6 AN ECONOMIC STUDY OF FIELD MICE. 



Outside the insect world the most striking examples of occasional 

 excessive nndtiplication of a species are afforded by rodents. The 

 invasions of rabbits and rats are familiar, but no rodents exhibit the 

 tendency more forcibly than the Mierotina'^ a subfamiW which 

 includes lemmings, voles, and muskrats. The lemmings and voles, 

 especially, are noted for those peculiar waves of increase that astonish 

 observers and bring disaster in their wake. The most noted example 

 is afforded by the someAvhat periodic migrations of lemmings 

 {Lemmus lemmus) in Norway and Sweden. 



These animals live in the higher plateaus of Scandinavia. Here 

 during several favorable years they increase in geometrical ratio 

 until the food supply gives out and hunger impels great hordes to 

 migrate into the lower valleys. Once started on their journey they 

 continue in the same general direction in spite of all obstacles. 

 They travel in vast armies, swimming lakes and streams, li\ang on 

 the products of the soil, and carrying calamity to farmers. They 

 breed on the journey, lingering only until the young are strong 

 enough to travel or until food is exhausted. They are constantly 

 the i^rey of natural enemies which gather in their wake, and are 

 destroyed ruthlessly by man; so that in spite of an enormous natural 

 increase, the vast army gradually melts away. Usually disease breaks 

 out and helps to decimate them, so that as a rule comparatively few 

 reach the final barrier to their march, the sea. After a short delay 

 the survivors, ignorant of the nature of the barrier, plunge into the 

 water and essay its passage, swimming until they perish." The 

 migrations usually cover a period of two years, but are sometimes 

 prolonged to three. None of the migrating animals return to their 

 homes, and they are. entirely absent in the lower valleys until the 

 next migration.^ 



The economic vole {Microtus ceconomus) of Siberia performs 

 somewhat similar migrations. Writing of it over a century ago 

 Thomas Pennant said : " They in certain years make great migra- 

 tions out of Kamtschatka ; they collect in the spring and go off in 

 incredible multitudes. Like the Lemmus, they go in a direct course 

 and nothing stops their progress, neither rivers or arms of t!ie sea: 

 in their passage they often fall a prey to ravenous fishes and birds, 

 but on land they are safe, as the Kamtschatkans pay a superstitious 

 regard for them; and when they find them Ijdng Aveak or half dead 

 with fatigue on the banks, after passing a river, they will give them 



a Prof. Robert Collett, of Christiania, Norway, I'ecords that in November, 

 1868, a steamer sailed for a quarter of an hour thi'ough a swarm of lemmings 

 which extended as far as the eye could reach over the Trondhjemsfjord. 

 (Journal Linnean Society of London, Vol. 13, p. 33, 1878.) 



6T. T. Somerville, Proc. Zool. Society of London, 1891, pp. r)55-658. Robert 

 Collett, Journal Linnean Soc. of London, Vol. 18, pp. 327-33-t, 1878. 



