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Show 36G GEOGRAPHICAL DISTHIBUTION, CnAP. XI. in this same belief, had not Agassiz an~ others called vivid attention to the Glacial period, whiCh, as we shall immediately see, affords a simple explanation of .these facts. 'Ve have evidence of almost every conceivable kind, organic and inorganic, that within a very rec~nt geoloaical period central Europe and North Amenca suffer~d under an' Arctic climate. The ruins of a house burnt by fire do not tell their tale m~re plai~ly, tha_n do the mountains of Scotland and V\ ales, WIth theu scored f1 anks, polished surfaces, and perched boulders, of the icy streams with which their valleys were lately filled. So greatly has the climate of Europe changed, that in Northern Italy, gigantic moraines, left by old glaciers, are now clothed by the vine and maize. Through .. out a large part of the United States, erratic boul~ers, and rocks scored by drifted icebergs and coast-ice, plainly reveal a former cold period. The former influence of the glacial cliinate on the distribution of the inhabitants of Euro]_Je, as explained with remarkable clearness by Edward Forbes, is substantially as follows. But we shall follow the changes more readily, by supposing a new glacial period to come slowly on, and then pass away, as formerly occurred. As the cold came on, and as each more southern zone became fitted~ for arctic beings and ill-fitted for their former more temperate inhabitants, the latter wonld be supplanted and arctic productions would take their places. The inhabitants of the more temperate regions would at the same time travel southward, unless they were stopped by barriers, in which case they would perish. The mountains would become covered with snow and ice, and their former Alpine inhabitants would descend to the plains. By the time that the cold had reached its maximum, we should have a uniform arctic fauna and flora, covering the central parts of Europe, as far CHAP. XI. DURING THE GLACIAl.~ PERIOD. 367 ~ou th'1 as. the Alps and Pyrenees, and even stretching Into Spain. The now temperate regions of the United St~tes would likewise be covered by arctic plants and animals, and these would be nearly the same with those of Europe; for the present circumpolar inhabitants which we suppose to have everywhere travelled south~ ward, are remarkably uniform round the world. We may suppose that the Glacial period came on a little earlier or later in North America than in Europe, so will the southern migration there have been a little earlier or later; but this will make no difference in the final result. As the warmth returned, the arctic forms would retreat northward, closely followed up in their retreat by the productions of the more temperate regions. And as the snow melted from the bases of the mountains, the arctic forms would seize on the cleared and thawed ground, always ascending higher and higher, as the warmth increased, whilst their brethren were pursuing their northern journey. Hence, when the warmth had fully returned, the same arctic species, which had lately lived in a body together on the lowlands of the Old and New Worlds, would be left isolated on distant mountainsum~ its (havin? been exterminated on all lesser heights) and In the arctiC regions of both hemispheres. Th~s we ca~ understand the identity of many plants at pmnts so Immensely remote as on the mountains of the United States and of Europe. We can thus also understand the fact that the Alpine plants of each mountain-range are more especially related to the arctic forms living due north or nearly due north of them: for the migration as the cold came on, and the re-migration on the returning warmth, will generally have been due south and north. The Alpine plants, for example, of Scotland, as remarked by Mr. If. C. Watson, |