6 HISTORY OF THE EUROPEAN FAUNA. 



appears at first a somewhat difficult task. Indeed, 

 the means of dispersal of the various groups of 

 animals are so different that it occurred to me it 

 might be better to deal with the mammals, the 

 birds, the reptiles, and so forth, all separately. This 

 idea I have attempted to follow to some extent, with 

 most satisfactory results. The British fauna of the 

 present day is no doubt complex, but no more so 

 than the fauna of the most recent of our geological 

 deposits the Pleistocene. However, when we go 

 back still further and look at the earlier Tertiary 

 remains, we find the fauna becoming less complex. 

 Northern species disappear, and the strata are 

 entirely filled with the remains of southern animals 

 and plants. Geologists indeed are quite unanimous 

 in their belief, that the fauna of the British Islands 

 during the earlier epochs of the Tertiary Era was a 

 southern one ; that it then gradually became more 

 temperate, until at last, in more recent times, decidedly 

 northern forms invaded the country. These seem 

 to have driven out to some extent at least the 

 southern species; but more recently again, the 

 southerners, reinforced by an eastern contingent, 

 appear to have gained territory and are advancing 

 into the area held by the northerners. The eastern 

 invasion does not seem to have affected Ireland at 

 all, and we find the country there divided between the 

 southern and northern animals. We can thus roughly 

 construct a map as I have done here, showing, by 

 means of horizontal and sloping lines, the principal 



