NOCTURNAL MIGRATION 195 



summer migrants to that country. In some cases no 

 doubt the weaker birds are turned out by the stronger. 

 Swallows arrive in Dlisseldorf early in April. For some 

 weeks they circle over the town, like a swarm of bees. 

 Early in May the swifts arrive, and soon become as 

 abundant as the swallows were, whilst the latter birds are 

 rarely seen during the summer. 



Too much has probably been made of the great lines 

 of migration, the highways which lead from the summer 

 to the winter quarters.* It has been asserted that there 

 is a connection 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 probability 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 submergfed land. 



I venture to think that the modus operandi of migra- 

 tion has been to a largfe 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 naturalist to record the sum of 

 many years' observation. Probably in no place in the 

 world are these desiderata so exactly fulfilled as upon the 



* This and the following paragraphs have been left intact, but how much 

 Mr. Seebohm was afterwards led to modify his views on the subject of migration 

 may be seen by reference to page 418 in Part II. of this volume. ^ — Ed. 



