1895.] MYODES LEMMUS. 38 


when they once reach the valleys, they meet with localities which 
are quite foreign to them. They then continue blindly on, endeavour- 
ing to find a home corresponding to that they have left, but which 
they never regain. The migratory individuals proceed hopelessly om 
ido å certaim death. 
Qnly in rare instances do they wander upwards, and it has, 
probably, never been clearly proved that individuals which reached 
the bottom of the valleys have ever succeeded in reaching å 
new plateau, on which they could settle down. 
It is otherwise in respect to the other producers of multi- 
tudes, such as Å. gregarius and Å. glareolus. "These inhabit the 
lowlands. The bulk of them are, too, pressed out to the sides 
in a prolific year, but the localities they come to duriug this, 
correspond to their own; they therefore settle down more quickly, 
and their migrations are Shorter. 
. It follows of itself that the migratory tendency does not 
affect all individuals. Some will invariably remain behind, all- 
though the mountains on the cessation of å migratory year ap- 
pear to us to be deserted. 
The migrations of the lemming are thus å natural conse- 
quence of their temporarily strong vitality. The so-called *mi- 
gratory instinct” is not peculiar to this species, but migration 
can, invariably, be proved to take place when any species of 
animal has abnormally increased in numbers. 
9. Centres of the Migrations, 
In some years M. lemmus brings more litters into the world 
than in others, without any migration resulting. If the condi- 
tions are favourable, å minor over production occurs, and causes 
minor migrations; such take place each year at one or another 
Spot in the country, and may be only indicated by a few indivi- 
duals appearing sporadically in the nearest valleys. 
Vid.-Selsk. Forh. 1895. No.3. 3 
