60 ORIGIN OF MIGRATION. 
return of winter. Thus the habit of going north in the 
spring and returning in the fall was eventually formed. 
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 
