PÅ 
2 

32 | R. COLLETT. - [No. 814 
The same is the case with other Arvicoles, such as Å. gla- 
reolus and Å. gregarius. But also with these an extension side- 
ways may be traced during prolific years, and instances are 
constantly reported of distriets then being suddenly inundated 
by the immigration of one or other of these species from the 
neighbouring territories. 
This migratory tendency is somewhat more pronounced in My- 
odes schisticolcr (the wood lemming). This little, striking, species, 
inhabits exclusively the spruce forests, where it moves about in 
mossy tracts amongst windfalls and stones; but owing to its ob- 
scure life, its habits are almost unknown, and its presence even 
is wellnigh unperceivable except just during prolific (and mi- 
gratory) years. Their migrations do not appear, however, to be 
of any great extent, and seldom proceed far beyond the out- 
skirts of the coniferous woods; but here they may be met with 
plentifully enough, when one has once got to know their noctur- 
nal habits. They advance in straggling hordes which recall to 
mind the marches of their larger relative; moreover they exhi- 
bit å similar tendency to take to the water in which they often 
are lost in great numbers. 
As before mentioned, M. lemmus probably stands at the 
head of all Norwegian rodents in its powers, under certain cir- 
cumstances, of producing an excessive number of individualsl. 
The enormous multitudes require increased space, amd the in- 
dividuals, which, under normal conditions, have each an excessively 
large tract at their disposal, cannot, om account of their disposition, 
bear the unaccustomed proximity of the numerous neighbours. In- 
voluntarily the individuals are pressed out to the sides until the 
edge of the mountain is reached. In aå short time they enjoy them- 
selves there, and the old individuals willingly bred in the upper re- 
gions of the forests, where, at other times, they are entirely wanting. 
New swarms, however, follow om; they could mot return, but the 
journey proceeds omvards down the sides of the mountains, and 
1 Analogies are, however, not wanting from other countries in Eu- 
rope, where Arvicola gregarius (aud occasionally Å. glareolus) appears 
able, during certain years, to yield å similar abnormal increase. 


