60 ORIGIN OP MIGRATION. 



tion in the southern hemisphere, where no evidences of 

 glaciation have as yet been discovered. 



As I have said, the existing conditions are the result 

 of changes which have been active for ages. No species, 

 therefore, has acquired its present summer range at one 

 step, but by gradually adding new territory to its breed- 

 ing ground. For example, certain of our Eastern birds 

 are evidently derived through Mexico, and in returning 

 to their winter quarters in Central America, they travel 

 through Texas and Mexico and are unknown in Florida 

 and the West Indies. Others have come to us through 

 Florida, and in returning to their winter quarters do not 

 pass through either Texas or Mexico. This is best illus- 

 trated by the Bobolink, an Eastern bird which, breeding 

 from New Jersey northward to Nova Scotia, has spread 

 westward until it has reached Utah and northern Mon- 

 tana. But — and here is the interesting point — ^these birds 

 of the far West do not follow their neighbors and migrate 

 southward through the Great Basin into Mexico, but, 

 true to their inherited habit, retrace their steps, and leave 

 the United States by the roundabout way of Florida, 

 crossing thence to Cuba, Jamaica, and Yucatan, and win- 

 tering south of the Amazon. The Bobolinks of Utah 

 did not learn this route in one generation ; they inherited 

 the experience of countless generations, slowly acquired 

 as the species extended its range westward, and in return- 

 ing across the continent they give us an excellent illustra- 

 tion of the stability of routes of migration. 



They furnish, too, an instance of one of the most 

 important factors in migration — that is, the certainty 

 with which a bird returns to the region of its birth. 

 This is further evidenced by certain sea birds which 

 nest on isolated islets to which they regularly return 

 each year. 



Of this wonderful " homing instinct," which plays so 



