Fossil Plants as Tests of Climate

Abstract

MR. DE RANCE'S note relating to the above subject in. NATURE, p. 294, mentions that “Heer has determined a magnificent flora of more than 350 species from these northerrt tertiaries, and that he at once pointed out the absence of tropical and subtropical forms.” My contention, founded on an attentive study of his determinations and of the original specimens in London and Dublin, and to some extent in Copenhagen, is that not fifty, or perhaps not the half of fifty, of these determinations are entitled to the smallest weight; and again that though at first he saw nothing subtropical in the flora, he subsequently declared the presence of palms, &c., upon utterly insufficient data. While, however, wishing to rid the “magnificent” flora of 300 or more useless and misleading encumbrances, I am far from wishing to depreciate the extraordinary significance and value of that which remains, and which clearly shows that in early Eocene times the coast of Greenland supported in certain places forests which included the redwood, the plane, and even the magnolia, associated with many more northern forms. This is consistent with the tropical vegetation existing during a part of the possibly contemporary lower tertiary period in the south of England. Both facts are sufficiently inexplicable, but there is no occasion to magnify the difficulties they present. As to the Greenland floras they have not been proved to contain any forest trees that might not, and which in fact do not, flourish in their modern representatives, when planted in certain favourable spots on the west coast of Ireland, and even of Scotland. We are not even obliged to assume that Greenland as a country was characterised by such vegetation, for this might be as erroneous as to regard Ireland or Scotland as countries generally characterised by forests of arbutus. The flora of a country is in fact most likely to be preserved in its most sheltered spots, in lake bottoms like parts of Killarney, or where small rivers quietly steal into the tidal waters of deeply recessed bays like those of Bantry and Kenmare, in forest pools like some in the Mount Stewarts' woods of Bute, and in the backwaters and marginal pools of the lower reaches of larger rivers; we are not only entitled, but we are bound to consider this to have been the case in Greenland, and to base our estimate of its climate in the lower tertiaries upon this view and no other. Now what geologists and physicists ought to do, and what they resolutely won't do, is before going farther afield for cause and effect, to take the map of the world on Mercator's projection, and consider how far, if the Atlantic were a closed ocean to the north, as we know it must have been, the required climatic conditions would be produced. The difference between the arbutus nooks of Ireland on the one side and the desolation of Labrador on the other is brought about solely by ocean currents. At the period of the Greenland floras the arctic currents were excluded, and consequently the whole Atlantic basin was filled with the circulation of equatorial and temperate waters only. The distribution of plants and animals renders it extremely probable that during much of the tertiary period, the antarctic waters were equally excluded from the Atlantic by land connecting Africa and South America. What, under these circumstances, would happen to the climate of the Atlantic littoral? It would, it appears to me, be more philosophical to dispose of this question, which is supported by a weight of evidence, before invoking shifting of the earth's axis, or other hypothetical causes supported by none.