OF ANIMALS. 107 



very few animals that are really cosmopolites, the most of these 

 creatures being colonized within limited regions. That such 

 should be the case, we can comprehend, if we study the circum- 

 stances which may oppose their progress. But this study is far 

 from furnishing us a satisfactory explanation of all cases of 

 limited circumscription of a species, and it is often impossible to 

 divine why certain animals remain restricted to a locality, when 

 nothing seems to oppose their propagation in neighbouring situa- 

 tions. 



Whatever may be the reason, the obstacles to the geographical 

 distribution of species are sometimes mechanical, and at others, 

 physiological ; among the first are seas and chains of lofty moun- 

 tains. To terrestrial animals seas of much extent are in general 

 an impassable barrier, and we perceive, all things being equal, 

 the mixture of two distinct faunas is always most intimate in pro- 

 portion as the regions to which they belong are, geographically, 

 most approximated, or in communication with each other, by 

 intermediate lands. The Atlantic Ocean prevents species peculiar 

 to tropical America, from extending to Africa, Europe, or Asia ; 

 and the fauna of the New World is entirely distinct from that of 

 the old continent, except in the highest latitudes, towards the 

 north pole. But there the land of the two continents is approxi- 

 mated, America being separated from Asia only by Behring's 

 Straits, and is connected to Europe by Greenland and Iceland : 

 on this account zoological exchanges can be more easily effected, 

 and we find there species common to both worlds ; for example, 

 the white bear, the reindeer, the castor, the ermine, the bald 

 eagle, &c. Chains of lofty mountains also constitute natural 

 barriers, which arrest the dispersion of species, and prevent the 

 admixture of faunae, proper to neighbouring zoological regions. 

 For instance, the opposite declivities of the Cordillera of the 

 Andes are inhabited by species which are for the most part dif- 

 ferent ; the insects of the Brazilian side, for example, are almost 

 all distinct from those found in Peru and New Granada. 



The dispersion of marine animals living near coasts is pre- 

 vented in the same manner by the geographical configuration of 

 the earth; but here it is sometimes a continuation of a long chain 

 of land, and sometimes a vast extent of open sea, which opposes 

 the dissemination of species. Thus most animals of the Medi- 

 terranean are also found in the European portion of the Atlantic, 

 but they do not extend to the seas of India, from which the Medi- 

 terranean is separated by the isthmus of Suez, nor can they 

 traverse the ocean to gain the shores of the New World. 



The physiological circumstances which tend to limit the dif- 

 ferent faunae are more numerous ; and without doubt, the first iu 

 consideration is the unequal temperature of different regions of 

 the earth. There are species which can bear an intense cold and 



