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Show 484 ISLAND LIFE. (PART II. extent without the aid of those powerful causes explained in our eiO'hth chapter-causes which acted as a constantly 0 . recurrent motive-power to produce that "contmuous cur-rent of vegetation" from north to south across the whole width of the tropics referred to by Sir Joseph Hooker. Those causes were, the repeated changes of climate which, during all <Yeoloaical time appear to have occurred in both hemispheres, b 0 ' d . culminating at rare intervals in glacial epochs, an whwh have been shown to depend upon changes of excentricity of the earth's orbit and the occurrence of summer or winter in aphelion, in conjunction with the slower and more irregular changes of geographical conditions; these combined causes acting chiefly through the agency of heat-bearing oceanic currents and of snow- and ice-collecting highlands. Let us now bri~fly consider how such changes would act in favouring the dispersal of plants. Elevation and dep1·ession of the Snow Line as aiding the migration of Plants.-We have endeavoured to show (in an earlier portion of this volume) that wherever geographical or physical conditions were such as to produce any considerable amount of perpetual snow, this would be increased whenever a high degree of excentricity concurred with winter in aphel1~on, and diminished during the opposite phase. On all mountain ranges, therefore, which reached above the snow-line, there would be a periodical increase and decrease of snow, and when there were extensive areas of plateau at about the same level, the lowering of the snow-line might cause such an increased accumulation of snow as to produce great glaciers and ice-fields, such as we have seen occurred in South Africa during the last period of high excentricity. But along with such depression of the ·line of perpetual snow there would be a corresponding depression of the alpine and sub-alpine zones suitable for the growth of an arctic and temperate vegetation, and, what is perhaps more important, the depression would necessarily produce a great extension of the area of these zones on all high mountains, thus affording a number of new stations suitable for such temperate plants as might first reach them. But just above and below the snow-line is the area of CilAP. xxm.] ARCTIC PLANTS IN NEW ZEALAND. 48,5 most powerful disintegration and denudation frorn the It t" f ' a ornate ac wn o frost and sun, of ice and water; and thus the more extend~d area would be subject to the constant occurrence of land-s!Ips, berg-falls, and floods, with their accompanying accumul. atwn~ of d~bris a~d of alluvial soil, affording innumerable statwns m whwh solitary wind-borne seeds miaht germinate and temporarily establish themselves. b T.his low.ering an~ rising of the snow-line each 10,500 years dunng penods of high excentricity, would occur in the northern and sou~hern ~e~ispheres alternately; and where there were high mountams Withm the tropics the two would probably overlap ~ach ot~ler, so that the northern depression would make itself felt m a shght d~gree even across the equator some way into the south~rn hemisphere, and vice versa; and even if the difference of the height of perpetual snow at the two extremes did not averaO'e more than a few hundred feet, this would be amply sufficient ~o su?ply .the new and unoccupied stations needful to facilitate the m1gratwn of plants. But the difference~ of. temperature in the two hemispheres ca~se~ by the sun bemg m perihelion in the winter of the one while It was in aphelion during the same season in the other would necessarily lead to increased aerial and marine currents' as already explained ; and whenever geographical condition~ were such as to favour the production of glaciation in any area t~es.e effects would become more powerful, and would further aid m the dispersal of the seeds of plants. Changes of Olimat('. favourable to JJfigration.-It is clear then that during p~riods w ben no glacial epochs were produced in th~ northern hemisphere, and even when a mild climate extended over the whole polar area, alternate chanaes of climate favourina t~e dispe~sal of plants would occur on all high mountains, and Wit~ partwular force on such as rise above the snow-line. But dun~g that lon~-~ontin~ed, though comparatively recent, phase of high excentnCI.ty whwh produced an extensive glaciation in the northern hemisphere and local glaciations in the southern these risings and lowerings of the snow-line on all mountai~ ~anges would have been at a maximum, and would have been mcreased by the depression of the ocean ·which must have |
