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Show 488 ISLAND LJF}1:;. (rAR'l' II. pen·o d were so much h·k .e ex1·s t'm g speci·e s that although they have generally recei·v e d f res h names they may ·.w ell hav. e been 1'd en t'w a1 ; and a 1a rae proportion of the vegetatiOn durmg the 0 . • . whole Tertiary period consisted of genera whwh are .~~Ill I. · a 1 But from what is now known of the rate of sub-aenal lVlllo· d' . . f l. denudation, we are sure, that during each I VISion. o t ns period many mountain chains must have .be.en cons1derabJy lowered, while we know that same of the existmg ranges have been greatly elevated. Ancient volcanoes, too, have .been destroyed by denudation, and new ones have been bmlt up, .so that we may be quite sure that ample means for the transmission of temperate plants across the tropics, may have existed in countries where they are now no longer to be found. The great mountain masses of Guiana and Brazil, for example, must have been far more lofty before the sedimentary covering was denuded from their granitic bosses and metamorphic peaks, and may have aided the southern migr.ation .of plants before ~he final elevation of the Andes. And If Afnca presents us with an example of a continent of vast antiquity, we may be sure that its areat central plateaux once bore far loftier mountain ranges e~e they were reduced to their present condition by long ages of denudation. Proofs of MigrationJ by way of the Andes.-We are now prepared to apply the principles a??ve laid dow~ to the. explanation of the character and affimhes of the vanous portiOns of the north temperate flora in the southern hemisphere, and especially in Australia and New Zealand. At the present time the only unbroken chain of highlands and mountains connecting the Arctic and north temperate with the Antarctic lands is to be found in the American continent, the only break of importance being the comparatively low 1 Out of forty-two genera from the Eocene of Sheppey enumerated by Dr. Ettingha.usen in the Geological Magazine for January. 1880,. only two or three appear to be extinct, while there is a mostextraordmary mtermixture of tropical and temperate forms-Musa, Nipa, and Victoria, with Corylus Prunus Acer &c. The rich Miocene flora of Switzerland, describ~d hy Pr~fessor 'Heer presents f,l, still larger proportion of liying ) ' genera. CHAr. xxm.] ARCTIC PLANTS IN NEW ~EALAND. 489 Isthmus of Panama, where there is a distance of about 300 miles occupied by rugged forest-clad hills, between the lofty peaks of V eragua and the northern extremity of the Andes of New Grenada. Such distances are, as we have already seen, no barrier to the diffusion of plants; and we should accordingly expect that this great continuous mountain-chain has formed the most effective agent in aiding the southward miaration of the Arctic and north temperate vegetation. We do find, in fact, not .only that a large number of northern genera and many species are scattered all along this line of route, but that at the end of the .long journey, in Southern Chile and Fuegia, they have established themselves in such numbers as to form an important part of the flora of those countries. From the lists given in the works already referred to, it appears that there are between sixty and seventy northern genera in Fueaia and ~out~ern C~ile, while about forty of the species are ab:olutely Identical With those of Europe and the Arctic reaions. Considering how comparatively little the mountain~ of South Temperate America are yet known, this is a very remarkable result, and it proves that the transmission of species must have gone on up to comparatively recent times. Yet, as only a few of these species are now found along the line of migration, we see that they only occupied such stations temporarily; and we may connect their disappearance with the passing away of the last glacial period which, by raising the snow-line, reduced the area on which alone they could exist, and exposed them to the competition of indigenous plants from the belt of country immediately below them. Now, just as these numerous species and genera have undoubtedly passed along the great American ranae of mountains 0 ' although only now found at its two extremes, so others have doubtless passed on further; and have found ·more suitable stations or less severe competition in the Antarctic continent and islands, in New Zealand, in Tasmania, and even in Australia itself. The route by which they may have reached these countries is easily marked out. Immediately south of Cape Horn, at a distance of only 500 miles, are the South Shetland Islands and Graham's Land, whence the Antarctic continent or ) |