b AN ECONOMIC STUDY OF FIELD .MK K. 
Outside the insect world the most striking example- of occasional 
excessive multiplication 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 Microtince, a subfamily 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 somewhat periodic migrations of lemmings 
(Lemmas 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, living 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 prey 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 Lemmas* they go in a direct course 
and nothing stops their progress, neither rivers or arms of the 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 lying weak or half dead 
with fatigue on the banks, after passing a river, they will give them 
a Prof. Robert Collett, of Christiania, Norway, records that in November, 
1868, a steamer sailed for a quarter of an hour through 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.) 
&T. T. Somerville. Proc. Zool. Society of London, 1891, pp. 655-658. Robert 
Collett. Journal Linnean Soe. of London, Vol. 13, pp. 327-334, 1878. 
