William Henry Flower,
Encyclopædia Britannica, Ninth Edition…
“ When any combination of circumstances has occasioned an increase of the numbers of the lemmings in their ordinary dwelling places, impelled by the restless or migratory instinct possessed in a less developed degree by so many of their congeners, a movement takes place at the edge of the elevated plateau, and a migration towards the lower-lying land begins. The whole body moves forward slowly, always advancing in the same general direction in which they originally started, but following more or less the course of the great valleys. They only travel by night ”
