EDWARD FORBES. 2 19 



prevailed in Northern Europe, and in the northern portions 

 of North America. At this time, our mountains were 

 covered with ice and snow, and the general state of 

 things must have been very much what we now see in 

 the Arctic regions. During this period of glacial cold, 

 a great migration southwards of Arctic animals and plants 

 took place, and these, rinding suitable life-conditions, 

 established themselves far south of their former limits. 

 Thus, during this period the Icelandic Scallop (Pecten 

 islandicus\ and various other shellfish which are now 

 found living in Arctic seas, migrated as far as the coasts 

 of Britain, and we find their remains in the glacial clays 

 of the estuary of the Clyde, and elsewhere round our 

 coasts. Similarly various alpine and Arctic plants 

 invaded our area, and took advantage of the cold to 

 establish themselves on the low grounds and hills. 

 Contemporaneously with this southward migration of 

 northern plants and animals, we must suppose a like 

 movement to have taken place on the part of the animals 

 and plants which had previously inhabited our area. 

 These latter would be driven gradually farther and farther 

 south by the increasing cold, and a clear field would in 

 this way be left for the Arctic invaders. 



When, however, the glacial period ended, and more 

 temperate conditions were gradually re-established, a 

 reverse movement would be set on foot The southern 

 forms of life would again move northwards, and piece 

 by piece reconquer the territory from which they had 

 been dispossessed; while the northern immigrants would 

 be driven, step by step, backwards towards the pole. But 

 as the climatic conditions became gradually less severe, 



