XIII 



No hard and fast line between migrants and non-migrants — 

 Swallows and partridges — Contrasted behaviour in two 

 mocking-birds — Spur-wing lapwing — An instinct in a state 

 of flux — Migration in other creatures — Fishes and insects — 

 Kirby and Spence speculate — Spiders — ^Mammals — Migration 

 a danger — Sand-grouse and the " Tartar invasions of Europe " 

 — A "sense of polarity" the origin of migration — A trace of 

 this sense in man. 



ONE of the first facts confronting us concern- 

 ing the migration of birds is that there is 

 no dividing line between migrants and non- 

 migrants. One would say off-hand that there is a 

 great difference indeed — a very hard and fast dividing 

 line between the swallow, let us say, and the partridge. 

 It is not so when we come to consider that swallows 

 are not always migratory; that there are regions 

 where they remain all the year round; that in some 

 countries this migration is only a partial one; that 

 even in a country so far north as England, where the 

 flies they subsist on do not exist in winter, the impulse 

 to migrate fails in some individuals, and these remain 

 in a torpid condition like bats and female bumble- 

 bees and wasps, and no doubt perish in most 

 instances before the return of warm weather. 



When we come to a milder climate, such as that of 

 the Argentine pampas, every observer must see that 



a considerable number of one of the most common 



205 



