8 HISTORY OF THE EUROPEAN FAUNA. 



facilitate the comprehension of the arguments used, to 

 give maps. Some of these represent the geographical 

 conditions at the particular epoch referred to in the 

 text, but they merely claim to give a general idea. 

 There was never any intention to make them corre- 

 spond with all the data of which we have geological 

 evidence. They are what I might call "diagrammatic." 

 In comparing them with reconstructions of former 

 physical geography such as have been attempted from 

 time to time, I hope geologists will therefore deal 

 leniently with the faults I may have committed, and 

 remember that the maps are " impressions," or " dia- 

 grams," and not faithful representations of all the 

 geographical revolutions witnessed by some of our 

 remote forefathers at any particular period. 

 r. The knowledge we gain from a study of the British 

 Tertiary deposits enables us to affirm positively that 

 both the eastern and the northern species arrived in 

 these islands comparatively recently, but that the 

 southern forms must have migrated northward from 

 the Continent long ages ago. Since the northern 

 and the eastern migrations that is to say, those 

 coming from the north and east were the last to 

 arrive in Northern Europe, the remains of the animals 

 contained in the most recent deposits of that portion 

 of our continent will furnish us with a clue as to the 

 extent of the area inhabited by them. This is not 

 all, however. It is also possible to discover from these 

 remains the direction which the animals that they 

 belonged to came from. As we shall learn later on, 



