CHAPTER XL 



THE GP]OGRAPlIICx\L DISTRIBUTION OF ANI- 

 MALS. 



The assemblage of animal life peopling any one locality 

 or area is called its fauna, as the plants of a place consti- 

 tute its flora. Where the physical geography — i.e., the con- 

 tour of the surface, the plains, valleys, and hills — is of iden- 

 tical character and the climate the same, the fauna is much 

 the same, but when these characteristics of soil and climate 

 cliange, as in passing from lowlands to highlands, or from 

 south to north, the assemblage of animals will be found 

 to change in a corresponding ratio. And as there are no 

 definite limits to any large area of the earth's surface, the 

 physical features of one area merging insensibly, as a rule, 

 into adjoining districts, so adjoining faunae merge into one 

 another, and a certain jjroportion of the species may range 

 through two or more faunal areas. 



There are in nature causes tending to restrain animals 

 within their faunal limits, and others tending to diffuse 

 them, or to cause them to migrate from their specific cen- 

 tres or centres of creation — namely, the point where the in- 

 dividuals of a species are most abundant, and where, ac- 

 cordingly, they are supjiosed to have originated. 



Barriers to the Spread of Animals from their Specific 

 Centres.— Among the most important are the oceans and 

 their basins. The animals of the opposite sides of the Pa- 

 cific Ocean are entirely unlike, no species being common to 

 the two sides ; while, of the immense numbers of animals 

 peopling the coast of Brazil and the opposite coast of Af- 

 rica, only two or three are known to be identical. Differ- 

 ence in climate is also a great barrier, the animals of the 



