OCTOBER. 169 



might seem at first sight to support the old idea that 

 they were guided on migration by some mysterious 

 "sense of direction," or were led by experienced 

 birds which "knew the way." These theories are 

 destroyed, however, by the facts that young birds 

 often migrate in advance of their elders, and that 

 both young and old travel far out of their ordinary 

 course when the wind blows excessively from east or 

 west. 



THE ORIGIN OF MIGRATION. 



Originally, no doubt, the birds migrated for 

 shorter distances, and only upon compulsion, remain- 

 ing near their breeding-haunts until actual want of 

 food compelled them to fly to save their lives. This 

 necessity would ordinarily arise when bitter winds 

 from north or east drove away all insect life, covered 

 the ground with snow, or locked it up with frost. 

 Flying before such a wind, the birds would travel to 

 the south and west, soon reaching warmer districts, 

 where they could halt, at any rate, till the cold wind 

 came again and drove them on. In the spring the 

 warm south wind a scorching hot wind it is where 

 the swallows spend their winter would fill the birds' 

 minds with desire to return to their pleasant summer 

 homes; and, flying with it, they could not help 

 travelling in the right direction. Of course, if the 

 northern hemisphere was an unbroken expanse of 

 land, the birds might be carried far to east or west 

 of their destinations, and only by the merest chance 

 when the winds of spring happened exactly to 

 reverse the course of those of autumn would any 



