374 GEOGRAPHICAL DISTRIBUTION. 



general parallelism in the conditions of Old and New 

 Worlds, liow 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 degrees, 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 latitude 35 degrees 

 with those north of 25 degrees, 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 ns 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 pro- 

 ductions. 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 op- 

 posite sides of lofty and continuous mountain-ranges, of 

 great deserts and even of large rivers, we find different 

 productions; though as mountain-chains, deserts, etc., are 

 not as impassable, or likely to have endured so long, as the 

 oceans separating continents, the differences are very in- 

 ferior in degree to those characteristic of distinct con- 

 tinents. 



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

 inhabitants of the eastern and western shores of South 

 America are very distinct, v»"ith extremely few shells, Crus- 

 tacea, or echinodermata in common; but Dr. Giinther has 



