PLATE III. 



The Advance of Spring 

 After H. Hildebrandsson 



The map illustrates the march of isotherm 48° F., "the climatic beginning 

 of spring," across Europe. 



The lines show the fortnightly advance of this spring temperature from 

 the Mediterranean Sea to the Arctic Ocean, and indicate approximately the 

 dates on which the various regions of the Continent became climatologically 

 suitable for the reception of their bird summer visitors. 



