590 ZOOLOGY SECT. 



(Larus), some Ducks (Anas), &c. are also cosmopolitan or world- 

 wide in distribution. On the other hand, the range of a species, 

 genus, or order may be restricted to a single limited district. The 

 genus Liopelma (New Zealand Frog) is found only in a small area 

 in the Auckland district ; the species Scdmo killinensis (Loch Killin 

 Charr) occurs only in Loch Killin in Inverness-shire ; the Order 

 Rhynchocephalia is confined to New Zealand. The entire range 

 may be broken up, as it were, into a number of stations, depending 

 sometimes on habitat, sometimes on unknown causes ; the Tuatara, 

 for instance, is found at present only in some half 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 dis- 

 persal 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 (Eudynamis 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. In- 

 fusoria, 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, etc. 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 



