30 ON THE STUDY OF ZOOGEOGRAPHICAL REGIONS, 



mapped out on a complete map of the world, and then studied in 

 relation to all the regions in which they occur. In those cases 

 in which they occur in only one region, they may be very similar 

 to entogenic contours, but will exhibit less density and extent. 



We may now define the three main types of contour as 

 follows : — 



i. Ectoyenic Contours. — The contours exhibited by groups 

 which evolved their present group-centres outside the region Z, 

 but have since invaded Z and form a definite part of its fauna 

 and flora. The zoocentres will be either completely outside Z, or 

 only slightly projecting into it, while the lower contour-lines will 

 extend farther and farther into Z. 



Generally, it will be found that the species forming the immi- 

 grant group are quite distinct from the main body of the group 

 still located in Y. Very often they are also generically distinct, 

 but the closer connection between the parent genus and its 

 ojffshoots will still be evident, and will necessitate the two being 

 taken together as a natural group, according to the rule already 

 laid down. 



li. Entogenic Contours. — The contours exhibited by groups 

 which evolved their present group-characters within the region Z. 

 The zoocentres will lie entirely within Z, while the lines of lower 

 contour will spread out more and more over the region, and one 

 or more of them may possibly pass outside the region (forming 

 the beginning of a new ectogenic contour for some other region). 



Groups with entogenic contours are essentially those that give 

 the distinctive character to a region, and on them the main zoo- 

 geographical regions of the earth are based. 



iii. Palceogenic Contours. The contours exhibited by groups 

 which are remnants of what were once far more widely spread 

 groups. Such contours may consist of one or more isolated areas 

 of low value, and usually exhibit discontinuity over more than 

 one region. These isolated areas may be regarded as the "sunken 

 peaks" (probably the zoocentres) of a once large and continuous 

 contour (just as an archipelago shows only the sunken peaks of 

 what once formed a continuous land-mass). 



It should be clearly recognised that these three types of 

 contour are definitely connected, and that intermediate forms 



