4 PROLOGUE 



without previous experience, without knowledge of 

 Hfe, ultimately to winter on predestined grounds of 

 the very existence of which they are completely 

 ignorant. 



Few achievements in the animal kingdom parallel 

 this, the most striking aspect of bird migration. As 

 we turn the pages of our book, we shall see some- 

 thing of the many difficulties that confront us when 

 we attempt to explain it in terms of modern sci- 

 entific knowledge. But the attempt is worth while 

 for it takes us far beyond the world of birds and 

 their migrations. It brings us face to face with the 

 fundamental problems of Life. A solution of but a 

 single phase may unexpectedly widen the present 

 boundaries of biological knowledge. 



