802 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



MORE DETAILED PROBLEMS OF GEOGRAPHICAL 



DISTRIBUTION. 



Leaving the general, and at present very obscure, problem 

 of the evolution of faunas, let us briefly notice some of the 

 more detailed questions of distribution. We shall content 

 ourselves with stating (i) a few of the outstanding facts; 

 (2) the factors determining why some animals are here and 

 others there ; and (3) the usually recognised zoo-geographical 

 regions. 



Outstanding facts. (a) Widely separated countries may 

 have an essentially similar fauna. Thus, there is much in 

 common between Britain and Northern Japan, and there 

 is so much agreement between the North European (Palae- 

 arctic) and the North American (Nearctic) fauna, that many 

 unite the two regions in one (Holarctic). 



(b} Closely adjacent countries may have quite different 

 faunas. Thus the Bahamas and Florida, Australia and 

 New Zealand, are peopled by very different animals. Two 

 little islands, Bali and Lombok, in the Malay Archipelago, 

 which are separated by "Wallace's Line," a strait only 

 fifteen miles wide at its narrowest part, differ from each 

 other in their birds and quadrupeds more widely than 

 Britain and Japan. 



(f) Regions with very different faunas are in many cases 

 connected by transition areas. Thus a journey from the 

 North of Canada to Brazil would show a fairly gradual 

 transition from an Arctic to a tropical fauna. 



(d) At the same time there are regions whose fauna is 

 exceedingly distinctive and sharply defined. Thus the 

 Mammalian fauna of Australia is distinctively Marsupial, 

 and nowadays only the American opossums and Cceno- 

 lestes occur beyond the Australasian limits. 



(e) Another striking fact is the " discontinuous distri- 

 bution " of certain types, by which we mean that examples 

 of a type may occur in widely separated regions without 

 there being any representatives in the intermediate area. 

 The general explanation is, that the type in question once 

 enjoyed a wide distribution, as the rock record shows, and 

 that the conditions favourable to survival have been found 

 in widely separated places. Thus of the genus Tapir 



