CHAP. xx. HELIGOLAND. 249 



nection between these routes and the position of submerged 

 continents across which the birds migrated in past ages. 

 Probably there is some such connection, but in all proba- 

 bility an accidental one. To prove the case it would be 

 necessary to show that migratory birds chose a longer route 

 across a shallow sea in preference to a shorter route across a 

 deep sea. It would be necessary also to prove that the 

 habit of migration is older than the subsidence of the 

 submerged land. 



I venture to think that the modus operandi of migration 

 has been to a large extent misunderstood. Few birds 

 migrate by day. By far the greater number of species 

 migrate by night. The number of places where nocturnal 

 migrations can be systematically observed is very small. 

 Two circumstances are requisite to make such observations 

 successful. First, a sufficiently large population sufficiently 

 interested in the event to permit no nocturnal migration to 

 pass undiscovered. Second, a sufficiently intelligent na- 

 turalist to record the sum of many years' observation. Pro- 

 bably in no place in the world are these desiderata so exactly 

 fulfilled as upon the island of Heligoland. Soon after my 

 return from the valley of the Petchora, Mr. Gatke, the cele- 

 brated ornithologist and artist, who has resided for so many 

 years on Heligoland, invited me to visit the island, to renew 

 the acquaintance of the grey plover, the little stint, the 

 blue-throat, the shore-lark, the little bunting, and others of 

 my Petchora friends, and to see something of the wonderful 

 stream of migration which sets in every autumn from the 

 Arctic regions to the sunny South, and flows abundantly past 



