CHAPTER XV 



ZOO-GEOGRAPHICAL DISTRIBUTION 



Space will not allow us to deal with this important question 

 as fully as it deserves. We shall, however, use the somewhat 

 novel method which I have already introduced in this subject [175]. 

 The six main zoo-geographical regions of the earth, as defined by 

 Sclater and Wallace, are accepted as valid, while Polynesia, 

 with its obviously negative or non-continental characters, is 

 treated as an appendix. The fauna of each region is then sub- 

 divided into three portions, which may be briefly defined as 

 follows : 



1. Palaeogenic Groups, or those archaic remnants whose 

 distribution is discontinuous, extending over one or more regions. 

 They are the last remains of groups which were once much more 

 widely spread. 



2. Entogenic Groups, or those groups which form the autoch- 

 thonous or peculiar fauna of each region. They are not necessarily 

 confined to a single region, since they frequently develop sufficient 

 energy to spread over two or more regions. A group is, however, 

 only entogenic in that region in which it forms a definite zoocentre 

 (region of greatest density), placed definitely within the region. 



3. Ectogenic Groups, or those groups which, being entogenic 

 in a neighbouring region, have invaded the region under discussion, 

 and have gained a footing in some part of it, thus modifying the 

 composition of the fauna of that part. No zoocentre is formed by 

 any group in the region in which it is ectogenic. 



It is the entogenic groups which give the special character to 

 the regional fauna. The ectogenic groups modify the fauna of 

 outlying parts of a region, but are absent or small over the greater 

 part of the region. Palaeogenic groups follow no definite rules 



