CASES OF ADAPTATIOjST 163 



less myriads that are seen passing over Heligoland are but 

 a minute fraction of those that really pass, high up and quite 

 out of sight. This is shown by the fact, that if, on a dark 

 night, it suddenly clears and the moon comes out, the swarms 

 of birds immediately cease. Another fact is, that, on what 

 the islanders call '' good nights,'^ the birds that come to rest 

 seem to drop down suddenly out of the sky. One other fact 

 is mentioned by Mr. Seebohm. It is that every year the reg- 

 ular migration season is preceded by a week or two, during 

 which a few stragglers appear ; and these are all old birds 

 and many of them slightly crippled, or partially moulted, or 

 without some of their toes, or onlv half a tail, or some other 

 defect. These are supposed to be mostly unmated birds or 

 those whose young have been destroyed. It is also supposed 

 that, during favourable weather (for the birds) migration goes 

 on continuously during the season of about six weeks, though 

 for the most part invisible at Heligoland, but often audible 

 when quite invisible. 



Xow, the fact of the young birds only appearing on Heligo- 

 land for the first week or so of the season of each species is 

 easily explicable. Rem.embering that the autumnal migration 

 includes most of the parent birds and such of their broods as 

 have sur^dved, it is probable that the latter will form at least 

 half or, more often, two-thirds of each migrating flock. But 

 the young birds, not having yet acquired the full strength 

 of the adults, and having had little, if any experience, in 

 long and continuous flights, a considerable proportion of them 

 on the occasion of their first long flight over the sea, on see- 

 ing the lighthouse and knowing already that lights imply land 

 and food-crops below them, and being also much fatigued, will 

 simply drop down to rest just as they are described as do- 

 ing. The old birds and the stronger young ones, however, 

 pass high over head, till they reach the north coast of Hol- 

 land, or, in some cases, pass over to our eastern coasts. We 

 must also remember that the loncrer the birds are in mak- 

 ing the journey overland, the more young birds are lost by 



