Elevation and Depression 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 aphelion, 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, because as we descend the average slopes become less abrupt,—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 most powerful disintegration and denudation, from the alternate action of frost and sun, of ice and water; and thus the more extended area would be subject to the constant occurrence of land-slips, berg-falls, and floods, with their
accompanying accumulations of débris and of alluvial soil, affording innumerable stations in which solitary wind-borne seeds might germinate and temporarily establish themselves.
This lowering and rising of the snow-line each 10,500 years during periods of high excentricity, would occur in the northern and southern hemispheres alternately; and where there were high mountains within the tropics the two would probably overlap each other, so that the northern depression would make itself felt in a slight degree even across the equator some way into the southern hemisphere, and vice versâ; and even if the difference of the height of perpetual snow at the two extremes did not average more than a few hundred feet, this would be amply sufficient to supply the new and unoccupied stations needful to facilitate the migration of plants. It is well known that all great mountain ranges have undergone such fluctuations, as proved by ice-marks below the present level of snow and ice.
But the differences of temperature in the two hemispheres caused by the sun being in perihelion in the winter of the one while it was in aphelion during the same season in the other, would necessarily lead to increased aërial and marine currents, as already explained; and whenever geographical conditions were such as to favour the production of glaciation in any area these effects would become more powerful, and would further aid in the dispersal of the seeds of plants.
Changes of Climate Favourable to Migration.—It is clear then, that during periods when no glacial epochs were produced in the northern hemisphere, and even when a mild climate extended over the whole polar area, alternate changes of climate favouring the dispersal of plants would occur on all high mountains, and with particular force on such as rise above the snow-line. But during that long-continued, though comparatively recent, phase of high excentricity which produced an extensive glaciation in the northern hemisphere and local glaciations in the southern, these risings and lowerings of the snow-line on all mountain ranges would have been at a maximum, and
would have been increased by the depression of the ocean which must have arisen from such a vast bulk of water being locked up in land-ice, and which depression would have produced the same effect as a general elevation of all the continents. At this time, too, aërial currents would have attained their maximum of force in both hemispheres; and this would greatly facilitate the dispersal of all wind-borne seeds as well as of those carried in the plumage or in the stomachs of birds, since we have seen, by the cases of the Azores and Bermuda, how vastly the migratory powers of birds are increased by a stormy atmosphere.
Migration from North to South has been long going on.—Now, if each phase of colder and warmer mountain-climate—each alternate depression and elevation of the snow-line, only helped on the migration of a few species some stages of the long route from the north to the south temperate regions, yet, during the long course of the Tertiary period there might well have arisen that representation of the northern flora in the southern hemisphere which is now so conspicuous. For it is very important to remark that it is not the existing flora alone that is represented, such as might have been conveyed during the last glacial epoch only; but we find a whole series of northern types evidently of varying degrees of antiquity, while even some genera characteristic of the southern hemisphere appear to have been originally derived from Europe. Thus Eucalyptus and Metrosideros have been determined by Dr. Ettingshausen from their fruits in the Eocene beds of Sheppey, while Pimelea, Leptomeria and four genera of Proteaceæ have been recognised by Professor Heer in the Miocene of Switzerland; and the former writer has detected fifty-five Australian forms in the Eocene plant beds of Häring (? Belgium).[[192]] Then we have such peculiar genera
as Pachychladon and Notothlaspi of New Zealand said to have affinities with Arctic plants, while Stilbocarpa—another peculiar New Zealand genus—has its nearest allies in the Himalayan and Chinese Aralias. Following these are a whole host of very distinct species of northern genera which may date back to any part of the Tertiary period, and which occur in every south temperate land. Then we have closely allied representative species 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 glacial 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 about this result.
Here, then, we 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 America, Australia, and New Zealand, thus adds another to the long series 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 Migration.—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