DEEPER PROBLEMS OF MIGRATION 287 



follow the distribution of minute crustaceans in the sea. 

 We recall the march of the lemmings from the Tundra, 

 and from the Scandinavian valleys, where they have 

 become too numerous ; they press westward in search of 

 food, through villages, over walls, across rivers, until they 

 reach the North Sea, which solves their population problem 

 But this is no more a true migration than are the invasions 

 of rats and voles that occur every now and then as the 

 result of some unwonted disturbance of the balance of 

 nature. 



Among mammals there are not a few records of periodic 

 movements, for instance, among deer, among bats, and 

 among cetaceans, but they do not all fulfil the criteria of 

 true migration. This is illustrated, however, by many 

 seals, though not by our common British Phoca vitulina, 

 which travel periodically to their breeding-places. 



In accepting the view that migration is instinctive, 

 we do not suppose that this explains much, or that there is 

 no learning to be done. The establishment of an instinct 

 requires analysis just like that of a habit, and there are many 

 instincts that are improved for the individual by experience. 

 We have to avoid the one extreme, well stated and forcibly 

 rejected by Mr. Dixon, that " birds migrate by instinct, 

 knowing not how or why, impelled along a course that 

 never errs or changes, flying from point to point with no 

 more mental effort than an arrow from a bow." The 

 prompting, this author says, is instinctive, but there is much 

 that the bird has to learn. The instinct is very variable 

 and often imperfect, for " birds blunder like human folk, 

 losing their way, and perishing in uncounted hosts/' We 

 have to avoid the other extreme of supposing that a success- 

 ful migratory journey is the outcome of sensory alertness 

 and good judgment. Somewhere between these two 

 extremes the truth probably lies. There is a migratory 



