CuAP. XI. GEOGRAPHICAL DISTRIBUTION. 323 



land in Australia, South Africa, and Western South America, 

 between latitudes 20"^ and 35°, we shall find parts extremely 

 similar in all their conditions, yet it would not he possible to 

 point out three faunas and floras more utterly dissimilar. Or 

 again, we may compare the productions of South America 

 south of hit, 35° with those north of 25°, which consequently 

 are separated by a space of ten deg'rces of latitude and are ex- 

 posed to considerably different conditions, yet they are incom- 

 I)arably more closely related to each other than they arc to the 

 productions of Australia or Africa under nearly the same cli- 

 mate. Analog'ous 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 be- 

 tween the productions of various regions. We sec this in the 

 great difference of nearly all the terrestrial productions of the 

 New and Old Worlds, excepting in the northern parts, Avhere 

 the land almost joins, and where, under a slightly-different cli- 

 mate, there might have been free mij^Tation for the northern 

 temperate forms, as there now is for the strictly arctic produc- 

 tions. We see the same fact in the great difference betwefMi 

 the inhabitants of Australia, Africa, and South America, under 

 the same latitude : for these countries are almost as much iso- 

 lated 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, etc., are not as impassable, or likely to have 

 endured so long, as the oceans separating continents, the differ- 

 ences are very inferior in degree to those characteristic of dis- 

 tinct continents. 



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

 habitants of the eastern and western shores of South America 

 arc very distinct, with extremely few fishes, shell, or crabs, in 

 common ; but Dr. GUnther has recently shown that on opjio- 

 site sides of the Isthmus of Panama, about thirty jier cent, of 

 the fishes are the same ; and this fact has led naturalists to be- 

 lieve that the istlnnus was formerly open. Westward of the 

 shores of America, a wide space of open ocean extends, with 

 not an island as a hahing-place for emigrants ; here we have a 

 barrier of another kind, and as soon as this is passed we meet 

 in the eastern islands of the Pacilic with another and totally 



