152 



Biology in America 



As the explorer setting forth upon an unknown journey 

 concerns himself first of all with his equipment and means 

 of travel, so must tlie student of l)i()-gcography consider pri- 

 nuirily the factors wliieli th^terminc the dispersal of plants 

 and animals throuf^hout the world. Animals of strong 

 Hight, like most birds and bats, are relatively unhindered in 

 their movements and coiisequently these groups are of world- 

 wide distribution. The greatest travelei- in tlie world is the 

 arctic tern which spends its summers amid the arctic snows, 

 where its newly hatched young have been found surrounded 

 by a wall of freshly fallen snow scraped out of the nest by 

 the parent bird, and journeys south 11,0(JU miles to spend 

 its winter on the shores of the antarctic continent. Many 



The Aectic Teen 

 The greatest traveller in \\\v world. !■ rom Cuoke, "Bird Migration," 

 in Bulletin Bureau of JViological Survey. 



other birds make semi-annual journeys of close to 10,000 miles 

 and the great majority of them travel long distances on their 

 migrations. Bats also migrate long distances, this fact of- 

 fering a possible explanation of their presence on some oceanic 

 islands, otherwise destitute of native mammals. 



Marine animals, especially fishes, are often widespread in 

 their distrilnition because of their powers of migration and 

 the relative absence of barriers within the sea, but fresh 

 water fishes are generally limited more or less closely to the 

 area in wiiieli Ihey occur. No general rule however can be 

 laid down. 



Among terrestrial animals the greatest travellers are the 

 mammals and these often perform long journeys, a habit 

 characteristic of the bison "that ever-journeying animal, 

 which moves in countless droves from i)oint to point of the 



