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Show 322 GEOGRAPlliCAL DISTRIBUTION. [CHAP. XI. foregoing remarks on distribution apply not only to strictly arctic forms but also to many sub-arctic and to some few northern te~perate forms, for some of these. are the same on the lower 1nountai.ns and on the pla1ns of North A1nerica and Europe ; and it may be reasonably asked 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 p~esent day, the sup-arctic and northern temperate productions of the Old and New Worlds are separated fron1 each other by the Atlantic Ocean and by the extreme northern part of the Pacific. During the Glacial period, when the inhabitants of the Old and New \V orlds lived further southwards than at present, they 1nust have been, still more completely separated by wider spaces of ocean. I believe the above difficulty n1ay be surmounted by looking to still earlier changes of climate of an opposite nature. We have good reason to believe that during the newer Pliocene period, before the Glacial epoch, and whilst the majority of the inhabitants of the world were specifically the same as no'v, the climate was warmer than at the present day. l-Ienee we may suppose that the organisms now living under the climate of latitude 60°, auring the Pliocene period lived 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 Polar Circle there is almost continuous land from western Europe, through Siberia, to eastern An1erica. _And to this continuity of the circumpolar land, and to the consequent freedom for inter-migration under a more favourable clhnate, I attribute the necessary amount of uniformity in the sub-arctic and northern temperate productions of the Old and New W or1ds, at a period anterior to the Glacial epoch. Believing, from reasons before alluded to, that ~ur continents have long remained in nearly the same relat1ve position, though subjected to large, but partial oscillat~ons of level, I am strongly inclined to extend the above v1ew, and to infer that during some earlier and still warmer pe- CHAP. XI.] DURING THE GLACIAL PERIOD . . 328 nod, such as the older PI' h 1ocene per' d t e same plants and animals 'nh .Io ' a 1a rge number of ous c~rcumpolar land ; and that ~tlted the almost continuboth In the Old and New W ld ooe plants and animals southwards as the climate b or s, ~egan slowly to migrat~ the commencement of the Gyan;el ess warm, long before as I believe, their descendant ama pler~od. We now see d1't1' 0n, I·n the central parts sf mEo st yIn a mo dI'f i ed con-' States. On this view we can o urope and the United with very little identity b ~nderstand the relationship North .America and E uro 'e e ween . the productions of remarkable, considering the'~~ relationship which is most their separation by the .Atla 1~. aoe of the two areas, and understand the singular fact~~~ ~an. We can further ser':ers, that the productions ofaiE ed on by several obduring the later tertiar sta urope and .America to each other than they :re af:h. were mor~ closely related these warmer periods the no ·the present tinle ; for during New Worlds will have be I 1 ern parts ?f the Old and by land, serving as a brid en a ;nost continuously united by cold~ for the inter-migraa~~s~fc~h r~n~ehred. impassable Durmg the slowly deere . mr In abitants. period, as soon as the specie:s.Ing warmth of the Pliocene the New and Old World I~ common, which inhabited Circle, they must have be:' migraied south of the Polar other. This separation as~ comp hetely cut off from each ductions are concerned' tool~r Is t elmore temperate proas the plants and anim'als . p ace ong ages ago. And have become mingled in thigrated southward, they wHl n~tive .American production: ~n~ ~eat region with the With them; and in the th ' n av~ had to compete the Old World. Conse :e:~l great region, with those of f~vourable for much moaifi t~ we have here every-thing !ion .than with the AI in c~ ~on,-. for far more modifi.cam a much more rece~t ee~ro uctions, left isolated, with-r~ s and on the arctic fand~d~t~ ~e seWveral mountain- 1 uas come, that when e wo . orlds. Hence W ductions of the temper:: ~~n;pare fthe now living proorlds, we find ver £ . gio?s o the New and 0 ld Gray has lately show~ th:t Identical! species. (though Asa more p ants are Identical than |