CHAPTER XXXVIII 

 DISTRIBUTION OF LAND ANIMALS 



Extent of the Subject. — The geographical distri- 

 bution of animals deals with the distribution of species 

 over the earth's surface, the conditions which govern their 

 existence in the region they occupy, and the agencies 

 favoring or hindering their dispersal into other areas. 

 It thus treats of distribution in space at the present time 

 and appears to be distinct from paleontology, or the dis- 

 tribution of species in past times based on the study of 

 fossil remains entombed in the rocks, and from geology 

 dealing with the physical conditions and changes of the 

 earth. In reality, these three sciences are closely linked, 

 and many puzzling features of geographical distribution 

 have been explained only after the migrations of extinct 

 species and the relations of ancient land masses were 

 understood. The family of camels, for example, is rep- 

 resented only in South America, Asia, and Northern 

 Africa. From their fossil remains it is known that in 

 ancient times they inhabited North America, that some 

 individuals migrated to South America, while others 

 crossed a land bridge into Asia, and thus with the ex- 

 tinction of the North American population their present 

 distribution is made clear. Moreover, every other species 

 of living animal comes from a long line of ancestors 

 whose migrations in bygone times have determined its 

 position on the earth to-day. 



Means of Dispersal. — The migrations of animals 

 are the results of agencies of varied character. The 

 highest terrestrial species, such as horses, camels, ele- 

 phants, bears, deer, and many more or less closely related 



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