INTRODUCTION. 



areas inhabited at the present time by the species of 

 northern, southern, and eastern origin (Fig. i). 



FIG. i. Map of the British Islands, indicating approximately the 

 areas inhabited by the northern, southern, and eastern animals. 

 The horizontal lines represent the areas of northern species, the 

 sloping lines those of southern and eastern ones. 



In the problems which are being discussed in this 

 work I have often found it of advantage, in order to 



