2,6 THE MIGRATIONS OF BIRDS 



In the next geological period, the Pleistocene or 

 Ice Age, huge sheets or glacial ice spread down from 

 the north across the land, advancing and retreating 

 during tens of thousands of years. Periods of glacial 

 invasion were followed for unknown reasons by 

 times of warmth, during which the ice retreated, 

 when forms that we now consider tropical and sub- 

 tropical were able to flourish in arctic regions. Fol- 

 lowing these, the ice again came south. The ad- 

 vance of the glacial front was slow, probably almost 

 imperceptible, so that the hand of cold crept slowly 

 over the land. Life, both plant and animal, shivered 

 in the unwonted chill. Some forms, possibly of wide 

 range and adaptability, retreated slightly; others 

 less pliant, unable to change from their accustomed 

 habit and range, perished and disappeared, leaving 

 no record except as their harder portions were en- 

 tombed and preserved beneath the ice. Congestion 

 in the steadily decreasing land area toward the 

 Equator crowded others, and through an increasing 

 competition brought about further extermination of 

 individuals and species. 



Let us look now for a moment at conditions during 

 the first, or Wisconsin, stage when ice extended 

 across Canada and the northern United States as far 

 as New Jersey, Pennsylvania, and Illinois. With the 

 exception of a few islands of land left bare through 

 some freak or accident, ice covered a vast area where 



