77 2 Ta\-erner, T//e Origin of Migratioii. Lj"ly 



habit that its development is not only possible but very probable, 

 and just what the student of evolution would expect. 



This scattering of the younger individuals, however it was 

 brought about, would then favor the extension of the migration 

 range by the ones thus driven to wander from their accustomed 

 haunts. As further substantiation of this, it is to be noticed that 

 birds found far from their natural haunts are usually immature 

 specimens. 



A young bird on its tirst spring migration, would naturally 

 return to the familiar place where it was raised. Being driven 

 away from here, it would wander about until it found a suitable 

 location for its own breeding — perhaps a mile, perhaps two, may- 

 be less, away from its original home. The succeeding years, 

 it would return to this new haunt, and the range of the species 

 could be extended by its offspring. Thus, each bird would follow 

 the route taken by its parents, and thus each point on a migration 

 route would indicate the place that was once the ultimate goal of 

 the migrations of its ancestors. 



Migrations to true oceanic islands are more difficult to explain 

 along these lines, but I do not think that they invalidate the 

 reasoning in any way. Migrating birds certainly have wonderful, 

 and as yet mysterious, senses of location and direction, and it is 

 not too much to say that a bird, once it has traveled a certain 

 journey, is usually able to find its way over the same path again. 

 A pair of birds have only to be storm-blown to one of these 

 isolated spots, breed there, and return with its progeny, to start a 

 tendency in their offspring to migrate to the same place again. 

 As long as the least tendency to an advantageous migration were 

 started, natural selection would confirm, increase, and fix the 

 habit firmly ; and along with this, the new senses, structures and 

 habits necessary to their accomplishment. It is unlikely, however, 

 that this type of migration could be started until after certain 

 powers and senses had been developed by migrations to other 

 localities. They must, therefore, be regarded as secondary move- 

 ments originally, though in some cases they have become now the 

 prime or only migrations of the species by the extermination of all 

 those individuals that adhered to the original routes. 



The return movement in the fall is the same thing, nearly, as 



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