The Migration of Birds 179 
Previous to the Glacial period, tropical conditions 
prevailed in the north temperate zone; hence tem- 
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 
