1 98 ANIMALS AT WORK AND PL A Y 



particular country was the natural home of all the 

 birds which we call ' spring migrants/ such as the 

 swallow and the nightingale ; and that when winter 

 came, bringing cold and hunger, these deserted their 

 home and sought a temporary shelter in warmer 

 climates, whence they hurried back to their dulces 

 nidos\ like ships' crews impatient to revisit a frozen 

 port which was nevertheless their home. This 

 accounted for the departure of the birds in 

 autumn and their return in spring. The visits of 

 northern birds in winter were only a result of the 

 same causes, working in more northern lands. It 

 was a theory as simple as Homer's notion of the 

 ocean stream circling in endless flow round the rim 

 of the inhabited world. When the naturalists grew 

 dissatisfied, and demanded more data, and began to 

 sit in lighthouses through the winter nights to note 

 the migrants coming and going, and sift reports from 

 those whose duty makes them watchers by the shore, 

 the apparent simplicity of birds' travel could no 

 longer be admitted. There seemed neither limit nor 

 law in the incessant and perplexing streams of bird 

 movement. The problem of the real ocean currents, 

 compared with the ancient belief in the constant 

 ocean stream, was simple in comparison with the 

 crossing and recrossing, the apparent contradiction 

 and want of purpose in the journeys of the birds. 



