GEOGRAPHICAL DISTRIBUTION OP ANIMALS 439 





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Fio. 257. On the shore of Narborough Island, one of the Galapagos Islands, Pacific 

 Ocean, showing peculiar species of sea-lions, lizards, and cormorants. Drawn 

 from a photograph made by Messrs. SNODGRASS and HELLER. 



348 Effect of barriers. It will be thus seen that geo- 

 graphical distribution is primarily dependent on barriers or 

 checks to the movement of animals. The obstacles met 

 in the spread of animals determine the limits of the spe- 

 cies. Each species broadens its range as far as it can. It 



