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 the student of bio-geography consider pri- 

 marily the factors which determine the dispersal of plants 

 and animals throughout the world. Animals of strong 

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

 their movements and consequently these groups are of world- 

 wide distribution. The greatest traveler in the 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 11,000 miles to spend 

 its winter on the shores of the antarctic continent. Many 



THE AECTIO TERN 



The greatest traveller in the world. From Cooke, ' ' Bird Migration, ' ' 

 in Bulletin Bureau of Biological Survey. 



1 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 distribution 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 which they 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 point to point of the 



