150 THE WORLD OF LIFE CHAP. 



facts observed at Heligoland show that no such inference is 

 required to explain the admitted fact. Let us see what 

 these additional facts are. 



The enormous rushes of migratory birds which rest at 

 Heligoland always occur at night, and are very intermittent. 

 They usually take place on dark nights, sometimes in 

 millions ; at other times, a week will sometimes pass with 



FIG. 22. THE 



LIGHTHOUSE AT 



HELIGOLAND ON A 



MIGRATION NIGHT. 



only a few stragglers. Of one such pitch-dark night Mr. 

 Seebohm writes : 



"Arrived at the lighthouse, an intensely interesting scene pre- 

 sented itself. The whole of the zone of light within range of the 

 mirrors was alive with birds coming and going. Nothing else was 

 visible in the darkness of the night, but the lanthorn of the light- 

 house vignetted in a drifting sea of birds. From the darkness in 

 the east, clouds of birds were continually emerging in an unin- 

 terrupted stream ; a few swerved from their course, fluttered for a 

 moment as if dazzled by the light, and then gradually vanished 

 with the rest in the western gloom. ... I should be afraid to 

 hazard a guess as to the hundreds of thousands that must have 

 passed in a couple of hours ; but the stray birds that the lighthouse- 

 man succeeded in capturing amounted to nearly 300." 



