102 Allen, Origin and Distribution of N. A. Birds. I April 



climatic barrier, and have thus been unable to reach any part of 

 the Eurasiatic continent. 



The Glacial Period and the change of climate it produced has 

 not only had a profound and permanent influence upon the dis- 

 tribution of life in the northern hemisphere, but proved a 

 powerful agent in the evolution of new types, and in the extinc- 

 tion of old ones ; it also left its impress upon the majority of 

 northern birds through the development in them of the habit of 

 migration : for it is now generally conceded that this habit must 

 have originated in consequence of the great reduction of temper- 

 ature beginning about the close of the Tertiary. Prior to this 

 period, owing to the comparatively uniform climatic conditions 

 then obtaining almost everywhere, there could have been no 

 necessity for extended periodic movements. Later a nearly 

 uniform temperature throughout the year gave place in northern 

 latitudes to warm summers followed by winters of great severity. 

 During the waning of the ice period the area offering a congenial 

 summer home to a great multitude of birds became greatly ex- 

 tended, from which, however, they were driven by semi-arctic 

 winters to seek favorable winter haunts further southward. 



We have here what seems a natural and reasonable hypothesis 

 for the origin of migration, and one now currently accepted by 

 ornithologists. In this way, it is believed, the habit of migra- 

 tion not only originated but has become established as an irre- 

 sistible hereditary impulse, as inherent and mandatory as the 

 'instinct' of reproduction. 



In this connection, a few words may not be out of place respect- 

 ing the question, Why do birds migrate? In considering this 

 question, it should be borne in mind that there is everywhere, and 

 has ever been, a constant and intense struggle for existence — that 

 throughout nature the birth rate is far above the possibilities of 

 permanent increase. Hence, in the bird world, as elsewhere, 

 every station affording favorable conditions for existence must be 

 occupied ; there can be no unutilized areas. Many birds are 

 organized to subsist only upon insects or soft fruits, or upon both 

 combined ; such food abounds in summer far to the northward of 

 where it can be obtained in winter. Thus some of our Swifts, 

 Swallows and Flycatchers range in summer almost to the borders 

 of the Arctic Circle, where for a few weeks in summer they find 

 abundance of food and a congenial temperature. Here they 



