300 PRINCIPLES OF ANIMAL BIOLOGY 



Migration. — An important factor in geography is the wanderings of 

 animals. Migration in some form or other is a fundamental attribute 

 of animals. There are several kinds of migration, which may be termed 

 periodic, sporadic, and normal. 



Periodic Migration. — Certain forms move in large numbers from 

 one place to another at different times of the year or at different times 

 in their life-history. The southward migration of many birds in the fall 

 and their return in the spring is an example of seasonal migration. The 

 freshwater eel migrates at times separated by an interval of years. In 

 its youth this animal ascends the rivers from the sea and lives there for 

 years but does not breed; upon reaching maturity it returns to the sea to 

 breed. This kind of migration may be called periodic, and it is doubtful 

 whether it is effective in extending the range. 



Sporadic Migration. — Somewhat allied to periodic movements perhaps 

 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 mammal related to the rats and mice. The 

 migration of this species has been described by Lyell as follows: "Once 

 or twice in a quarter of a century the}^ appear in vast numbers, advancing 

 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 Lap- 

 land to the Bothnian Gulf, where they are drowned in the same manner. 

 They are followed in their journey by bears, wolves and foxes, which prey 

 upon them incessantly. They generally move in lines, which arc 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 the Pallas 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 disturbed 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 outVjreak 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 l']ngland, 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 th(! 

 invaders br(;d the first year they were not so well established that they 

 could have reared young. Such sporadic outbreaks are apparently of 



