THE BIOLOGICAL CONQUEST OK TIIL WolU.O 



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barriers may be in vertical zones, extending from tlie ocean level to 

 mountain tops, as well as horizontal, spreading out north and south 

 from the equator in zones of latitude. 



Map showing ancient and modern ranges of the elephants and their ancestors. 

 The shaded area shows the former habitat of the maniniolh and mastodon, 

 ant^estor of the modern elephant. A land connection probal)Iy existed I)etween 

 Asia and North America. Note the restricted range of the present-day elephants 

 indicated by heavy shading. How can this be accounted for.^ 



Sometimes natural barriers occur, such as high mountain ranges 

 with eternal snow, deserts with unfavorable conditions of moisture, 

 or in the case of water-distributed animals such as fishes, high water- 

 falls may prevent them from moving up a stream beyond a cciiain 

 point. The barrier for one organism, however, might l)e a highway 

 for another. A desert would be an impassable barrier- to a squirn^l 

 but not to a camel. 



Geographical barriers have not always been fixed. Geological 

 history reveals the fact that some land surfaces were once occujiied by 

 water and what is now water may have been land. The presence of 

 fo.ssil sea.shells in the Panama Canal area indicates that the Isthmus 

 was formerly submerged, and there is evidence that as late as Eocene 

 times there was a land connection acro.ss Bering Straits. As bar- 

 riers have changed so has the resulting distribution of organisms. 

 Distribution often indicates the geography of the i)ast. .Mnnbers 

 of the same genus may differ widely in certain isolated localities, as, 

 for example, the tapirs found in tropical America and the Malay 



