CHAPTER V 

 Summary 



In conclusion we may briefly review the line of 

 thought that has been adopted in the foregoing 

 pages. 



The fact that birds are intimately adapted to 

 journeying in the air forms the fundamental basis 

 for the stupendous migrations that are so charac- 

 teristic of many species. Air surrounds the entire 

 globe and, apart from storms which are temporary, 

 is without barriers. It is the ideal medium for 

 continuous travel. Ability to use it, however, does 

 not constitute migration. It merely makes exten- 

 sive migrations possible. A bird living in an en- 

 vironment congenial throughout the year will re- 

 main non-migratory. It may even forfeit its ability 

 to use the air and become flightless in non-com- 

 petitive surroundings. 



The environment must, therefore, be looked upon 

 as an integral factor in the evolution of migrations. 

 Particular attention has been paid to conditions in 

 northern regions of the northern hemisphere and 

 an attempt has been made to ascertain how they 

 could have induced the custom of migration. The 



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