GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 499 



first we may mention seas and high chains of mountains. 

 For terrestrial animals, in fact, seas of a certain extent form in 

 general an insurmountable barrier; and we see that, all things 

 being equal in other respects, the mixture of two distinct 

 faunae is always the more intimate that the regions to which 

 they belong are more geographically approximated, or are 

 placed in communication by intermediate lands. Thus the 

 Atlantic Ocean prevents the species proper to Tropical Ame- 

 rica from spreading into Africa, Europe, and Asia ; and the 

 fauna of the New World is completely distinct from that of 

 the Old, unless it be in the more elevated latitudes towards 

 the northern pole; but there the lands approach : America is 

 only separated from Asia by the straits of Behring, and holds 

 relations with the north of Europe through Greenland and 

 Iceland ; thus zoological exchanges could take place much 

 more easily, and it is there, in fact, that we find the species 

 common to the two worlds, such as the white bear, the rein- 

 deer, the beaver, the ermine, the pelerine falcon, the white- 

 headed eagle, &c. Lofty chains of mountains constitute also 

 natural barriers which often arrest the dispersion of species, 

 and prevent the fusion of faunse peculiar to neighbouring 

 zoological regions. Thus the two slopes of the Cordilleras of 

 the Andes are inhabited by species generally distinct; and 

 the insects of Brazil, for example, are almost all distinct from 

 those we meet with in Peru or in New Granada. The dis- 

 persion of marine animals living near the coast is shackled 

 in the same way by the geographical configuration of the 

 globe ; but here it is sometimes a long contiguity of land, 

 sometimes a vast extent of the deep sea, which opposes itself 

 to the dissemination of species. Thus most of the animals 

 of the Mediterranean are also found in the European portion 

 of the Atlantic, but have not been able to reach the Indian 

 seas, from which the Mediterranean is separated by the isthmus 

 of Suez ; nor to traverse the Atlantic Ocean, to spread them- 

 selves on the coasts of the New World. 



633. The physiological circumstances which tend to 

 limit the different fauna? are more numerous ; but that which 

 presents itself in the first place is unquestionably the unequal 

 temperature of the different regions of the globe. There are 

 species which can support equally well an intense cold and 

 tropical heat ; man and the dog, for example ; but there 

 are others which in this respect are less favoured by nature, 

 and which do not prosper, or even cannot exist, but under the 



