Around the globe, millions of creatures depend on their mobility to survive. From the tiniest organisms to majestic mammals, they know that they must migrate or die trying. The National Geographic Channel's "Great Migrations," which premieres Nov. 7, follows the astonishing and awe-inspiring paths of the sperm whale, the wildebeest, the monarch butterfly and the red crab. Every year, more than a million wildebeest and 200,000 zebras must chase the seasonal rains, in a 300-mile loop around Tanzania and Kenya.