254 SIBERIA IN EUROPE. CHAP. XX. 



assumed, by blind instinct ; that they are guided by pro- 

 minent landmarks with which they have gradually become 

 familiar; and that many birds which are not gregarious at 

 any other time of the year become so during the periods of 

 migration, in order to avail themselves of the experience of 

 the veteran travellers of their own or of other species. The 

 desire to migrate is a hereditary instinct originally formed 

 and continually kept up by the necessity to do so in order 

 to maintain a struggle for existence against the changes of 

 temperature, but the direction in which to migrate must be 

 learned afresh by each individual. The theory that migra- 

 tion ordinarily takes place at high elevations is supported by 

 the fact that it is only in dark or cloudy weather that 

 migration on a large scale is observed. It is supposed 

 that the landmarks being obscured by clouds, the birds 

 are obliged to descend to see their way, for it is observed 

 that as soon as the clouds begin to break, the migration 

 apparently comes to an end. On dark nights the stream 

 of migration suddenly stops when the moon rises. Each 

 bird has its time of migration. Weather has, apparently, 

 nothing to do with this date. Good weather does not 

 seem to hasten the birds to their breeding haunts, nor 

 bad weather retard their starting. If the suitable con- 

 junction of circumstances occur during the season of a 

 certain bird's migration, that bird visits the island. If 

 the season goes by without such conjunction, the bird 

 does not visit the island. The period of its migration is 

 over. The migration of this species has taken place at 

 high altitudes, it may be, or by other routes ; and it is in 



