238 HISTORY OF THE EUROPEAN FAUNA. 



difficulty to the supposition that some of the so- 

 called Newer Pliocene Crags of the east coast of 

 England were laid down at the same time by the 

 same sea. 



This would also explain how the Arctic species 

 come to inhabit the Caspian, as the old Aralo-Caspian 

 Sea could have had some communication (Fig. 12, 

 p. 156) with the North European Sea. And this again 

 offers an explanation of the otherwise mysterious 

 occurrence of the Caspian Dreyssensia polymorpJia 

 in the lower continental boulder-clay. 



The climatic reasons for the supposition that the 

 boulder-clay is a marine deposit have already been 

 given (p. 66). However, it may be asked what about 

 the glacial flora which has been proved to have existed 

 all over the plains of Northern Europe ? what about 

 the relics of this same flora which still linger on in a 

 few localities to the great delight of the systematic 

 botanist ? They have been spoken of as indications of 

 a former Arctic climate in Europe. The presence of 

 an Arctic species such as Dryas octopetala in any of 

 the pleistocene deposits is often looked upon as an 

 absolute proof of a very severe climate having pre- 

 vailed at the time they were laid down. Professor 

 Geikie tells us that the South of England was 

 clothed with an Arctic flora, when the climate be- 

 came somewhat less severe than it had been during 

 the climax of the glacial cold (p. 398). Relics of 

 such a flora have been detected at Bovey Tracey, in 

 Devonshire, the Arctic plants found comprising 



