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Show CHAPTER X. I fl. f · · en ses 1'n changing the 'habitations of spcecis-Powers n ueuco o morgamc .. u • . . . of diffusion indispensable, that each species may mamtam Its g~ound-How changes in tho physical geography affect . the distribution of spec1e~-Rnte of the change of species cannot be uniform, however regular the action of the inorganic causes-Illustration derived from subsiden:es by earth~uakes-from the elevation of land by the same-from the formation of new 1sland!l-from the wearing through of an isthmus-Each change in the phyllical geography of large regions must occasion the extinction of ~pecies-Eff~cts o~ a general alteration of climate on the migration of species-Gradual refngerahon causes species in the northern and southern hemispheres to become distinct-Elevation of temperature the reverse-Effects in the distribution of species which must result from vicissitudes in climate inconsistent with the theory of transmutation. HAviNG shown in the last chapter how considerably the numerical increase or the extension of the geographical range of any one species must derange the numbers and distribution of others, let us now direct our attention to the influence which the inorganic causes described in our first volume are continually exerting on the habitations of species. So great is the ins~ability of the earth's surface, that if Nature were not continually engaged in the task of sowing seeds and colonizing animals, the depopulation of a certain portion of the habitable sea and land would in a few years be considerable. Whenever a river transports sediment into a lake or sea, the aquatic animals and plants which delight in deep water are expelled : the tract, however, is not allowed to remain useless, but is soon peopled by species which require more light and heat, and thrive where the water is shallow. Every addition made to the land by the encroachment of t~e delta of a river banishes many subaqueous species from their native abodes; but the new-formed plain is not permitted to lie unoccupied, being instantly covered with terrestrial vege· tation. The ocean devours continuous lines of sea-coast, and Ch.X.] POWERS OF MIGRATION INDISPENSABLE, 1~9 precipitates forests or rich pasture-land into the waves· but this space is not lost to the animate creation, for shells and seaweed soon adhere to the new-made cliffs, and numerous fish people the channel which the current has scooped out for itself. No sooner has a volcanic isle been thrown up than some lichens begin to grow upon it, and it is sometimes clothed with v-erdure while smoke and ashes are still occasionally thrown from th; crater. The cocoa, pandanus, and mangrove take root upon the coral reef before it has fairly risen above the waves. The burning stream of lava that descends from Etna rolls throuO'h the. stately f~re~t, and converts to ashes every tree and he~b whiCh .stand m 1ts way ; but the black strip of land thus desolated, Is covered again, in the course of time, with oaks, pines, and chestnuts, as luxuriant as those which the fiery torrent swept away. ·" Every flood and landslip, every wave which a hurricane or earthquake throws upon the shore, every shower of volcanic dust an~ ashes which buries a country far and wide to the depth ~f many feet, ever~ advance of the sand-flood, every conversiOn of salt-water mto fresh when rivers alter their main channel of discharge, every permanent variation in the rise or fall of tides in an estuary-these and countless other causes ~isplace in th~ course of a few centuries certain plants and ammals from statiOns which they previously occupied. If, therefore, the Author of Nature had not been prodiO'al of those • b numerous contrivances before alluded to, for spreading all classes of organic beings over the earth-if he had not ordained that the fluctuations of the animate and inanimate creation shou.ld be in perfect harmony with each other, it is evident that considerable spa~es, now the most habitable on the globe, would soon be as devmd of life as are the Alpine snows, or the dark abysses of the ocean, or the movinO' sands of the Sahara TI · · b • . 1e powers then of m1grat1on and diffusion conferred on a~Imals and plants, are indispensable to enable them to maintam tl~eir ground, and would be necessary even though it were never mtended that a species should gradually extend its geo. |