THE COMING OF THE BIRDS 91 



routes. There is really no such thing as "instinct." 

 The habits of the birds have been evolved under 

 pressure or guidance from without. So there is 

 nothing to "tell" them that land and water have 

 changed very much since the days of the Ice Age, 

 and year by year they follow the old routes, many of 

 which ought to have been altered long ago. 



