EFFECTS OF THE GLACIAL PERIOD 421 



summer of those climates at that period. Probably 

 during the darkest months of midwinter, if the cool season 

 of the pre-glacial period may be called winter, some local 

 migrations took place, and birds wandered back again for 

 a month or two into the adjoining districts, but these 

 little journeys can scarely be dignified with the name of 

 migration. 



In process of time, however, the temperature of the 

 earth appears to have cooled to such an extent that as 

 each pole came to be in aphelion during winter, the winter 

 became so severe that those birds who did not learn to 

 migrate to southern climes perished for lack of food during 

 the cold season. These periods of severe winters lasted 

 for 10,500 years, and were followed by similar periods of 

 mild winters when the cold was transferred to the opposite 

 pole, the complete revolution of the precession of the 

 equinoxes taking about 21,000 years. Then came the 

 glacial period, a period supposed to have lasted 120,000 

 years, when the relative positions of the various planets 

 in the solar system so increased the eccentricity of the 

 earth's orbit, and so exaggerated the severity of the 

 winters, that in consequence of the effects of cold being 

 cumulative (ice and snow not running away as water does) 

 the severity of the winter became at length so great that 

 summer was unable to melt the whole of the previous 

 winter's snow and ice. A permanent glacier having once 

 been formed at the North Pole, and havingf once bridofed 

 over the Arctic Ocean to the continent, would rapidly 

 increase so long as the cause of its existence continued ; 

 and the evidence of geology goes far to prove that, at the 

 height of the glacial epoch, the field of ice measured five 

 or six thousand miles across. As this immense ^lacier 

 marched southwards the palaearctic birds were driven 

 before it, and whilst most of them still came annually to 



