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Show 164 EFFECT OF CHANGES IN PHYSICAL GEOGRAPHY [Ch. X. bl d the ocean for a supply sufficient to ]onO'er a e to raw on . r> • 'l'b · 't must sink and leave dry a certam restore Its equt 1 rwm, 1 • ' • . . f 1 d d 'ts borders The current wh1ch now portwn o an aroun 1 • . fl 1 t f the Black Sea into the :McdttetTanean ows constant y ou o . ld \ l · r"pidly anrl the level oft he Merllter-wou t 1en rus 1 m more " ' ld b tl reby l )revented from falling so low; but ranean wou e 1e . h 1 l f 1 B\ k Sea would for the same reason, swk, so t e eve o t 1e ac ' ' t l1 at w l1 en, b y a con t1'n ued series of elevatory movements, the S tt·a·t ts o f G'b Jt had become completely closed up, we 1 ra ar nn•g 1 1 t expec t 1a rge and level ~~ andy steppes to s.u rround both the Ruxine and Mediterranean, like those occurnng at present on the skirts of the Caspian, and the :sea of Aral. The geo· graphical range of hundreds of aquatic species would ~e thereby circumscribed, and that of hundreds of terrestt'Ial plants and animals extended. A line of submarine volcanos crossing the channel of some strait, and gradually choking it up with ashes an.d lava, might produce a new barrier as effectually as .a senes of ear~hquakes; especially if thermal springs,. plentl.fu11y ~barged w1th carbonate of lime, silica, and other mmeralmgrcd1ents, should promote the rapid multiplication of corals a~1~ she11s, ~nd cement them torrether with solid matter prectpttated durmg 0 • • the intervals between eruptions. Suppose m this manner a stoppage to be caused of the Bahama Channel between the bank of that name and the coast of Florida. This insignificant revolution, confined to a mere spot in the bottom of the ocean, would, by diverting the main current of the Gulf-stream, give rise more effectua1ly than the opening of the Straits of Panama before supposed, to extensive changes in the climate and distribution of animals and plants inhabiting the northern hemisphere. · . A repetition of elevatory movements of earthquakes rmght continue over an area as extensive as Europe, for thousands Of aO'eS at the bottom of the ocean in certain regions, and b ' • produce no visible effects ; whereas, if they should. operate m some shallow parts of the Pacific, amid the coral archipelagos, Ch. X.] ON THE DISTRIBUTION OF SPECIES. 165 they would soon give birth to a new continent. Hundreds o~ volcanic .islands may be thrown up and become covered with vegetatwn, without causinrr more than local :fluctuations in the animate world; but if a cl1ain like the Aleutian archipelago or the Kurile isles, run for a distance of many hundred miles, so as to form an almost uninterrupted communication between two continents, or two distant islands, the migrations of pla.nts, birds, insects, and even of some quadrupeds, may cause m a short time an extraordinary series of revolutions tending to augment the range of some animals and plants an~ to limit that of others. A new archipelacro micrht be fo;med • t> t> In the Mediterranean, the Bay of Biscay, and a thousand other localities, and might produce less important events than one rock which should rise up between Australia and J ava• so placed that winds and currents might cause an interchange of the plants, insects, and birds,. of the latter countries. If we turn from the igneous to the aqueous agents, we find the same t~ndency to an irregular rate of change, naturally connected With the strictest uniformity in the energy of those causes. W~en the sea, for example, gradually encroaches upon both sides of a narrow isthmus, as that of Sleswick separating the North Sea from the Baltic, where, as we stated: the cl~ff's on b~th the opposite coasts are wasting away*, no mater~al alteratwn ~·esults for thousands of years, save only that there 13 a progressive conversion of a small strip of land into water. A few feet only, or a few yards, are annually removed; ~ut when at last the partition shall be broken down, and the tides of the ocean shall enter by a direct passage into the inland sea, instead of going by a circuitous route through the Cattegat, a body of salt-water will sweep up as far as the Gulfs. of Bothnia and Finland, the waters of which are now brackish, or almost fresh ; and this revolution will be attended by t.he.local annihilation of many species. Similar consequences must have resulted on a small scale wh en th e sea opened its way through the is't hmus of Staveren' * Vol. i. p. 289. |