OCR Text |
Show 370 GEOGRAPHICAL DISTRIBUTION, CHAP. XT. habitants of the Old and New Worlds lived furth~r southwards than at present, they must have been still more completely separated by wider spaces of ocean. I believe the above difficulty may be surmounted ?Y looking to still earlier changes of clim~te of an oppo~Ite nature. We have good reason to beheve th~t dunng the newer Pliocene period, before the Glamal epoch, and whilst the majority of the inhabitants of. the world were specifically the same as now, the chmate was warmer than at the present day. Hence we may suppose that the organisms no~ living u~der _the climate of latitude 60°, during the Phocene penod hved further north under the Polar Circle, in latitude 66°-67° ; and that the strictly arctic productions then lived on the broken land still nearer to the pole. Now if we look at a globe, we shall see that under the ~olar Circle there is almost continuous land from western Europe, through Siberia, to eastern America. And to this continuity of the circumpolar land, and to the consequent ~reedom for intermigration under a more favou:able. ch:n:-ate, I attribute the necessary amount of uniformity In the sub-arctic and northern temperate productions of the Old and New Worlds, at a period anterior to the Glacial epoch. Believing, from reasons before alluded to, that our continents have long remained in nearly the same re~ative position, though subjected to _lar~e, but partial oscillations of level, I am strongly 1nchned to exte~d the above view, and to infer that during some ~arlier and still warmer period, such as the older Ph~cene period, a large number of the same plants and arumals inhabited the almost continuous circumpolar land; and that these plants and animals, both in the Old and New Worlds, began slowly to migrate southwards as the climate became less warm, long before the com- CHAP. XI. DURING THE GLACIAL PERIOD. H71 mencement of the Glacial period. We now see as I believe, their descendants, mostly in a modified' condition, in the central parts of Europe and the United States. On this view we can understand the relationship, with very little identity, between the productions of North America and Europe,-a relationship which is most remarkable, considering the distance of the two areas, and their separation by the Atlantic Ocean. vVe can further understand the singular fact remarked on by several observers, that the productions of Europe and America during the later tertiary stages were more closely related to each other than they are at the present time ; for during these warmer periods the northern parts of the Old and New Worlds vvill have been almost continuously united by land, serving as a bridge, since rendered impassable by cold, for the inter-migration of their inha bi tan ts. During the slowly decreasing warmth of the Pliocene period, as soon as the species in common, which inhabited the New and Old Worlds, migrated south of the Polar Circle, they must have been completely cut off from each other. This separation, as far as the more temperate productions are concerned, took place long ages ago. And as the plants and animals migrated southward, they will hav_e become mingled in the one great region with the n~tlve American productions, and have had to compete With them; and in the other great region, with those of. the Old World. Consequently we have here everything favourable for much modification -for far more ~odi:fication than with the Alpine p;oductions, left Isolated, within a much more recent period, on the several mountain-ranges and on the arctic lands of the two Worl~s.. Hence it has come, that when we compare the now hv1ng productions of the temperate regions of the New and Old Worlds, we find very few identical |