810 ZOOLOGY 



cases in which it can he demonstrated that their paths are 

 along ancient shore lines. As Wallace says : " It is easy 

 to see how the migrations that had once taken place over 

 continuous land would be kept up first over lagoons and 

 marshes, then over a narrow channel, and subsequently 

 over a considerable sea, no one generation of birds ever 

 perceiving any difference in the route." The migration 

 routes of North America have not been well studied. The 

 Atlantic coast is one line ; the Mississippi valley, for- 

 merly, no doubt, an arm of the sea, is another route along 

 which even sea-birds now migrate. The question how 

 birds recognize and are able to follow their migration 

 routes, is a difficult one ; they seem to have a marvellous 

 sense of direction. There is great need of a more perfect 

 knowledge of migration phenomena in North America. 

 Every one who has learned to distinguish accurately the 

 species of birds can contribute to this knowledge by keep- 

 ing records of the time of appearance of migrants in the 

 spring and the fall. The American Ornithologists' Union 

 has for many years solicited observations on this subject 

 from voluntary collaborators. 



v 



Bird Flight. - The difficulties which must be overcome 

 in order to fit birds for flight are, first, that of sustaining 

 the heavy body in a medium of such low specific gravity as 

 the air and, secondly, that of progression in this medium. 

 To diminish the difficulty of sustaining the body, the 

 specific gravity is reduced to a minimum by great air- 

 spaces in the body, which exist even in the hollow bones. 

 The body is kept from being overturned in the air by the 

 position of the wings, which are placed high up on the 

 trunk while the digestive organs, breastbone, and breast 

 muscles are placed low. To aid locomotion, the general 



