G8 THE WANDERINGS OF ANIMALS [CH. 



circumstances these rodents suddenly increase, enor- 

 mous numbers radiate from the highlands of Norway 

 into the lowlands. At any given spot they seem to 

 keep to the same direction, but taken as a whole, 

 they disperse to anywhere. The overcrowded con- 

 dition of their homes impels them to leave, and this 

 impulse they follow blindly, even, it is alleged, into 

 the sea. But they do not attempt to settle in a new 

 district. A year or two after such an irruption not 

 a lemming is to be found, and where, during the 

 stampede they came across suitable districts, they 

 found these already occupied by resident lemmings. 

 The same applies to the countless numbers of sand- 

 grouse, Syrrhaptes paradvxiis, which in the spring 

 of 1888 came from somewhere in Central Asia and 

 spread over the whole of Europe, like a wave reach- 

 ing from Norway to England and Spain. Many did 

 breed in suitable places in the same year, few in 1889 

 when thev were exterminated. 



i 



Such and similar irruptions have no doubt taken 

 place often during the world's history, and yet such 

 sporadic stampedes into a foreign country hardly 

 ever lead to regular settlements, especially if such 

 a country already possesses a kindred fauna of its 

 own. Almost every year some North American birds 

 are reported on the west coast of Europe ; but 

 although such stragglers must have come over to 

 Europe for thousands of years, the respective species 



