| OCR Text |
Show 372 GEOGRAPHICAL DISTRIBUTION, CHAP. XI. species (though Asa Gray has lately shown that more plants are identical than was formerly suppo~ed), but we :find in every great class many forms, which son:e naturalists rank as geographical races, and others as ellstinct species; and a host of closely allied or re~resentative forms which are ranked by all naturalists as specifically distinct. As on the land so in the waters of the sea, a slow southern migratio~ of a marine fauna,. which. during the Pliocene or even a somewhat earher penod, was nearly uniform along the continuous shores ?f th~ Polar Circle, will account, on the theory of modi:fica tion, for many closely allied forms now living in areas completely sundered. Thus, I think, we can understand the presence of many existing and tertiary representative forms on the eastern and western shores of temperate North America ; and the still more striking case of many closely allied crustaceans (as described in Dana's admirable work), of some :fish and other marine animals, in the Mediterranean and in the seas of J apan,-areas now separated by a continent and by nearly a hemisphere of equatorial ocean. These cases of relationship, without identity, of the inhabitants of seas now disjoined, and likewise of the past and present inhabitants of the temperate lands of North America and Europe, are inexplicable on the theory of creation. We cannot say that they have been created alike, in correspondence with the nearly similar physical conditions of the areas ; for if we compare, for instance, certain parts of South America with the southern continents of the Old World, we see countries closely corresponding in all their physical conditions ' but with their inhabitants utterly dissimi.l ar~ But we must return to our more immediate subJect, the Glacial period. I am convinced that Forbes's view CHAP. XI. DURING THE GLACIAL PERIOD. 373 may be largely extended. In Europe we have the plainest evidence of the cold period, from the western shores of Britain to the Oural range, and southward to the Pyrenees. We may infer, from the frozen mammals and nature of the mountain vegetation, that Siberia was similarly affected. Along the Himalaya, at points 900 miles apart, glaciers have left the marks of their former low descent; and in Sikkim, Dr. I-Iooker saw maize growing on gigantic ancient moraines. South of the equator, we have some direct evidence of former glacial action in New Zealand; and the same plants, found on widely separated mountains in this island, tell the same story. If one account which has been published can be trusted, we have direct evidence of glacial action in the south-eastern corner of Australia. Looking to America; in the northern half, ice-borne fragments of rock have been observed on the eastern side as far south as lat. 06°-37°, and on the shores of the Pacific, where the climate is now so different, as far south as lat. 46°; erratic boulders have also been notw. ed on the Rocky Mountains. In the ' Cord' illera of Equatorial South America, glaciers once extended far below their present level. In central Chile I was astonished at the structure of a vast mound of detritus, about 800 feet in height, crossing a valley of the Ande.s; and this I now feel convinced was a gigantic moraine, left far below any existing glacier. Further south on both sides of the continent, from lat. 41° to th~ southernmost extremity, we have the clearest evidence of former glacial action, in huge boulders transported far from their parent source. . vVe do not know that the Glacial epoch was strictly s~mul~aneous at these several far distant points on oppoSite sides of the world. But we have good evidence in almost every case, that the epoch was included within |
