396 ORIGIN OF SPECIES 



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 Amer- 

 ica south of lat. 35° with those north of 25°, which conse- 

 quently 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 differ- 

 ences between the productions of various regions. We see 

 this in the great differences in nearly all the terrestrial pro- 

 ductions 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 mi- 

 gration 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 moun- 

 tain-ranges, of great deserts and even of large rivers, we 

 find different productions; though as mountain-chains, des- 

 erts, &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 

 inhabitants of the eastern and western shores of South 

 America are very distinct, with extremely few shells, Crus- 

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

 recently shown that about thirty per cent, of the fishes are 

 the same on the opposite sides of the isthmus of Panama; 



