DISTRIBUTION IN TIME. 51 



The geographical distribution of animals is concerned with 

 the determination of the areas within which every species of 

 animal is at the present day confined. Some species are found 

 almost everywhere, when they are said to be " cosmopolitan ; " 

 but, as a rule, each species is confined to a limited and definite 

 area. Not only are species limited in their distribution, but it 

 is possible to divide the globe into a certain number of geo- 

 graphical regions or " zoological .provinces," each of which is 

 characterised by the occurrence in it of certain associated 

 forms of animal life. 



The geographical distribution of land animals is conditioned 

 partly by the existence of suitable surroundings, and partly by 

 the presence of barriers preventing migrations. Thus, certain 

 contiguous regions might be equally suitable for the existence 

 of the same animals, but they might belong to different zoolo- 

 gical provinces, if separated by any impassable barrier, such as 

 a lofty chain of mountains. Owing to their power of flight, the 

 geographical distribution of birds is much less limited than 

 that of mammals ; and many migratory birds may be said to 

 belong to two zoological provinces. In spite of their powers 

 of locomotion, however, birds are limited by the necessities of 

 their life to definite areas, and a zoological province may be 

 marked by its birds just as well as by its quadrupeds. 



The geographical distribution of an animal at the present 

 day by no means necessarily coincides with its former exten- 

 sion in space. Many species are known which now occupy a 

 much more restricted area than they did formerly, owing to 

 changes in climate, the agency of man, or other causes. Simi- 

 larly, there are species whose present area is much wider than 

 it was originally. 



At the present day, naturalists usually adopt either the zoo- 

 logical provinces proposed by Prof. Huxley, or those proposed 

 by Mr Sclater, both arrangements possessing certain features 

 in common. Prof. Huxley proposes to divide the earth's sur- 

 face into four primary zoological provinces, as follows, each 

 possessing its own "fauna," or characteristic assemblage of 

 animals : 



I. OrnithogcEa, or the Novo-Zelanian Province, comprising 

 only New Zealand. 



II. Antarctogaa, or the Australian Province, comprising 

 Australia, Tasmania, and the Negrito Islands. 



III. Dendrogiza, or the Austro-Columbian Province, includ- 

 ing South America, Central America, and Mexico. 



IV. Arctogcea, including all the rest of the world, and having 

 as sub-provinces, 



