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Show 302 GEOGRAPHICAL DISTRIBUTION. f0HA.P. XI. CHAPTER XI. GEOGRAPHICAL DISTRIBUTION. Present distribution cannot be accounted for by differences in physical c?nditiona-Importance of barriers-Affinity of the productions of the same contment-Centres of creation-Means of dispersal, by changes of climate a_nd of ~he level of ~he land, and by occasional means-Dispersal during the Glacial penod co-extensive with the world. IN considering the distribution of organic beings over the face of the globe, the first great fac~ ~hi?h ~trikes us .is, that neither the similarity nor the dissimilanty of the n;habitants of various reO'ions can be accounted for by thmr climatal and other physical conditio~s. Of late, almo~t every author who has studied th~ subJect has come to this conclusion. The case of America alone would almost suffice to prove its truth: for if we ~xclude the n?rthern parts where the circumpolar land IS almost continu~u~, all authors agree t~at on~ o~ th~ mo.st fundamental diVIsions in geograp~Ical distnbution lS that between the New and Old Worlds ; yet if we travel over the ;ast American continent from the central parts of the United States to its extre~e southern point, we. me~t ~ith t~e most diversified conditions ; the most hu1nid d1stncts, arid deserts lofty mountains, grassy plains, forests, marshes, lakes, ~nd great rivers, u~der alrnost e\e:y t~n1perature. There is hardly a chmate or condition 1n the Old World which cannot be paralleled in the N.ew-at l.ea~t as closely as the same species generally require ; for 1t IS a most rare case to find a group of organisms confine~ to any small spot, having conditions _peculiar in only a shght degree· for instance small areas 1n the Old World could be poidted out hott~r than any in the New World, yet 0BAP. XI.] GEOGRAPHICAL DISTRIBUTION. 303 these are not inhabited by a peculiar fauna or flora. Notwithstanding this paralleli~m in t~e conditions of the Old and New Worlds, how Widely different are their living pro d uctl.o ns .' . In the southern hemisphere, if we co1npare large tracts of land in Australia, South Africa, and western South America, between latitudes 25° and 35°, we shall find parts extremely similar in all their conditions, yet it would not be possible to point out three faunas and floras more utterly dissimilar. Or again we may compare the productions of South America south of lat. 35° with those north of 25°, which consequently inhabit a considerably different climate, and they will be found incomparably more closely related to each other, than they are to the productions of Australia or Africa under nearly the same climate. Analogous facts could be given with respect to the inhabHants of the sea. A second great fact which strikes us in our general review is, that barriers of any kind, or obstacles to free mjgration, are related in a close and important manner to the differences between the productions of various regions. We see this in the great difference of nearly all the terrestdal productions of the New and Old Worlds, excepting in the northern parts, where the land almost joins and where, under a slightly different climate, there might bave been free migration for the northern temperate forms as there now is for the strictly arctic productions. We' see the same fact in the great difference between the inhabitants of Australia, Africa, and South America under the same latitude: for these countries are almost as much isolated from each other as is possible. On each continent, also, we see the same fact ; for on the opposite sides of lofty and c.ontinuous mountain~ranges, and of great deserts, and ~ometimes even of large rivers, we £.nd different productiOn~ ; though as mountain-chains, deserts, &c., are not as Impassable, or likely to have endured so long as ~he ~cea?-s separating continents, the differences are verr 1nfer10r 1n degree to those characteristic of distinct continents. Turning to the sea, we £.nd the same law. No two |
