英语听力:自然百科 大迁徙 Great Migrations—7(在线收听

 Then the brilliant masses take to the wing, heading to a place they've never been. Their only map, imprinted on their genes. A mysterious interplay of sun on eyes and antennae, an uncanny sense of magnetic north guides the way. 

 
It will take five months for three generations to get the species as far north as Canada and a fourth super-generation to get them back to Mexico. Such a multi-generational migration is incredibly rare. It takes the leading generation three weeks to fly 500 miles into the southern U.S..
 
These intrepid elders sense the deaths are approaching fast. There is urgency now to pass on the migratory baton. The female, having carried her eggs from Mexico, now searches for the monarch’s life-giving grail. 
 
The milkweed sprouting all around, her offspring will be laid on it, hatch on it, nurse on its toxic milk and become toxic themselves, a defense they will flaunt with their singular beauty. 
 
She is dying, but with luck, her children, grandchildren and great grandchildren will carry on the journey as far north as Canada and then an astounding 2,000 miles back to Mexico.
  原文地址:http://www.tingroom.com/lesson/zrbaike/2010/259335.html