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Show 166 EFFECT OF CIIANGES IN PIH'SICAL GEOGRAPHY [Ch.X. f · nion between an inland in the thirteenth century, ormmg an u lake and the ocean, and opening, in the course of one century, a shallow strait more than half as wide as the narrowest part of that which divides England from France. It will almost seem superfluous, after .W.e have,.t~us tr~ced the important modifications in the conditwn of nvmg bem.gs which flow from changes of trifling extent, t~ argue that en~Ire revo1 u t1. 0ns nn.g 1 1 t be brought about ' if the chmate and pl1.y s1cal geography of the whole globe were greatly ~ltered. Species we have stated are, in general, local, some bemg confined to extremely small spots, and depending for their existe~ce on a combination of causes which, if they are to be met w1th elsewhere, occur only in some very remote region. Hence it must happ:n that when the nature of these localities is changed the species will perish ; for it will rarely hap~en that the cau~~ ~hich alters the character of the district w1ll afford new facihttes to the species to establish itself elsewhere. If we attL·ibute the origin of a great part of the desert of Africa to the o-radual progress of moving sands, driven eastward by the :esterly winds, we may safely infer that a variety of species must have been annihilated by ~his c~use alon~. The sand-flood bas been inundating, from lime Immemorial, the rich lands on the west of the Nile, and we have only to multiply this effect a sufficient numbet· of times, in order to und~rstand how, in the lapse of ages, a whole group of terrestrial animals and plants may become extinct. This desert, without including Bornou and Darfour, extends, according to the calculation of Humboldt, over one hundred and ninety-four thousand square leagues, an area far more than double that of the Mediterranean, which occupies only seventynine thousand eight hundred square leagues. In a small por· tion of so vast a space, we may infer, from analogy, that there were many peculiar species of plants and ani~al~ whi~h must have been banished by the sand, and their hab1tatwns mvad~d by the camel and by birds and insects formed for the arid sands. Ch.X.J ON THE DISTRIBUTION OF SPECIES. 167 There is evidently nothing in the nature of the catastrophe to favour the escape of the former inhabitants to some adjoining province; nothing to weaken, in the bordering lands, that powerful barrier against emigration-pre-occupancy. Nor, even if the exclusion of a certain group of species from a given tract were compensated by an extension of their range over a new country, would that circumstance tend to the conservation of species in general; for the extirpation would merely then be transferred to the region so invaded. If it be imagined, for example, that the aboriginal quadrupeds, birds, and other animals of Africa emigrated in consequence of the advance of drift-sand, and colonized Arabia, the indigenous Arabian species must have given way before them, and have been reduced in number or destroyed. Let us next suppose that, in some central and more elevated parts of the great African desert, the upheaving power of earthquakes should be exerted throughout an immense series of ages, accompanied, at certain intervals, by volcanic erup .. tions such ' as gave rise at once, in 1755, to a mountain one thousand seven hundred feet high, on the Mexican plateau. 'Vhen the continued repetition of these events had caused a mountain-chain, it is obvious that a complete transformation in the state of the climate would be brought about throughout a vast area. We will imagine the summits of the new chain to rise so as to be covered, like Mount Atlas, for several thousand feet, with snow during a great part of the year. The melting of these snows, during the greatest heat, would cause the rivers to swell in the season when the greatest drought now prevails; the waters, moreover, derived from this source, would always be of lower temperature than the surrounding atmosphere, and would thus contribute to cool the climate. During the nu~ merous earthquakes and volcanic eruptions which would attend the gradual formation of the chain, there would be many floods, caused by the bursting of temporary lakes and by the melting of snows by lava. These inundations would deposit alluvial |