CHAP. XII.] GEOGRAPHICAL DISTRIBUTION. 297 



mountains, grassy plains, forests, marshes, lakes, and great rivers, 

 under almost every temperature. There is hardly a climate or 

 condition in the Old World which cannot be paralleled in the 

 New at least as closely as the same species generally require. 

 No doubt small areas can be pointed out in the Old World hotter 

 than any in the New World ; but these are not inhabited by a 

 fauna different from that of the surrounding districts; for it is 

 rare to find a group of organisms confined to a small area, of 

 which the conditions are peculiar in only a slight degree. Not- 

 withstanding this general parallelism in the conditions of the 

 Old and New Worlds, how widely different are their living 

 productions ! 



In the southern hemisphere, if we compare large tracts of land 

 in Australia, South Africa, and western South America, between 

 latitudes 25 and 35, we shall find parts extremely similar in all 

 their conditions, yet it would not be possible to point out three 

 faunas and floras more utterly dissimilar. Or, again, we may 

 compare the productions of South America south of lat. 35 with 

 those north of 25, which consequently are separated by a space 

 of ten degrees of latitude, and are exposed to considerably different 

 conditions ; yet they are incomparably more closely related to 

 each other than they are to the productions of Australia or Africa 

 under nearly the same climate. Analogous facts could be given 

 with respect to the inhabitants of the sea. 



A second great fact which strikes us in our general review is, 

 that barriers of any kind, or obstacles to free migration, are 

 related in a close and important manner to the differences between 

 the productions of various regions. We see this in the great 

 difference in nearly all the terrestrial productions of the New 

 and Old Worlds, excepting in the northern parts, where the land 

 almost joins, and where, under a slightly different climate, there 

 might have been free migration for the northern temperate forms, 

 as there now is for the strictly arctic productions. We see the 

 same fact in the great difference between the inhabitants of 

 Australia. Africa, and South America under the same latitude ; 

 for these countries are almost as much isolated from each other 

 as is possible. On each continent, also, we see the same fact ; 

 for on the opposite sides of lofty and continuous mountain- 

 ranges, of great deserts and even of large rivers, we find different 

 productions ; though as mountain-chains, deserts, &c., are not as 

 impassable, or likely to have endured so long, as the oceans 

 separating continents, the differences are very inferior in degree 

 to those characteristic of distinct continents. 



Turning to the sea, we find the same law. The marine inhabi- 

 tants of the eastern and western shores of South America are 

 very distinct, with extremely few shells, Crustacea, or echino-- 



