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Show 368 GEOGRAPHICAL DISTRIBUTION, CHAP. XI. and those of the Pyrenees, as remarked by Ramond, are more especially allied to the plants of northern Scandinavia · those of the United States to Labrador; those of the m' ountains of Siberia to the arctic regions of that country. These views, grounded as they are on the perfectly well-ascertained occurrence of a former Glarial period, seem to me to explain in so satisfactory a manner the present distribution of the Alpine and A.rctic productions of Europe and America, that when in other regions we find the same species on distant mountain- summits, we may almost conclude without other evidence, that a colder climate permitted their forn1er migration across the low intervening tracts, since become too warm for their existence. If the climate, since the Glacial period, has ever been in any degree warmer than at present (as some geologists in the United States believe to have been the case, chiefly from the distribution of the fossil Gnathodon), then the arctic and temperate productions will at a very late period have marched a little further north, and subsequently have retreated to their present homes; but I have met with no satisfactory evidence with respect to this intercalated slightly warmer period, since the Glacial period. The arctic forms, during their long southern migra-tion and re-migration northward, will have been exposed to nearly the same climate, and, as is especially to be noticed, they will have kept in a body together; consequently their mutual relations will not have been much disturbed, and, in accordance with the principles inculcated in this volume, they will not have been liable to much modification. But with our Alpine productions, left isolated from the moment of the returning warmth, first at the bases and ultimately on the summits of the m.ountains, the case will have been somewhat dif- CHAP. XI. DURING THE GLACIAL PERIOD. 369 fe· rent';1 1 fho r it is not likely that all the sam e arc t1' c spe-cies WI ave been left on mountain ranges distant from each other, and have survived there ever since. th will, also, in all probability have become min()'l a' 'etly · AI' be Wil anment p. ine species ' which must have ex1·s t e d on the mountains ?efore the commencement of the Glacial epoch, and whiCh during its coldest period will h r b een t emporar1'1 y drI' ven down to the plains; they wai'lel, ~lso, have been ~xposed to somewhat different climatal ~nfluences. Thei~· mutual relations will thus have been In some degree disturbed; consequently they will h b 1. bl · . ave een Ia e to ~odification; and this we find has been the ca~e; for If we compare the present Alpine plants and_ animals of the several great European mountainranges, though very many of the species are identically the same, some present varieties, some are ranked as do~btful forms, and some few are distinct yet closely alhed or representative species. I~ illustrating what, as I believe, actually took place during the Glacial period, I assumed that at its commencement the arctic productions were as uniform round the polar regions as they are at the present day But the foregoing remarks on distribution apply not only to strictly arctic forms, but also to many sub-arctic and to some few northern temperate forms, for some of the~e are the same on the lower mountains and on the plains of North Anlerica and Europe; and it may be reasonably. ask~d how I account for the necessary degree of uniformity of the sub-arctic and northern temperate forms round the world, at the commencement of the Glacial period. At the present day, the sub-arctic and northern temperate productions of the Old and ~ e 1 w ~ orlds are separated from each other by the t antiC. Ocean and by the extreme northern part of the Pacific. During the Glacial period, when the in-R 3 |
