| OCR Text |
Show 486 ISL,AN D LIFE. (PAil.'!' Il. arisen from such a vast bulk of water being locked up in landice, and which depre::>sion would have produced tho .sa~e effect as a general elevation of all the contine~ts. A~ this time, too, ae'" n·a 1 currents would have attained their maxim£u m·1 · of force · b th hemispheres · and this would greatly aCI 1tate the In 0 ' h . d. dispersal of all wind-borne seeds as w~ll as ?f t ose carne m the plumage or in the stomachs of bu~s, smce .we have seen how vastly the migratory powers of birds are mcreased by a stormy atmosphere. . Jfigration from NoTth to Smtih has been long ~o~n.~ on.-Now, if each phase of colder and warmer mounta1~-chmate-each alternate depression and eleva.tion of the snow-hue, only helped 011 t.he migration of a few species some sta~es of the lo~g route from the north to the south temperate regwns, yet, durmg the IonO' course of the Tertiary period there might well have arisen that represente:."Ltion of the northern flora in the southern hemisphere which is now so conspicuous. For it is ver~ important to remark that it is not the existing flora alone that IS represented, such as miO'ht have been conveyed during the last gbcial epoch 0 • • only; but we find a whole senes of northern types evidently of varying degrees of antiquity, while even some genera characteristic of the southern hemisphere appear to have been originally <lerived from Europe. Thus E1walyptus and lrfetrosideros have been determined by Dr. Ettinghausen from their fruits in the Eocene beds of Sheppey, while Pimelea, Leptmneria and four o·enera of Proteacere have been recognised by Professor Heer in b • . the Miocene of Switzerland ; and the former wnter has detected fifty-five Australian forms in the Eocene pla.nt beds of Haring (? Belgium).1 Then. we have such peculiar genera as Pachychla- 1 Sir Joseph Hooker informs me that he considers these identifications worthless, and Mr. Bentham has also written very strongly against the value of similar identificatio'ns by Hecr and Unger. Giving due weight to tl1e opinions of these eminent botanists we must admit that Australian genera have not yet been dernonstmled to lu,tve existed in Europe during the Tertiary period; but, on the other h~md, the evidence that they diJ so appears to have some weight, on account of the improbability that the numerous resemblances to Australian plants which ]Javo been noticed by different observers should all be illusory; while the well established faet of the former wide distribution of rn~ny tropical o,r ,now restricted types of CHAP. XXIII.] AUCTIC PLANTS IN NEvV ZEALAND. 487 don and Notothlaspi of New Zealand said to have affinities with Arctic plants, while Stilboca·rpa-another peculia.r New Ze~land genus-has its nea.rcst allies in the Himalayan and C.ln~ese Aral~as. Following these are a whole host of very distmct species of northern genera which may date back to any part of the Tertiary period, and which occur in every south . tempera.te land. Then we have closely allied representative spec10s of European or Arctic plants; and, lastly, a number of identical species,-and these two classes are probably due entirely to the action of the last great glacjul epoch, whose long continuance, and the repeated fluctuations of climate with which it commenced and terminated rendered it an agent of sufficient power to have brought abo~t this result. Here, then, ¥Ye have that constant or constantly recurrent process of dispersal acting throughout long periods with varying power-that " continuous current of vegetation " as it has been termed, which the facts demand; and the extraordinary phenomenon of the species and genera of European and even of Arctic plants being represented abundantly in South Africa Australia, and New Zealand, thus adds another to the Ion~ . 0 senes of phenomena which are rendered intelligible by frequent alternations of warmer and colder climates in either hemisphere, culminating, at long intervals and in favourable situations, in actual glacial epochs. · Geological changes as aiding rnigration. -It will be well also to notice here, that there is another aid to dispersion, dependent upon the changes effected by denudation during the long periods included in the duration of the species and genera of plants. A considerable number of the plants of Europe of the Miocene plants and animals, so frequently illustrated in the present volum~, removes the antecedent improbability which is supposed to attach to such identifications. I am myself the more inclined to accept them, because, according to tho views here advocated, such migrations must have taken place at remote as well as at recent epochs ; and the preservation of some of the~::e types in Australia while they have become extinct in Europe, is exactly paralleled by numerons facts in the distribution of ·animals which have b~en already referred to in Chapt.er XIX., and elsewhere in this volume, and also repeatedly in my larger work. |
