270 RECREATIONS OF A NATURALIST 



Dr A. R. Wallace, that there is a connection be- 

 tween the routes taken and the position of sub- 

 merged land over which the birds migrated in past 

 ages. Possibly this may be so, though probably the 

 connection is accidental. At anyrate, to prove 

 the case, it would be necessary to show that the 

 habit of migration is older than the subsidence of 

 the submerged land, and that the migrants prefer a 

 longer route over a shallow sea to a shorter route 

 over a deep one. 



From what has been actually observed, it would 

 appear that birds on passage, as a rule, follow the 

 coast lines, the courses of great rivers, and the low 

 country lying at the base of mountain chains. 



As regards the direction taken by migrating 

 flocks, I shall presently invite attention to what 

 Engflish naturalists have been able to advance on 

 this point, through the instrumentality of the light- 

 house keepers ; but, before taking leave of the 

 Continental authorities just quoted, it may be well 

 to consider their views (shared as they are by many 

 of our ornithologists) respecting the cause of migra- 

 tion, and the. faculty by which it is performed. 



It would seem that there are at least three 

 primary causes for the migration of birds: (i) 

 failure of food at certain seasons ; (2) change of 

 temperature; and (3) what may be termed "home- 

 affection,'' or a longing to return to the real home, 

 which (as before stated) is the country where 

 it rears its young, or where it has itself been 

 reared. 



It is t\\e first of these causes apparently which 



