68 THE RIDDLE OF MIGRATION 



tify these with a considerable show of probabiHty by 

 analysing them as they are making migrations be- 

 fore our very eyes at the present moment. 



The Jurassic epoch, in which Archaeopteryx and 

 Archaeornis make their appearance, was character- 

 ised by the mildness of its climate. Even the ex- 

 treme north was genial and there was no — or at the 

 most but a slight — accumulation of ice even at the 

 poles. Antarctica, now a blizzard-swept waste of 

 snow and ice, produced cycads and other temperate 

 vegetation while Greenland and Alaska boasted 

 enormous forests of species of trees which today 

 characterize the temperate zones of the globe. It 

 is probable that Archaeopteryx and its relatives en- 

 joyed a sub-tropical climate in Bavaria. Condi- 

 tions during the Cretaceous and Eocene, in which 

 the earliest representatives of modern toothless 

 birds first appear, remained much the same but the 

 end of the Eocene saw the inception of a very slow 

 progressive change to cooler conditions which cul- 

 minated in the ice ages of the Pleistocene. The 

 fossil record has preserved the story of the gradual 

 southward retreat of such tropical reptiles as croco- 

 diles and such plants as palms, of the evolution of 

 arctic forms of mollusca and other animals and an 

 invasion, from the north, of yet other forms, com- 

 ing south before the advancing ice-sheets. The 



