38 THE MIGRATIONS OF BIRDS 



climate continued; and as centuries passed, mo- 

 mentary necessity became habit, and regular lines 

 of migration were established. 



Where glacial action has not figured definitely, we 

 may look to other factors, which affect similar eco- 

 logic changes, to explain migratory movements 

 among birds. Extension of forest areas, or of plains, 

 or growth of marshes may permit extension among 

 avian species that seize on the new range as terri- 

 tory in which they may breed. After young are 

 reared, they may wander or withdraw — move- 

 ments which, with the passage of time, become 

 fixed and hereditary. 



Such to me seem logical hypotheses, which afford 

 explanation of the wonderful phenomenon of bird 

 migration. It is admitted that they are not fully 

 satisfactory and that there are a number of points 

 that are not definitely covered. Explanation of these 

 cannot now be attempted. We must recognize that 

 Man's actual experience of these happenings, to 

 which we may look for assistance in considering 

 them, is wholly negligible, since the number of years 

 of our recorded study and observation do not repre- 

 sent a dot of perceptible size in the long procession 

 of centuries that have covered the development of 

 these things. 



