region (Fiji, Tonga, and Samoa).
(4) The endemic genera of the Hawaiian Compositæ are mainly American in their affinities. The relationship of the solitary Tahitian genus (Fitchia) is still a subject of discussion.
(5) In the Hawaiian Islands, as well as in Tahiti and Rarotonga, the plants of the endemic genera of Compositæ are, as a rule, arborescent or shrubby; and in the first two localities they are mainly restricted to the higher levels.
(6) In discussing the mode of dispersal of the achenes of the original genera we have also to explain why the process of dispersal has been in the main suspended.