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Show 168 EFFECT OF CHANGES IN PIIYSTCAL GEOGRAPHY (Ch. X. matter far an d WI' d e over tlle original sands at a. ll levels, . as the country assume d varw. us sh apes ' and was modrfied agam and agam. by t 11 e mov.m g po'~ er from below, and the aqueous erosw. n o f t h e sur1ca ce a b o ve · At lenOo 'th the S. ahara w. ould. b. e L! •.tern1 1• ze d , 1. rr1. ga t c d by rivers •a nd streamlets mtersectmg It Ill every d1. rect.w n, an d cove r·ed by J. unOo 'le and morasses, so th. at the am. ma1 s an d p 1a n t s which now people northern Afnca wou ld d1. sappear, an d the reoai on would gr. adually becom. e .f it.t ed .Lt!o r t 11 e recep tI' on o f a po· pulation . of specieS perfectly d1ss1miiar in their forms, habits, and organization. . . r.rhere are always some peculiar and cl~a~a.ctenstic features in the physical geography of each large .division of the glob~; and on these peculiarities the state of a~1mal and vegeta~le h~e 1· s d epenc1 e n t . If, therefore , we admit mcessant fluctuatiOns m the physical geography, we must,. at the same. time, .concede the successive extinction of terrestrial and aquatic species to be part of the economy of our system. WI:en some great class ?f stations is in excess in certain latitudes, as, for example, m wide savannahs, arid sands, lofty mountains, or inland seas, we find a cm·respondinO' development of species adapted for such circumstances. Inb North America, where there is a ch a'm of vast inland lakes of fresh-water, we find an extraordinary abundance and variety of aquatic birds, fresh-water fish, testacea, and small amphibious reptiles, fitted for such a climate. The greater part of these would perish if the lakes were destroyed,- an event that might be brought about by s.ome of the least of those important revolutions contemplated m geo· logy. It might happen that no fresh-water lakes of .con·esponding magnitude might then exist on the globe ; but If they occurred elsewhere, they might be situated in New Holland, Southern Africa, Eastern Asia, or some region so distant as to be quite inaccessible to the North American specie~; or. they might be situated within the tropics, in a climate umnhab1table by species fitted for a temperate zone; or, finally, we .may presume that they would be pre-occupied by indigenous tnbes. To pursue this train of reasoning farther is unnec~ssary; the Ch.X.] ON THE DISTRIBUTION OF SPECIES. 169 reader has only to reflect on what we have said of the habita· tions and the stations of organic beings in general, and to consider them in relation to those effects which we have con- . templated in om· first volume as resulting from the igneous and aqueous causes now in action, and he will immediately }Jerceive tbatJ amidst the vicissitudes of the earth's surface, species cannot be immortal, but must perish one after the other like the individuals which compose them. There is no pos: sibility of escaping from this conclusion, without resorting to some hypothesis as violent as that of Lamarck, who imagined, as we have before seen, that species are each of them endowed with indefinite powers of modifying their organization, in conformity to the endless changes of circumstances to which they are exposed. Some of the effects which must attend every general alteration of climate are sufficiently peculiar to claim a separate consideration before concluding the present chapter. We have before stated that, during seasons of extraordinary severityJ many northern birds, and, in some countries, many quadrupeds, migrate southwards. If these cold seasons were to become frequent, in consequence of a gradual and general refrigeration of the atmosphere, such migrations would be more and more regular, untilJ. at length, many animals, now confined to the arctic regions, would become the tenants of the temperate zone ; while the inhabitants of the latter would app~oach n~arer to the equator. At the same time, many species prevwusly established on high mountains, would begin to descend, in every latitttde, towards the middle reO'ions and 1 . b J t lose wh1eh were confined to the flanks of mountains would make their way into the plains. Analogous chanO'es would also take place in the vegetable kingdom. 0 . If, on the contrary, the heat of the atmosphere be on the mcr:ase, the plants and animals of low gmunds would ascend to higher levels, the equatorial species would migrate into the temperate zone, and those of the temperate into the arctic circle. But although some species might thus be preserved, every |