3$ • THE MIGRATION OF BIRDS, 



Migrations of birds became increasingly pronounced^ 

 and its culminating point was reached when the 

 North Polar world became covered with a vast icy 

 mantle, and all living things were either killed or 

 banished to more southern latitudes. During the 

 initial stages of the Glacial Epoch, when eccentricity 

 was not so high as at a later phase, alternations of 

 warm and cold periods were caused by winter occur- 

 ring in perihelion or aphelion. But as long as 

 eccentricity remained high and at its climax, pre- 

 cession had little effect upon the glaciated Pole, and 

 only a partial amelioration of climate took place 

 during the inter-glacial periods in more southern 

 regions. When eccentricity was considerably dimin- 

 ished precession had again a more marked effect 

 upon the climate, and a warmer temperature pre- 

 vailed than is now the case. Since the passing away 

 of the Glacial Epoch the eccentricity of the earth's 

 orbit rapidly decreased, and for the past 60,000 

 years it has remained uniformly low, with this 

 mevitable consequence, that the changes of climate 

 produced by precession have been correspondingly 

 slight, and the climate of the north temperate zone 

 has remained in an exceptional state of stability. 

 One inevitable result of the Glacial Epoch was to 

 place Migration on a very different basis from what 

 It had occupied before. All the Prae-Pliocene order 

 of things had passed away ; continents had become 

 more closely knit together ; seas had vanished ;. 

 archipelagoes disappeared ; warm currents had been 

 diverted and checked ; Polar land elevated. 



