64 THE WANDERINGS OF ANIMALS [CH. 



unoccupied ground. The annual repetition became 

 an established habit, at last an ineradicable instinct. 



The prime impulse is want of food. The new 

 growing grass on the prairie attracts every year those 

 creatures which live upon pasture. The tropical belt 

 of the world is so crowded that there is the keenest 

 competition, whilst in the temperate and cold regions 

 occurs a long winter unfit for the support of many 

 kinds, whereupon in the summer these same regions 

 are covered with new vegetation, with its concomitant 

 abundance of insects and other invertebrates. The 

 tables are decked again, and these opportunities are 

 not wasted. 



Hard and fast differences between periodical mi- 

 grations, sporadic changes of the abode, and other 

 fluctuations do not exist, they are a question of 

 degree. The grey plover of America breeds within 

 the arctic circle and winters in tropical countries ; 

 many other birds shift their abode only a few hundred 

 miles. To complicate matters further, it is not neces- 

 sary that the migration be undertaken periodically, 

 more than once, by the same individual. The common 

 eel ascends the rivers as an ' elver,' in its youth ; 

 years after, it returns to the sea, there to breed and 

 die, whilst many other fishes come and go, year after 

 year. Many birds are still immature in their second 

 year, and yet they migrate like their adult relations. 

 It seems permissible to take this fact as an indication 



