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Show 160 EFFECT OF CHANGES IN PHYSICAL GEOGRAPHY (Ch. X. h . 1 B t a facility of shifting their quarters grap 1ca range. u · h b' be·m a once g1· ven, 1· t canna t fail to happen that. them a ltants f 0 o one provm• ce s h ou ld occasionally penetrate mto some ot.h er, s.m ce t h e stronges t of th os e barriers w.h ich we before descnbed as separatm. g d1' s tm' e t I.e g ions , are all hable to be thrown down one a f ter t h e o th er., during the vicissitudes of the earth's surface. . . The numbers and distribution of particular species are affected in two ways, by changes in the physical geograp~y of the earth. First, these changes promote or retard the migra- t1. 0ns of speci·e s ; se condlJv , they alter the physical conditions of the localities which species inhabit. If the ocean should gradually wear its way through an isthmus, like that of Suez, it would open a passage for the intermixture of the aquatic tribes of two seas previously disjoined, and would, at the same time, close a free communication which the te:restrial plants and animals of two continents had before enJoyed. These would be, perhaps, the most important consequences in regard to the distribution of species, which would result from the breach made bv the sea in such a spot ; but there would be others of a distinct nature, such as the conversion of a certain tract of land which formed the isthmus into sea. This space previously occupied by terrestrial plants and animals would.be immediately delivered over to the aquatic, a local revolutiOn which miO'ht have happened in innumerable other parts of ~he globe, wi~hout being attended by any alteration in the blendmg together of species of two distinct provinces. This observation leads us to point out one of the most interesting conclusions to which we are led by t~e conte~plation of the vicissitudes of the inanimate world m relatiOn to those of the animate. It is clear that if the agency of inorO'anic causes be uniform as we have supposed, they must ope- ra0t e very I• rregularly on the state o f orgam·c b ei'n Oo'S ' so that .t he rate according to which these will change in particular regwns will not be equal in equal periods of time. We are not about to advocate the doctrine of general catas· Ch.X.J ON THE DISTRIDUTION OF SPECIES. 161 trophes re.c urring at certain intervals ' as in the ancient oriental cosmogomes, nor do we doubt that if very considerable periods of equal duration could be taken into our consideration and compared one with another, the rate of chanO'e in the livinO' as well · tl · · 0 0 . as m 1e morgamc world, would be nearly uniform;' but If we regard each of the causes separately, which we know to be at present the most instrumental in remodelling the state of ~he surface, we shall find that we must expect each to be in actwn :or t~ousands of years, without producing any extensive alt~ratwns m tl~e habitable surface, and then to give rise, durmg a very bnef period, to important revolutions. We shall illustrate . this principle by a few of the most remarkable examples whbh present themselves. In the course of the last century, as we have before pointed out, a considerable number of instances are recorded of the solid surface, whether covered by water or not, having been permanently sunk or upraised by the power of earthquakes. Most of these convulsions are only accompanied by temporary :fluctuations in the state of limited districts, and a continued repetition of the~e. events fo~ thousands of years might not produce any declSlve change m the state of many of those great zooloO'ical or botanical provinces of which we have sketched the boundaries. When, for example, large parts of the ocean and even of inland seas are a thousand fathoms or upwards in depth it is a matter of no moment to the animate crention that vast tracts should be heaved up many fathoms at certain intervals or should .subside to the same amount. Neither can any mat:rial re.volutwn be produced in South America either in the terrestrial or the marine plants or animals by a series of shocks on the coast of Chili, each of which, like that of Penco in 1750 should uplift the coast about twenty-five feet. ~or if th; ground sinks fifty feet at a time, as in the harbour of Port Royal, in Jamaica, in 169~, will such alterations of level work ~ny general fluctuations in the state of organic beinO's inhabitIng the West India islands, or the Caribbean Sea 0 It is only when these subterranean powers • by shiftinsr Vor,, II. ' .... M |