326 MIGRATION OF BIRDS. 



under the waves, the birds would continue to take the same 

 route as before, but would extend their flight from shore to 

 shore. The Mediterranean Sea is also crossed by several 

 stretches of submerged land which formerly connected 

 Europe with Africa, and it is found that the principal 

 routes taken at the present time by migratory birds when 

 crossing that sea are over these stretches. 



It is probable that, owing to the causes above men- 

 tioned, certain species of birds have migrated to a greater 

 or lesser extent ever since they appeared in any consider- 

 able numbers on the surface of the earth, and that a 

 hereditary tendency to migrate in autumn and spring has 

 become a part of their nature, in the same way as a 

 hereditary tendency to point game has become a part of 

 the nature of a well-bred pointer, for a young dog of this 

 kind which has never seen game will often point it on the 

 first occasion on which the dog is taken into the field. 



Birds while on migration fly very high in clear 

 weather, when, doubtless, they can see a great extent of 

 the surrounding country. That they are guided on their 

 journey by experienced leaders who have migrated in 

 previous years, and who thus know the landmarks to 

 guide them on their way, is very probable. It is not 

 likely that the young intuitively know the route to be 

 taken on migration any more than a young untrained 

 homing pigeon knows the direction in which to fly to 

 reach its cote, when it is conveyed a long distance away 

 from its native haunt. A homing pigeon is gradually 

 trained to return from long distances by taking it away 

 from its dovecot for short distances in the first place, and 

 increasing these by degrees to enable it to become acquainted 

 with the landmarks of the intervening country. Many 

 homing pigeons are lost if they are flown during foggy 

 weather, thus showing that they are guided in their flight 



