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Ch. XLII.] VICISSITUDES IN THE E.IRTH'S SUEFACE. 



445 



tlie J previously occupied. If, therefore, the Author of Nature 

 had not been prodigal of those 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 



r 



even though, it were never intended that a species 



animate and inanimate creation should be in perfect harmony 

 with each other, it is evident that considerable spaces, now^ 

 the most habitable on the globe, would soon be as devoid 

 of life as are the Alpine snows, the dark abysses of the 

 ocean, or the moving sands and salt plains of the Sahara. 



The powers, then, of migration and diffusion, conferred, as 

 already shown, on animals and plants, are indispensable to 

 enable them to maintain their ground, and would be necessary, 



should 



gradually extend its geographical range. But a facility of 

 shifting their quarters being once given, it cannot fail to 

 happen that the inhabitants of one province should occasion- 

 ally penetrate into some other ; since the strongest of those 

 barriers which I before described as separating distinct regions 

 are all liable to be thrown down, one after the other, during 

 the vicissitudes of the earth's surface. 



We have seen in the Twelfth Chapter"^ how vast a suc- 

 cession of changes in the physical geography of the globe has 

 been revealed to us by geology. Although these changes are 

 incessant they proceed at so slow a rate that mankind at large 

 are wholly unconscious of their reality. It would not be easy 

 for the naturalist to take account of the advantage which one 

 species may gain over another in the course of a few centuries, 

 even at those points on the borders of two distinct provinces 



At such 



where the struggle for existence is most 



points the rate of change must 



outstrip the 



average 



pace at which it proceeds in the organic world generally. 

 If the ocean should gradually wear its way throuo-h an 

 isthmus, like that of Suez, it would open a passage for the 



intermixture 



Mediter 



ranean and Eed Sea) previously disjoined, and would, at the 



communication 



animals 



before enjoyed. 



^ Vol. I. p. 248. 



