THE FACTORS DETERMINING DISTRIBUTION. 803 



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. 



(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 

 means of dispersal. Thus, so far as we know, the ( )ld 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. 



(<) Distribution is in part determined by the actual changes (geological, 

 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 Cccnolestes. 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 of many birds and mammals, sometimes 

 indeed reducing it to ////, 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 

 changes of the earth and its climate, and (6) the bionomic 

 relations. 



