The Migration of Birds 179 



Previous to the Glacial period, tropical conditions 

 prevailed in the north temperate zone; hence tern-: 

 perate climate must have extended nearly to the 

 north pole; but even here probably a sufficient dif- 

 ference existed between summer and winter to render 

 a certain degree of migration necessary. These con- 

 ditions must have continued for centuries, until the 

 coming of the Glacial period, which, brief as it was, 

 geologically speaking, wrought wonderful changes in 

 the zoological world. » 



Let us stop for a moment to consider how significant 

 a factor the Glacial period in North America must 

 have been in bringing about this slow but sure change 

 in the habits of our birds. From some point in the 

 north which has not yet been determined the ice- 

 field began to stretch slowly southward, until Canada 

 was buried; and finally a portion of the United States, 

 including the region of the Great Lakes eastward to 

 the Atlantic Ocean, southward into New Jersey and 

 Pennsylvania, and westward to the Missouri, was 

 also completely covered by the ice sheet. Similar ice 

 sheets prevailed in northern Europe and Asia. It 

 seems probable that the migration of birds was one 

 of the great results of the Glacial period. Had the 

 Glacial period never occurred, and had the temperate 

 conditions of the north frigid zone remained as they 



