606 ZOOLOGY SECT. 



dozen small islets, each of which is a station, while the whole of 

 them constitute the range of the species. 



Barriers. A barrier in zoo-geography implies any permanent 

 obstacle to the dispersal of a species. For instance, the Tasman 

 Sea is an impassable barrier to the passage of most animals be- 

 tween Australia and New Zealand, strong-flying birds being the 

 only species able to cross it. On the other hand, the number of 

 Birds able to pass so narrow a strait as the British Channel is very 

 considerable, while still narrower arms of the sea may be crossed 

 by a large proportion of Mammals, Insects, &c. Thus barriers are 

 of unequal importance according to the particular animals under 

 discussion ; wide deserts and lofty snow-covered mountain ranges 

 are impassable to most species ; to some even a narrow river or arm 

 of the sea is insuperable. 



Means of Dispersal. Most Mammals and many Reptiles can 

 swim across rivers and arms of the sea if not too broad ; thus 

 narrow straits and rivers are of no significance as barriers to the 

 dispersal of many animals. On the other hand, rivers of even 

 moderate breadth are insuperable barriers to Monkeys, which are 

 unable to swim. 



Aerial animals can overcome many of the barriers impassable to 

 flightless forms. Birds of strong flight often pass over immense 

 stretches of ocean. For instance, a Cuckoo (Eu&ynamis taitensis) 

 habitually winters in Fiji and other Pacific islands, and spends 

 its summer in New Zealand, traversing the enormous distance 

 of 1,500 miles twice a year. Many Insects also are able to fly 

 great distances, especially when carried by gales. 



Mechanical dispersal is an important factor in the case of many 

 animals. Small Crustacea and Molluscs may be carried great 

 distances in the mud adhering to the feet of Birds. Infusoria, the 

 eggs of Rotifers, and other microscopic forms may be transported 

 in the dried condition by wind. Birds and Insects are frequently 

 blown out to sea and carried for immense distances, and Mammals, 

 Reptiles, &c., may be widely distributed by being carried on drift- 

 wood, or on floating islands or " rafts " formed of large masses of 

 matted vegetation, such as are often detached by storms in the 

 tropics. Finally, the dispersal of many fixed or shore-haunting 

 animals is ensured by their free-swimming larvae. 



Importance of the various Groups of Animals in Zoo- 

 Geography. In close dependence on the means of dispersal we 

 have the fact that the various groups of animals are of very 

 unequal value in the study of distribution. The greater the 

 facilities for the transport of any species across a given barrier, 

 the less significance will attach to its occurrence on both sides of the 

 barrier. Conversely, when a species, having few or no facilities for 

 dispersal, is found on opposite sides of an important barrier, the 

 natural conclusion is either that the barrier is of comparatively 



