608 



MR. P. C. MITCHELL ON A FORMULA TO 



[Nov. 4, 



be drawn with four strokes of the pen thus indicates the regions in 

 their relative positions, fig. 1. 



The four subregions in each region except in the Nearctic 

 region lie, in a general way, two to the north, two to the south, two 

 to the east, two to the west. In the Palaearctic region, for instance, 

 the North-European and Siberian subregions are the northern pair ; 

 the Mediterranean and Manchurian the southern pair ; the North- 

 European and Mediterranean are the western, the Siberian and 

 Manchurian the eastern subregions. If the subregions are numbered 



1 o o 



thus : — 



North-European 1 



Mediterranean 2 



Siberian 3 



Manchurian 4 



and if the numbers be placed as indicated in figure 2, it will be seen 

 that the numbers are in the relative position of the regions they 

 denote. 



Fig. 2. 



3 



4 



The numbers 1, 2, 3, 4 are placed in similar relative position in 

 the spaces corresponding to the other regions except the Nearctic, and 

 obviously represent subregions as in the following list : — 



Ethiopian Region .... 1. West Africa. 



2. South Africa. 



3. East Africa. 



4. Madagascar. 



Oriental 1. Hindostan. 



2. Ceylon. 



3. Indo-China. 



4. Indo-Malay. 



