ZOO-GEOGRAPHICAL REGIONS. 763 



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 (geological, 

 climatic, &c.) 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 a Mesozoic 

 mammalian fauna which has been exterminated everywhere else, except 

 in the case of the American opossums. 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 nil, or increasing it to desperation. 



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. 



Zoo- Geographical Regions. 



I shall simply quote a paragraph from Professor Heil- 

 prin's work The Geographical and Geological Distribution 

 of Animals (Internat. Sci. Series. London, 1887), a very 

 valuable book for the student, especially as it considers dis- 

 tribution in space and time together. 



" By most naturalists (Wallace, Sclater, and others) the 

 terrestrial portion of the earth's surface is recognised as 

 consisting of six primary zoological regions, which corre- 

 spond in considerable part with the continental masses of 

 geographers. These six regions are : 



" i. The Palcearctic, which comprises Europe, temperate 

 Asia (with Japan), and Africa north of the Atlas Moun- 

 tains ; also Iceland, and the numerous oceanic islands of 

 the North Atlantic : 



