42 THE MIGRATION OF BIRDS 



later the shortening of the journey, through lengthen- 

 ing of the short cut, would lead the birds to Vera Cruz 

 and later still to Yucatan. It may be questioned, 

 what object could the birds have in risking an over- 

 sea voyage, away from chance of food and hope of 

 rest, when the land-bridge remained open for them ? 

 Each individual or group of individuals which ar- 

 rived at any particular place a little in advance of 

 the migrating multitudes of its own species, or others 

 which fed upon the same kind of food, would certainly 

 gain advantage, and would be the most likely to 

 develop strong flight and the power of endurance in 

 its descendants ; it would indeed be a winner in life's 

 race. 



Great weight has been placed upon the use of 

 land-bridges and the hereditary habit of crossing 

 seas where these land-bridges once existed but have 

 been submerged during the great geological changes 

 in the earth's surface. Many have insisted that 

 wherever migrants cross the sea they do so along 

 submerged coast-lines or over submerged land- 

 bridges, arguing that the gradual evolution which 

 has made the advantageous adoption of a habit of 

 migration possible was unable to eliminate the 

 hereditary tendency to follow the exact route by 

 which their ancestors passed from place to place. 

 That there have been considerable alterations in 

 coast-lines and in the general distribution of land 



