762 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



(b.) Closely adjacent countries may have quite different 

 faunas. Thus, the Bahamas and Florida, Australia and 

 New Zealand are peopled by very different animals. But 

 the best illustration is that of 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. They differ from each other in their birds 

 and quadrupeds far more widely than Britain and Japan. 



(c.) 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 there is only one family of Marsupials the 

 American opossums found beyond the Australasian limits. 



(e.) Another striking fact is the " discontinuous dis- 

 tribution " 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, 

 there are some four species in South and Central America, 

 while the only other species occurs in Malacca and Borneo. 

 Similarly the Camelidae are represented by one genus in 

 the Old World and another in South America, and the 

 insectivorous Centetidae are represented by five genera in 

 Madagascar, and one in Cuba and Hayti. 



The Factors determining Distribution. 



There are six factors which combine to determine the particular distri- 

 bution of an animal. These may be conveniently considered in pairs. 



(a) Distribution is in part determined by the constitution of the 

 animal and the physical conditions of the region. Thus snakes 

 dimmish rapidly in numbers towards the poles, their constitution being 

 in most cases ill-adapted to withstand cold ; thus crayfishes are absent 

 from districts where the fresh water does not contain sufficient lime salts 

 for their needs. 



(b) Distribution is in part determined by the position of the animal's 

 original home (which is often an unknown fact), and by the available 



