BARRIERS AND MEANS OF DISPERSAL 65 



5 km. from land. 36 The hippopotamus and polar bear seem to be the 

 best swimmers among the terrestrial mammals. The hippopotamus 

 swims the strait between the mainland and Zanzibar, a distance of 

 30 km.; a polar bear has been observed swimming in the open ocean 

 30 km. from the nearest resting place. Reindeer also swim well and 

 readily enter the water; they are said to have been taken by ships on 

 frequent occasions, far from land. 37 Twenty kilometers seems to be 

 the maximum distance that can be negotiated by the red deer. 38 Con- 

 versely the Palk Strait, 30 km. wide, has kept the tiger out of Ceylon. 

 Amphibians are killed by salt water. 



The speed of movement on land is relatively unimportant for the 

 distribution of animals, on account of the amount of time available. 

 Although earthworms and snails move slowly, they have occupied all 

 the area in northern Europe that was covered by the ice sheet since 

 its retreat. Effective and active wandering by land snails is attested 

 by the occurrence of the photopositive helicid snails on isolated areas 

 of Muschelkalk and Cretaceous rocks in the North German Plain, 39 

 and by the relatively rich and uniform snail population of the scat- 

 tered ruins in the Black Forest, which are otherwise poor in snails. 40 

 In the United States the colonies of the introduced Helix hortensis 

 expand rapidly. 39 Introduced European earthworms have displaced the 

 native forms near the cities in California, Chile, Australia, and else- 

 where, and have spread a considerable distance inland. Almost nothing 

 is left, on the Antilles, of the native oligochaete fauna. 41 



Mountains and deserts form climatic barriers, and greater powers 

 of motion do not necessarily enable animals to cross them. When 

 connections, such as ice masses, are temporary, powers of rapid mo- 

 tion may enable animals to spread from the mainland to otherwise 

 inaccessible islands and from island to island. Thus the caribou have 

 spread from the Melville Peninsula to Baffin Land and further north- 

 ward, the reindeer from the mainland to Nova Zembla, and the 

 wolves have followed them. The arctic fox seen by Nansen more than 

 100 km. north of Sannikow Land in the New Siberia Archipelago illus- 

 trates the distances to which swift runners may travel over the ice. 

 Flying animals are least limited in dispersal by physical factors. 

 The air offers no barriers; only the low temperature and lessened 

 density at high altitudes may affect flight. Deserts, mountains of 

 moderate height, and seas of not too great extent are not barriers for 

 good fliers. However, there is great variation in the power of flight in 

 different groups of flying animals. 



Among the insects, forms with limited flight are usually restricted 

 in distribution. The number of genera of limited distribution, inhabit- 



