Migration 



appearance considerably : their backs become darker, their 

 under sides lighter and their eyes larger. The silver eels, 

 as they are now called, like the elvers, migrate only by 

 night, and on moonless nights at that ; occasionally they 

 travel massed together in balls. After spawning the eels 

 die, so that, unlike the salmon, they migrate only once 

 during their adult lives. Frequently eels have been 

 observed travelling overland from place to place, wriggling 

 through the herbage, after the manner of a snake. These 

 land journeys can hardly be termed migration. 



Amongst quadrupeds migration is by no means common. 

 The reindeer is a migrant. In Spitzbergen, where there 

 are large herds of these animals, the reindeer seek the 

 valleys during the short Arctic summer, but in autumn 

 they turn their steps back to the sea coast, where they 

 feed mainly upon seaweed. It is said that a female 

 always leads the migratory herds. In Arctic America, 

 where the reindeer is known as the caribou, there is a 

 regular southward migration in winter, but in summer the 

 herds return to their northern uplands. 



The South African springbok is also an occasional 

 migrant. When the absence of rain causes its almost 

 desert home to become absolutely barren, it migrates 

 eastwards to more fertile regions. Travellers have 

 described enormous herds of these migratory antelopes 

 moving in a solid mass of over half-a-mile in width and 

 taking over two hours to pass a given point. They have 

 even related that the herds were of such enormous size 

 that a lion, attempting to prey upon one of their number, 

 became so surrounded by the migrants that escape was 

 impossible and he was forced to march along with them. 



Thirst is the compelling force in the case of the spring- 

 boks, and not long ago thousands upon thousands 

 migrated to the sea, drank of the salt water and died in 

 countless numbers ; for miles upon miles the shores of 

 the Indian Ocean were strewn with their dead and 

 decaying bodies. 



