GEOGRAPHIC DISTRIBUTION 319 



tion anywhere which seems to demand periodic migration as its explana- 

 tion. The mere fact that the animals have to return from the place 

 to which they periodically travel nullifies any effect which such move- 

 ments might have on the size of the range. Migration would have to 

 be accompanied by some physiological change in order to extend the 

 area occupied. 



Sporadic Migration. — Somewhat allied to periodic movements pei'- 

 haps are the sudden outbreaks or irruptions of a species that may occur, 

 during which the range is widely extended. The classic example is that 

 of the Lapland lemming, a small rodent. The migration of this species 

 has been described by Lyell as follows. 



" Once or twice in a quarter of a century they appear in vast numbers, advanc- 

 ing along the ground and 'devouring every green thing.' Innumerable bands 

 march from Kolen, through Northland and Finmark, to the Western Ocean, 

 which they immediately enter; and after swimming about for some time, perish. 

 Other bands take their route through Swedish Lapland to the Bothnian Gulf, 

 where they are drowned in the same manner. They are followed in their journey 

 by bears, wolves and foxes, Avhich prey upon them incessantly. They generally 

 move in lines, which are about three feet from each other, and exactly parallel, 

 going directly forward through rivers and lakes ; and when they meet with stacks 

 of hay or corn gnawing their way through them instead of passing around." 



Another case of sudden movements is afforded by Pallas's sand 

 grouse. This species inhabits the steppes of central Asia, extending 

 into northern China and the Kirghiz Steppes north of the Aral Sea in 

 the winter. At least since 1859 the bird has been in a restless and dis- 

 turbed state and great waves of individuals have moved out from the 

 normal range. In an irruption in 1859 some of them reached Poland, 

 Holland, and the British Isles. Another outbreak in 1863 apparently 

 involved thousands of individuals, and the birds reached Italy and the 

 Pyrenees in the south of Europe, Scandinavia and Archangel in the north, 

 and the British Isles and the Faroes in the west. Still another wave 

 occurred in 1888 and at this time flocks appeared in England, Scotland, 

 and Ireland. After each wave the species soon disappeared from the 

 invaded countries. The extinction may have been due to slaughter by 

 man; but while some of the invaders bred the first year, they were not 

 so well established that they could have reared young. 



Such sporadic outbreaks are apparently of the same nature as those 

 which have been observed within the range of a species. An example 

 is the mouse plague of 1907-1908 in the Humboldt Valley, Nevada. 

 These mice (Microtus montanus), which live in scattered colonies in 

 swampy places, are not usually abundant enough to attract notice. 

 They produce half a dozen at a litter and four to six litters per year, 

 but ordinarily are kept in check. In the year named, however, some 



