36 THE MIGRATION OF BIRDS. 



animal food might be picked up on the Arctic 

 coasts ; or again, the ages when winter had become 

 so severe that all the land was buried deep in snow, 

 and the ocean itself sealed with ice. These varying 

 details, however, do not prevent us from going back 

 to a remote past, when probably the birds inhabiting 

 temperate and even northern regions were sedentary. 

 That this was probably the case in Eocene times is 

 reflected in the fact of the wondrous wealth of vege- 

 tation, indicating a corresponding high degree of 

 temperature then prevailing, when palms clothed 

 the English plains, when crocodiles basked in the 

 rivers, and a glorious sub-tropical climate reigned 

 supreme. Or even more recently in Miocene ages, 

 when chestnuts and magnolias, walnuts and vines 

 flourished in Greenland, and the swamp-cypress and 

 the water-lily grew on now desolate and ice-doomed 

 Spitzbergen — when the Lower Miocene cUmate of 

 Switzerland, for instance, resembled that of North 

 Africa to-day. To understand the subject clearly, 

 it will now be necessary briefly to review the changes 

 which have undoubtedly occurred during this 

 remote past of Tertiary time, and of which we 

 have abundant geological and palaeontological evi- 

 dence, and which are still further confirmed by 

 astronomical calculation. 



From Dr. Croll's published tables [Philosophical 

 Magazine, xxxvi. 1868), showing the amount of 

 the Earth's eccentricity of orbit for the past three 

 million years, it appears that a long-continued 

 period of exceptionally high eccentricity occurred 



