84 DR- BLANFORD'S realms chap. 



can be traced, it results as a conclusion that from a given area of 

 origin the group in question migrated in all directions where 

 XDOssible to a varying degree ; it then died out in intervening 

 tracts, or was left only in a certain part of its former and more 

 extensive area of range. 



Zoological Regions. — Seeing that each species of animal has 

 its own definite range, it is clear that the earth's surface can be 

 apportioned into divisions wliich are characterised by their 

 animal inhabitants. We sliall divide the earth into realms, 

 which are the largest divisions ; then into regions ; and 

 finally into subregions. It must be borne in mind that the 

 various groups of tlie animal kingdom are of different ages, 

 geologically speaking, and have therefore had less or more time, 

 as the case may be, to settle down into their present distribution, 

 and that different animals differ greatly in their rate of multi- 

 plication, their power of migration, and their susceptibility 

 to the effectiveness of various natural and other barriers to 

 distribution. It is not, tlierefore, possible to divide the 

 world into realms and regions which shall express the fticts of 

 distribution of the entire animal kingdom. Sucli divisions, 

 which are common in text-books of zoology having but a small 

 section devoted to zoogeography, are at best mere approximations 

 and averages ; no good is gained by taking such a comprehen- 

 sive view of the matter, as the essential object of subdividing 

 the earth's surface is thereby lost sight of The zoogeographical 

 division of the earth which will be adopted here is that origin- 

 ally recommended by Dr. Blanford, and now accepted by a 

 number of authorities. There are three " realms," to which a 

 fourth may perhaps be added — though on negative grounds, and 

 merely for the purpose of emphasising the parts of the world to 

 whicli mammals have not gained access. The realms are again 

 divisible into regions, at least in the case of one of them, and 

 the regions may be again separated into more or less distinct 

 subregions or provinces. The three primary divisions or realms 

 which contain mammals are the ISTotogaean, including Australia 

 and certain islands to the north of it ; the Xeogaean, or the 

 South American continent and Central America ; the Arcto- 

 gaean, including the continents of North ^Vmerica, Europe, Asia, 

 and Africa, together with the adjacent islands, such as the 

 West Indies, East Indies (exclusive of those which fall within 



