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Show 504 ISLAND LIFE. (PART II. and that the radically different .distribution of land and sea in the northern and southern hemispheres has gener~lly le~ to great d1. vers1' ty of cr 1 1n ate in the Arctic .a nd An·t archti C regwnbs . Tho 1J:' 0rm and ar.ran cremont of the contments IS s <;>wn to o 0 • such as to f:a vour the tra' nsfer of warm oceamc currents to tho nort1 1 f:a r 1· n exce ss of those which move towards tho south, and wh enever th eS e Currents had free pa. ssage. through the northern 1a no1 -masses t o tha"' polar · area, a mild climate. m.u st hav.e pro- vaL• 1e d over the whole northern hemisphere.. It IS only In very recen t tu. nes tha t the o(' freat ·nort.h ern contments have .b ecome so com p1 e t eI y co nsolidated as they now are, thus s.h uttmg .o ut t 11 e warm wa t er from their interiors ' and rendenng .p ossible a w1'd e-spread an d intense oO 'lacial epoch. But tlus gro. at climatal change was a,ctually brought about by the h1gh excen t n·c i' t, y w l1I' ch occurred ab. ou. t 2.0 0,000 years ago;. and it is doubtful if a similar glacmtwn In equally low .latitudes could be produced by means of any such geographical c~mbinations as actually occur, without the concurrence of a high excentricity. . A survey of the present condition of the ~arth supp?rts tlns view, for though we have enormous mountam ranges m eve:y latitude, there is no glaciated country south of Greenland. m N. Lat. Gl o. But directly we go back. a. very short penod, we find the superficial evidences of glaCiatiOn to an enormous extent over three-fourths of the globe. In the Alps and Pyrenees, in the Brjtish Isles and Scandinavia, in Spain and the Atlas, in the Caucasus and the Himalayas, in Eastern North America and west of tho Rocky Mountains, in tho Andes, in the Mountains of Brazil, in South Africa, and in New Zealand, hucre moraines and other unmistakable ice-marks attest the ~versal descent of the snow-line for several thousand feet um 1· h below its present level. If we reject the i.nfluence o~ ~1g excentricity as the cause of this almost umversal glaciati~n, we must postulate a general elevation. of. al~ th.ese moun taws about the same time-for the close s1m1lanty m the state of preservation of the ice-marks and. the kn.own activity of denu~ dation as a destroying agent, forbid the Idea that they bclono to widely separated epochs. It has, indeed, been suggested, CHAP. XXl"V.] SUMMARY AND CONCLUSION. 505 that denudation alone has lowered these mountains so much durin~ the ~uarternary epoch, that they were previously of suffiCI~nt he1ght to account for the glaciation of all of them, but this hardly needs refutation; for it is clear that denudation could not at the same time have removed some thousands of feet of roc~ from many hundreds of square miles of lofty snow-collectmg plateaus, and yet have left moraines, and blocks, and even glacial strire, undisturbed and uneffaced on the slopes and in the valleys of these same mountains. The theory of geological climates set forth in this volume w bile founded on Mr. Croli' s researches, differs from all that I~ ave !et been made public, _in clearly tracing out the comparative Influence of geographical and astronomical revolutions showing that, while the former have been the chief, if not th~ exclusive, causes of the long-continued mild climates of the Arctic regions, the concurrence of the latter has been essential to the production of glacial epochs in the temperate zones, as well as of those local glaci~tions in low latitudes, of w.hich there is such an abundance of evidence. The next question discussed is that of geological time as bearing on the development of the organic world. The periods of time usually demanded by geologists have been very great, and it was often assumed that there was no occasion to limit them. But the theory of development demands far more ; for the earliest fossiliferous rocks prove the existence of many and varied forms of life which require unrecorded ages for their development-ages probably far longer than those which have elapsed from that period to the present day. The physicists, however, deny that any such indefinitely long periods are available. The sun is ever losing heat far more rapidly than it can be renewed from any known or conceivable source. The earth is a cooling body, and must once have been too hot to support life; while the friction of the tides is checking the earth's rotation, and this cannot have gone on indefinitely without making our day much longer than it is. A limit is therefore placed to the age of the habitable earth, and it has been thought that the time so allowed is not sufficient for the long processes of geological change and organic development. It is therefore |
