904 GEOGRAPHICAL DISTRIBUTION OF ANIMALS 



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 distribution of an animal. 

 These may be conveniently considered in pairs. 



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

 animal and by the physical conditions of the region. Thus snakes 

 diminish 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. 



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

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

 means of dispersal. Thus, so far as we know, the Old World has been 

 the exclusive home of the anthropoid apes, and there they have 

 remained ; thus bats, being able to fly, have a more cosmopolitan 

 distribution than most other mammals ; thus amphibians, being unable 

 to withstand salt water, are absent from almost all oceanic islands. 



(c) Distribution is in part determined by the actual changes (geo- 

 logical, climatic, etc.) which have affected different regions, and by 

 " bionomic " factors, i.e. the relations between the animal in question 

 and other organisms, whether animals, plants, or man. Thus it is 

 plain that we cannot understand the fauna of Australia without knowing 

 the geological fact that part of this island was once connected with 

 the Oriental continent by a bridge of land across the Java Sea. The 

 Australasian mammalian fauna consists of survivals and descendants 

 of Mesozoic Marsupials which have been exterminated everywhere else, 

 except the American opossums and Coenolestes. The original Australian 

 mammals were saved, not by any virtue of their own, but by the earth- 

 change which insulated them. Similarly, it is the geologist who helps 

 us' to understand the faunal diversity on the two sides of " Wallace's 

 Line," or the absence of amphibians, reptiles, and mammals from the 

 Canaries. That much will also depend on the animal's power of 

 surviving the struggle for existence in different regions is too obvious 

 to require exposition. We need only think of the way in which man 

 has in a few years altered the distribution t)f many birds and mammals, 

 sometimes indeed reducing it to nil, or increasing it with disastrous 

 results. 



To sum up : the chief factors determining geographical 

 distribution are — (i) the constitution of the animal, (2) the 

 physical conditions of the region, (3) the position of the 

 original home, (4) the means of dispersal, (5) the historical 



