History of the Vegetation of the Chatham Islands.
I do not intend to discuss at any length the history of the vegetation of the Chatham Islands, and the affinities between its species and those of other parts of the New Zealand area. A comparison of the species common to New Zealand and the Chatham Islands, detailing exactly any differences, however
slight, which distinguish the forms of the Chathams from those of New Zealand, would be a matter of high biological interest, but one which is hardly possible in the present state of knowledge regarding the plants. Judging by previous experience, species of the Chathams supposed to be endemic may be eventually shown to exist in other parts of the New Zealand biological area* when its botany is more fully investigated; while unrecorded species of local distribution may very possibly be discovered in the Chatham group. So far as genera are concerned, the Chatham Islands possess only one endemic genus, Myosotidium. Taking next the species of Buchanan's list (3), omitting a few the occurrence of which appears doubtful and adding others recorded here and elsewhere (37, 39, 40, 41) since its publication, there are about 166 species and distinct varieties of flowering-plants and fifty-one of vascular cryptogams in the Chatham Islands, giving a total of 217,† of which about thirty-one—i.e., 14 per cent.—are considered by me to be endemic, while one, Leucopògon (Styphylea) richei, is, according to Baron Von Mueller, identical with an Australian species. All the remainder occur in some part or other of the New Zealand region; indeed, with the exception of Rhopalostylis baueri(?), Myrsine chathamica, and Pratia arenaria, they are all to be met with in New Zealand proper. But many of the endemic species are so very closely related to New Zealand forms that it will always be a matter of opinion whether such are at best varieties, and not species at all.
From the above it may be seen clearly that there is little difference so far as species are concerned between the Chatham Islands and New Zealand, and, if the differences between related plants be taken as a measure of the length of time since they deviated from a parent stock, it seems right to consider the flora of the Chathams as a recent offset from that of New Zealand.
As to how the New Zealand plants made their way to the Chathams in the first instance geology teaches us that New Zealand at one time extended very much further to the east than at present, and that it is not unlikely that there was actual land-connection between the two groups of islands (33, p. 177), or if not land, then merely a narrow piece of sea across which the plants could easily migrate by means of birds, wind, and the other agencies discussed by Hemsley (28). Even if the ocean barrier had always been as wide as at present it seems quite possible that plants could find their
[Footnote] * Lepyrodia traversii, Hymenanthera chathamica, and Myrsine chathamica were formerly considered endemic The latter was collected by Mr. G. M. Thomson at the head of Wilson Bay, Stewart Island (50A).
[Footnote] † Drude gives only sixty-two indigenous phanerogams (17).
way from New Zealand to the Chathams without much difficulty. The sparrow and the blackbird, both now a nuisance on Chatham Island, reached that land, as pointed out before, unaided by man; the smoke of bush-fires from the North Island of New Zealand at times fills the air of Chatham Island when the wind blows from the north-east; finally, logs have again and again been cast up on all the sea-beaches. Old logs of Podocarpus totara are to be found in considerable numbers in the vicinity of the north coast buried under sand and in the beds of creeks, while in some places they occur at some distance inland, although there is no reason to think that such trees grew on the island.
But I do not think we need postulate any carriage over a broad tract of ocean for the Chatham Island plants; on the contrary, the geological evidence in favour of a wide extension of New Zealand eastwards is very strong, while zoological* and botanical support is not wanting. With regard to this latter, I am not in a position to make any further additions, either confirmatory or the contrary, to my recently published statements (10) regarding the difference in the life-history of certain so-called species according as they are indigenous to New Zealand or the Chatham Islands, so I reserve dealing with this part of my subject until such time as a number of seedling plants now under control are sufficiently developed for me to speak in a definite and exact manner as to their behaviour. All that can be said now on this head is that an examination of the conditions of life on Chatham Island has convinced me that local edaphic influences have played a greater part in modifying the vegetation than I had supposed, and that in consequence, although some of the plants of the Chatham Islands may much resemble certain Pliocene plants of New Zealand, the flora as a whole is not identically what it was in the Pliocene period, for some species must have deviated very considerably from the original type.
One of the difficulties that suggests itself in the way of accepting actual land-connection between New Zealand and the Chathams, and which supports the view that the breadth of ocean between the two lands must always have been wide, is the absence in the Chatham Islands of so many characteristic New Zealand genera; for it seems inconceivable, for example, that so common a New Zealand plant as Cordyline australis, or that so few of the other New Zealand species having fruits readily carried by birds, should not have reached the Chathams, even if there had
[Footnote] * One of the strongest zoological proofs is the migration from Chatham Island of the New Zealand shining cuckoo, a New Zealand migratory bird. As to the significance of this, see Captain Hutton's paper, “Our Migratory Birds” (338)
been a narrow piece of water to cross. If, however, a large area containing an abundant plant population becomes by degrees much restricted the struggle for existence must become much keener, and only those plants can survive, as certain stations become of very limited area, which can drive their competitors out of such stations; or, if themselves forced to move elsewhere, possess powers of rapid adaptation to the new conditions; or, what is often more important, some structure designed primarily for another purpose may, being put to a secondary use, enable them to hold their own in another station. Under the above circumstances many plants must be eradicated altogether, and others must take refuge in the most inhospitable places, while others, again, may easily occupy a quite different station from that to which they are accustomed. According to this view, the species of plants in the Chatham Islands must have decreased enormously in numbers since the time when the Chathams formed part of Greater New Zealand.*
How a very slight change in edaphic and climatic conditions can affect the vegetation of Chatham Island is exhibited by the two distinct regions of vegetation, the tableland and the lowland, each of which contains plants unknown or very scarce in the other. With quite a small modification of the above conditions numbers of plants would perish.
Several plants are extremely local; for instance, Gunnera monoica has been found in only one place in Chatham Island (11), although the ground in many places would seem to be an ideal station for it. There seems no reason why there should be only a very few trees of Coprosma robusta on the island when C. chathamica is one of the commonest trees. Discaria toumatou, found now in one or two places, should surely have become established on the fixed sand-dunes; but these, being suitable for forest growth, would not allow it to occupy what is a characteristic habitat in New Zealand. Other trees or shrubs of very limited distribution are Dodonœa viscosa, Myoporum lœtum, Leptospermum scoparium, and Plagianthus divaricatus. Make the tableland a little wetter and Olearia chathamica would become simply a rock plant, and with the weathering of the rock on which it grew might be eradicated. Reduce the level of the island a few metres and Sophora would exist no longer. Bearing facts such as the above in
[Footnote] * For a different view of the case, see Mr. Cheeseman's paper on the “Flora of the Kermadec Islands” (6). The occurrence on Chatham Island of Coprosma chathamica, so closely related to C. petiolata of the Kermadecs, and of Rhopalostylis baueri in both regions, if the identification of the latter be correct, suggests that they travelled along the coast, which would, in the event of an east and north-east extension of New Zealand, join the Kermadecs and the Chathams.
mind, it is not unreasonable to suppose that Chatham Island contained at one time many more species than is now the case, and that even such a ubiquitous tree as Cordyline australis may have been there and been destroyed in the struggle for existence caused by the shrinking of the land.